The Leading Intraseasonal Variability Mode of Wintertime Surface Air Temperature over the North American Sector

Abstract

In this study, detailed characteristics of the leading intraseasonal variability mode of boreal winter surface air temperature (SAT) over the North American (NA) sector are investigated. This intraseasonal SAT mode, featured by two anomalous centers with an opposite sign, one over central NA and another over East Siberia (ES) / Alaska, bears great resemblance to the “warm Arctic - cold continent” pattern of the interannual SAT variability over NA. This intraseasonal SAT mode and associated circulation exert pronounced influences on regional weather extremes, including precipitation over the northwest coast of NA, sea ice concentration over the Chukchi and Bering Seas, as well as extreme warm and cold events over the NA continent and Arctic region. Surface warming and cooling signals of the intraseasonal SAT mode are connected to temperature anomalies in a deep tropospheric layer up to 300hPa with a decreasing amplitude with altitude. Particularly, a coupling between the troposphere and stratosphere is found during evolution of the intraseasonal SAT variability, although whether the stratospheric processes are essential in sustaining the leading intraseasonal SAT mode is difficult to be determined based on observations alone. Two origins of wave sources are identified in contributing to vertically propagating planetary waves near Alaska, one over ES/Alaska associated with local intraseasonal variability, another from the subtropical North Pacific via Rossby wave trains induced by tropical convective activity over the western Pacific, possibly associated with the Madden-Julian Oscillation.

Pacific sea surface temperature anomalies as important boundary forcing in driving the interannual Warm Arctic-Cold Continent pattern over the North American sector

“…Figure 6a presents regressed anomalous SST (shading) and sea ice (contours) associated with the observed WACC variability. The observed WACC pattern is closely linked to sea ice loss over CBS as previously reported (e.g., Kug et al 2015;Blackport et al 2019;Guan et al 2020a), although the causality is difficult to be determined based on the observations due to the two-way interactions between Arctic sea ice and atmosphere. The WACC pattern over the NA sector is also found to be associated with negative SST anomalies over the central and western NP along 408N and surrounding positive anomalies over the eastern part of the NP basin and CBS, as well as a small patch of warm SST anomalies over the tropical western Pacific (TWP) near 1608E.…”

“…The internal variability of atmospheric circulation over the mid-to high latitudes of Eurasia and NA continents is often manifested by the vigorous subseasonal variability. For example, a similar WACC pattern in SAT anomalies has been recently reported as a leading subseasonal SAT variability mode to link Arctic sea ice changes and winter SAT anomalies over midlatitude continents (e.g., Lin 2018;Guan et al 2020b), representing a crossscale influence on the interannual WACC variability (Sorokina et al 2016;Guan et al 2020a).…”

“…The observed and simulated SAT anomalies of the leading SVD mode, which explains 40% of the total squared covariance of the observed and simulated SAT variations, capture the WACC pattern over the NA sector-that is, warm anomalies centered over eastern Siberia (ES)/Alaska and cold anomalies over central NA-along with the anomalous Alaskan high in bridging the two anomalous SAT centers. As previously mentioned, the anomalous anticyclonic circulation is expected to sustain the WACC pattern by advecting cold air from the Arctic into central NA, and warm and moist air from the south into the CBS (e.g., Kug et al 2015;Guan et al 2020a). While the warming anomalies over the Arctic are well simulated, the amplitude of cold anomalies over central NA is significantly underestimated in models by about 50% (Fig.…”

“…In contrast to the pronounced warming and rapid sea ice loss over the Arctic in recent decades, frequent occurrence of cold harsh winters has been observed over Eurasia and central North America (NA), jointly featuring a ''warm Arctic, cold continents'' (WACC) pattern (e.g., Overland et al 2011;Cohen et al 2014;Kug et al 2015;Sun et al 2016). A WACC pattern has also been identified as a prevailing interannual variability mode in surface air temperature (SAT) anomalies during boreal winter over the mid-to high latitudes of Eurasia and NA (Kug et al 2015;Blackport et al 2019;Mori et al 2019a;Guan et al 2020a; see Fig. 1a for an example of the WACC pattern over the NA sector).…”

A Connection of Winter Eurasian Cold Anomaly to the Modulation of Ural Blocking by ENSO

“…(negative)-phase NAO (NAO + [NAO − ]) events or the negative (positive) PNA events are associated with the negative (positive) phase of ENSO (Li & Lau, 2012;Lin et al, 2005;Müller & Roeckner, 2006). While the ENSO modulates the North American weather and climate via the changes in the PNA (Li et al, 2019;Straus & Shukla, 2002) and North Pacific Oscillation (Guan et al, 2020), it can also influence the winter Eurasian climate and weather through stratospheric processes (Butler et al, 2014).…”