North American winter‐spring storms: Modeling investigation on tropical Pacific sea surface temperature impacts

Abstract

An increased frequency and intensity of winter and spring storms have recently manifested over a broad area of North America—along the east coast of the U.S. especially, though global mean storm tracks are suggested to shift northward. To understand these changes, we have conducted atmospheric model experiments, examining the response of North American storm activity to the elevated tropical Pacific sea surface temperature (SST) associated with El Niño. The results indicate that, when tropical Pacific SST increases, there are more numerous intense storms over southwestern, southeastern, and northwestern North America, but fewer weaker storms over the northeast. Transient eddy analysis of the general circulation demonstrates consistent changes, suggesting systematic changes from large‐scale general circulation to synoptic‐scale storms. These changes can be attributed to enhanced lower tropospheric baroclinicity, to which the southward shift and an intensification of extratropical jet streams make a major contribution.

The impact of eastern equatorial Pacific convection on the diversity of boreal winter El Niño teleconnection patterns

“…The anomalies in synoptic eddy activity evident in Fig. 17a agree well with the canonical storm track response to El Niño reported in previous studies (Hoerling and Ting 1994;Straus and Shukla 1997;Harnik et al 2010;Seager et al 2010;Basu et al 2013;Grise et al 2013). The normalized storm track anomalies for EPN events (Fig.…”

Persistence of pressure patterns over North America and the North Pacific since AD 1500

“…This reconstruction, which allows dominant atmospheric circulation features of the 20th century to be compared with earlier time periods, indicates that over the past several decades, GPH anomaly patterns have persistently clustered in configurations that mirror observed and modelled climate changes, such as the positive trend in the instrumental PNA index 35 (Figs 4 and 5). These trends in synoptic circulation patterns have important implications for storminess, snowpack and streamflow in North America [33][34][35] , and the dominant pressure patterns characterized here will allow for an examination of associated terrestrial climate impact patterns in future research.…”

“…Evidence from climate models suggests that future regional changes in hydroclimate will depend on multiple drivers of atmospheric moisture transport 6 . Although uncertainties still exist, observations and modelling experiments point to an increase in the frequency and intensity of El Niño, with an associated southward shift and intensification of the subtropical jet stream, an enhanced PNA pattern and an increase in winter and spring storms across much of the U.S. and western Canada 33 , as well as a decrease in zonal flow and weakened westerlies into North America 34 . This reconstruction, which allows dominant atmospheric circulation features of the 20th century to be compared with earlier time periods, indicates that over the past several decades, GPH anomaly patterns have persistently clustered in configurations that mirror observed and modelled climate changes, such as the positive trend in the instrumental PNA index 35 (Figs 4 and 5).…”

Understanding the contrasting North Atlantic Oscillation anomalies of the winters of 2010 and 2014

“…Since the westerly jet is more equatorward (poleward) located for the 2010 (2014) case, the waves propagate more equatorward (poleward). Note, in particular, the more southern route of synoptic eddies over North America which is a systematic feature of El Niño events [ Seager et al , ; Basu et al , ]. Eddies' tilts located upstream of the wave packet envelope (i.e., west of 140°W) are mostly the same for the two cases, but they significantly differ more downstream.…”