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C. Maes, G. Madec, and P. Delecluse

Abstract

An OGCM is used to investigate the importance of lateral mixing in the tropical Pacific Ocean circulation. Horizontal subgrid-scale physics is parameterized by the usual Laplacian operator. Three simulations are performed using three different orders of magnitude for lateral eddy viscosity and diffusivity coefficients: 104, 103, and 102 m2 s−1. The upper layer response is found sensitive to lateral diffusion as well as the rest of the general circulation. Decreasing lateral mixing coefficients raises the mean kinetic energy level and the input of energy by the wind, and enhances the vertical dissipation. This weakens the equatorial meridional cell and induces a reduction of 20 Sv in the transport of the equatorial upwelling.

These results are due to the nonlinear interplay between horizontal and vertical diffusion. The nature of the Equatorial Undercurrent (EUC) is found particularly sensitive to the relative importance of the diffusive conditions. Lateral mixing dominates the different regimes of the EUC when the strongest diffusion coefficient is used. Under these conditions, the EUC dynamics is similar to a boundary layer regime where strong meridian and vertical circulation insulates the equatorial dynamics. Conversely, a weakness of horizontal diffusion leads to enhancement of the vertical diffusion role, even in the core of the EUC. In the eastern inertial regime, vertical diffusion can replace horizontal diffusion when dissipation is needed.

While the dynamics is severely altered between the simulations, the SST pattern errors with the climatology are found quite similar, in conflict with the results reported by Nevertheless, at the basin scale, lateral mixing conditions affect the meridian heat transport. Similar diffusive and advective heat transport amplitudes are found with the strongest lateral coefficient, while advective meridian heat transport dominates in both other experiments. Moreover, the different terms of the surface heat budget are found sensitive to the lateral diffusion: when lateral diffusion coefficient is sufficiently low, the balance implies the heating by the transient heat transport and the cooling by both monthly mean advection and vertical diffusion. Indeed, in the equatorial cold tongue, vertical diffusion is large enough to distribute heat down to the thermocline below the mixed layer.

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J-P. Boulanger, P. Delecluse, C. Maes, and C. Lévy

Abstract

A high-resolution oceanic general circulation model (OGCM) of the three tropical oceans is used to investigate long equatorial wave activity in the Pacific Ocean during the 1985–94 TOGA period. The ARPEGE atmospheric general circulation model simulated zonal wind stress forcing and the OPA OGCM simulated dynamic height are interpreted using techniques previously applied to data. Long equatorial waves of the first baroclinic mode (Kelvin and first-mode Rossby waves) are detected propagating in the model outputs during the entire period. A seasonal cycle and interannual anomalies are computed for each long equatorial wave. In the east Pacific basin, long equatorial wave coefficients are dominated by seasonal variations, while west of the date line they display strong interannual anomalies. Interannual long-wave anomalies are then compared to wave coefficients simulated by a simple wind-forced model. The results presented here indicate the major role played by wind forcing on interannual timescales in generating long equatorial waves. Discrepancies between the simple wave model and the OPA first-mode Rossby coefficients allow one to draw limitations of interpreting sea surface variability in terms of waves of the first baroclinic mode alone. Finally, the simple wave model cannot fully explain the Kelvin wave amplitude near the western boundary, nor the first-mode Rossby wave amplitude near the eastern boundary. However, coherency between the Kelvin and first-mode Rossby wave coefficients at both boundaries lead to the conclusion that reflection occurs in this model and contributes to the wave amplitudes as they propagate away from the boundaries.

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J. Boutin, Y. Chao, W. E. Asher, T. Delcroix, R. Drucker, K. Drushka, N. Kolodziejczyk, T. Lee, N. Reul, G. Reverdin, J. Schanze, A. Soloviev, L. Yu, J. Anderson, L. Brucker, E. Dinnat, A. Santos-Garcia, W. L. Jones, C. Maes, T. Meissner, W. Tang, N. Vinogradova, and B. Ward

Abstract

Remote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 × 100 km2. In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability.

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