Pacific Meridional Modes and Tropical Decadal Variability: Past and Future

Meridional Modes (MMs) are coupled ocean-atmosphere disturbances that are energized in the sub-tropics through the wind-evaporation-sea surface temperature (WES) thermodynamic feedback. In the Pacific, the growth and propagation of the MMs from the extra-tropics to the equatorial latitudes is recognized as an important mechanism for triggering El Niño Southern Oscillation (ENSO) and energizing ENSO-like decadal variability. Using observational and climate modeling data, we show that MMs propagating from both the southern and northern hemisphere explain a large and independent fraction of tropical decadal variance over the historical period 1940-2017. As the mean state of the tropics warms under anthropogenic forcing, the nonlinear relationship between sea surface temperature and evaporation amplifies the WES thermodynamic feedback, which in turn leads to more energetic MMs, stronger ENSO-like decadal variability, and a tighter coupling between tropics and extra-tropics. Consistent with this trend, MMs have played a key role in recent record-breaking anomalies associated with the North Pacific marine heatwave of 2014-15 and the 2015-16 El Niño. Given that amplification of the WES is also predicted for other oceanic basins under a warming climate, the increase in ocean-atmosphere coupling may provide a robust physical framework to explore future changes in climate variability on global scales.