B.: Decline of the subpolar North Atlantic circulation during the 1990s

• Main Author: Peter B. Rhines
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: The subpolar North Atlantic undergoes a; Labrador Sea; NADW; North Atlantic Deep Water; North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.645.8246; http://stephenschneider.stanford.edu/Publications/PDF_Papers/HakkinenRhines2004.pdf

Observations of sea surface height reveal that substantial changes have oc-curred 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. Extreme fluctuations in atmospheric forcing of the subpolar zone of the North Atlantic Ocean, reflected by the North Atlantic Oscil-lation (NAO), have occurred over the past 30 years (1). These changes have demonstrably affected the water column and ocean circula-tion, as we show here with altimetry and in situ measurements. The subpolar gyre of the North Atlantic circulates cyclonically between 50 ° and 65°N and contains strong boundary currents. It is a region of intense interaction between ocean and atmosphere: Wintertime cold winds re-move heat at rates of several hundred watts per square meter, resulting in deep convec-tion reaching as far as 2500 m below the surface. Newly formed subpolar waters com-bine with dense Nordic Sills overflows to provide the origins of North Atlantic Deep Water (NADW).