Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing

Resistance of rocky intertidal communities to oceanic climate fluctuations

A powerful way to predict how ecological communities will respond to future climate change is to test how they have responded to the climate of the past. We used climate oscillations including the Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation, and El Niño Southern Oscillation (ENSO) and variation in upwelling, air temperature, and sea temperatures to test the sensitivity of nearshore rocky intertidal communities to climate variability. Prior research shows that multiple ecological processes of key taxa (growth, recruitment, and physiology) were sensitive to environmental variation during this time frame. We also investigated the effect of the concurrent sea star wasting disease outbreak in 2013-2014. We surveyed nearly 150 taxa from 11 rocky intertidal sites in Oregon and northern California annually for up to 14-years (2006-2020) to test if community structure (i.e., the abundance of functional groups) and diversity were sensitive to past environmental variation. We found little to no evidence that these communities were sensitive to annual variation in any of the environmental measures, and that each metric was associated with < 8.6% of yearly variation in community structure. Only the years elapsed since the outbreak of sea star wasting disease had a substantial effect on community structure, but in the mid-zone only where spatially dominant mussels are a main prey of the keystone predator sea star, Pisaster ochraceus. We conclude that the established sensitivity of multiple ecological processes to annual fluctuations in climate has not yet scaled up to influence community structure. Hence, the rocky intertidal system along this coastline appears resistant to the range of oceanic climate fluctuations that occurred during the study. However, given ongoing intensification of climate change and increasing frequencies of extreme events, future responses to climate change seem likely.

Climate change impacts on mesophotic regions of the Great Barrier Reef

Climate change projections for coral reefs are founded exclusively on sea surface temperatures (SST). While SST projections are relevant for the shallowest reefs, neglecting ocean stratification overlooks the striking differences in temperature experienced by deeper reefs for all or part of the year. Density stratification creates a buoyancy barrier partitioning the upper and lower parts of the water column. Here, we mechanistically downscale climate models and quantify patterns of thermal stratification above mesophotic corals (depth 30 to 50 m) of the Great Barrier Reef (GBR). Stratification insulates many offshore regions of the GBR from heatwaves at the surface. However, this protection is lost once global average temperatures exceed ~3 °C above preindustrial, after which mesophotic temperatures surpass a recognized threshold of 30 °C for coral mortality. Bottom temperatures on the GBR (30 to 50 m) from 2050 to 2060 are estimated to increase by ~0.5 to 1 °C under lower climate emissions (SSP1-1.9) and ~1.2 to 1.7 °C under higher climate emissions (SSP5-8.5). In short, mesophotic coral reefs are also threatened by climate change and research might prioritize the sensitivity of such corals to stress.

Historical changes in wind-driven ocean circulation drive pattern of Pacific warming

The tropical Pacific warming pattern since the 1950s exhibits two warming centers in the western Pacific (WP) and eastern Pacific (EP), encompassing an equatorial central Pacific (CP) cooling and a hemispheric asymmetry in the subtropical EP. The underlying mechanisms of this warming pattern remain debated. Here, we conduct ocean heat decompositions of two coupled model large ensembles to unfold the role of wind-driven ocean circulation. When wind changes are suppressed, historical radiative forcing induces a subtropical northeastern Pacific warming, thus causing a hemispheric asymmetry that extends toward the tropical WP. The tropical EP warming is instead induced by the cross-equatorial winds associated with the hemispheric asymmetry, and its driving mechanism is southward warm Ekman advection due to the off-equatorial westerly wind anomalies around 5°N, not vertical thermocline adjustment. Climate models fail to capture the observed CP cooling, suggesting an urgent need to better simulate equatorial oceanic processes and thermal structures.

Robust changes in global subtropical circulation under greenhouse warming

The lower tropospheric subtropical circulation (SC) is characterized by monsoons and subtropical highs, playing an important role in global teleconnections and climate variability. The SC changes in a warmer climate are influenced by complex and region-specific mechanisms, resulting in uneven projections worldwide. Here, we present a method to quantify the overall intensity change in global SC, revealing a robust weakening across CMIP6 models. The weakening is primarily caused by global-mean surface warming, and partly counteracted by the direct CO2 effect. The direct CO2 effect is apparent in the transient response but is eventually dominated by the surface warming effect in a slow response. The distinct response timescales to global-mean warming and direct CO2 radiative forcing can well explain the time-varying SC changes in other CO2 emission scenarios. The declined SC implies a contracted monsoon range and drying at its boundary with arid regions under CO2-induced global warming.

Forced changes in the Pacific Walker circulation over the past millennium

The Pacific Walker circulation (PWC) has an outsized influence on weather and climate worldwide. Yet the PWC response to external forcings is unclear1,2, with empirical data and model simulations often disagreeing on the magnitude and sign of these responses3. Most climate models predict that the PWC will ultimately weaken in response to global warming4. However, the PWC strengthened from 1992 to 2011, suggesting a significant role for anthropogenic and/or volcanic aerosol forcing5, or internal variability. Here we use a new annually resolved, multi-method, palaeoproxy-derived PWC reconstruction ensemble (1200-2000) to show that the 1992-2011 PWC strengthening is anomalous but not unprecedented in the context of the past 800 years. The 1992-2011 PWC strengthening was unlikely to have been a consequence of volcanic forcing and may therefore have resulted from anthropogenic aerosol forcing or natural variability. We find no significant industrial-era (1850-2000) PWC trend, contrasting the PWC weakening simulated by most climate models3. However, an industrial-era shift to lower-frequency variability suggests a subtle anthropogenic influence. The reconstruction also suggests that volcanic eruptions trigger El Niño-like PWC weakening, similar to the response simulated by climate models.

Anthropogenic fingerprints in daily precipitation revealed by deep learning

According to twenty-first century climate-model projections, greenhouse warming will intensify rainfall variability and extremes across the globe1-4. However, verifying this prediction using observations has remained a substantial challenge owing to large natural rainfall fluctuations at regional scales3,4. Here we show that deep learning successfully detects the emerging climate-change signals in daily precipitation fields during the observed record. We trained a convolutional neural network (CNN)5 with daily precipitation fields and annual global mean surface air temperature data obtained from an ensemble of present-day and future climate-model simulations6. After applying the algorithm to the observational record, we found that the daily precipitation data represented an excellent predictor for the observed planetary warming, as they showed a clear deviation from natural variability since the mid-2010s. Furthermore, we analysed the deep-learning model with an explainable framework and observed that the precipitation variability of the weather timescale (period less than 10 days) over the tropical eastern Pacific and mid-latitude storm-track regions was most sensitive to anthropogenic warming. Our results highlight that, although the long-term shifts in annual mean precipitation remain indiscernible from the natural background variability, the impact of global warming on daily hydrological fluctuations has already emerged.

A well-oxygenated eastern tropical Pacific during the warm Miocene

The oxygen content of the oceans is susceptible to climate change and has declined in recent decades1, with the largest effect in oxygen-deficient zones (ODZs)2, that is, mid-depth ocean regions with oxygen concentrations <5 μmol kg-1 (ref. 3). Earth-system-model simulations of climate warming predict that ODZs will expand until at least 2100. The response on timescales of hundreds to thousands of years, however, remains uncertain3-5. Here we investigate changes in the response of ocean oxygenation during the warmer-than-present Miocene Climatic Optimum (MCO; 17.0-14.8 million years ago (Ma)). Our planktic foraminifera I/Ca and δ15N data, palaeoceanographic proxies sensitive to ODZ extent and intensity, indicate that dissolved-oxygen concentrations in the eastern tropical Pacific (ETP) exceeded 100 µmol kg-1 during the MCO. Paired Mg/Ca-derived temperature data suggest that an ODZ developed in response to an increased west-to-east temperature gradient and shoaling of the ETP thermocline. Our records align with model simulations of data from recent decades to centuries6,7, suggesting that weaker equatorial Pacific trade winds during warm periods may lead to decreased upwelling in the ETP, causing equatorial productivity and subsurface oxygen demand to be less concentrated in the east. These findings shed light on how warm-climate states such as during the MCO may affect ocean oxygenation. If the MCO is considered as a possible analogue for future warming, our findings seem to support models suggesting that the recent deoxygenation trend and expansion of the ETP ODZ may eventually reverse3,4.

Future Indian Ocean warming patterns

Most future projections conducted with coupled general circulation models simulate a non-uniform Indian Ocean warming, with warming hotspots occurring in the Arabian Sea (AS) and the southeastern Indian Ocean (SEIO). But little is known about the underlying physical drivers. Here, we are using a suite of large ensemble simulations of the Community Earth System Model 2 to elucidate the causes of non-uniform Indian Ocean warming. Strong negative air-sea interactions in the Eastern Indian Ocean are responsible for a future weakening of the zonal sea surface temperature gradient, resulting in a slowdown of the Indian Ocean Walker circulation and the generation of southeasterly wind anomalies over the AS. These contribute to anomalous northward ocean heat transport, reduced evaporative cooling, a weakening in upper ocean vertical mixing and an enhanced AS future warming. In contrast, the projected warming in the SEIO is related to a reduction of low-cloud cover and an associated increase in shortwave radiation. Therefore, the regional character of air-sea interactions plays a key role in promoting future large-scale tropical atmospheric circulation anomalies with implications for society and ecosystems far outside the Indian Ocean realm.

North Atlantic cooling triggered a zonal mode over the Indian Ocean during Heinrich Stadial 1

Abrupt changes in the Atlantic meridional overturning circulation (AMOC) are thought to affect tropical hydroclimate through adjustment of the latitudinal position of the intertropical convergence zone (ITCZ). Heinrich Stadial 1 (HS1) involves the largest AMOC reduction in recent geological time; however, over the tropical Indian Ocean (IO), proxy records suggest zonal anomalies featuring intense, widespread drought in tropical East Africa versus generally wet but heterogeneous conditions in the Maritime Continent. Here, we synthesize proxy data and an isotope-enabled transient deglacial simulation and show that the southward ITCZ shift over the eastern IO during HS1 strengthens IO Walker circulation, triggering an east-west precipitation dipole across the basin. This dipole reverses the zonal precipitation anomalies caused by the exposed Sunda and Sahul shelves due to glacial lower sea level. Our study illustrates how zonal modes of atmosphere-ocean circulation can amplify or reverse global climate anomalies, highlighting their importance for future climate change.

Enhanced ocean oxygenation during Cenozoic warm periods

Dissolved oxygen (O₂) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming^1,2. However, models provide an unclear picture of future ODZ change in both the near term and the long term^3–6. The paleoclimate record can help explore the possible range of ODZ changes in warmer-than-modern periods. Here we use foraminifera-bound nitrogen (N) isotopes to show that water-column denitrification in the eastern tropical North Pacific was greatly reduced during the Middle Miocene Climatic Optimum (MMCO) and the Early Eocene Climatic Optimum (EECO). Because denitrification is restricted to oxygen-poor waters, our results indicate that, in these two Cenozoic periods of sustained warmth, ODZs were contracted, not expanded. ODZ contraction may have arisen from a decrease in upwelling-fuelled biological productivity in the tropical Pacific, which would have reduced oxygen demand in the subsurface. Alternatively, invigoration of deep-water ventilation by the Southern Ocean may have weakened the ocean’s ‘biological carbon pump’, which would have increased deep-ocean oxygen. The mechanism at play would have determined whether the ODZ contractions occurred in step with the warming or took centuries or millennia to develop. Thus, although our results from the Cenozoic do not necessarily apply to the near-term future, they might imply that global warming may eventually cause ODZ contraction.

By using foraminifera-bound nitrogen isotopes, it is shown that, during two warm periods of the Cenozoic, oxygen-deficient zones contracted rather than expanded, suggesting that global warming may not necessarily lead to increased oceanic anoxia.

Varying contributions of fast and slow responses cause asymmetric tropical rainfall change between CO2 ramp-up and ramp-down

Tropical rainfall is important for regional climate around the globe. In a warming climate forced by rising CO₂, the tropical rainfall will increase over the equatorial Pacific where sea surface warming is locally enhanced. Here, we analyze an idealized CO₂ removal experiment from the Carbon Dioxide Removal Model Intercomparison Project (CDRMIP) and show that the tropical rainfall change features a stronger pattern during CO₂ ramp-down than ramp-up, even under the same global mean temperature increase, such as the 2 °C goal of the Paris Agreement. The tropical rainfall during CO₂ ramp-down increases over the equatorial Pacific with a southward extension, and decreases over the Pacific intertropical convergence zone and South Pacific convergence zone. The asymmetric rainfall changes between CO₂ ramp-down and ramp-up result from time-varying contributions of the fast and slow oceanic responses to CO₂ forcing, defined as the responses to abrupt CO₂ forcing in the first 10 years and thereafter, respectively, in the abrupt-4xCO2 experiment. The fast response follows the CO₂ evolution, but the slow response does not peak until 60 years after the CO₂ peak. The slow response features a stronger El Niño-like pattern, as the ocean dynamical thermostat effect is suppressed under stronger subsurface warming. The delayed and stronger slow response leads to stronger tropical rainfall changes during CO₂ ramp-down. Our results indicate that returning the global mean temperature increase to below a certain goal, such as 2 °C, by removing CO₂, may fail to restore tropical convection distribution, with potentially devastating effects on climate worldwide.

Surface warming-induced global acceleration of upper ocean currents

How the ocean circulation changes in a warming climate is an important but poorly understood problem. Using a global ocean model, we decompose the problem into distinct responses to changes in sea surface temperature, salinity, and wind. Our results show that the surface warming effect, a robust feature of anthropogenic climate change, dominates and accelerates the upper ocean currents in 77% of the global ocean. Specifically, the increased vertical stratification intensifies the upper subtropical gyres and equatorial currents by shoaling these systems, while the differential warming between the Southern Ocean upwelling zone and the region to the north accelerates surface zonal currents in the Southern Ocean. In comparison, the wind stress and surface salinity changes affect regional current systems. Our study points a way forward for investigating ocean circulation change and evaluating the uncertainty.

Projections of faster onset and slower decay of El Niño in the 21st century

Future changes in the seasonal evolution of the El Niño-Southern Oscillation (ENSO) during its onset and decay phases have received little attention by the research community. This work investigates the projected changes in the spatio-temporal evolution of El Niño events in the 21^st Century (21 C), using a multi-model ensemble of coupled general circulation models subjected to anthropogenic forcing. Here we show that El Niño is projected to (1) grow at a faster rate, (2) persist longer over the eastern and far eastern Pacific, and (3) have stronger and distinct remote impacts via teleconnections. These changes are attributable to significant changes in the tropical Pacific mean state, dominant ENSO feedback processes, and an increase in stochastic westerly wind burst forcing in the western equatorial Pacific, and may lead to more significant and persistent global impacts of El Niño in the future.

Asymmetrical response of summer rainfall in East Asia to CO2 forcing

Understanding the regional hydrological response to varying CO₂ concentration is critical for cost-benefit analysis of mitigation and adaptation polices in the near future. To characterize summer monsoon rainfall change in East Asia in a changing CO₂ pathway, we used the Community Earth System Model (CESM) with 28 ensemble members in which the CO₂ concentration increases at a rate of 1% per year until its quadrupling peak, i.e., 1468 ppm (ramp-up period), followed by a decrease of 1% per year until the present-day climate conditions, i.e., 367 ppm (ramp-down period). Although the CO₂ concentration change is symmetric in time, the amount of summer rainfall anomaly in East Asia is increased 42% during a ramp-down period than that during a ramp-up period when the two periods of the same CO₂ concentration are compared. This asymmetrical rainfall response is mainly due to an enhanced El Niño-like warming pattern as well as its associated increase in the sea surface temperature in the western North Pacific during a ramp-down period. These sea surface temperature patterns enhance the atmospheric teleconnections and the local meridional circulations around East Asia, resulting in more rainfall over East Asia during a ramp-down period. This result implies that the removal of CO₂ does not guarantee the return of regional rainfall to the previous climate state with the same CO₂ concentration.

Over-projected Pacific warming and extreme El Niño frequency due to CMIP5 common biases

Extreme El Niño events severely disrupt the global climate, causing pronounced socio-economic losses. A prevailing view is that extreme El Niño events, defined by total precipitation or convection in the Niño3 area, will increase 2-fold in the future. However, this projected change was drawn without removing the potential impacts of Coupled Model Intercomparison Project phase 5 (CMIP5) models’ common biases. Here, we find that the models’ systematic biases in simulating tropical climate change over the past century can reduce the reliability of the projected change in the Pacific sea surface temperature (SST) and its related extreme El Niño frequency. The projected Pacific SST change, after removing the impacts of 13 common biases, displays a ‘La Niña-like’ rather than ‘El Niño-like’ change. Consequently, the extreme El Niño frequency, which is highly linked to the zonal distribution of the Pacific SST change, would remain mostly unchanged under CMIP5 warming scenarios. This finding increases confidence in coping with climate risks associated with global warming.

A very likely weakening of Pacific Walker Circulation in constrained near-future projections

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51-72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models' credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

Recent increases in tropical cyclone precipitation extremes over the US east coast

The impacts of inland flooding caused by tropical cyclones (TCs), including loss of life, infrastructure disruption, and alteration of natural landscapes, have increased over recent decades. While these impacts are well documented, changes in TC precipitation extremes-the proximate cause of such inland flooding-have been more difficult to detect. Here, we present a latewood tree-ring-based record of seasonal (June 1 through October 15) TC precipitation sums (ΣTCP) from the region in North America that receives the most ΣTCP: coastal North and South Carolina. Our 319-y-long ΣTCP reconstruction reveals that ΣTCP extremes (≥0.95 quantile) have increased by 2 to 4 mm/decade since 1700 CE, with most of the increase occurring in the last 60 y. Consistent with the hypothesis that TCs are moving slower under anthropogenic climate change, we show that seasonal ΣTCP along the US East Coast are positively related to seasonal average TC duration and TC translation speed.

Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects

[Figure: see text].

How tropical cyclone flooding caused erosion and dispersal of mercury-contaminated sediment in an urban estuary: The impact of Hurricane Harvey on Buffalo Bayou and the San Jacinto Estuary, Galveston Bay, USA

Hurricane Harvey (Harvey), a slow-moving storm, struck the Texas coast as a category 4 hurricane. Over the course of 53 days, the floodwaters of Harvey delivered 14 × 10⁹ m³ of freshwater to Galveston Bay. This resulted in record flooding of Houston bayous and waterways, all of which drained into the San Jacinto Estuary (SJE,) with its main tributaries being Buffalo Bayou and the San Jacinto River. The lower SJE and lower Buffalo Bayou has experienced up to 3 m of land subsidence in the past 100 years and, as a result, prior to Hurricane Harvey, up to 2 m of sediment within the upper seabed contained an archive of high concentrations of Total Hg (HgT) and other particle-bound and porewater contaminants. Within the SJE, Harvey eroded at least 48 cm of the sediment column, resulting in the transport of an estimated 16.4 × 10⁶ tons of sediment and at least 2 tons of Hg into Galveston Bay. This eroded sediment was replaced by a Harvey storm deposit of 7.73 × 10⁶ tons of sediment and 0.96 tons within the SJE, mostly sourced from Buffalo Bayou. Considering that the frequency of slow-moving tropical cyclones capable of delivering devastating rainfall may be increasing, then one can expect that delivery of Hg and other contaminants from the archived sediment within urbanized estuaries will increase and that what happened during Harvey is a harbinger of what is to come.

Solar geoengineering may not prevent strong warming from direct effects of CO2 on stratocumulus cloud cover

Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO₂ can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.

Walker circulation response to extratropical radiative forcing

Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude-to-Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns.

Characterization of south central Pacific Ocean wind regimes in present and future climate for pearl farming application

In the South Pacific (SP) pearl farming atolls, wind is the main driver of lagoon water circulation, affecting dispersal and survival of pearl oyster larvae. To characterize typical wind conditions in the SP, wind regime classifications are performed from regional climate simulations using the WRF model, for present-day and for the end of the 21st century under RCP8.5 scenario conditions. At the daily time-scale, 4 regimes are identified: a trade-wind, a north-easterly, and two easterly regimes. Their characteristics are driven by large-scale circulation and climate modes of variability. In future projection, all regimes are characterized by a ~15% wind speed increase, while directions and occurrence frequencies undergo marginal changes. At the monthly time-scale that corresponds to pearl oyster pelagic larval duration, nine wind regimes are determined including three regimes with wind reversals. These regimes can be used to model typical lagoon conditions during larval dispersal.

Consequences of drought severity for tropical live oak (Quercus oleoides) in Mesoamerica

In two Costa Rican and three Honduran sites that vary in rainfall and soil properties, we used natural isotopes, a soil water balance model, and broad-scale climate-based drought indices to study shifts in water use with ontogeny from seedlings to mature tropical live oak (Quercus oleoides) trees. Water use patterns help to explain persistence of this broadly distributed species in Mesoamerica and to evaluate likely threats of ongoing climate changes. At the end of the dry season, soil δ¹⁸ O profiles can be described by one-phase exponential decay curves. Minimum values reflect geographic origins of the last significant rain event, and curvature is inversely related to canopy closure, demonstrating its role in controlling topsoil evaporation. Partitioning of soil water sources for transpiration was analyzed with a mixing model. In the Costa Rican sites, in a relatively dry year, saplings and mature trees took up water from the upper soil. In a relatively wet year in the Honduran sites, we observed deeper water extraction. In all sites, soil storage dampens extreme variation in water availability. The size dependence of water uptake with larger stems exploiting deeper layers is translated into variation in bulk leaf δ¹³ C-based water use efficiency (WUE) with the exception of mature trees. From 1932 to 2015, drought severity was evaluated with the Standardized Precipitation Evapotranspiration Index (SPEI) concurrently with simulations of the soil water balance model. Drought occurrence increased, regardless of the time period, averaged across 6, 12, or 24 months. All ontogenetic stages in all populations experienced frequent water limitation. We found evidence for linear trends toward aridification with increases of return periods of drought for October SPEI-24 declining from 42 to 6 yr in Costa Rica and from 21 to 7 yr in Honduras and recent occurrence of multiyear droughts from 2013 to 2016. October SPEI-12 and SPEI-24 were significantly related to the Oceanic Niño Indices demonstrating that local inter-annual variations in drought severity in Mesoamerica are modulated by large-scale climate forces. Drought severity in the near-term future depends on the extent to which the Pacific will adopt a more La Niña-like vs. a more El Niño-like state under ongoing climatic changes.

Radioisotopes demonstrate changes in global atmospheric circulation possibly caused by global warming

In this paper, we present a new method to study global atmospheric processes and their changes during the last decade. A cosmogenic radionuclide measured at ground-level, beryllium-7, is utilized as a proxy to study atmospheric dynamics. Beryllium-7 has two advantages: First, this radionuclide, primarily created in the lower stratosphere, attaches to aerosols that are transported downwards to the troposphere and travel around the globe with the general atmospheric circulation. By monitoring these particles, we can provide a global, simple, and sustainable way to track processes such as multi-annual variation of the troposphere, tropopause heightening, position and speed of atmospheric interface zones, as well as the poleward movement and stalling patterns of jet streams. Second, beryllium-7 is a product of cosmic rays which are themselves directly linked to solar activity and the earth magnetic field. This study shows whether beryllium-7 observed concentration changes are correlated with such natural processes or independent of them. Our work confirms that major changes in the atmospheric circulation are currently ongoing, even though timeseries are too short to make climatological assessments. We provide solid evidence of significant and progressive changes of the global atmospheric circulation as well as modifications of tropopause heights over the past decade. As the last decade happened to be the warmest on record, this analysis also indicates that the observed changes are, at least to some extent, attributable to global warming.

Large shift of the Pacific Walker Circulation across the Cenozoic

Fluctuations in the Pacific Walker Circulation (PWC), a zonally oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions. Through a suite of coupled model simulations on tectonic timescales, we demonstrate that the PWC was ∼38° broader and ∼5% more intense during the Early Eocene relative to present. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient. However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remaining steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12°) and weakening (by ∼22%), which we show here is linked with the closure of the tropical seaways. Moreover, our results suggest that rising CO₂ favors a weaker PWC under the same land-sea configurations, a robust feature across the large spread of Cenozoic climates considered here, supporting a weakening of the PWC in a warmer future.

On the essentials of drought in a changing climate

Droughts of the future are likely to be more frequent, severe, and longer lasting than they have been in recent decades, but drought risks will be lower if greenhouse gas emissions are cut aggressively. This review presents a synopsis of the tools required for understanding the statistics, physics, and dynamics of drought and its causes in a historical context. Although these tools have been applied most extensively in the United States, Europe, and the Amazon region, they have not been as widely used in other drought-prone regions throughout the rest of the world, presenting opportunities for future research. Water resource managers, early career scientists, and veteran drought researchers will likely see opportunities to improve our understanding of drought.

Hydrological cycle changes under global warming and their effects on multiscale climate variability

Despite a globally uniform increase in the concentrations of emitted greenhouse gases, radiatively forced surface warming can have significant spatial variations. These define warming patterns that depend on preexisting climate states and through atmospheric and oceanic dynamics can drive changes of the hydrological cycle with global-scale feedbacks. Our study reviews research progress on the hydrological cycle changes and their effects on multiscale climate variability. Overall, interannual variability is expected to become stronger in the Pacific and Indian Oceans and weaker in the Atlantic. Global monsoon rainfall is projected to increase and the wet season to lengthen despite a slowdown of atmospheric circulation. Strong variations among monsoon regions are likely to emerge, depending on surface conditions such as orography and land-sea contrast. Interdecadal climate variability is expected to modulate the globally averaged surface temperature change with pronounced anomalies in the polar and equatorial regions, leading to prolonged periods of enhanced or reduced warming. It is emphasized that advanced global observations, regional simulations, and process-level investigations are essential for improvements in understanding, predicting, and projecting the modes of climate variability, monsoon sensitivity, and energetic fluctuations in a warming climate.

Proposing the solar-wind energy flux hypothesis as a driver of inter-annual variation in tropical tree reproductive effort

PREMISE

The El Niño Southern Oscillation (ENSO) affects tropical environmental conditions, potentially altering ecosystem function as El Niño events interact with longer-term climate change. Anomalously warm equatorial Pacific Ocean temperatures affect rainfall and temperature throughout the tropics and coincide with altered leaf flush phenology and increased fruit production in wet tropical forests; however, the understanding of mechanisms underlying this pattern is limited. There is evidence that increases in tropical tree reproduction anticipate El Niño onset, motivating the continued search for a global driver of tropical angiosperm reproduction. We present the solar-wind energy flux hypothesis: that physical energy influx to the Earth's upper atmosphere and magnetosphere, generated by a positive anomaly in the solar wind preceding El Niño development, cues tropical trees to increase resource allocation to reproduction.

METHODS

We test this hypothesis using 19 years of data from Luquillo, Puerto Rico, correlating them with measures of solar-wind energy.

RESULTS

From 1994 to 2013, the solar-wind energy flux into Earth's magnetosphere (E_in ) was more strongly correlated with the number of species fruiting and flowering than the Niño 3.4 climate index, despite Niño 3.4 being previously identified as a driver of interannual increases in reproduction.

CONCLUSIONS

Changes in the global magnetosphere and thermosphere conditions from increased solar-wind energy affect global atmospheric pressure and circulation patterns, principally by weakening the Walker circulation. We discuss the idea that these changes cue interannual increases in tropical tree reproduction and act through an unidentified mechanism that anticipates and synchronizes the reproductive output of the tropical trees with El Niño.

Rainfall variations in central Indo-Pacific over the past 2,700 y

We present a high-resolution, replicated speleothem δ¹⁸O record from Klang Cave in southern Thailand that characterizes rainfall variation in NCIP over the past 2,700 y. This record reveals notable dry climate conditions during the current and past warm periods, similar to the observations in SCIP, which resemble enhanced El Niño-like conditions. Using a newly developed ITCZ shift index, we find a southward shifted ITCZ during the early MWP and the CWP. Our results suggest that detecting changes in rainfall due to anthropogenic forcing still remains indistinguishable from natural variability in the northern tropics.

Tropical rainfall variability is closely linked to meridional shifts of the Intertropical Convergence Zone (ITCZ) and zonal movements of the Walker circulation. The characteristics and mechanisms of tropical rainfall variations on centennial to decadal scales are, however, still unclear. Here, we reconstruct a replicated stalagmite-based 2,700-y-long, continuous record of rainfall for the deeply convective northern central Indo-Pacific (NCIP) region. Our record reveals decreasing rainfall in the NCIP over the past 2,700 y, similar to other records from the northern tropics. Notable centennial- to decadal-scale dry climate episodes occurred in both the NCIP and the southern central Indo-Pacific (SCIP) during the 20th century [Current Warm Period (CWP)] and the Medieval Warm Period (MWP), resembling enhanced El Niño-like conditions. Further, we developed a 2,000-y-long ITCZ shift index record that supports an overall southward ITCZ shift in the central Indo-Pacific and indicates southward mean ITCZ positions during the early MWP and the CWP. As a result, the drying trend since the 20^th century in the northern tropics is similar to that observed during the past warm period, suggesting that a possible anthropogenic forcing of rainfall remains indistinguishable from natural variability.

Maritime Continent water cycle regulates low-latitude chokepoint of global ocean circulation

The Maritime Continent (MC) is a low-latitude chokepoint of the world oceans with the Indonesian throughflow (ITF) linking the Indo-Pacific oceans, influencing global ocean circulation, climate, and biogeochemistry. While previous studies suggested that South-China-Sea freshwaters north of the MC intruding the Indonesian Seas weaken the ITF during boreal winter, the impact of the MC water cycle on the ITF has not been investigated. Here we use ocean-atmosphere-land satellite observations to reveal the dominant contribution of the MC monsoonal water cycle to boreal winter-spring freshening in the Java Sea through local precipitation and runoff from Kalimantan, Indonesia. We further demonstrate that the freshening corresponds to a reduced southward pressure gradient that would weaken the ITF. Therefore, the MC water cycle plays a critical role regulating ITF seasonality. The findings have strong implications to longer-term variations of the ITF associated with the variability and change of Indo-Pacific climate and MC water cycle.

Slowdown of the Walker circulation at solar cycle maximum

The Pacific Walker Circulation (PWC) fluctuates on interannual and multidecadal timescales under the influence of internal variability and external forcings. Here, we provide observational evidence that the 11-y solar cycle (SC) affects the PWC on decadal timescales. We observe a robust reduction of east-west sea-level pressure gradients over the Indo-Pacific Ocean during solar maxima and the following 1-2 y. This reduction is associated with westerly wind anomalies at the surface and throughout the equatorial troposphere in the western/central Pacific paired with an eastward shift of convective precipitation that brings more rainfall to the central Pacific. We show that this is initiated by a thermodynamical response of the global hydrological cycle to surface warming, further amplified by atmosphere-ocean coupling, leading to larger positive ocean temperature anomalies in the equatorial Pacific than expected from simple radiative forcing considerations. The observed solar modulation of the PWC is supported by a set of coupled ocean-atmosphere climate model simulations forced only by SC irradiance variations. We highlight the importance of a muted hydrology mechanism that acts to weaken the PWC. Demonstration of this mechanism acting on the 11-y SC timescale adds confidence in model predictions that the same mechanism also weakens the PWC under increasing greenhouse gas forcing.

Regulation of atmospheric circulation controlling the tropical Pacific precipitation change in response to CO2 increases

The spatial pattern of precipitation responses to CO₂ concentration increases significantly influences global weather and climate variability by altering the location of tropical heating in a warmer climate. In this study, we analyze the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model projections of tropical Pacific rainfall response to quadrupled increase of CO₂. We found that the precipitation changes to the CO₂ concentration increase cannot be interpreted by a weakening or strengthening of large-scale east-west coupling across the tropical Pacific basin, i.e., Walker circulation. By calculating the water vapor transport, we suggest instead that different responses of the Walker and Hadley circulations to the increasing CO₂ concentration shape the details of the spatial pattern of precipitation in the tropical Pacific. Therefore, more regionally perturbed circulations over the tropical Pacific, which is influenced by the mean state change in the tropical Pacific and the enhanced precipitation outside the tropical Pacific, lead to greater increases in precipitation in the western equatorial Pacific as compared to the eastern tropical Pacific in a warmer climate.

Oman coral δ18O seawater record suggests that Western Indian Ocean upwelling uncouples from the Indian Ocean Dipole during the global-warming hiatus

The Indian Ocean Dipole (IOD) is an interannual mode of climate variability in the Indian Ocean that has intensified with 20^th century global-warming. However, instrumental data shows a global-warming hiatus between the late-1990s and 2015. It is presently not clear how the global-warming hiatus affects modes of climate variability such as the IOD, and their basin-wide ocean-atmosphere teleconnections. Here, we present a 26-year long, biweekly record of Sr/Ca and δ¹⁸O from a Porites coral drilled in the Gulf of Oman. Sea surface temperature (SST_anom) is calculated from Sr/Ca ratios, and seawater δ¹⁸O (δ¹⁸O_sw-anom) is estimated by subtracting the temperature component from coral δ¹⁸O. Our δ¹⁸O_sw-anom record reveals a significant regime shift in 1999, towards lower mean δ¹⁸O_sw values, reflecting intensified upwelling in the western Indian Ocean. Prior to the 1999 regime shift, our SST_anom and δ¹⁸O_sw-anom show a clear IOD signature, with higher values in the summer of positive-IOD years due to weakened upwelling. The IOD signature in SST_anom and δ¹⁸O_sw-anom disappears with the overall intensification of upwelling after the 1999 regime shift. The inferred increase in upwelling is likely driven by an intensified Walker circulation during the global-warming hiatus. Upwelling in the Western Indian Ocean uncouples from the IOD.

Interdecadal Variations in the Walker Circulation and Its Connection to Inhomogeneous Air Temperature Changes from 1961–2012

The tropical Pacific Walker circulation (PWC) is fundamentally important to global atmospheric circulation, and changes in it have a vital influence on the weather and climate systems. A novel three-pattern decomposition of a global atmospheric circulation (3P-DGAC) method, which can be used to investigate atmospheric circulations including the PWC, was proposed in our previous study. Therefore, the present study aims to examine the capability of this 3P-DGAC method to acquire interdecadal variations in the PWC and its connection to inhomogeneous air temperature changes in the period from 1961–2012. Our findings reveal that interdecadal variations in the PWC, i.e., weakening (strengthening) between the periods 1961–1974 and 1979–1997 (1979–1997 and 1999–2012), can be observed using the zonal stream function (ZSF) derived from the 3P-DGAC method. Enhancement of the PWC is also associated with the strengthening and weakening of zonal circulations in the tropical Indian Ocean (IOC) and Atlantic (AOC), respectively, and vice versa, implying a connection between these zonal overturning circulations in the tropics. The interdecadal variations in the zonal circulations correspond well to inhomogeneous air temperature changes, i.e., an enhancement of the PWC is associated with a warming (cooling) of the air temperature from 1000 to 300 hPa in the western (mid–eastern) Pacific Ocean and a cooling (warming) of the air temperature in the tropopause in the western (mid–eastern) Pacific Ocean. Furthermore, a novel index for the PWC intensity based on air temperature is defined, and the capability of the novel index in representing the PWC intensity is evaluated. This novel index is potentially important for the prediction of the PWC by using dynamic equations derived from the 3P-DGAC method.

Rainfall trends and variation in the Maasai Mara ecosystem and their implications for animal population and biodiversity dynamics

Rainfall exerts a controlling influence on the availability and quality of vegetation and surface water for herbivores in African terrestrial ecosystems. We analyse temporal trends and variation in rainfall in the Maasai Mara ecosystem of East Africa and infer their implications for animal population and biodiversity dynamics. The data originated from 15 rain gauges in the Mara region (1965–2015) and one station in Narok Town (1913–2015), in Kenya’s Narok County. This is the first comprehensive and most detailed analysis of changes in rainfall in the region of its kind. Our results do not support the current predictions of the International Panel of Climate Change (IPCC) of very likely increases of rainfall over parts of Eastern Africa. The dry season rainfall component increased during 1935–2015 but annual rainfall decreased during 1962–2015 in Narok Town. Monthly rainfall was more stable and higher in the Mara than in Narok Town, likely because the Mara lies closer to the high-precipitation areas along the shores of Lake Victoria. Predominantly deterministic and persistent inter-annual cycles and extremely stable seasonal rainfall oscillations characterize rainfall in the Mara and Narok regions. The frequency of severe droughts increased and floods intensified in the Mara but droughts became less frequent and less severe in Narok Town. The timings of extreme droughts and floods coincided with significant periodicity in rainfall oscillations, implicating strong influences of global atmospheric and oceanic circulation patterns on regional rainfall variability. These changing rainfall patterns have implications for animal population dynamics. The increase in dry season rainfall during 1935–2015 possibly counterbalanced the impacts of resource scarcity generated by the declining annual rainfall during 1965–2015 in Narok Town. However, the increasing rainfall extremes in the Mara can be expected to create conditions conducive to outbreaks of infectious animal diseases and reduced vegetation quality for herbivores, particularly when droughts and floods persist over multiple years. The more extreme wet season rainfall may also alter herbivore space use, including migration patterns.

Influence of internal climate variability on Indian Ocean Dipole properties

The Indian Ocean Dipole (IOD) is the dominant mode of interannual variability over the tropical Indian Ocean (IO) and its future changes are projected to impact the climate and weather of Australia, East Africa, and Indonesia. Understanding the response of the IOD to a warmer climate has been largely limited to studies of individual coupled general circulation models or multi-model ensembles. This has provided valuable insight into the IOD’s projected response to increasing greenhouse gases but has limitations in accounting for the role of internal climate variability. Using the Community Earth System Model Large Ensemble (CESM-LE), the IOD is examined in thirty-five present-day and future simulations to determine how internal variability influences properties of the IOD and their response to a warmer climate. Despite small perturbations in the initial conditions as the only difference between ensemble members, significant relationships between the mean state of the IO and the IOD arise, leading to a spread in the projected IOD responses to increasing greenhouse gases. This is driven by the positive Bjerknes feedback, where small differences in mean thermocline depth, which are caused by internal climate variability, generate significant variations in IOD amplitude, skewness, and the climatological zonal sea surface temperature gradient.

A global slowdown of tropical-cyclone translation speed

As the Earth's atmosphere warms, the atmospheric circulation changes. These changes vary by region and time of year, but there is evidence that anthropogenic warming causes a general weakening of summertime tropical circulation^1-8. Because tropical cyclones are carried along within their ambient environmental wind, there is a plausible a priori expectation that the translation speed of tropical cyclones has slowed with warming. In addition to circulation changes, anthropogenic warming causes increases in atmospheric water-vapour capacity, which are generally expected to increase precipitation rates ⁹ . Rain rates near the centres of tropical cyclones are also expected to increase with increasing global temperatures^10-12. The amount of tropical-cyclone-related rainfall that any given local area will experience is proportional to the rain rates and inversely proportional to the translation speeds of tropical cyclones. Here I show that tropical-cyclone translation speed has decreased globally by 10 per cent over the period 1949-2016, which is very likely to have compounded, and possibly dominated, any increases in local rainfall totals that may have occurred as a result of increased tropical-cyclone rain rates. The magnitude of the slowdown varies substantially by region and by latitude, but is generally consistent with expected changes in atmospheric circulation forced by anthropogenic emissions. Of particular importance is the slowdown of 30 per cent and 20 per cent over land areas affected by western North Pacific and North Atlantic tropical cyclones, respectively, and the slowdown of 19 per cent over land areas in the Australian region. The unprecedented rainfall totals associated with the 'stall' of Hurricane Harvey^13-15 over Texas in 2017 provide a notable example of the relationship between regional rainfall amounts and tropical-cyclone translation speed. Any systematic past or future change in the translation speed of tropical cyclones, particularly over land, is therefore highly relevant when considering potential changes in local rainfall totals.

El Niño–Southern Oscillation and its impact in the changing climate

Extensive research has improved our understanding and forecast of the occurrence, evolution and global impacts of the El Niño–Southern Oscillation (ENSO). However, ENSO changes as the global climate warms up and it exhibits different characteristics and climate impacts in the twenty-first century from the twentieth century. Climate models project that ENSO will also change in the warming future and have not reached an agreement about the flavor, as to the intensity and the frequency, of future ENSO conditions. This article presents the conventional view of ENSO properties, dynamics and teleconnections, and reviews the emerging understanding of the diversity and associated climate impacts of ENSO. It also reviews the results from investigations into the possible changes in ENSO under the future global-warming scenarios.

Stabilised frequency of extreme positive Indian Ocean Dipole under 1.5 °C warming

Extreme positive Indian Ocean Dipole (pIOD) affects weather, agriculture, ecosystems, and public health worldwide, particularly when exacerbated by an extreme El Niño. The Paris Agreement aims to limit warming below 2 °C and ideally below 1.5 °C in global mean temperature (GMT), but how extreme pIOD will respond to this target is unclear. Here we show that the frequency increases linearly as the warming proceeds, and doubles at 1.5 °C warming from the pre-industrial level (statistically significant above the 90% confidence level), underscored by a strong intermodel agreement with 11 out of 13 models producing an increase. However, in sharp contrast to a continuous increase in extreme El Niño frequency long after GMT stabilisation, the extreme pIOD frequency peaks as the GMT stabilises. The contrasting response corresponds to a 50% reduction in frequency of an extreme El Niño preceded by an extreme pIOD from that projected under a business-as-usual scenario.

It is unclear how extreme positive Indian Ocean Dipole will respond to 1.5 °C of warming. Here the authors show that the frequency of these events increases linearly with warming, doubling at 1.5 °C from the pre-industrial level, but plateaus thereafter.

Late Holocene slowdown of the Indian Ocean Walker circulation

Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19-23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming.

Decadal Western Pacific Warm Pool Variability: A Centroid and Heat Content Study

We examine several characteristics of the Western Pacific Warm Pool (WP) in the past thirty years of mixed interannual variability and climate change. Our study presents the three-dimensional WP centroid (WPC) movement, WP heat content anomaly (HC) and WP volume (WPV) on interannual to decadal time scales. We show the statistically significant correlation between each parameter's interannual anomaly and the NINO 3, NINO 3.4, NINO 4, SOI, and PDO indices. The longitudinal component of the WPC is most strongly correlated with NINO 4 (R = 0.78). The depth component of the WPC has the highest correlation (R = -0.6) with NINO3.4. The WPV and NINO4 have an R-Value of -0.65. HC has the highest correlation with NINO3.4 (R = -0.52). During the study period of 1982-2014, the non-linear trends, derived from ensemble empirical mode decomposition (EEMD), show that the WPV, WP depth and HC have all increased. The WPV has increased by 14% since 1982 and the HC has increased from -1 × 108 J/m2 in 1993 to 10 × 108 J/m2 in 2014. While the largest variances in the latitudinal and longitudinal WPC locations are associated with annual and seasonal timescales, the largest variances in the WPV and HC are due to the multi-decadal non-linear trend.

On the role of ozone feedback in the ENSO amplitude response under global warming

The El Niño-Southern Oscillation (ENSO) in the tropical Pacific Ocean is of key importance to global climate and weather. However, state-of-the-art climate models still disagree on the ENSO's response under climate change. The potential role of atmospheric ozone changes in this context has not been explored before. Here we show that differences between typical model representations of ozone can have a first-order impact on ENSO amplitude projections in climate sensitivity simulations. The vertical temperature gradient of the tropical middle-to-upper troposphere adjusts to ozone changes in the upper troposphere and lower stratosphere, modifying the Walker circulation and consequently tropical Pacific surface temperature gradients. We show that neglecting ozone changes thus results in a significant increase in the number of extreme ENSO events in our model. Climate modeling studies of the ENSO often neglect changes in ozone. We therefore highlight the need to understand better the coupling between ozone, the tropospheric circulation, and climate variability.

Hydroclimate changes across the Amazon lowlands over the past 45,000 years

Reconstructing the history of tropical hydroclimates has been difficult, particularly for the Amazon basin-one of Earth's major centres of deep atmospheric convection. For example, whether the Amazon basin was substantially drier or remained wet during glacial times has been controversial, largely because most study sites have been located on the periphery of the basin, and because interpretations can be complicated by sediment preservation, uncertainties in chronology, and topographical setting. Here we show that rainfall in the basin responds closely to changes in glacial boundary conditions in terms of temperature and atmospheric concentrations of carbon dioxide. Our results are based on a decadally resolved, uranium/thorium-dated, oxygen isotopic record for much of the past 45,000 years, obtained using speleothems from Paraíso Cave in eastern Amazonia; we interpret the record as being broadly related to precipitation. Relative to modern levels, precipitation in the region was about 58% during the Last Glacial Maximum (around 21,000 years ago) and 142% during the mid-Holocene epoch (about 6,000 years ago). We find that, as compared with cave records from the western edge of the lowlands, the Amazon was widely drier during the last glacial period, with much less recycling of water and probably reduced plant transpiration, although the rainforest persisted throughout this time.

Projected changes in prevailing winds for transatlantic migratory birds under global warming

A number of terrestrial bird species that breed in North America cross the Atlantic Ocean during autumn migration when travelling to their non-breeding grounds in the Caribbean or South America. When conducting oceanic crossings, migratory birds tend to associate with mild or supportive winds, whose speed and direction may change under global warming. The implications of these changes for transoceanic migratory bird populations have not been addressed. We used occurrence information from eBird (1950-2015) to estimate the geographical location of population centres at a daily temporal resolution across the annual cycle for 10 transatlantic migratory bird species. We used this information to estimate the location and timing of autumn migration within the transatlantic flyway. We estimated how prevailing winds are projected to change within the transatlantic flyway during this time using daily wind speed anomalies (1996-2005 and 2091-2100) from 29 Atmosphere-Ocean General Circulation Models implemented under CMIP5. Autumn transatlantic migrants have the potential to encounter strong westerly crosswinds early in their transatlantic journey at intermediate and especially high migration altitudes, strong headwinds at low and intermediate migration altitudes within the Caribbean that increase in strength as the season progresses, and weak tailwinds at intermediate and high migration altitudes east of the Caribbean. The CMIP5 simulations suggest that, during this century, the likelihood of autumn transatlantic migrants encountering strong westerly crosswinds will diminish. As global warming progresses, the need for species to compensate or drift under the influence of strong westerly crosswinds during the initial phase of their autumn transatlantic journey may be diminished. Existing strategies that promote headwind avoidance and tailwind assistance will likely remain valid. Thus, climate change may reduce time and energy requirements and the chance of mortality or vagrancy during a specific but likely critical portion of these species' autumn migration journey.