Reconstruction of summer temperature variation from maximum density of alpine pine during 1917-2002 for west Sichuan Plateau, China

Having analyzed the tree ring width and maximum latewood density of Pinus densata from west Sichuan, we obtained different climate information from tree-ring width and maximum latewood density chronology. The growth of tree ring width was responded princi- pally to the precipitation in current May, which might be influenced by the activity of southwest monsoon, whereas the maximum latewood density reflected summer temperature （June-September）. According to the correlation relationship, a transfer function had been used to reconstruct summer temperature for the study area. The explained variance of reconstruction is 51% （F=52.099, p〈0.0001）. In the reconstruction series： before the 1930s, the climate was relatively cold, and relatively warm from 1930 to 1960, this trend was in accordance with the cold-warm period of the last 100 years, west Sichuan. Compared with Chengdu, the warming break point in west Sichuan is 3 years ahead of time, indicating that the Tibetan Plateau was more sensitive to temperature change. There was an evident summer warming signal after 1983. Although the last 100-year running average of summer temperature in the 1990s was the maximum, the running average of the early 1990s was below the average line and it was cold summer, but summer drought occurred in the late 1990s.

CLIMATIC FEATURES OF SUMMER TEMPERATURE IN NORTHEAST CHINA UNDER WARMING BACKGROUND

By using,summer temperature data in 26 stations from 1951 to 2003, the variation characteristics of summer temperature in Northeast China （NET） were analyzed based on the background of climate wanning. The results showed that the warming in summer was 0.15~C/10a in Northeast China, which was higher than that on the global, Northern Hemisphere or Northeast Asia scale in the recent 50 years. The responses of NET to global warming were shown in 3 aspects mainly. Firstly, it became warm and the average temperature increased in summer; secondly, the temperature variability increased, which displayed the increase of climatic instability; thirdly, the disaster of low temperature decreased and high temperature damage increased obviously, but the disaster of low temperature still existed in some areas under global warming background, which would be worthy of notice further.

Regional Summer Temperature Decrease against Global Warming in China, Landform Effect?

The data of 16o national meteorological observatory （NMO） stations with long-term monthly temperature data for China were analyzed in this study to show the basin-centered summer temperature decrease against global warming in the past half century. The summer and winter isotherm structures of 1950s and 1990s worked out by interpolation show the isotherm structure variations： the isotherm structure generally moves northward in winter, but in summer it is characterized with separate high-temperature and low-temperature centers and the isotherm structure moves inward the centers with global warming, indicating that the temperature in the highland areas increases but that in the lowland areas decreases in the summer of the duration. The possible mechanism of the basin-centered temperature decrease in summer is discussed in this paper.

How similar are annual and summer temperature variability in central Sweden?

Tree-ring based temperature reconstructions have successfully inferred the past inter-annual to millennium scales summer temperature variability. A clear relationship between annual and summer temperatures can provide insights into tile variability of past annual mean tem- perature from the reconstructed summer temperature. However, how similar are summer and annual temperatures is to a large extent still unknown. This study aims at investigating the relationship between annual and summer temperatures at different timescales in central Sweden during the last millennium. The temperature variability in central Sweden can represent large parts of Scandinavia which has been a key region for dendroclimatological research. The observed annual and summer temperatures during 1901-2005 were firstly decomposed into different frequency bands using ensemble empirical mode decomposition （EEMD） method, and then the scale-dependent relationship was quantified using Pearson correlation coefficients. The relationship between the observed annual and summer temperatures determined by the instrumental data was subsequently used to evaluate 7 climate models. The model with the best performance was used to infer the relationship for the last millennium. The results show that the relationship between the observed annual and summer temperatures becomes stronger as the timescale increases, except for the 4--16 years timescales at which it does not show any relationship. The summer temperature variability at short timescales （2--4 years） shows much higher variance than the annual variability, while the annual temperature variability at long timescales （〉32 years） has a much higher variance than the summer one. During the last millennium, the simulated summer temperature also shows higher variance at the short timescales （2-4 years） and lower variance at the long timescales （〉1024 years） than those of the annual temperature. The relationship between the two temperatures is generally close at the long timescales, and weak at the short timescales. Overall the summer temperature variability cannot well reflect the annual mean temperature variability for the study region during both the 20th century and the last millennium. Furthermore, all the climate models examined overestimate the annual mean temperature variance at the 2--4 years timescales, which indicates that the overestimate could be one of reasons why the volcanic eruption induced cooling is larger in climate models than in proxy data.

Seasonal Prediction of Summer Temperature over Northeast China Using a Year-to-Year Incremental Approach

We present a model for predicting summertime surface air temperature in Northeast China（NESSAT） using a year-to-year incremental approach.The predicted value for each year＇s increase or decrease of NESSAT is added to the observed value within a particular year to yield the net forecast NESSAT.The seasonal forecast model for the year-to-year increments of NESSAT is constructed based on data from 1975- 2007.Five predictors are used：an index for sea ice cover over the East Siberian Sea,an index for central Pacific tropical sea surface temperature,two high latitude circulation indices,as well as a North American pressure index.All predictors are available by no later than March,which allows for compilation of a seasonal forecast with a two-month lead time.The prediction model accurately captures the interannual variations of NESSAT during 1977-2007 with a correlation coefficient between the predicted and observed NESSAT of 0.87（accounting for 76%of total variance） and a mean absolute error（MAE） of 0.3℃.A cross-validation test during 1977-2008 demonstrates that the model has good predictive skill,with MAE of 0.4℃and a correlation coefficient between the predicted and observed NESSAT of 0.76.

Summer Extreme Temperatures over East China during 1984-2004 Simulated by LASG/IAP Regional Climate Model CREM

The authors examine extreme summer temperatures over East China during 19844004 using a regional climate model named CREM （the Climate version of Regional Eta-coordinate Model）, which was developed by LASG/IAP. The results show that the main features of the extreme summer temperatures over East China are reproduced well by CREM, and the skill for the minimum temperature is higher than that for the maximum tem- perature, especially along the Yangtze-Huai River Valley （YHV）. The simulated extreme temperatures are lower than those of observation, especially for the maximum temperature. The bias of extreme temperatures is consistent with the cold bias of the climatological mean summer surface air temperature. The skill of the model in simulating the interannual variability of extreme temperatures increases from north to south. The simulated interannual variation of the minimum temperature is more reasonable than the maximum temperature. The underestimation of net solar radiation at the surface leads to a cold bias of the climatological mean temperature. Furthermore, the model underestimates the light and moderate rain, while overestimates heavy rain. It causes the simulated minimum temperature more reasonable than the maximum temperature.

Global Annual Mean Surface Air Temperature Anomalies and Their Link with Indian Summer Monsoon Failures

Analysis of the global mean annual temperature anomalies based on land and marine data for the last 88 years (1901-1988) of this century has been carried out with a view to find any relationship with failures in Indian summer monsoon rainfall. On the climatological scale (i.e. 30 years) it has been noticed that there is an abnormal increase in the frequency of drought years during epochs of global warming and cooling, while it is considerably less when global temperatures are near normal. Results are unchanged even when the data are filtered out for ENSO (El-Nino Southern Oscillation) effect.It has also been noticed that during warm and cold epochs in global temperatures the amount of summer monsoon rainfall decreases as compared to the rainfall during a normal temperature epoch.

Characteristics Analysis of the Temperature inside and outside the Themometer Shelter in Changsha during Summer

A statistical analysis on the simultaneous observation data of the temperature inside and outside the themometer shelter in Changsha during summer of 2011 and 2012 was carried out. The results show that the temperature outside the themometer shelter is higher（ lower） than that inside the themometer shelter in the daytime（ nighttime）. However,there exists a similar variation trend for the average temperature and the extremely maximum temperature inside and outside the themometer shelter in summer. The average temperature and the extremely maximum temperature outside the themometer shelter are higher than that inside the themometer shelter. Furthermore,there exists a difference between the temperatures inside and outside the themometer shelter during different weather conditions. The difference of average temperature reaches 1. 2 ℃ in rainy day,2. 8 ℃ in cloudy day and 3. 1 ℃ in sunny day,and the extremely maximum temperature difference reaches 6. 4 ℃. Especially for the days of high temperature ≥35 ℃,there is 61 high temperature days inside the themometer shelter,while there is 125 high temperature days inside the themometer shelter during 2011- 2012. Furthermore,the extremely maximum temperatures inside and outside the themometer shelter are 38. 9 and42. 0 ℃ respectively. Rainfall and showery rain are the most effective ways to relieve the high temperature in summer. Therefore,the difference between the temperatures inside and outside the themometer shelter should be taken into account in the high temperature forecast and public meteorological service.

Circulation anomalies in the mid–high latitudes responsible for the extremely hot summer of 2018 over northeast Asia

This study investigated the contributions of mid–high-latitude circulation anomalies to the extremely hot summer(July and August;JA)of 2018 over Northeast Asia(NEA).The JA-mean surface air temperature in 2018 was 1.2°C higher than that of the 1979–2018 climatology,with the amplitude of such an anomaly almost doubling the interannual standard deviation,making 2018 the hottest year during the analysis period 1979–2018.The abnormal warming over NEA was caused by a local positive geopotential height anomaly reaching strongest intensity in JA 2018.Further investigation suggested that the upper-tropospheric circulation anomalies over northern Europe and the Caspian Sea were crucial to forming this NEA circulation anomaly through initiating downstream wave trains.Particularly,the geopotential heights over these two regions were concurrently at their highest in JA 2018,and therefore jointly contributed to the profound circulation anomaly over NEA and the hottest summer on record.Due to these two teleconnection patterns,the temperature anomalies in NEA are closely related to those in both northern Europe and the Caspian Sea,where the similarly extreme warming also happened in 2018.

Impacts of Polar Vortex,NPO,and SST Configurations on Unusually Cool Summers in Northeast China.Part I:Analysis and Diagnosis

This study unveils the evolution of two major early signals in the North Pacific atmosphere-ocean system that heralded abnormal high-pressure blockings and cold-vortex activities across Northeast China, based on an analysis of the configurations of major modes including the polar vortex, the North Pacific Oscillation （NPO）, and SST in the preceding winter and spring and atmospheric low-frequency disturbances in Northeast China. We analyzed these aspects to understand the atmosphere ocean physical coupling processes characterized by the two early signals, and here we explain the possible mechanisms through which dipole circulation anomalies affect the summer low-temperature processes in Northeast China. We further analyzed the interdecadal variation background and associated physical processes of the two early signals.

INTERDECADAL VARIATIONS OF INTERACTION BETWEEN NORTH PACIFIC SSTA AND EAST ASIAN SUMMER MONSOON

Identification of key SST zones is essential in predicting the weather / climate systems in East Asia. With the SST data by the U.K. Meteorological Office and 40-year geopotential height and wind fields by NCAR / NCEP, the relationship between the East Asian summer monsoon and north Pacific SSTA is studied, which reveals their interactions are of interdecadal variation. Before mid-1970’s, the north Pacific SSTA acts upon the summer monsoon in East Asia through a great circle wavetrain and results in more rainfall in the summer of the northern part of China. After 1976, the SSTA weakens the wavetrain and no longer influences the precipitation in North China due to loosened links with the East Asian summer monsoon. It can be drawn that the key SST zones having potential effects on the weather / climate systems in East Asia do not stay in one particular area of the ocean but rather shift elsewhere as governed by the interdecadal variations of the air-sea interactions. It is hoped that the study would help shed light on the prediction of drought / flood spans in China.

Mechanism of Varve Formation and Paleoenvironmental Research at Lake Bolterskardet, Svalbard, the Arctic

On the basis of observation of thin sections and ^137Cs data, laminations in sediment are interpreted to be varves in Bolterskardet Lake （78°06′ N, 16°01′ E）, Svalbard, the Arctic. Varves appear under a petrologic microscope as couplets of dark-silt and light-clay layers. The mechanism of varve formation is surmized as follows： each silt layer is the production of sediment inflow interpreted as mainly derived from snowmelt during summer; each clay layer was deposited in a stillwater environment during an ice-cover period. A light -clay layer provides an important index bed to identify the annual interface. The high accumulation rates, long period of ice cover, and topographically closed basin are probably all critical factors in forming and preserving varves. Varve thickness is known to be controlled mainly by summer temperature. The variation of varve thickness in Lake Bolterskardet can then be used to reconstruct summer temperature. The varve series show that there has been distinct decade-scale variability of summer temperature over the past 150 years. Warm periods occurred in the 1860s, around 1900, the 1930s, 1950s, and 1970s, and in the last 20 years. The varved sediments of Lake Bolterskarde preserve an ideal record for high-resolution paleoclimatic and paleoenvironmental research in this data-sparse area.

The 2009 Summer Low Temperature in Northeast China and Its Association with Prophase Changes of the Air-Sea System

Under the background of global warming, summer （JJA） low temperature events in Northeast China had not occurred for about 15 yr since 1994, but one such event took place in 2009. By using the NCEP/NCAR reanalysis data, the 100-yr station temperature data at Harbin and Changchun, and the Hadley Center sea surface temperature （SST） data, this paper intends to reveal the cause, circulation background, and influencing factors of this event. Analysis of both horizontal and vertical circulations of a low-value system over Northeast China in summer 2009 during the low temperature event shows that anomalous activities of the Northeast China cold vortex （NECV） played the most direct role. A decadal cooling trend of - 0.8℃ （10 yr）-1 over 1999-2008 at Changchun and Harbin was found, which is obviously out-of-phase with the linear warming trend （0.2℃ （10 yr）-1） over 1961-2000 for Northeast China in response to the global warming. The previous winter North Pacific polar vortex （NPPV） area index, significantly positively related to the observed summer temperatures of Harbin and Changchun, was also in a significantly declining tendency. These provide favorable decadal backgrounds for the 2009 low temperature event. Different from the average anomaly field of 500-hPa height for summer 1994-2008 in Northeast China, in the summer of 2009, the Arctic Oscillation （AO） showed a strong negative phase distribution, and significant negative height anomalies dominated Northeast Asia, Aleutian Islands, and North Atlantic. Furthermore, the negative phase of North Pacific Oscillation （NPO） in the winter of 2008 was obviously strong, and it maintained in the spring of 2009. Meanwhile, the SSTA in the equatorial eastern-central Pacific Ocean in the winter of 2008 showed a La Nina phase, but the strength of the La Nina weakened obviously in the spring of 2009. The abnormally strong activities of NECV in June and July of 2009 were related to the disturbances of stationary waves that replaced the original ultra-long waves over the North Pacific region in April and May 2009. The singular value decomposition （SVD） and harmonic analysis results suggest that the anomalous phase of NPO is an important precursor for summer temperature variations over Northeast China, and also a stable planetary-scale component that can be extracted from the atmospheric circulation in addition to the chaotic components on the synoptic scale.

Evaluation of multidimensional simulations of summer air temperature in China from CMIP5 to CMIP6 by the BCC models: From trends to modes

To determine whether the capability of the CMIP6 version of Beijing Climate Center(BCC)models(BCC-ESMI and BCC-CSM2-MR)in simulating China summer surface air temperature(SAT)has improved,we presented a multidimensional evaluation of the summer SAT in China including the trends,modes,and influencing factors.Critical comparisons are also made with the results of CMIP5(BCC-CSM1.1 and BCC-CSMl.lm).In general,the CMIP6,especially BCC-CSM2-MR,has smaller deviations in the trends,the means,the mutations,the maximum centers,the variances,and the spatial patterns of the dominant modes from observatio relative to those of CMIP5.However,the BCC CMIP6 models still underestimate the SAT variation in the Qinghai-Tibetan Plateau and the northeastern regions of China,and the performance is unsatisfactory with respect to the physical drivers of the dominant modes.Importantly,all the BCC models can capture the spatio-temporal characteristics of the first mode well and can,in general,characterize the spatial pattern of the second mode,but none of the models perform well in the principal component of the second mode(PC2)due to the low performance with respect to the interannual variation of PC2.Furthermore,the factors influencing the leading two modes are evaluated.The two CMIP6 can simulate better the Northern Hemisphere subtropical high northern boundary affecting the first mode.Another factor,the Asia polar vortex area,can only be simulated better by two low-resolution models(BCC-CSM1.1 and BCC-ESM I).For the second mode,all four models simulate the influence of Asian zonal circulation well,but poorly simulate that of the southern Indian Ocean dipole due to a large deviation in the Indian Ocean surface temperature.

EFFECTS OF THE SEA-ICE ALONG THE NORTH PACIFIC ON SUMMER RAINFALL IN CHINA

By using statistic method,the effects of the abnormal sea-ice along the North Pacific on both summer rainfall and temperature in China and the corresponding distribution of atmospheric circulation during the period from late winter to early spring are explored in this article.The analysis indicates that the percentage of the sea-ice area along the North Pacific is the biggest during February-April,so is the distribution of the standard variance.Thus,the period of February-April is the natural winter of the sea-ice along the North Pacific.A quasi-four-year oscillation exists in the sea-ice-area-index during this period. The abnormal sea-ice area has significant effects on both summer rainfall and temperature of China.At the 500 hPa geopotential height field corresponding to the years with large values of sea- ice areas,the polar vortex in May is obviously stretched to east and south.The subtropical high over West Pacific strengthens enormously and extends northwards,and the Mongolian trough deepens.The deepening maintains till June and July.The summer meridional cell over the East Asia increases,with the southerly being able to reach high latitudes.As a result,the rainfall increases over the areas north of the Yangtze River and decreases over the areas south of the Yangtze River obviously in China.In the temperature field,it is shown that the temperature is lower over most areas in northern China.and higher in southern China.There are opposite situations in the years when sea-ice index is small.

Heat wave mitigation of ecosystems in mountain areas-a case study of the Upper Yangtze River basin

Background:Natural ecosystems,such as forests and grasslands,can mitigate heat waves,but research on heat wave mitigation in mountain ecosystems is lacking.Considering the upper reaches of the Yangtze River basin(URYB)as the study area,we first divided the URYB into mountainous and non-mountainous areas based on a digital elevation model(DEM).Then,we used temperature to identify heat waves and used the temperature and humidity index to identify suitable days.Finally,the differences between mountainous and non-mountainous areas were compared,and regional development strategies are proposed.Result:(1)The frequency and duration of heat waves increased by 0.433/y(P<0.1),0.07/y(P<0.05),while the number of suitable days in June,July,and August gradually decreased slightly by 0.0096/y(P<0.1),0.0125/y(P<0.1),0.004/y(P<0.1),respectively from 1986 to 2015.(2)The average monthly number of suitable days was the highest in mountainous areas during the summer from 1986 to 2015.Mountainous areas have an advantage in mitigating heat waves owing to landform-based changes in the local climate.(3)In addition to the landform,altitude was the main factor responsible for mitigating heat waves in mountainous areas.Conclusion:Parts of the plateau and whole mountain areas were found to be best for avoiding extremely high temperatures based on the number of suitable summer days from 1986 to 2015.We suggest formulating tourism promotion strategies,and strengthening tourism infrastructure,considering landscape protection and creation to promote sustainable regional development.