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.

Brief summary:Based on the 20 th century atmospheric reanalysis, winters with more frequent blocking, in a band of blocked latitudes from Greenland to British Isles, are found to persist over several decades and correspond to a warm North Atlantic Ocean, in-phase with Atlantic multi-decadal ocean variability.https://ntrs.nasa.gov/search.jsp?R=20110008410 2018-05-11T14:52:52+00:00Z 2 AbstractAtmospheric blocking over the northern North Atlantic involves isolation of large regions of air from the westerly circulation for 5-14 days or more. From a recent 20 th century atmospheric reanalysis (1,2) winters with more frequent blocking persist over several decades and correspond to a warm North Atlantic Ocean, in-phase with Atlantic multi-decadal ocean variability (AMV). Ocean circulation is forced by wind-stress curl and related air/sea heat exchange, and we find that their space-time structure is associated with dominant blocking patterns: weaker ocean gyres and weaker heat exchange contribute to the warm phase of AMV.

Analysis of surface drifter tracks in the North Atlantic Ocean from the time period 1990 to 2007 provides evidence that warm subtropical waters have recently increased their penetration toward the Nordic seas. Prior to 2000, the warm water branches of the North Atlantic Current fed by the Gulf Stream turned southeastward in the eastern North Atlantic. Since 2001, these paths have shifted toward the Rockall Trough, through which the most saline North Atlantic waters pass to the Nordic seas. These surface drifters are able to overcome the Ekman drift, which would force them southward under the westerly winds dominating the subpolar Atlantic, yet the changes in path cannot be accounted for by changes in Ekman drift. Eddy kinetic energy from satellite altimetry shows increased energy along the shifted drifter pathways across the Mid‐Atlantic Ridge since 2001. These near‐surface changes have occurred during continual weakening of the North Atlantic subpolar gyre, as seen by altimetry. They are also consistent with the observed increase in temperature and salinity of the waters flowing northward into the Nordic seas. These findings suggest the changes in the vertical structure of the northern North Atlantic Ocean, its dynamics, and exchanges with the higher latitudes. Wind stress and its curl changes are discussed as a possible forcing of the changes in the pathways of the subtropical waters.

[1] As a part of the Arctic Ocean Model Intercomparison Project, results from 10 Arctic ocean/ice models are intercompared over the period 1970 through 1999. Models' monthly mean outputs are laterally integrated over two subdomains (Amerasian and Eurasian basins), then examined as functions of depth and time. Differences in such fields as averaged temperature and salinity arise from models' differences in parameterizations and numerical methods and from different domain sizes, with anomalies that develop at lower latitudes carried into the Arctic. A systematic deficiency is seen as AOMIP models tend to produce thermally stratified upper layers rather than the ''cold halocline'', suggesting missing physics perhaps related to vertical mixing or to shelf-basin exchanges. Flow fields pose a challenge for intercomparison. We introduce topostrophy, the vertical component of VÂr r r rD where V is monthly mean velocity and r r r rD is the gradient of total depth, characterizing the tendency to follow topographic slopes. Positive topostrophy expresses a tendency for cyclonic ''rim currents''. Systematic differences of models' circulations are found to depend strongly upon assumed roles of unresolved eddies.