Diagnosis of seasonal variations of tropical cyclogenesis over the South China Sea using a genesis potential index

This study examines the seasonal variations of tropical cyclogenesis over the South China Sea (SCS) using a genesis potential (GP) index developed by Emanuel and Nolan. How different environmental factors (including low-level vorticity, mid-level relative humidity, vertical wind shear, and potential intensity) contribute to these variations is investigated. Composite anomalies of the GP index are produced for the summer and winter monsoons separately. These composites replicate the observed seasonal variations of the observed frequency and location of tropical cyclogenesis over the SCS. The degree of contribution by each factor in different regions is determined quantitatively by producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology. Over the northern SCS, potential intensity makes the largest contributions to the seasonal variations in tropical cyclogenesis. Over the southern SCS, the low-level relative vorticity plays the primary role in the seasonal modulation of tropical cyclone (TC) genesis frequency, and the vertical wind shear plays the secondary role. Thermodynamic factors play more important roles for the seasonal variations in tropical cyclogenesis over the northern SCS, while dynamic factors are more important in the seasonal modulation of TC genesis frequency over the southern SCS.     

Modulation of tropical cyclogenesis over the South China Sea by ENSO Modoki during boreal summer

This study examines the modulation of tropical cyclogenesis over the South China Sea(SCS) by the El Nio-Southern Oscillation(ENSO) Modoki during the boreal summer. Results reveal that there were more tropical cyclones(TCs) formed over the SCS during central Pacific warming years and less TC frequency during central Pacific cooling years. How different environmental factors(including low-level relative vorticity, mid-level relative humidity, vertical wind shear, and potential intensity) contribute to this influence is investigated, using a genesis potential(GP) index developed by Emanuel and Nolan. Composite anomalies of the GP index are produced for central Pacific warming and cooling years separately, which could account for the changes of TC frequency over the SCS in different ENSO Modoki phases. The degree of contribution by each factor is determined quantitatively by producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology. The results suggest that the vertical wind shear and low-level relative vorticity, which are associated with the ENSO Modoki-induced anomalous circulations in Matsuno-Gill patterns, make the largest contributions to the ENSO Modoki modulation of tropical cyclogenesis over the SCS as implied by the GP index. These results highlight the important roles of dynamic factors in the modulation of TC frequency over the SCS by the ENSO Modoki during the boreal summer.     

Analyzing the influences of two types of El Nino on Tropical Cyclone Genesis with a modified genesis potential index

To understand the impacts of large-scale circulation during the evolution of El Nino cycle on tropical cyclones(TC) is important and useful for TC forecast.Based on best-track data from the Joint Typhoon Warning Center and reanalysis data from National Centers for Environmental Prediction for the period 1975-2014,we investigated the influences of two types of El Nino,the eastern Pacific El Nino(EP-El Nino) and central Pacific El Nino(CP-E1 Nino),on global TC genesis.We also examined how various environmental factors contribute to these influences using a modified genesis potential index(MGPI).The composites reproduced for two types of El Nino,from their developing to decaying phases,were able to qualitatively replicate observed cyclogenesis in several basins except for the Arabian Sea.Certain factors of MGPI with more influence than others in various regions are identified.Over the western North Pacific,five variables were all important in the two El Nino types during developing summer(July-August-September) and fall(OctoberNovember-December),and decaying spring(April-May-June) and summer.In the eastern Pacific,vertical shear and relative vorticity are the crucial factors for the two types of El Nino during developing and decaying summers.In the Atlantic,vertical shear,potential intensity and relative humidity are important for the opposite variation of EP-and CP-E1 Ninos during decaying summers.In the Southern Hemisphere,the five variables have varying contributions to TC genesis variation during peak season(January-February-March) for the two types of El Nino.In the Bay of Bengal,relative vorticity,humidity and omega may be responsible for clearly reduced TC genesis during developing fall for the two types and slightly suppressed TC cyclogenesis during EP-El Nino decaying spring.In the Arabian Sea,the EP-El Nino generates a slightly positive anomaly of TC genesis during developing falls and decaying springs,but the MGPI failed to capture this variation.     

Analyzing the influences of two types of El Niño on tropical cyclone genesis with a modified genesis potential index

To understand the impacts of large-scale circulation during the evolution of El Niño cycle on tropical cyclones (TC) is important and useful for TC forecast. Based on best-track data from the Joint Typhoon Warning Center and reanalysis data from National Centers for Environmental Prediction for the period 1975–2014, we investigated the influences of two types of El Niño, the eastern Pacific El Niño (EP-El Niño) and central Pacific El Niño (CP-El Niño), on global TC genesis. We also examined how various environmental factors contribute to these influences using a modified genesis potential index (MGPI). The composites reproduced for two types of El Niño, from their developing to decaying phases, were able to qualitatively replicate observed cyclogenesis in several basins except for the Arabian Sea. Certain factors of MGPI with more influence than others in various regions are identified. Over the western North Pacific, five variables were all important in the two El Niño types during developing summer (July–August–September) and fall (October–November–December), and decaying spring (April–May–June) and summer. In the eastern Pacific, vertical shear and relative vorticity are the crucial factors for the two types of El Niño during developing and decaying summers. In the Atlantic, vertical shear, potential intensity and relative humidity are important for the opposite variation of EP- and CP-El Niños during decaying summers. In the Southern Hemisphere, the five variables have varying contributions to TC genesis variation during peak season (January–February–March) for the two types of El Niño. In the Bay of Bengal, relative vorticity, humidity and omega may be responsible for clearly reduced TC genesis during developing fall for the two types and slightly suppressed TC cyclogenesis during EP-El Niño decaying spring. In the Arabian Sea, the EP-El Niño generates a slightly positive anomaly of TC genesis during developing falls and decaying springs, but the MGPI failed to capture this variation.

     

An Alternative Index for the Onset of the Summer Monsoon over the South China Sea

1　Introduction　TheindicesfortheAsianmonsoonhavebeenstud iedinmanyworks .Recently ,thechoiceofpropermonsoonindiceshasreceivedexceptionalattentionandraisedcontroversy (WebsterandYang ,1 992 ;Goswa mietal.,1 999;Goswami,2 0 0 0 ;Wang ,2 0 0 0 ) .Us ingzona…     

Numerical study of the seasonal salinity budget of the upper ocean in the Bay of Bengal in 2014

Impact factors on the salinity budget,especially the eddy salt fluxes and smaller-scale diffusive salt fluxes for the upper 50 m of the Bay of Bengal(BoB) in 2014 are investigated using a box model based on the Regional Ocean Modeling System(ROMS) daily outputs.The model results reproduce that the precipitation and river runoffs are the dominant factors modulating the sharp salinity decrease during the summer monsoon season.The analysis shows that the salinity increase after the summer monsoon is mostly due to the meridional advective and diffusive salt fluxes.The vertical advective salt flux,which is sensitive to the different signals of the wind stress curl,plays an important role in balancing the salinity change induced by the meridional advective salt flux during both the summer and winter monsoon seasons.Distinctive spatial mesosc ale structures are presented in the eddy salt flux throughout the year,and their contributions are sizeable(over 30% in the meridional direction and about 10%-30% in the vertical direction).The meridional eddy salt flux is larger in the monsoon seasons than that in the inter-monsoon seasons,and in a positive pattern near the western boundary during the winter monsoon and autumn inter-monsoon.The vertical eddy salt flux makes an important contribution to the salinity budget,especially along the coastal area and around the Andaman and Nicobar Islands.The vertical eddy salt flux becomes large when a tropical cyclone passes the area.     

Impacts of the two types of El Niño on Pacific tropical cyclone activity

It is well known that Tropical cyclone(TC) activities over the Pacific are affected by El Nino events. In most studies El Nino phenomena have been separated into east Pacific warming(EPW) and central Pacific warming(CPW) based on the location of maximum SST anomaly. Since these two kinds of El Nino have different impacts on Pacific tropical cyclone activities, this study investigates different features of TC activities and the genesis potential index(GPI) during EPW years and CPW years. Four contrib- uting factors, i.e., the low-level absolute vorticity, the relative humidity, the potential intensity and the vertical wind shear, are exam- ined to determine which factors are most important in causing the anomalous TC activities. Our results show that during EPW years in July–August(JA0), TC activities are more frequent with stronger intensity over the Western North Pacific(WNP) and Eastern North Pacific(ENP). The maximum anomaly center of TC activities then drifts eastward significantly in September–October(SO0). However, centers of anomalous TC activity barely change from JA0 to SO0 during CPW years. In January–February–March(JFM1) of the decaying years of warming events, TC frequency and intensity both have positive anomaly over the South Pacific. The anoma- lies in EPW years have larger amplitude and wider spatial distribution than those in CPW years. These anomalous activities of TC are associated with GPI anomaly and the key factors affecting GPI anomaly for each ocean basin are quite different.     

Variation of Thornthwaite moisture index in Hengduan Mountains,China

The Thornthwaite moisture index, an index of the supply of water(precipitation) in an area relative to the climatic demand for water(potential evapotranspiration), was used to examine the spatial and temporal variation of drought and to verify the influence of environmental factors on the drought in the Hengduan Mountains, China. Results indicate that the Thornthwaite moisture index in the Hengduan Mountains had been increasing since 1960 with a rate of 0.1938/yr. Annual Thornthwaite moisture index in Hengduan Mountains was between –97.47 and 67.43 and the spatial heterogeneity was obvious in different seasons. Thornthwaite moisture index was high in the north and low in the south, and the monsoon rainfall had a significant impact on its spatial distribution. The tendency rate of Thornthwaite moisture index variation varied in different seasons, and the increasing trends in spring were greater than that in summer and autumn. However, the Thornthwaite moisture index decreased in winter. Thornthwaite moisture index increased greatly in the north and there was a small growth in the south of Hengduan Mountains. The increase of precipitation and decrease of evaporation lead to the increase of Thornthwaite moisture index. Thornthwaite moisture index has strong correlation with vegetation coverage. It can be seen that the correlation between Normalized Difference Vegetation Index(NDVI) and Thornthwaite moisture index was positive in spring and summer, but negative in autumn and winter. Correlation between Thornthwaite moisture index and relative soil relative moisture content was positive in spring, summer and autumn, but negative in winter. The typical mountainous terrain affect the distribution of temperature, precipitation, wind speed and other meteorological factors in this region, and then affect the spatial distribution of Thornthwaite moisture index. The unique ridge-gorge terrain caused the continuity of water-heat distribution from the north to south, and the water-heat was stronger than that from the east to west part, and thus determined the spatial distribution of Thornthwaite moisture index. The drought in the Hengduan Mountains area is mainly due to the unstable South Asian monsoon rainfall time.     

An interdecadal change of tropical cyclone activity in the South China Sea in the early 1990s

Tropical cyclone (TC) genesis in the South China Sea (SCS) during 1979-2008 underwent a decadal variation around 1993. A total of 55 TCs formed in the SCS from May to September during 1994- 2008, about twice that during 1979-1993 (27). During the TC peak season (July-September, JAS), there were 43TCs from 1994-2008, but only 17 during 1979-1993. For July in particular, 13TCs formed from 1994-2008, but there were none during 1979-1993. The change in TC number is associated with changes of key environmental conditions in atmosphere and ocean. Compared to 1979-1993, the subtropical high was significantly weaker and was displaced more eastward during 1994-2008. In the former period, a stronger subtropical high induced downward flow, inhibiting TC formation. In the latter period, vertical wind shear and outgoing longwave radiation all weakened. Mid-level (850-500hPa) humidity, and relative vorticity were higher. Sea surface temperature and upper layer heat content were also higher in the area. All these factors favor TC genesis during the latter period. The decadal change of TC genesis led to more landfalling TCs in Southern China during the period 1994-2008, which contributed to an abrupt increase in regional rainfall.     

成都风湿发病的气象分析

应用数诸天气学的方法，对风湿病发生时的气象条件和天气系统进行分析归纳，得出成都地区风湿发生的高峰期主要是春末夏初，秋末冬初，风湿的发生主要与气象因子中的相对湿度，温度及其变化有很大的关系。成都地区与风湿发作的有关天气系统有西南低涡、南支槽、蒙古高压、高原低涡、高原切变线等，且在冷锋过境或持续低温阴雨的天气形势下会诱发风湿病发作，由此可通过数据统计的方法和天气学的方法来预测风湿病发病期，为预防风湿病提供依据。     

Does the Asian monsoon modulate tropical cyclone activity over the South China Sea?

To investigate whether the Asian monsoon influences tropical cyclone (TC) activity over the South China Sea (SCS), TCs (including tropical storms and typhoons) over the SCS are analyzed using the Joint Typhoon Warning Center dataset from 1945 to 2009. Results show an increasing trend in the frequencies of TC-all (all TCs over the SCS) and TY-all (all typhoons over the SCS), due mainly to an increase in the number of TCs moving into the SCS after development elsewhere. Little change is seen in the number of TCs that form in the SCS. The results of wavelet analysis indicate that the frequency of typhoons (TY) shows a similar oscillation as that of TCs, i.e., a dominant periodicity of 8-16 years around the 1970s for all TC activity, except for TC-mov (TCs that moved into the SCS from the western North Pacific). To examine the relationship between typhoon activity and the summer monsoon, a correlation analysis was performed that considered typhoons, TCs, and five monsoon indexes. The analysis reveals statistically significant negative correlation between the strength of the Southwest Asian summer monsoon and typhoon activity over the SCS, which likely reflects the effect of the monsoon on TC formation in the western North Pacific (WNP) and subsequent movement into the SCS. There is a statistically significant negative correlation between TY-loc (typhoons that developed from TCs formed over the SCS) and the South China Sea summer monsoon and Southeast Asian summer monsoon.     

Impacts of SST anomalies in the Indian-Pacific basin on Northwest Pacific tropical cyclone activities during three super El Ni?o years

This study investigated the impact of sea surface temperature(SST)in several important areas of the Indian-Pacific basin on tropical cyclone(TC)activity over the western North Pacific(WNP)during the developing years of three super El Ni?o events(1982,1997,and 2015)based on observations and numerical simulations.During the super El Ni?o years,TC intensity was enhanced considerably,TC days increased,TC tracks mostly recurved along the coasts,and fewer TCs made landfall in China.These characteristics are similar to the strong ENSO-TC relationship but further above the climatological means than in strong El Ni?o years.It indicates that super El Ni?o events play a dominant role in the intensities and tracks of WNP TCs.However,there were clear differences in both numbers and positions of TC genesis among the different super El Ni?o years.These features could be attributed to the collective impact of SST anomalies(SSTAs)in the tropical central-eastern Pacific and East Indian Ocean(EIO)and the SST gradient(SSTG)between the southwestern Pacific and the western Pacific warm pool.During 2015,the EIO SSTA was extremely warm and the anomalous anticyclone in the western WNP was enhanced,resulting in fewer TCs than normal.In 1982,the EIO SSTA and spring SSTG showed negative anomalies,followed by an increased anomalous cyclone in the western WNP and equatorial vertical wind shear.This intensified the conversion of eddy kinetic energy from large-scale flows,favorable for the westward shift of TC genesis.Consequently,anomalous TC activities during the super El Ni?o years resulted mainly from combined SSTA impacts of different key areas over the Indian-Pacific basin.     

Characteristics of Thermal and Geopotential Height Differences Between Continent and Ocean and Its Role in the Strength of the Asian Summer Monsoon

The interdecadal factors affecting the summer monsoon winds over Somalia and the South China Sea were studied. Global geopotential heights and wind velocity fields of the 850-hPa and 200-hPa pressure levels, as well as sea surface temperature anomaly data and correlation coefficients were analyzed. The monsoons over Somalia and the South China Sea were found to be two different monsoon systems, operating on different mechanisms and being affected by different ocean-atmosphere interactions. The intensity of the Asian subtropical summer monsoon is influenced by the intensity of the summer monsoon over Somalia in the month of June and by the intensity of the summer monsoon over the South China Sea in the months of June and July. The summer monsoon wind strength over Somalia is affected by regional factors, such as the heating of the Tibetan plateau, and by global mechanisms, such as the subtropical heat exchange with Antarctica. The summer monsoon over the South China Sea is affected by different ocean-atmosphere interactions. The Somalia and subtropical summer monsoons have wind blowing down the pressure gradient from area over ocean to that over land, like typical summer monsoons. The South China Sea summer monsoon has winds that blow down the pressure gradient from area over land to that over ocean. The South China Sea summer monsoon is affected by the Kuroshio Current off the east coast of Japan.     

Super typhoon activity over the western North Pacific and its relationship with ENSO

This paper analyzes the characteristics of super typhoons(STYs)over the western North Pacific(WNP)from 1965 to2005 and describes the seasonal variability of STY activity.The relation between STY activity and the El Nio-Southern Oscillation(ENSO)as well as the possible reason for the influence of the ENSO on STY activity are also investigated.The results showed thatabout one fifth of the tropical cyclones(TCs)over the WNP could reach the rank of STY.Most STYs appeared from July to Novem-ber while there was a highest ratio between number of STYs and total number of TCs in November.Most STYs appeared east of thePhilippine Sea.In El Nio years,affected by sea surface temperature(SST),monsoon trough and weak vertical wind shear,TC for-mation locations shifted eastward and there were more STYs than in La Nia years when the affecting factors changed.     

Spatial and temporal variation of drought index in a typical steep alpine terrain in Hengduan Mountains

This study describes the spatial and temporal variation of a drought index and makes inferences regarding the environmental factors that influence this variability in the Hengduan Mountains. A drought index is typically used to determine the moisture conditions and the magnitude of water deficiency in a given area. Based on data from 26 meteorological stations over the period 1960-2012, the spatial and temporal variations of the drought index were analyzed using a thin plate smoothing splines method that considered elevation as a covariate. The drought index was estimated based on the potential evapotranspiration (E₀) as defined by the Penman Monteith model modified by FAO (1998). The results of the reported analysis showed that the drought index in the Hengduan Mountains has been decreasing since 1960 at a rate of -0.008/a. This represented a progressive shift from the "sub-humid" class, which typified the wider area in the Hengduan Mountains, toward the "humid" class, which appeared in the Hengduan Mountains areas. The drought index was relatively high in the north and low in the south and the variation of the drought index varied with seasons. The drought index showed increasing trends in summer and autumn and it is greater in autumn than in summer, while it showed a decreasing trend in spring and winter. Drought index is inversely proportional to the soil relative humidity and Normalized Difference Vegetation Index (NDVI).     

Comparison of remote sensing data with in-situ wind observation during the development of the South China Sea monsoon

Wind measurements derived from QuikSCAT data were compared with those measured by anemometer on Yongxing Island in the South China Sea (SCS) for the period from April 2008 to November 2009. The comparison confirms that QuikSCAT estimates of wind speed and direction are generally accurate, except for the extremes of high wind speeds (>13.8m/s) and very low wind speeds (<1.5m/s) where direction is poorly predicted. In-situ observations show that the summer monsoon in the northern SCS starts between May 6 and June 1. From March 13, 2010 to August 31, 2010, comparisons of sea surface temperature (SST) and rainfall from AMSR-E with data from a buoy located at Xisha Islands, as well as wind measurements derived from ASCAT and observations from an automatic weather station show that QuikSCAT, ASCAT and AMSR-E data are good enough for research. It is feasible to optimize the usage of remote-sensing data if validated with in-situ measurements. Remarkable changes were observed in wind, barometric pressure, humidity, outgoing longwave radiation (OLR), air temperature, rainfall and SST during the monsoon onset. The eastward shift of western Pacific subtropical high and the southward movement of continental cold front preceded the monsoon onset in SCS. The starting dates of SCS summer monsoon indicated that the southwest monsoon starts in the Indochinese Peninsula and forms an eastward zonal belt, and then the belt bifurcates in the SCS, with one part moving northeastward into the tropical western North Pacific, and another southward into western Kalimantan. This largely determined the pattern of the SCS summer monsoon. Wavelet analysis of zonal wind and OLR at Xisha showed that intra-seasonal variability played an important role in the summer. This work improves the accuracy of the amplitude of intra-seasonal and synoptic variation obtained from remote-sensed data.     

Seasonal variation in imposex intensity of Thais clavigera

Imposex, specifically caused by TBT pollution, refers to the superimposition of male sexual characteristics in gastropod females. Seasonal variation of imposex intensity in Thais clavigera from both slightly and severely contaminated sites in Hong Kong waters was studied from 1988 to 1999. The male penis length showed significant difference between both sites and seasons. It was shortest during late autumn and early winter (October to December) and longest during spring and early summer (February to June). Female penis length also showed significant difference between sites. It did not change seasonally, however. The RPS (Relative Penis Size) index was the highest during autumn and early winter, and the lowest during spring and early summer. The VDS (Vas Deferens Sequence) index remained stable throughout the sampling period. This study showed that VDS index is a better indicator when we compare relative intensity of imposex. The comparison can only be meaningful provided the samples from different locations are taken during the same season.     

Interannual variability of the spring Wyrtki Jet

Feature s of the interannual variability of the spring Wyrtki Jet in the tropical Indian Ocean are revealed using observation data and model output.The results show that the jet has significant interannual variation,which has a significant correlation with winter El Nino Modoki index(R=0.62).During spring after an El Nino(La Nina) Modoki event,the Wyrtki Jet has a positive(negative) anomaly,forced by a westerly(easterly) wind anomaly.The result of a linear-continuously stratified model shows that the first two baroclinic modes explain most of the interannual variability of the spring Wyrtki Jet(-70%) and the third to fifth modes together account for approximately 30%.Surface wind anomalies in the tropical Indian Ocean are related to the Walker circulation anomaly associated with El Nino/La Nina Modoki.The interannual variability of the spring Wyrtki Jet has an evident impact on sea surface salinity transport before the onset phase of the summer monsoon in the Indian Ocean.     

Spatio-temporal Characteristics of Atmospheric Pollution and Cause Analysis of Haze Events in Sichuan Basin,China

This study analyzed the spatio-temporal variability of air quality data for six standard air pollutants(Particulate Matter 2.5(PM_(2.5)), Particulate Matter 10(PM_(10)), SO_2, NO_2, CO, and O_3) in the Sichuan Basin(SCB), China from 2015 to 2018 in relation to the formation of haze using conventional meteorological data(temperature, wind speed, and relative humidity), satellite data(fire point data, vertical profiles of aerosol subtypes, and aerosol optical depth), planetary boundary layer height, and backward trajectories. The results indicated that the spatio-temporal evolution of the air quality index(AQI) had notable seasonality for the pollution severity in descending order: winter, spring, summer, and autumn. Autumn and winter severe haze events occurred in November and January, respectively, and were caused by higher local pollution emissions under stagnant air conditions. Spring severe haze events occurred in May and were caused by dust from Northwest China and local regions. Severe summer haze events occurred in July and were caused by local burning. Therefore, the analyses showed that local burning, stagnant meteorological conditions, air mass transport and anthropogenic pollution emissions played a key role in haze in the SCB. This study provides scientific insights for fully analyzing heavy air pollution in SCB, China, and also provides a scientific basis for pollution research in regions of complex terrain as basins and mountains.     

Seasonal variation in imposex intensity of Thais clavigera

Imposex, specifically caused by TBT pollution, refers to the superimposition of male sexual characteristics in gastropod females. Seasonal variation of imposex intensity inThais clavigera from both slightly and severely contaminated sites in Hong Kong waters was studied from 1988 to 1999. The male penis length showed significant difference between both sites and seasons. It was shortest during late autumn and early winter (October to December) and longest during spring and early summer (February to June). Female penis length also showed significant difference between sites. It did not change seasonally, however. The RPS (Relative Penis Size) index was the highest during autumn and early winter, and the lowest during spring and early summer. The VDS (Vas Deferens Sequence) index remained stable throughout the sampling period. This study showed that VDS index is a better indicator when we compare relative intensity of imposex. The comparison can only be meaningful provided the samples from different locations are taken during the same season. Supported by the NSFC (40006007).