Istrian Coastal Countercurrent and its year-to-year variability

In this paper we demonstrate the existence of a descending surface current which usually appears in August along the Istrian coast line and runs count er to the general Adriatic-wide cyclonic flow. This is achieved by computin g the relative (with respect to the 30-dbar surface) geostrophic currents a s well as absolute currents and net transports from hydrographic data colle cted monthly to seasonally at six stations of the Po-Rovinj profile in the northern Adriatic. We name it the Istrian Coastal Countercurrent (ICCC). If s mean relative speed is high, 7 cm s(-1) (relative currents at the profile on average stay below 5 cm s(-1) in winter and below 8 cm s(-1) in 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-point ing to a possible existence of an anticyclonic gyre off Rovinj-were observe d. The existence of the ICCC is confirmed by direct current measurements, w hich show that the descending coastal current was intense in 1977 and 1983, and that it did nor develop in 1976, 1978 and 1987. The intensity of me ICCC in August over the 1966-1992 interval was signific antly correlated to the air-sea hear flux observed with the - 7 months time lag in combination with the Po River discharge rate with the - 1 month tim e 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 hi ndcasted for 1976, 1978 and 1987. It seems that high intensity of the ICCC coincides with oceanographic conditions which favour near anoxia or mucilag e events, observed in northern Adriatic in 1977, 1988, 1989, 1991 and 1997.