In the sea ice physics section, we use a combination of observations and numerical model experiments to improve our understanding of the dynamics of the coupled sea ice ocean system. One of the key topics of these investigations is the response of the sea ice and the ocean to the atmospheric forcing, i.e. the wind, air temperatures and the fluxes of heat and freshwater between these climate subsystems.
Fresh water content, variability in a future climate scenario run with the earth system model MPI-ESM. The solid blue line shows the total Arctic (liquid + sea ice) freshwater content (integrated in the area in blue on the map) (Graphic: T. Kovacz, Alfred-Wegener-Institut)
The Arctic Ocean receives freshwater by run-off from Asia north of the Himalaya. Large rivers carry freshwater from this enormous catchment area into the Eurasian shelf seas. Ocean waters with salinities below the average salinity of the Arctic Ocean also contribute positively to the fresh water balance of the Arctic Ocean. Such waters flow in from the Nordic Seas.
An interchange of waters between the Arctic and the Nordic seas results in in sea saw fluctuations in fresh water content between the basins. To reach an equilibrium, the Arctic Ocean must on average export freshwater. Major pathways of the export are the East Greenland Current and sea ice flux through Fram Strait. Inflow of saline water from the South is equivalent to a fresh water export. Fresh water from the Arctic strengthens the stratification in the Nordic Seas and the Labrador Sea. Strong stratification prevents downward transport that usually occurs in these oceans where the northern downward branch of the Atlantic Meridional Overturning Circulation (AMOC) is normally situated.
To better understand the interbasin exchanges of fresh water we employ ocean sea ice and large-scale Earth System models. The models are validated using sea ice drift estimates from satellite remote sensing and regional airborne sea ice thickness measurements. Oceanographic moorings in Fram Strait provide estimates of liquid fresh water transport. Once validated, the models can be used for numerical experiments that reveal the forcing of fresh water transport and the relationship between fresh water storage in the Arctic Ocean and release of fresh water to the Nordic Seas in export events. The large-scale consequences of such events for the oceanic circulation and regional sea level changes are also studied in such experiments.
The Arctic Ocean surface circulation is characterized by the Transpolar Drift, which carries water and sea ice from the Siberian Shelves to exit at the Fram Strait, and the Beaufort Gyre, a large clockwise rotating gyre in the Canadian Basin (Fig 1). This circulation system is fed by inflows of freshwater from the rivers, and by marine water from the Pacific and the Atlantic Oceans through the gateways of Bering Strait, Fram Strait and the Barents Sea Opening. Below this surface water layer water of Atlantic origin circulates counterclockwise in the Canadian and the Eurasian Basin along the slopes of the bottom topography (Fig 2). These circulation features are known today as being variable in location, extent and intensity, sometimes in in the direction of the flows.
One method to study the changing circulation on the large scale is the use of tracers which mark specific water masses on their paths through the Arctic. Such a tracer is 129 Iodine, an anthropogenic radionuclide which is dumped as waste into the Irish Sea and the North Sea by nuclear reprocessing facilities. Measuring the changing distribution of this tracer in the Arctic and comparing the measurements with model simulations, allows us to monitor and better understand the changing circulation and its driving forces.
Investigations based on measurements of the distribution of 129 Iodine in the Arctic and model simulations with AWI’s coupled sea ice – ocean model NAOSIM, driven with realistic atmospheric data (NCEP) have revealed large scale changes of circulation patterns (Karcher et al, 2012). As a consequence of the high state of the Arctic Oscillation (AO), a mode of intense anti-clockwise wind fields over the Arctic Ocean, the first half of the 1990s saw an exceptionally small Beaufort Gyre and a relocation of the Transpolar Drift from the Eurasian region to the Canadian Basin. At the same time the mid-depth circulation of Atlantic Water below the surface circulation, showed a widespread intrusion of water from the Atlantic into the Canadian Basin, along the continental slope, marked with high concentrations of the tracer 129 Iodine (See Fig 3a and b for 1995 at 240 m depth). The strong inflow of Atlantic Water in the Canadian Basin continued into the 2000s (Fig 3c) while the Beaufort Gyre and the Transpolar Drift slowly started to bounce back into their typical position and size (see Fig 1), and the AO was less anomalous after 1995. As it turned though, the following period saw an intense clockwise wind field regionally over the Canadian Basin with continued for the next two decades and led to a large extend and increased depth of the low saline surface water rotating in the Beaufort Gyre (e.g. Proshutinsky et al. 2009, Rabe et al., 2011). The model simulations suggested that his would have profound consequence for the circulation of the mid-depth Atlantic Water Layer, effectively shutting down the intrusion of Atlantic derived Water into the Canadian Basin with the boundary currents (Karcher et al., 2012). Recent updates to 2015 and beyond show that this situation has continued, and apart from a weak intrusion from the North, the Canadian Basin is shut off from ventilation with Atlantic Water (see Fig 3d for 2015). In a new publication jointly with John Smith from Bedford Institute for Oceanography in Canada, the model simulations will be compared to the measurements from the international observational campaign GEOTRACES in 2015, showing the persistence of the shut off simulated by the model, is confirmed by those 129 Iodine measurements.
Fig. 3a (Graphic: Alfred-Wegener-Institut)
Fig. 3b (Graphic: Alfred-Wegener-Institut)
Fig. 3c (Graphic: Alfred-Wegener-Institut)
Figure 1: Surface circulation of the Arctic Ocean (Graphic: Alfred-Wegener-Institut)
Figure 2: Mid-depth circulation in the Arctic Ocean, which is fed from the Atlantic. 129 Iodine releases (inlet) from the nuclear facilities in Sellafield and La Hague, which mark the Atlantic origin waters (Karcher et al, 2012) (Graphic: Alfred-Wegener-Institut)
Fig. 3d (Graphic: Alfred-Wegener-Institut)
Figure 3: Concentration of 129 Iodine [107 at/l] at 240m depth, tracing water of Atlantic origin, a) observed in 1995; simulated (NAOSIM) b) in 1995; c) in 2000 (all from Karcher et al, 2012, data from Smith et al. 1999); d) updated for 2015.
Literature: Karcher, M., Smith, J. N., Kauker, F., Gerdes, R. and Smethie Jr, W. (2012): Recent changes in Arctic Ocean circulation revealed by 129‑Iodine observations and modelling, Journal of Geophysical Research ‑ Oceans. doi: 10.1029/2011JC007513
Rabe, B. , Karcher, M. , Schauer, U. , Toole, J. M. , Krischfield, R. A. , Pisarev, S. , Kauker, F. , Gerdes, R. and Kikuchi, T. (2011): An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006‑2008 period , Deep‑Sea Research I, 58185, 173 . doi: 10.1016/j.dsr.2010.12.002
Sea ice drift and ocean currents are to a large degree driven by the wind. Sea ice thickness, sea ice concentration and sea ice roughness influence the momentum flux from the atmosphere to the ocean (Castellani et al., 2015 and Itkin et al., 2014) The Beaufort Gyre, the Transpolar Drift and finally the export of sea ice and relatively fresh water through Fram Strait depend on these sea ice properties. A feedback loop connects oceanic heat transport, winds and sea ice transport in the Greenland Sea (Kovacs et al.2020). We expect similar feedback loops to exist in other regions of the Arctic Ocean and the Nordic Seas.
Literature: Castellani, G. , Gerdes, R. , Losch, M. and Lüpkes, C. (2015): Impact of Sea-Ice Bottom Topography on the Ekman Pumping / G. Lohmann , H. Meggers , V. Unnithan , D. Wolf-Gladrow , J. Notholt and A. Bracher (editors) , In: Towards an Interdisciplinary Approach in Earth System Science, (Springer Earth System Sciences), Heidelberg [u.a.], Springer, 251 p., ISBN: 978-3-319-13865-7 . doi: 10.1007/978-3-319-13865-7_16
Kovacs, T., Marshal, J. and Gerdes, R. (2020, under revision)
Itkin, P., Karcher, M. and Gerdes, R. (2014): Is weaker Arctic sea ice changing the Atlantic water circulation? Journal of Geophysical Research: Oceans, 119 (9), pp. 5992-6009 . doi: 10.1002/2013JC009633
