Istrian Coastal Countercurrent and its Year-to-Year Variability

Abstract

In this paper we demonstrate the existence of a descending surface current which usually appears in August along the Istrian coast line and runs counter to the general Adriatic-wide cyclonic flow. This is achieved by computing the relative (with respect to the 30-dbar surface) geostrophic currents as well as absolute currents and net transports from hydrographic data collected monthly to seasonally at six stations of the Po-Rovinj profile in the northern Adriatic. We name it the Istrian Coastal Countercurrent (ICCC). Its mean relative speed is high, 7cms−1(relative currents at the profile on average stay below 5cms−1in winter and below 8cms−1in summer), inducing a net outflow near the Istrian coast.The ICCC was strong in 1968, 1977, 1983, 1984, 1988, 1989, 1991, 1992, 1996 and 1997, weak or absent in 1971–1973, 1982, 1986, 1993–1995. Whenever the ICCC was well pronounced, specific dynamic and hydrographic features—pointing to a possible existence of an anticyclonic gyre off Rovinj—were observed. The existence of the ICCC is confirmed by direct current measurements, which show that the descending coastal current was intense in 1977 and 1983, and that it did not develop in 1976, 1978 and 1987.The intensity of the ICCC in August over the 1966–1992 interval was significantly correlated to the air–sea heat flux observed with the −7 months time lag in combination with the Po River discharge rate with the −1 month time lag and to the sea surface temperature recorded in the area with the −5 months time lag. On the correlation basis a weak or non-existent ICCC is hindcasted for 1976, 1978 and 1987. It seems that high intensity of the ICCC coincides with oceanographic conditions which favour near anoxia or mucilage events, observed in northern Adriatic in 1977, 1988, 1989, 1991 and 1997.