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Wintertime deepening of the North Atlantic mixed layer 1960-2004
Semyon A. Grodsky, James A. Carton, and Hailong Liu
January 3, 2007
Submitted to Geophysical Research Letters
Department of Atmospheric and Oceanic Science,
University of Maryland at College Park
Corresponding author:
senya@atmos.umd.edu
1
ABSTRACT
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SST in the subpolar North Atlantic has dropped by as much as 0.5 oC in the past 45 years
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with much of this drop occurring in the winter months. A number of hypotheses have
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been put forward to explain this drop including the impacts of changes in surface
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freshwater flux, horizontal advection, or changes in the rate of formation of upper ocean
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water masses. Here a new monthly analysis of mixed layer properties based on the World
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Ocean Atlas 2005 data set is used to examine trends in the seasonal maximum mixed
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layer depth. The examination reveals a gradual deepening of the winter-early spring
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mixed layer at a rate in excess of 15 m decade-1 during the period 1960-2004 at latitudes
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north of 50oN with the greatest deepening occurring in the southern Labrador Sea. This
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deepening trend has occurred during decades characterized by a gradual increase in
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wintertime storminess and thus turbulent mixing and surface heat loss, as reflected in a
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strengthening of the North Atlantic Oscillation. Results from numerical simulation with a
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one dimensional mixed layer model support the hypothesis that increases in entrainment
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cooling of the mixed layer resulting from the deepening may explain the decrease in
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wintertime SST at subpolar latitudes.
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1. INTRODUCTION
In recent decades while much of the ocean including the subtropical gyre of the
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North Atlantic has been warming, the subpolar gyre has been cooling [Dickson et al.,
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2000]. Several explanations have been proposed for this unusual cooling. Mysak et al.
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[1996] propose an important role for the impact on density of the export of ice and fresh
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water from the Arctic driven by anomalous westerly winds. Read and Gould [1992]
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alternatively emphasize the role of deep convection in the Labrador basin in introducing
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anomalous thermohaline forcing. Häkkinen and Rhines [2004] show that anomalous local
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surface flux in the subpolar gyre since the 1990s is of the right size and phase to explain
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the anomalous cyclonic circulation of the subpolar gyre which in turn introduces
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anomalous horizontal advection [see also Flatau et al., 2001]. Here we report on the
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results of a new examination of the ocean mixed layer at subseasonal frequencies in order
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to better understand long-term changes in the properties of the upper layers of the
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subpolar gyre and their connection to changing SST.
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The oceanic mixed layer is a quasi-homogeneous layer separating the atmosphere
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from the interior ocean. This layer undergoes seasonal changes reflecting the seasonal
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changes of surface forcing [e.g. de Boyer Montegut et al., 2004]. In summer the mixed
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layer is shallow due to high sunlight and low winds. But in winter and early spring
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intense wind stirring and negative buoyancy flux cause the mixed layer to deepen
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progressively due to convective mixing. During this season the behavior of the mixed
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layer is different in the eastern and western subpolar gyre. In the Norwegian basin of the
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eastern gyre the mixed layer cannot penetrate through the base of the nearsurface water
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mass known as Atlantic Water (~300m). In this eastern region mixed layer temperature is
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thus governed by horizontal advection [Nilsen and Falck, 2006]. In the western gyre, in
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contrast, convection can reach much greater depths and thus entrainment cooling
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becomes a dominant term in the mixed layer heat budget. In this western region year-to-
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year and longer changes in local atmospheric forcing, which regulate the rate of
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convection, may explain much of the observed changes in the temperature of the
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convective mixed layer [Dickson et al. 1996; Lazier et al., 2002].
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The wintertime surface fluxes in the North Atlantic largely reflect the position and
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intensity of the atmospheric storm tracks. Since the 1960s the storminess of the North
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Atlantic has increased remarkably, while winds in the subtropics have become quiescent,
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as reflected in an increase in the North Atlantic Oscillation (NAO) Index [Hurrell, 1995].
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This multi-decadal shift between contrasting NAO extremes has been accompanied by
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enhanced wind-induced stirring of the water column as well as anomalous surface
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turbulent heat loss throughout the Nordic Seas [Dickson et al., 2000]. By destabilizing the
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water column these changes intensify convection within the mixed layer and thus
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increase entrainment across the mixed layer base.
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A number of modeling studies have examined the impact of changes in surface
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forcing on wintertime mixed layer properties including depth and temperature [e.g.
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Alexander et al., 2000; Tandon and Zhou, 2004; Wetzel et al., 2005]. For example, Wetzel
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et al. [2005] confirm that surface forcing associated with high NAO conditions leads to
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anomalously deep mixed layers in the subpolar gyre and tropics and shallow mixed layer
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in the subtropics. Alexander et al. [2000] present coupled GCM modeling results
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indicating the importance of entrainment cooling of the mixed layer in regulating
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anomalous wintertime SST in their model.
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Here we revisit the causes of subseasonal changes in the mixed layer of the North
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Atlantic using a new analysis of mixed layer depth to examine the role of surface fluxes
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in controlling subseasonal variability of the mixed layer properties. Despite significant
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data limitations we find a pattern of the wintertime mixed layer deepening in the western
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and central North Atlantic. This pattern has developed in response to intensifying winds
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and shifting storm tracks. Results from a one dimensional mixed layer model are
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consistent with the hypothesis that the anomalous mixed layer deepening may explain the
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observed changes in subpolar SST.
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2. DATA AND METHODS
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This study relies on a number of observational data sets. Vertical profiles of temperature
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and salinity for the period 1960 through 2004 come from the World Ocean Database
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2005 (WOD05) [Boyer et al., 2006]. We focus on this period because of the increase in
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data beginning in the 1960s. Mixed layer depth is evaluated from each profile as the
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depth at which temperature changes by T from the temperature at 10m depth. Our
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choice of the value of T follows de Boyer Montegut et al. [2004] with one
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modification. While de Boyer Montegut et al. choose T = -0.2 oC, we choose | T | =0.2
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o
C to allow shallow mixed layers to be detected in regions of temperature inversions
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(typically at high latitude: see Supplements for more details). Also, following de Boyer
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Montegut et al., in areas where climatological salinity stratified barrier layers occur we
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correct the depth estimates for these based on climatological monthly estimates.
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For most atmospheric parameters we rely on the NCEP/NCAR monthly reanalysis
o
o
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of Kalnay et al. [1996], available on a 2.5 x2.5 grid throughout our period of interest.
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The NAO Index of North Atlantic winter storm tracks (December-March) is based on
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500mb geopotential height anomalies following Barnston and Livezey [1987] and has
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been obtained from the National Centers for Environmental Prediction/Climate
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Prediction Center. Monthly SST is the UK Met Office/Hadley Centre HadISST1 analysis
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[Rayner et al. 2003].
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The local impact of the mixed layer deepening on mixed layer temperature is
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evaluated through simulations using the one-dimensional hybrid mixed layer model of
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Chen et al. [1994]. This model is based on the Kraus-Turner-type bulk mixed layer
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physics in which the depth of the mixed layer is determined by a turbulent energy balance
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equation, while the temperature of the mixed layer is determined by a temperature budget
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equation driven by surface fluxes and entrainment. These balances are augmented in the
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Chen et al. formulation by the addition of convection and Richardson Number-dependent
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mixing. The model is forced by monthly mean surface fluxes starting from the January
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climatological temperature and salinity determined from the WOD05 data set.
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3. RESULTS
We begin by comparing the winter-early spring MLD averaged over the two
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nearly equally long intervals (1960-1979) and (1980-2004). Fig. 1a shows that during the
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45-year period the mixed layer deepens north of 50oN and west of the Greenwich
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meridian. The deepening is stronger in the southern Labrador basin where it exceeds 80m
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or 30% of its seasonal average, and where the bottom of winter convection deepens by
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some 400 to 800m during the high NAO interval (Fig.1b). East of the Greenwich
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meridian, including the Norwegian Basin there is little change in MLD. An examination
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of winter air-sea interaction in this basin by Nilsen and Falck [2006] shows that there is
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no significant correlation between atmospheric forcing and MLD. Here Nilsen and Falck
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suggest the mixed layer is confined to the layer containing Atlantic Water layer and thus
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MLD may not exceed the depth of the Atlantic Water, ~300m in winter.
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South of 50oN in the west the mixed layer deepens by 20 to 40m along the Gulf
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Stream front. In this region the depth of the mixed layer at least partly reflects shifts of
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the Gulf Stream intensity and front position rather than local air-sea exchanges. Taylor
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and Stephens, [1998] and Joyce et al. [2000] have shown that shifts in the front, in turn,
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respond to changes in the large-scale patterns of winds in the North Atlantic. High values
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of the NAO Index, favor northward shifts of the Gulf Stream front and deeper subtropical
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MLD, and vice versa.
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We explore the connection between basin-scale meteorology and basin-scale
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changes in winter-spring MLD with a joint Empirical Orthogonal Function (EOF)
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analysis of MLD and mixed layer temperature (Fig. 2). Spatial structure of the leading
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EOF (14% of explained variance) captures mixed layer deepening in the subpolar gyre as
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well as some deepening in the subtropics along the Gulf Stream front while the 2nd EOF
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(8% of variance, not shown) captures meridional shifts of the leading spatial structure. In
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the past 45 years there has been a gradual strengthening of this leading pattern (Fig.2c)
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corresponding to the gradual deepening of the MLD at subpolar latitudes noted in Fig. 1.
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The strengthening of this pattern also corresponds to a strengthening of the NAO with a
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ratio of around 1 and time correlation of r = 0.5. However, this correspondence is
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confined to the decadal trend: at interannual periods the correspondence between these
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variables is weak (time correlation at interannual periods is r = 0.02, while the 95%
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confidence level of zero correlation is 0.15).
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An alternative approach of calculating the least squares projection of the NAO
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Index time series on oceanic variables is presented in Fig. 3. The projection on MLD
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resembles the spatial patterns of the turbulent kinetic energy production by wind stirring
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and net surface heat flux with a strong positive response north of 50oN where it deepens
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by 30m in response to a unit increase of the NAO Index (Fig.3a). In the subtropics there
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is some statistically insignificant shoaling in the east and deepening in the west along the
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Gulf Stream front. Similarly, the projection on SST (Fig.3b) resembles the spatial
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patterns of the turbulent kinetic energy production by wind stirring and net surface heat
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flux except in the western subtropics where currents are strong (Figs. 3c, 3d). These
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similarities suggest that throughout much of the basin the mixed layer responds to local
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forcing. In contrast, along the region of the Gulf Stream front the mixed layer depth and
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temperature vary in phase suggesting the importance of horizontal transport. In the
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tropics both wind stirring and surface buoyancy flux contribute to the mixed layer energy
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budget, while relative importance of wind stirring decreases for a deeper mixed layers at
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high latitudes.
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We next discuss the relationship between year-to-year changes in seasonal MLD
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and seasonal SST. Annually averaged, SST in the North Atlantic has been warming
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during the 45-year period, with most of the warming occurring in the summer at a rate of
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0.1 oC decade-1 (Fig.4b). The warming is stronger in summer because the shallow
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summer mixed layer is able to respond rapidly to an increase in surface flux. In contrast,
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the cooling at subpolar latitudes is primarily a phenomenon of the winter months. This
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winter effect is apparent at subtropical latitudes as well.
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The contribution of entrainment-induced cooling during cold seasons is evident in
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the results of the Chen et al. [1994] model. The results indicate that an anomalous 100m
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seasonal deepening of the mixed layer is accompanied by a 0.7 oC drop in SST (Fig.5).
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Thus the anomalous deepening of the mixed layer of 20 m decade-1 at 55o N will cool the
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mixed layer at a rate of 0.14oC decade-1. This relationship is consistent with the negative
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phase relationship between MLD and SST in Figs. 3a, 3b and explains the winter-early
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spring reversal of SST trends at latitudes where the NAO index is positively correlated
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with winds and surface heat loss (Fig.4b).
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4. SUMMARY
In this work we examine changes in the winter-early spring mixed layer depth in
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the North Atlantic during 1960-2004 to understand the causes of long-term trends in SST.
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While SSTs have been warming throughout much of the North Atlantic, this warming has
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been concentrated in the summer months when the mixed layer is quite shallow. In
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contrast, SSTs have been dropping at subpolar latitudes and this decrease in SST has
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occurred primarily in winter months. During the 45-year period the wintertime expression
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of North Atlantic winds (the North Atlantic Oscillation) shifted from its negative phase in
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the 1960s to its positive phase in the recent decade. This shift reflects shifts in the
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position and intensity of the midlatitude storm track, as well as strengthening of the
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midlatitude westerly surface winds and the tropical northeasterly trade winds.
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The first part of this paper examines trends in seasonal MLD. We find a striking
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deepening trend in winter-early spring in excess of 15m decade-1 at subpolar latitudes. An
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examination of the spatial structure of the trends in winter-spring MLD suggests that
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throughout most of the basin except the western subtropics, the patterns result from
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changes in local meteorological forcing.
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The second part of this paper examines the impact of changes in MLD on SST.
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We exploit the apparent local nature of the mixed layer dynamics and introduce a one
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dimensional mixed layer model based on Chen et al. [1994] mixed layer physics. 45-year
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simulations with this model at 55oN shows that the excess entrainment associated with
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the observed trend in winter-spring mixed layers likely accounts for cooling at a rate of
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around 0.15oC decade-1 during these winter months.
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Acknowledgements. The authors gratefully acknowledge funding support from the
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National Science Foundation (OCE0351319) and the NASA Physical Oceanography
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Program.
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Figure 1. Difference between DJFM averages (1980-2004) and (1960-1979): (a) mixed
layer depth [m] (colors), (b) maximum mixed layer depth [m]. Climatological DJFM
mixed layer depth (1960-2004) is overlain in (a) as contours. Data gaps are shaded in
gray.
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Figure 2. Leading joint empirical orthogonal function of DJFM MLD and SST. (a,b)
spatial patterns, (c) time series (solid). DJFM NAO Index (dashed). This EOF explains
14% of variance.
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Figure 3. Time regression of the DJFM NAO Index with DJFM (a) mixed layer depth
[m]; (b) sea surface temperature [oC]; (c) mixed layer turbulent kinetic energy production
by wind stirring, 2mu*3w * C p  w /( gnD) , [Wm-2]. It is scaled using the Krauss-Turner
like bulk parameterization for deepening mixed layers, D is climatological MLD,  is
thermal expansion coefficient, m=0.4, n=0.18; and (d) net surface heat flux into the ocean
[Wm-2]. Regression is evaluated for anomalies from the climatological seasonal cycle
during 1960-2004. Shading in (a) indicates statistical significance at the 95% level.
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Figure 4. Linear trend slope of (a) MLD, (b) SST 60-10oW zonal averages. Trends are
evaluated during 1960-2000. MLD trend exceeding 15m decade-1 is shaded. SST trends
exceeding 0.15 oC decade-1 and below -0.05 oC decade-1 are shaded in light/dark gray,
respectively.
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Figure 5. Relationship between simulated wintertime (December-March) anomaly mixed
layer depth and SST. Data are spatially averaged over 60-10W, 50-60N box. Data are
simulated by a 1D mixed layer model of Chen et al. [1994] forced by the monthly mean
NCEP/NCAR reanalysis winds and heat flux.
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