The forcing of sea ice and the near-surface ocean circulation in the Arctic depends strongly on the atmospheric circulation at the atmosphere-ocean interface. A weak anticyclone appears in the annual mean field of sea level pressure over the Arctic Ocean, but Arctic pressures vary widely (and can be dominated by cyclonic circulations) on synoptic, seasonal and interannual timescales. During the most recent decade, Arctic pressures have been persistently below the long-term mean, while North Atlantic pressure features as measured by the North Atlantic Oscillation (NAO) index have been unusually strong. Thompson and Wallace's(1998) "Arctic Oscillation" mode of wintertime variability supports the notion that the Arctic and North Atlantic atmospheric circulations are coupled. In this presentation, we evaluate the strength of the Arctic-North Atlantic linkage as a function of timescale (daily to decadal) and its seasonality.
The seasonality of the Arctic and subtropical North Atlantic high pressure systems shows a consistent out-of-phase relationship between the two features: the Arctic anticyclone is strongest in winter/spring and weakest in summer, while the North Atlantic subtropical high is weakest in winter/spring and strongest in summer. Correlations between the interannual variations of monthly Arctic SLP and the NAO are positive in all months, but are strongest in February-April (r = 0.68 for March) and weakest in September-October (r = 0.10 for October). For nearly all calendar months, Arctic SLP is more strongly correlated with Atlantic subpolar pressures (the Icelandic low) than with the Atlantic subtropical high pressure system as indexed by the Azores SLP.
Additional results to be presented are the relationships between the occurrence of extreme daily pressures in the central Arctic and the North Atlantic, as well as the associations with North Pacific SLP variations. We will also address the implications of the low-frequency SLP variations for the Arctic freshwater budget, specifically the sea ice outflow through Fram Strait and the net freshening of the Arctic ocean as deduced from precipitation (less evaporation) in the recent atmospheric reanalyses of NCEP and ECMWF.