NORTH-ATLANTIC CIRCULATION AND VARIABILITY, REVIEWED FOR THE CNLS CONFERENCE

The circulation and water mass structure of the North Atlantic are rev iewed, with emphasis on the large-scale overturning cell which produce s North Atlantic Deep Water (NADW). Properties and transports for its major components (Nordic Seas Overflow Water, Labrador Sea Water, Medi terranean Water, Antarctic Intermediate Water and Antarctic Bottom Wat er) are reviewed. The transport estimates and properties of NADW coupl ed with the observed meridional heat transport in the Atlantic limit t he temperature of northward flow which replenishes the NADW to the ran ge 11-15 degrees C. The high salinity of the North Atlantic compared w ith other ocean basins is important for its production of intermediate and deep waters; about one third of its higher evaporation compared w ith the North Pacific is due to the Mediterranean. The evaporation/pre cipitation balance for the North Atlantic is similar to the Indian and South Atlantic Oceans; the difference between the North and South Atl antic may be that high evaporation in the North Atlantic affects much greater depths through Mediterranean Water production. Also described briefly is variability of water properties in the upper layers of the subtropical/subpolar North Atlantic, as linked to the North Atlantic O scillation. The oceanographic time series at Bermuda is then used to s how decadal variations in the properties of the Subtropical Mode Water , a thick layer which lies in the upper 500 m. Salinity of this layer and at the sea surface increases during periods when the North Atlanti c westerlies weaken between Iceland and the Azores and shift southwest ward. (The North Atlantic Oscillation index is low during these period s). Temperature at the surface and in this layer are slightly negative ly correlated with salinity, decreasing when salinity increases. It is hypothesized that the salinity increases result from incursion of sal ine water from the eastern subtropical gyre forced by the southward mi gration of the westerlies, and that the small temperature decreases ar e due to increased convection in the Sargasso Sea, also resulting from the southward shift of the westerlies.