First and second baroclinic mode responses of the tropical Indian Ocean to interannual equatorial wind anomalies

The combined and individual responses of the first and second baroclinic mode dynamics of the tropical Indian Ocean to the well-known Indian Ocean Dipole mode (IOD) wind anomalies are investigated. The IOD forced first baroclinic Rossby waves arrive at the western boundary in three months, while the reflected component from the eastern boundary with opposite phase arrives in five to six months, both carry input energy to the west. The inclusion of the second baroclinic mode slows down the wave propagation by mode coupling and stretches the energy spectrum to a relatively longer time scale. The total energy exists in the equatorial wave guide for at least five months from the forcing, as much as 10% of that of the atmospheric input, which mainly dissipates at the western boundary. The individual responses of the ocean to IOD interannual wind anomaly show that the significant modes of oceanic anomalies are confined to a wave guide of 10° on either side of the equator.     

孟加拉湾上层环流研究综述

综述了孟加拉湾上层环流研究的主要成果并指出,研究海区环流与季风转换不完全同步。在西南季风期间,南、北海区各有一气旋式环流;在秋季季风过渡期间,出现海湾尺度的气旋式环流;在东北季风期间,气旋式环流减弱北移,南部则为一反气旋式环流控制;春季与秋季的情形相反,整个湾出现一海湾尺度的反气旋式环流。研究海区环流的变异主要受季风、赤道远地作用和浮力通量等复杂外源作用的影响。东印度沿岸流的季节变化与季风转换也不同步,局地风、内部Ekman抽吸、远地沿岸风及赤道远地作用的影响对沿岸流周年变化有重要作用。孟加拉湾上层环流年际变化显著,此年际变化主要受赤道风场的影响。     

南海冬、夏季环流的三维数值模拟

本文利用一个斜压三维陆架海模式——HAMSOM模式对12月份和8月份的南海环流进行数值模拟,结果为:对上层流场,在12月份,在西沙群岛－中沙群岛海区间呈现一个气旋式环流,在越南中部东岸存在一支南向西边界流,在金兰湾的远海为一局地反气旋涡,在南海南部,主要表现为万安滩的气旋式大弯曲(气旋涡)及在北康暗沙北侧的反气旋涡;在8月份,在东沙群岛－中沙群岛－吕宋岛西侧海域间存在一大尺度的气旋涡,在南海西部主要表现为以西沙群岛南部的气旋涡与金兰湾－礼乐滩间的反气旋式大环流相对峙的局面,同时在万安滩东侧有－气旋涡.由于斜压效应、底形效应的作用,使冬、夏季的南海南部中层流场几乎与上层流场相反.     

Observed Variability of Thermohaline Fields,Currents and Eddies in the Western Bay of Bengal During BOBMEX-99

Field measurements during the Bay of Bengal Monsoon Experiment (BOBMEX-99), data from a deep sea moored buoy, and satellite altimeter were used to describe variability in the hydrographic and meso-scale features in the Bay of Bengal (BoB) during the summer monsoon of 1999. The thermohaline fields showed two regions of upsloping of isopleths centered at 82°E and 84.75°E, ～110 km and 450 km away from the coast, respectively, followed by downsloping. The upsloping/downsloping of isopleths and the alternating currents was part of cyclonic and anti-cyclonic circulation patterns in the western BoB. In this region, both wind and current were important in the dynamics of coastal upwelling. The observations showed a relationship between the propagating waves and eddy on variability of thermohaline fields. On an annual cycle, four Kelvin waves were observed in the BoB, but only the downwelling Kelvin wave formed during October entered the Arabian Sea. During the monsoon season, four eddies were formed in the western BoB, of which the anticyclonic eddy centered at 15°N, 84°E and the cyclonic eddy centered at 17.5°N, 84.5°E were prominent. The baroclinic instability caused by the opposing currents along the east coast and the wind stress curl favored the formation of eddies. Okhubo-Weiss and Isern-Fontanet parameter confirmed the presence of eddies in the BoB.     

Interannual variability of the North Pacific Subtropical Countercurrent: role of local ocean–atmosphere interaction

Seasonal and interannual variability of the Subtropical Countercurrent (STCC) in the western North Pacific are investigated using observations by satellites and Argo profiling floats and an atmospheric reanalysis. The STCC displays a clear seasonal cycle. It is strong in late winter to early summer with a peak in June, and weak in fall. Interannual variations of the spring STCC are associated with an enhanced subtropical front (STF) below the surface mixed layer. In climatology, the SST front induces a band of cyclonic wind stress in May north of the STCC on the background of anticyclonic curls that drive the subtropical gyre. The band of cyclonic wind and the SST front show large interannual variability and are positively correlated with each other, suggesting a positive feedback between them. The cyclonic wind anomaly is negatively correlated with the SSH and SST below. The strong (weak) cyclonic wind anomaly elevates (depresses) the thermocline and causes the fall (rise) in the SSH and SST, accelerating (decelerating) STCC to the south. It is suggested that the anomalies in the SST front and STCC in the preceding winter affect the subsequent development of the cyclonic wind anomaly in May. Results from our analysis of interannual variability support the idea that the local wind forcing in May causes the subsequent variations in STCC.     

副热带一热带太平洋密跃层水量交换年际变化的模拟研究

利用一个全球海洋环流模式在3组风应力资料的强迫下模拟分析了副热带太平洋向热带太平洋密跃层水量输送的年际变化特征及其和风应力的关系,并设计数值试验,研究了密跃层水量输送的变化机制.结果表明,副热带太平洋向赤道太平洋的密跃层水量输送具有显著的年际变化.在年际时间尺度上,南北太平洋西边界密跃层水量输送都起着补偿内部路径输送的...     

Numerical study on the summer circulation of the upper South China Sea and its establishment

A coupled single-layer/two-layer model is employed to study the South China Sea (SCS) upper circulation and its response before and after the onset of summer monsoon. It is found that, in summer, due to the β effect and the first baroclinic mode of the wind-driven current, a northward western boundary jet current is formed along the Indo-China Peninsula coast, and it leaves the coast at about 13° N and diffuses towards northeast; next to the Indo-China Peninsula, a large anticyclonic     

Primary Study of the Mechanism of Eddy Shedding from the Kuroshio Bend in Luzon Strait

The mechanism of the anticyclonic eddy's shedding from the Kuroshio bend in Luzon Strait has been studied using a nonlinear 2 1/2 layer model, in a domain including the North Pacific and South China Sea. The model is forced by steady zonal wind in the North Pacific. Energy analysis is adopted to detect the mechanism of the eddy shedding. Twelve experiments with unique changes of wind forcing speed (to obtain different Kuroshio transports at Luzon Strait) were performed to examine the relationship between the Kuroshio transport (KT) and the eddy shedding events. In the reference experiment with KT of 22.7 Sv (forced with zonal wind idealized from the annual mean wind stress from the COADS data set), the interval of eddy shedding is 70 days and the shed eddy centers at (20°N, 117.5°E). When the Kuroshio bend extends westward, the southern cyclonic perturbation grows so rapidly as to form a cyclonic eddy (18.5°N, 120.5°E) because of the frontal instability in the south of the Kuroshio bend. In the evolution of the cyclonic eddy, it cleaves the Kuroshio bend and triggers the separation of the anticyclonic eddy. In statistical terms, anticyclonic eddy shedding occurs only when KT fluctuates within a moderate range, between 21 Sv and 28 Sv. When the KT is larger than 28 Sv, a stronger frontal instability south of the Kuroshio bend tends to generate a cyclonic eddy of size similar to the width of the Luzon Strait. The bigger cyclonic eddy prevents the Kuroshio bend from extending into the SCS and does not lead to eddy shedding. On the other hand, when the KT decreases to less than 21 Sv, the frontal instability south of the Kuroshio bend is so weak that the size of corresponding cyclonic eddy is smaller than half the width of the Luzon Strait. The cyclonic eddy, lacking power, fails to cleave the Kuroshio bend and cause separation of an anticyclonic eddy; as a result, no eddy shedding occurred then, either.     

Intrinsic and forced interannual variability of the Gulf of Alaska mesoscale circulation

The response of the Gulf of Alaska (GOA) circulation to large-scale North Pacific climate variability is explored using three high resolution (15 km) regional ocean model ensembles over the period 1950-2004. On interannual and decadal timescales the mean circulation is strongly modulated by changes in the large scale climate forcing associated with PDO and ENSO. Intensification of the model gyre scale circulation occurs after the 1976-1977 climate shift, as well as during 1965-1970 and 1993-1995. From the model dynamical budgets we find that when the GOA experiences stronger southeasterly winds, typical during the positive phase of the PDO and ENSO, there is net large-scale Ekman convergence in the central and eastern coastal boundary. The geostrophic adjustment to higher sea surface height (SSH) and lower isopycnals lead to stronger cyclonic gyre scale circulation. The opposite situation occurs during stronger northwesterly winds (negative phase of the PDO).Along the eastern side of the GOA basin, interannual changes in the surface winds also modulate the seasonal development of high amplitude anticyclonic eddies (e.g. Haïda and Sitka eddies). Large interannual eddy events during winter-spring, are phase-locked with the seasonal cycle. The initial eddy dynamics are consistent with a quasi-linear Rossby wave response to positive SSH anomalies forced by stronger downwelling favorable winds (e.g. southwesterly during El Niño). However, because of the fast growth rate of baroclinic instability and the geographical focusing associated with the coastal geometry, most of the perturbation energy in the Rossby wave is locally trapped until converted into large scale nonlinear coherent eddies. Coastally trapped waves of tropical origin may also contribute to positive SSH anomalies that lead to higher amplitude eddies. However, their presence does not appear essential. The model ensembles, which do not include the effects of equatorial coastally trapped waves, capture the large Haïda and Sitka eddy events observed during 1982 and 1997 and explain between 40% and 70% of the tidal gauges variance along the GOA coast.In the western side of the GOA basin, interannual eddy variability located south of the Alaskan Stream is not correlated with large scale forcing and appears to be intrinsic. A comparison of the three model ensembles forced by NCEP winds and a multi-century-long integration forced only with the seasonal cycle, shows that the internal variability alone explains most of the eddy variance. The asymmetry between the eddy forced regime in the eastern basin, and the intrinsic regime in the western basin, has important implications for predicting the GOA response to climate change. If future climate change results in stronger wintertime winds and increased downwelling in the eastern basin, then increased mesoscale activity (perhaps more or larger eddies) might occur in this region. Conversely, the changes in the western basin are not predictable based on environmental forcing. Eastern eddies transport important biogeochemical quantities such as iron, oxygen and chlorophyll-a into the gyre interior, therefore having potential upscale effects on the GOA high-nutrient-low-chlorophyll region.     

Major variability modes and wind patterns over the Sea of Japan and adjacent areas

The expansion of wind fields observed at fixed times (four times daily) in complex empirical orthogonal functions is performed for the Japan Sea area (34°–53° N, 127°–143° E). The wind fields are taken from the 1998–2004 NCEP/NCAR Reanalysis data with better spatial resolution (1° × 1°) than the standard product, which are publicly available on the Internet. Major modes of wind variability in the Japan Sea area are identified. The modes determine a general direction of air-mass transport throughout a year, zonal and meridional modulation, and a cyclonic and an anticyclonic eddy component. Objective classification of wind fields with respect to the prevailing flow direction is performed, and wind stress and wind-curl patterns are obtained for major events in the cold and warm periods of the year. The pattern obtained can be used in hydrodynamic numerical models of the general circulation of the Japan Sea.     

Predictability of Interannual Variability in the Kuroshio Transport South of Japan Based on Wind Stress Data over the North Pacific

It is expected that a roughly two-year forecast of the Kuroshio transport variation can be made from a past record of wind stress data over the ocean, since it takes nearly ten years for the first-mode baroclinic Rossby wave to traverse the entire basin in the midlatitude North Pacific (∼30°N). We therefore investigated the predictability using an ocean general circulation model driven by the wind stress data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. Referring to a hindcast experiment as the control run, we carried out fifteen forecast experiments, the initial conditions of which are taken from the hindcast experiment at intervals of two years during the period from the end of 1969 to the end of 1997. Each of the forecast experiments is driven only by wind stress in the year preceding each experiment. The forecasted Kuroshio transport anomaly south of Japan agrees better with the hindcasted one during the first two years of the forecast in most cases. In some cases, however, significant disagreements occur, most of which are likely due to larger unpredictable variations caused by wind stress anomalies near Japan. At the end of forecast year 2, the anomaly correlation coefficient is about 0.7, and rms of the forecast error is smaller than rms of the hindcasted anomaly. These results indicate that the prediction of the interannual variability in the Kuroshio transport could be made two years in advance at a statistically significant level. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sea Surface Height Variability in the Indian Ocean from TOPEX/POSEIDON Altimetry and Model Simulations

Sea Surface Height (SSH) variability in the Indian Ocean during 1993-1995 is studied using TOPEX/POSEIDON (T/P) altimetry data. Strong interannual variability is seen in the surface circulation of the western Arabian Sea, especially in the Somali eddy structure. During the Southwest (SW) monsoon, a weak monsoon year is characterized by a single eddy system off Somalia, a strong or normal monsoon year by several energetic eddies. The Laccadive High (LH) and Laccadive Low (LL) systems off southwest India are observed in the altimetric SSH record. The variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal, is also detected. Evidence is found for the propagation of Kelvin and Rossby waves across the northern Indian Ocean; these are examined in the context of energy transfer to the western boundary currents, and associated eddies. A simple wind-driven isopycnal model having three active layers is implemented to simulate the seasonal changes of surface and subsurface circulation in the North Indian Ocean and to examine the response to different wind forcing. The wind forcing is derived from the ERS-1 scatterometer wind stress for the same period as the T/P altimeter data, enabling the model response in different (active/weak) monsoon conditions to be tested. The model output is derived in 10-day snapshots to match the time period of the T/P altimeter cycles. Complex Principal Component Analysis (CPCA) is applied to both altimetric and model SSH data. This confirms that long Rossby waves are excited by the remotely forced Kelvin waves off the southwest coast of India and contribute substantially to the variability of the seasonal circulation in the Arabian Sea.     

Sea Surface Height Variability in the Indian Ocean from TOPEX/POSEIDON Altimetry and Model Simulations

Sea Surface Height (SSH) variability in the Indian Ocean during 1993-1995 is studied using TOPEX/POSEIDON (T/P) altimetry data. Strong interannual variability is seen in the surface circulation of the western Arabian Sea, especially in the Somali eddy structure. During the Southwest (SW) monsoon, a weak monsoon year is characterized by a single eddy system off Somalia, a strong or normal monsoon year by several energetic eddies. The Laccadive High (LH) and Laccadive Low (LL) systems off southwest India are observed in the altimetric SSH record. The variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal, is also detected. Evidence is found for the propagation of Kelvin and Rossby waves across the northern Indian Ocean; these are examined in the context of energy transfer to the western boundary currents, and associated eddies. A simple wind-driven isopycnal model having three active layers is implemented to simulate the seasonal changes of surface and subsurface circulation in the North Indian Ocean and to examine the response to different wind forcing. The wind forcing is derived from the ERS-1 scatterometer wind stress for the same period as the T/P altimeter data, enabling the model response in different (active/weak) monsoon conditions to be tested. The model output is derived in 10-day snapshots to match the time period of the T/P altimeter cycles. Complex Principal Component Analysis (CPCA) is applied to both altimetric and model SSH data. This confirms that long Rossby waves are excited by the remotely forced Kelvin waves off the southwest coast of India and contribute substantially to the variability of the seasonal circulation in the Arabian Sea.     

The Circulation in the Upper and Intermediate Layers of the South China Sea

The circulation in the basin of the South China Sea is reproduced using a four-layer numerical model. Current fields in the second (upper) and third (intermediate) layers are emphasized. Three eddies coexist in the upper layer in summer. The circulation pattern in this layer is similar to that in the first (surface) layer. In winter, a cyclonic circulation occupies the entire basin of the South China Sea in the upper layer as in the surface layer. On the other hand, the circulation pattern in the intermediate layer is fairly different from that in upper two layers especially in winter. A double-gyre pattern appears in the intermediate layer during winter. The pattern is caused by the propagation of the baroclinic Rossby wave of the second mode. This wave is excited at onset of the winter monsoon wind. Such circulation pattern well explains the observed salinity distribution in the intermediate layer. Although the double-gyre pattern in the intermediate layer is revealed even in summer in this model, it is restricted in the western part of the basin. Besides, its current speed is small compared to that in winter.     

Seasonal variability of water transport through the Straits of Gibraltar, Sicily and Corsica, derived from a high-resolution model of the Mediterranean circulation

The variability of the water transport through three major straits of the Mediterranean Sea (Gibraltar, Sicily and Corsica) was investigated using a high-resolution model. This model of the Mediterranean circulation was developed in the context of the Mercator project.The region of interest is the western Mediterranean between the Strait of Gibraltar and the Strait of Sicily. The major water masses and the winter convection in the Gulf of Lions were simulated. The model reproduced the meso-scale and large-scale patterns of the circulation in very good agreement with recent observations. The western and the eastern gyres of the Alboran Sea were observed but high interannual variability was noticed. The Algerian Current splits into several branches at the longitude of the Strait of Sicily level, forming the Tyrrhenian branch, and, the Atlantic Ionian Stream and the Atlantic Tunisian Current in the eastern Mediterranean. The North Current retroflexed north of the Balearic Islands and a dome structure was observed in the Gulf of Lions. The cyclonic barotropic Algerian gyre, which was recently observed during the MATER and ELISA experiment, was evidenced in the simulation.From time-series of 10-day mean transport, the three straits presented a high variability at short time-scales. The transport was generally maximum, in April for the Strait of Gibraltar, in November for the Strait of Sicily, and in January for the Strait of Corsica. The amplitudes of the transport through the Straits of Gibraltar (0.11 Sv) and Sicily (0.30 Sv) presented a weaker seasonal variability than that of the Strait of Corsica (0.70 Sv).The study of the relation between transport and wind forcing showed that the transport through the Strait of Gibraltar is dependent on local zonal wind over short time-scales (70%), which was not the case for the other straits (less than 30%). The maximum (minimum) of the transport occurred for an eastward (westward) wind stress in the strait. An interannual event was noticed in November–December 2001, which corresponded to a very low transport (0.3 Sv), which was characterised by a cyclonic circulation in the western Alboran Sea. That circulation was also reproduced by the model for other periods than winter during the interannual simulation.The transport through the Strait of Sicily is not influenced by local wind.The wind stress curl of the northwestern Mediterranean influenced the transport through the Strait of Corsica.     

一个考虑潮汐、中尺度涡和地形影响的南海底部环流诊断模型

本文构造了一个考虑潮汐、中尺度涡和地形影响下的南海底部环流诊断模型。在该模型中,潮汐混合和涡致混合引起的垂直速率用一个类似的改进参数化方案来表示。该模型结果显示在南海深层吕宋海峡"深水瀑布"和斜压影响最大,潮汐作用和中尺度涡影响次之,风场的影响最小。斜压影响的整体效应与其他因素相反。潮汐混合与涡致混合具有明显的地形依赖性。潮汐混合主要集中在南海北部海盆地形较为陡峭的陆坡区和南海中部海山区,而涡致混合主要集中在海盆西边界区以及中部海山区。在不考虑吕宋海峡"深水瀑布"、潮汐和中尺度涡的情况下(对应吕宋海峡关闭),南海底部环流为反气旋式环流。考虑吕宋海峡"深水瀑布"后,南海底层环流为气旋式环流,而潮汐混合和涡致混合起到加强整个气旋式环流强度的作用。此外,该模型还给出了南海底部环流量级大小与地形坡度之间的密切关系,即地形坡度较大的地方,其流速也大。这对于现场观测有着一定的参考意义。最后,本文用尺度分析的方法从理论上分析了该模型的适用性,证实了该模型具有一定的可靠性。     

Trajectory of Mesoscale Eddies in the Kuroshio Recirculation Region

Trajectories of mesoscale eddies in the Kuroshio recirculation region were investigated by using sea surface height (SSH) anomaly observed by the TOPEX/POSEIDON and ERS altimeters. Cyclonic and anticyclonic eddies have been traced on maps of the filtered SSH anomaly fields composed from the altimeter observations every ten days. Both the cyclonic and anticyclonic eddies propagate westward in the Kuroshio recirculation region from a region south of the Kuroshio Extension. The propagation speed of these eddies has been estimated as about 7 cm s⁻¹, which is much faster than the phase speed theoretically estimated for the baroclinic first-mode Rossby wave in the study area. It was also found that in the Izu-Ogasawara Ridge region, most of eddies pass through the gap between the Hachijojima Island and Ogasawara (Bonin) Islands, and some of the eddies decay around the Izu-Ogasawara Ridge. It seems that the trajectory of the eddies is crucially affected by the bottom topography. In the region south of Shikoku and east of Kyushu, some of the eddies coalesce with the Kuroshio. It is also suggested that this coalescence may trigger the path variation of the Kuroshio in the sea south of Japan. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mesoscale Eddies Observed by TOLEX-ADCP and TOPEX/POSEIDON Altimeter in the Kuroshio Recirculation Region South of Japan

Mesoscale eddies in the Kuroshio recirculation region south of Japan have been investigated by using surface current data measured by an Acoustic Doppler Current Profiler (ADCP) installed on a regular ferry shuttling between Tokyo and Chichijima, Bonin Islands, and sea surface height anomaly derived from the TOPEX/POSEIDON altimeter. Many cyclonic and anticyclonic eddies were observed in the region. Spatial and temporal scales of the eddies were determined by lag-correlation analyses in space and time. The eddies are circular in shape with a diameter of 500 km and a temporal scale of 80 days. Typical maximum surface velocity and sea surface height anomaly associated with the eddies are 15–20 cm s^–1 and 15 cm, respectively. The frequency of occurrence, temporal and spatial scales, and intensity are all nearly the same for the cyclonic and anticyclonic eddies, which are considered to be successive wave-like disturbances rather than solitary eddies. Phase speed of westward propagation of the eddies is estimated as 6.8 cm s^–1, which is faster than a theoretical estimate based on the baroclinic first-mode Rossby wave with or without a mean current. The spatial distribution of sea surface height variations suggests that these eddies may be generated in the Kuroshio Extension region and propagate westward in the Kuroshio recirculation region, though further studies are needed to clarify the generation processes.     

Interannual and Decadal Variabilities of NPIW Salinity Minimum Core Observed along JMA’s Hydrographic Repeat Sections

We investigated the variation of the North Pacific Intermediate Water (NPIW) distribution in the western North Pacific, focusing on the intermediate salinity minimum (S < 34.2) core observed along the meridional hydrographic sections including the 137°E repeat section by the Japan Meteorological Agency. This core is a cross-section of a low salinity tongue extending westward along the recirculation in the subtropical gyre. The core size shows remarkable variabilities in interannual and decadal time scales. The salinity change in the density layer during the period of core expansion (shrinking) represents the spatial salinity change in the tongue toward the west (east). Thus, we conclude that the core size variation is associated with the zonal wobble of the tongue having thicker distribution to the east, rather than temporal changes of the water mass itself. The core size at 137°E is well correlated with the meridional gradient of the depth in the isopycnal surface at the salinity minimum representing the recirculation intensity, suggesting a relation with the intensity of the subtropical gyre. A significant lag-correlation between the gradient and the wind forcing over the North Pacific suggests that the first mode baroclinic Rossby waves excited in the central North Pacific propagated westward to change the intensity of the recirculation in interannual time scales. In decadal time scales, it is found that the wind stress curl and heat flux fields in the North Pacific precede the recirculation by about 11 years.     

Mechanism of currents through the straits of the Japan Sea: Mean state and seasonal variation

The mechanism governing the mean state and the seasonal variation of the transports through the straits of the Japan Sea is studied using a newly presented, simple analytical model and a basin scale general circulation model (GCM). The GCM reproduces the transports through the straits of the Japan Sea realistically owing to its fine horizontal resolution of about 20 km and realistic topography. A series of experiments conducted by changing surface forcing shows that the annual mean wind-driven circulation in the North Pacific Ocean is most responsible for the formation of the mean transports. It is also found that the seasonal variation of the alongshore component of monsoonal wind stress over the North Pacific basin, especially that over the Okhotsk Sea, is responsible for the seasonal variation of the transports. The simple analytical model can explain these simulated features very well. The physical concept of this model is based on the formation of the around-island circulation through the adjustment of coastally trapped waves and Rossby waves and geostrophic control at the narrow straits. It solves the sea surface heights (SSHs) at the edge of each strait and the transport through it. The value of the line integral of the SSH along the island is determined by the baroclinic Rossby waves approaching the island from the east and the alongshore wind stress around the island. The basin scale seasonal variation of SSH along the coast induced by the variation of the alongshore monsoonal wind stress can also be incorporated into this model by giving the SSH anomaly at the northeastern point of the Soya Strait. Thus, it is suggested that both the mean state and the seasonal variation are caused mainly by wind stress forcing. Minor modification by the seasonal heat flux forcing brings the amplitude and the phase of the seasonal variation closer to the observed values.