Spectral Analysis of Submesoscale Energy Cascades Using Subtropical North Atlantic Sea Surface Temperature Fields

Abstract:

Mixing parameterizations of current climate models could be improved by a better understanding of energy cascades in the submesoscale ocean (1 – 10 km). This study uses spectral analysis to compare energy cascades in two different data sets that resolve processes of about 1 km scale. The first data set consists of 20 years of in situ measurements from an acoustic Doppler current profiler (ADCP, temperature and velocity) and about 10 years of in situ data from a thermosalinograph (TSG, temperature and salinity) both mounted on the Oleander, a container vessel that makes weekly trips from New Jersey to Bermuda. The second data set consists of global sea surface skin temperature data from the Visible-Infrared Imager-Radiometer Suite (VIIRS) mounted on the Soumi-NPP NASA spacecraft. The slopes of the ADCP and TSG potential energy spectra are between -2.4 and -2.6, the similarity suggesting they represent the same physical processes. Because the TSG produces higher resolution data than the ADCP, the TSG energy spectrum better resolves processes at scales < 10 km. When separated by region, the TSG energy spectra have different slopes in three distinct regions along the Oleander track: on the continental shelf and in the Gulf Stream the slopes are about -2.7, and in the Sargasso Sea the slope is about -2.5. All slopes determined from the temperature structure functions for the same regions are between 1.0 and 1.3, with the Sargasso Sea slope being the lowest of the four (the three separated regions and the entire Oleander track). In theory, the energy spectrum slope (n) and the structure function slope (p) should be related by n = p + 1 if the underlying physical processes are both isotropic and homogeneous. The deviations from theory may result from failure of this assumption and/or from the contribution of multiple processes to the spectra, which are combined differently for structure functions than for energy spectra. Of interest here is that our estimates show consistent differences in slopes in the different regions. Comparisons will be made between TSG and VIIRS temperature structure functions to determine if surface properties are consistent with in situ data.