Winter convection

The horizontal structure of the field of the minimal water temperatures (the core of the CIL) of the Black Sea in the extreme months of the annual cycle (February and August) is presented in Figs. 6a and 6b. The February field of the minimal water temperatures (Fig. 6a) is similar to the surface temperature field (Fig. 5a). Meanwhile, a detailed analysis shows that they are not fully identical from the northwest to the southeast the excess of the surface temperature over the minimal value grows up to 1.0 °C. The depth of the temperature minimum location increases in the same direction down to 70-80 m. This points to the absence of CIL water renewal owing to the winter convective mixing over some part of the Black Sea area. 

Of particular interest for the winter convection and the formation of the winter water layer are the temperatures of maximum density and the freezing points. Reported in Table 20.3 are the freezing points, tf, and the temperatures of maximum density, t D, for brackish seawater of Reference Composition (Millero et al., 2008) with absolute salinity, 5a, between 0 and 25 g/kg, computed from the lAPWS formulations for water (lAPWS, 1996), ice (lAPWS, 2006), and the saline Gibbs function (Feistel, 2007). 

The essential fluctuations of the mean monthly water temperature were observed at aU depths of the Aral Sea. In deeper layers this happened due to the effect of winter convection which, in turn, is connected with the severity of winters. Low temperatures in the bottom layers were registered most often after cold winters with much ice, and higher temperatures - after warm winters. The severity and duration of winters also influence the annual mean temperature of air and water. 

The most important process forming the hydrological structure of waters in the Aral Sea before its desiccation was a convective mixing in autumn and winter. Regardless of the fact that the Aral Sea locates in the southern latitudes the winter convection here was developing over the whole water area of the sea and engaged all water layers. 

Therefore, the deep waters in the western trough were formed as a result of the combined effect of local waters going down during winter convection and advec-tion of waters from the eastern and central parts of the sea in the bottom layer. 

However, in the spring of 2004, the entire column was nearly uniform in salinity at about 86 ppt (see Table 3), indicating that winter convection was likely to had been able to destroy the meromictic conditions, and the enhanced stratification was an intermittent rather than permanent feature. By the late summer of 2004, a new type of stratification had arisen, which persisted in aummns until the fall of 2006. This stratification was mainly three-layered, involving two salinity maxima, one at the surface, and the other one at the bottom, separated by a relatively fresh intermediate layer (Fig. 6). Despite the salinity inversion just below the upper mixed layer, the column was maintained in a stable state by a steep thermocline. The deeper salinity maximum was best developed in 2005, but a hint of it was also observed in the fall of 2006, when there was a marked bottom mixed layer, indicating the presence of strong currents in the bottom layer (which was also confirmed by direct current measurements). 

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