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Yueyue Yu, Rongcai Ren, and Ming Cai

Abstract

This study investigates the dynamical linkage between the meridional mass circulation and cold air outbreaks using the ERA-Interim data covering the period 1979–2011. It is found that the onset date of continental-scale cold air outbreaks coincides well with the peak time of stronger meridional mass circulation events, when the net mass transport across 60°N in the warm or cold air branch exceeds ~88 × 109 kg s−1. During weaker mass circulation events when the net mass transport across 60°N is below ~71.6 × 109 kg s−1, most areas of the midlatitudes are generally in mild conditions except the northern part of western Europe. Composite patterns of circulation anomalies during stronger mass circulation events greatly resemble that of the winter mean, with the two main routes of anomalous cold air outbreaks being along the climatological routes of polar cold air: namely, via East Asia and North America. The Siberian high shifts westward during stronger mass circulation events, opening up a third route of cold air outbreaks through eastern Europe, where lies the poleward warm air route in the winter-mean condition. The strengthening of the Icelandic low and Azores high during stronger mass circulation events acts to close off the climatological-mean cold air route via western Europe; this is responsible for the comparatively normal temperature there. The composite pattern for weaker mass circulation events is generally reversed, where the weakening of the Icelandic low and Azores high, corresponding to the negative phase of the North Atlantic Oscillation (NAO), leads to the reopening and strengthening of the equatorward cold air route through western Europe, which is responsible for the cold anomalies there.

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Yueyue Yu, Ming Cai, Chunhua Shi, and Rongcai Ren

Abstract

It is well established that sudden stratospheric warming (SSW) events tend to be accompanied by continental-scale, surface cold-air outbreaks (CAOs) in midlatitudes in boreal winter. However, SSW events occur at most one to two times per winter, whereas CAOs occur three to seven times over each of the North American and Eurasian continents. Using the ERA-Interim dataset for 37 winters (November–March) from 1979 to 2016, we reveal that SSW events correspond to a large-amplitude or long-lasting subset of pulse-like, anomalously strong, stratospheric mass circulation events. The anomalously strong, stratospheric mass circulation events (referred to as PULSE events) occur more than nine times in an average winter. The “displacement” versus “split” types of SSWs tend to correspond to the “wavenumber 1” versus “wavenumber 2” types of PULSEs, though the relationship between split-type SSWs and wavenumber-2-type PULSEs is weaker. Like SSW events, PULSEs also have a close relationship with CAOs. The robust relationship with CAOs still holds for the PULSE events not accompanied by SSW events. Using PULSE events, we determine that more than 70% of CAOs in the 37 winters occur in the week before and after a PULSE event, with a false alarm rate of CAO occurrence of about 25.7%. SSW events, however, are associated with only about 5.7% of CAOs, with a false alarm rate of 21.7%. Therefore, the linkage between individual continental-scale CAOs and PULSE events represents a more generalized relationship between the stratospheric circulation anomalies and surface weather. PULSE signals should also be considered as a potentially useful stratospheric indicator of the occurrence of individual CAO events.

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Yueyue Yu, Rongcai Ren, Jinggao Hu, and Guoxiong Wu

Abstract

This study reports a mass budget analysis on the year-to-year variability of the winter [December–February (DJF)]-mean Arctic (60°–90°N) surface pressure (Ps) using the 33-yr daily Interim ECMWF Re-Analysis (ERA-Interim; 1979–2011). The analysis reveals that the interannual variability of mass transported into the Arctic region in upper layers plays a dominant role in the interannual variability of the winter-mean Arctic Ps anomalies. When winter-mean Arctic Ps anomalies are positive, both the transport of mass into the Arctic region in the upper layer by the poleward branch of meridional mass circulation and the transport of mass out of the Arctic region in the lower layer by the equatorward branch tend to strengthen and vice versa. In the earlier winter months from November to December, mass anomalies transported in overwhelm those transported out, explaining the mass source of winter-mean Arctic Ps anomalies. The coupling between adiabatic mass transport by meridional mass circulation and diabatic processes explains why, over the Arctic region, yearly variations of winter Ps are positively correlated with mass anomalies in the upper layer (above 290 K) and near the surface (below 260 K) but negatively correlated with mass anomalies in the middle and lower troposphere (between 260 and 290 K). In winters with positive (negative) Arctic Ps anomalies, wave activity, particularly in wavenumbers 1 and 2, is stronger (weaker) in the extratropical stratosphere in the earlier winter months from November to January, coincident with the interannual variability of the meridional mass circulation intensity in winter seasons.

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Yueyue Yu, Ming Cai, Rongcai Ren, and Huug M. van den Dool

Abstract

This study investigates dominant patterns of daily surface air temperature anomalies in winter (November–February) and their relationship with the meridional mass circulation variability using the daily Interim ECMWF Re-Analysis in 1979–2011. Mass circulation indices are constructed to measure the day-to-day variability of mass transport into the polar region by the warm air branch aloft and out of the polar region by the cold air branch in the lower troposphere. It is shown that weaker warm airmass transport into the upper polar atmosphere is accompanied by weaker equatorward advancement of cold air in the lower troposphere. As a result, the cold air is largely imprisoned within the polar region, responsible for anomalous warmth in midlatitudes and anomalous cold in high latitudes. Conversely, stronger warm airmass transport into the upper polar atmosphere is synchronized with stronger equatorward discharge of cold polar air in the lower troposphere, resulting in massive cold air outbreaks in midlatitudes and anomalous warmth in high latitudes. There are two dominant geographical patterns of cold air outbreaks during the cold air discharge period (or 1–10 days after a stronger mass circulation across 60°N). One represents cold air outbreaks in midlatitudes of both North America and Eurasia, and the other is the dominance of cold air outbreaks only over one of the two continents with abnormal warmth over the other continent. The first pattern mainly corresponds to the first and fourth leading empirical orthogonal functions (EOFs) of daily surface air temperature anomalies in winter, whereas the second pattern is related to the second EOF mode.

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Ming Cai, Yueyue Yu, Yi Deng, Huug M. van den Dool, Rongcai Ren, Suru Saha, Xingren Wu, and Jin Huang

Abstract

Extreme weather events such as cold-air outbreaks (CAOs) pose great threats to human life and the socioeconomic well-being of modern society. In the past, our capability to predict their occurrences has been constrained by the 2-week predictability limit for weather. We demonstrate here for the first time that a rapid increase of air mass transported into the polar stratosphere, referred to as the pulse of the stratosphere (PULSE), can often be predicted with a useful degree of skill 4–6 weeks in advance by operational forecast models. We further show that the probability of the occurrence of continental-scale CAOs in midlatitudes increases substantially above normal conditions within a short time period from 1 week before to 1–2 weeks after the peak day of a PULSE event. In particular, we reveal that the three massive CAOs over North America in January and February of 2014 were preceded by three episodes of extreme mass transport into the polar stratosphere with peak intensities reaching a trillion tons per day, twice that on an average winter day. Therefore, our capability to predict the PULSEs with operational forecast models, in conjunction with its linkage to continental-scale CAOs, opens up a new opportunity for 30-day forecasts of continental-scale CAOs, such as those occurring over North America during the 2013/14 winter. A real-time forecast experiment inaugurated in the winter of 2014/15 has given support to the idea that it is feasible to forecast CAOs 1 month in advance.

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