Indian Ocean warming modulates Pacific climate change

Abstract

It has been widely believed that the tropical Pacific trade winds weakened in the last century and would further decrease under a warmer climate in the 21st century. Recent high-quality observations, however, suggest that the tropical Pacific winds have actually strengthened in the past two decades. Precise causes of the recent Pacific climate shift are uncertain. Here we explore how the enhanced tropical Indian Ocean warming in recent decades favors stronger trade winds in the western Pacific via the atmosphere and hence is likely to have contributed to the La Niña-like state (with enhanced east-west Walker circulation) through the Pacific ocean-atmosphere interactions. Further analysis, based on 163 climate model simulations with centennial historical and projected external radiative forcing, suggests that the Indian Ocean warming relative to the Pacific's could play an important role in modulating the Pacific climate changes in the 20th and 21st centuries.

Fig. 1.

Warming rates in the tropical IO and Pacific over the past three decades. Shown are monthly anomalies and linear trends of observed SST in the tropical Pacific (120°–280°E, 20°S–20°N, red lines) and IO (40°–120°E, 20°S–20°N, blue lines) based on two independent analyses (A, B). The linear trends of the IO-minus-Pacific SST differences are 0.063 ± 0.031 and 0.053 ± 0.027 °C per decade, respectively. Error bars denote 95% confidence intervals according to Student t test. Results based on HadISST 1.1 data (Fig. S8) and averaged over different latitude bands (e.g., 10°S–10°N or 30°S–30°N) are similar.

Fig. 2.

Pacific climate change in recent decades. Shown are 2000–2009 mean anomalies of (A) SST (contour), surface winds (thick vectors indicate ≤5% significance according to one-tailed Student t test), and precipitation (mm/d, color shaded) in the tropical Pacific, (B) the atmospheric Walker circulation, and (C) upper ocean temperature in the equatorial Pacific (5°S–5°N). The anomalies are calculated relative to the climatology of 1983–2006. The solid (dashed) line in (C) denotes climatological (2000-2009 mean) 20 °C isotherm, which represents the thermocline depth. Results based on the linear trends of these fields during 1982–2009 and other available ocean and atmosphere reanalysis datasets (Fig. S10) are similar.

Fig. 3.

Influence of multidecadal IO warming on the Pacific climate. Shown are (A) zonal surface wind and (B) SST changes in the equatorial Pacific (5°S–5°N). Black lines denote the observed anomalies of 2000–2009. Gray line in A denotes the wind anomalies reproduced by the SINETX-F climate model with global observed SST forcing (Fig. S9A). The red line in A displays the influence of the IO SST rise (2000–2009 minus 1982–1999) on the Pacific surface wind via atmospheric processes, based on model experiments in which only the tropical IO observed SST is assimilated into the coupled model, whereas climatological SST is prescribed elsewhere. The green (blue, orange, and purple) lines indicate influences of the IO warming in presence of the Pacific local ocean–atmosphere coupling, based on model sensitivity experiments in which 0.025 (0.05, 0.075, and 0.1) °C per decade warming trend is prescribed in the tropical IO. These are 50-y mean differences between the sensitivity experiments and the control run; in those simulations, we prescribe SST in the IO but keep elsewhere free ocean–atmosphere coupling (Methods). Thick lines indicate ≤10% significance according to Student t test.

Fig. 4.

Pacific wind changes modulated by the IO warming in the simulations of the CMIP3 and CMIP5 models. (A) Scatter plot of the linear trends of the Pacific zonal wind (150°–240°E, 10°S–10°N) vs. those of the SST warming difference between the tropical eastern Pacific (200°–280°E, 20°S–20°N) and IO (40°–120°E, 20°S–20°N). Gray line denotes the least-square linear fit of the trends based on 163 models of 10 climate simulation/projection experiments (correlation is 0.63). (B) Multidecadal changes of the Pacific wind (solid lines) and eastern Pacific-minus-IO warming difference (dashed lines), with the linear trends being removed. These are multimodel ensemble mean results based on the CMIP3 20C3M and SRESA1B experiments. Thick lines indicate ≤10% significance according to one-tailed Student t test. (C) Correlations between the multidecadal Pacific zonal wind and eastern Pacific-minus-IO SST difference. Red bars (black lines) denote the median (individual model) correlations.