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4 Sea Temperature

The seasonal and inter-annual variation of the water mass dynamic and distribution can be
practically followed by their temperature signatures, both vertically and horizontally, when
there is water stratification in the summer and water mixing in the winter period.

The main driving processes are: the progression of the AIS and its eastward extension, the
upwellings south of Sicily, the warming and cooling of the shallow continental shelf waters.
Indeed, significant variability can be observed from year to year and even the annual cycle is
rather complex. When considering the simulated results of Sorgente et al. (2003) for
seasonal variability of sea surface temperature (at 5m depth), figure 8 shows a shift in the
mean temperature of about 6°C between winter (February) and summer (August) mainly as a
consequence of surface heating.

During winters the mixing processes result in the homogenization of the water column up to
depths in excess of 100m, and with temperatures on average 0.5°C higher in the southern
part of the Sicily Channel. During this time, the thermal structure is fairly homogeneous,
especially beyond 35° N (including the north side of the Sicily Channel). The temperature
does not exceed 15°C.

During the summertime, the nearshore well stratified surface layer (averages 20m in depth)
above the cooler and relatively fresher Modified Atlantic Water (MAW), due to solar heating,
reaches temperatures between 20 and 26°C. In summer the simulated potential temperature
is characterised mainly by upwelling events along the southern coast of Sicily, bringing
cooler water to the surface. This is in contrast to the overall increase in temperature over the
region.

Warm waters have a mean temperature of over 26°C. The contrast in temperature of the
MAW exiting the Sicilian Channel with the warmer Ionian water produces a sharp
temperature gradient which is often evidenced over the Malta escarpment by conspicuous
thermal features on the sea surface temperature. The progression of the AIS and its south-
eastward extension is delineated by the annual mean 19.86°C isotherm.

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