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- Title
Mechanism of Interannual Cross‐Equatorial Overturning Anomalies in the Pacific Ocean.
- Authors
Rao, Devanarayana R. M.; Tandon, Neil F.
- Abstract
The meridional overturning circulation (MOC) transports heat and mass between the tropics and the extratropics. Recent research has shown that the variability of the Indo‐Pacific MOC dominates the variability of the global MOC on interannual timescales, and this variability is characterized by a prominent cross‐equatorial cell (CEC) spanning the tropics. This CEC is a potentially important influence on interannual climate variability, but the mechanism responsible for the CEC is not understood. This study seeks to elucidate the mechanism of the CEC using two observational estimates of the ocean. Our analysis shows that the CEC can be explained by the following mechanistic chain: (a) Anomalies in the atmospheric circulation and hydrological cycle produce equatorially antisymmetric density anomalies in the upper Pacific Ocean (above approximately 500 m); (b) these density anomalies generate equatorially antisymmetric anomalies of sea surface height (SSH); (c) these SSH anomalies generate a cross‐equatorial flow above approximately 1,000 m; and (d) this anomalous cross‐equatorial flow drives compensating flow below approximately 1,000 m. This mechanism contrasts with that responsible for anomalous cross‐equatorial overturning on seasonal timescales, which is primarily the Ekman response to equatorially antisymmetric anomalies of zonal wind stress. On interannual timescales, the zonal wind stress anomalies associated with the CEC are equatorially symmetric, and steric SSH variations are the dominant driver of the CEC. These insights may lead to improved understanding and prediction of interannual climate variability. Plain Language Summary: Earth exhibits strong year‐to‐year variations in climate that have important consequences for economies and livelihoods. Understanding these year‐to‐year, or "interannual," variations requires understanding variations in the transport of energy by the atmosphere and the ocean. This study focuses on the ocean portion of this transport. Recent research has shown that interannual variations in global ocean transport primarily arise from transport variations in the Indian and Pacific Oceans. Furthermore, this Indo‐Pacific variability is characterized by a prominent cross‐equatorial cell (CEC) spanning the full depth of the Indian and Pacific Oceans from approximately 18°S to 20°N latitude. We perform an analysis of two ocean models blended with observational data (a standard approach when observational data are sparse) in order to clarify the mechanism responsible for the CEC. Our analysis reveals that the CEC arises because processes in the atmosphere produce changes in upper‐ocean density near the equator, which in turn changes the ocean pressure gradient, which in turn drives flow across the equator in the upper ocean and compensating flow in the opposite direction in the deep ocean. Such physical insights may lead to improved understanding and prediction of interannual climate variations. Key Points: Variability of the Pacific meridional overturning circulation is characterized by a prominent cross‐equatorial cell (CEC) in the tropicsThe positive phase of the CEC results from a steric equatorially antisymmetric anomaly in sea surface heightThe resulting anomalous northward cross‐equatorial flow in the upper Pacific is compensated by anomalous southward flow in the deep Pacific
- Subjects
PACIFIC Ocean; MERIDIONAL overturning circulation; OCEANOGRAPHY; OCEAN convection; OCEAN currents
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 10, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2021JC017509