Low-Frequency Sea Level Variability in the Northeastern Mediterranean

Abstract

Analyses of time series of one year of sea level data from 11 stations in the northeastern Mediterranean (Adriatic, Ionian and Aegean seas) are presented in terms of local atmospheric pressure forcing. The study focuses on low-frequency oscillations with time scales ranging from one day to several weeks, but shorter than the seasonal scale. The summer and winter seasons are analyzed separately. An empirical orthogonal function analysis of both sea level and atmospheric pressure resulted in a separation of lower frequency oscillations of planetary-wave time scale (expressed by the first mode) from the higher frequency synoptic time scale variability (included in the second mode). The first mode is related to the in-phase sea level or atmospheric pressure variations of the entire area, while the second mode represent variations for which the Adriatic Sea is out of phase with both the Ionian and Aegean seas. Only those first two modes represent a regionally coherent signal subtracting more than 90% of the total variances of both sea level and atmospheric pressure. The sea level E0Fs are closely related to respective atmospheric pressure modes both in space and time. Departures from the isostatic response, evidenced in the low frequency range of atmospheric planetary waves are indicated not to be due to geostrophic control in straits. The second-mode time dependence is shown to be related to the surface pressure changes induced by the regional cyclonic activity. The location of the zero crossing of the atmospheric pressure second mode over Otranto Strait is explained in terms of the prevalent cyclone paths.