Seasonal variability of tropical cyclones generated over the South China Sea

The seasonal variability of tropical cyclones （CTCs） generated over the South China Sea （SCS） from 1948 to 2003 is analyzed. It peaks in occurrence in August and few generate in late winter （from January to March）. The seasonal activity is attributed to the variability of atmosphere and ocean environments associated with the monsoon system. It is found that the monsoonal characteristics of the SCS basically determine the region of tropical cyclone （TC） genesis in each month.

Wave hindcast under tropical cyclone conditions in the South China Sea:sensitivity to wind fields

Reliable wave information is critical for marine engineering.Numerical wave models are useful tools to obtain wave information with continuous spatiotemporal distributions.However,the accuracy of model results highly depends on the quality of wind forcing.In this study,we utilize observations from five buoys deployed in the northern South China Sea from August to September 2017.Notably,these buoys successfully recorded wind field and wave information during the passage of five tropical cyclones of different intensities without sustaining any damage.Based on these unique observations,we evaluated the quality of four widely used wind products,namely CFSv2,ERA5,CCMP,and ERAI.Our analysis showed that in the northern South China Sea,ERA5 performed best compared to buoy observations,especially in terms of maximum wind speed values at 10 m height(U10),extreme U10 occurrence time,and overall statistical indicators.CFSv2 tended to overestimate non-extreme U10 values.CCMP showed favorable statistical performance at only three of the five buoys,but underestimated extreme U10 values at all buoys.ERAI had the worst performance under both normal and tropical cyclone conditions.In terms of wave hindcast accuracy,ERA5 outperformed the other reanalysis products,with CFSv2 and CCMP following closely.ERAI showed poor performance especially in the upper significant wave heights.Furthermore,we found that the wave hindcasts did not improve with increasing spatiotemporal resolution,with spatial resolution up to 0.5°.These findings would help in improving wave hindcasts under extreme conditions.

Influences of MJO-induced Tropical Cyclones on the Circulation-Convection Inconsistency for the 2021 South China Sea Summer Monsoon Onset

The South China Sea Summer Monsoon(SCSSM)onset is characterized by an apparent seasonal conversion of circulation and convection.Accordingly,various indices have been introduced to identify the SCSSM onset date.However,the onset dates as determined by various indices can be very inconsistent.It not only limits the determination of onset dates but also misleads the assessment of prediction skills.In 2021,the onset time as identified by the circulation criteria was 20 May,which is 12 days earlier than that deduced by also considering the convection criteria.The present study mainly ascribes such circulation-convection inconsistency to the activities of tropical cyclones(TCs)modulated by the Madden-Julian Oscillation(MJO).The convection of TC“Yaas”(2021)acted as an upper-level diabatic heat source to the north of the SCS,facilitating the circulation transition.Afterward,TC“Choi-wan”(2021)over the western Pacific aided the westerlies to persist at lower levels while simultaneously suppressing moist convection over the SCS.Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation.The skillful prediction of the MJO during late spring may provide an opportunity to accurately predict the establishment of the SCSSM several weeks in advance.

Detection of cyclonic eddy generated by looping tropical cyclone in the northern South China Sea: a case study

A case study on the cyclonic eddy generated by the tropical cyclone looping over the northern South China Sea (NSCS) is presented, using TOPEX/POSEIDON altimeter data and AVHRR sea surface temperature (SST) data. Three cases relating to the tropical cyclone events (Typhoon Kai-Tak in July 2000, Tropical Storm Russ in June 1994 and Tropical Storm Maria in August-September 2000) over the NSCS have been analyzed. For each looping tropical cyclone case, the cyclonic eddy with an obvious sea level depression appears in the sea area where the tropical cyclone takes a loop form, and lasts for about 2 weeks with a slight variation in location. The cold core with the SST difference greater than 2℃against its surrounding areas is also observed by the satellite-derived SST data.

The characteristic differences of tropical cyclones forming over the western North Pacific and the South China Sea

The best track dataset of tropical cyclones in the western North Pacific （WNP） and the South China Sea （SCS） from 1977 to 2005 during the satellite era, the NCEP/NCAR reanalysis dataset and the extended reconstructed sea surface temperature dataset are employed in this study. The main climatological characteristics of tropical cyclone formation over the WNP and the SCS are compared. It is found that there is obviously different for the locations of tropical cyclone origins, achieving the lowest central pressure and termination points between over the WNP and over the SCS. The annual number of tropical cyclones forming over the SCS is obviously less than over the WNP, and there is a significant negative correlation with the correlation coefficient being - 0.36 at the 5% significance level between over the WNP and over the SCS. The mean speed of tropical cyclone moving is 6.5 m/s over the WNP and 4.6 m/s over the SCS. The mean lowest central pressure of tropical cyclones is obviously weaker over the SCS than over the WNP. The tropical cyclone days per year, mean total distance and total displacement of tropical cyclone traveled over the WNP are all obviously longer than those over the SCS. Tropical cyclone may intensify to Saffir - Simpson hurricane scale 5 over the WNP, but no tropical cyclone can intensify to Saffir - Simpson hurricane scale 3 over the SCS. The changing ranges of the radii （R15,R16） of the 15.4 m/s winds them and the 25.7 m/s winds over the WNP are obviously wider than those over the SCS, and the median values of the radii over the WNP are also larger than those over the SCS. For the same intensity of tropical cyclones, both radii have larger medians over the WNP than over the SCS. The correlations of annual mean tropical cyclone size parameters between over the WNP and over the SCS are not significant. At the same time, the asymmetric radii of tropical cyclones over the WNP are different from those over the SCS.

Contribution of South China Sea Tropical Cyclones to an Increase in Southern China Summer Rainfall Around 1993

The increase in southern China summer rainfall around 1993 was accompanied by an increase in tropical cyclones that formed in the South China Sea. This study documents the connection of these two features. Our analysis shows that the contribution of tropical cyclones that formed in the South China Sea to southern China summer rainfall experienced a significant increase around 1993, in particular, along the coast and in the heavy rain category. The number of tropical cyclones that formed in the western North Pacific and entered the South China Sea decreased, and their contribution to summer rainfall was reduced in eastern part of southern China （but statistically insignificant）. The increase in tropical cyclone-induced rainfall contributed up to -30＆ of the total rainfall increase along the coastal regions. The increase of tropical cyclones in the South China Sea appears to be related to an increase in local sea surface temperature.

Climatological characteristics of monsoons and tropical cyclone activities over China seas and their influences on hydrological and seasonal structures over the South China Sea

The aim of this paper is to study whether the features of perennial summer over the South China Sea remain constant all the year round or not , and whether there are any seasonal differences throughout the year or not. According to the characteristics of remarkable monsoon and frequent typhoon, the influences of monsoon and tropical cyclone on the hydrological features and the seasonal structure over the South China Sea are analysed by using examples. It may be considered that in the perennial summer area over the South China Sea, it is summer all the year round, but it does not remain constant throughout the year. On the basis of index dates of developing stages for winter and summer monsoons as well as the seasonal characteristics of typhoon frequency,the perennial summer season over the South China Sea may be divided into four periods, namely, early summer, midsummer, sweltering summer and late summer. The concrete classification and the hydrological seasonal feature of each period are discussed.

Interannual variability in the sea surface cooling induced by tropical cyclones in the South China Sea

Sea surface cooling induced by tropical cyclones(TCs)is an important component of air-sea interactions.Using coordinate transformation and composite analysis methods,we examined the interannual variability in TCinduced sea surface cooling(TCSSC)in the South China Sea(SCS).The frequency of surface cooling cases was over 86%and that of surface warming cases was less than 14%.The magnitude of TCSSC was defined as the absolute value of TCSSC.The maximum magnitude of TCSSC occurred on the right side of the TC track,and the mean magnitude of TCSSC decreased by 0.04℃/a from 2006 to 2018.The interannual variability in TCSSC was highly correlated with the TC translation speed and pre-TC mixed layer depth.Notably,TCSSC got enhanced in El Nino years of 2007,2010,and 2015.The El Nino types were suggested to determine the occurring periods of strong TCSSC via controlling the positions of SCS anticyclones,which brought pre-TC shallow mixed layer and caused strong TCSSC via vertical mixing process during El Nino events.To quantify how the anticyclone influences TCSSC,we need to use mixed layer heat balances model in the next study.

Climate change trend and causes of tropical cyclones affecting the South China Sea during the past 50 years

Tropical cyclones(TCs)in the South China Sea(SCS)cause serious disasters and loss every year to the coastal and inland areas of southern China.The types of TCs are usually difficult to forecast,and studies on the understanding of the TCs affecting the SCS are lacking.In this study,the authors use the TC data during 1965–2017 from two best-track datasets to analyse the climatic characteristics in terms of the frequency,the track activity,and the influencing indexes of the TCs affecting the SCS and investigate the possible causes.The results show that,during 1965–2017,there were 535 TCs affecting the SCS,mainly occurring from June to November of each year,with the annual average frequency exhibiting a significant downward trend.Meanwhile,the frequency of the track activity in most areas of the SCS also demonstrate a remarkable decreasing trend but an increase in the Gulf of Tonkin and the Taiwan Strait.The large-scale environmental anomalous westerlies and the decrease of humidity in the mid-and low-level over the northern part of the SCS are likely the main causes for the decrease in frequency and the track activity.In addition,the analysis using the cyclone activity index shows that the influence of the before mentioned TCs in southern China gradually decreases,while the influence of TCs in the SCS show a decreasing trend during past decades.

SURFACE OBSERVATIONS IN THE TROPICAL CYCLONE ENVIRONMENT OVER THE SOUTH CHINA SEA

In this paper, the observational data from Marine and Meteorological Observation Platform （MMOP） at Bohe, Maoming and buoys located in Shanwei and Maoming are used to study the characteristics of air-sea temperature and specific humidity difference and the relationship between wind and wave with the tropical cyclones over the South China Sea （SCS）. The heat and momentum fluxes from eddy covariance measurement （EC） are compared with these fluxes calculated by the COARE 3.0 algorithm for Typhoon Koppu. The results show that at the developing and weakening stages of Koppu, both these differences between the sea surface and the near-surface atmosphere from the MMOP are negative, and data from the buoys also indicate that the differences are negative between the sea surface and near-surface atmosphere on the right rear portion of tropical cyclones （TCs） Molave and Chanthu. However, the differences are positive on the left fi＇ont portion of Molave and Chanthu. These positive differences suggest that the heat flux is transferred from the ocean to the atmosphere, thus intensifying and maintaining the two TCs. The negative differences indicate that the ocean removes heat fluxes from the atmosphere, thus weakening the TCs. The wind-wave curves of TCs Molave and Chanthu show that significant wave height increases linearly with 2-min wind speed at 10-m height when the wind speed is less than 25 m/s, but when the wind speed is greater than 25 m/s, the significant wave height increases slightly with the wind speed. By comparing the observed sensible heat, latent heat, and friction velocity from EC with these variables from COARE 3.0 algorithm, a great bias between the observed and calculated sensible heat and latent heat fluxes is revealed, and the observed friction velocity is found to be almost the same as the calculated friction velocity.

Effects of the East Asian Summer Monsoon on Tropical Cyclone Genesis over the South China Sea on an Interdecadal Time Scale

Tropical cyclone （TC） genesis over the South China Sea （SCS） during 1965–2004 was analyzed. The locations of TC genesis display evident seasonal changes, with the mean position of formation situated north of 15 °N in summer （June–July–August） and south of 15 °N in autumn （September–October–November）. The TC genesis in summer underwent dramatic interdecadal variations, with more and less TC frequency during 1965–1974/1995–2004 and 1979–1993, respectively. In contrast, a significant interannual variation of TC genesis with a period of ～4 years was observed in autumn. This study investigated the relationship of SCS TC genesis to the East Asian jet stream （EAJS） and the western North Pacific subtropical high （WNPSH） on an interdecadal time scale. Analysis and comparison of the impacts of the EAJS and the WNPSH on vertical wind shear changes indicate that changes in the WNPSH and EAJS intensity rather than EAJS meridional location are responsible for changes in TC genesis on an interdecadal time scale. Corresponding to a weaker EAJS, anomalous Rossby wave energy at upper levels displays equatorward propagation at midlatitudes and poleward propagation in the subtropics. This induces anomalous convergence and divergence of wave activity fluxes in East Asia around 30 °N and the SCS, respectively. The anomalous divergence of wave activity fluxes reduces easterlies at upper levels over the SCS, which is favorable to TC genesis.

The Upper Ocean Thermal Structure and the Genesis Locations of Tropical Cyclones in the South China Sea

The relationship between the upper ocean thermal structure and the genesis locations of tropical cyclones (TCs) in the South China Sea (SCS) is investigated by using the Joint Typhoon Warning Center (JTWC) best-track archives and high resolution (1/4 degree) temperature analyses of the world's oceans in this paper. In the monthly mean genesis positions of TCs from 1945 to 2005 in the SCS, the mean sea surface temperature (SST) was 28.8℃ and the mean depth of 26℃ water was 53.1 m. From the monthly distribution maps of genesis positions of TCs, SST and the depth of 26℃ water in the SCS, we discovered that there existed regions with SST exceeding 26℃ and 26℃ water depth exceeding 50 m where no tropical cyclones formed from 1945 to 2005 in the SCS, which suggests that there were other factors unfavorable for TC formation in these regions.

Role of surface warming in the northward shift of tropical cyclone tracks over the South China Sea in November

Tropical cyclones （TCs） formed in the Northwest Pacific Ocean （NWP） can cross the South China Sea （SCS） sometimes. It is found that the TC tracks in the SCS in November are shifted to the north after 1980 compared with those before 1980. Both data analyses and numerical simulations show that the surface warming in the SCS may contribute to this more northward shift. The warming produces a cyclonic atmosphere circulation anomaly in the northwestern SCS and an associated southerly in the central SCS steering the TCs to the north.

Precipitation Microphysical Processes in the Inner Rainband of Tropical Cyclone Kajiki (2019) over the South China Sea Revealed by Polarimetric Radar

Polarimetric radar and 2D video disdrometer observations provide new insights into the precipitation microphysical processes and characteristics in the inner rainband of tropical cyclone(TC)Kajiki(2019)in the South China Sea for the first time.The precipitation of Kajiki is dominated by high concentrations and small(<3 mm)raindrops,which contribute more than 98%to the total precipitation.The average mass-weighted mean diameter and logarithmic normalized intercept are 1.49 mm and 4.47,respectively,indicating a larger mean diameter and a lower concentration compared to the TCs making landfall in eastern China.The ice processes of the inner rainband are dramatically different among different stages.The riming process is dominant during the mature stage,while during the decay stage the aggregation process is dominant.The vertical profiles of the polarimetric radar variables together with ice and liquid water contents in the convective region indicate that the formation of precipitation is dominated by warm-rain processes.Large raindrops collect cloud droplets and other raindrops,causing reflectivity,differential reflectivity,and specific differential phase to increase with decreasing height.That is,accretion and coalescence play a critical role in the formation of heavy rainfall.The melting of different particles generated by the ice process has a great influence on the initial raindrop size distribution(DSD)to further affect the warm-rain processes.The DSD above heavy rain with the effect of graupel has a wider spectral width than the region without the effect of graupel.

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 43 TCs fi＇om 1994-2008, but only 17 during 1979-1993. For July in particular, 13 TCs 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 （850-500 hPa） 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.

CHARACTERISTICS OF INTENSITY CHANGE IN TROPICAL CYCLONES AFFECTING THE SOUTH CHINA SEA FROM 1977 TO 2007

The best track data of tropical cyclones (TCs) provided by Regional Specialized Meteorological Center (RSMC) Tokyo for the South China Sea (SCS) from 1977 to 2007 are employed to study the spatiotemporal variations (for a period of 12 hours) and the rapid (slow) intensification (RI/SI) of TCs with different intensity. The main results are as follows. (1) Over this period, the tropical storms (TSs) and severe tropical storms (STSs) mostly intensify or are steady while the typhoons (TYs) mostly weaken. The stronger a TC is initially, the more observation of its intensification and the less its variability will be; the more observation of its weakening is, the larger its variability will be. (2) The TC intensifies the fastest at 0000 UTC while weakening the fastest at 1200 UTC. (3) In the intensifying state, TSs, STSs, and TYs are mainly active in the northeastern, central-eastern, and central SCS respectively. The weakening cases mainly distribute over waters east off Hainan Island and Vietnam and west off the Philippines. Some cases of TSs and STSs weaken over the central SCS. (4) The RI cases form farther south in contrast to the SI cases. The RI cases are observed in regions where there are weaker vertical shear and easterly components at 200 hPa. The RI cases also have stronger mid-and lower-level warm-core structure and smaller radii of 15.4 m/s winds. The SI cases have slightly higher SST.

IMPACT OF CONVECTION OVER THE SOUTH CHINA SEA ON TROPICAL CYCLONE MOTION OVER THE WESTERN NORTH PACIFIC DURING SUMMER MONSOON

The intraseasonal oscillation(ISO) of the South China Sea(SCS, 105-120°E, 5-20°N) convection and its influences on the genesis and track of the western North Pacific(WNP) tropical cyclones(TCs) were explored, based on the daily average of NCEP/NCAR reanalysis data, the OLR data and the western North Pacific tropical cyclone best-track data from 1979 to 2008. The mechanism of the influences of ISO on TC movement and the corresponding large-scale circulation were discussed by a trajectory model. It was found as follows.(1) During the SCS summer monsoon, the SCS convection exhibits the ISO features with active phases alternating with inactive phases. The monsoon circulation patterns are significantly different during these two phases. When the SCS convection is active(inactive), the SCS-WNP monsoon trough stretches eastward(retreats westward) due to the activity(inactivity) of SCS monsoon, and the WNP subtropical high retreats eastward(stretches westward), which enhances(suppresses) the monsoon circulation.(2) The amount of TC genesis in the active phase is much more than that in the inactive phase. A majority of TCs form west of 135 °E during the active phases but east of 135 °E in the inactive phases.(3) The TCs entering the area west of 135 °E and south of 25 °N would move straight into the SCS in the active phase, or recurve northward in the inactive phase.(4) Simulation results show that the steering flow associated with the active(inactive)phases is in favor of straight-moving(recurving) TCs. Meanwhile, the impacts of the locations of TC genesis on the characteristics of TC track cannot be ignored. TCs that occurred father westward are more likely to move straight into the SCS region.

RELATIONSHIP BETWEEN TROPICAL ISO AND TROPICAL CYCLONE ACTIVITY OVER THE SOUTH CHINA SEA

The relationship between the tropical intra-seasonal oscillation(ISO) and tropical cyclones(TCs) activities over the South China Sea(SCS) is investigated by utilizing the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR) global reanalysis data and tropical cyclone best-track data from 1949 to 2009.The main conclusions are:(1)A new ISO index is designed to describe the tropical ISO activity over the SCS,which can simply express ISO for SCS.After examining the applicability of the index constructed by the Climate Prediction Center(CPC),we find that the convection spatial scale reflected by this index is too large to characterize the small-scale SCS and fails to divide the TCs activities over the SCS into active and inactive categories.Consequently,the CPC index can't replace the function of the new ISO index;(2)The eastward spread process of tropical ISO is divided into eight phases using the new ISO index,the phase variation of which corresponds well with the TCs activities over the SCS.TCs generation and landing are significantly reduced during inactive period(phase 4-6) relative to that during active period(phase 7-3);(3)The composite analyses indicate distinct TCs activities over the SCS,which is consistent with the concomitant propagation of the ISO convective activity.During ISO active period,the weather situations are favorable for TCs development over the SCS,e.g.,strong convection,cyclonic shear and weak subtropical high,and vice versa;(4)The condensation heating centers,strong convection and water vapor flux divergence are well collocated with each other during ISO active period.In addition,the vertical profile of condensation heat indicates strong ascending motion and middle-level heating over the SCS during active period,and vice versa.Thus,the eastward propagation of tropical ISO is capable to modulate TCs activities by affecting the heating configuration over the SCS.

Monthly prediction of tropical cyclone activity over the South China Sea using the FGOALS-f2 ensemble prediction system

The monthly prediction skill for tropical cyclone(TC)activity in the South China Sea(SCS)during the typhoon season(July to November)was evaluated using the FGOALS-f2 ensemble prediction system.Specifically,the prediction skill of the system at a 10-day lead time for monthly TC activity is given based on 35-year(1981–2015)hindcasts with 24 ensemble members.The results show that FGOALS-f2 can capture the climatology of TC track densities in each month,but there is a delay in the monthly southward movement in the area of high track densities of TCs.The temporal correlation coefficient of TC frequency fluctuates across the different months,among which the highest appears in October(0.59)and the lowest in August(0.30).The rank correlation coefficients of TC track densities are relatively higher(R>0.6)in July,September,and November,while those in August and October are relatively lower(R within 0.2 to 0.6).For real-time prediction of TCs in 2020(July to November),FGOALS-f2 demonstrates a skillful probabilistic prediction of TC genesis and movement.Besides,the system successfully forecasts the correct sign of monthly anomalies of TC frequency and accumulated cyclone energy for 2020(July to November)in the SCS.

THE CHARACTERISTICS OF TEMPORAL AND SPATIAL DISTRIBUTION OF TROPICAL CYCLONE FREQUENCIES OVER THE SOUTH CHINA SEA AND ITS AFFECTING OCEANIC FACTORS IN THE PAST 50 YEARS

The characteristics of temporal and spatial distribution of tropical cyclone frequencies over the South China Sea areas and its affecting factors in the past 50yrs are analyzed based on typhoon data that provided by CMA and Simple Ocean Data Assimilation （SODA）. The results show that the tropical cyclone frequencies from June to October show concentrated geographic distribution, for they mainIy distribute over the SCS area from 15 - 20°N. The characteristics present significant interdecadal changes. The impact of oceanic factors on the tropical cyclone frequencies in the SCS area is mainly realized by La Nina and La Nifia-like events before 1975 but mainly by E1 Nino and E1 Nifio-like events after 1975.