Climatic warming of Atlantic intermediate waters

Interdecadal temperature variability of the Atlantic Ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990 s. A comprehensive reanalysis of North Atlantic sections and the inclusion of South Atlantic sections show that warming seen previously in the North A tlantic extends to the South Atlantic. The largest statistically significan t changes occur on pressure surfaces between 1000 and 2000 decibars (db). O ver this pressure range and for latitudes between 32 degreesS and 36 degree sN, temperatures have warmed by similar to0.5 degreesC century(-1). At 48 d egreesN a cooling of similar to3 degreesC century(-1) occurred between the 1950s and 1980s. These isobaric temperature trends are decomposed into ones along surfaces o f constant neutral density, and ones due to the vertical movement of neutra l surfaces. The two components are associated with different processes. In the southern North Atlantic (8 degrees -36 degreesN) the subthermocline war ming between the 1950s and 1980s appears to be due primarily to downward di splacements of neutral surfaces, while the South Atlantic changes occur pri marily along density surfaces. The downward displacements in the North Atla ntic occur throughout the 1000-2000-db layer, suggesting a volumetric incre ase (decrease) in the water masses above (below) the intermediate layer. Si nce calculated wind-driven displacements of the thermocline do not agree wi th this analysis, a change in deep water formation rates is the most likely explanation. The South Atlantic warming trend can be extended further back in time and is due to isopycnal advection, which has a much slower signal propagation speed than does the displacement mechanism for the North Atlant ic changes. This suggests that warming in Atlantic intermediate waters is due not only to climatic forcing changes over the last four decades, but also to changes on centennial timescales. These oceanic climate changes have origins in bo th the northern and southern polar seas.