Anthropogenic Influence on Monsoon and Walker Circulations: the Interplay between energetic constraints and dynamical mechanisms

Yen-Ting Hwang

seminar
Mar. 12, 2024

11:10 am – 12:00 pm MDT

Webcast

Main content

The approach to calculating the atmospheric energy budget has become an essential theoretical foundation for understanding variations in tropical rainfall under climate change. From a global mean perspective, radiative cooling constrains convective heating and thus global mean precipitation. From a zonal mean perspective, shifts in the tropical rain belt are linked to the interhemispheric asymmetry of energy sources and sinks. The energetic perspective serves as a powerful diagnostic tool, linking changes in atmospheric circulation with climate forcings, feedbacks, and ocean heat uptake. Meanwhile, atmospheric and oceanic dynamics interact with these energy sources and sinks at various timescales and shape the transient evolution of energetics. I will share two studies that explore these themes. First, all climate models project a weakened Walker Cell and an El Niño-like warming pattern under climate change, which is in stark contrast to observed trends. While anthropogenic aerosols have been suggested as a cause, the prolonged cooling trend over the equatorial Pacific appears in conflict with Northern Hemisphere aerosol emission reduction since the 1980s. Here, using CESM, we show that the superposition of fast and slow responses to aerosol emission change – an increase followed by a decrease – can sustain the La Niña-like condition for a longer time than expected. The rapid adjustment of Hadley Cell to aerosol reduction triggers joint feedback between low clouds, wind, evaporation, and sea surface temperature in the Southeast Pacific, leading to a wedge-shaped cooling that extends to the central equatorial Pacific. Meanwhile, the northern subtropical cell gradually intensifies, resulting in equatorial subsurface cooling that lasts for decades. In the second part of the talk, we will shift our focus to the fast and slow responses of the South Asian monsoon circulation to CO2 forcing, which is known to be the major source of uncertainty for future rainfall projections in the region in CMIP models. Contrary to the well-established 'wet-gets-wetter' perspective, we demonstrate that the effects of increasing water vapor are compensated by the circulation slowdown. Most of the rainfall changes are instead driven by a robust northward shift of the convection zone toward South Asia. This northward shift is robust in the fast response of the 4xCO2 experiments and the linear trend in the SSP3.7 scenario. Through energetic analysis and purposefully designed experiments, the northward shift component can be traced to remote extratropical energy sources and sinks (e.g., cloud radiative effects and Southern Ocean heat uptake). These energy sources and sinks are physically robust and can be detected in observational records. However, they are expected to weaken as the Southern Ocean warms up and the AMOC weakens.

Yen-Ting Hwang