A study of deep ocean convection and the sea level variability in the North Atlantic

Li, Feili
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University of Delaware
Aspects of convection in the Labrador Sea and the circulations from the mid- to high-latitude North Atlantic are investigated using a variety of in-situ, satellite, and atmospheric reanalysis data products. Deep ocean convection in the Labrador Sea has transitioned from a period of intensification during the early 1990s into current stage of weakening with, however, higher variability in strength. Changes in hydrographic properties were used to investigate the evolution of deep convection at the monthly to interannual timescales. The atmospheric forcing characterized by the North Atlantic Oscillation (NAO) index has an important role in setting deep convection variability. On one hand, enhanced atmospheric forcing favors the extremely strong convective activity in the Labrador Sea as happened in the winter of 2008. On the other hand, over longer timescales, the cumulative NAO index is significantly correlated to the interannual variations in the heat content and mixed layer depths in the central Labrador Sea. Moreover, although ongoing warming in the intermediate layers tends to impede rigorous convection down to 1500m depth by steadily adding buoyancy, shallow convection will most likely remain active in the near future. The linkages between the horizontal and the overturning circulations in the North Atlantic were investigated in terms of sea level changes based on altimeter observations. A dipole pattern, centered between the Northern Atlantic subpolar region and the region near the Gulf Stream (GS), was observed in the linear trends of the sea surface height anomaly (SSHA). This dipole pattern is essentially associated with the interannual to decadal SSHA oscillations of the two regions. The low-frequency SSHA variability in the subpolar regions more effectively responds to the cumulative NAO forcing and leads that of the GS region by 29 months. Moreover, there is a remarkable reversal of the SSHA trends from the 1990s to the 2000s. This dipole pattern in sea level anomaly is a characteristic spatial pattern for the overturning circulation in the North Atlantic, whose changes closely relate to latitudinal coherence of different processes involved. Besides the dipole pattern in the sea level anomaly, the SSHA was used to investigate various processes at annual and longer timescales. The sea level variations in the subpolar gyre (SPG) are dominated by the annual cycle and the long-term increasing trend. In comparison, the SSHA along the GS is dominated by variability at intra-seasonal and annual timescales. The sea level rise in the SPG developed at a reduced rate in the 2000s compared to rates in the 1990s, which was accompanied by an increase in spectral energy starting from around 2002 after a period of energy loss in the system. These changes in both apparent trend and low-frequency SSHA oscillations reveal the importance of low-frequency variability in the SPG. To identify the possible contributing factors for these changes, the heat content balance (equivalent variations in the sea level) in the subpolar region was examined. The results indicate that horizontal circulations may primarily contribute to the interannual to decadal variations, while the air-sea heat flux is important at annual timescale. Furthermore, the low-frequency variability in the SPG might be related to the propagation of the Atlantic meridional overturning circulation (AMOC) variations from the deep-water formation region to mid-latitudes in the North Atlantic.