High resolution altimeter gridded fields for coastal and regional studies: applications in the Western Mediterranean

Abstract

International audiencePrevious studies have shown that the merging of multiple altimeter missions provide improved description of the mesoscale variability but they have also pointed out that the present configuration still lacks of enough resolution for correctly sampling regional and coastal features. In this context, we develop and test alternative methods to generate high resolution altimeter maps by using a two step method in which smaller scales are added close to the altimeter tracks. In a first step, the large scale signals (~100 km) are removed by subtracting the standard gridded Sea Level Anomaly maps (AVISO) to along track data (unfiltered and unsubsampled). In a second step, the residuals are submitted to an objective analysis scheme with correlation scales adjusted to smaller mesoscale dynamics. A method including bathymetric constraint has been tested to provide a pseudo-dynamical boundary condition and to increase reliability in the coastal region. SST data information from satellite sensors are also integrated in the covariance function to gain resolution. The methods are applied over the 2002-2010 period in the NW Mediterranean, a challenging area due to the low signal to noise ratio but where expertise and independent in situ data are available. As expected, the new fields display smaller 58 features not present in the AVISO product with higher levels of Eddy Kinetic Energy close to the tracks. The mean power spectra (averaging all tracks and passes) also presents an increase in energy for wavelengths smaller than 200 km. Additionally, several tests were performed changing the correlation scales and altimeter, and the results did not present important sensitivity for small changes in the parameters. A correlation scale of 20 km and measurement noise variance of 9 cm2 were finally selected. The dataset is then compared to independent data such as drifters launched in the area, several campaigns with in-situ data and glider information, revealing an improvement in the resolution of some small scale features. However, the method does not improve significantly the accuracy when statistical comparisons with drifters are performed in all the area. A crossvalidation with independent along track altimeter data shows a modest reduction of the differences between interpolated maps and along track measurements when the statistics are computed for interpolation errors smaller than 20% of the total variance. Finally we explore the potential of combining altimeter data with tide gauges located along the Mallorca coast in order to improve coastal features. Some illustrations of preliminary results will be presente