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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),
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |