Analysis of a general circulation model product: 1. Frontal systems in the Brazil/Malvinas and Kuroshio/Oyashio regions
Authors: Garzoli Silvia L. , Garraffo Zulema , Podesta Guillermo , Brown Otis 
Published: February 2008

In the present paper (part 1 of 2), the product of the Semtner and Chervin general circulation model (GCM) is compared with available observations in the frontal areas of the Brazil/Malvinas and the Kuroshio/Oyashio confluences. The dimensionality of the systems studied is reduced by using the empirical orthogonal functions (EOF) and frontal density methods. The two sets of data utilized to validate the model are the sea surface temperature (SST) from the satellite observations and temperature fields product from the GCM at levels 1 (12.5 m), 2 (37.5 m) and 6 (160 m). Comparisons are made between the dominant empirical modes and the locus of maximum probability for observations and model product. The model reproduces intense thermal fronts at the surface and in the upper layers. In the upper layer (level 1) they are induced by the internal dynamics of the model and not by the restoring of the model to climatology alone. The variability of these fronts is less pronounced in the model than in the observations. The dominant period in the observations is annual with contributions of semiannual and high frequency oscillations. In the model, the dominant variability is also annual at all analyzed levels. A semiannual oscillation contributed to a lower degree and is related to eddies that, in the model, have an annual and semiannual periodicity. For the regions examined, the location of the fronts are reproduced in the model within differences of 4° to 5° with observations. In the Brazil/Malvinas region, the Confluence front is reproduced approximately 4° towards the west of the observed front. This appears to be due to the resolution of the model that, in a 0.5° × 0.5° grid, does not resolve the sharp slope at the edge of the Argentine continental shelf. The maximum southward penetration of the warm tongue of Brazil waters occurs in the model approximately 4° towards the north. This is related to the fact that, in the model, the Malvinas transport doubles the one derived from the observations. This might be due to the effect of a large modeled transport for the Circumpolar Current or, again, to a poor resolution of the topography. In the Kuroshio area, the Oyashio front, which in observations is more pronounced at the surface than in the lower layers, is well reproduced in the surface temperature field. On the contrary, the Kuroshio front, more intense in the lower layers but still marked in the satellite observations, is visible in the model only below 160 m. The front is not present in the surface temperature field but, as a consequence of the thermal wind balance, an intense eastward flow at the location of the Kuroshio Extension is observed in the model velocity field. When compared with the observations, the location of the Extension is shifted approximately 5° towards the south. This indicates a shift in latitude between the modeled and observed latitude of separation. The resolution of the model is marginal to reproduce the process of eddy formation, but large scale eddies are observed in the model in both analyzed areas. They are generated as a pinch of the main flow with an annual and semiannual periodicity. We conclude that some of the differences between model and observations, like the differences found in the locations of the fronts, and the diminished variability, will decrease with a higher resolution.