On the Role of Horizontal Temperature Advection for the Interdecadal Arctic Warming Trend

Many studies argue that wintertime Arctic amplification arises though a delayed feedback process wherein the summertime sea-ice loss, which is amplified by the sea-ice albedo feedback mechanism, causes the ocean to warm and release heat back into the atmosphere during the subsequent winter. In this study, we use observational data from ERA-interim to investigate the contributions from various terms in the thermodynamic energy equation toward the wintertime Arctic surface warming trend observed during the past several decades. Terms examined include horizontal and vertical temperature advection, adiabatic warming/cooling, vertical mixing, latent heat release and longwave heating/cooling. We analyze data provided on the lowest model level, which corresponds to a height of about 10 meters, for the time period from 1990-2012.

We begin our study by analyzing the midlatitude winter sea level pressure (SLP) anomaly trend, following the perspective that advection of the climatological temperature by the anomalous wind has made an important contribution to the enhanced winter warming trend observed over the Barents and Kara Seas, and Baffin Bay, and also to the pronounced cooling trend observed in recent decades over Siberia. Specifically, the SLP anomaly trend is characterized by two distinct patterns: a negative NAO-like dipole over the North Atlantic and a pronounced high-pressure pattern over Siberia. By projecting daily SLP data onto these SLP trend patterns, we produce two statistically independent timeseries that respectively quantify the daily variation of the North Atlantic and Siberia SLP anomaly trend patterns. We then use multiple linear regression, with the regression coefficients corresponding to the intraseasonal relationship between the two SLP anomaly time series and the surface air temperature (SAT), to estimate the contributions that these SLP trend patterns have on the Arctic and Siberian SAT trends. Our results show that the SLP anomaly trend patterns explain about 25-50% of the observed Arctic winter warming trend and more than 75% of the cooling trend over Siberia. These results suggest that a large contribution to the Arctic warming and Siberian cooling trends arises from a long-term increase in the frequency of days with these SLP anomaly patterns.

We also examine lagged daily composites against the two independent SLP trend timeseries’ discussed in the previous paragraph. Our composites reveal that the SAT trend patterns undergo growth and decay over a period of about 2 weeks. The composite thermodynamic energy equation is examined in order to investigate the physical processes that drive the warming and cooling trends. It is found that horizontal temperature advection accounts for the warming trend observed over the Arctic. This temperature advection is strongly opposed by longwave radiative cooling and vertical mixing. This result is in sharp contrast with the widely-held viewpoint that Arctic warming is caused by turbulent sensible and latent heat fluxes, since it implies that these heat fluxes actually dampen the Arctic warming that is driven by horizontal temperature advection. The Siberian cooling is found to be driven by vertical mixing and adiabatic cooling, possibly associated with vertical motion forced by topography. The composite thermodynamic energy equation is also applied throughout the vertical column of the atmosphere and shows that horizontal temperature advection is much stronger near the surface than it is aloft. This implies that Arctic amplification should not be attributed to surface processes based solely on the fact that the temperature trend is bottom-heavy.