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Mechanisms of Summertime Subtropical Southern Indian Ocean Sea Surface Temperature Variability: On the Importance of Humidity Anomalies and the Meridional Advection of Water Vapor

A. M. ChiodiJoint Institute for the Study of the Atmosphere and the Ocean, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

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D. E. HarrisonJoint Institute for the Study of the Atmosphere and the Ocean, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

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Abstract

It is well known that some austral summertime subtropical Indian Ocean sea surface temperature (SST) variability correlates with rainfall over certain regions of Africa that depend on rainfall for their economic well-being. Recent studies have determined that this SST variability is at least partially driven by latent heat flux variability, but the mechanism has not been fully described. Here, the mechanism that drives this SST variability is reexamined using analyses of operational air–sea fluxes, ocean mixed layer modeling, and simple atmospheric boundary layer physics. The SST variability of interest is confirmed to be mainly driven by latent heat flux variability, which is shown, for the first time, to be mainly caused by near-surface humidity variability. This humidity variability is then shown to be fundamentally driven by the anomalous meridional advection of water vapor. The meridional wind anomalies of interest are subsequently found to occur when the subtropical atmospheric anticyclone is preferentially located toward one of the sides (east/west) of the basin.

* PMEL Contribution Number 2927 and Joint Institute for the Study of the Atmosphere and the Ocean Contribution Number 1321

Corresponding author address: A. M. Chiodi, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Box 357941, Seattle, WA 98115. Email: chiodi@ocean.washington.edu

Abstract

It is well known that some austral summertime subtropical Indian Ocean sea surface temperature (SST) variability correlates with rainfall over certain regions of Africa that depend on rainfall for their economic well-being. Recent studies have determined that this SST variability is at least partially driven by latent heat flux variability, but the mechanism has not been fully described. Here, the mechanism that drives this SST variability is reexamined using analyses of operational air–sea fluxes, ocean mixed layer modeling, and simple atmospheric boundary layer physics. The SST variability of interest is confirmed to be mainly driven by latent heat flux variability, which is shown, for the first time, to be mainly caused by near-surface humidity variability. This humidity variability is then shown to be fundamentally driven by the anomalous meridional advection of water vapor. The meridional wind anomalies of interest are subsequently found to occur when the subtropical atmospheric anticyclone is preferentially located toward one of the sides (east/west) of the basin.

* PMEL Contribution Number 2927 and Joint Institute for the Study of the Atmosphere and the Ocean Contribution Number 1321

Corresponding author address: A. M. Chiodi, NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Box 357941, Seattle, WA 98115. Email: chiodi@ocean.washington.edu

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