Propagation of Rossby Waves over Ridges Excited by Interannual Wind Forcing in a Western North Pacific Model

Numerical experiments with a multi-level general circulation model have been performed to investigate basic processes of westward propagation of Rossby waves excited by interannual wind stress forcing in an idealized western North Pacific model with ocean ridges. When the wind forcing with an oscillation period of 3 years is imposed around 180°E and 30°N, far from Japan, barotropic waves excited by the wind can hardly cross the ridges, such as the Izu-Ogasawara Ridge. On the other hand, a large part of the first-mode baroclinic waves are transmitted across the ridges, having net mass transport. The propagation speed of the first-mode baroclinic wave is accelerated (decelerated) when an anticyclonic (cyclonic) circulation is formed at the sea surface, due to a deeper (shallower) upper layer, and to southward (slightly northward) drift of the circulation. Thus, when the anticyclonic circulation is formed on the northern side of the cyclonic one, they propagate almost together. The second-mode baroclinic waves converted from the first-mode ones on the ridges arrive south of Japan, although their effects are small. The resulting volume transport variation of the western boundary current (the Kuroshio) reaches about 60% of the Sverdrup transport variability estimated from the wind stress. These characteristics are common for the interannual forcing case with a longer oscillation period. In the intraseasonal and seasonal forcing cases, on the other hand, the transport variation is much smaller than those in the interannual forcing cases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Upwelling and ocean structures off Algoa Bay and the south-east coast of South Africa

As the Agulhas Current flows along the south-east coast of South Africa, a number of processes operate that bring cold, deep water up onto the narrow shelf. As a consequence, upwelling along the coastline is enhanced farther southward and downstream. This situation is investigated off Algoa Bay and along the south-east coast to Port Alfred, where measurements demonstrate that marked temperature variability occurs at the coastline, particularly in summer when temperature structures are more intense and easterly-component winds more common. There is no indication that upwelling is more prevalent at Port Alfred; increasing variability farther south is evident at Woody Cape/Cape Padrone, where the coastline veers westwards, forming the eastern boundary of Algoa Bay. Here it is found that, after a wind change to north-easterly, cold water is upwelled along the shoreline between 19 hours and 2.5 days later. Such upwelling progresses north-eastwards with the movement of the wind and weather systems, although colder water also moves south-westwards into Algoa Bay. Winds, currents, sea level and sea temperatures are highly correlated, with fluctuations in sea level measuring >50 cm being associated with coastal trapped waves (CTWs). Such barotropic wind-driven CTWs are frequently active during upwelling, although it is unclear whether there is any interaction between the two phenomena.     

Sea level variation in the eastern Asia

Mean sea level variations in the eastern Asia region during 1950 to 1991 are investigated with the use of observed sea level data at 16 stations. It is suggested from the data analysis, that the main cause of long-term sea level variation in this region may be the plate tectonic processes. The mean sea levels along the eastern coasts of Japan and the Philippines, and that along the southern coast of Indonesia have risen due to the subsidence of Pacific, Philippine and Australian plates under the Eurasian plate, respectively. On the other hand, the mean sea levels along the western coasts of Japan and the Philippines, and that along the northern coast of Indonesia have fallen. The distribution map of mean sea level rise at the year 2030 from 1985 in this region is presented on the basis of the results of this work and IPCC (1990).     

Response of the low-frequency fluctuations of sea level to atmospheric forcing along the coasts of the Huanghai Sea and the East China Sea

In this paper, the low-frequency fluctuations of sea level and their relationship to atmospheric forcing along the coasts of the Huanghai Sea and the East China Sea are studied. Spectrum analyses are made for the time series of daily mean sea level, atmospheric pressure and wind stress at seven coastal stations. It is found that at all the stations, the main part of the energy of the sea level fluctuations, within the (2-60)-day period, is concentrated on the (12-60)-day period band and that an obvious spectral peak appears at the 3-day period. Along the coast of the Huanghai Sea, variations in the sea level are greater in winter than in summer. In winter, along the coasts of the Huanghai Sea and the East China Sea there is a kind of sea level fluctuations propagating southwards. Among the many factors causing sea level variation, the most obvious one is atmospheric pressure, followed next by the alongshore wind stress.     

Coastal-Trapped Waves with Several-Day Period Caused by Wind along the Southeast Coast of Honshu, Japan

Generation and propagation of several-day period fluctuations along the southeast coast of Honshu, Japan, were investigated by analyzing sea level data and by using a numerical model. The sea level data obtained at twelve stations from Choshi to Omaezaki in fall in 1991, showed energy peaks at the 3–6 day period at the eastern stations in this coast. Time lags of the 3–6 day period fluctuations between station and station indicate westward propagation along the coast. However, the energy level of the 3–6 day period fluctuations suddenly decreased south of the Izu Peninsula. Numerical experiments using a two-layer model were performed to clarify the generation and propagation mechanism of the several-day period fluctuations by periodical wind in fall. The amplitude distributions of observed sea level were qualitatively explained by a coastal-trapped wave (CTW) in the numerical experiment. From the discussions on propagation of a free wave, CTW with the characteristics of a shelf wave generated by the wind along the northeast of the Boso Peninsula was separated into two types of wave at the southeast of the peninsula. One is an internal Kelvin wave with large interface displacement and the other is the shelf wave propagating over the northern part of the Izu Ridge. The sudden decrease in the surface displacement with the 3–6 day period observed at the western stations is considered to be due to the local effect of the wind and phase relation between the internal Kelvin wave and shelf wave.     

Propagating tsunami wave and subsequent resonant response signals detected by HF radar in the Kii Channel,Japan

Signals from the tsunami waves induced by the March 11, 2011 moment magnitude (M_w) 9.0 Tohoku-Oki earthquake and from subsequent resonances were detected as radial velocity variability by a high-frequency ocean surface radar (HF radar) installed on the eastern coast of the Kii Channel, at a range of about 1000 km from the epicenter along the eastern to southern coasts of Honshu Island. A time–distance diagram of band-passed (9–200 min) radial velocity along the beam reveals that the tsunami waves propagated from the continental shelf slope to the inner channel as progressive waves for the first three waves, and then natural oscillations were excited by the waves; and that the direction of the tsunami wave propagation and the axis of the natural oscillations differed from that of the radar beam. In addition, spectral analyses of the radial velocities and sea surface heights obtained in the channel and on the continental shelf slope suggest complex natural oscillation modes excited by the tsunami waves.     

Continental shelf waves off the Fukushima coast

Numerical experiments are performed on shelf waves forced by wind stress with a spectral peak around a period of 100 hr. Water depth in the numerical model is a function of offshore distance only and resembles a bathymetric profile off the Fukushima coast. A pair of vortices alined in the offshore direction and a large vortex are reproduced and they propagate southward outside the forced region. Judging from the propagation speed, the former corresponds to the second-mode and the latter to the first-mode shelf waves. In the forced region, the propagation speed of a trough and a ridge is slow, 3–5km hr^–1. These propagation characteristics reproduce those observed along the Fukushima coast and this propagation speed corresponds to that of second-and third-mode shelf waves. Thus, it is concluded that the periodical current fluctuations observed in the inshore region along the Fukushima coast are due to motions associated with the second-and third-mode shelf waves.     

太平洋涌浪分布特征及其对海-气边界层的影响

本文对海-气边界层波致风机制的相关理论进行了阐述,并利用ERA-40再分析资料给出了太平洋谱峰速度、波龄、波陡等描述涌浪和波致风机制物理量的年际和季节空间分布特征。分析表明:东太平洋赤道地区等海域涌浪速度最大且涌浪由南向北传播明显;太平洋波边界层高度基本呈现出东高西低的分布形势;波致风机制主要发生在赤道热带海域,北半球夏季波致风机制偏强,冬季偏弱,南半球反之;北半球北部海域夏季更易发生波致风机制,赤道附近海域相反;南海为风浪与涌浪组成的混合浪,对其波候等相关研究有必要分开进行讨论。     

大亚湾冬季水位的亚潮变化及其与南海的耦合

本文应用常规时间序列谱分析方法和频域的多输入线性模型研究了冬季广东省大亚湾内水位的亚潮变化及其与大亚湾本地和外海远处各种强迫作用因素间的关系。结果表明：冬季在亚湾亚潮水位的能量主要集中在６．４ｄ和３．６ｄ频带,而在１０．７ｄ频带还有一较弱谱峰,同期广东沿海风的低频能量也主要集中于２－７ｄ频段。造成冬季亚潮水位变化的原因包含了大亚湾本地气象条件的影响,但主要是远地因素作用于大亚湾的结果。外海影响一方     

渤、黄、东海8个主要分潮的数值模拟研究

应用MIKE数值模拟软件,采用无结构三角形网格,建立一套计算区域包括整个渤海、黄海、东海以及东海大陆架和琉球群岛的高分辨率数值模型,考虑了实际水深和岸线,外海开边界采用西北太平洋大模型结果的潮位提供,模拟了东中国海潮波的波动过程,对潮波垂直运动过程进行调和分析,得到了渤海、黄海、东海的M2,S2,K1,O1以及N2,K2,P1,Q1八个主要分潮的传播和分布特征。利用中国沿海14个潮位站的调和常数对模型结果进行了验证,验证结果显示模型较为准确可靠。研究结果表明:4个主要半日潮(全日潮)在渤、黄、东海的传播情形基本相似,即潮波在渤海、黄海、东海沿岸的传播性质上类似沿岸开尔文波的传播形态,并且成功再现了计算海域的4个半日分潮无潮点和2个全日分潮无潮点。全日潮振幅各无潮点附近振幅最小,而海湾的波腹区振幅最大,东海潮差呈现近岸方向振幅大、离岸方向振幅小,浙闽沿海振幅也较大,黄海振幅相对较小,渤海振幅在辽东湾和渤海湾顶最大,两个无潮点周边振幅较小。     

Measurements of the atmospheric turbulence in the coastal zone of the sea during weak wind from a mountainous coast

Results of measurements of the atmospheric turbulence in the layer between 1.5 and 21 m above sea level and the drag coefficient of the sea surface as the wind blows from a 4-km-long mountainous slope with a mean inclination of 11° are presented. The measurements of wind-speed profiles and its fluctuations at several levels, waves, and the main meteorological parameters were carried out in autumn 2005 and 2008 from a stationary platform located in the Black Sea at a distance of approximately 1 km from the southern coast of Crimea. It is shown that during weak synoptic wind a low-level wind jet develops at night over the sea with a maximum velocity up to 5–6 m/s at a level of approximately 6 m over the sea induced by the katabatic wind over the coastal slope. According to the approximate estimates, the horizontal scale of the low-level jet can reach a few tens of kilometers. This flow is characterized by the dissipation rate of the turbulence energy independent of height and low-frequency velocity fluctuations related to the gravity waves and advection of turbulence from the coast. It is shown that the lower part of the boundary layer (up to a height of 3 m) is adjusted to the sea-surface roughness. The dependencies of the drag coefficient on the wind speed or wave age are steadier than in the data for the open sea. However, the age of the waves is not a universal parameter at long and short fetches.     

Numerical experiment on <Emphasis Type="Italic">Kyucho</Emphasis> around the Tango Peninsula induced by Typhoon 0406

A numerical experiment using a three dimensional level model was performed to clarify the mechanism generating a strong coastal current, Kyucho, induced by the passage of Typhoon 0406 around the tip of the Tango Peninsula, Japan in June 2004. Wind stress accompanied by Typhoon 0406 was applied to the model ocean with realistic bottom topography and stratification condition. The model well reproduced the characteristics of Kyucho observed by Kumaki et al. (2005), i.e., the strong alongshore current with maximum velocity of 53 cm s⁻¹ and its propagation along the peninsula with propagation speed of about 0.6 m s⁻¹ one half-day after the typhoon’s passage. Coastal-trapped waves (CTW) accompanied by downwelling were induced along the northwest coast of the peninsula by the alongshore wind stress. The energy density flux due to the CTW flowed eastward along the coast, and indicated scattering of the CTW around the eastern coast of the peninsula. In addition, significant near-inertial internal gravity waves were also caused in the offshore region from the west of the Noto Peninsula to the north of the Tango Peninsula by the typhoon’s passage. The energy flux density of the near-inertial fluctuations flowed southward off the Fukui coast, and part of the energy flux was trapped on the tip of the Tango Peninsula, flowing with the coast on its right. It was found that the strong current, Kyucho, at the northeastern tip of the Tango Peninsula was generated by superposition of the near-inertial internal gravity waves and subinertial CTW.     

Subinertial and seasonal variations in the Soya Warm Current revealed by HF ocean radars,coastal tide gauges,and bottom-mounted ADCP

Subinertial and seasonal variations in the Soya Warm Current (SWC) are investigated using data obtained by high frequency (HF) ocean radars, coastal tide gauges, and a bottom-mounted acoustic Doppler current profiler (ADCP). The HF radars clearly captured the seasonal variations in the surface current fields of the SWC. Almost the same seasonal cycle was repeated in the period from August 2003 to March 2007, although interannual variations were also discernible. In addition to the annual and interannual variations, the SWC exhibited subinertial variations with a period of 5–20 days. The surface transport by the SWC was significantly correlated with the sea level difference between the Sea of Japan and Sea of Okhotsk for both the seasonal and subinertial variations, indicating that the SWC is driven by the sea level difference between the two seas. The generation mechanism of the subinertial variation is discussed using wind data from the European Centre for Medium-range Weather Forecasts (ECMWF) analyses. The subinertial variations in the SWC were significantly correlated with the meridional wind stress component over the region. The subinertial variations in the sea level difference and surface current delay from the meridional wind stress variations by one or two days. Sea level difference through the strait caused by wind-generated coastally trapped waves (CTWs) along the east coast of Sakhalin and west coast of Hokkaido is considered to be a possible mechanism causing the subinertial variations in the SWC.     

Continental shelf waves off the fukushima coast Part I: Observations

Current records obtained in the inshore region along the Fukushima coast are analyzed. The existence of periodical current fluctuations whose period is about 100 hours and whose amplitude is as large as 15–25cm s^–1 is recognized. Auto-spectral analyses are made also for sea level, atmospheric pressure and wind records. Each spectrum has significant peaks at the similar period to the current spectrum. The wind spectrum has a broad peak compared with the current. The periodical current fluctuations propagate southward with speed of 3–5 km h^–1. These propagation speeds seem to correspond to those of the second-and third-mode shelf waves.     

Modeling of the branches of the Tsushima Warm Current in the Eastern Japan Sea

The branches of the Tsushima Warm Current (TWC) are realistically reproduced using a three-dimensional ocean general circulation model (OGCM). Simulated structures of the First Branch and the Second Branch of the TWC (FBTWC and SBTWC) in the eastern Japan Sea are mainly addressed in this study, being compared with measurement in the period September–October 2000. This is the first numerical experiment so far in which the OGCM is laterally exerted by real volume transports measured by acoustic Doppler current profiler (ADCP) through the Tsushima Straits and the Tsugaru Strait. In addition, sea level variation measured by tide-stations along the Japanese coast as well as satellite altimeters is assimilated into the OGCM through a sequential data assimilation method. It is demonstrated that the assimilation of sea level variation at the coastal tide-stations is useful in reproducing oceanic conditions in the nearshore region. We also examine the seasonal variation of the branches of the TWC in the eastern Japan Sea in 2000. It is suggested as a consequence that the FBTWC is continuous along northwestern Honshu Island in summertime, while it degenerates along the coast between the Sado Strait and the Oga Peninsula in other seasons. On the other hand, a mainstream of the SBTWC exists with meanders and eddies in the offshore region deeper than 1000 m to the north of the Sado Island throughout the year.     

Characteristics of coastal-trapped waves induced by typhoon along the southeast coast of Honshu,Japan

Mooring observations using ADCP, electromagnetic current meters and thermometers were performed to clarify the vertical and horizontal structure of coastal-trapped waves (CTWs) on continental shelf and slope on the eastern side of Sagami Bay, Japan, in August and September 2003. A strong inflow associated with CTW caused by Typhoon 0315 (CTW15) was observed with remarkable downwelling. The maximum current due to CTW15 was over 100 cm s⁻¹, confined to the upper layer shallower than 90 m. The CTW (CTW10) induced by Typhoon 0310, was associated with the coastal upwelling and maximum outflow was 33 cm s⁻¹; the currents were extended near the bottom at 230 m depth. Remarkable discrepancies were found between the current structures of CTWs. CTW15 was explained by superposing the second CTW mode on the first CTW mode, whereas CTW10 was explained by the first CTW mode. The generation and propagation processes of both CTWs were reproduced by numerical experiments using a three-dimensional level model. The model results indicated that the difference of modal characteristics between CTW15 and CTW10 already exists in the CTW generation region and are due to difference of the wind direction, i.e., the typhoon’s path.     

Vertical structure variability in a seasonal simulation of a medium-resolution regional model of the Eastern South Pacific

A seasonal simulation from a medium-resolution ocean general circulation mode (OGCM) is used to investigate the vertical structure variability of the Southeast Pacific (SEP). The focus is on the extra-tropical Rossby wave (ETRW) variability and associated forcing mechanism. Some aspects of the model mean state are validated from available observations, which justifies a vertical mode decomposition of the model variability. The analysis of the baroclinic mode contributions to sea level indicates that the gravest mode is dominant over most of the domain at all frequencies. Annual variability is on average twice as large as the semi-annual variability which is confined near the coast for all the modes. The first baroclinic mode contribution to the annual cycle exhibits a clear westward propagation north of the critical latitude. The higher-order modes only contribute near the coast where they are associated with vertically propagating energy. The residual variability, which is the energy at all timescales other than annual and semi-annual periods peaks offshore between 20°S and 30°S for all baroclinic modes. The third baroclinic mode also exhibits a relative maximum variability off the coast of Peru south of the critical latitude of the annual cycle (13°S), where the Peru–Chile Undercurrent is the most intense. Sensitivity experiments to the atmospheric and boundary forcing suggest that the residual variability results from the non-linear interaction between annual Rossby waves and the mean flow, while the annual ETRWs in the model result from the summed-contribution from both the local wind stress and remote equatorial forcing. Overall the study extends the classical analysis of sea level variability in the SEP based on linear theory, and suggests that the peculiarities of the baroclinic modes need to be taken into account for interpreting the sea level variability and understanding its connection with the equatorial variability.     

山东半岛东部南北岸典型砂质海岸沉积、地貌的横向差异及成因分析———以海阳万米海滩岸段和威海国际海水浴场岸段为例

根据2011年在海阳万米海滩岸段与威海国际海水浴场岸段调查获得的夏、冬两季海岸实测地形剖面与沉积物粒度数据,并收集相关水文资料,对南北两海岸地貌与沉积差异性进行分析,探讨了半岛东部南北岸典型砂质海岸动力环境的差异。研究结果表明,南部海岸宽广平缓,发育滩脊、滩肩、沙波纹等地貌,沉积物在水下岸坡上段以中粗砂、中细砂为主,水下岸坡以下段以粉砂、黏土质粉砂为主;北部海岸地形陡,发育滩肩陡坎、水下沙坝等地貌,沉积物以砾质砂为主。导致这些差异的动力为风、波浪、潮汐及沿岸流堆积。     

东亚沿岸近期相对海平面的升降趋势

根据东亚沿岸４５个水位观测站的长序列水位资料，用不同的计算区域平均海平面升降的方法，估计了该区海平面升降趋势。结果表明，从本世纪５０年代初至９０年代初，整个海区平均而言海平面呈上升趋势。海平面升降的区域性变化较大：中国沿岸除山东半岛外，其他海区平均是上升的，在日本群岛南部和朝鲜半岛南部沿岸，由几种方法得出的结果多数是上升的，但上升幅度很小。本文对东亚沿岸海平面升降的估计结果与Ｂａｒｎｅｔｔ的相应估计差别较大，其主要原因是Ｂａｒｎｅｔｔ选站较少，且选的站集中在该区南北两端，中间部分无资料；估计方法虽有影响，但属次要的。