Model-observation and reanalyses comparison at key locations for heat transport to the Arctic: Assessment of key lower latitude influences on the Arctic and their simulation

Blue-Action Work Package 2 (WP2) focuses on lower latitude drivers of Arctic change, with a focus on the influence of the Atlantic Ocean and atmosphere on the Arctic. In particular, warm water travels from the Atlantic, across the Greenland-Scotland ridge, through the Norwegian Sea towards the Arctic. A large proportion of the heat transported northwards by the ocean is released to the atmosphere and carried eastward towards Europe by the prevailing westerly winds. This is an important contribution to northwestern Europe's mild climate. The remaining heat travels north into the Arctic. Variations in the amount of heat transported into the Arctic will influence the long term climate of the Northern Hemisphere. Here we assess how well the state of the art coupled climate models estimate this northwards transport of heat in the ocean, and how the atmospheric heat transport varies with changes in the ocean heat transport. We seek to improve the ocean monitoring systems that are in place by introducing measurements from ocean gliders, Argo floats and satellites. These state of the art computer simulations are evaluated by comparison with key trans-Atlantic observations. In addition to the coupled models ‘ocean-only’ evaluations are made. In general the coupled model simulations have too much heat going into the Arctic region and the transports have too much variability. The models generally reproduce the variability of the Atlantic Meridional Ocean Circulation (AMOC) well. All models in this study have a too strong southwards transport of freshwater at 26°N in the North Atlantic, but the divergence between 26°N and Bering Straits is generally reproduced really well in all the models. Altimetry from satellites have been used to reconstruct the ocean circulation 26°N in the Atlantic, over the Greenland Scotland Ridge and alongside ship based observations along the GO-SHIP OVIDE Section. Although it is still a challenge to estimate the ocean circulation at 26°N without using the RAPID 26°N array, satellites can be used to reconstruct the longer term ocean signal. The OSNAP project measures the oceanic transport of heat across a section which stretches from Canada to the UK, via Greenland. The project has used ocean gliders to great success to measure the transport on the eastern side of the array. Every 10 days up to 4000 Argo floats measure temperature and salinity in the top 2000m of the ocean, away from ocean boundaries, and report back the measurements via satellite. These data are employed at 26°N in the Atlantic to enable the calculation of the heat and freshwater transports. As explained above, both ocean and atmosphere carry vast amounts of heat poleward in the Atlantic. In the long term average the Atlantic ocean releases large amounts of heat to the atmosphere between the subtropical and subpolar regions, heat which is then carried by the atmosphere to western Europe and the Arctic. On shorter timescales, interannual to decadal, the amounts of heat carried by ocean and atmosphere vary considerably. An important question is whether the total amount of heat transported, atmosphere plus ocean, remains roughly constant, whether significant amounts of heat are gained or lost from space and how the relative amount transported by the atmosphere and ocean change with time. This is an important distinction because the same amount of anomalous heat transport will have very different effects depending on whether it is transported by ocean or the atmosphere. For example the effects on Arctic sea ice will depend very much on whether the surface of the ice experiences anomalous warming by the atmosphere versus the base of the ice experiencing anomalous warming from the ocean. In Blue-Action we investigated the relationship between atmospheric and oceanic heat transports at key locations corresponding to the positions of observational arrays (RAPID at 26°N, OSNAP at ~55N, and the Denmark Strait, Iceland-Scotland Ridge and Davis Strait at ~67N) in a number of cutting edge high resolution coupled ocean-atmosphere simulations. We split the analysis into two different timescales, interannual to decadal (1-10 years) and multidecadal (greater than 10 years). In the 1-10 year case, the relationship between ocean and atmosphere transports is complex, but a robust result is that although there is little local correlation between oceanic and atmospheric heat transports, Correlations do occur at different latitudes. Thus increased oceanic heat transport at 26°N is accompanied by reduced heat transport at ~50N and a longitudinal shift in the location of atmospheric flow of heat into the Arctic. Conversely, on longer timescales, there appears to be a much stronger local compensation between oceanic and atmospheric heat transport i.e. Bjerknes compensation

Atmospheric forcing on the Canadian Arctic Archipelago freshwater outflow and implications for the Labrador Sea variability

International audienceThe variability of the freshwater export through the Canadian Arctic Archipelago (CAA) is analyzed using a hindcast simulation forced by surface atmospheric forcing from the ERA40 reanalysis (1958-2001). Although the two channels representing the archipelago in the model are both sensitive to the along-channel sea surface height (SSH) gradient, they appear to have very distinct behaviors. The outflow to Lancaster Sound is shown to be largely controlled by the magnitude of the upstream SSH gradient across McClure Strait. The gradient shows a close link to the wind stress curl in the western Arctic but also to a large-scale SSH anomaly pattern which has a strong signal over the shelf to the south of McClure Strait. The latter has, however, little statistical connection to the SSH variability in the Beaufort Gyre. By contrast, the outflow through Nares Strait responds preferentially to SSH variations in the northern Baffin Bay which are remotely forced by air-sea heat exchanges in the Labrador Sea. The variability is largely coherent between the two outflows and is controlled by a dipolar atmospheric pattern reminiscent of the North Atlantic/Arctic Oscillation. When entering the subpolar gyre, the CAA freshwater outflow remains confined to the Labrador shelf with little impact on the salinity of the interior Labrador Sea and potentially on the convection. The latter is represented by a distinct mode of salinity variability in the western subpolar gyre which is rather influenced by the variability of the sea ice export through Fram Strait

Oceanic heat anomalies and Arctic sea-ice variability

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Variability of the ice export through Fram Strait in 1993–98: the winter 1994/95 anomaly

The origin of the large positive anomaly of the Fram Strait sea ice export which occurred in winter 1994/95 is analysed on the basis of a model simulation of the Arctic sea ice cover over the period 1993–98. The overall intra-annual and interannual variability in the model is in good agreement with observational estimates and the 1994/95 anomaly is well reproduced with an amplitude amounting to half of the mean winter value. Model results suggest that, concomitant to anomalous export velocities, larger than usual ice thickness in the strait contributes to the outstanding amplitude of the anomaly. Analysis on the ice thickness evolution in the strait indicates that the thick ice advected in Fram Strait at the end of the fall of 1994 originates in the anomalous cyclonic wind stress which prevailed during the preceding summer. This anomalous wind stress resulted in persistent convergence of the ice flow against the northern coasts of Canada and Greenland and in the formation of a large thickness anomaly north of Greenland. The anomaly then feeds the Fram Strait ice flow during those following winter months when the local wind forcing in the strait favours ice drift from the north-west. Our results suggest that short-term wind stress variations resulting in local thickness changes to the north of Fram Strait can lead to substantial variability of the Fram Strait ice export

Response of the eastern North Atlantic subpolar gyre to the North Atlantic Oscillation

International audienceThe salinity changes in the subpolar gyre (SPG) in response to the North Atlantic Oscillation (NAO) are studied in idealized numerical experiments. In the eastern SPG, anomalies of similar amplitude as those observed during the 1995-1996 shift of the NAO are mainly driven by the local response to the wind stress, through the set-up of an anticyclonic "intergyre" anomaly. In positive NAO, this anomalous circulation advects altogether (i) fresh, cold water from the western to the eastern SPG, contributing there to the formation of negative salinity anomalies, and (ii) warm, saline subtropical water to the south of Newfoundland, forming positive anomalies there. The latter are subsequently transported with the North Atlantic Current to the eastern SPG where they could act to weaken the low-salinity signal. The occurrence of this signal is concomitant with the acceleration of the gyre but, in contrast to earlier findings, is not subject to it

Oceanic heat anomalies and Arctic sea-ice variability

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Sensitivity of the meridional transport in a 1.5-layer ocean model to localized mass sources

The response of a 1.5-layer ocean model forced by localized stochastic mass sources is studied. The focus is on the sensitivity of the spectral characteristics of the meridional transport to the location and the extent of the source region. In all the experiments, performed in hemispheric and interhemispheric basins, the spectra show a peak at interannual time scale revealing the existence of an oscillation. The period of the oscillation is defined by the zonal extent of the forcing, whereas its amplitude is affected by its location. When the source region is located in the northwestern corner of the basin, the peak emerges clearly on the spectrum of the meridional transport, whereas it is strongly reduced when the source region is located in open ocean. The extension to an inter-hemispheric basin increases the energy at the period of the oscillation, but the introduction of the equatorial dynamics does not affect the spectral characteristics of the response for periods longer than 1 year

Low-frequency variability of the separated western boundary current in response to a seasonal wind stress in a 2.5-layer model with outcropping

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Interaction of a Coastal Kelvin Wave with the Mean State in the Gulf Stream Separation Area

The interaction of a coastal Kelvin wave with the mean state in the Gulf Stream separation area is studied using a hierarchy of numerical models including both low- and high-resolution 2.5-layer models and a coarse-resolution ocean general circulation model (OGCM) in a simple configuration. When the Kelvin front reaches the separation area in the low-resolution 2.5-layer model, an anomaly of opposite sign emerges and remains in the upper layer of the separation area. The mechanism leading to its buildup is the following: the variations of thickness in the active midlayer due to the propagation of the Kelvin wave induce current variations that act as a source of thickness anomalies for the upper layer (negative feedback). This source term is proportional to the mean vorticity gradient. The latter therefore must be large enough to obtain a significant response, which explains why this phenomenon occurs only in the separation area. The anomaly remains in the separation area because the advection by the mean zonal current is balanced by the current anomalies due to the variations of thickness in the surface layer. A very similar response is obtained in the high-resolution case and with the OGCM; thus, the mechanism leading to this evolution seems also largely independent of the model. However, in the OGCM, the surface current is sufficiently strong to advect the anomaly in the surface layers (above 200-m depth). Note finally that these anomalies do not prevent the Kelvin waves from pursuing their travel southward and then eastward toward the eastern boundary where Rossby waves are radiated. Numerous recent results based on this adjustment mechanism therefore would be robust