Understanding Precipitation Variation with ENSO from a General Circulation Perspective

The variation of global precipitation distribution patterns caused by El Niño-Southern Oscillation (ENSO) is evident, yet the underlying mechanisms remain unclear. Our research strives to illuminate the change of general circulation pattern induced by ENSO and their impact on global rainfall patterns. We achieve this by using the Transformed Eulerian Mean (TEM) framework. TEM circulation, referred to as the residual mean circulation, can be conceptualized as the mass-weighted total mass transport that combines Eulerian mean and eddy transport. Thus, to the extent that precipitation is caused by moisture transport by large-scale motions, TEM should be closely related to the total precipitation pattern. Our result indeed shows a close alignment between the ENSO-induced zonal mean precipitation difference and TEM circulation vertical motion variation.

Under steady states, the meridional residual mean velocity in the free atmosphere is balanced by the divergence of Eliassen – Palm Flux (EP flux) which is comprised of eddy heat flux and eddy momentum flux components. The effect of eddy heat flux dominates the TEM variation between the subtropics and the midlatitudes, while eddy momentum flux is more important in the tropics. Our result also shows that the key eddy heat flux anomalies are upgradient of the temperature gradient anomaly. These counter-intuitive eddy heat flux anomalies are mostly carried out by zonal wavenumber 3 eddies. These results suggest that planetary-scale wave refractions play an important role in setting the zonal mean precipitation anomalies associated with the ENSO.