The freshwater cycle in the Arctic

Whereas most of the global upper-ocean stratification is determined by temperature with warmer water at the sea surface, the Arctic Ocean stratification is determined by salinity, whose effect on density is larger than that of temperature at low water temperatures. Thus, already small changes in the Arctic freshwater cycle can have a large impact on the Arctic climate. Additionally, changes in freshwater export from the Arctic into the North Atlantic can affect the thermohaline circulation and consequently have the potential to influence the global climate.

 

To study the influence of changes in the atmospheric large-scale circulation on the interannual variability of the compoentents of the Arctic freshwater cycle, we use a regionally coupled atmosphere-ocean-sea ice model, consisting of the ocean-sea ice model MPIOM, the regional atmosphere model REMO and the hydrological discharge model HD. This model has a closed freshwater cycle and high horizontal resolution in the Arctic. The computational grids of the models are shown in Fig.1. For details on the coupling we refer to Sein et al. (2015) and Elizalde (2011).

 

The leading modes in winter mean sea level pressure account for large parts of the interannual variability of the freshwater within the Arctic, such as the export of solid and liquid freshwater through Fram Strait and the Canadian archipelago. However, the variability in river runoff, driven by the variability of the Eurasian fraction, is not captured by these modes. In the winter preceeding a year with increased Eurasian runoff, an anomalous strong Icelandic low deflects the transport of moisture and heat northward. Hence, the winter is warmer and wetter than usual over the Eurasian continent. Overall increased precipitation during winter, late spring and summer lead to increased river runoff. In summer, increased inflow of moist air originating from the Pacific leads to enhanced runoff in eastern Siberia. Additionally, enhanced summer cyclone activity over northern Europe leads to enhanced runoff (Fig.2). This enhanced cyclone activity entering western Siberia supplies more moisture than usual, not only to the western Siberian rivers, but also to eastern Siberia.

 

Fig. 1: Computational grids from MPIOM (blue) and REMO (red).

Fig. 2: Regression coefficient between Eurasian runoff and 3 months running mean of vertically integrated moisture transport of 6-hourly data in kg/m/s per standard deviation of Eurasian runoff. The term moist.transp.(SON) refers to the mean value of vertically integrated moisture transport for the period September-November, moist.transp.(NDJ) to the transport for November-January, and so forth.

 

More details can be found in Niederdrenk et al. (2016).

 

References:

 

Elizalde Arellano A. (2011) The water cycle in the Mediterranean region and the impacts of climate change. Reports on Earth System Science (PhD thesis) (link)

Niederdrenk, A. L. (2013). The Arctic hydrologic cycle and its variability in a regional coupled climate model. Reports on Earth System Science (PhD thesis) (link)

Sein, D. V., U. Mikolajewicz, M. Gröger, I. Fast, W. Cabos, J. G. Pinto, S. Hagemann, T. Semmler, A. Izquierdo, and D. Jacob (2015). Regionally coupled atmosphere-ocean-sea ice- marine biogeochemistry model ROM: 1. description and validation, J. Adv. Model. Earth Syst., 7, 268-304, doi:10.1002/2014MS000357 (link)