AIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGION

AIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGIONAIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGIONAIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGIONAIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGIONAIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGIONAIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGION

Đại học Quốc Gia Hà Nội VIET NAM NATIONAL UNIVERSITY, HANOI Đinh Văn Ưu Dinh Van Uu Tương tác biển-khí Khu vực nhiệt đới ấn độ Thái bình dương AIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGION Nhµ xuÊt Đại học Quốc Gia Hà Nội - 2003 VIET NAM NATIONAL UNIVERSITY PRESS - 2003 VIET NAM NATIONAL UNIVERSITY, HANOI Dinh Van Uu Table of contents INTRODUCTION Chapter I SMALL SCALE AIR-SEA INTERACTION 10 AIR- SEA INTERACTION IN THE INDIAN-PACIFIC TROPICAL REGION 1.1 Introduction 10 1.2 Surface Processes 10 1.2.1 Equation of motion with Viscosity 12 1.2.2 Turbulence in boundary layer 13 1.2.3 Turbulent Stresses: The Reynolds Stress 14 1.2.4 Thickness of the lower atmospheric boundary layer 15 1.2.5 Vertical profile of wind speed within the surface layer 16 1.2.6 Characteristics of the lower atmospheric boundary layer on the sea 18 1.3 Influence of the stratification and wind wave on the turbulent characteristics of the lower atmospheric boundary layer 21 1.3.1 Stratification and its influence on the turbulent characteristics of the lower atmospheric boundary layer 21 1.3.2 Dependence of the sea surface roughness on wind wave characteristics 23 1.4 VIET NAM NATIONAL UNIVERSITY PRESS - 2003 Calculation of turbulent fluxes on sea surface 25 1.4.1 Direct Calculation of Fluxes 25 1.4.2 Gradient method 26 1.4.3 Indirect Calculation of Fluxes: Bulk Formulas 28 1.4.4 Field and model estimates of the sea surface drag 30 1.4.5 Climatic estimation of the turbulent fluxes 32 1.4.6 Calculation fluxes in the storm condition 35 2.5.1 Introduction 59 Chapter II OCEAN’S RESPONSE TO THE ATMOSPHERE 36 2.5.2 The Oceanic Mixed Layer and Thermocline 59 Dynamic interaction and Ekman layer 37 2.5.3 Geographical Distribution of Surface Temperature and Salinity 63 2.1.1 Response of the upper ocean to winds and inertial motion 37 2.5.4 Thermal transformation of the lower atmospheric boundary layer on 2.1.2 Wind circulation and Ekman Layer at the Sea Surface 38 2.1.3 Langmuir Circulation 41 2.1.4 Influence of stability in the Ekman Layer 42 2.1.5 Ekman Mass Transports 42 2.1 2.4.4 2.5 Meridional Heat Transport 57 Thermodynamic interaction of air-sea boundary layers and the upper ocean active layer 59 the sea -67 3Chapter III AIR SEA INTERACION IN THE INDIAN-PACIFIC TROPICAL ZONE 69 3.1 Introduction 69 3.2 Thermal Land-Ocean-Atmosphere interaction and general atmospheric and ocean circulation 70 2.1.6 2.2 Application of Ekman Theory 43 2.2.1 Boundary layers model with constant turbulent viscosity 45 2.2.2 Boundary layers model with variable turbulent viscosity 47 2.3 3.2.1 Importance of the Ocean in Earth’s Heat Budget 71 3.2.2 The development of a thermal circulation 71 3.2.3 The Coriolis effect (deflection caused by Earth’s rotation) 74 3.2.4 Sea level pressure and wind 75 3.2.5 Intertropical Convergence Zone (ITCZ) 76 3.2.6 Atmospheric circulation in tropical zone 78 3.2.7 General ocean circulation 81 Hydrodynamic interaction model of air-sea boundary layers 45 Wind Waves as the ocean’s response to atmosphere 48 2.3.1 Introduction 48 2.3.2 Linear Theory of Ocean Surface Waves 49 2.3.3 Waves and the Concept of a Wave Spectrum 51 2.3.4 Generation of Waves by Wind 52 2.3.5 Influence of atmospheric stratification on wave development 52 2.3.6 Wave Forecasting 53 3.3 2.4 Scales of variability in the ocean-atmosphere system 91 3.3.1 Diurnal and synoptic scales 92 3.3.2 Seasonal to interannual time scales 95 3.3.3 Interannual cycle 96 The Oceanic Heat Budget 54 3.4 2.4.1 Introduction 54 2.4.2 Factors influencing Heat-Budget Terms 54 2.4.3 Geographic Distribution of Terms in the Heat Budget 56 Variability of the monsoon system 98 3.4.1 Monsoon activity in the tropical ocean-atmosphere system 98 3.4.2 Variability of the Asian-Australian monsoon system 101 3.5 1051 Interannual climate variability in the Indian-Pacific tropical zone -100 3.5.1 ENSO activity and variability 105 3.5.2 The Quasi-Biennial zonal wind Oscillation (QBO) 112 3.5.3 Decadal variability in the Pacific and Indian Oceans 116 3.5.4 The variability in the Bien Dong (South China) Sea 121 INTRODUCTION REFERENCES 128 Air and water are the two fluids we know most about They are both essential to the maintenance of our lives, providing a hospitable environment for all living things We have a direct, lifelong experience in observing how they behave and how we utilize them In the natural environment we can see, feel or hear examples of almost all the kinds of fluid flows that we will study in this book The atmosphere is a layer of gas held to the surface of the earth by gravitational attraction Most of the mass of the atmosphere is confined to the first 15 kilometers above sea level, yet the small amount above this level is responsible for filtering out the deadly high energy radiation from the sun which would otherwise destroy life The interaction of the atmosphere with sunlight helps to maintain the earth's surface temperature above that of an airless planet, like the moon (This increase in temperature, called the greenhouse effect, threatens to grow in the future because of anthropogenic emissions of heat absorbing gases.) The motion of the atmosphere that we observe around us is driven by the diurnal pattern of heating by the sun and cooling by radiation to outer space Over the year, these heating/cooling patterns shift to different latitudes, giving rise to annual climate variations An intimate part of this process is the evaporation of water from the earth's surface, the formation of clouds from this water at high elevations and its precipitation back to the surface This distillation of ocean water, moved to land by winds, provides the sweet water that maintains terrestrial life Local weather provides a variety of wind motions Sometimes the wind speed is quite small, especially at night when radiative cooling stabilizes the atmosphere But storms driven by precipitation of water vapor, such as thunderstorms and hurricanes, can have very high wind speeds Cold air is more dense than warm air, so that a cold air mass tends to flow under a warm air mass, forming a cold front Large scale weather patterns drift past our locality, bringing changes that are not greatly affected by local conditions As we can see from watching the daily television weathercast, the main features of the weather pattern extend over many thousands of kilometers, a distance hundreds of times larger the atmospheric height Yet the changes of pressure, temperature and humidity are much greater in the vertical direction than in the horizontal direction, despite the much greater horizontal size of a weather pattern The pull of gravity is so strong over large distances that it forces the atmosphere to flow mostly in the horizontal direction Because we are so small compared to the vertical and horizontal dimensions of the atmosphere, we can observe how the wind blows in a tiny portion of the atmosphere that is nearby us It is noticeable that the wind speed and direction are somewhat variable, especially over time intervals of less than a minute These changes are much more rapid than the changes accompanying a weather pattern, which may take a day to pass us by Fluid flows that exhibit variability over time and length scales which are small compared to that of the overall flow are called turbulent flows The atmospheric wind is a turbulent flow The atmospheric motion is responsible for diluting air pollutants, such as those emitted by power plants and automobiles When these pollutant streams are marked by smoke, we can readily observe how the smoke intensity decreases as the wind turbulence mixes the pollutant stream with clean air, diluting the strength of the pollutant within the plume (or, if you prefer, dirtying more and more of the atmosphere) Most of these pollutants mix no higher than a few kilometers in the atmosphere , and are eventually carried far downwind and deposited back to the earth's surface Some, however, not soon return to the earth and instead mix gradually throughout the entire atmosphere, including the stratosphere Some of these gases lead to the destruction of stratospheric ozone and to increased average surface temperature The mixing properties of the atmosphere are extremely important in determining the degree of atmospheric pollution in urban areas The water in the ocean, lakes and rivers, as well as that in underground aquifers, is called the hydrosphere The volume of fresh water on the continents is small compared to the oceanic volume, but it is the part of the hydrosphere that is most important to the maintenance of terrestrial life The management and use of this water for agricultural and other purposes form an important branch of engineering But the ocean is important too It provides the source of precipitation over the land and, in its surface layer, an environment for the 10 growth of microscopic plants and animals that form the base of the oceanic food chain The ocean tends to make the climate more uniform in the latitudinal direction by moving warm tropical waters towards the poles and displacing cold polar water towards the equator We are all familiar with the downhill motion of streams and rivers, flowing toward the sea The energy in this flow can be tapped by building dams that force the river flow through turbines to generate electric power Sometimes this energy is dramatically dissipated as the river plunges over a precipice to form a waterfall The violently turbulent motion at the base of the falls converts the river's directed energy into heat When the river reaches the sea, its fresh water, being lighter than the sea water, floats on top of the sea, sometimes many miles beyond the river's mouth before it is diluted with sea water Most of the fresh water on the continents is out of sight, below ground It exists in the pores between mineral deposits and is fed by precipitation that percolates through the ground under the influence of gravity The fluid velocity in the underground aquifers is much lower than it is in rivers, the water being impeded by the frictional force of the porous medium through which it flows Underground water is often the source of potable water Locating underground water and pumping it from the ground for human use is limited by the characteristics of the underground aquifer Preserving the purity of this water from the contamination by toxic fluids buried or dumped on the surface and subsequently traveling down into the aquifer is a major problem worldwide At the edge of the ocean we see the ocean waves crashing on the shore The waves carry to the shore energy generated by 11 the wind blowing over the ocean surface Of course, the ocean surface doesn't move (on the average) in the direction of the waves, but it oscillates as the wave passes by Ocean waves are called gravity waves because the pull of gravity at the air-sea interface is responsible for the propagation of these waves, which not penetrate far below the ocean surface The other oceanic motion we notice at the sea shore is the tidal rise and fall of the sea surface This twice-a-day cycle is caused by the difference in gravitational pull of the moon (and to a lesser extent, the sun) on opposite sides of the earth The differential gravity force gives rise to a bulging of the ocean surface in the direction of the moon, which passes a given location twice in the lunar day of 25 hours The tidal motion, consisting of both a vertical and horizontal oscillation, may be amplified greatly along the continental coastline, sometimes by a factor of ten above the general oceanic amplitudes Oceans may contain localized currents, like the Gulf stream, that are mighty rivers flowing across a nearly stationary ocean Earthquakes can generate tsunami waves that travel many thousands of kilometers before crashing ashore, sometimes wreaking devastation on lowlying coastlines Even hurricanes can generate storm surge waves that cause coastal flooding The rise and fall of the tide can be utilized to produce mechanical power, but at the present time this is seldom economical compared to river power Mechanisms have been devised to extract power from ocean waves, but this has also proved to be uneconomical But the forces exerted on ships and wave barriers by the ocean waves can be very substantial, and protecting them against such forces very expensive Knowledge 12 of the dynamics of the ocean is important to many of mankind's pursuits Air-sea interaction provides a comprehensive account of how the atmosphere and the ocean interact to control the global climate, what physical laws govern this interaction, and its prominent mechanisms In recent years air-sea interaction has emerged as a subject in its own right, encompassing small-scale and large-scale processes in both air and sea By developing its subject from basic physical (thermodynamic) principles, the book is broadly accessible to a wide audience It is mainly directed towards graduate students and research scientists in meteorology, oceanography, and environmental engineering The book will be of value on entry level courses in meteorology and oceanography, and also to the broader physics community interested in the treatment of transfer laws, and thermodynamics of the atmosphere and ocean Current research in air-sea interaction is attempting to apply our understanding of molecular and turbulence scale processes to global scale phenomena There are many unknowns in the fundamental processes that control the global climate For example, what processes control the amount and distribution of water in the atmosphere; what is the effect of cloud variability on the sea surface temperature? How changes in the ocean circulation affect the atmospheric circulation and, hence, the distribution of wind stress, temperature and precipitation, and how does this feed back to the ocean? These, and many other related questions, encurage some of the studies of the interaction of the atmosphere and the ocean Major uncertainties remain in our understanding of the fundamental processes of air-sea interaction, particularly, in heterogeneous and nonequilibrium conditions; for example, we 13 not know enough about the relationships between the directional wave spectra, surface fluxes and the properties of the oceanic and atmospheric boundary layers to develop satisfactory predictive models The ocean and atmosphere are interdependent, or coupled, because of the dependence of the atmosphere on heat and moisture at the sea surface and the dependence of the ocean circulation on the wind Studies rarely combine investigations of both environments to determine the extent of the fedbacks between the ocean and the atmosphere Understanding the coupled ocean-atmosphere system depends largely on the scales of interaction between the two fluids and the processes that provide the strongest feedbacks Advances are most likely through multidisciplinary process studies that connect the upper ocean and lower atmosphere For global studies more comprehensive parameterizations of the surface processes are required as well as improvements in satellite retrievals and assimilation in numerical models The high spatial and temporal variability of surface processes needs to be properly considered In situ measurements are revealing very complex horizontally and vertically heterogeneous fields that cannot be resolved by current remote sensing techniques High resolution models, which include the physics of the processes that contribute to this variability, combined with satellite data seems to be the best tool for global analysis and prediction The large-scale dynamics of the ocean and the atmosphere are closely related Energy is transferred from the atmosphere to the ocean surface mixed layer driving the circulation of the upper ocean In turn, energy from the ocean is fed back to the atmosphere affecting the atmospheric circulation, the weather 14 and the climate The concept is deceptively simple, but as we explore the coupled earth system, we are frequently limited by our lack of understanding of the interchanges between the atmosphere and ocean Kraus and Businger (1994) highlighted several areas that continue to require more attention, for example, the interaction of the wind and surface waves, the parameterization of subgrid scale processes in large-scale circulation models, and the transfer of gases across the air-sea interface It has been convenient to divide air-sea interaction studies into two categories: small- and large- scale ocean-atmosphere interactions However, this often belies the fundamental precept that the basis of the interaction of the atmosphere and the ocean is the exchange of matter and energy across a material interface - the sea surface An exchange that, for the most part, occurs on molecular scales, involving both turbulent and laminar processes modified by wave breaking, surface tension, the structure of the planetary boundary layer and the ocean mixed layer and other effects A satisfactory understanding of these processes remains elusive, but is essential if we are to address adequately the larger scale oceanatmosphere problems In the past many proponents of large-scale studies, such as global climate and ocean circulation relied heavily on the veracity of the parameterization of small-scale air-sea exchange processes, often overlooking the uncertainties in the basic measurements and their interpretation More recently, we have recognized the importance of connecting small-scale process studies, investigating the exchange of heat, moisture, momentum and trace constituents across the air-sea interface, 15 with the large-scale problems of global climate change and ocean circulation that rely heavily on numerical models and highly averaged fields Studies, such as the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) and the World Ocean Circulation Experiment (WOCE), are leading the way by highlighting the importance of process studies for a satisfactory understanding of global climate and ocean general circulation problems The small-scale exchange processes are generally related to the global-scale problems via parameterizations of the fluxes that use mean quantities obtained by measuring on various platforms such as buoys, ships and satellites These parameterizations are also used extensively in operational meteorological models as well as many research general circulation models of the coupled ocean-atmosphere system Large uncertainties exist in the derivations of the bulk parameterizations due to the difficulty of measuring surface fluxes directly, and the difficulty of applying these measurements to scales greater than a few hundred kilometers and several hours The problems are particularly acute in the tropics where low wind speeds and very high sea surface temperatures result in primarily buoyancy-driven fluxes that are not well parameterized by most prevailing methods, and in coastal regions where fetch, topography and water depth vary considerably While there is resistance to the establishment of canonical parameterizations of the fluxes, the TOGA COARE research community recognized that such an approach has the advantage of focusing the attention of a large group of researchers to exchange and compare data and rapidly transfer 16 this information to a broader community whose interests require surface fluxes Despite these efforts, it is unlikely that in this decade of large scale climate and ocean circulation studies we will know the surface fluxes as well as enough we would like A continued effort will be required to improve our knowledge of heat, moisture, momentum and trace constituent fluxes, to increase our understanding of the uncertainties in the fluxes we measure, and to improve our knowledge of the relationship between boundary layer processes on each side of the interface to the surface exchange mechanisms research to the global and Indian-Pacific regional issues of atmosphereocean climate and its variations The Indian- Pacific tropical zone is most important heat storage of the World Ocean with the highest value of the mean sea surface temperature (SST); there is the Western Pacific/ Eastern Indian or AsianAustralian warm pool The formation of the warm pool and its variation is the favourable condition for typhoon formation and development in the western part of the Pacific Ocean The variability of the global-scale ocean and atmosphere circulation including Trade’s wind, the Walker circulation, the Asian-Australian monsoon system, the El Niño/Southern Oscillation (ENSO) and sea surface temperature (SST) is related to the oscillations of the ocean-atmosphere system This course will acquentance physical oceanography and meteorology students with the one-dimensional theories of turbulent boundary layers, to observations of the planetary boundary layer of the atmosphere, and to observations of the surface mixed layer of the ocean Coupled one-dimensional models of air-sea interaction will be studied with a view towards understanding the importance of interactions of the turbulent boundary layers with each other, and with the interior of their respective fluids Finally, we will review current progress in our understanding of the surface processes, and the application of this 17 18 2.2 Chaptre SMALL SCALE AIR-SEA INTERACTION 2.1 Introduction This part of the course will introduce physical oceanography and meteorology students to the one-dimensional theories of turbulent boundary layers, to observations of the planetary boundary layer of the atmosphere, and to observations of the surface mixed layer of the ocean Necessary concepts of turbulence theory will be studied, along with the probability and statistical tools that are appropriate Coupled one-dimensional models of air-sea interaction will be studied with a view towards understanding the importance of interactions of the turbulent boundary layers with each other, and with the interior of their respective fluids Finally, the subject of the influence of horizontal variability will be opened, with the objective of appreciating the role that ocean and atmosphere dynamics play in modulating the thermodynamics of the turbulent boundary layers, and as an introduction to the associated part of the course on Large-Scale Ocean-Atmosphere Interactions: Air-sea interaction and tropical meteorology and oceanography 19 Surface Processes Interactions between the ocean and atmosphere occur at the air-sea interface The ocean surface is a material interface that is a barrier to the exchange of heat, moisture, momentum and trace constituents Away from the surface both fluids are usually in turbulent motion, but near the interface turbulence is suppressed and the transport is controlled primarily by molecular processes To quantify the exchanges at the interface it is necessary to understand how the turbulent layers of the ocean and the atmosphere are connected via the molecular sublayers in either side of the sea surface In turn, we need to understand how the turbulent layers transport the properties of the interface into the interior of these fluids, the extent to which changes in the interior feed back to the interface, and how processes at the surface affect the structure of the deep ocean and free atmosphere The fundamental processes that connect the atmosphere and the ocean are the energy input to the ocean by the wind, the net freshwater flux, expressed primarily as precipitation minus evaporation, and the net surface heat flux As Charnock (1951) pointed out the energy transmitted by the wind to the ocean is a tiny fraction of the radiation received at the surface, yet wind-driven currents largely determine the regions where the ocean energy is fed back into the atmosphere that sets the pattern of cloudiness, which in turn determines the radiation input The ocean-atmosphere system is intrinsically coupled, although feedbacks across the air-sea interface are often masked by temporal and spatial differences As in the example above, we may understand the processes that connect the ocean and the atmosphere, but we not understand well enough the distribution of energy within the 20 This schema illustrated two equations for ice covrage in the Northern Ice Ocean (I) and heat storage of warm current branch in north Atlantic (Q): dI   m(Q  Q0 ) dt (3.9) dQ  n( I  I ) dt Figure 3.34 Schema of circulation system for Atlantic Northern Ice Oceans (from Doronin, 1980): The period of this cycle could be estimated according 2 following formula:   , its approximated value is about 3.5 mn years  - Florida warm water,  Gulf Stream warm current branch, H – New Foundland,  Interaction of the tropics and subtropics is another potentially important means by which the upper ocean processes influence climate variability, possibly including the decadal modulation of ENSO - north Atlantic warm current,  - Labrador cold current,  deep water circulation in the Northern Ice Ocean The acceleration of water circulation is conduced to decrease the remaining time of water in the warm tropical region before reaching Gulf Stream thus the water temperature is not reached its normal value In this case the temperature difference between west and east regions of the ocean will be decreased too, wind decreases and the remaining time of water in the warm tropical region begins increase The water reaching Gulf Stream has temperature warmer than its normal value then the east-west temperature gradient and its anomaly increase that increases meridional wind and new cycle will be developed again The period of this cycle is changed from years to years 241 4.5.4 The variability in the Bien Dong (South China) Sea The western Pacific/South East Asia region has an important influence on global climate because the heating of atmosphere associate with the heavy rains of the AsianAustralian monsoon system drives mean and variable globalscale Ocean and Atmosphere Circulation The Bien Dong (South China) Sea with the maximum depth greater than 5000 m is the largest marginal sea in Western Pacific This region is most important heat storage of the World Ocean with the highest value of the Mean Sea Surface Temperature (SST); there is the Western Pacific/ Eastern Indian or AsianAustralian Warm pool (figure 3.35) The mechanism of formation of the warm pool and its variation is very important in order to understand processes 242 air-sea interaction in the South East Asian monsoon region and global climate variability The high heat storage of the SCS and of the adjacent Indonesian Seas is the favourable condition for typhoon formation and development in the western part of the Pacific Ocean The convergence of moisture supplied by the surrounding oceans in the InterTropical Convergence Zone (ITCZ) gives the diabatic heat sources, migrating seasonally across the equator with reversing monsoon The Asian-Australian monsoon system is one of the most important circulation systems in the general circulation of the global atmosphere Interannual variation in the intensity and location of these heat sources are responsible for the quasibiennial oscillation and ENSO This variation is associated with the intensity and location of atmospheric cells of the Walker and Hadley circulation in the South East Asian region The water circulation in marginal seas such as the Bien Dong Sea and the complex straits and seas of the Maritime Continent allow a strongly variable flow from the Pacific to Indian Ocean: Pacific Indian Throughflow The water circulation of the South China Sea (SCS) is principally determined by reversing monsoon winds The seasonal circulation pattern in the SCS and its adjacent seas has been investigated by several researchers such as Wyrtki, 1961, Pohlmann T., 1987, Ping-Tung Shaw and Shenn-Yu Chao, 1994, etc They have pointed out that the SCSC circulation is related to water exchanges between the SCS and the Kurioshio through the Louzon Strait, between the SCS and Indonesian seas through the Mindoro Strait (to Sulu Sea) and Karimata Strait (to Java Sea) and between the SCS and the East China Sea through Taiwan strait 243 The variability of the marginal circulation has not, in general, been adequately observed with respect to interannual variability, and in some areas with respect to the seasonal cycle The air-sea flux fields of sufficient accuracy are not available and the ocean thermohaline structure observations are too sparse to provide adequate spiritual coverage for mapping without appeal to models for either statistical or dynamical interpolation and such models may not be accurate enough The bifurcation latitude of the North Equatorial Current (NEC) at the Philippine coast is affected by the amount of transport around the Philippines For the ocean-scale circulation model the absence of the analysis thermohaline structure information and the absence of the Pacific to Indian Ocean throughflow may seriously distort the model boundary currents The marginal sea circulation models have become more realistic; they useful perspectives on certain aspects of the real world Figure 3.35 Distribution of the mean sea surface temperature in the South-eastern Asian seas (Youhai He and Cuihua Guan, 1998) 244 The variation of the wind direction is very important not only by reversing monsoon wind from one season to another (fig.3.36) but there is also strong variation from one region to another during one season (Helleman & Rosenstein 1983) During the SW monsoon the variation of the wind directions and velocities is very important because the position of the ITCZ is more variable and the wind come from Indochina continent is influenced by orographic transformation due to local conditions (relief, coastal line ) The SW monsoon season is also season of typhoon and tropical cyclone with strong perturbation of the wind field in the sea and in the adjacent regions There are two seasons of the precipitation in this region with about 70 % of the total amount accumulated in the SW monsoon season Along the Vietnamese coastal zone the precipitation maximum and its time have a high correlation with the position of the ITCZ Beside of the wind and the precipitation-evaporation variation, the air- sea flux of the heat is strong variable in the space and in the time The seasonal variation of the air temperature for north-west region is very strong in compare with the southern region of the sea and another tropical region During the NE monsoon the air temperature difference between the north and the south region of the sea is very big, its difference my reaches about 15-20C Due to the low air temperature and strong wind, the total amount of the heat fluxes in the north-west coastal region is about 150 - 200 wt/m2 from sea surface to the atmosphere At the same time, in the south-east region, the sea water always receives the heat fluxes from atmosphere The Variational Inverse Method (VIM) was developed in the GeoHydrodynamic and Environment (GHER), Liege University and applied to make Mediterranean Ocean Data Base (MODB) For the South China Sea we used VIM with the grid size 15’x15’ longitude- latitude and 30 depth levels In the figures 3.37 showed the analyzed surface temperature and salinity distributions for two monsoon seasons using VIM Figure 3.36 Wind stress on the Bien Dong Sea during winter (top) and summer (bottom) from Uu, 2001 245 246 The Asian-Australian monsoon is one of the most remarkable phenomena of the ocean-atmosphere-land system in response to a seasonal march of solar insolation with complex feedback mechanisms The exchange fluxes in the ocean-atmosphere-land interfaces are dependent of atmospheric and oceanic variables such as land and sea surface temperature (SST), winds, etc In particular, the SST is one of the most important variables in the prediction of monsoon and marine system Interannual variation in the intensity and location of the heat sources are responsible for the quasibiennial oscillation (QBO) and ENSO This variation is associated with the intensity and location of atmospheric cells of the Walker and Hadley circulation in the equatorial zone, especially in the South East Asian region The variations of the SST in the region of Indonesian seas may influence in interannual variability of the monsoon Capturing the physics of the interactions between the monsoon diabatic heat sources and the warm pool heat sources in numerical models of the coupled oceanatmosphere-land system is critical for improve ENSO and monsoon forecasts Figures 3.37 Analysed surface layer temperature fields during winter (top) and summer (bottom) seasons from Uu D.V and Brankart, 1997 247 The shape of the warm pool along the boundary is influenced by the surface western Pacific boundary currents In particular, the bifurcation of the North Equatorial Current near 14N into the Mindanao Current and the Kurioshio appears to be subject to considerable interannual variability related to the QBO and ENSO (Lukas et al., 1996) The simulation results (Metzger and Hurburt, 1996) show that when the Sulu archipelago is opened, a net cyclonic flow develops around the Philippines, which is essentially an extension of the northern Pacific tropical gyre The bifurcation latitude of the north Equatorial Current at the Philippines coast is also affected by the amount of transport through the Sulu archipelago 248 The water circulation of the South China (SCS) and adjacent seas is principally determined by reversing monsoon winds (fig 3.38) The seasonal circulation pattern in the SCS and its adjacent seas has been investigated by several researchers They have pointed out that the SCS circulation is related to water exchanges between the SCS and the Kurioshio through the Luzon Strait, between the SCS and Indonesian seas through the Mindoro Strait (to Sulu Sea) and Karimata Strait (to Java Sea) and between the SCS and the East China Sea through Taiwan strait Intrusion of the Kurioshio to South China Sea (Faris and Wimbush, 1996) and its variation makes the complexity of the basin-scale circulation in the SCS and adjacent seas The results of the three-dimensional thermohaline and circulation modelling of the SCS show the possibility to estimate the water fluxes exchanged between Pacific and Indian Oceans via Indonesian seas and the interaction between SCS and Kuroshio Current (KC) system There are two principal branches of the circulation in the SCS: one is permanently passing by eastern part of the sea from Luzon strait to Sulu Sea and another passing western part of the sea in direction to Java Sea in winter and from Java Sea in summer The simulated permanent loop current in the north-eastern region of the SCS shows the high intensity of the interaction between SCS and KC system (fig 3.39, 3.40) Figure 3.38 the water circulation in the South China Sea in winter (top) and summer (bottom) after Wyrtki (1961) 249 The Kuroshio penetrates through the Luzon Strait in to the northern South China Sea forming a loop current When this happens part or all the Kuroshio flows to the west through the Luzon strait just north of 20N The water may travel as far west about 117E before turning north and then east In the passage to Java Sea the current during southwest monsoon is conducted to warm water transport from Indian Ocean to SCS 250 (c) (d) Figure 3.39 The water circulation in the South China Sea in winter after Fang and Fang 1998 (a) and model simulation (c) after Uu, 2001 251 Figure 3.40 the water circulation in the South China Sea in summer after Fang and Fang 1998 (b) and model simulation (d) after Uu, 2001 252 The simulated salinity fields (Uu, 2001) shows that the north Pacific subtropical maximum salinity water mass enters into north part of the South China sea following the Kuroshio loop current through Luzon Strait The occurrence of the Kuroshio loop current was modelled during the winter and summer monsoon season The simulated results show that the Kurioshio, the western boundary current of the North Pacific, has been found frequently to intrude into the northern SCS through the Luzon strait, which is the only deep and wide passage connecting the semi-enclosed SCS to western North Pacific The intruded Kurioshio moves westward, impinges upon the continental slope near the Dongsha Islands, and then splits The major part turns right to move eastward and leaves the SCS through the northern portion of the Luzon Strait, while the splinter turns left and moves southward, forming the SCS Branch (SCSB) The north-eastward current is the South China Sea Warm Current (SCSWC) The SCSWC is just onshore of the southwestward SCSB, but moves in opposite direction Their climatological appearance is shown in figure 3.41 for two 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