Atlantic to Mediterranean Sea Level Difference Driven by Winds near Gibraltar Strait

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 Experiment (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 barotropic 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 Mediterranean 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.