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Influence of sea ice cover on high latitude precipitation: Inferences from precipitation isotope measurements and a 2D model


The most widely cited climate feedback in the Arctic region is ice cover. Warming climate reduces the sea ice extent, which causes a lower surface albedo, resulting in more absorbed insolation and further warming – a positive feedback. Conversely, warming is also likely to result in increased Arctic evaporation and precipitation, leading to increased snow cover and a higher Arctic terrestrial albedo, which would cause cooling – a negative feedback. The balance between these feedbacks must be understood and quantified in order to predict climate response to influences such as increased greenhouse gases. Here, we use measurements of high latitude precipitation isotopes and a 2D model to investigate interannual variability in the contributions of subtropical and Arctic vapor sources to Arctic precipitation. In a previous study, we used isotopic ratios alone to investigate the sources of moisture to the Arctic. We found significant positive relationships between ice area and the d-excess of precipitation on both interannual and seasonal timescales, an expected result under the assumption that sea ice prevents evaporation from the sea surface and consequently reduces the contribution of Arctic moisture with low d-excess values to Arctic precipitation. In this work, we go a step further with an attempt to estimate the influence of sea ice cover on Arctic evaporation using a 2D model and constraining it with high latitude isotopic measurements. The 2D model is a vertical-meridional mass conservation model for H2O, HDO, and H218O with prescribed atmospheric circulation and temperatures. For each isotope, the rates of surface evaporation, sublimation, precipitation, and reevaporation of falling hydrometeors are calculated, and values of the humidity and isotopic concentrations of both vapor and hydrometeors are computed interdependently with the four process rates.. The model fractionation associated with the four processes is based primarily on the work of Jouzel and Merlivat. Specifically, we include the effect of supersaturation in snow formation. We tune the steady state solutions to interannual means of observed precipitation and evaporation amounts at low latitudes, and to isotopic compositions of precipitation at low and high latitudes, with seasonal boundary conditions of temperature and circulation. By varying the Arctic sea ice area, which regulates surface evaporation, to obtain agreement with the constraints of precipitation isotope observations for one season in a specific year, we can extract from the model the amounts of Arctic precipitation that originated in subtropical and Arctic vapor source areas. We find that the Arctic vapor amount is significantly correlated with interannual variations of the sea-ice extent, which leads to inferences about the significance of the evaporation feedback. Furthermore, this study confirms that vapor sourcing can have an impact on isotopic ratios in ice cores and inherent implications for paleoclimatic interpretation.