Winter temperature extremes in China and their possible causes

International Journal of Climatology, 2016

Two measures of intra-seasonal variability, indicated respectively by standard deviations (SD) and day-today (DTD) fluctuations denoted by absolute differences between adjacent 2-day periods, as well as their relationships with large-scale circulation patterns were investigated in China during 1962-2008 on the basis of homogenized daily temperature records from 549 local stations and reanalysis data. Our results show that both the SD and DTD of daily minimum temperatures (T min) in summer as well as the minimum and maximum temperatures in winter have been decreasing, while the daily maximum temperature (T max) variability in summer is fluctuating more, especially over southern China. In summer, an attribution analysis indicates that the intensity of the Western Pacific Subtropical High (WPSH) and high-level East Asian Subtropical Jet stream (EASJ) are positively correlated with both SD and DTD, but the correlation coefficients are generally greater with the SD than with the DTD of the daily maximum temperature, T max. In contrast, the location of the EASJ shows the opposite correlation pattern, with intensity regarding the correlation with both SD and DTD. In winter, the Arctic Oscillation (AO) is negatively correlated with both the SD and DTD of the daily minimum temperature, but its intra-seasonal variability exhibits good agreement with the SD of the T min. The Siberian High acts differently with respect to the SD and DTD of the T min , demonstrating a regionally consistent positive correlation with the SD. Overall, the large-scale circulation can well explain the intra-seasonal SD, but DTD fluctuations may be more local and impacted by local conditions, such as changes in the temperature itself, the land surface, and so on.

International Journal of Climatology, 2015

The impact of the boreal summer intraseasonal oscillation (BSISO) on extreme hot and cool events was investigated, by analyzing the observed and reanalysis data for the period from 1983 to 2012. It is found that the frequency of the extreme events in middle and high latitudes is significantly modulated by the BSISO convection in the tropics, with a 3-9-day lag. During phases 1 and 2 when the BSISO positive rainfall anomaly is primarily located over a northwest-southeast oriented belt extending from India to Maritime Continent and a negative rainfall anomaly appears in western North Pacific, the frequency of extreme hot events is 40% more than the frequency of non-extreme hot events. Most noticeable increase appears in midlatitude North Pacific (north of 40°N) and higher-latitude polar region. Two physical mechanisms are primarily responsible for the change of the extreme frequency. First, an upper-tropospheric Rossby wave train (due to the wave energy propagation) is generated in response to a negative heating anomaly over tropical western North Pacific in phases 1 and 2. This wave train consists of a strong high pressure anomaly center northeast of Japan, a weak low pressure anomaly center over Alaska, and a strong high pressure anomaly center over the western coast of United States. Easterly anomalies to the south of the two strong midlatitude high pressure centers weaken the climatological subtropical jet along 40°N, which is accompanied by anomalous subsidence and warming in North Pacific north of 40°N. Second, an enhanced monsoonal heating over South Asia and East Asia sets up a transverse monsoonal overturning circulation, with large-scale ascending (descending) anomalies over tropical Indian (Pacific) Ocean. Both the processes favor more frequent extreme hot events in higher-latitude Northern Hemisphere. An anomalous atmospheric general circulation model is used to confirm the tropical heating effect.

Advances in Atmospheric Sciences, 2011

Trends in the frequencies of four temperature extremes (the occurrence of warm days, cold days, warm nights and cold nights) with respect to a modulated annual cycle (MAC), and those associated exclusively with weather-intraseasonal fluctuations (WIF) in eastern China were investigated based on an updated homogenized daily maximum and minimum temperature dataset for 1960-2008. The Ensemble Empirical Mode Decomposition (EEMD) method was used to isolate the WIF, MAC, and longer-term components from the temperature series. The annual, winter and summer occurrences of warm (cold) nights were found to have increased (decreased) significantly almost everywhere, while those of warm (cold) days have increased (decreased) in northern China (north of 40°N). However, the four temperature extremes associated exclusively with WIF for winter have decreased almost everywhere, while those for summer have decreased in the north but increased in the south. These characteristics agree with changes in the amplitude of WIF. In particular, winter WIF of maximum temperature tended to weaken almost everywhere, especially in eastern coastal areas (by 10%-20%); summer WIF tended to intensify in southern China by 10%-20%. It is notable that in northern China, the occurrence of * Corresponding author: YAN Zhongwei, yzw@tea.ac.cn Temperature Extremes and Weather-Intraseasonal Fluctuations 2 warm days has increased, even where that associated with WIF has decreased significantly. This suggests that the recent increasing frequency of warm extremes is due to a considerable rise in the mean temperature level, which surpasses the effect of the weakening weather fluctuations in northern China. , 2010: Trends in temperature extremes in association with weather-intraseasonal fluctuations in eastern China. Adv.

The Arctic Oscillation (AO), which depicts a most dominant large-scale seesaw between the mid-latitudes and Arctic atmospheric mass, influences climate over Eurasia, North America, eastern Canada, North Africa, and the Middle East, especially during boreal winter. This review, with a special focus on the East Asian region, summarizes the climatic impact of AO. It begins with a description of the spatial structure of AO and the related climatic anomalies. The relationship of winter AO with the simultaneous East Asian winter climate (e.g. the East Asian winter monsoon (EAWM), cold surges/cold waves, and precipitation) and its instability are then followed. It is generally accepted that, through impacting the Siberian high, westerly wind, blocking frequency, Rossby wave activities etc., a positive phase of winter AO is associated with a weaker-than-normal EAWM, warmer conditions in East Asia, less frequency of cold surges/cold waves, increasing (decreasing) of winter precipitation in south (north) parts of East Asia; and vice versa. Notably, the pathways that the winter AO exerts impact are different. Besides, the AO-EAWM and the AO-cold surges/cold wave linkages have spatial and temporal variations. Subsequently , an overview of the inter-seasonal linkages between the East Asian summer monsoon with the preceding spring/winter AO is presented. There is a generally accepted knowledge that a positive spring AO is followed by significant positive summer precipitation anomalies in southern China and western Pacific as well as negative ones in the lower valley of Yangtze River and southern Japan. Finally, this review synthesizes the impact of win-ter/spring AO on the East Asian spring climate (e.g. dust storm, temperature, and precipitation) and discusses the potential predictive value of AO. The projection of AO and its impact on the East Asian climate in future has been barely explored. We conclude that, along with the long-term observation data, the linkage between AO and the East Asian climate on the sub-seasonal and decadal time scales, how tropical and extratropical forcing modulates the linkage and how the linkage evolves under future warming conditions should be more investigated. Notably, the change of AO during 1990–2013 winters could explain the Eurasian cooling but failed to explain the Arctic warming. In the future, the effect of Ural blocking on Arctic and Eurasian climate and their connection might be a hot topic.

Geophysical Research Letters, 2005

1] The interannual variability of winter surface air temperature (SAT) in the Northern Hemisphere (NH) associated with the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) is studied. The AO and the NAO show different impacts on winter NH SAT variations. The AO affects the SAT over the Euro-Asian and African continents, whereas the NAO is more regional with the major effect on the SAT in the western North Africa. This discrepancy can be reflected in other atmospheric variables such as sea level pressure and geopotential height fields as well. The analyses in this paper also suggest that the AOrelated signal can penetrate deeply into the stratosphere while the NAO one is largely a tropospheric phenomenon.

Using ensemble empirical mode decomposition (EEMD) for detecting the interdecadal changes in the East Asian winter monsoon (EAWM), this study identi es that the intensity of the EAWM experienced remarkable transi- tion around mid-1980s and late 2000s: a strong period (P1, 1960–1986), a weak period (P2, 1987–2007), and a strong period (P3, 2008–2015). A distinctive cold (warm) surface temperature anomaly is found over the Eurasian continent including East Asia, and cold (warm) sea surface tempera- ture (SST) anomalies are present over the North Paci c (NP) during P1 (P2). In contrast with P1, the EAWM is charac- terized by a large temperature di erence between Eurasian continent and NP, and a negative Paci c Decadal Oscillation (PDO)-like SST anomaly pattern is found over the NP dur- ing P3. During three periods, the Siberian high (SH) plays an important role in deciding the intensity of the EAWM. In addition, the EAWM exists under the in uence of an enhanced atmospheric circulation associated with a posi- tive North Paci c Oscillation (NPO) and a negative PDO during P2 and P3, respectively. Accordingly, to recognize the relative importance of each combined e ect which it dif- fers for times, the combined e ect of SH and NPO (SH and PDO) variability on the weakening (strengthening) EAWM is explored during P1 and P2 (P2 and P3). Consequently, while the atmospheric variabilities of SH and NPO bring about the weakening of EAWM after the mid-1980s, the SST variability related to PDO with the SH variabilty is involved in the strengthening of EAWM in recent decade.

Geophysical Research Letters, 2007

1] We estimate the contributions of natural and anthropogenic forcings to the observed summer cooling over the Eastern China through a set of controlled experiments of an atmospheric general circulation model. The control run is forced by historical natural and anthropogenic agents, as well as by the Hadley Center global sea surface temperature. The model suggests a weak influence by the solar radiation change on this cooling, but a significant contribution by the sulfate aerosol to this cooling, through both dynamic and thermal processes. The inclusion of the sulfate aerosol induces a positive gradient of air temperature in the middle-upper troposphere, which results in a northward shift in the 200 hPa East Asian westerly jet stream and an increase of the East Asian summer monsoon, leading to more cloud cover and precipitation in the Eastern China therefore surface cooling over the region.

Asia-Pacific Journal of Atmospheric Sciences, 2018

The combined effect of the Madden-Julian oscillation (MJO) and Arctic oscillation (AO) on the temperature variation over Asia is investigated using the thermodynamic budget equation. Because of the distinct geographical origin of the two atmospheric modes, the influence of AO is more dominant in the higher latitude, whereas the MJO impact is more predominant in the lower latitude. Hence, the physical process responsible for the surface cold anomaly is different for northern and southern Asia. Cold anomaly appears in most of Asia 20-25 days after the MJO phase 6 (corresponding to the phase 2-3). However, more strengthened cold anomaly occurs over northern Asia under the negative AO state and it is caused by advection of temperature anomaly by climatological northerly wind associated with the East Asia winter monsoon flow. On the other hand, much stronger cold anomaly is seen over southern Asia under the positive AO state for the same lag day of the initial MJO phase 6. Aside from the upward overturning circulation forced by the tropical MJO over the subtropics and lower midlatitudes, the weakening of the East Asia subtropical jet by the positive AO induces additional upward motion over southern Asia to adjust the thermal wind balance. The combined effect of the MJO and AO also influences on the occurrence of extreme cold event. Under the negative (positive) AO phase, the extreme cold event occurrence probability over northern (southern) Asia increases by 90% (60%) compared to that for all winter days. The relative increase rate for the MJO phases 2-3 is ~30% over southern Asia. The cold event occurrence probability for the combined modes is about the twice that for only MJO impact, suggesting that an incorporation of both modes enhance the predictability of extreme cold event.

Meteorology and Atmospheric Physics, 2005

Statistical tests were used to detect the variations of winter (December to February), summer (June to August), and annual mean temperatures of 160 representative stations in China during the period from 1951 to 2000. Changes of temperature in the observed time series that exceed the confidence level of 0.01 were classified as significant temperature rises or drops. The stations with significant temperature rises were classified into several temperature rise types, by the use of a clustering method. A search was made for sudden, or abrupt, temperature changes at each of the 160 stations. Possible reasons for the observed temperature variations are discussed. The statistical analysis reveals that for the 160 stations, significant rises occur in 53.1% of their annual mean temperatures, 51.9% of their winter temperatures, and 13.8% of their summer temperatures. Temperature rises increase with latitude. In contrast, significant drops occur in annual mean temperatures at 1 station, winter temperatures at 3 stations, and summer temperatures at 5 stations. Approximately 50% of stations with significant temperature variations are in the form of abrupt changes. The abrupt temperature changes all occurred in the 1970s, most often in 1979 and 1980. The abrupt changes are shown to be related to the subtropical high in South China Sea. The temperature rises in winter are shown to be linked to the presence of an anomalously strong zonal circulation in Eurasia, and a weak polar vortex, since the 1980s. The summer temperature rise in South China also has a link with the increase in size and intensity of the subtropical high, over the same period. Some of the station temperature drops in summer were related to a weaker summer monsoon. The annual mean temperature rise at 71.8% of the stations was influenced mainly by their winter temperature rises. For 8.2% of the stations, the annual mean temperature rise was connected to their summer temperature rise, and for 12.9% of the stations it was related to both the summer and winter temperature rises.

2015

The present study investigates the overall changes in occurrences of winter cold surges over Southeast China for the period 1961-2012, using instrumental observations, reanalysis and model simulation datasets. Based on objectively defined criteria, cold surges were classified into 3 types according to their dynamical origin as inferred from daily evolution patterns of surface pressure systems with a focus on the Siberian High (SH): type A with an amplification of a quasi-stationary SH associated with high-pressure anomalies over the Ural mountains, type B with a developing SH associated with fast traveling upper-level waves, and type C with a high-pressure originated in the Arctic. Examination of the long-term change in cold surge occurrences shows different interdecadal variations among the 3 types. During 1961-2012, type A events (37.8%) decreased, while type B events, accounting for the majority (52.5%) of total winter cold surges, increased slightly. The contribution by type C to the total occurrence of the cold surges was small (8.8%) compared to that of A and B, but it became more frequent in the latest decade, related to the tendency of the Arctic Oscillation (AO) being more in its negative phase. Overall, we found slightly increased occurrences of cold surges over Southeast China since the early 1980s, despite the weakened SH intensity and warmer mean temperature compared to previous decades. The climate model projections of the phase 5 of the Coupled Model Intercomparison Project (CMIP5) suggests similar trend in the late 21st century under warmer climate.