Precursor Role of Winter Sea-Ice in the Labrador Sea for Following-Spring Precipitation over Southeastern North America and Western Europe

The role of winter sea-ice in the Labrador Sea as a precursor for precipitation anomalies over southeastern North America and Western Europe in the following spring is investigated. In general terms, as the sea ice increases, the precipitation also increases. In more detail, however, analyses indicate that both the winter sea-ice and the sea surface temperature(SST)anomalies related to increases in winter sea-ice in the Labrador Sea can persist into the following spring. These features play a forcing role in the spring atmosphere, which may be the physical mechanism behind the observational relationship between the winter sea-ice and spring precipitation anomalies. The oceanic forcings in spring include Arctic sea-ice anomalies and SST anomalies in the tropical Pacific and high-latitude North Atlantic. Multi-model Coupled Model Intercomparison Project Phase 5 and Atmospheric Model Intercomparison Project simulation results show that the atmospheric circulation response to the combination of sea-ice and SST is similar to that observed, which suggests that the oceanic forcings are indeed the physical reason for the enhanced spring precipitation. Sensitivity experiments conducted using an atmospheric general circulation model indicate that the increases in precipitation over southeastern North America are mainly attributable to the effect of the SST anomalies, while the increases over Western Europe are mainly due to the sea-ice anomalies. Although model simulations reveal that the SST anomalies play the primary role in the precipitation anomalies over southeastern North America, the observational statistical analyses indicate that the area of sea-ice in the Labrador Sea seems to be the precursor that best predicts the spring precipitation anomaly.

Changing characteristics and spatial differentiation of spring precipitation in Southwest China during 1961-2012

In this study,we analyze spring precipitation from 92 meteorological stations spanning between 1961 and 2012 to understand temporal-spatial variability and change of spring precipitation over Southwest China.Various analysis methods are used for different purposes,including empirical orthogonal function（EOF） analysis and rotated EOF（REOF） for analyzing spatial structure change of precipitation anomaly,and the Mann-Kendall testing method to determine whether there were abrupt changes during the analyzed time span.We find that the first spatial mode of the precipitation has a domain uniform structure;the second is dominated by a spatial dipole;and the third contains five variability centers.The 2000 s is the decade with the largest amount of precipitation while the 1990 s is the decade with the smallest amount of precipitation.The year-to-year difference of that region is large：the amount of the largest precipitation year doubles that of the smallest precipitation year.We also find that spring precipitation in Southwest China experienced a few abrupt changes：a sudden increase at 1966,a sudden decrease at 1979,and a sudden increase at 1995.We speculate that the spring precipitation will increase gradually in the next two decades.

CLIMATOLOGICAL CHARACTERISTICS OF SPRING PRECIPITATION OVER SOUTHERN CHINA AND ITS INTRASEASONAL OSCILLATION

Based on observations and reanalysis data,the characteristics of the evolution of climatological spring precipitation over Southern China(SPSC) and the associated climatological intraseasonal oscillation(CISO) and atmospheric circulation are studied.Results show that SPSC increases in an oscillatory way.Although the evolution of SPSC is similar in different regions,there are also differences.In different regions of Southern China,the onset dates of the rain season are from the 12 th to 24 th pentad and the peak dates are after the 20 th pentad.CISO is an important component of SPSC,which is not only statistically significant,but also accompanies a dynamically coherent structure.The peak wet/dry phase of each CISO cycle corresponds to a significant rainfall increasing/decreasing period and modulates the evolution of SPSC.The rainfall growth in the second half of March and mid-April is the result of the modulation.The wet/dry phase of CISO is accompanied by low-level convergent(upper-level divergent) and cyclonic(anti-cyclonic) circulation,which favors ascending motion to develop over Southern China.

Correlation and SVD Analysis of Anomalous Spring Precipitation in Northwest China and Sea Surface Temperature in Key Region in Recent 50 Years

[Objective] The aim was to study the relationship between spring precipitation anomaly in Northwest China and sea surface temperature anomaly (SSTA) in Key region in recent 50 years. [Method] Based on monthly average precipitation in Northwest China and global monthly sea surface temperature (SST) grid data, the effects of SSTA in equatorial central and eastern Pacific on spring precipitation in Northwest China were discussed by means of correlation and SVD analysis. [Result] For spring precipitation in Northwest China, the key period of SST was from August to September in the former year, and the equatorial central and eastern Pacific (125°-85° W, 5° S-10° N) was named 'Key region'. Correlation analysis showed that there was obviously positive correlation between spring precipitation in Northwest China and SST (monthly average from August to September in the former year) in equatorial central and eastern Pacific, especially Key region. SVD analysis revealed that spring precipitation anomaly in Northwest China distributed in phase, while SST in equatorial central and eastern Pacific from August to September in the former year had higher positive correlation coefficient, and there was obviously positive couple correlation in Key region. [Conclusion] The study could provide theoretic foundation for the prediction of drought and flood in Northwest China.

Influence of the Eastern Indian Ocean Warm Pool Variability on the Spring Precipitation in China

The relationship between the variability of the Eastern India Ocean Warm Pool (EIWP) and the spring precipitation in China is studied in the paper based on an analysis of the Simple Ocean Data Assimilation (SODA) Sea Surface Temperature (SST) data, the reanalysis data of monthly grid wind field at 925 hPa with a resolution of 2.5° latitude and longitude from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR),and the monthly mean rainfall data from 160 observational stations in China. The results show that there is a strong correlation between the EIWP variability and the spring precipitation in China. The area, volume and intensity indices of the EIWP are negatively correlated with the spring precipitation in southwestern China, while they are positively correlated with the spring precipitation in the rest of China, especially in the northeast. For this correlation between the EIWP variability and the spring precipitation in China, it is found that the correlative relationship is mainly connected with the variations of the moisture transport by the warm air flow, which is under the influence of the EIWP variability, into the inland of China in spring. Two causative factors may influence this transport. One is the variation of the moisture transport carried by the warm air flow from the Arabian Sea influenced by the EIWP variability. The other is the variation of the equator-crossing flow (70°-90°E) influenced by the EIWP anomaly in the previous winter which exerts its effect on the moist warm air transported from the Southern Hemisphere. The position and intensity of the Western North Pacific Subtropical High (WNPSH)variability caused by EIWP variation also influence the spring precipitation in China.

Reduced Spring Precipitation Bias and Associated Physical Causes over South China in FGOALS-f3 Climate Models:Experiments with the Horizontal Resolutions

Considerable spring precipitation occurs over South China(SC),a region that is adjacent to large-scale Asian topography and oceans.Its reasonable simulation is crucial for improving regional climate predictability.This study investigates spring precipitation biases over SC and their possible causes in atmosphere-only and coupled Flexible Global Ocean–Atmosphere–Land System finite-volume version 3(FGOALS-f3) models with different horizontal resolutions.The performance of spring precipitation simulation over SC varies across different FGOALS-f3 model versions,with the best reproducibility in the high-resolution coupled model(25 km).In the low-resolution atmosphere-only model(100–125 km),the precipitation dry bias over SC is closely linked to overestimated surface sensible forcing over the eastern Tibetan Plateau(TP),which weakens the subtropical anticyclone over the western Pacific(SAWP) through regional circulation responses.By contrast,the high-resolution atmosphere-only model further amplifies surface thermal forcing in the Asian continents,causing intensified land–sea thermal contrast between the Southeast Asian continents and western Pacific,enhanced southerly winds and SAWP,and increased water vapor transport into SC.Meanwhile,the reduced middle–high level cold bias over 10°–30°N in the high-resolution atmosphere-only model intensifies the East Asian westerly jet and ascent over SC,leading to enhanced spring precipitation there.The high-resolution coupled model simulation not only reduces sea surface cold bias over the Bay of Bengal,thus intensifying the Indian–Burma trough and strengthening low-level water vapor transport into SC,but also enhances ascent over SC.As a result,the high-resolution coupled model better reproduces the magnitude and pattern of spring precipitation over SC than its atmosphere-only model.Compared with low-resolution models,the domain-mean spring precipitation dry bias decreases by 11.2% over SC in the high-resolution atmosphere-only model and by 35.9% in the coupled model.These results demonstrate that the high-resolution FGOALS-f3 models can improve simulations of the influencing atmospheric circulations and spring precipitation over SC.

Diversity on the Interannual Variations of Spring Monthly Precipitation in Southern China and the Associated Tropical Sea Surface Temperature Anomalies

There is a continuous and relatively stable rainy period every spring in southern China(SC).This spring precipitation process is a unique weather and climate phenomenon in East Asia.Previously,the variation characteristics and associated mechanisms of this precipitation process have been mostly discussed from the perspective of seasonal mean.Based on the observed and reanalysis datasets from 1982 to 2021,this study investigates the diversity of the interannual variations of monthly precipitation in spring in SC,and focuses on the potential influence of the tropical sea surface temperature(SST)anomalies.The results show that the interannual variations of monthly precipitation in spring in SC have significant differences,and the correlations between each two months are very weak.All the interannual variations of precipitation in three months are related to a similar western North Pacific anomalous anticyclone(WNPAC),and the southwesterlies at the western flank of WNPAC bring abundant water vapor for the precipitation in SC.However,the WNPAC is influenced by tropical SST anomalies in different regions each month.The interannual variation of precipitation in March in SC is mainly influenced by the signal of El Nino-Southern Oscillation,and the associated SST anomalies in the equatorial central-eastern Pacific regulate the WNPAC through the Pacific-East Asia(PEA)teleconnection.In contrast,the WNPAC associated with the interannual variation of precipitation in April can be affected by the SST anomalies in the northwestern equatorial Pacific through a thermally induced Rossby wave response.The interannual variation of precipitation in May is regulated by the SST anomalies around the western Maritime Continent,which stimulates the development of low-level anomalous anticyclones over the South China Sea and east of the Philippine Sea by driving anomalous meridional vertical circulation.

THE EFFECT OF EQUATORIAL CENTRAL PACIFIC SSTAS ON THE INTERANNUAL VARIATION OF PRECIPITATION OVER SOUTHWEST CHINA DURING SPRING

The interannual variations of rainfall over southwest China(SWC) during spring and its relationship with sea surface temperature anomalies(SSTAs) in the Pacific are analyzed, based on monthly mean precipitation data from 26 stations in SWC between 1961 and 2010, NCEP/NCAR re-analysis data, and Hadley global SST data. Sensitivity tests are conducted to assess the response of precipitation in SWC to SSTAs over two key oceanic domains, using the global atmospheric circulation model ECHAM5. The interannual variation of rainfall over SWC in spring is very significant.There are strong negative(positive) correlation coefficients between the anomalous precipitation over SWC and SSTAs over the equatorial central Pacific(the mid-latitude Pacific) during spring. Numerical simulations show that local rainfall in the northwest of the equatorial central Pacific is suppressed, and a subtropical anticyclone circulation anomaly is produced, while a cyclonic circulation anomaly in the mid-latitude western Pacific occurs, when the equatorial Pacific SSTAs are in a cold phase in spring. Anomalous northerly winds appear in the northeastern part of SWC in the lower troposphere. Precipitation increases over the Maritime Continent of the western equatorial Pacific, while a cyclonic circulation anomaly appears in the northwest of the western equatorial Pacific. A trough over the Bay of Bengal enhances the southerly flow in the south of SWC. The trough also enhances the transport of moisture to SWC. The warm moisture intersects with anomalous cold air over the northeast of SWC, and so precipitation increases during spring. On the interannual time scale, the impacts of the mid-latitude Pacific SSTAs on rainfall in SWC during spring are not significant, because the mid-latitude Pacific SSTAs are affected by the equatorial central Pacific SSTAs; that is,the mid-latitude Pacific SSTAs are a feedback to the circulation anomaly caused by the equatorial central Pacific SSTAs.

Strengthened influence of the East Asian trough on spring extreme precipitation variability over eastern Southwest China after the late 1980s

The relationship between variations in the East Asian trough(EAT)intensity and spring extreme precipitation over Southwest China(SWC)during 1961-2020 is investigated.The results indicate that there is an interdecadal increase in the relationship between the EAT and spring extreme precipitation over eastern SWC around the late1980 s.During the latter period,the weak(strong)EAT corresponds to a strong and large-scale anomalous anticyclone(cyclone)over the East Asia-Northwest Pacific region.The EAT-related anomalous southerlies(northerlies)dominate eastern SWC,leading to significant upward(downward)motion and moisture convergence(divergence)over the region,providing favorable(unfavorable)dynamic and moisture conditions for extreme precipitation over eastern SWC.In contrast,during the former period,the EAT-related circulation anomalies are weak and cover a relatively smaller region,which cannot significantly affect the moisture and dynamic conditions over eastern SWC;therefore,the response in extreme precipitation over eastern SWC to EAT is weak over the period.The interdecadal change in the relationship between eastern SWC spring extreme precipitation and the EAT could be related to the interdecadal change in the EAT variability.The large(small)variability of the EAT is associated with significant(insignificant)changes in spring extreme precipitation over eastern SWC during the latter(former)period.

Wavelet analysis of spring climate characteristics in arid aeolian area of agro-pastoral ecotone in China

The unique regional climate characteristics are among the main reasons for the frequent wind-sand activity in arid and cold areas in the agro-pastoral ecotone in Inner Mongolia, China. This paper focuses on the time series of temperature and precipitation in spring when sandstorms often occur in the area. Based on meteorological data for a 46-year period from 1959 to 2004, multi-scale variations and abrupt changes in temperature and precipitation were analyzed with the Mexican hat function （MHF） wavelet method, showing the multi-scale variation characteristics of temperature and precipitation, as well as the periods and change points at different time scales. The relationship between temperature and precipitation was obtained using the wavelet analysis method. Obvious staggered features of the variations of spring temperature and precipitation were observed in this agro-pastoral ecotone. The strongest oscillation periods of spring temperature variations were 1 and 22 years, while for precipitation, the strongest oscillation periods of variations were 2, 8, and 22 years. In addition, lower spring temperature corresponded to lower precipitation, whereas higher temperature yielded higher precipitation rate.