The monsoon as a self-regulating coupled ocean—atmosphere system

Interdecadal Changes in the Major Modes of Asian–Australian Monsoon Variability: Strengthening Relationship with ENSO since the Late 1970s*

The present paper develops an integral view of the year-to-year variability across the entire Asian–Australian monsoon (A–AM) system, which covers one-third of the global tropics between 40° and 160°E. Using season-reliant empirical orthogonal function (S-EOF) analysis, the authors identified two major modes of variability for the period 1956–2004. The first exhibits a prominent biennial tendency and concurs with the turnabout of El Niño–Southern Oscillation (ENSO), providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation. The second mode leads ENSO by one year. The remote El Niño forcing, the monsoon–warm pool ocean interaction, and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode. The monsoon–ocean interaction is characterized by a positive feedback between the off-equatorial convectively coupled Rossby waves and the underlying sea surface temperature (SST) “dipole” anomalies.

Since the late 1970s the overall coupling between the A–AM system and ENSO has become strengthened. The relationships between ENSO and the western North Pacific, East Asian, and Indonesian monsoons have all become enhanced during ENSO’s developing, mature, and decaying phases, overriding the weakening of the Indian monsoon–ENSO anticorrelation during the developing phase. Prior to the late 1970s (1956–79), the first mode shows a strong biennial tendency, and the second mode does not lead ENSO. After 1980, the first mode shows a weakening biennial tendency, and the second mode provides a strong precursory signal for ENSO. These interdecadal changes are attributed to increased magnitude and periodicity of ENSO and the strengthened monsoon–ocean interaction.

Refractory periods and climate forcing in cholera dynamics

Outbreaks of many infectious diseases, including cholera, malaria and dengue, vary over characteristic periods longer than 1 year. Evidence that climate variability drives these interannual cycles has been highly controversial, chiefly because it is difficult to isolate the contribution of environmental forcing while taking into account nonlinear epidemiological dynamics generated by mechanisms such as host immunity. Here we show that a critical interplay of environmental forcing, specifically climate variability, and temporary immunity explains the interannual disease cycles present in a four-decade cholera time series from Matlab, Bangladesh. We reconstruct the transmission rate, the key epidemiological parameter affected by extrinsic forcing, over time for the predominant strain (El Tor) with a nonlinear population model that permits a contributing effect of intrinsic immunity. Transmission shows clear interannual variability with a strong correspondence to climate patterns at long periods (over 7 years, for monsoon rains and Brahmaputra river discharge) and at shorter periods (under 7 years, for flood extent in Bangladesh, sea surface temperatures in the Bay of Bengal and the El Niño-Southern Oscillation). The importance of the interplay between extrinsic and intrinsic factors in determining disease dynamics is illustrated during refractory periods, when population susceptibility levels are low as the result of immunity and the size of cholera outbreaks only weakly reflects climate forcing.