Air-sea heat flux exchange over the South China Sea under different weather conditions before and after southwest monsoon onset in 2000

With the data observed from the Second SCS Air-Sea Flux Experiment on the Xisha air-sea flux research tower, the radiation budget, latent, sensible heat fluxes and net oceanic heat budgets were caculated before and after summer monsoon onset. It is discovered that, after summer monsoon onset, there are considerable changes in air-sea fluxes, especially in latent heat fluxes and net oceanic heat budget. Furthermore, the analyzed results of five synoptic stages are compared. And the characteristics of the flux transfer during different stages around onset of South China Sea monsoon are discussed. The flux change shows that there is an oceanic heat accumulating process during the pre-onset and the break period, as same as oceanic heat losing process during the onset period. Moreover, latent fluxes, the water vapor moving to the continent, even the rainfall appearance in Chinese Mainland also can be influenced by southwester. Comparing Xisha fluxes with those obtained from the Indian Ocean and the western Pacific Ocean, their differences may be observed. It is the reason why SSTs can keep stable over the South China Sea while they decrease quickly over the Arabian Sea and the Bay of Bengal after monsoon onset.

THE CHARACTERISTICS OF AIR-SEA HEAT FLUX EXCHANGE DURING THE GENERATION AND DEVELOPMENT OF LOCAL TYPHOONS OVER THE SOUTH CHINA SEA

A South China Sea(SCS) local TC(SLT) is defined as a tropical cyclone(TC) that forms within the SCS region and can reach the grade of tropical storm(TS) or above. The statistical features of the SLTs from 1985 to 2007 are analyzed first. It is found that over the SCS about 68% of the TCs can develop into TSs. The SLT intensity is relatively weak and associated with its genesis latitude as well as its track. The SLT monthly number presents a seasonal variation with two peaks in May and July to September. Based on the daily heat flux data from the Woods Hole Oceanographic Institution_Objectively Analyzed air-sea Fluxes(WHOI_OAFlux) in the same period, the air-sea exchange during the process of generation and development of the SLT is studied. Results show that the heat fluxes released to the atmosphere increase significantly day by day before cyclogenesis. The ocean to the south to the TC center provides the main energy. Along with the development of SLT, the regions with large heat fluxes spread clockwise to the north of TC, which reflects the energy dispersion property of vortex Rossby waves in the periphery of the TC. Once the SLT forms the heat fluxes are not intensified as much. During the whole process, the net heat, latent heat and sensible heat flux display a similar evolution, while the latent heat flux makes a main contribution to the net heat flux. The maximum air-sea heat exchange always occurs at the left side of the TC moving direction, which may reflect the influence of the SCS summer monsoon on TC structure.

Differences in Precipitation and Related Wind Dynamics and Moisture and Heat Features in Separate Areas of the South China Sea before and after Summer Monsoon Onset

Using surface and balloon-sounding measurements, satellite retrievals, and ERA5 reanalysis during 2011–20, this study compares the precipitation and related wind dynamics, moisture and heat features in different areas of the South China Sea(SCS) before and after SCS summer monsoon onset(SCSSMO). The rainy sea around Dongsha(hereafter simply referred to as Dongsha) near the north coast, and the rainless sea around Xisha(hereafter simply referred to as Xisha) in the western SCS, are selected as two typical research subregions. It is found that Dongsha, rather than Xisha, has an earlier and greater increase in precipitation after SCSSMO under the combined effect of strong low-level southwesterly winds, coastal terrain blocking and lifting, and northern cold air. When the 950-h Pa southwesterly winds enhance and advance northward, accompanied by strengthened moisture flux, there is a strong convergence of wind and moisture in Dongsha due to a sudden deceleration and rear-end collision of wind by coastal terrain blocking. Moist and warm advection over Dongsha enhances early and deepens up to 200 h Pa in association with the strengthened upward motion after SCSSMO, thereby providing ample moisture and heat to form strong precipitation. However, when the 950-h Pa southwesterly winds weaken and retreat southward, Xisha is located in a wind-break area where strong convergence and upward motion centers move in. The vertical moistening and heating by advection in Xisha enhance later and appear far weaker compared to that in Dongsha, consistent with later and weaker precipitation.

Evidence for water exchange between the South China Sea and the Pacific Ocean through the Luzon Strait

An analysis of historical oxygen data provides evidence on the water exchange between theSouth China Sea (SCS) and the Pacific Ocean (PO). In the vicinity of the Luzon Strait (LS) , the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 m (intermediate layer) , while the situation is reversed above 700 m (upper layer) and below 1 500 m (deep layer). The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers, supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical. Within the SCS basin, the oxygen distribution indicates widespread vertical movement, including the upwelling in the intermediate layer and the downwelling in the deep layer.

Application of LICOM to the numerical study of the water exchange between the South China Sea and its adjacent oceans

On the basis of 900-year integration of a global ocean circulation model-LICOM driven by ECMWF reanalysis wind data with uniform 0.5°-grids, a quantitative estimate of the annual and monthly mean water exchange of the South China Sea （SCS） with its adjacent oceans through 5 straits is obtained. Among them, the annual transport is the largest in the Luzon Strait, then in the Taiwan Strait, and then in the Sunda Shelf, in the Balabac Strait and in the Mindoro Strait in turn, the largest monthly transport variation appears in the Luzon Strait and Sunda shelf. It is shown that the mass transport through the Taiwan Strait is affected by monsoon, while the transport through the Luzon Strait may be associated with the bifurcation position of the North Equatorial Current off the east Philippines shore; the transports in the Luzon Strait and Sunda Shelf are out of phase in direction but well correlated in magnitude. The annual and monthly mean heat and salinity exchange of the SCS through the straits are also calculated and shown to be in phase with the mass transport. The Kuroshio water carries about 0.43 PW heat transport and 151.33 kt/s salinity transport into the SCS, while most of them is carried out of the SCS through the Taiwan Strait and Sunda Shelf annually. The further model integration based on the 900-year integration for another 44 a from 1958 to 2001 driven by real wind data （ERA40 data） shows that the monthly mean mass transport via these straits varies annually with a large variation range, which may be associated with the seasonal and interannual variations in the current field and sea surface height in the SCS. The mean mass transport through the Taiwan Strait, Luzon Strait, Mindoro Strait, Balabac Strait and Sunda Shelf is 2.012×10^6, -4.063×10^6, -0.124×10^6, -0.083×10^6 and 2.258×10^6 m^3/s, respectively.

Water Masses in the South China Sea and Water Exchange between the Pacific and the South China Sea

Water masses in the South China Sea (SCS) were identified and analyzed with the data collected in the summer and winter of 1998. The distributions of temperature and salinity near the Bashi Channel (the Luzon Strait) were analyzed by using the data obtained in July and December of 1997. Based on the results from the data collected in the winter of 1998, waters in the open sea areas of the SCS were divided into six water masses: the Surface Water Mass of the SCS (S), the Subsurface Water Mass of the SCS (U), the Subsurface-Intermediate Water Mass of the SCS (UI),the Intermediate Water Mass of the SCS (I), the Deep Water Mass of the SCS (D) and the Bottom Water Mass of the SCS(B). For the summer of 1998, the Kuroshio Surface Water Mass (KS) and the Kuroshio Subsurface Water Mass (KU) were also identified in the SCS. But no Kuroshio water was found to pass the 119.5°E meridian and enter the SCS in the time of winter observations. The Sulu Sea Water (SSW) intruded into the SCS through the Mindoro Channel between 50-75 m in the summer of 1998. However, the data obtained in the summer and winter of 1997 indicated that water from the Pacific had entered the SCS through the nor-thern part of the Luzon Strait in these seasons, but water from the SCS had entered the Pacific through the southern part of the Strait. These phenomena might correlate with the 1998 El-Nio event.

The seasonal variability of an air-sea heat flux in the northern South China Sea

The seasonal variabilities of a latent-heat flux （LHF）, a sensible-heat flux （SHF） and net surface heat flux are examined in the northern South China Sea （NSCS）, including their spatial characteristics, using the in situ data collected by ship from 2006 to 2007. The spatial distribution of LHF in the NSCS is mostly controlled by wind in summer and autumn owing to the lower vertical gradient of air humidity, but is influenced by both wind and near-surface air humidity vertical gradient in spring and winter. The largest area-averaged LHF is in autumn, with the value of 197.25 W/m 2 , followed by that in winter; the third and the forth are in summer and spring, respectively. The net heat flux is positive in spring and summer, so the NSCS absorbs heat; and the solar shortwave radiation plays the most important role in the surface heat budget. In autumn and winter, the net heat flux is negative in most of the observation region, so the NSCS loses heat; and the LHF plays the most important role in the surface heat budget. The net heating is mainly a result of the offsetting between heating due to the shortwave radiation and cooling due to the LHF and the upward （outgoing） long wave radiation, since the role of SHF is negligible. The ratio of the magnitudes of the three terms （shortwave radiation to LHF to long-wave radiation） averaged over the entire year is roughly 3：2：1, and the role of SHF is the smallest.

Response of the South China Sea summer monsoon onset to air-sea heat fluxes over the Indian Ocean

We objectively define the onset date of the South China Sea (SCS) summer monsoon, after having evaluated previous studies and considered various factors. Then, interannual and interdecadal characteristics of the SCS summer monsoon onset are analyzed. In addition, we calculate air-sea heat fluxes over the Indian Ocean using the advanced method of CORARE3.0, based on satellite remote sensing data. The onset variation cycle has remarkable interdecadal variability with cycles of 16 a and 28 a. Correlation analysis between air-sea heat fluxes in the Indian Ocean and the SCS summer monsoon indicates that there is a remarkable lag correlation between them. This result has important implications for prediction of the SCS summer monsoon, and provides a scientific basis for further study of the onset process of this monsoon and its prediction. Based on these results, a linear regression equation is obtained to predict the onset date of the monsoon in 2011 and 2012. The forecast is that the onset date of 2011 will be normal or 1 pentad earlier than the normal year, while the onset date in 2012 will be 1-2 pentads later.

Variation of Air-Sea Heat Fluxes over the Western Pacific Warm Pool Area and Its Relationship with the South China Sea Summer Monsoon Onset

Based on oceanic and atmospheric parameters retrieved by satellite remote sensing using a neural network method, air-sea heat fluxes over the western Pacific warm pool area were calculated with the advanced the advanced Coupled Ocean-Atmosphere Response Experiment 3.0 (COARE3.0) bulk algorithm method. Then, the average annual and interannual characteristics of these fluxes were analyzed. The rela- tionship between the fluxes and the South China Sea (SCS) summer monsoon onset is highlighted. The results indicate that these fluxes have clear temporal and spatial characteristics. The sensible heat flux is at its maximum in the Kuroshio area, while the latent heat flux is at its maximum in the North Equatorial Current and Kuroshio area. The distribution of average annual air-sea heat fluxes shows that both sensible and latent heat fluxes are maximized in winter and minimized in summer. The air-sea heat fluxes have obvious interannual variations. Correlation analysis indicates a close lag-correlation between air-sea heat fluxes in the western Pacific warm pool area and at the SCS summer monsoon onset. The lagcorrelation can therefore predict the SCS summer monsoon onset, providing a reference for the study of precipitation related to the monsoon.

On the Upper Oceanic Heat Budget in the South China Sea:Annual Cycle

The upper oceanic heat budget in the South China Sea (SCS) is studied on the basis of ocean surface heat flux, upper sea heat storage and horizontal oceanic heat transport calculated from Comprehensive Ocean and Atmosphere Data Set. Several useful conclusions can be obtained and they are helpful for us to understand the climatologically thermal condition in the SCS. The annual variation of net heat budget reflects the adjustment and sudden change of the monsoon circulation over the SCS. The variation of upper oceanic heat storage of the SCS is tightly connected with the oceanic heat transport as well as the vertical movement in the SCS and so on.

On the relationship between convection intensity of South China Sea summer monsoon and air-sea temperature difference in the tropical oceans

The annual, interannual and inter-decadal variability of convection intensity of South China Sea (SCS) summer monsoon and air-sea temperature difference in the tropical ocean is analyzed, and their relationship is discussed using two data sets of 48-a SODA (simple ocean data assimilation) and NCEP/NCAR. Analyses show that in wintertime Indian Ocean (WIO), springtime central tropical Pacific (SCTP) and summertime South China Sea-West Pacific (SSCSWP), air-sea temperature difference is significantly associated with the convection intensity of South China Sea summer monsoon. Correlation of the inter-decadal time scale (above 10 a) is higher and more stable. There is inter-decadal variability of correlation in scales less than 10 a and it is related with the air-sea temperature difference itself for corresponding waters. The inter-decadal variability of the convection intensity during the South China Sea summer monsoon is closely related to the inter-decadal variability of the general circulation of the atmosphere. Since the late period of the 1970s, in the lower troposphere, the cross-equatorial flow from the Southern Hemisphere has intensified. At the upper troposphere layer, the South Asian high and cross-equatorial flow from the Northern Hemisphere has intensified at the same time. Then the monsoon cell has also strengthened and resulted in the reinforcing of the convection of South China Sea summer monsoon.

Implication of the South China Sea Throughflow for the Interannual Variability of the Regional Upper-Ocean Heat Content

In this study the interannual variability of the upper-ocean heat content in the South China Sea （SCS） was revisited using simple ocean data assimilation （SODA） combined with objective analyzed data sets that included the horizontal and vertical structures. The results confirmed that the upper-ocean heat content in the SCS is lower than normal during the mature phase of E1 Nifio events, and two super E1 Nifio events, 1982/1983 and 1997/1998 were also included. The variability of the heat content was consistent with the variability of the dynamic height anomalies. The SCS throughflow （SCSTF） plays an important role in regulating the interannual variability of the heat content, especially during the mature phase of E1 Nifio events.

Diagnostic Study of Apparent Heat Sources and Moisture Sinks in the South China Sea and its Adjacent Areas during the Onset of 1998 SCS Monsoon

The apparent heat sources (?Q1 ?) and moisture sinks (?Q2 ?) are calculated based on the reanalyzed data of the South China Sea Monsoon Experiment (SCSMEX) from May 1 to August 31, 1998. It is found that the formation and distribution of the atmospheric heat sources are important for the monsoon onset. The earlier onset of the SCS monsoon is the result of enduring atmospheric heating in the Indo–China Peninsula and South China areas. The atmospheric heating firstly appears in the Indo–China Peninsula area and the sensible heat is the major one. The 30–50 day periodic oscillation of atmospheric heat sources between the SCS area and the western Pacific warm pool has a reverse phase distribution before the middle of July and the low frequency oscillation of heat sources in SCS area has an obvious longitudinal propagation. The 30–50 day low frequency oscillation has vital modificatory effects on the summer monsoon evolution during 1998. Key words Apparent heat sources - Apparent moisture sinks - The South China Sea monsoon - Diagnostic Study Sponsored by the National Key Project of Fundamental Research “ SCSMEX” and the Research Fund for the Doctoral Program of Higher Education: “ Study of the Air-sea Interaction in the SCS Monsoon Region”.

SPATIAL PATTERN OF THE AIR-SEA INTERACTION NEAR THE SOUTH CHINA SEA DURING WINTER

This paper delineates the coupled and principal pattrns of sea surface temperture (SST) and surface wind near the South China Sea (SCS), and discusses the mechanisms of air-sea coupling near the SCS and their asspcoiation with the Asian monsoon. Singular value decomposition (SVD) and single field principal component analysis (PCA) are applied to the so and wind anomalies from the 1979 - 1995 NCEP/NCAR reanalysis data.The leading SVD mode explains a predominant amount of squared covariance between the SST and zonal or meridional wind. During winte, the meridional wind’s relation to the SST is betterr than the relation of zonal wind to ase. Despite the large magnitude of the squared covariance between SST and zonalor meridional wind, the spatial patterns of the first mode of SVD between the SST and meridional wind are similar. They both exhibit ellipe-shaped variance with the center near the SCS and a northeast-southwest oriented main axis. The spatial patterne of the leading mode of SVD between the SST and zonal wind are also similar to a certain degree. The zonal wind is not as closely correlated to the SST as the meridional wind is. These results suggest that the meridional wind and SST are stronly coupled during the winter season, and that there is a certain coupled action system in the SCS.

Performances of Seven Datasets in Presenting the Upper Ocean Heat Content in the South China Sea

In this study, the upper ocean heat content （OHC） variations in the South China Sea （SCS） during 1993- 2006 were investigated by examining ocean temperatures in seven datasets, including World Ocean Atlas 2009 （WOA09） （climatology）, Ishii datasets, Ocean General Circulation ModeI for the Earth Simulator （OFES）, Simple Ocean Data Assimilation system （SODA）, Global Ocean Data Assimilation System （GODAS）, China Oceanic ReAnalysis system （CORA） , and an ocean reanalysis dataset for the joining area of Asia and Indian-Pacific Ocean （AIPO1.0）. Among these datasets, two were independent of any numerical model, four relied on data assimilation, and one was generated without any data assimilation. The annual cycles revealed by the seven datasets were similar, but the interannual variations were different. Vertical structures of temperatures along the 18~N, 12.75~N, and 120~E sections were compared with data collected during open cruises in 1998 and 2005-08. The results indicated that Ishii, OFES, CORA, and AIPO1.0 were more consistent with the observations. Through systematic shortcomings and advantages in presenting the upper comparisons, we found that each dataset had its own OHC in the SCS.

Heat oscillation in the upper ocean of the southern South China Sea

Data used in this study are temperature/depth profiles taken over the upper 400 m of the ocean in the southern South China Sea (4°-14° N, 106°-120° E) for the period 1961-1973. The data are analyzed on the grid 2 (latitude) by 2 (longitude) in space and bimonthly in time. The vertically averaged temperature (TAV) over the upper 100 m of the ocean is calculated as the estimate of the heat content in the upper ocean.The TAV is cooler in the northwest region of the study area and warmer in the southeast in the annual and seasonal mean figures. The first EOF (Empirical Orthogonal Function) of anomalous TAV accounts for 41 % of the total variance for the period 1961-1973. The time function associated with it displays a significant interannual changes in the heat content, with 2-4 a oscillation period and associated with the ENSO events. During ENSO event TAV increases with the tendency of increasing towards equator along the basin. This anomalous states also exist in the water layers below 100 m depth. The isotherm is usually deepened during ENSO period. The deepened amplitude of the isotherm decreases with depth, and varies with ENSO events, seasons and regions. The reason for that is related to weak monsoon in El Nino year and associated eddy activity. Besides this, there is a gain in heat in the upper ocean because of the strong subtropical high during ENSO period.

Imprint of the ENSO on Rainfall and Latent Heating Variability over the Southern South China Sea from TRMM Observations

Analyses of the Tropical Rainfall Measuring Mission （TRMM） datasets revealed a prominent interannual variation in the convective-stratiform rainfall and latent heating over the southern South China Sea （SCS） during the winter monsoon between 1998 and 2010. Although the height of maximum latent heating remained nearly constant at around 7km in all of the years, the year-to- year changes in the magnitudes of maximum latent heating over the region were noticeable. The interannual variations of the convee- tive-stratiform rainfall and latent heating over the southern SCS were highly anti-correlated with the Nifio-3 index, with more （less） rainfall and latent heating during La Nifia （El Nifio） years. Analysis of the large-scale environment revealed that years of active rain- fall and latent heating corresponded to years of large deep convergence and relative humidity at 600hPa. The moisture budget diag- nosis indicated that the interarmual variation of humidity at 600hPa was largely modulated by the vertical moisture advection. The year-to-year changes in rainfall over the southern SCS were mainly caused by the interannual variations of the dynamic component associated with anomalous upward motions in the middle troposphere, while the interannual variations of the thermodynamic com- ponent associated with changes in surface specific humidity played a minor role. Larger latent heating over the southern SCS during La Nifia years may possibly further enhance the local Hadley circulation over the SCS in the wintertime.

INTERANNUAL VARIABILITY OF HEAT CONTENT, AND ITS RELATIONSHIP WITH THE SUMMER MONSOON INTENSITY, IN THE SOUTH CHINA SEA

This study analyzes the Ishii 700 m heat content(HC) in the South China Sea(SCS).During the 1978-2012 period,the HC in the SCS changed dramatically on interannual timescales.Three main findings emerged from the analysis.1)The first spatial pattern of the empirical orthogonal function(EOF1)was consistently distributed over most of the SCS,whereas that of the second empirical orthogonal function(EOF2) showed a dipole signal.2)The HC anomalies in the SCS were closely related to the SCS summer monsoon intensity.When the HC over most of the SCS increased(decreased) in previous winter,the SCS summer monsoon was strengthened(weakened).Therefore,the HC behavior in the SCS during previous winter can well predict the intensity of the SCS summer monsoon.3)HC anomalies in the SCS largely influence the monsoon and Walker circulations,in turn affecting the western Pacific subtropical high and finally the SCS summer monsoon.

SUBTROPICAL HIGH, LATENT HEATING BY CONVECTION AND ASSOCIATION WITH DISSIPATION AND MAINTENANCE OF SOUTH CHINA SEA TYPHOONS

Numerical modeling and experiments are conducted for the South China Sea typhoons Helen (1995) and Willie (1996) with an auto-adaptive mesh model. It is shown that durating the stage of dissipation the typhoons are mainly related with the subtropical high rather than the topography. The high is sensitive to the intensity change of the typhoon so that the former weakens as the latter strengthens and vice versa. Maintaining the typhoon as a main factor, the release of latent heat is in reversed proportion with the subtropical high in terms of the intensity. It is found that the storm tends to be maintained if it moves close to the westerly trough after landfall.

A comparison of the CMIP5 models on the historical simulation of the upper ocean heat content in the South China Sea

Seventeen models participating in the Coupled Model Intercomparison Project phase 5（CMIP5） activity are compared on their historical simulation of the South China Sea（SCS） ocean heat content（OHC） in the upper 300 m. Ishii＇s temperature data, based on the World Ocean Database 2005（WOD05） and World Ocean Atlas 2005（WOA05）, is used to assess the model performance by comparing the spatial patterns of seasonal OHC anomaly（OHCa） climatology, OHC climatology, monthly OHCa climatology, and interannual variability of OHCa. The spatial patterns in Ishii＇s data set show that the seasonal SCS OHCa climatology, both in winter and summer, is strongly affected by the wind stress and the current circulations in the SCS and its neighboring areas. However, the CMIP5 models present rather different spatial patterns and only a few models properly capture the dominant features in Ishii＇s pattern. Among them, GFDL-ESM2 G is of the best performance. The SCS OHC climatology in the upper 300 m varies greatly in different models. Most of them are much greater than those calculated from Ishii＇s data. However, the monthly OHCa climatology in each of the 17 CMIP5 models yields similar variation and magnitude as that in Ishii＇s. As for the interannual variability, the standard deviations of the OHCa time series in most of the models are somewhat larger than those in Ishii＇s. The correlation between the interannual time series of Ishii＇s OHCa and that from each of the 17 models is not satisfactory. Among them, BCC-CSM1.1 has the highest correlation to Ishii＇s, with a coefficient of about 0.6.