Climatic Warming of Atlantic Intermediate Waters*

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Abstract
Interdecadal temperature variability of the Atlantic Ocean is investigated by differencing hydrographic sections taken from the 1920s through the 1990s. A comprehensive reanalysis of North Atlantic sections and the inclusion of South Atlantic sections show that warming seen previously in the North Atlantic extends to the South Atlantic. The largest statistically significant changes occur on pressure surfaces between 1000 and 2000 decibars (db). Over this pressure range and for latitudes between 32°S and 36°N, temperatures have warmed by ∼0.5°C century−1. At 48°N a cooling of ∼3°C century−1 occurred between the 1950s and 1980s. These isobaric temperature trends are decomposed into ones along surfaces of constant neutral density, and ones due to the vertical movement of neutral surfaces. The two components are associated with different processes. In the southern North Atlantic (8°–36°N) the subthermocline warming between the 1950s and 1980s appears to be due primarily to downward displacements of neutral surfaces, while the South Atlantic changes occur primarily along density surfaces. The downward displacements in the North Atlantic occur throughout the 1000–2000-db layer, suggesting a volumetric increase (decrease) in the water masses above (below) the intermediate layer. Since calculated wind-driven displacements of the thermocline do not agree with 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 Atlantic 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 both the northern and southern polar seas.