Tropospheric temperature interannual variations associated with decadal changes in the North Atlantic Oscillation

Abstract

This paper studies the causes and mechanisms of the formation of extreme anomalies in the tropospheric temperature associated with the North Atlantic Oscillation (NAO). Our approach is based on understanding that, in the annual cycle, continental-scale tropospheric temperature anomalies (planetary waves with longitudinal wave numbers of 1–3) can both intensify under the direct action of heat inflow as an energy source for these anomalies (radiation cooling/heating) and weaken as a result of the destructive action of heat inflow under temperature advections with the opposite (to the heat inflow) sign [4, 5]. According to the monthly mean data of the NCEP/NCAR reanalysis over the 40-year period, seasonal air temperature anomalies have been studied at the level 850 hPa (T850) in different regions of Eurasia. It has been confirmed that the negative NAO phase in winter is favorable for preserving negative T850 anomalies in the east of the continent at this time of year, whereas the positive NAO phase is favorable for negative T850 anomalies in the west. However, it has been revealed that this dependence was critically violated during the winter seasons approximately two years before an extreme event. This was explained by the fact that, in those years, the NAO influence on winter T850 anomalies was limited. This paper formally considers a certain mechanism of anomalous heat inflow as an energy source for these anomalies with functions of the formation (intensification) of negative T850 anomalies in winter and positive T850 anomalies in summer, as well as with a function of the limitation of the influence of the predominant dynamic mode on some regions of the continent. It is shown that, in the 1960s, T850 anomalies with negative NAO indices in the east of the continent were governed by a hypothetic mechanism of heat inflow as an energy source for anomalies; in 1980s, at prolonged positive NAO indices, T850 anomalies in the west of the continent could also be governed by this mechanism. This paper, within the accepted degree of detail, demonstrates the process of limitation of the NAO influence in some years (1966, 1967, 1987, and 1988), which leads to an unbalance of the anomalies and a possible extreme phenomenon. It is demonstrated that, in some seasons, the anomalies were not governed by the hypothetic mechanism of the heat inflow under the action of large NAO changes and a complete upset of the annual cycle of anomalies. Determining the first indicators of the unbalance, which can lead to extreme anomalies, is shown to be difficult if it is based only on an analysis of winter seasons (as is the case with most of the works) without invoking the annual trends of the tropospheric temperature and the NAO index.