The Ocean Response to Low-Frequency Interannual Atmospheric Variability in the Mediterranean Sea. Part II: Empirical Orthogonal Functions Analysis

Abstract

This paper deals with the statistical analysis of the nine years of model simulation described in Part I. Here the focus is on different applications of EOF analysis in the time domain, pointing out the spatial?temporal scales of the Mediterranean general circulation variability. The analysis is carried out either in 2D or 3D space and is based on the singular value decomposition technique. Seasonal and interannual variability of the Mediterranean Basin occur on the subbasin gyre spatial scales. Two major events of interannual variability occurring during the winters of 1981 and 1986 are identified through the analysis of the barotropic/baroclinic circulation. Barotropic streamfunction EOF analysis shows that after strong winter wind anomaly events, which enhance cyclonic circulation, the basin relaxes to opposite sign vorticity regimes. The analysis confirms that the largest barotropic anomalies are locked to the winter season. The temperature 3D EOF analysis highlights that three vertical amplitude modes can represent all the variance in the upper 400 m of the water column. The second mode, which has basin-wide scales with meridional gradients, is surface intensified and contains the seasonal cycle anomalies. The surface ocean signal propagates into the mixed layer within a period of three months. The dynamic height field interannual variability is pronounced at the subbasin-scale gyres and it shows anomalies in summer of 1981 and 1986, six months after the winter forcing anomaly events. The study of the correlation between surface heat fluxes and SST variability (extended EOF) shows that winter anomalous cooling events (1981, 1987) can generate different effects on the SST response involving both large-scale advection/diffusion processes and subduction events.