Changes in both surface sources (P - E + R and sea ice melting) and lateral transports can lead to freshwater content changes in the Arctic Ocean. Surface fluxes are likely to be the dominant factors for future changes as global warming and the atmospheric hydrological cycle intensify. For the 20th century changes, lateral transports may have played a more important role (Wu et al. 2007). Proshutinsky et al. (2002) suggested that anomalous freshwater storage within the anticyclonic Beaufort Gyre can be potentially larger than changes in river runoff and sea ice export. Häkkinen and Proshutinsky (2004) and Hátún et al. (2005) emphasise the contribution of the Atlantic water inflow. Jungclaus et al. (2005), Wu and Wood (2007) have all realised the importance of freshwater exchange between the Arctic and the subpolar North Atlantic in affecting basin scale freshwater content changes. Wu and Wood (2007) have shown that anomalous atmospheric conditions such as the winter of 1971/72 may cause a circulation regime change within the Arctic/subpolar North Atlantic Ocean system that has a long lasting effect on water exchanges through the Greenland-Iceland-Scotland (GIS) straits. Freshwater redistribution following such circulation changes can lead to substantial freshwater content changes comparable to the recent freshening trend reported by Dickson et al. (2002), Curry and Mauritzen (2005). The GSA can now be well simulated in climate models (Haak et al. 2003; Koenigk et al. 2006a; Wadley and Bigg 2004, 2006). Haak et al. (2003) suggested from their model simulations that the GSA is linked to anomalous sea ice export through Fram Strait driven by anomalous atmospheric circulation. On the other hand Houghton and Visbek (2002), Wadley and Bigg (2006) have recently questioned the advective nature of the GSA.

15.7 Conclusions

Increasing greenhouse gas concentrations in the atmosphere have a disproportionate impact on polar climates relative to global warming. Enhanced warming due to polar amplification, first pointed out by Manabe and Stouffer (1980), is now a well recognised phenomenon (see, e.g., Holland and Bitz 2003; ACIA 2005). Additional freshwater input due to increased moisture transport from the subtropics and river discharges has made another distinction for the polar regions under an accelerating global hydrological cycle (Wu et al. 2005; Stocker and Raible 2005). Changes in the global freshwater cycle will directly affect the distribution of water resources worldwide (see, Oki and Kanae 2006, for a recent review). Changing patterns and severity of droughts and floods will be parts of its climate impact on regional scales. Extra freshwater input into the Arctic/subarctic oceans has another worrying consequence on the climate system. This is its potential of diluting the northern polar oceans where deep convection occurs and the associated weakening the Atlantic thermohaline circulation (THC, e.g. Vellinga and Wood 2002; Wu et al. 2004; Curry and Mauritzen 2005). Moreover, meltwater input from a disintegrating Greenland ice sheet could further accelerate the THC weakening (Fichefet et al. 2003; Jungclaus et al. 2006b).

There are already signs of systematic changes in the Arctic/subarctic freshwater cycle (Dickson et al. 2002; Curry et al. 2003; Curry and Mauritzen 2005; Peterson et al. 2006). In order to understand and attribute the observed changes to different causes, long climate records and comprehensive computer models are needed to expand our research into further depth and accuracy. Having described the progress in simulating the terms in the Arctic hydrological budget above, it is clear that there are weak areas in both observations and modelling. Because the polar regions are highly sensitive parts of the global hydrological cycle, we need observations to be more reliable, continuous with better coverage for monitoring global changes. We need climate models to resolve more detailed processes and feedbacks in simulating precipitation, evaporation, sea ice and land hydrology. We need better estimates of the magnitude and variability of the Arctic/subarctic hydrological budgets. As modellers, we would like to use increasingly more observational measurements to validate and constrain climate model simulations. In the meantime, we would also like to use our models to help understand the mechanisms of observed variability and change, to attribute them to different possible causes, and to use model projections to guide future observational efforts.

There are competing sources of freshwater adding to the Arctic/subarctic oceans as global warming continues. At the present, there are considerable uncertainties even for the climatological means for the individual contributors from both observational estimates and climate model simulations (see Fig. 15.1). Large differences also exist between model simulated budget terms and observationally based estimates, as well as among different models. Those uncertainties in the means will undoubtedly overshadow any predicted budget and trends. We should aim to achieve an observationally constrained, multi-model ensemble prediction of the Arctic freshwater budget such as the one shown in Table 15.1 in the near future. It will enable us to answer the following questions: What is the likely upper bound of freshwater input into the Arctic/subarctic oceans? How much of that is likely to be realised over the next 50 or 100 years? Which component is likely to play a leading role? Besides, how can we best use the Arctic as an indicator for monitoring the global hydrological cycle? To complete these tasks will require concerted efforts from both the observational and the modelling communities.

Acknowledgements Peili Wu and Richard Wood are funded by the UK Department of Environment, Food and Rural Affairs under the Climate Prediction Programme (PECD/7/12/37). Tore Furevik has been supported by the Norwegian Research Council through the NoClim project. We thank Jeff Ridley and Jonathan Gregory for helpful comments on the review of Greenland ice sheet.


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