engleski

Meteorological and oceanographic conditions in the northern Adriatic Sea during the period June 1999-July 2002: Influence on the mucilage phenomenon

Mucilage events have been documented several times during the last two centuries in the northern Adriatic Sea (NA), but their frequency has significantly increased during the last 15 years (since 1988). The shallow NA is strongly influenced by meteorological conditions and by freshwater discharges, mainly from the Po River. Consequently, the oceanographic properties of water masses change considerably during the seasons and from year-to year. In this work, the main meteorological and oceanographic conditions in the NA during the period June 1999-July 2002 are described and related to the mucilage phenomenon (formation of large organic aggregates and gelatinous surface layers). Approximately monthly cruises along three transects in the NA were performed. Remarkable meteorological situations were selected from large-scale ECMWF analyses and from the Local Analysis and Prediction System (LAPS ; Emilia Romagna Meteorological Service), while sea conditions were analysed from the Wave Adriatic Model (WAM) output. Data for several meteorological stations in the region were used. Average seasonal cycles of sea temperature and salinity simulated with statistical models, based on historical data collected in the NA since 1972, were used to define thermal and haline anomalies. The monthly anomaly variability of maximum and minimum air temperatures, and rainfall (amount and number of rain days) did not appear to be relevant for the mucilage phenomenon outbreak. However, the thermohaline gradients in the region were more developed in 2002, and, even if at some lower intensity, in 2000, i.e. proportionally to the intensity and duration of the mucilage events, compared to 2001 (short living event) and 1999 (no event). These more pronounced gradients were due to a combination of several unusual conditions: sharp heating of the sea surface in May-June, domination of eastwards transport of freshened waters formed in the Po Delta area, and intrusion of very high salinity intermediate waters, originated in the Cretan Sea and Levantine Basin. Moreover, in winter of both 2000 and 2002 very dense and cold water formed and remained in the bottom layer up to spring, contributing to increase the stratification degree of the water column. The duration of the mucilage events and 3 their spatial distribution in the region depend strongly on meteorological changes. Recurrent anticyclonic conditions, characterized by low wind and calm sea, favour extended events in time (up two months in 2002). In contrast, unusual bad weather, particularly due to strong "bora" wind, can be determining in sharp decay of the event (e.g. in July 2000).

meteorological conditions; salinity; temperature; oceanographic conditions; mucilage

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