On the contributions of atmospheric pressure and wind to daily sea level in the northern Adriatic Sea

Abstract

Daily sea level variability in the Adriatic Sea is studied from different data sets using Empirical Orthogonal Functions, in connection with atmospheric pressure and wind stress. The first mode explains 56–69% of total variance and consists of uniform sea level variability all over the basin, correlated with atmospheric pressure through the inverse barometer effect. The second mode explains 13–16% of variance and accounts for an along-basin sea level gradient, which is correlated with the meridional wind stress component. The first two Principal Components are used as proxies to pressure- and wind-induced components of storm surges in the northern Adriatic. The analysis of the frequency of the most intense events in the 1957–2005 period shows that the wind contribution to storm surges has decreased, while no significant trends are found in the contribution of atmospheric pressure.

Introduction

Sea level variations represent a major issue in the context of current studies on climatic change. Their potentially harmful impact on human life and activities in several coastal regions of the world depends on the general mean sea level rise, but can be locally enhanced by an increased frequency of extreme events. Relative sea level changes, i.e. those observed relative to a ground-based benchmark, result from variations of several factors acting on different time scales, such as waves, tides, atmospheric forcing, ocean currents, mass and heat changes of the ocean, geophysical processes affecting the land level and the geoid, as discussed by Plag (2006) and Jevrejeva et al. (2008).

In the Adriatic Sea the northern region is very sensitive to sea level changes since most of the coastal areas is low and subject to floods. The sea level time series of the Adriatic stations exhibit different long-term trends that may strongly depend on local factors, such as the vertical motion of the tide gauge related to local geology and tectonic movements (Zerbini et al., 1996, Bergant et al., 2005). In addition to natural subsidence, the northwestern Adriatic coast, including the Venice Lagoon and the area around Marina di Ravenna, has been affected by anthropogenic subsidence connected with the extraction of underground water and gas, particularly during the 1930–1970 period (Carbognin et al., 2004).

On seasonal and longer time scales sea level variability is modulated by changes in the thermohaline structure of the basin, affecting the water density and volume, induced by changes in the hydrological balance and the atmospheric forcing, and the ocean circulation through the Otranto Strait, which connects the basin with the rest of the Mediterranean Sea. On the synoptic time scales atmospheric pressure and wind are the most effective forcing terms. The main wind regimes are characterized by Bora and Sirocco. Bora blows from the North-East to East sector across the basin, generally in connection with anticyclones over central or eastern Europe. Sirocco blows from South-East and is usually associated to cyclones over the western Mediterranean. Its flow is channelled by the orography that surrounds the basin, namely the Apennines along the Italian peninsula and the Dynaric Alps along the Dalmatian coast, and it is a major factor responsible for storm surges in the northern Adriatic. A comprehensive review of the Adriatic oceanography and air–sea interactions can be found in Cushman-Roisin et al. (2001).

Several authors dealt with the interactions and relationships between sea level and atmospheric pressure and wind in the Adriatic Sea. Longer than daily time scales have been analysed, for instance, by Lascaratos and Gačić (1990), Raicich and Crisciani (1999), Pasarić et al. (2000), Beretta et al. (2005) and Bergant et al. (2005). Also events that develop on daily and sub-daily time scales have been studied, namely storm surges and seiches (e.g. Buljan and Zore-Armanda, 1976, Raicich et al., 1999, Raicich, 2003, Pirazzoli and Tomasin, 2008) and meteorological tsunamis (Vilibić et al., 2004).

The response of sea level to the atmospheric pressure and wind forcing on time scales relevant to storm surges can be modelled by means of depth-integrated equations of motions (e.g. Bowden, 1983). Neglecting Earth’s curvature, in a reference system where x is positive eastwards and y northwards, the responses to atmospheric pressure (p_a) only and meridional wind stress (τ_y) only (similarly for zonal wind stress τ_x) can be written in finite form as

$$\mathrm{\Delta }\eta =-\frac{1}{g\rho }\mathrm{\Delta }{p}_a$$

$$\frac{\mathrm{\Delta }\eta }{\mathrm{\Delta }y}=\frac{{\tau }_y}{g\rho (\eta +h)}$$

respectively, where h is water depth, η is sea surface elevation, ρ is water density and g is gravity acceleration. In the Adriatic Sea |η| is rarely greater than 1 m and the minimum depth (sufficiently far from the coast) is about 20 m, in the northern basin, therefore (η+h)≈h in Eq. (2). Under this approximation, linear relationships exist between sea level and atmospheric pressure anomalies (the inverse barometer effect) and between sea level gradient and wind stress, respectively.

In this work we study the time variability of Adriatic sea level using daily means, aiming at recognizing the contributions of atmospheric pressure and wind to storm surges in the northern basin, which determine only part of the basin dynamics. Nevertheless, the statistical analysis, performed on the basis of Eqs. (1), (2), is able to extract significant signals related to the atmospheric forcing.

Data and methods are described in Section 2. Section 3 presents and discusses the results of the analysis of spatial and temporal variability of daily mean sea level and of the statistical analysis of extreme events. Conclusive remarks are proposed in Section 4.