Atlantic to Mediterranean Sea Level Difference Driven by Winds near Gibraltar Strait DIMITRIS MENEMENLIS,ICHIRO FUKUMORI, AND TONG LEE Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California (Manuscript received 11 July 2005, in final form 17 May 2006) ABSTRACT Observations and numerical simulations show that winds near Gibraltar Strait cause an Atlantic Ocean to Mediterranean Sea sea level difference of 20 cm peak to peak with a 3-cm standard deviation for periods of days to years. Theoretical arguments and numerical experiments establish that this wind-driven sea level difference is caused in part by storm surges due to alongshore winds near the North African coastline on the Atlantic side of Gibraltar. The fraction of the Moroccan coastal current offshore of the 284-m isobath is deflected across Gibraltar Strait, west of Camarinal Sill, resulting in a geostrophic surface pressure gradient that contributes to a sea level difference at the stationary limit. The sea level difference is also caused in part by the along-strait wind setup, with a contribution proportional to the along-strait wind stress and to the length of Gibraltar Strait and adjoining regions and inversely proportional to its depth. In the 20–360-day band, average transfer coefficients between the Atlantic–Alboran sea level difference and surface wind stress at 36°N, 6.5°W, estimated from barometrically corrected Ocean Topography Experi- ment (TOPEX)/Poseidon data and NCEP–NCAR reanalysis data, are 0.10  0.04 m Pa 1 with 1  5-day lag and 0.19  0.08 m Pa 1 with 5  4-day lag for the zonal and meridional wind stresses, respectively. This transfer function is consistent with equivalent estimates derived from a 1992–2003 high-resolution barotro- pic simulation forced by the NCEP–NCAR wind stress. The barotropic simulation explains 29% of the observed Atlantic–Alboran sea level difference in the 20–360-day band. In turn, the Alboran and Medi- terranean mean sea level time series are highly correlated,   0.7 in the observations and   0.8 in the barotropic simulation, hence providing a pathway for winds near Gibraltar Strait to affect the mean sea level of the entire Mediterranean. 1. Introduction The sea level difference between the Atlantic Ocean and the Mediterranean Sea has been attributed mostly to the following: tides (e.g., Brandt et al. 2004), atmo- spheric pressure fluctuations (e.g., Tsimplis and Josey 2001), steric contributions (e.g., Cazenave et al. 2002), and geostrophic or hydraulic controls within the Strait of Gibraltar (e.g., Ross and Garrett 2000). Some studies (Fukumori et al. 2007; García-Lafuente et al. 2002a,b; Garrett 1983), however, suggest that winds in the vicin- ity of Gibraltar Strait may be a fifth major cause of Atlantic to Mediterranean sea level difference. García- Lafuente et al. (2002a) describe a meteorological forc- ing event that interrupts inflow in the Strait of Gibral- tar. They estimate an approximate contribution from wind forcing of 0.3 Sv (Sv  10 6 m 3 s 1 ), which is added to a 0.5–0.6-Sv contribution from atmospheric pressure. Fukumori et al. (2007) analyze altimetric sea level ob- servations, from which tidal and atmospheric pressure signals have been removed, and identify a near-uniform basin-wide sea level fluctuation of the Mediterranean Sea with periods ranging from days to years and with amplitudes of up to 0.2 m peak to peak. They estimate that these fluctuations are caused by a net mass flux through the Strait of Gibraltar with a peak 10-day- averaged amplitude of 0.26 Sv, corresponding to a depth-averaged current of 0.08 m s 1 at the Camarinal Sill. By comparison, Astraldi et al. (1999) report that tidal currents within the strait have velocities higher than 2.5 m s 1 , that atmospheric pressure fluctuations cause currents with magnitudes of about 0.4 m s 1 , and that there are baroclinic currents with magnitudes of about 0.5 m s 1 that are induced by the specific budget of the Mediterranean Sea. From theoretical considerations, Garrett (1983) esti- mates that wind-driven storm surges on the continental Corresponding author address: Dimitris Menemenlis, Jet Pro- pulsion Laboratory, California Institute of Technology, Mail Stop 300–323, 4800 Oak Grove Dr., Pasadena, CA 91109. E-mail: menemenlis@jpl.nasa.gov FEBRUARY 2007 MENEMENLIS ET AL. 359 DOI: 10.1175/JPO3015.1 © 2007 American Meteorological Society JPO3015