MULTIYEAR VARIABILITY IN THE NEAR-SURFACE TEMPERATURE STRUCTURE OF THE MIDLATITUDE WESTERN NORTH-ATLANTIC OCEAN

Between 1966 and 1995, subsurface temperature data have been collected in the western North Atlantic Ocean using expendable bathythermograph s. Data coverage is sparse in both time and space, but evidence for de cadal variability in the upper 400 m of the water column is found. The data were averaged by month onto a 2 degrees of latitude by 4 degrees of longitude grid. Thirty-one quadrangles in the region bounded by 17 degrees N and 43 degrees N and 78 degrees W and 66 degrees W have suf ficient data to provide consistent results. Anomaly time series at 0, 100, 200, 300, and 400 m were estimated by subtracting a mean monthly climatology. The individual records were detrended and filtered to hig hlight the longer-period signals. The analysis resulted in 25-year rec ords (1969-1993) for study. Within the thermocline of the subtropical gyre and the Gulf Stream at 100 and 200 m, periods of predominately po sitive temperature anomaly end in 1971, 1982, and 1990, while periods of negative anomaly end in 1976 and 1985. Only the events ending in 19 71, 1976, and 1990 are in the majority of the records at 300 and 400 m . Most of the events also appear in the sea surface temperature (SST) records but are somewhat masked by significant noise at the surface. M eridional-vertical temperature sections through the subtropical gyre s how that transitions from negative to positive anomaly events are char acterized by a deepening of the isotherms throughout the section and t ransitions from positive to negative events by a rising of the isother ms. Significant lateral migration of the axis of the Gulf Stream, alth ough possibly masked by the 2 degrees averaging, is not necessary to e xplain either type of event. The transitions in the SST and 100-m temp erature time series occur at essentially the same time as the transiti ons in an index of the North Atlantic Oscillation (NAG) that has also been detrended (i.e., 1971, 1976, 1980, 1984, 1988). The 1971, 1976, a nd 1988 NAO events are also observed at 300 and 400 m as described ear lier. Periods of positive subsurface temperature anomaly are coinciden tal with periods of positive NAO index, and periods of negative subsur face temperature anomaly are coincidental with periods of negative NAO index. Thus earlier results showing connections between the NAO and w estern Atlantic SST at decadal timescales are now extended to at least 400 m in the water column. Trends were computed from the individual 2 5-year records. The trends at all depths are predominately negative no rth of 38 degrees N and positive south of 38 degrees N. Inferences fro m the horizontal distribution of the trends and results from earlier s tudies suggest that the 1969-1993 period may be a phase of a 30- to 50 -year signal observed in the northern Atlantic since the beginning of the century.