Convective activity in the Labrador Sea: Preconditioning associated with decadal variability in subsurface ocean stratification

Mizoguchi, Ken-ichi; Morey, Steven L.; Zavala‐Hidalgo, Jorge; Suginohara, Nobuo; Häkkinen, Sirpa; O’Brien, James J.

doi:10.1029/2002jc001735

Cited by 5 publications

(3 citation statements)

References 39 publications

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“…This memory leads to a close coupling between the response of the ocean's lateral processes and the vertical mixing forced by the atmosphere. In a coupled ice-ocean model, Mizoguchi et al (2003) further demonstrate the importance of the interior's autumnal stratification in modeling interdecadal variability of the deep convection cycle.…”

Section: Introductionmentioning

confidence: 97%

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

Chanut

Barnier

Large

et al. 2008

Journal of Physical Oceanography

135

194

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International audienceThe cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (⅓°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective overturning event produces convective eddies (CEs) of 20-30 km in diameter, as observed and produced in more idealized models, with a distinct seasonal cycle of EKE peaking in April. Sensitivity experiments show that each of these eddy types plays a distinct role in the heat budget of the central Labrador Sea, hence in the convection cycle. As observed in nature, deep convective mixing is limited to areas where adequate preconditioning can occur, that is, to a small region in the southwestern quadrant of the central basin. To the east, west, and south, BCEs flux heat from the BCS at a rate sufficient to counteract air-sea buoyancy loss. To the north, this eddy flux alone is not enough, but when combined with the effects of Irminger Rings, preconditioning is effectively inhibited here too. Following a deep convective mixing event, the homogeneous convection patch reaches as deep as 2000 m and a horizontal scale on the order of 200 km, as has been observed. Both CEs and BCEs are found to play critical roles in the lateral mixing phase, when the patch restratifies and transforms into Labrador Sea Water (LSW). BCEs extract the necessary heat from the BCS and transport it to the deep convection site, where it fluxed into convective patches by CEs during the initial phase. Later in the phase, BCE heat flux maintains and strengthens the restratification throughout the column, while solar heating establishes a near-surface seasonal stratification. In contrast, IRs appear to rarely enter the deep convection region. However, by virtue of their control on the surface area preconditioned for deep convection and the interannual variability of the associated barotropic instability, they could have an important role in the variability of LSW

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Section: Introductionmentioning

confidence: 97%

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

Chanut

Barnier

Large

et al. 2008

Journal of Physical Oceanography

135

194

View full text Add to dashboard Cite

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“…In the early 1980s convection was also strongly reduced by a freshwater anomaly (Belkin et al 1998), yet this occurred during a high NAO period (Curry et al 1998). Several model studies have been carried out with the aim of determining the dominant factor of the two in shutting down convective activity in the Labrador Sea during the GSA, but the results are conflicting (Hä kkinen 1999;Haak et al 2003;Mizoguchi et al 2003).…”

Section: Introductionmentioning

confidence: 99%

“…The convergence of buoyant water, on the other hand, is associated with a lateral influx from the boundary currents surrounding the Labrador Sea (Straneo 2006a). Variations in the boundary current characteristics, due to changes either in the freshwater carried at the surface or in the warm, salty Irminger water found below it, can thus also influence convective activity (Lazier 1980;Dickson et al 1988;Curry et al 1998;Hä kkinen 1999;Houghton and Visbeck 2002;Mizoguchi et al 2003;Straneo 2006a). Many studies of the distant past, recent history, and future scenarios point to large freshwater anomalies as means of shutting down convection and affecting the AMOC, but the details on how this happens are unclear.…”

Section: Introductionmentioning

confidence: 99%

Mechanisms behind the Temporary Shutdown of Deep Convection in the Labrador Sea: Lessons from the Great Salinity Anomaly Years 1968–71

2012

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From 1969 to 1971 convection in the Labrador Sea shut down, thus interrupting the formation of the intermediate/dense water masses. The shutdown has been attributed to the surface freshening induced by the Great Salinity Anomaly (GSA), a freshwater anomaly in the subpolar North Atlantic. The abrupt resumption of convection in 1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here oceanic and atmospheric data collected in the Labrador Sea at Ocean Weather Station Bravo and a one-dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail. These results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet. The analysis herein shows that the shutdown was initiated by the GSA-induced freshening as well as the mild 1968/69 winter. After the shutdown had begun, however, the continuing lateral freshwater flux as well as two positive feedbacks [both associated with the sea surface temperature (SST) decrease due to lack of convective mixing with warmer subsurface water] further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air-sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surface water. In 1972 convection resumed because of both the extreme atmospheric forcing and advection of saltier waters into the convection region.

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Nobuo Suginohara: A distinguished ocean modeller and leader in modern Japanese oceanography

You

Fukasawa

Yasuda

et al. 2010

Deep Sea Research Part II: Topical Studies in Oceanography

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Convective activity in the Labrador Sea: Preconditioning associated with decadal variability in subsurface ocean stratification

Cited by 5 publications

References 39 publications

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

Mechanisms behind the Temporary Shutdown of Deep Convection in the Labrador Sea: Lessons from the Great Salinity Anomaly Years 1968–71

Nobuo Suginohara: A distinguished ocean modeller and leader in modern Japanese oceanography

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