Alexandra Bozec

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

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.

Impact of penetrative solar radiation on the diagnosis of water mass transformation in the Mediterranean Sea

[1] We applied a revised diagnosis of water mass formation and mixing to a 1/8° resolution ocean model of the Mediterranean Sea. The diagnosis method used and presented by Iudicone et al. (2008) is similar to that developed by Walin (1982) and applied to the Mediterranean Sea by Tziperman and Speer (1994), to which we added a penetrative solar radiation. Both the prognostic model and the diagnostic method were in agreement with respect to the solar flux parameterization. Major changes were observed in the yearly budget of water mass transformation when the penetrative solar radiation is taken into account in the diagnosis. Annual estimates of water mass formation rates were decreased by a factor of two, with values within the range [-3.7 Sv, 1.5 Sv] compared to [-6 Sv, 3 Sv]. This decrease resulted from a lower seasonal variation when penetrative solar radiation was included. This can be explained by the fact that the solar radiation flux acted over a wider range of seawater density leading to lower net values over a given density interval. The major impact of the penetrative solar radiation occurred during spring and summer. Newly formed dense water was then transformed into lighter water with a rate reaching a value about 50% of that of the water mass formation rate in winter. Another consequence was that mixing processes which counteract formation rate in yearly budget of water mass formation rates, were overestimated. We showed that, in spring and summer, about a third of the transformation took place below the surface layer.

Fast Wind-Induced Migration of Leddies in the South China Sea

AbstractEddies off the Strait of Luzon (termed here as “Leddies,” analogous to “Teddies” originating from the Indonesian Throughflow) are formed rapidly and migrate swiftly. Their migration rate (~10–20 cm s−1) is an order of magnitude faster than that of most eddies of the same scale (~1 cm s−1). On the basis of observations, it has been suggested earlier that the rapid generation process is due to the southeast monsoon.Here, the authors place this earlier suggestion on a more solid ground by developing both analytical and process-oriented numerical models. Because the eddies are formed by the injection of foreign, lighter Kuroshio water into the South China Sea (SCS), the eddies are modeled as lenses: that is, “bullets” that completely encapsulate the mass anomaly associated with them. It turns out that the rings migrate at an angle α (between 0° and 90°) to the right of the wind direction {i.e., tan−1[(2 − γ)f2R/8g′CD, where in conventional notation γ is the vorticity, R the eddy radius, and CD the int...

Chapter 7 Regional atmospheric, marine processes and climate modelling

Publisher Summary The Mediterranean region is rather unique in respect to its geographical position: north of the largest desert in the world, the Sahara and south of a large temperate climate region, Europe. It is, therefore, a transition area between tropical and mid-latitude climates. As a transition area, the Mediterranean region shows important local climate variability and rather large gradients, both in the South-North and East-West directions. The chapter explores that the Mediterranean Sea is a concentration basin with an evaporation rate much larger than the rainfall rate and river runoff, leading to increase in salt content. It is also a source of heat to the atmosphere with annual decreases of temperature for water masses. Numerical modeling, both global and regional, is an important tool to understand physical mechanisms controlling climate change and variability at different spatio-temporal scales. It also provides the unique possibility to construct physically based and comprehensive future climate scenarios, the starting point for many socio-economical impact considerations. It presents several studies on the physical mechanisms controlling the Mediterranean climate variation and change. The current status of the Mediterranean regional climate modeling and the preliminary results of a regional coupled model are presented in the chapter.

A Puzzling Disagreement between Observations and Numerical Models in the Central Gulf of Mexico

AbstractTwo large, independent sets of direct observations in the central Gulf of Mexico show a mean near-surface flow of ~10 cm s−1 to the west, concentrated in the northern and southern Gulf. Numerical models that the authors have examined do not produce this mean westward flow. The observed speeds appear to be almost an order of magnitude larger than the estimated errors; this paper studies the observations to estimate carefully the possible errors involved and compares the observations with model results. The flow to the west in the southern Gulf is presumably wind driven on the shallow parts of the shelf, and, in slightly deeper water at the outer edges of the shelf, is possibly the result of southward Sverdrup interior flow driven by the negative curl of the wind stress. In another possibly related issue, long-term deep current-meter observations in the northern Gulf at ~1000 m and below find flow to the west, whereas some models find flow to the east. The flow proposed here assumes a mean flow to t...