Extreme spring cold spells in North China during 1961–2014 and the evolving processes

This study presents the major features of extreme spring cold spells(ESCS)in North China during 1961–2014 and the evolving processes.During an ESCS,northern Eurasia is controlled by large-scale positive SLP anomalies and an anomalous anticyclone that continuously transport northern cold air to North China,causing significant,persistent,lower-than-normal daily temperatures.In fact,these positive SLP and anticyclonic wind anomalies begin to appear over northwestern Europe about 10 days prior to the ESCS.The anticyclonic wind anomalies keep moving southeastward and expand to the west of Lake Baikal until two days before the ESCS.Then,the center of the anomalous low-level anticyclone moves over Lake Baikal,inducing northerly wind anomalies that transport cold air into North China.In the following days,as the positive SLP anomalies weaken,the anomalous anticyclone and related northerly winds appear to wane,and the ESCS gradually comes to an end.The results may prove meaningful for the prediction and early warning of ESCSs.

Spatio-temporal Variation of Arctic Sea Ice in Summer from 2003 to 2013

The variation in Arctic sea ice has significant implications for climate change due to its huge influence on the global heat balance. In this study, we quantified the spatio-temporal variation of Arctic sea ice distribution using Advanced Microwave Scanning Radiometer(AMSR-E) sea-ice concentration data from 2003 to 2013. The results found that, over this period, the extent of sea ice reached a maximum in 2004, whereas in 2007 and 2012, the extent of summer sea ice was at a minimum. It declined continuously from 2010 to 2012, falling to its lowest level since 2003. Sea-ice extent fell continuously each summer between July and mid-September before increasing again. It decreased most rapidly in September, and the summer reduction rate was 1.35 × 10~5 km^2/yr, twice as fast as the rate between 1979 and 2006, and slightly slower than from 2002 to 2011. Area with >90% sea-ice concentration decreased by 1.32 × 10~7 km^2/yr, while locations with >50% sea-ice concentration, which were mainly covered by perennial ice, were near the North Pole, the Beaufort Sea, and the Queen Elizabeth Islands. Perennial Arctic ice decreased at a rate of 1.54 × 10~5 km^2 annually over the past 11 years.

北斗星和北极星

夜晚，北方天空有两个容易辨认的星座，一为大熊星座，另一为小熊星座（图１）。图１大熊星座的排列，形如水杓，且位于北天，故又称北斗星。北斗星指的是北斗七星，而不是指某一颗星。小熊星座的排列，与北斗星相似，但其亮度比不上北斗星。因此，最能吸引古今中外人们注...

Reconstruction of autumn sea ice extent changes since AD1289 in the Barents-Kara Sea, Arctic

Using high-resolution ice core and tree ring proxies for sea ice extent （SIE）, we reconstructed a robust time series of autumn SIE over the Barents-Kara （B-K） sector of the Arctic from AD 1289-1993. After intercomparing the results and statistical parameters using the ordinary least squares regression （OLSR）, the principle component regression （PCR） and the partial least squares regression （PLSR） methods, SIE time series were synthesized into a more robust series using the weighted average method, which used the explained variances as weights. The results showed that from the end of the 13th century to the end of 18th century, the autumn B-K SIE was large, with large variations and a slightly expanding trend overall. This reflected significant multideeadal oscillations under the Little Ice Age （LIA） background. The B-K SIE began to decrease at the end of the 18th century, and a shrinking trend became significant during the second half of the 19th century, which lasted into the 1930s- 1940s. The 1930s-1940s was a period with a relatively low SIE in the B-K Sea, and the SIE had a short period of expansion from the 1940s-1970s. However, the B-K SIE has continuously and significantly shrank since the 1970s. The reduction in the B-K SIE since the end of the 18th century has been unprecedented in both duration and speed over the last 700 years. The B-K SIE has retreated significantly since the 1970s, with a speed 6.18 times greater than the former mean retreating speed. The industrial revolution may be a dominant factor in this result. The Arctic SIE in recent years may be the lowest it has been over the last millennium.

Arctic Oscillation anomaly in winter 2009/2010 and its impacts on weather and climate

Using the Arctic Oscillation(AO) index,the exceptional winter(DJF) of 2009 has been analyzed.The middle-to-high latitudes of the Northern Hemisphere suffered from a nearly zonally symmetric anomaly of temperature and pressure.This situation revealed that two negative AO events occurred in the winter of 2009/2010,with unprecedented low values in January 2009 and February 2010.The negative AO event in January 2009 can be further divided into two stages:the first stage was mainly driven by enhanced upward-propagating planetary waves,which led to a weak stratospheric polar vortex associated with a downward-propagating negative AO signal;the second stage was caused by a lower tropospheric positive temperature anomaly in the high latitudes,which maintained the positive geopotential height anomaly of the first stage.The two successively occurring stages interacted and caused the lower troposphere to experience a strong and lengthy persistence of the negative AO event.We consider that the second event of negative AO in February 2010 is related to the downward-propagating negative AO after sudden stratospheric warming.Eleven long-persistence negative AO events were analyzed using reanalysis data.The results suggest that the negative AO in the troposphere might have been caused by stratospheric sudden warming,a downward-propagating weak stratospheric circulation anomaly or dynamic processes in the troposphere.Further study shows that the negative phase of the AO in the winter of 2009/2010 corresponded to a wide range of temperature and precipitation anomalies in the Northern Hemisphere.Therefore,to improve the accuracy of weather forecasting and climate prediction,more attention should be paid to the AO anomaly and its impact.

Relationship between zonal position of the North Atlantic Oscillation and Euro-Atlantic blocking events and its possible effect on the weather over Europe

The North Atlantic Oscillation(NAO) exhibited a marked eastward shift in the mid-1970 s. Observations show that the extreme weather events in Europe have emerged frequently in the past decades. In this paper, based upon the daily NAO index, we have calculated the frequency of in-situ NAO events in winter during 1950-2011 by defining the Eastern-type NAO(ENAO) and Western-type NAO(WNAO) events according to its position at the east(west) of 10°W. Then, the composites of the blocking frequency, temperature and precipitation anomalies for different types of NAO events are performed. Results show that the frequency of Euro-Atlantic blocking events is distributed along the northwest-southeast(southwest-northeast) direction for the negative(positive) phase. Two blocking action centers in Greenland and European continent are observed during the negative phase while one blocking action center over south Europe is seen for the positive phase. The action center of blocking events tends to shift eastward as the NAO is shifted toward the European continent. Moreover, the eastern-type negative phase(ENAO) events are followed by a sharp decline of surface air temperature over Europe(especially in central, east, and south Europe), which have a wider and stronger impact on the weather over European continent than the western-type negative phase(WNAO) events do. A double- branched structure of positive precipitation anomalies is seen for the negative phase event, besides strong positive precipitation anomalies over south Europe for ENAO event. The eastern-type and western-type positive phase(ENAO+ and WNAO+) can lead to warming over Europe. A single-branched positive precipitation anomaly dominant in central and north Europe is seen for positive phase events.