Decline of Subpolar North Atlantic Circulation During the 1990s

TLDR

Observations of sea surface height show substantial changes in the mid‑ to high‑latitude North Atlantic over the past decade. Weak 1990s buoyancy forcing weakened thermohaline circulation, allowing the domed subpolar isopycnal structure to decay and reducing circulation, consistent with altimetric and hydrographic observations. Altimeter and current‑meter data indicate that subpolar sea surface height rose in the 1990s while gyre circulation weakened, a trend that appears stronger in the late 1990s than in the 1970s–1980s and extends deep into the water column.

Abstract

Observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid- to high-latitude North Atlantic Ocean. TOPEX/Poseidon altimeter data show that subpolar sea surface height increased during the 1990s, and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation. Combining the data from earlier satellites, we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s. Direct current-meter observations in the boundary current of the Labrador Sea support the weakening circulation trend of the 1990s and, together with hydrographic data, show that the mid- to late 1990s decline extends deep in the water column. Analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations: A weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation.