月基甚低频射电干涉仪Tripole天线特性研究

对月基甚低频射电干涉仪所用Tripole矢量有源天线进行理论分析,计算了不同激励下的辐射方向图以及给定方向图时所需的激励。不同于传统的天线阵列方法,单一的Tri-pole(三阵子)天线即可实现对入射波达波方向(DOA)的估计。运用改进的基于单位矩阵束(UMP)和最小二乘法(LS)的算法实现了对Tripole天线达波方向的估计,该算法适用于对入射波不同频率分量的DOA估计,计算量小,且估计精度更高,仿真结果表明了算法的有效性。

Impact of the North Atlantic Sea Surface Temperature Tripole on the East Asian Summer Monsoon

A strong （weak） East Asian summer monsoon （EASM） is usually concurrent with the tripole pattern of North Atlantic SST anomalies on the interannual timescale during summer, which has positive （negative） SST anomalies in the northwestern North Atlantic and negative （positive） SST anomalies in the subpolar and tropical ocean. The mechanisms responsible for this linkage are diagnosed in the present study. It is shown that a barotropie wave-train pattern occurring over the Atlantic-Eurasia region likely acts as a link between the EASM and the SST tripole during summer. This wave-train pattern is concurrent with geopotential height anomalies over the Ural Mountains, which has a substantial effect on the EASM. Diagnosis based on observations and linear dynamical model results reveals that the mechanism for maintaining the wave-train pattern involves both the anomalous diabatic heating and synoptic eddy-vorticity forcing. Since the North Atlantic SST tripole is closely coupled with the North Atlantic Oscillation （NAO）, the relationships between these two factors and the EASM are also examined. It is found that the connection of the EASM with the summer SST tripole is sensitive to the meridional location of the tripole, which is characterized by large seasonal variations due to the north-south movement of the activity centers of the NAO. The SST tripole that has a strong relationship with the EASM appears to be closely coupled with the NAO in the previous spring rather than in the simultaneous summer.

Unprecedented East Asian warming in spring 2018 linked to the North Atlantic tripole SST mode

An unusually warm East Asia in spring 2018,when exceptionally high surface air temperatures were recorded in large areas of Asia,such as northern China,southern China,and Japan,was investigated based on the ERA-Interim reanalysis.The East Asian warming anomalies were primarily attributed to a tripole mode of North Atlantic SST anomalies,which could have triggered anomalous Rossby wave trains over the North Atlantic and Eurasia through modulating the North Atlantic baroclinic instability.Atlantic-forced Rossby waves tend to propagate eastward and induce anomalously high pressure and anticyclonic activity over East Asia,leading to a northward displacement of the Pacific subtropical high.As a result,descending motion,reduced precipitation,and increased surface solar radiation due to less cloud cover appear over East Asia,accompanied by remarkably warm advection from the ocean to southern China,northern China,and Japan.The transportation of anomalously warm advection and the feedbacks between soil moisture and surface temperature were both favorable for the recordbreaking warmth in East Asia during spring 2018.The seasonal‘memory’of the North Atlantic tripole SST mode from the previous winter to the following spring may provide useful implications for the seasonal prediction of East Asian weather and climate.

The north-east North Atlantic Tripole implicated as a predictor of the August precipitation decadal variability over north China

Monthly precipitation over north China in August(NCAP)is the second highest in the year,and it is important to understand its driving mechanisms to facilitate reliable forecasting.The NCAP displays a significant decadal variability of a cycle about 10-year and negatively correlates with the July north-east North Atlantic Tripole(NAT)over the decadal timescales.This study shows that the Eurasian decadal teleconnection(EAT)acts as a bridge that links the July NAT with NCAP decadal variability.This coupled ocean–atmosphere bridge(COAB)mechanism,through which the July NAT influences the decadal variability of NCAP,can be summarized as follows.The cumulative effect of the NAT drives the EAT to adjust atmospheric circulation over north China and the surrounding regions,and so regulates precipitation in north China by influencing local water vapor transport.When the July NAT is in a negative(positive)phase,the EAT pattern has a positive(negative)pattern,which promotes(weakens)the transmission of water vapor from the sea in the south-east to north China,thus increasing(decreasing)NCAP over decadal timescales.The decadal NCAP model established based on the July NAT can effectively predict the NCAP decadal variability,illustrating that the July NAT can be implicated as a predictor of the NCAP decadal variability.

Teleconnection between Sea Ice in the Barents Sea in June and the Silk Road,Pacific–Japan and East Asian Rainfall Patterns in August

In contrast to previous studies that have tended to focus on the influence of the total Arctic sea-ice cover on the East Asian summer tripole rainfall pattern, the present study identifies the Barents Sea as the key region where the June sea-ice variability exerts the most significant impacts on the East Asian August tripole rainfall pattern, and explores the teleconnection mechanisms involved. The results reveal that a reduction in June sea ice excites anomalous upward air motion due to strong near-surface thermal forcing, which further triggers a meridional overturning wave-like pattern extending to midlatitudes.Anomalous downward motion therefore forms over the Caspian Sea, which in turn induces zonally oriented overturning circulation along the subtropical jet stream, exhibiting the east–west Rossby wave train known as the Silk Road pattern. It is suggested that the Bonin high, a subtropical anticyclone predominant near South Korea, shows a significant anomaly due to the eastward extension of the Silk Road pattern to East Asia. As a possible descending branch of the Hadley cell, the Bonin high anomaly ultimately triggers a meridional overturning, establishing the Pacific–Japan pattern. This in turn induces an anomalous anticyclone and cyclone pair over East Asia, and a tripole vertical convection anomaly meridionally oriented over East Asia. Consequently, a tripole rainfall anomaly pattern is observed over East Asia. Results from numerical experiments using version 5 of the Community Atmosphere Model support the interpretation of this chain of events.

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.