Coupling between the North Atlantic subpolar gyre vigor and forest fire activity in northern Scandinavia

Abstract. The circulation strength of the North Atlantic subpolar gyre varies on a range of timescales, it regulates the northwards oceanic heat transport and influences weather and climate over Scandinavia. We test the hypothesis that persistent atmospheric circulation anomalies favorable for extensive forest fire activity in the northern Scandinavian boreal region are predominantly associated with weaker subpolar gyre strength on subannual timescales. We included both winter and summer drought as important precursors for forest fire occurrence in the boreal region. Three ensemble members of climate model simulations covering the time period 850–2005 AD are considered. Years of widespread and severe drought in northern Scandinavia are identified using the monthly drought code as a summer-drought indicator, and winter drought is identified as the 5th percentile of coldest and driest winters. The statistical significance of anomalous ocean- and atmospheric circulation is tested for these years, both during and prior to the main fire season.

Analysis of the ensemble of three simulations did not yield a generalized result, hence the hypothesis cannot be confirmed for subannual timescales. For the three simulations we find respectively that the fire-prone years are associated with subpolar gyre circulation that is on average stronger, weaker or insignificantly changed compared with the mean state. The 5th percentile of most extreme dry and cold winters has a strong relation to the winter North Atlantic oscillation (NAO), but not with the gyre circulation state. We find a number of extremely cold/dry winters occurring during the Little Ice Age (LIA, 16th–19th centuries AD), and infer that winter drought may have played a significant role in promoting forest fire activity at this time.

Our results highlight the importance of resolving the past fire seasonality in the northern Scandinavian domain, and developing compound drought indicators for winter and spring.