Variational Assimilation of Altimeter Data in a Multilayer Model of the Tropical Pacific Ocean

Anthony T. Weaver Atmospheric, Oceanic and Planetary Physics, Department of Physics, Clarendon Laboratory, Oxford, United Kingdom

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David L. T. Anderson Atmospheric, Oceanic and Planetary Physics, Department of Physics, Clarendon Laboratory, Oxford, United Kingdom

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Abstract

A four-dimensional variational method is used to examine the extent to which a time sequence of altimeter measurements can determine the subsurface flow in a linear multilayer model of the tropical Pacific Ocean. The experiments are all of the identical-twin type. Complete maps of sea level extracted from the model in a control integration play the role of the altimeter observations in the assimilation experiments. The results of the experiments indicate that, over timescales of months, the sea level information can be effectively propagated into the subsurface, particularly in the dynamically active equatorial region. Several degrees off the equator, however, where waves propagate more slowly, the recovery of the subsurface flow in models containing more than two vertical modes is significantly more difficult. The sensitivity of these results to the lengths of the data sampling and assimilation periods is discussed.

* Current affiliation: Laboratoire d’Océanographie Dynamique et de Climatologie, (CNRS/ORSTOM/UPMC), Université Paris VI, Paris, France.

Current affiliation: European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom.

Corresponding author address: Dr. Anthony T. Weaver, Laboratoire d’Océanographie Dynamique et de Climatologie, (CNRS/ORSTOM/UPMC), Université Paris VI, boîte 100, 4 place Jussieu, 75252 Paris Cedex 05, France.

Abstract

A four-dimensional variational method is used to examine the extent to which a time sequence of altimeter measurements can determine the subsurface flow in a linear multilayer model of the tropical Pacific Ocean. The experiments are all of the identical-twin type. Complete maps of sea level extracted from the model in a control integration play the role of the altimeter observations in the assimilation experiments. The results of the experiments indicate that, over timescales of months, the sea level information can be effectively propagated into the subsurface, particularly in the dynamically active equatorial region. Several degrees off the equator, however, where waves propagate more slowly, the recovery of the subsurface flow in models containing more than two vertical modes is significantly more difficult. The sensitivity of these results to the lengths of the data sampling and assimilation periods is discussed.

* Current affiliation: Laboratoire d’Océanographie Dynamique et de Climatologie, (CNRS/ORSTOM/UPMC), Université Paris VI, Paris, France.

Current affiliation: European Centre for Medium-Range Weather Forecasts, Reading, United Kingdom.

Corresponding author address: Dr. Anthony T. Weaver, Laboratoire d’Océanographie Dynamique et de Climatologie, (CNRS/ORSTOM/UPMC), Université Paris VI, boîte 100, 4 place Jussieu, 75252 Paris Cedex 05, France.

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