North Atlantic Winter Climate Regimes: Spatial Asymmetry, Stationarity with Time, and Oceanic Forcing

Abstract

The observed low-frequency winter atmospheric variability of the North Atlantic-European region and its relationship with global surface oceanic conditions is investigated based on the climate and weather regimes paradigm. Asymmetries between the two phases of the North Atlantic Oscillation (NAO) are found in the position of the Azores high and, to a weaker extent, the Icelandic low. There is a significant eastward displacement or expansion toward Europe for the NAOϩ climate regime compared to the NAOϪ regime. This barotropic signal is found in different datasets and for two quasi-independent periods of record (1900-60 and 1950-2001); hence, it appears to be intrinsic to the NAOϩ phase. Strong spatial similarities between weather and climate regimes suggest that the latter, representing long time scale variability, can be interpreted as the time-averaging signature of much shorter time scale processes. Model results from the ARPEGE atmospheric general circulation model are used to validate observed findings. They confirm in particular the eastward shift of the Atlantic centers of action for the NAOϩ phase and strongly suggest a synoptic origin as it can be extracted from daily analyses. These results bring together present-day climate variability and scenario studies where such an NAO shift was suggested, as it is shown that the last three decades are clearly dominated by the occurrence of NAOϩ regimes when concentrations of greenhouse gases are rapidly increasing. These findings highlight that the displacement of the North Atlantic centers of action should be treated as a dynamical property of the North Atlantic atmosphere and not as a mean longitudinal shift of climatological entities in response to anthropogenic forcings. The nonstationarity with time of the atmospheric variability is documented. Late-century decades differ from early ones by the predominance of NAO climate regimes versus others. In such a context, comments on the relevance of the station-based NAO index is provided. Both tropical and extratropical sea surface temperature (SST) anomalies alter the frequency distribution of the North Atlantic regimes. Evidence is presented that the so-called ridge regime is preferably excited during La Niña events, while the NAO regimes are associated with the North Atlantic SST tripole. The ARPEGE model results indicate that the tropical branch of the SST tripole affects the NAO regimes occurrence. Warm tropical SST anomalies are more efficient at exciting NAOϪ regimes than cold anomalies are at forcing NAOϩ regimes. The extratropical portion of the North Atlantic SST tripole also seems to play a significant role in the model, tending to counteract the dominant influence of the tropical Atlantic basin on NAO regimes.

Relationship between monthly rainfall in northwest Iberian Peninsula and North Atlantic sea surface temperature

“…Actually, it can only be observed when significances of 90% are considered. The tripole is the leading pattern of SST variability in the North Atlantic (Czaja and Frankignoul, 1999;Marshall et al, 2001) and has been related with atmospheric circulation, and in particular with North Atlantic Oscillation (NAO) pattern in winter (Czaja and Frankignoul, 2002;Czaja et al, 2003;Cassou et al, 2004b). In particular, the two northern lobes could affect the storm track, while the southern lobe could affect Hadley circulation (Czaja et al, 2003;Losada et al, 2007).…”

Upscaling European surface temperatures to North Atlantic circulation‐pattern statistics

“…See (Wanner et al, 2001) for a review of NAO. The NAO+ and NAO− patterns are similar to those found by (Cassou et al, 2004), although the exact orientation of the NAO dipole differs (their patterns are found by cluster analysis of monthly winter means of sea-level pressure data from National Centers for Environmental Prediction (NCEP) reanalysis).…”

Impacts of the EA and SCA patterns on the European twentieth century NAO–winter climate relationship

“…As a result, most modern NAO indices are derived using linear approaches such as the principal component (PC) time series of the leading empirical orthogonal function (EOF) of regional gridded SLP data (Moore et al, 2013;Wang et al, 2012), or by nonlinear techniques such as cluster analysis (Cassou et al, 2004). In any case, a positive negative) state of the NAO is associated with stronger (weaker) than average westerly winds across the middle latitudes of the Atlantic onto Europe.…”