Interannual Variation of the Onset of Yunnan’s Rainy Season and Its Relationships with the Arctic Oscillation of the Preceding Winter

Based on an analysis of the circulation in May associated with the interannual variation of the onset of Yunnan’s rainy season, this study examined the rela-tionship between Arctic Oscillation (AO) and the onset timing of the rainy sea-son by using the NCEP/NCAR reanalysis and observational precipitation data for 1961-2010. The results indicated that, on an interannual time scale, intense Asian summer monsoon and an active EU-pattern wave train circulation in its positive phase, associated with a cold cyclonic cell covering the western part of the East Asian subtropical westerly jet (EASWJ), jointly contributed to the onset of the rainy season in May. Otherwise, the onset might be suppressed. The cold cyclonic cell over East Asia likely led to the southward shift and enhancement of EASWJ as well as its secondary circulation around the jet entrance, which could provide a favorable dynamic and thermal condition for rainfalls in Yunnan as was revealed in previous studies on 10 - 30-day time scale. Further examination showed that the preceding wintertime AO played a significant role in the timing of the onset of the rainy season before the mid-1980s’ by mostly modulating the wave-train-like circulation over East Asia in May. During that time period, when the AO index of the previous winter was positive (negative), Yunnan’s rainy season tended to begin earlier (later) than normal. Correspond-ingly, the precipitation in May was also closely linked to wintertime AO.

North Atlantic forcing of autumn drought in Southwest China

Drought often lasts long and is thus closely related to slowly varying external forcing such as sea surface temperature(SST).Here,based on observed precipitation and SST data along with NCEP-DOE reanalysis data,the possible impacts of North Atlantic SST on drought formation in Southwest China are investigated.Results show that northeast-southwest-orientated dipole SST anomalies in the mid-high latitudes of the North Atlantic are closely related to autumn drought in Southwest China;the linear correlation coefficient between them reaches 0.48 during 1979-2020,significant at the 0.001 level.The dipole SST anomalies trigger southeastward-propagating Rossby waves and induce barotropic cyclonic circulation anomalies over India and the western Tibetan Plateau.This enhances the upward motion in northern India and the western Tibetan Plateau and causes a compensating downdraft,reduced precipitation,and consequent drought formation in Southwest China.

Analysis of the Causes of a Low Cloud and Low Visibility Weather at Cangyuan Washan Airport

At 11:00 am on August 5, 2017, Cangyuan Washan Airport experienced low cloud and low visibility weather, accompanied by aircraft turbulence, which affected the normal operation of flights, which was closely related to the meteorological conditions at that time. Using the hourly reanalysis data of the European Centre for Medium-range Weather Forecast (ECMWF) Reanalysis 5 (ERA5), including Geopotential height, temperature, precipitation, wind field, specific humidity, vorticity and other elements, with a spatial resolution of 0.25° × 0.25°, this paper focuses on the horizontal distribution and vertical configuration of various physical quantities before and after the occurrence of low cloud and low visibility weather at the airport. The results indicate that the main influencing system of this low cloud and low visibility weather is the westward tropical depression. Before the occurrence of low cloud and low visibility weather, low-level water vapor converges and is accompanied by precipitation. The temperature decreases with precipitation, the near-surface wind direction changes, and the wind speed decreases.

The Relationship Between Indian Ocean SST and Tropical Cyclone Genesis Frequency over North Indian Ocean in May

Tropical cyclone(TC)activities in the North Indian Ocean(NIO)peak in May during the pre-monsoon period,but the TC frequency shows obvious inter-annual variations.By conducting statistical analysis and dynamic diagnosis of long-term data from 1948 to 2016,the relationship between the inter-annual variations of Indian Ocean SST and NIO TC genesis frequency in May is analyzed in this paper.Furthermore,the potential mechanism concerning the effect of SST anomaly on TC frequency is also investigated.The findings are as follows:1)there is a broadly consistent negative correlation between NIO TC frequency in May and SST in the Indian Ocean from March to May,with the key influencing area located in the southwestern Indian Ocean(SWIO);2)the anomalies of SST in SWIO(SWIO-SST)are closely related to a teleconnection pattern surrounding the Indian Ocean,which can significantly modulate the high-level divergence,mid-level vertical motion and other related environmental factors and ultimately influence the formation of TCs over the NIO;3)the increasing trend of SWIO-SST may play an essential role in the downward trend of NIO TC frequency over the past 69 years.

A Mechanism for the Interannual Variation of the Early Summer East Asia-Pacific Teleconnection Wave Train

Based on the 500-hPa geopotential height, surface air temperature, and China summer rain-belt type data from 1978 to 2002, the spatial spectrum function sets which well represent the variation of large scale atmospheric circulations were obtained using the least square method. A mechanism for the interannual variation of the East Asia-Pacific teleconnection （EAP） wave train in early summer was identified with the low-order spectral method and the hypothesis-test method. The results indicate that, when nonlinear wave- wave and wave-flow interactions on large scale are stronger in the inner dynamic process of the atmosphere, there are obvious nonlinear features in the evolution of the atmospheric circulation, and the EAP exhibits a negative-positive-negative （＂- ＋ -＂） spatial distribution in low to high latitudes in early summer. The corresponding EAP index is positive, which leads to a northward shift of the western Pacific subtropical high （WPSH） and the China rain-belt is located in the Huaihe River valley and its north in summer. On the contrary, when nonlinear wave-wave and wave-flow interactions on large scale are weaker, there appears a linear feature in the evolution of the atmospheric circulation, and the EAP shows a positive-negative-positive （＂＋ - ＋＂） spatial distribution in low to high latitudes. The corresponding EAP index is negative, which inhibits the WPSH against moving northward, and the China rain-belt is located in the Huaihe and Yangtze River valleys and their south.

Cold Air Activities in July 2004 and Its Impact on Intense Rainfalls over Southwest China

The severe rainfall events in the mid-summer of July 2004 and the roles of cold air in the formation of heavy precipitation are investigated by using daily observational precipitation data of China and NCEP/NCAR reanalysis. The results show that the severe rainfalls in Southwest China are closely related to the cold air activities from the mid-high latitudes, and the events take place under the cooperative effects of mid-high latitude circulation and low latitude synoptic regimes. It is the merging of a cold vortex over mid-latitudes with the northward landing typhoon and eastward Southwest China Vortex, as well as the abrupt transformation from a transversal trough into an upright one that causes three large alterations of mid-high atmospheric circulation respectively in the early and middle ten days of this month. Then, the amplitude of long waves soon magnifies, leading to the unusual intrusion of cold air to low-latitude areas in the mid-summer. Meanwhile, the warm and humid southwest summer monsoon is quite active. The strong interactions of cold air and summer monsoon over Southwest China result in the large-scale convective rainfalls on the southern side of cold air. With regard to the activities of cold air, it can influence rainfalls in three prominent ways. Firstly, the incursion of upper-level cold air is often accompanied by partial southerly upper-level jet. The ascending branch of the corresponding secondary circulation, which is on the left front side of the jet center, provides the favorite dynamic upward motion for the rainfalls. Secondly, the southward movement of cold air contributes to the establishment of atmospheric baroclinic structure, which would lead to baroclinic disturbances. The atmospheric disturbances associated with the intrusion of cold air can destroy the potential instability stratification, release the convective available potential energy （CAPE） and finally cause convective activities. In addition, the advection processes of dry and cold air at the upper level along with the advection of humid and warm air at the lower level are rather significant for the reestablishment of potential instability in the precipitation area, which is one of the crucial factors contributing to persistent rainfalls.