The Annual Cycle in the Tropical Pacific Ocean Based on Assimilated Ocean Data from 1983 to 1992

Thomas M. Smith Climate Prediction Center/National Centers for Environmental Prediction, NWS/NOAA, Washington, D.C.

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Muthuvel Chelliah Climate Prediction Center/National Centers for Environmental Prediction, NWS/NOAA, Washington, D.C.

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Abstract

An analysis of the tropical Pacific Ocean from January 1983 to December 1992 is used to describe the annual cycle, with the main focus on subsurface temperature variations. Some analysis of ocean-current variations are also considered. Monthly mean fields are generated by assimilation of surface and subsurface temperature data into an ocean general circulation model. Data used in the analysis include satellite sea surface temperature observations and surface and subsurface temperature observations from ships and buoys. Comparisons with observations show that the analysis reasonably describes large-scale ocean thermal variations. Ocean currents are not assimilated and do not compare as well with observations. However, the ocean-current variations in the analysis are qualitatively similar to the known variations given by others. The authors use harmonic analysis to separate the mean annual cycle and estimate its contribution to total variance.

The analysis shows that in most regions the annual cycle of subsurface thermal variations is larger than surface variations and that these variations are associated with changes in the depth of the thermocline. The annual cycle accounts for most of the total surface variance poleward of about 10° latitude but accounts for much less surface and subsurface total variance near the equator. Large subsurface annual cycles occur near 10°N associated with shifts of the intertropical convergence zone and along the equator associated with the annual cycle of equatorial wind stress. The hemispherically asymmetric depths of the 20°C isotherms indicate that the large Southern Hemisphere warm pool, which extends to near the equator, may play an important role in thermal variations on the equator.

Abstract

An analysis of the tropical Pacific Ocean from January 1983 to December 1992 is used to describe the annual cycle, with the main focus on subsurface temperature variations. Some analysis of ocean-current variations are also considered. Monthly mean fields are generated by assimilation of surface and subsurface temperature data into an ocean general circulation model. Data used in the analysis include satellite sea surface temperature observations and surface and subsurface temperature observations from ships and buoys. Comparisons with observations show that the analysis reasonably describes large-scale ocean thermal variations. Ocean currents are not assimilated and do not compare as well with observations. However, the ocean-current variations in the analysis are qualitatively similar to the known variations given by others. The authors use harmonic analysis to separate the mean annual cycle and estimate its contribution to total variance.

The analysis shows that in most regions the annual cycle of subsurface thermal variations is larger than surface variations and that these variations are associated with changes in the depth of the thermocline. The annual cycle accounts for most of the total surface variance poleward of about 10° latitude but accounts for much less surface and subsurface total variance near the equator. Large subsurface annual cycles occur near 10°N associated with shifts of the intertropical convergence zone and along the equator associated with the annual cycle of equatorial wind stress. The hemispherically asymmetric depths of the 20°C isotherms indicate that the large Southern Hemisphere warm pool, which extends to near the equator, may play an important role in thermal variations on the equator.

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