The Early Twentieth-Century Warming in the Arctic—A Possible Mechanism

Abstract

The huge warming of the Arctic that started in the early 1920s and lasted for almost two decades is one of the most spectacular climate events of the twentieth century. During the peak period 1930-40, the annually averaged temperature anomaly for the area 60Њ-90ЊN amounted to some 1.7ЊC. Whether this event is an example of an internal climate mode or is externally forced, such as by enhanced solar effects, is presently under debate. This study suggests that natural variability is a likely cause, with reduced sea ice cover being crucial for the warming. A robust sea ice-air temperature relationship was demonstrated by a set of four simulations with the atmospheric ECHAM model forced with observed SST and sea ice concentrations. An analysis of the spatial characteristics of the observed early twentieth-century surface air temperature anomaly revealed that it was associated with similar sea ice variations. Further investigation of the variability of Arctic surface temperature and sea ice cover was performed by analyzing data from a coupled ocean-atmosphere model. By analyzing climate anomalies in the model that are similar to those that occurred in the early twentieth century, it was found that the simulated temperature increase in the Arctic was related to enhanced wind-driven oceanic inflow into the Barents Sea with an associated sea ice retreat. The magnitude of the inflow is linked to the strength of westerlies into the Barents Sea. This study proposes a mechanism sustaining the enhanced westerly winds by a cyclonic atmospheric circulation in the Barents Sea region created by a strong surface heat flux over the icefree areas. Observational data suggest a similar series of events during the early twentieth-century Arctic warming, including increasing westerly winds between Spitsbergen and Norway, reduced sea ice, and enhanced cyclonic circulation over the Barents Sea. At the same time, the North Atlantic Oscillation was weakening.

The Arctic Amplification Debate

“…Studies by Johannessen et al (2004), based on analyses of surface data combined with simulations by two state-of-the-art global climate models, along with those by Bengtsson et al (2004) and Delworth and Knutson (2000), support this assertion. They conclude that the earlier warming involved changes in the THC, which in turn affected sea ice extent.…”

Arctic Climate Tipping Points

“…Past and present observations indicate that abrupt shifts can occur in the ocean-atmosphere-sea ice system in the Barents-Kara Seas (Bengtsson et al 2004;Semenov et al 2009). A shift to a state like that hypothesised for the 1920s abrupt warming seems feasible, in which much reduced year-round ice cover causes a strong winter ocean-atmosphere heat flux, which in turn supports a locally cyclonic circulation, enhancing westerly winds that support the inflow of warm ocean waters, in a self-amplifying positive feedback loop (Bengtsson et al 2004).…”

“…A shift to a state like that hypothesised for the 1920s abrupt warming seems feasible, in which much reduced year-round ice cover causes a strong winter ocean-atmosphere heat flux, which in turn supports a locally cyclonic circulation, enhancing westerly winds that support the inflow of warm ocean waters, in a self-amplifying positive feedback loop (Bengtsson et al 2004). Arguably such a transition might already be underway, but there is currently insufficient information to link a tipping point to global temperature.…”

“…These events involved a self-amplifying process in which increased sea-ice cover altered atmospheric wind patterns in a way that reinforced the reduction in inflow and the build up of the sea-ice (Bengtsson et al 2004;Semenov et al 2009). Then in the last century, an abrupt Arctic warming occurred in the 1920s, which persisted until the 1940s, with up to 1.7°C warming across 60-90°N at its peak in the 1930s (Alley et al 2003).…”

“…Then in the last century, an abrupt Arctic warming occurred in the 1920s, which persisted until the 1940s, with up to 1.7°C warming across 60-90°N at its peak in the 1930s (Alley et al 2003). This warming has been linked to changes in the opposite direction in the Barents Sea, this time increased inflow of warm Atlantic water, sea-ice retreat, and wind changes that were self-amplifying (Bengtsson et al 2004). The warming occurred during an interval of strong regional anthropogenic climate forcing due to deposition of black carbon (soot) on Arctic snow (McConnell et al 2007), so perhaps a role for anthropogenic forcing in triggering this event should be reconsidered.…”