Intraseasonal Variability of the Upper-Ocean Thermal Structure Observed at 0° and 165°E

Chidong Zhang Department of Atmospheric Sciences and Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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Abstract

In response to perturbations in surface wind and energy fluxes associated with the atmospheric Madden–Julian oscillation (MJO), the thermal structure of the upper ocean (surface to 300 m) in the equatorial western Pacific exhibits prominent and intriguing intraseasonal variability. Distinct features occur in two regimes: the surface regime from the surface to 75 m and the thermocline from 100 to 300 m. The intraseasonal variability in temperature is highly coherent within each regime. The two regimes, however, are generally decoupled in the sense that the mechanisms for the intraseasonal variability in each are different and the temperature perturbations in one regime do not significantly affect those in the other. Intraseasonal perturbations in the surface regime are closely related to fluctuations in surface wind speed and wind work, implying the effects of air–sea heat fluxes and turbulent mixing. Intraseasonal surface warming and cooling may penetrate to 75 m or deeper, where the thermal stratification can be strong. In the thermocline regime, variations in temperature result from vertical displacement of the thermocline induced by zonal wind stress forcing. The intraseasonal variations in the two regimes and their phase relations are subjected to considerable modulations by the mean zonal wind. Outstanding intraseasonal signals in the upper ocean are also found for isotherm depths and ocean heat content. These observations are based on a 7-yr dataset collected from the Tropical Atmosphere–Ocean buoy moored at 0° and 165°E. The implications of these observations to ocean mixed layer thermodynamics and heat balance of the western Pacific warm pool are discussed.

+ Current affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Corresponding author address: Dr. Chidong Zhang, MPO/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098.

Email: czhang@rsmas.miami.edu

Abstract

In response to perturbations in surface wind and energy fluxes associated with the atmospheric Madden–Julian oscillation (MJO), the thermal structure of the upper ocean (surface to 300 m) in the equatorial western Pacific exhibits prominent and intriguing intraseasonal variability. Distinct features occur in two regimes: the surface regime from the surface to 75 m and the thermocline from 100 to 300 m. The intraseasonal variability in temperature is highly coherent within each regime. The two regimes, however, are generally decoupled in the sense that the mechanisms for the intraseasonal variability in each are different and the temperature perturbations in one regime do not significantly affect those in the other. Intraseasonal perturbations in the surface regime are closely related to fluctuations in surface wind speed and wind work, implying the effects of air–sea heat fluxes and turbulent mixing. Intraseasonal surface warming and cooling may penetrate to 75 m or deeper, where the thermal stratification can be strong. In the thermocline regime, variations in temperature result from vertical displacement of the thermocline induced by zonal wind stress forcing. The intraseasonal variations in the two regimes and their phase relations are subjected to considerable modulations by the mean zonal wind. Outstanding intraseasonal signals in the upper ocean are also found for isotherm depths and ocean heat content. These observations are based on a 7-yr dataset collected from the Tropical Atmosphere–Ocean buoy moored at 0° and 165°E. The implications of these observations to ocean mixed layer thermodynamics and heat balance of the western Pacific warm pool are discussed.

+ Current affiliation: Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Corresponding author address: Dr. Chidong Zhang, MPO/RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098.

Email: czhang@rsmas.miami.edu

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