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.

Wind Spatial Structure Triggers ENSO’s Oceanic Warm Water Volume Changes

“…The WWV magnitude and pycnocline spatial patterns for both the instantaneous and adjusted responses are largely consistent when conducting the experiments of this section with the comprehensive global coupled ocean-sea ice model GFDL-MOM025 [Fig. S4; see Zeller et al (2019) for a model description]. Thus, we conclude that the results presented here are not significantly impacted by the simplicity of the ocean model utilized.…”

Mechanisms of Tropical Pacific Decadal Variability

“…However, observational and modelling results based on temperature observations only showed that these anomalies decayed prior to reaching the equator 47 , casting doubt on the feasibility of this mechanism. Further works suggested that the South Pacific could be more suitable for the ̅ ′ mechanism [48][49][50] , due to its larger and more direct equatorward transport [51][52][53][54] , Indeed, the presence of the Intertropical Convergence Zone (ITCZ) in the tropical North Pacific alters the depth of the pycnocline and creates a "potential vorticity barrier" 54 that limits the interior equatorward flow 54,55 (Fig. 3).…”

Subsurface Cooling in the Tropical Pacific Under a Warming Climate

“…Zeller et al. (2019) also indicate that the North Hemisphere (NH) STC appears to lead the South Hemisphere (SH) STC by 3 months during El Niño and La Niña events, especially the NH surface branch, so the NH surface transport is also the main factor affecting the mass exchange between the tropics and subtropics on the interannual timescale. Instead of the general advection through the meridional overturning circulation, the advection of the spiciness anomalies at a certain isopycnal surface is another process that can demonstrate the effect of the subtropical variability on tropical climate (Zeller et al., 2021).…”