Cooling in the western Bering Sea in 1999: quick propagation of La Nina signal or compensatory processes effect?

In 1999, synoptic and hydrological conditions in the western Bering Sea wer e characterized by negative SST and air temperature anomalies, extensive ic e coverage and late melting. Biological processes were also delayed. In 199 9, the average zooplankton biomass was 1.76 g/m(3), approximately half the average 3.07 g/m(3) in 1998. Pacific salmon migrated to the northeastern Ka mchatka streams two weeks later. This contrasts with 1997 (spring and summe r) and 1998 (summer) when positive SST anomalies were widely distributed th roughout the northwestern Bering Sea shelf. Since the second half of the 19 90s, seasonal atmospheric processes developed over the western Bering Sea t hat were similar to those of the cold decades of the 1960-1970s. A meridion al atmospheric circulation pattern began to replace zonal transport. Colder Arctic air masses have shifted over the Bering Sea region and shelf water temperatures have cooled considerably with the weakening of zonal atmospher ic circulation. Temperature decreased in the cold intermediate layer during its renewal in winter. Besides, oceanic water inflow intensified into the Bering Sea in intermediate layers. Water temperature warmed to 4 degreesC a nd a double temperature maximum existed in the warm intermediate layer in l ate summer in both 1997 and 1998. Opposing trends of cold water temperature and a warm intermediate layer led to an increase of vertical gradients in the main thermocline and progressing frontogenesis. It accelerates frontal transport and can be regarded as a chief cause of increased water exchange with the Pacific Ocean. (C) 2001 Elsevier Science Ltd. All rights reserved.