Quantifying the Contribution of Ocean Dynamics to SST Anomaly Formation

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The goal of the study is to use observational data to determine the role of mixed layer horizontal advection in generating sea surface temperature (SST) anomalies from monthly to interannual timescales. The primary concern is in quantifying the advection of the climatological SST gradient by geostrophic current anomalies, and the resulting SST anomaly (SSTA) signal in the climatological mixed-layer. This signal is of interest to the question of whether SST anomalies are predictable months or more in advance. There is also strong evidence that it can be measured with current observational data. A secondary goal will be to quantify the role of the mean geostrophic currents in advecting the SSTA. The region of study will be all ice-free, deep-water from about 65S to 5S and 5N to 65N. The spatial scales investigated, for the primary goal, will range from a few degrees latitude and longitude to ocean basin scale (though the advective heat flux will be resolved to a few 10s of kilometers, as explained below). For the secondary goal the advective heat flux resolution in many regions will be limited to scales larger than hundreds of kilometers due to data limitations. The spatial patterns and fraction of SSTA variance generated by mixed-layer, geostrophic advection will be determined. Unprecedented spatial resolution for advective heat fluxes: estimates of advection will be based upon the advective heat flux into grid cells outlined by the satellite ground tracks of the T/P mission, hereafter the diamond grid cells. This allows for the estimation of the surface geostrophic current with the along track sea surface height anomaly (SSHA) data obtained from the T/P mission (data available from October 1992) and its follow on program, Jason-1 (data available from January 2002). Compared to previous studies using gridded altimeter data, this method avoids the necessity of spatial interpolation inherent in the gridding. For the primary goal involving current anomalies advecting climatological SST this allows for about an order of magnitude improvement in spatial resolution. (An interpolation in time, of up to 5 days, is unavoidable. A quantitative assessment of both interpolation error and altimeter error is provided herein.) Different SST products will be evaluated, including the nominally 9 km resolution climatological SST obtained from the NASA/NOAA AVHRR Oceans Pathfinder SST dataset. The advection term will be time integrated to obtain the advective contribution to the SSTA at each diamond grid cell. A misleading picture of the relative importance of the SSTA forcing terms can arise by comparing the relative magnitude or variance of the forcing terms. This occurs because of the widely different decorrelation times of the various forcing terms, and is explained in theory in a recent article by the authors, and verified by their heat budget analysis of the Kuroshio Extension area. Time integrating a forcing term allows for direct evaluation of its contribution to the signal variance. Finally, the SSTA generated by advection at each diamond grid cell will be analyzed in novel ways, revealing different aspects of the advection term. First it will be projected onto the leading empirical orthogonal functions (EOFs) of SSTA. This allows one to quantify the fraction of variance attributed to advection on an EOF by EOF basis. An EOF analysis of the advection generated SST anomaly will also be performed to reveal the dominant spatial patterns. Finally the dependence upon temporal and spatial scale will be assessed.Broader Impacts: The results will be useful for either providing definitive support of the null hypothesis that anomalous advection of the climatological SST gradient is not important in generating SSTA on seasonal to interannual timescales, or providing a more rigorous assessment of advection generated SST. Low spatial resolution results on the role of mean currents will also be obtained. Together these results will be crucial for guiding future research on the variability and predictability of the World Ocean SSTA and the climate that it influences.