Recent increases in Arctic freshwater flux affects Labrador Sea convection and Atlantic overturning circulation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of ocean thermohaline circulation. Melting of Greenland's ice sheet is freshening the North Atlantic; however, whether the augmented freshwater flux is disrupting the AMOC is unclear. Dense Labrador Sea Water (LSW), formed by winter cooling of saline North Atlantic water and subsequent convection, is a key component of the deep southward return flow of the AMOC. Although LSW formation recently decreased, it also reached historically high values in the mid-1990s, making the connection to the freshwater flux unclear. Here we derive a new estimate of the recent freshwater flux from Greenland using updated GRACE satellite data, present new flux estimates for heat and salt from the North Atlantic into the Labrador Sea and explain recent variations in LSW formation. We suggest that changes in LSW can be directly linked to recent freshening, and suggest a possible link to AMOC weakening.

Figure 1. Study region showing oceanographic sections and major currents around Greenland.

Red and orange arrows indicate Atlantic-origin water and blue arrows indicate Arctic-origin water. BC, Baffin Current; CDS, Cape Desolation Section; CFS, Cape Farewell Section; EGC, East Greenland Current; HBC is Hudson's Bay Current; IC, Irminger Current; LC, Labrador Current; NAC, North Atlantic Current; PS, Paamiut Section; WGC is West Greenland Current. Three-dimensional structure of major water masses in the Labrador Sea is shown in Supplementary Fig. 1.

Figure 2. Mass change of Greenland estimated from GRACE for the period 2002–2014.

Black curve shows data, grey shading indicates monthly uncertainty and red curve shows the best fitting constant acceleration model. Onset time of acceleration defined when the rate of mass change is zero, in 1996 (red arrow), with mass arbitrarily set to zero.

Figure 3. Heat and salt fluxes of Irminger Water for the period 1949–2013.

(a) Heat and (b) salt fluxes of Irminger Water are measured at three sections in southwest Greenland. Locations of three sections are shown in Fig. 1. CDS, Cape Desolation Section; CFS, Cape Farewell Section; PS, Paamiut Section. Solid line represents a 3-year running average, yearly data shown by plus signs. Red arrow marks the onset time of accelerated mass loss for Greenland estimated from GRACE (Fig. 2).

Figure 4. Freshwater flux from Greenland and CAA and Arctic sea ice for the period 1979–2013.

For Arctic sea ice, we plot only changes in flux (see text). The sum of these sources (Total) is also plotted. Grey shading indicates propagated uncertainty (see Supplementary Note 1).

Figure 5. Thickness of LSW and total freshwater flux and salt flux of Irminger Water.

Grey solid line indicates the thickness of LSW, black solid line indicates total freshwater flux and dotted line indicates salt flux of Irminger Water. Thickness and salt flux are smoothed with a 3-year running mean. Thickness is obtained from the objective analysis of EN4.0.2 data set from the UK Met Office Hadley Center. Thickness is averaged over 50° N–65° N and 38° W–65° W. Expression of salt flux in terms of freshwater flux is shown in Supplementary Fig. 7.