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

Abstract

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

Evaluation of a Finite-Element Sea-Ice Ocean Model (FESOM) set-up to study the interannual to decadal variability in the deep-water formation rates

“…The comparison of the the modelled and observational derived March MLD revealed that the model setup is able to reproduce the general location of the main deep water formation areas in the Labrador Sea and Greenland Sea. The the main deep convection area in the model Labrador Sea is slightly shifted to the northwest which could be explained by a lack of eddy induced mixing with the West Greenland Current in the Labrador Sea caused by the limited horizontal resolution as described by Chanut et al (2008). There, the existence of eddies that mix with the warm Irminger Current, the so-called Irminger Rings, can limit the northward extent of the main deep convection area.…”

A 1/36° model of the Solomon Sea embedded into a global ocean model: On the setting up of an interactive open boundary nested model system

“…AGRIF have already been used in several studies. 26,27,28 The primary goal of setting up an AGRIF nest is to perform a local increase in spatial and temporal resolution in order to capture details of the flow. The refined grid ("child grid") is embedded in the coarser "mother grid", sharing a connection zone where both the child and the mother bathymetries are identical.…”

Aluminium in the North Atlantic Ocean and the Labrador Sea (GEOTRACES GA01 section): roles of continental inputs and biogenic particle removal

“…In the eastern North Atlantic dAl concentrations in surface waters range between 1-5 nM (Measures et al, 2008) and in contrast to the western North Atlantic, a dAl mid depth maximum (>30 nM) is observed associated with Mediterranean Overflow Waters (MOW) (Measures et al, 2015;Rolison et al, 2015). Globally, the highest dAl 10 concentrations have been measured in the Mediterranean Sea (up to 174 nM) (Chou and Wollast, 1997;Rolison et al, 2015) and the subtropical North Atlantic (up to 60 nM) (Schlosser et al, 2014), while the lowest concentrations (< 1nM) were found in the Southern Ocean (Middag et al, 2011), Pacific Ocean (Orians and Bruland, 1986) and high latitude North Atlantic (Middag et al, 2015b).…”

“…Thus, the enhanced pAl 5 levels could be attributed to sediment resuspension events caused by the south flowing Labrador Current (Mertz et al, 1993) possibly in combination with eddies that could also transfer terrigenous inputs into the Labrador Sea (Chanut et al, 2008;Hátún et al, 2007).…”