Density compensation minimizes the impact of the Labrador Sea convection to the AMOC

May 13, 2020
A schematic of the transport structure across the Labrador Sea
Figure 1. A schematic of the transport structure across the Labrador Sea. Warm and salty waters enter the Labrador Sea via the West Greenland Current (WGC) and exit in the Labrador Current (LC) with cold and fresh anomalies. The property changes are a result from the exchange between the boundary current and the basin interior, where the cold and fresh Labrador Sea Water (LSW) – the product of convection – is located. The resultant sharp tilt of isotherm from the WGC to the LC (red dashed line) suggests a strong transformation with respect to temperature space (red arrow). In contrast, the isopycnal slope is comparable on both sides of the basin due to density compensation, which results in a weak diapycnal transformation (black arrow) (click image to enlarge).
The Atlantic Meridional Overturning Circulation (AMOC) is a basin-scale circulation whose mean state and variability are critical for the climate system. Previous modeling studies have shown that the convective activities in the Labrador Sea drive the variability of the AMOC strength. However, recent observations from the Overturning in the Subpolar North Atlantic Program (OSNAP) have revealed a minimal contribution of the Labrador Sea convection to the subpolar AMOC strength during 2014-2016, a period with intense Labrador Sea convection events. Why did the recent observations conflict with the earlier studies?
To answer this question, Zou and coauthors analyzed the transport and property fields across the Labrador Sea using the OSNAP observations (2014-2016) and an ocean reanalysis dataset GloSea5 (1993-2016). They compared the transport structure in density space with that in temperature and salinity space. A summarizing schematic of the transport structure is shown in Figure 1. Warm and salty waters enter the Labrador Sea via the West Greenland Current. While traveling around the basin, these waters continuously exchange their properties with the basin interior, where the convection-produced cold and fresh Labrador Sea Water is located. As a result, the waters become much colder and fresher when they exit the basin via the Labrador Current. In contrast to the significant temperature and salinity changes, the study finds that the boundary current density remains largely unaffected. This implies that impacts of temperature and salinity changes on density are mostly compensated. Specifically, an increase of the density induced by the colder temperature is counteracted by a decrease of the density caused by the fresher salinity. 
The density compensation in the Labrador Sea has important consequence on the strength of the overturning circulation, which is quantified as the diapycnal (i.e. across density surfaces) transformation rate across the Labrador Sea. With comparable density slopes across the section in Figure 1, the diapycnal transformation is weak because of the strong isopycnal (i.e. along density surfaces) circulation: transport of the inflow via the West Greenland Current is mostly canceled out by the transport of the outflow via the Labrador Current. The net isopycnal transport, denoting the diapycnal transformation rate, is as small as 2Sv (1Sv=106m3/s) and therefore contributes minimally to the total AMOC strength, which is ~14Sv. If density is not compensated and is a function of temperature (red dashed line in Figure 1), the significantly tilted isopycnal (and isotherm) would lead to a strong diapycnal transformation rate (~13Sv), thereby exaggerating the impact of convection on the AMOC.
This work highlights the critical role of density compensation by temperature and salinity in determining the overturning circulation strength in the Labrador Sea. It emphasizes the importance of correct simulations on both temperature and salinity fields to improve our future predictions on the AMOC. 
Written by 
Sijia Zou, Woods Hole Oceanographic Institution

Sijia Zou1,2, M. Susan Lozier2, Feili Li3,2, Ryan Abernathey4, Laura Jackson5

1Woods Hole Oceanographic Institution
2Duke University
3Georgia Institute of Technology
4Columbia University
5Met Office, UK