Monsoonal effects on the hydrography of the upper waters (

Abstract

In the past, upwelling related to the southeast monsoon has been postulated around the Outer Banda Arc and from the Aru Basin onto the Sahul Shelf. These areas were surveyed during the Snellius-II Expedition, in August 1984 (southeast monsoon) and in February/March 1985 (northwest monsoon). During both cruises, a well-developed mixed layer was generally found, with a thickness of on average 40 to 50 m but dependent on ambient wind conditions and salinity profile. The salinity of the surface waters of the Arafura Sea was below 34, due to the rivers of Irian Jaya. In August, the westward surface current appeared accompanied by a counterdirected bottom current along the eastern slope of the Aru Basin, as water originating from 100 to 150 m was found on the Sahul Shelf up to depths of 40 to 60 m. The intrusion covered hundreds of kilometres both north and south of the Aru Islands, and probably gave significant enrichment along the coast of Irian Jaya by mixing with the less saline upper water. Repeated sampling of stations at one to three week intervals showed that upwelling also occurred in the Aru Basin and, weakly, in the eastern Banda Sea. Density planes at depths of 50 to 150 m moved upwards at speeds between 0.3 and 3 m·d −1, presumably causing large-scale entrainment of waters from the thermocline into the mixed layer. Surfacing of ‘cold’ water was not observed, but at some sites the top of the thermocline was within 25 m of the surface. In the Aru Basin, the mean depth of σ t (∼25°C) increased from 60 m to more than 110 m between August 1984 and February 1985. In the eastern Banda Sea this density plane only dropped from 80 to 90 m depth, but there downwelling still proceeded at a rate of 1 m·d −1. Analysis of data of earlier cruises (9 in total) supports the alternation of up- and downwelling, with highest amplitude in the Aru Basin. Large interannual variations occur up to 300 m, in probable relation to El Niño. In February 1985, the T-S values in the 150 to 300 m layer were somewhat higher than in August 1984, indicating influx of Indian Ocean water during the northwest monsoon.