Ocean's largest chlorophyll-rich tongue is extending westward (2002-2022)

Upwelling in the Equatorial Pacific nurtures an expansive, westward-stretching chlorophyll-rich tongue (CRT), supporting 18% of the annual global new production. Surrounding the CRT are the oligotrophic subtropical gyres to the north and south, which are suggested to be expanding under global warming. Yet, how this productive CRT has changed, expanding or contracting, remains unknown. By applying the empirical mode decomposition (EMD) method to 20-year monthly measurements of chlorophyll-a concentration from MODIS-Aqua satellite (2002-2022), we demonstrate that the CRT exhibited a significant westward extension, at an average expanding rate of 1.87 ( ± 0.82) × 105 km2/yr. The westward extension of the CRT is attributed to strengthened equatorial upwelling and a strengthened South Equatorial Current from 2002 to 2022, driven by intensified easterly trade winds as the Pacific Decadal Oscillation predominantly remains in its negative phase during this period. Interestingly, EMD analysis on central locations of the Pacific gyres suggested simultaneous extension of the gyres and the CRT during 2002-2022, with the gyres extending poleward. Our findings imply a broader cover of productive water along the equator, while its impact on tropical climate, ecosystems, and carbon cycle deserves further investigation.

Coupled dynamics of the North Equatorial Countercurrent and Intertropical Convergence Zone with relevance to the double-ITCZ problem

The Intertropical Convergence Zone (ITCZ) is a salient feature of tropical atmospheric circulation characterized by intense convective clouds and rainfall north of the equator. Climate models, however, commonly experience the double-ITCZ problem-the tendency to produce another strong precipitation band but south of the equator. Here, we describe coupled interaction between the ITCZ and the North Equatorial Countercurrent (NECC) that is relevant to this problem. This current is a major component of the tropical Pacific upper-ocean circulation, which flows against easterly trade winds and transports warm water from the western Pacific eastward. Its core follows the latitudinal position of the ITCZ. Trade winds converge toward the ITCZ, creating a local minimum in zonal winds and generating positive wind stress curl that maintains an eastward current despite westward winds. We show that interaction between the ITCZ and the NECC involves positive feedback: a stronger NECC advects warm water from the western Pacific to the colder east thus increasing sea surface temperature (SST) along its path, intensifying convection within the ITCZ and hence strengthening wind stress curl, further strengthening the NECC. To demonstrate this wind stress curl-advection-SST-precipitation (WASP) feedback, we conduct climate model experiments in which we progressively strengthen the surface Ekman component of the NECC and observe the intensification of the ITCZ and the entire NECC. Consequently, a weak NECC leads to a weak ITCZ, which can contribute to the double-ITCZ problem in climate models, since weak wind convergence north of the equator enables stronger convergence in the south.