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  • Author or Editor: David L. T. Anderson x
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Anthony T. Weaver and David L. T. Anderson

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.

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David L. T. Anderson and Andrew M. Moore

Abstract

The relative information content of mass and velocity measurements for initializing low-frequency equatorially trapped waves is considered using analytical arguments and a numerical model For the Kelvin wave, mass and velocity data are equally useful, but this is not true for the Rossby waves. It is the relative amount of potential energy and kinetic energy possessed by equatorial waves that determines the relative usefulness of mass and velocity data for initializing these waves. The effects of dissipation on the adjustment process is considered. It is shown for the Kelvin wave that mass and velocity data are equally useful even when dissipation is present, but the Rossby wave adjustment is sensitive to the level and form of dissipation used.

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David L. T. Anderson and Robert A. Corry

Abstract

In a previous study Anderson and Corry used a wind-driven two-layer model to study the effects of topography and islands on the seasonal variation of western boundary currents. The work is continued here with topography, geography and winds appropriate to the North Atlantic to examine the seasonal cycle of the Florida Straits transport. A summer maximum of transport is predicted consistent with observations. The area of importance and processes giving rise to the seasonal cycle are considered.

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Julio Sheinbaum and David L. T. Anderson

Abstract

A linear reduced-gravity model of the tropical pacific is used to assimilate XBT data. The model cannot fit the data in the eastern equatorial Pacific for the whole assimilation period. Several experiments with real and simulated data are performed to investigate the source of this deficiency, which may be in the model or the wind stress used to force the model. It is shown that on the basis of the simple model physics we cannot unambiguously partition the error between model and forcing in the real data assimilation experiments although simulated data experiments do permit discrimination between model and forcing errors. Because the data is incomplete and does not permit a unique determination of the initial state, the use of prior information in the form of first-guess fields and/or smoothing constraints is examined. The filtering characteristics of the optimization algorithm are also discussed by looking at the evolution of the initial conditions as a function of the iteration number.

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Julio Sheinbaum and David L. T. Anderson

Abstract

A variations method based on the adjoint equation technique is used to assimilate data in a relatively simple linear reduced gravity model of the tropical Pacific. Real XBT data are used by identifying the depth of the 16°C isotherm depth with the model layer depth. It is shown that the XBT data contain large scale information that corrects the model first guess. However, the model is not capable of fitting the data in the eastern Pacific for the whole assimilation period. Regions not seeded by the data are explicitly shown and the impact of data from different times on the initial state is also discussed.

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Jérôme Vialard, Christophe Menkes, David L. T. Anderson, and Magdalena Alonso Balmaseda

Abstract

Tropical instability waves (TIWs) appear as monthly oscillations of the currents, sea level, and sea surface temperature of the eastern equatorial Pacific. They are understood as unstable waves feeding on the kinetic and potential energy of the mean currents. A general circulation model is shown to reproduce the main features associated with TIWs. It is then used to investigate the dynamical regime of TIWs, by assessing their sensitivity to oceanic initial conditions. Locally in space and time, small perturbations can grow enough to modify significantly the phase of the TIW field, suggesting some chaotic behavior. When considered over the whole active TIW region, however, the phases of the perturbed and unperturbed experiments remain in agreement. This suggests that TIW activity in this model is more consistent with a limit cycle behavior than with fully developed turbulence and that irregular behavior of TIWs mostly stems from external forcing by the wind. A stronger result is that TIWs in experiments starting from very different initial conditions come back into phase after a few years. This is consistent with the suggestion that TIWs might be phase-locked to the wind forcing. Quiescent periods of the TIW's cycle play an important role in the decay of TIW's phase disagreement but cannot explain the phase-locking mechanism entirely. The implications of these results and their sensitivity to the forcing are discussed.

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