Arctic Sea Ice and Freshwater Changes Driven by the Atmospheric Leading Mode in a Coupled Sea Ice–Ocean Model

Abstract

Observational and modeling studies have indicated recent large changes of sea ice and hydrographic properties in the Arctic Ocean. However, the observational database is sufficiently sparse that the mechanisms responsible for the recent changes are not fully understood. A coupled Arctic ocean–sea ice model forced by output from the NCEP–NCAR reanalysis is employed to investigate the role that the leading atmospheric mode has played in the recent changes of the Arctic Ocean. A modified Arctic Oscillation (AO) index is derived for the region poleward of 62.5°N in order to avoid ambiguities in the distinction between the conventional AO and the North Atlantic Oscillation index. The model results indicate that the AO is the driver of many of the changes manifested in the recent observations. The model shows reductions of Arctic sea ice area and volume by 3.2% and 8.8%, respectively, when the AO changes from its negative to its positive phase. Concurrently, freshwater storage decreases by about 2%, while the sea ice and freshwater exports via Fram Strait increase substantially. The changes of sea ice and freshwater storage are strikingly asymmetric between the east and the west Arctic. Notable new findings include 1) the interaction of the dynamic and thermodynamic responses in the sense that changes of sea ice growth and melt are driven by, and feed back negatively to, the dynamically (transport) driven changes of sea ice volume; and 2) the compatibility of the associated freshwater changes with recently observed changes in the salinity of the upper Arctic Ocean, thereby explaining the observed salinity variations by a mechanism that is distinct from, but complementary to, the altered circulation of Siberian river water. In addition, the enhanced freshwater export could be a contributing factor to the increased salinity in the Arctic Ocean. The results of the simulations indicate that Arctic sea ice and freshwater distributions change substantially if one phase of the AO predominates over a decadal timescale. However, such results are based on an idealization of the real-world situation, in which the pattern of forcing varies interannually and the number of positive-AO years varies among decades.

Simulated Response of the Arctic Freshwater Budget to Extreme NAO Wind Forcing

“…During the NAO1 integration the total flux of freshwater through these straits was, on average, 906 and 495 km 3 yr 21 higher than the NAO2 integration, respectively. The increase at Fram Strait is entirely due to an increase in sea ice export, and we find that the 43% increase in sea ice export compares very well with the 56% change noted in the modeling study of Zhang et al (2003) examining the response of the Arctic to the different phases of the NAO. Despite the export of sea ice increasing at Fram Strait during the NAO1 integration, the volume of liquid freshwater transport was lower at this time and arises because the volume of freshwater flowing northward into the Arctic in the WSC increased by 24 km 3 yr 21 .…”

“…Hence, they cannot respond and evolve with feedbacks associated with change in the regional oceanice model. Furthermore, with the exception of the wind velocity data, all of the remaining forcing variables are taken from the period 1992-2001 when the NAO was by and large positive, and an increasing number of midlatitude synoptic storms penetrated into the Arctic, increasing the northward moisture and heat transport (Zhang et al 2004). During the period of NCEP forcing (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) the air temperature over the Arctic rose by approximately 0.58C, in contrast to the 1960s (when the NAO was negative) when the air temperature was, on average, over 18C colder (Shein et al 2006).…”

“…We note that many modeling studies of the Arctic freshwater budget discussed previously suffered from two shortcomings that likely lead to inaccuracies. First, and most importantly, previous model studies restore salinity and/or temperature to climatological values to avoid model drift (Zhang et al 1998b;Maslowski et al 2000;Zhang et al 2003;Karcher et al 2005). The strength of the restoring typically varies from days to months in these simulations, but any restoring makes deriving an accurate freshwater budget of the Arctic difficult.…”

“…The direct response of the Arctic freshwater budget to the positive and negative phases of the NAO pattern were recently examined by Zhang et al (2003), and then by Houssais et al (2007), using regional coupled oceanice models of the Arctic, forced with NCEP data. To create atmospheric forcing fields for the NAO, the NCEP data were regressed onto the positive and negative phases of the NAO index over the last 50 years.…”

“…This pattern is associated with an intensification of the North 1 We note that the NAO and the AO are nearly indistinguishable, with a monthly temporal correlation of 0.95 Deser (2000), so we focus on the NAO index based on the difference of normalized winter (December-March) sea level pressure between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland (Hurrell 1995). Atlantic storm track and an increased northward atmospheric heat transport. The reduced Ekman convergence can no longer retain the freshwater in the Beaufort gyre, causing it to be exported into the North Atlantic (Proshutinsky et al 2002;Zhang et al 2003;Hä kkinen and Proshutinsky 2004;Peterson et al 2006). At the same time, an intensified sea level pressure gradient in Fram Strait drives sea ice into the North Atlantic (Kwok et al 2004).…”