Seasonal Variations in Winds and Currents in the Southern Caspian Sea and the Importance of the Alborz Mountains
摘要
The circulation of the Caspian Sea, the largest enclosed aquatic basin in the world, is still somewhat mysterious. Recent studies have largely relied on numerical circulation models forced by global wind reanalysis products. Many of these papers lament the lack of any long-term observations of currents for comparison, and this is especially true in the southern basin. However, from late 2012 to early 2014, a series of field studies were conducted by the Iranian Ports and Maritime Authority in the southern basin of the Caspian, with currents measured at 5 sites, and a high-resolution gridded regional wind product was created for the same period. Here we use this combined dataset to describe the monthly-to-seasonal scales of variability of currents and winds near the coast. These low-frequency currents on the shelf are mostly barotropic at speeds of 0 to 10 cm s−1, while low-frequency winds are as high as 3 m s−1. Although the 14 month period of observations, containing a single seasonal cycle, is not enough to provide an unambiguous link, seasonal cycles in spatially-variable shelf currents are consistent in phase and magnitude with local forcing by seasonally and spatially-variable local winds, balanced by bottom friction. These local winds are heavily influenced by the surrounding topography, being steered sharply eastwards near the coast by the Alborz mountains in a manner not reflected in global reanalyses, and are also affected on regional scales by land heating in some of the coastal plains. In consequence of this local balance with spatially-variable winds the coastal currents will have convergences and divergences along the coastline that may in turn affect large-scale basin circulation patterns. In addition, flow dynamics are more complicated near Cape Sefidrood as the coastal current often separates from the coast there to form eddies.