An Overview of Dry-wet Climate Variability among Monsoon-Westerly Regions and the Monsoon Northernmost Marginal Active Zone in China

Climate in China's Mainland can be divided into the monsoon region in the southeast and the westerly region in the northwest as well as the intercross zone, i.e., the monsoon northernmost marginal active zone that is oriented from Southwest China to the upper Yellow River, North China, and Northeast China. In the three regions, dry-wet climate changes are directly linked to the interaction of the southerly monsoon flow on the east side of the Tibetan Plateau and the westerly flow on the north side of the Plateau from the inter-annual to inter-decadal timescales. Some basic features of climate variability in the three regions for the last half century and the historical hundreds of years are reviewed in this paper. In the last half century, an increasing trend of summer precipitation associated with the enhancing westerly flow is found in the westerly region from Xinjiang to northern parts of North China and Northeast China. On the other hand, an increasing trend of summer precipitation along the Yangtze River and a decreasing trend of summer precipitation along the monsoon northernmost marginal active zone are associated with the weakening monsoon flow in East Asia. Historical documents are widely distributed in the monsoon region for hundreds of years and natural climate proxies are constructed in the non-monsoon region, while two types of climate proxies can be commonly found over the monsoon northernmost marginal active zone. In the monsoon region, dry-wet variation centers are altered among North China, the lower Yangtze River, and South China from one century to another. Dry or wet anomalies are firstly observed along the monsoon northernmost marginal active zone and shifted southward or southeastward to the Yangtze River valley and South China in about a 70-year timescale. Severe drought events are experienced along the monsoon northernmost marginal active zone during the last 5 centuries. Inter-decadal dry-wet variations are depicted by natural proxies for the last 4-5 centuries in several areas over the non-monsoon region. Some questions, such as the impact of global warming on dry-wet regime changes in China, complex interactions between the monsoon and westerly flows in Northeast China, and the integrated multi-proxy analysis throughout all of China, are proposed.

Geographical Patterns and Temporal Variations of Regional Dry and Wet Heatwave Events in China during 1960–2008

Daily maximum/minimum temperatures and relative humidity records from 510 stations in China for the period 1960–2008 were used to investigate geographical patterns and temporal variations of heatwave （HW） events. Dry and wet HW events were compared by different definitions. Regionally, both dry and wet HW events are commonly located in southeastern China in the monsoon area, with neither type occurring in the northeast part of Northeast China and Southwest China, while the north-northwest region of the country experiences dry HW events and a few wet HW events. In the southeast of the country, site dry HW events occurred from April to September and mostly in June, while site wet HW events occurred from April to October and mostly in September. In total, 163 regional wet HW events were identified. The ten longest regional wet HW events lasted for more than 20 days, while the mean duration for 163 events was about 11 days. For the top ten events, six occurred after the 1990s, compared with four before this time. Global surface warming was clear since 1979, but the frequency and severity of regional wet HW events were relatively low in the 1980s, increasing remarkably since the 1990s. Possible reasons for this might be the strong interdecadal and interannual variations in regional atmospheric circulations, as well as water transport related directly to temperature contrasts in different regions, rather than global-mean temperature changes.

Interannual Thermocline Signals and El Nio-La Nia Turnabout in the Tropical Pacific Ocean

One of the fundamental questions concerning the nature and prediction of the oceanic states in the equatorial eastern Pacific is how the turnabout from a cold water state （La Nino） to a warm water state （El Nino） takes place, and vice versa. Recent studies show that this turnabout is directly linked to the interannual thermocline variations in the tropical Pacific Ocean basin. An index, as an indicator and precursor to describe interannual thermocline variations and the turnabout of oceanic states in our previous paper （Qian and Hu, 2005）, is also used in this study. The index, which shows the maximum subsurface temperature anomaly （MSTA）, is derived from the monthly 21-year （1980-2000） expendable XBT dataset in the present study. Results show that the MSTA can be used as a precursor for the occurrences of E1 Nino （or La Nino） events. The subsequent analyses of the MSTA propagations in the tropical Pacific suggest a one-year potential predictability for E1 Nino and La Nino events by identifying ocean temperature anomalies in the thermocline of the western Pacific Ocean. It also suggests that a closed route cycle with the strongest signal propagation is identified only in the tropical North Pacific Ocean. A positive （or negative） MSTA signal may travel from the western equatorial Pacific to the eastern equatorial Pacific with the strongest signal along the equator. This signal turns northward along the tropical eastern boundary of the basin and then moves westward along the north side of off-equator around 16°N. Finally, the signal returns toward the equator along the western boundary of the basin. The turnabout time from an E1 Nino event to a La Nino event in the eastern equatorial Pacific depends critically on the speed of the signal traveling along the closed route, and it usually needs about 4 years. This finding may help to predict the occurrence of the E1 Nino or La Nino event at least one year in advance.

Characteristics and Changes of Cold Surge Events over China during 1960-2007

This paper demonstrates regional characteristics, a long-term decreasing trend, and decadal variations in the frequency of cold surge events based on daily mean temperature and daily minimum temperature data in China's Mainland from 1960 to 2008. During these 48 years four high frequency centers of cold surge events were located in Xinjiang, central North China, northeast China, and southeast China. A main frequency peak of cold surge events occurs in autumn for the four regions and another peak is detected in spring over northeast China and southeast China. The regional pattern of cold surge frequencies is in accordance with the perturbation kinetic energy distribution in October December, January, and February April. The long-term decreasing trend ( 0.2 times/decade) of cold surge frequencies in northeast China and decadal variations in China are related to the variations of the temperature difference between southern and northern China in the winter monsoon season; these variations are due to the significant rising of winter temperatures in high latitudes.

Identifying Regional Prolonged Low Temperature Events in China

This study examined regional prolonged low temperature （PLT） events in China from the observational station data for the period 1960–2008 using the new criteria. The new definition of a site PLT event is that the daily minimum temperature does not exceed the 10th percentile threshold of the local daily minimum temperature climatology for at least 5 days at a station. The regional PLT event is defined as at least five adjacent stations exhibiting site PLT simultaneously for 5 d. Under the new definition, 552 regional PLT events were identified, and three indices： duration, extent, and intensity, as well as a comprehensive index （CI） were used to quantify the event severity. In addition, geographical patterns and temporal variations of regional PLT events were investigated using three event categories： strong, moderate, and weak. Spatially, strong events were mainly located in the north of Xinjiang and along the Yangtze River to the south of the Yangtze River; moderate events occurred in Xinjiang and south of the Yangtze River; and weak events occurred south of the Yellow River. The variation for the annual frequency of regional PLT events in China in the last 49 years showed a significant decreasing trend with a rate of-1.99 times per decade, and the significant transition decade was the 1980s.

Ranking Regional Drought Events in China for 1960-2009

The spatiotemporal variations of the site and regional droughts in China during 1960–2009 were analyzed by applying a daily composite-drought index （CDI） to 722 stations in China's Mainland. Droughts frequently happened in a zone extended from Southwest China to the Yellow River, North China, and the southwestern part of Northeast China, with two centers of high frequency in North China and Southwest China. In Southwest and South China, droughts tend to happen during the winter. In North China and along the Yellow River, droughts mainly occur during the winter and during May–June. During the past 50 years, the geographical distribution of site drought events showed high frequencies （0.9–1.3 times per year） in the upper Yellow River basin and North China, comparing with moderate frequencies （0.6–0.9 times per year） in Southwest China and the southwestern part of Northeast China and with lower frequencies over the middle and lower Yangtze River basin. And the frequencies increased over China's Mainland except for the upper reaches of the Yangtze River. A regional drought （RD） event is a widespread and persistent event that covers at least five adjacent sites and lasts for at least 10 days. There were 252 RD events in the past 50 years—five times per year. Most RD events lasted for 100 days and covered 100 stations, but the longest and largest RD event lasted for 307 days from 6 September 1998 to 9 July 1999 and covered 327 stations from North to Southwest China.

Trends in Graded Precipitation in China from 1961 to 2000

Daily precipitation rates observed at 576 stations in China from 1961 to 2000 were classified into six grades of intensity, including trace （no amount）, slight （≤ 1 mm d^-1）, small, large, heavy, and very heavy. The last four grades together constitute the so called effective precipitation （〉 1 mm d^-1）. The spatial distribution and temporal trend of the graded precipitation days are examined. A decreasing trend in trace precipitation days is observed for the whole of China, except at several sites in the south of the middle section of the Yangtze River, while a decreasing trend in slight precipitation days only appears in eastern China. The decreasing trend and interannual variability of trace precipitation days is consistent with the warming trend and corresponding temperature variability in China for the same period, indicating a possible role played by increased surface air temperature in cloud formation processes. For the effective precipitation days, a decreasing trend is observed along the Yellow River valley and for the middle reaches of the Yangtze River and Southwest China, while an increasing trend is found for Xinjiang, the eastern Tibetan Plateau, Northeast China and Southeast China. The decreasing trend of effective precipitation days for the middle- lower Yellow River valley and the increasing trend for the lower Yangtze River valley are most likely linked to anomalous monsoon circulation in East China. The most important contributor to the trend in effective precipitation depends upon the region concerned.

An Integrated Analysis of Dry-Wet Variability in Western China for the Last 4-5 Centuries

The dry-wet variability in western China and its spatiotemporal structure during the last 4-5 centuries was examined using 24 climate proxies from sediments, ice cores, historical documents, and tree rings. Spatial patterns and temporal evolutions of dryness and wetness were not only extracted from the proxy data using rotated empirical orthogonal function （REOF） analysis for the last 4 centuries, but also for instrumental data in the last 40 years. The leading five REOF modes indicate that 5 dry-wet variation centers exist in western China. Moreover, long-term variability in dryness and wetness is seen on long （centennial） to short （inter-decadal） timescales. An out-of-phase relationship for the inter-decadal variation was observed between the Hetao-upper Yangtze River region and north Xinjiang, indicating influences on dry-wet variations of the East Asian summer monsoon and the westerly winds over the two regions, respectively. A particularly long dry spell was found in the central Tibetan Plateau in the 19th century. A predominance of wet decades in the last 4 centuries was found in the arid and Hetao regions. Three regional dry-wet series with annual resolution in north Xinjiang, the upper Yellow River valley, and the Hetao area were constructed for analyses of the last 500 years. Dry-wet oscillations with periodicities of 16, 50, and 150 years in north Xinjiang, 50 years in the upper Yellow River valley, and 70-80 years in the Hetao region were identified by wavelet analysis. In general, these periods correspond to large-scale oscillations found in the climate system, are mainly related to ocean-atmosphere interaction.

Periodic oscillations in millennial global-mean temperature and their causes

Time series of solar radiation and north Pacific sea surface temperature(SST) index were used to analyze their causality relationship with various periodic oscillations in reconstructed millennial global-mean temperature series.The three long-term periods of the Medieval Warm Period(MWP),Little Ice Age(LIA) and recent Global Warming Period(GWP) were distinct in the temperature series.21-year,65-year,115-year and 200-year oscillations were derived from the temperature series after removing three long-term climatic temperatures.The phases of temperature oscillations significantly lagged behind oceanic SST and solar radiation variability.The recent decadal warm period was caused by the quasi-21-year temperature oscillation.At this century-cross period,the four oscillations reached their peaks simultaneously,which did not occur during the last millennium.Based on the long-term trend during the GWP and the four periodic oscillations,global-mean temperature is expected to drop to a new cool period in the 2030s and then a rising trend would be towards to a new warm period in the 2060s.

Reconstructed index of summer monsoon dry-wet modes in East Asia for the last millennium

Monthly precipitation datasets collected at 160 stations in China as well as the monthly winds and humidity data derived from the US National Centers for Environmental Prediction (NCEP) were used to construct the relationship between six summer dry-wet modes in eastern China and the summer monsoon airflow northward advance in East Asia. A millennial series of the monsoon dry-wet index (MDWI) was reconstructed based on Wang's six summer dry-wet modes in eastern China since 950 AD. A high (low) index indicates that the strong (weak) East Asian summer monsoon airflow can reach northern (southern) China and cause above (below) normal precipitation. Interdecadal periodic variations, such as the approximate 70-year oscillation, can be found in the MDWI series. In the last millennium, northern China has experienced persistent decadal wet periods and persistent decadal dry periods. At present, the MDWI is a low period on the interdecadal time scale so above-normal precipitation is observed in southern China and below-normal precipitation in northern China.

Changes in fossil-fuel carbon emissions in response to interannual and interdecadal temperature variability

Relationships on interannual and interdecadal timescales among global mean air temperature,CO2 concentrations and fossil-fuel carbon emissions in four major developed countries (the United States,the United Kingdom,France,and Germany) were analyzed.On an interannual timescale,the United States fossil-fuel carbon emissions tend to increase during cold winters and decrease during warm winters,which is opposite to the situation in summer.On an interdecadal timescale,cold (warm) periods both in the United States and globally agree with high (low) periods of fossil-fuel carbon emissions,with the temperature variability leading by 5–7 years.The leading correlation on the interdecadal timescale and the asymmetry in seasonal correlation on the interannual timescale indicate that temperature variability is a possible cause of changes in fossil-fuel carbon emissions.

How would global-mean temperature change in the 21st century?

The time series of HadCRUT3 global-mean surface air temperature(GSAT) anomaly,Pacific decadal oscillation(PDO) index,and the equatorial Pacific sea surface temperature(SST) were utilized,and their long-term trends and multiple time-scale periodic oscillations were explored in this study.A long-term trend with a warming rate about 0.44°C /century during 1850-2008,two cool floors occurred respectively around 1910 and during 1950-1970,and three warm flats happened in the 1870s,1940s and since the year 1998 were found in the GSAT.In this duration,the variability of GSAT can be well reconstructed by the quasi-21-year,the quasi-65-year,and century-scale oscillations.The recent decadal warm flat is caused by their positive phase overlapping from these three oscillations.The maximum rising temperature reached 0.26°C was simulated in 2004 by the three oscillations.The quasi-21-year and the quasi-65-year oscillations were possibly caused by solar radiation and internal variability of the ocean-atmosphere system.Therefore,an outlook of GSAT for the 21st century was made based on the long-term trend and these three oscillations.It was expected that a cool floor and a warm flat of the GSAT would appear in the 2030s and 2060s,respectively.However,the highest warming range is predicted about 0.6°C,it is less than the threshold 2°C and IPCC projection.