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Response of the Equatorial Thermocline to Extratropical Buoyancy Forcing

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  • 1 Center for Climatic Research and Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin
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Abstract

The GFDL Modular Ocean Model and the Miami Isopycnal Ocean Model are used to investigate the response of the equatorial thermocline to extratropical buoyancy forcing. Passive tracers and analytical theories are also used to shed light on the dynamics of the thermocline response. The major findings are the following. (i) The midlatitude region seems to be the optimal region for surface buoyancy forcing to affect the equatorial thermocline. This occurs because, first, thermocline anomalies in the midlatitudes can penetrate into the equator very efficiently; second, buoyancy forcing generates a strong local response in the midlatitudes. (ii) Dynamic waves as well as thermocline ventilation contribute to the response in the equatorial thermocline. Consequently, equatorward penetration is substantially greater for a temperature anomaly than for a passive tracer. (iii) Midlatitude forcing generates a significant temperature response in the equatorial thermocline for forcing periods longer than decadal. (iv) For a low latitude (10°–20°) buoyancy forcing, the equatorial thermocline could be dominated by a temperature anomaly that has the opposite sign to the surface forcing because of the strong higher mode baroclinic response in the ventilated thermocline. Finally, the relevance of this work to observations and climate variability is also discussed.

Corresponding author address: Z. Liu, Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 W. Dayton St., Madison, WI 53706.

Email: zliu3@facstaff.wisc.edu

Abstract

The GFDL Modular Ocean Model and the Miami Isopycnal Ocean Model are used to investigate the response of the equatorial thermocline to extratropical buoyancy forcing. Passive tracers and analytical theories are also used to shed light on the dynamics of the thermocline response. The major findings are the following. (i) The midlatitude region seems to be the optimal region for surface buoyancy forcing to affect the equatorial thermocline. This occurs because, first, thermocline anomalies in the midlatitudes can penetrate into the equator very efficiently; second, buoyancy forcing generates a strong local response in the midlatitudes. (ii) Dynamic waves as well as thermocline ventilation contribute to the response in the equatorial thermocline. Consequently, equatorward penetration is substantially greater for a temperature anomaly than for a passive tracer. (iii) Midlatitude forcing generates a significant temperature response in the equatorial thermocline for forcing periods longer than decadal. (iv) For a low latitude (10°–20°) buoyancy forcing, the equatorial thermocline could be dominated by a temperature anomaly that has the opposite sign to the surface forcing because of the strong higher mode baroclinic response in the ventilated thermocline. Finally, the relevance of this work to observations and climate variability is also discussed.

Corresponding author address: Z. Liu, Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 W. Dayton St., Madison, WI 53706.

Email: zliu3@facstaff.wisc.edu

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