Hemispheric Asymmetry of the Pacific Shallow Meridional Overturning Circulation

Abstract

The shallow subtropical cells (STCs) in the Pacific Ocean are thought to modulate the background state that the El Niño-Southern Oscillation (ENSO) operates in. This modulation is proposed to impact the frequency and intensity of ENSO events and their teleconnections. We use a high-resolution ocean model to investigate the volume transports associated with the STC branches along 5° N and 5° S. We find three prominent differences between the Southern hemisphere (SH) STC and the Northern hemisphere (NH) STC: (i) the NH STC varies 26% stronger than the SH STC; (ii) the NH STC appears to lead the SH STC by 3 months which causes the NH and SH STCs to play different roles during the course of El Niño and La Niña events; and (iii) in spite of the relative symmetry of the wind stress trends, the STCs have differing decadal trends, with the SH STC clearly dominating the changes in the post-1993 period. To investigate the mechanisms driving the STC variability, we identify winds that are linearly and nonlinearly related to ENSO to force the ocean model. The hemispheric difference in interannual variance as well as the phase difference between the STCs can be explained with ENSO forcing. Our results suggest ENSO to be an important factor in modulating its own background state, with a prominent role for the winds that are nonlinearly related to ENSO. The decadal trends and their interhemispheric disparity, however, cannot be reproduced by our targeted ENSO experiments.