Equatorial Pacific dynamics : lateral mixing and tropical instability waves

Abstract

This thesis presents a study focused on two main aspects of the equatorial Pacific dynamics: One is the parameterization of lateral mixing and its impact in numerical simulations of the equatorial Pacific ocean. The other is the Tropical Instability Wave (TIW) characteristics in eastern equatorial Pacific region and the associated coupled ocean-atmosphere interactions. The main objectives of this research project are achieved using both model and satellite observations.

An ocean general circulation model (OGCM) is adapted to represent the tropical Pacific ocean. Experiments are performed to assess the impact of the form of lateral mixing of momentum and tracers on the state of an equatorial ocean. It is found that the large-scale structure of both the velocity and temperature fields are very sensitive to the imposed parameterization of lateral mixing in the model. With uniform values for the viscosity and diffusion coefficient across the domain, a decrease in these coefficients increases the activity of TIW, resulting in an increase in the Sea Surface Temperature of the cold tongue, and thus overcoming the cold bias often found in ocean models of the equatorial Pacific. However there is an associated increase in the strength of the Equatorial Under Current (EUC) to unrealistic levels. It is found that the strength of the EUC can be limited, whilst having little affect on the TIW activity, by applying an enhanced level of mixing in the vicinity of the equator. This enhanced mixing is used to model the effect of the observed interleaving of water masses across the equator.

Descriptions of TIW variability characteristics in the tropical Pacific ocean as function of the large scale climate conditions and interannual variability, such as ENSO, are made based on satellite data. As others have found, the TIW ocean-atmosphere coupling is caused by atmospheric boundary layer (ABL) instability and mixing. Our observational results suggest that this mechanism of wind-SST coupled variability may occur not only during La Nina years, when TIWs are more active, but whenever the TIWs are active. There is evidence that the TIW activity increases when under strengthened wind stress conditions either in La Nina years or, by analogy, when numerical simulations are carried out under stronger wind stress.

The coupled ocean-atmosphere experiments, using a simplified ABL scheme, under different coupling strengths, suggests that this system is able to accurately simulate the TIW atmospheric imprints using a standard coupling coefficient. The active coupling strengthening produces a negative feedback on the TIWs. The TIW activity tend to be reduced in a cooler Cold Tongue region and anomalous values reaching -0.3^oC in some equatorial regions, when compared with the control (CTL) experiment. However, the mechanisms which fully explain the ABL effects in the TIWs characteristics need to be further investigated.

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