The Ocean Response to Low-Frequency Interannual Atmospheric Variability in the Mediterranean Sea. Part I: Sensitivity Experiments and Energy Analysis

Abstract

In this study a general circulation model is used in order to investigate the interannual response of the Mediterranean Basin to low-frequency interannual variability in atmospheric forcing for the period 1980?88. The model incorporates a realistic scheme for the air?sea interaction physics, has 31 levels in the vertical, and a quarter of a degree horizontal resolution. The simulations show the strong seasonal and interannual signal of the upper thermocline Mediterranean general circulation. Interannual variability of the basin has an eventlike character (anomalous winter wind curl for 1981 and 1986, heat flux winter anomalies in 1981 and 1987) and it is mainly forced by wintertime anomalies;for example, it is locked to the seasonal cycle. The Ionian and the eastern Levantine areas are found to be more prone to interannual changes. The Gibraltar mass transport undergoes small seasonal changes around an average value of 0.95 Sverdrup (Sv) while the Sicily Strait transport is characterized by much stronger seasonal and interannual fluctuations around an average of 1.5 Sv. Sensitivity experiments to atmospheric forcing show that large anomalies in winter wind events can shift the timing of occurrence of the seasonal cycle. Energy cycles involve exchanges between barotropic, baroclinic kinetic energy, and available potential energy reservoirs. The analysis presented here shows that the direction of conversion between these reservoirs depends on the wind strength. During winter, energy is stored in the available potential energy pool and the kinetic energy is directly forced by winds. During summer kinetic energy grows at the expense of available potential energy following the well-known baroclinic instability conversion process.