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Atlantic Overturning Circulation and Labrador Sea Convection Affected by Recent Increases in the Influx of Arctic Freshwater

The Atlantic Meridional Overturning Circulation (AMOC) is known to be an important component of ocean thermohaline circulation. The North Atlantic is freshening as a result melting of the Greenland ice sheet; however it is unclear if the flux of freshwater is disrupting the AMOC. A key component of the deep southward return flow of the AMOC, which is the dense Labrador Sea Water (LSW), formed by the saline North Atlantic water and subsequent convection that has been cooled in winter. The connection of the LSW to the influx of freshwater is made unclear because it reached historically high values in the mid-1990s but has decreased recently. In this paper Yang et al. present the results of the derivation of a new estimate for the freshwater flux from Greenland by the use of updated GRACE satellite data, present new estimates of flux for heat and salt from the North Atlantic into the Labrador Sea and a new explanation of recent variations in the formation of the LSW. Yang et al. suggest that recent freshening can be directly linked to changes in the LSW, as well as a possible link between the LSW and the weakening of the AMOC.

The Atlantic Meridional Overturning Circulation has long been accepted as having 2 stable modes (Stommel, 1961; Rooth, 1982; Broecker, Peteet & Rind, 1985), and that the AMOC could be weakened or shut down by anthropogenic warming (Broecker, 1987; Wood, Keen & Mitchell & Gregory, 1999). The North Atlantic is being freshened by recent accelerated melting of the Greenland ice sheet (Jiang, Dixon & Wdowinski, 2010; Rignot et al., 2011; Enderlin et al, 2014; Velicogna, Sutterley & van den Broeke, 2014; Yang, Dixon & Wdowinski, 2013). Numerical model experiments, so-called ‘hosing experiments’’, have been carried out in which freshwater may be distributed over broad or narrow regions of the North Atlantic, in order to study the sensitivity of the AMOC to freshwater flux (Fichefet et al., 2003; Jungclaus et al., 2006; Stouffer et al., 2006; Hu, Meehl, Han &Yin, 2011; Swingedouw et al., 2013; Ridley, Huybrechts, Gregory & Lowe, 2005; Brunnabend, Schröter, Riebroek & Kusche, 2015). It is suggested by some of these studies that the strength of the AMOC is sensitive to melting of the Greenland ice (Fichefet et al., 2003; Brunnabend, Schröter, Riebroek & Kusche, 2015), though other studies have suggested that the AMOC is not sensitive to such melting (Jungclaus et al., 2006; Hu, Meehl, Han & Yin, 2011; Ridley, Huybrechts, Gregory & Lowe, 2005). It is suggested by a few of the studies that the AMOC could be affected by freshwater additions as low as 0.1 Sv (100 mSv) (18-20), or possibly less (Fichefet et al., 2003; Brunnabend, Schröter, Riebroek & Kusche, 2015).

According to Yang et al. it is difficult to measure directly changes of the AMOC: currents comprising the deeper, southwards flowing portions can be diffuse and/or temporarily variable, and subtle, long-term changes in the oceanic properties can be masked by instrumental drift. Yang et al. also say it is difficult to distinguish changes forced by anthropogenic warming from natural variability. It is also difficult to numerically model the AMOC: if model grids are too coarse to reflect realistic oceanic processes and geographic constraints, as well as feedbacks among atmosphere, ocean and cryosphere (land and sea ice) are not well known.

The Labrador Sea is a key location for the formation of one of the dense, deepwater components of the AMOC as a result of winter convection; though the process is sensitive to surface conditions (Yashayaev & Loder, 2009).  It is suggested by Wood et al. (Wood, Keen, Mitchell & Gregory, 1999) there is a possibility of shutdown in the Labrador Sea convection in response to global warming. A theoretical stability analysis has been provided by Kuhlbrodt et al. (Kuhlbrodt, Titz, Feudel & Rahmstorf, 2001), and they suggest increased freshwater input can shut down winter convection in the Labrador Sea. Yang et al. say it is difficult to observe directly winter convection here as a result of its extreme conditions and small spatial scale.

In this paper Yang et al. consider recent changes in the Labrador Sea resulting from increased freshwater flux. Yang et al. derive a new estimate for increased flux of freshwater into the sub-polar North Atlantic, and suggest that most of this increase is being focused towards the Labrador Sea, thereby magnifying its impact and increasing the likelihood of significant effects on the AMOC, as a result of the clockwise nature of ocean circulation around Greenland (Joyce & Proshutinsky, 2007),

Sources & Further reading

  1. Yang, Q., T. H. Dixon, P. G. Myers, J. Bonin, D. Chambers and M. R. van den Broeke (2016). "Recent increases in Arctic freshwater flux affects Labrador Sea convection and Atlantic overturning circulation." Nature Communications 7: 10525.


Author: M. H. Monroe
Last updated:  06/09/2016
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