Impact of the spatial distribution of the atmospheric forcing on water mass formation in the Mediterranean Sea

Abstract

[1] The impact of the atmospheric forcing on the winter ocean convection in the Mediterranean Sea was studied with a high-resolution ocean general circulation model. The major areas of focus are the Levantine basin, the Aegean-Cretan Sea, the Adriatic Sea, and the Gulf of Lion. Two companion simulations differing by the horizontal resolution of the atmospheric forcing were compared. The first simulation (MED16-ERA40) was forced by air-sea fields from ERA40, which is the ECMWF reanalysis. The second simulation (MED16-ECMWF) was forced by the ECMWF-analyzed surface fields that have a horizontal resolution twice as high as those of ERA40. The analysis of the standard deviations of the atmospheric fields shows that increasing the resolution of the atmospheric forcing leads in all regions to a better channeling of the winds by mountains and to the generation of atmospheric mesoscale patterns. Comparing the companion ocean simulation results with available observations in the Adriatic Sea and in the Gulf of Lion shows that MED16-ECMWF is more realistic than MED16-ERA40. In the eastern Mediterranean, although deep water formation occurs in the two experiments, the depth reached by the convection is deeper in MED16-ECMWF. In the Gulf of Lion, deep water formation occurs only in MED16-ECMWF. This larger sensitivity of the western Mediterranean convection to the forcing resolution is investigated by running a set of sensitivity experiments to analyze the impact of different time-space resolutions of the forcing on the intense winter convection event in winter 1998-1999. The sensitivity to the forcing appears to be mainly related to the effect of wind channeling by the land orography, which can only be reproduced in atmospheric models of sufficient resolution. Thus, well-positioned patterns of enhanced wind stress and ocean surface heat loss are able to maintain a vigorous gyre circulation favoring efficient preconditioning of the area at the beginning of winter and to drive realistic buoyancy loss and mixing responsible for strong convection at the end of winter.

Design and validation of MEDRYS, a Mediterranean Sea reanalysis over the period 1992–2013

“…Many small-size islands and a particularly complex coastline limit the low-level air flow, channeling potentially strong and recurring regional winds (Mistral, Tramontane, Bora, Etesian, Sirocco;Herrmann et al, 2011). The role of the spatial resolution of the forcing has been highlighted as a key aspect of the representation of Mediterranean Sea phenomena such as local winds (Sotillo et al, 2005;Ruti et al, 2007;Herrmann et al, 2011;Lebeaupin Brossier et al, 2012), opensea deep convection (Herrmann and Somot, 2008;Béranger et al, 2010), shelf-cascading (Dufau-Julliand et al, 2004;Langlais et al, 2009), coastal upwelling (Estournel et al, 2009;Casella et al, 2011), permanent circulation features (Estournel et al, 2003;Ourmières et al, 2011) or intermittent eddies (Marullo et al, 2003;Ciappa, 2009;Rubio et al, 2009). The infra-diurnal temporal resolution of the forcing has also been identified as necessary to represent key phenomena such as large salinity anomalies following intense rainfall events (Lebeaupin Brossier et al, 2012) or the sea surface temperature (SST) diurnal cycle (Lebeaupin Brossier et al, 2011.…”

“…Over the Mediterranean Sea, the added-value of highresolution models has been shown in particular concerning the representation of the heat and water budget terms Josey et al, 2011), of wind field especially close to the coast and islands (Sotillo et al, 2005;Ruti et al, 2007;Herrmann and Somot, 2008;Langlais et al, 2009;Herrmann et al, 2011;Vrac et al, 2012) and of the events of strong air-sea fluxes (Herrmann and Somot, 2008;Béranger et al, 2010;Lebeaupin-Brossier et al, 2012). Dynamical downscaling of reanalyses have therefore been used to force long-term hindcast simulations (Beuvier et al, , 2012Herrmann et al, 2010;Solé et al, 2012;Vervatis et al, 2013;Auger et al, 2014;Harzallah et al, 2014).…”

Influence of sea surface roughness length parameterization on Mistral and Tramontane simulations

“…Since this winds are caused by similar synoptic situations, they often occur at the same time (Georgelin et al, 1994;Guenard et al, 2005). They play a crucial role for deep water formation in the Gulf of Lion and for the understanding of the Mediterranean Sea circulation (Schott et al, 1996;Béranger et al, 2010). On Mistral and Tramontane days, simulations with the regional climate model COSMO-CLM (CCLM) with 0.088 • grid spacing were found to be able to simulate Mistral and Tramontane wind patterns slightly overestimating 10 m wind speed compared to satellite and buoy observations (Obermann et al, 2016).…”

Forecasting the mixed-layer depth in the Northeast Atlantic: an ensemble approach, with uncertainties based on data from operational ocean forecasting systems

“…Measuring the accuracy of the mixed-layer depth is an essential diagnostic for quantifying limitations in the model schemes or parameterizations (Giordani et al, 2005;Keerthi et al, 2013;Tozuka and Cronin, 2014). (ii) The mixed layer is the ocean layer which is directly in interaction with the atmosphere and the study of the mixed layer can reveal biases or unappropriated formulation of the atmospheric forcing of the ocean model (Béranger et al, 2010;Giordani, 2011). (iii) Most of the primary production in the ocean occurs in the mixed layer.…”