Impact of Preceding Summer North Atlantic Oscillation on Early Autumn Precipitation over Central China

This study examined the impact of the preceding boreal summer(June–August) North Atlantic Oscillation(NAO) on early autumn(September) rainfall over Central China(RCC). The results show that a significant positive correlation exists between the preceding summer NAO and the early autumn RCC on the interannual timescale. In order to understand the physical mechanism between them, the role of ocean was investigated. It was found that the strong summer NAO can induce a tripole sea surface temperature anomaly(SSTA) in the North Atlantic; this SSTA pattern can persist until early autumn. The diagnostic analysis showed that the tripole SSTA pattern excites a downstream Atlantic-Eurasian(AEA) teleconnection, which contributes to an increase in RCC. The circulation anomalies related to SSTA caused by the weak NAO are opposite, so the RCC is less than normal. The results imply that the preceding summer NAO may be regarded as a forecast factor for the early autumn RCC.

Teleconnection between Winter Arctic Oscillation and Southeast Asian Summer Monsoon in the Pre-Industry Simulation of a Coupled Climate Model

A 600-year pre-industrial simulation with Bergen Climate Model(BCM)Version 2 is used to investigate the linkage between winter Arctic Oscillation(AO)and the Southeast Asian summer monsoon(SEASM)on the inter-decadal timescale.The results indicate an in-phase relationship between the AO and SEASM with periods of approximately 16–32 and 60–80 years.During the positive phase of winter AO,an anomalous surface anti-cyclonic atmosphere circulation appears over North Pacific in winter.The corresponding anomalies in ocean circulation and surface heat flux,particularly the latent and sensible heat flux,resemble a negative Pacific Decadal Oscillation(PDO)-like sea surface temperature(SST)pattern.The AO-associated PDO-like winter SST can persist into summer and can therefore lead to inter-decadal variability of summer monsoon rainfall in East and Southeast Asia.

Potential Correlation between the Decadal East Asian Summer Monsoon Variability and the Pacific Decadal Oscillation

This study discusses the potential contribution of the Pacific decadal oscillation(PDO)to the weakening of the East Asian summer monsoon(EASM)and the evident correlation between the positive PDO and"Southern flood and Northern drought(SFND)"summer rainfall pattern over East China.The mechanism behind this contribution is also discussed.

Dominant modes of geopotential height in the northern hemisphere in summer on interdecadal timescales

Empirical orthogonal function （EOF） analysis is performed on the field of the northem hemisphere geopotential height at 200-hPa using a 54-year （1958-2011） record of summer data on an interdecadal time scale. The first dominant mode, which shows smooth semi-hemispheric variation with maximum action centers in the western hemisphere in the mid-latitudes over the eastern Pacific, North America, and the North Atlantic, is related to global warming. The second mode, which has a pronounced tropical-extratropical alternating pattern with active centers located over the eastern hemisphere from Western Europe across East Asia to the western Pacific, has a close relationship with the Arctic Oscillation. Further analysis results indicate that the two dominant modes show good correlation with the Arctic sea ice concentration （SIC）, with correlation coefficients between these two modes and the first two EOF modes of the Arctic SIC reaching 0.88 and 0.86, respectively.

Interdecadal change in the South Asian summer monsoon rainfall in 2000 and contributions from regional tropical SST

The drying trend in the South Asian summer monsoon(SASM)area has been a focus of monsoon rainfall studies in the last two decades.However,this study reveals that a signi cant interdecadal change in the SASM rainfall occurred in approximately the year 2000.Obvious spatial inhomo-geneity was a feature of this change,with increased rainfall over the southern part of the India Pakistan border area that extends from the Arabian Sea,as well as in the western Bay of Bengal.Furthermore,there was decreased rainfall over the southern SASM and the western coast of the Indian Peninsula.Numerical experiments using CAM4 show that global SST changes can induce general changes in the SASM circulation consistent with observations.The tropical Pacific/Indian Ocean SST anomalies dominated the Walker and the regional Hadley circulation changes,respectively,while the descending motion anomalies over the southern SASM were further enhanced by the warmer tropical Atlantic SSTs.Moreover,the spatial inhomogeneity of this interdecadal change in the SASM rainfall needs further study.

Interdecadal change of the connection between winter North Pacific Oscillation and summer precipitation in the Huaihe River valley

The relationship between the winter North Pacific Oscillation (NPO) and the following summer precipitation in the Huaihe River valley before and after the mid-1970s is investigated by using the Chinese 160-station precipitation data and NCEP/NCAR reanalysis data from 1951 to 2008.It is found that their linkage appears to have an apparent interdecadal variation.Before the mid-1970s, there was a prominent out-of-phase relationship between the winter NPO and the summer precipitation in the Huaihe River valley.However, such a relationship is significantly weakened afterwards.The change of atmospheric circulations related to the winter NPO before and after the mid-1970s is further addressed.Before the mid-1970s, a strong (weak) winter NPO was followed by the summer situations with anomalous low-level cyclonic (anticyclonic) circulation over the western Pacific and descending (ascending) over the Huaihe River valley.Meanwhile, the water vapor transporting to the Huaihe River valley was reduced (enhanced).These conditions are unfavorable (favorable) for the precipitation occurring in the Huaihe River valley, and thus the local precipitation was decreased (increased).After the mid-1970s, however, the impact of winter NPO on the summer atmospheric circulation system associated with the rainfall in the Huaihe River valley becomes diluted, thereby weakening its linkage to the summer precipitation in the Huaihe River valley.