Satellite observation and model simulation of upper ocean biophysical response to Super Typhoon Nakri

In this paper, we use multi-satellite remote sensing data and Regional Ocean Model System (ROMS) to examine the physical and biological responses to Typhoon Nakri during a period from 1 to 8 June 2008 in the western North Pacific. From satellite observations, we find remarkable surface cooling (∼5 °C) and distinct phytoplankton enhancement after Nakri passage. It is interesting to note that in contrast to the well-documented rightward bias of the surface cooling response, the maximum biological response to Nakri occurred on the left of the Nakri’s track rather than the right side, where the most distinct cooling response occurred. To reveal the dynamic mechanism for this unusual phenomenon, we have done numerical experiments with and without the influence of preexisting cyclonic circulation and previous typhoon. The results from simulations show that both the physical and biological responses to Nakri are greatly affected by the preexisting cyclonic circulations. The discrepancy between the locations of maximum physical and biological responses mainly results from the advanced uplift and current advection of nutrient fields by previous typhoon Rammasun, which passed through the area about two weeks before Nakri.     

Ocean response to typhoon Nuri (2008) in western Pacific and South China Sea

Typhoon Nuri formed on 18 August 2008 in the western North Pacific east of the Philippines and traversed northwestward over the Kuroshio in the Luzon Strait where it intensified to a category 3 typhoon. The storm weakened as it passed over South China Sea (SCS) and made landfall in Hong Kong as a category 1 typhoon on 22 August. Despite the storm’s modest strength, the change in typhoon Nuri’s intensity was unique in that it strongly depended on the upper ocean. This study examines the ocean response to typhoon Nuri using the Princeton Ocean Model. An ocean state accounting for the sea-surface temperature (SST) and mesoscale eddy field prior to Nuri was constructed by assimilating satellite SST and altimetry data 12 days before the storm. The simulation then continued without further data assimilation, so that the ocean response to the strong wind can be used to understand processes. It is found that the SST cooling was biased to the right of the storm’s track due to inertial currents that rotated in the same sense as the wind vector, as has previously been found in the literature. However, despite the comparable wind speeds while the storm was in western Pacific and SCS, the SST cooling was much more intense in SCS. The reason was because in SCS, the surface layer was thinner, the vorticity field of the Kuroshio was cyclonic, and moreover a combination of larger Coriolis frequency as the storm moved northward and the typhoon’s slower translational speed produced a stronger resonance between wind and current, resulting in strong shears and entrainment of cool subsurface waters in the upper ocean.     

双模态风垂直切变对台风结构和强度的作用分析

本文应用WRF(v 3.4)模式输出资料,揭示了风垂直切变(Vertical Wind Shear:VWS)在垂直方向上的波状变化特征,这种波状变化在台风不同发展时期又有不同形态,其中在持续强盛期呈双模态分布.应用VWS引起的次级垂直环流影响台风对流分布和强度变化的基本原理,用模式资料分析发现:对流层中层具有的VWS是整层VWS的主要部分,台风强度变化滞后VWS的形态突变6h左右;双模态波状变化的VWS产生的次级环流和台风垂直环流的配置不同使台风强对流带结构变得不对称及眼墙区对流强度在垂直分布上变得不均匀,随着持续强盛期涡旋运动的增强,强对流带分布又趋于对称.又根据VWS形成的垂直方向上涡度力分布不均匀引起台风内中尺度滚轴状对流带不稳定发展原理,分析表明:对流层中、低层的涡度力有利于对流不稳定增强,垂直速度的最大值与风垂直廓线的拐点在同一高度上,这与理论模型的结论一致.因此,VWS的波状变化分布特征不仅影响台风强对流带中尺度结构的改变,也对台风持续强盛具有重要作用;同时也是台风内滚轴状对流带不稳定的可能启动机制.     

Impact of ocean–wave coupling on typhoon-induced waves and surge levels around the Korean Peninsula: a case study of Typhoon Bolaven

Typhoon-induced waves and surges are important when predicting potential hazards near coastal regions. In this paper, we applied a coupled modeling system for ocean–wave interaction to examine prediction capabilities for typhoon-induced waves and surges around the Korean Peninsula. To identify how ocean–wave coupling impacts wave and surge simulations during typhoon conditions, a set of comparative experiments was performed during Typhoon Bolaven (2012): (1) a fully coupled ocean–wave model, (2) a one-way coupled ocean–wave model without surface current feedback and ocean-to-wave water levels, and (3) a stand-alone ocean model without considering wave-based sea surface roughness (SSR). When coupled with the ocean model, the surface current reduced significantly the wave height on the right-hand side of the advancing typhoon track and improved prediction accuracy along the southern coast of Korea. Compared with the observed surge levels, the simulated surge height yielded improved results for peak height magnitude and timing compared with the uncoupled model. For wave-to-surge feedback, we found that wave-induced SSR plays an important role by modulating wind stress in the surface layer. The modulated wind stress directly affected the surge height, which improved surge peak prediction during the typhoon.     

Summer circulation structure and formation mechanism in the Beibu Gulf

Due to limited in situ data and diagnostic numerical models, the summer circulation structure and formation mechanism in the Beibu Gulf have always been in controversy in the past 50 years. Therefore, a new three-dimensional hindcast model was built within the northwestern South China Sea(SCS), forced with the daily averaged wind, heat flux, lateral flux, as well as tidal harmonic and eight major rivers discharges. And the east boundary was set up far away off the Qiongzhou Strait(QS). Lastly, the model results were consistent with not only the synchronous observation data from the project 908 but also the historical observed data. As a result, the summer circulation structure was revealed that the southern Gulf was occupied by an anticyclonic eddy whereas the northern Gulf was dominated by a cyclonic gyre. Although the circulation major structure was stable, its area and strength had yearly and monthly oscillation. The other three sensitive experiments indicated that the circulations in the southern and northern Gulf were driven by the SCS circulation and monsoon wind, respectively. After the theoretical analysis of the potential vorticity budget, it was further revealed the circulation in the northern Gulf was driven by the positive wind stress curl in summer. Besides, the river discharge was also significant as the vertical circulation had two layer structures outside the mouth of the Red River. Generally, this work calls for the further research on other subjects, such as ocean biogeochemical or marine fisheries.     

Parameterization of typhoon-induced ocean cooling using temperature equation and machine learning algorithms: an example of typhoon Soulik (2013)

This study proposed three algorithms that can potentially be used to provide sea surface temperature (SST) conditions for typhoon prediction models. Different from traditional data assimilation approaches, which provide prescribed initial/boundary conditions, our proposed algorithms aim to resolve a flow-dependent SST feedback between growing typhoons and oceans in the future time. Two of these algorithms are based on linear temperature equations (TE-based), and the other is based on an innovative technique involving machine learning (ML-based). The algorithms are then implemented into a Weather Research and Forecasting model for the simulation of typhoon to assess their effectiveness, and the results show significant improvement in simulated storm intensities by including ocean cooling feedback. The TE-based algorithm I considers wind-induced ocean vertical mixing and upwelling processes only, and thus obtained a synoptic and relatively smooth sea surface temperature cooling. The TE-based algorithm II incorporates not only typhoon winds but also ocean information, and thus resolves more cooling features. The ML-based algorithm is based on a neural network, consisting of multiple layers of input variables and neurons, and produces the best estimate of the cooling structure, in terms of its amplitude and position. Sensitivity analysis indicated that the typhoon-induced ocean cooling is a nonlinear process involving interactions of multiple atmospheric and oceanic variables. Therefore, with an appropriate selection of input variables and neuron sizes, the ML-based algorithm appears to be more efficient in prognosing the typhoon-induced ocean cooling and in predicting typhoon intensity than those algorithms based on linear regression methods.     

State analysis using the Local Ensemble Transform Kalman Filter (LETKF) and the three-layer circulation structure of the Luzon Strait and the South China Sea

A new circulation model of the western North Pacific Ocean based on the parallelized version of the Princeton Ocean Model and incorporating the Local Ensemble Transform Kalman Filter (LETKF) data assimilation scheme has been developed. The new model assimilates satellite data and is tested for the period January 1 to April 3, 2012 initialized from a 24-year simulation to estimate the ocean state focusing in the South China Sea (SCS). Model results are compared against estimates based on the optimum interpolation (OI) assimilation scheme and are validated against independent Argo float and transport data to assess model skills. LETKF provides improved estimates of the western North Pacific Ocean state including transports through various straits in the SCS. In the Luzon Strait, the model confirms, for the first time, the three-layer transport structure previously deduced in the literature from sparse observations: westward in the upper and lower layers and eastward in the middle layer. This structure is shown to be robust, and the related dynamics are analyzed using the results of a long-term (18 years) unassimilated North Pacific Ocean model. Potential vorticity and mass conservations suggest a basin-wide cyclonic circulation in the upper layer of the SCS (z?>??570 m), an anticyclonic circulation in the middle layer (?570 m?≥?z?>??2,000 m), and, in the abyssal basin (<?2,000 m), the circulation is cyclonic in the north and anticyclonic in the south. The cyclone–anticyclone abyssal circulation is confirmed and explained using a deep-layer reduced-gravity model as being caused by overflow over the deep sill of the Luzon Strait, coupled with intense, localized upwelling west of the strait.     

Impact of eddies on ocean diapycnal mixing in Gulf Stream region

Based on high-resolution,Array for Real-time Geostrophic Oceanography(Argo)profiles and Sea Level Anomaly(SLA)data,this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region.The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches4.0×10-5 m2 s-1.This is significantly higher than the 1.6×10-5 m2 s-1 outside eddies and 0.8×10-5 m2 s-1 in cyclonic eddies.Probabilities of diapycnal diffusivity greater than 10-4 m2 s-1 within anticyclonic and cyclonic eddies and outside eddies are29%,5%and 12%,respectively.However,magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order,10-5 m2 s-1.Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior,and 22 were observed during or shortly after strong winds.The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced,near-inertial wave energy propagates downward in anticyclonic eddies.The deeper part of 12 profiles(below 540 m)in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m,among which three profiles had overturns reaching 20 m.Enhanced mixing may have occurred in deep layers of some profiles,although it was not evident in average conditions.     

大气季节内振荡的活动与江淮流域夏季旱涝

观测资料的分析极为清楚地表明,江淮流域的夏季降水有着极为明显的低频变化,周期为30-60d和近20d的振荡是其最基本的特征,尤其是在多雨的年份.对应江淮夏季多雨(涝)年和少雨(旱)年,大气环流的分析表明其大气季节内振荡(IS0)的形势有着显著的差异.例如在多雨(少雨)年,在长江以南的850hPa上为一个低频(IS0)反气旋(气旋)性环流控制,而中国北部和日本一带为气旋(反气旋)性环流,从而在江淮流域形成较强的低频辐合(辐散)气流;在200hPa的青藏高原上却为一个低频气旋(反气旋)性环流所控制.分析还表明,对应多雨年,在江淮流域有明显的由中高讳度向南传播和由低玮度向北传播的大气低频振荡的汇合情况;而对应于少雨年,由中高纬度向南传播的低频系统较不明显,在江淮流域低频系统的汇合也较为不清楚.     

台风激发的第二类地脉动特征及激发模式分析

0.003~1 Hz频段的地脉动主要来源于海浪运动与固体地球的耦合作用,台风引起的强烈海浪运动往往可使地脉动能量显著增强.由于涉及大气-海洋-固体地球三个圈层之间的复杂动量传递与耦合过程,迄今为止,关于台风激发地脉动的具体源区位置及激发机制尚存在争议.本文选取日本、中国东南沿海及台湾地区的地震台站波形连续记录,研究了2008年台风"森拉克"和"黑格比"激发地脉动的时频特征,开展相应数值模拟,并与观测数据进行了对比分析研究.结果表明台风激发第二类地脉动存在两种主要模式:(1)近岸源区激发,即台风引起波浪入射至海岸反射并与后续来波相互作用形成驻波作用于海底而激发;(2)台风中心附近源区激发,即台风中心移动过程中不同时期激发的同频率波浪相向传播、相互作用产生驻波作用于海底而激发,源区位置主要集中于台风中心左后方.此外,结合波浪再分析数据、台风风场特征,我们进一步对第二类地脉动激发过程中的影响因素进行了分析,发现:第一种模式激发的地脉动与近岸源区波浪场强度、观测点至源区距离及台风中心至海岸线距离等因素相关;而第二种模式激发的地脉动则主要受台风中心附近波浪场的频率成分与传播方向影响.     

On the variability of the flow along the Meso-American Barrier Reef system: a numerical model study of the influence of the Caribbean current and eddies

A high resolution (3–8 km grid), 3D numerical ocean model of the West Caribbean Sea (WCS) is used to investigate the variability and the forcing of flows near the Meso-American Barrier Reef System (MBRS) which runs along the coasts of Mexico, Belize, Guatemala and Honduras. Mesoscale variations in velocity and temperature along the reef were found in seasonal model simulations and in observations; these variations are associated with meandering of the Caribbean current (CC) and the propagation of Caribbean eddies. Diagnostic calculations and a simple assimilation technique are combined to infer the dynamically adjusted flow associated with particular eddies. The results demonstrate that when a cyclonic eddy (negative sea surface height anomaly (SSHA)) is found near the MBRS the CC shifts offshore, the cyclonic circulation in the Gulf of Honduras (GOH) intensifies, and a strong southward flow results along the reef. However, when an anticyclonic eddy (positive SSHA) is found near the reef, the CC moves onshore and the flow is predominantly westward across the reef. The model results help to explain how drifters are able to propagate in a direction opposite to the mean circulation when eddies cause a reversal of the coastal circulation. The effect of including the Meso-American Lagoon west of the Belize Reef in the model topography was also investigated, to show the importance of having accurate coastal topography in determining the variations of transports across the MBRS. The variations found in transports across the MBRS (on seasonal and mesoscale time scales) may have important consequences for biological activities along the reef such as spawning aggregations; better understanding the nature of these variations will help ongoing efforts in coral reef conservation and maintaining the health of the ecosystem in the region.     

Hydrodynamic modelling of mesoscale eddies in the Black Sea

A three dimensional structure of mesoscale circulation in the Black Sea is simulated using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System. A number of sensitivity tests reveal the response of the model to changes in the horizontal resolution, time steps, and diffusion coefficients. Three numerical grids are examined with x-fine (3.2 km), fine (6.7 km) and coarse (25 km) resolution. It is found that the coarse grid significantly overestimates the energy of the currents and is not adequate even for the study of basin-scale circulation. The x-fine grid, on the other hand, does not give significant advantages compared to the fine grid, and the latter is used for the bulk of simulations. The most adequate parameters are chosen from the sensitivity study and used to model both the basin-scale circulation and day-to-day variability of mesoscale currents for the months of May and June of 2000. The model is forced with actual wind data every 6 h and monthly climatic data for evaporation, precipitation, heat fluxes and river run-off. The results of the fine grid model are compared favourably against the satellite imagery. The model adequately reproduces the general circulation and many mesoscale features including cyclonic and anticyclonic eddies, jets and filaments in different parts of the Black Sea. The model gives a realistic geographical distribution and parameters of mesoscale currents, such as size, shape and evolution of the eddies.     

Mesoscale dynamics and walleye pollock catches in the Navarin Canyon area of the Bering Sea

Large canyons incise the shelf break of the eastern Bering Sea to be preferred sites of the cross-shelf exchange. The mesoscale eddy activity is particularly strong near the shelf-break canyons. To study the mesoscale dynamics in the Navarin Canyon area of the Bering Sea, the time series of velocities derived from AVISO satellite altimetry between 1993 and 2015, drifters, Argo buoys, and ship-borne data are analyzed. We demonstrate that the strength of anticyclonic eddies along the shelf edge in spring and summer is determined by the wind stress in March–April. The increased southward wind stress in the central Bering Sea forced a supply of low-temperature and low-salinity outer shelf water to the deep basin and formation of the anticyclonic mesoscale circulation seaward of the Navarin Canyon. Enhanced northwestward advection of the Bering Slope Current water leads to increase in an ice-free area in March and April and increased bottom-layer temperature at the outer shelf. The strong (weak) northwestward advection of the eastern Bering Sea waters, determined by eastern winds in spring, creates favorable (unfavorable) conditions for the pollock abundance in the western Navarin Canyon area in summer.     

Upper layer structure of typhoon

Summary Upper layer structure of the Typhoon No. 5405 ? Grace ? in 1954 was investigated on synoptic weather maps. Two anticyclonic eddies was clearly shown on both sides of the cyclonic center. Assist. Prof.T. Masui, Yokohama National University, Yakinoshita 929,The City of Kamakura, Kanagawa-ken, Japan.     

Eddy energy sources and mesoscale eddies in the Sea of Okhotsk

Based on eddy-permitting ocean circulation model outputs, the mesoscale variability is studied in the Sea of Okhotsk. We confirmed that the simulated circulation reproduces the main features of the general circulation in the Sea of Okhotsk. In particular, it reproduced a complex structure of the East-Sakhalin current and the pronounced seasonal variability of this current. We established that the maximum of mean kinetic energy was associated with the East-Sakhalin Current. In order to uncover causes and mechanisms of the mesoscale variability, we studied the budget of eddy kinetic energy (EKE) in the Sea of Okhotsk. Spatial distribution of the EKE showed that intensive mesoscale variability occurs along the western boundary of the Sea of Okhotsk, where the East-Sakhalin Current extends. We revealed a pronounced seasonal variability of EKE with its maximum intensity in winter and its minimum intensity in summer. Analysis of EKE sources and rates of energy conversion revealed a leading role of time-varying (turbulent) wind stress in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk in winter and spring. We established that a contribution of baroclinic instability predominates over that of barotropic instability in the generation of mesoscale variability along the western boundary of the Sea of Okhotsk. To demonstrate the mechanism of baroclinic instability, the simulated circulation was considered along the western boundary of the Sea of Okhotsk from January to April 2005. In April, the mesoscale anticyclonic eddies are observed along the western boundary of the Sea of Okhotsk. The role of the sea ice cover in the intensification of the mesoscale variability in the Sea of Okhotsk was discussed.     

Annual variation of sea surface height,dynamic topography and circulation in the South China Sea

TOPEX/Poseidon satellite altimetry data from 1993 to 1999 were used to study mean annual variation of sea surface height anomaly (SSHA) in the South China Sea (SCS) and to reproduce its climatological monthly surface dynamic topography in conjunction with historical hydrographic data. The characters and rules of seasonal evolution of the SCS dynamic topography and its upper circulation were then discussed. Analyses indicate that annual variation of the SCS large-scale circulation could be divided into four major phases. In winter (from November to February), the SCS circulation is mainly controlled by double cyclonic gyres with domination of the northern gyre. Other corresponding features include the Kuroshio intrusion from the Luzon Strait and the northeastward off-shelf current in the area northwest off Kalimantan Island. The double gyre structure disassembled in spring (from March to April) when the northern gyre remains cyclonic, the southern gyre becomes anticyclonic, and the general circulation pattern shows a dipole. There is no obvious large-scale closed gyre inside the SCS basin in both summer (from May to July) and autumn (from August to October) when the SCS Monsoon Jet dominates the circulation, which flows northeastward across the SCS. Even so, circulation patterns of these two phases diverse significantly. From May to July, the SCS monsoon jet flows northward near the Vietnam coast and bends eastward along the topography southeast off Hainan Island at about 18°N forming an anticyclonic turn. It then turns northeastward after crossing the SCS. From August to October, however, the monsoon Jet leaves the coast of Vietnam and enters interior of the basin at about 13°N, and the general circulation pattern becomes cyclonic. The Kuroshio intrusion was not obvious in spring, summer and autumn. It is suggested from these observations that dynamic adjustment of the SCS circulation starts right after the peak period of the prevailing monsoon.     

Seasonal Variations of Currents over the Western Slope of the Middle Caspian Bed

Results of field observations of current dynamics in the frontal zone of the western Middle Caspian are given. The cyclonic circulation over the western slope in winter is shown to be a unidirectional intense current with velocities up to 100 cm/s. In summer, the current slows down and separates into branches—it turns southwestward and westward at the slope depth down to 150 m, southward and southeastward at the depth of ~100–350 m, and eastward at larger depths. In summer, shelf currents interact with the flow of Middle Caspian cyclonic circulation, resulting in that anticyclonic vortices reach the shelf.     

Rapid intensification of Super Typhoon Haiyan: the important role of a warm-core ocean eddy

Super Typhoon Haiyan devastated portions of Southeast Asia, particularly the Philippines, on November 8, 2013. In this paper, observational data are used to analyze the intensification process of Super Typhoon Haiyan. Observational data showed that Typhoon Haiyan intensified and the maximum sustained winds increased to 59 ms^?1 after it encountered a double warm-core ocean eddy, while the central pressure of the typhoon dropped from 970 hPa to 920 hPa. Numerical simulations and observational data show that the presence of the warm-core eddy combined with SST increases due to climate change led to the rapid intensification of Super Typhoon Haiyan. Comparing these two factors, the warm-core ocean eddy, which brings significantly more heat into the upper ocean, plays the leading role in the intensification, with climate warming making a lesser contribution. Moreover, due to the increased thickness of the mixed layer associated with the warm-core ocean eddy, Super Typhoon Haiyan did not significantly decrease the sea surface temperature to the east of the Philippines, as is typical of typhoons, and the largest decrease was approximately 1 °C.     

Lagrangian study of temporal changes of a surface flow through the Kamchatka Strait

Using Lagrangian methods, we analyze a 20-year-long estimate of water flux through the Kamchatka Strait in the northern North Pacific based on AVISO velocity field. It sheds new light on the flux pattern and its variability on annual and monthly time scales. Strong seasonality in surface outflow through the strait could be explained by temporal changes in the wind stress over the northern and western Bering Sea slopes. Interannual changes in a surface outflow through the Kamchatka Strait correlate significantly with the Near Strait inflow and Bering Strait outflow. Enhanced westward surface flow of the Alaskan Stream across the 174°E section in the northern North Pacific is accompanied by an increased inflow into the Bering Sea through the Near Strait. In summer, the surface flow pattern in the Kamchatka Strait is determined by the passage of anticyclonic and cyclonic mesoscale eddies. The wind stress over the Bering basin in winter–spring is responsible for eddy generation in the region.     

台风“梅花”诱发平流层重力波的数值模拟与AIRS观测

为了分析台风这类强对流诱发平流层重力波的过程,本文利用中尺度数值模式WRF-ARW(V3.5)和卫星高光谱红外大气探测器AIRS数据对2011年第9号强热带气旋"梅花"的重力波特征进行了分析.首先,针对模式输出的垂直速度场资料的分析表明,台风在对流层各个方向上几乎都具有诱发重力波的能量,而在平流层内则呈现出只集中于台风中心以东的半圆弧状波动,且重力波到达平流层后其影响的水平范围可达1000km.此外,平流层波动与对流层雨带在形态、位置以及尺度上均具有一定的相似性.其次,对风场的分析结果表明,不同高度上波动形态的差异主要是由于重力波垂直上传的过程中受到了平流层向西传的背景风场以及风切变的调制作用,揭示了重力波逆着背景流垂直上传的特征.随后,基于FFT波谱分析的结果表明,"梅花"诱发的平流层重力波水平波长中心值达到了1000km,周期在15~25h,垂直波长主要在8~12km.最后,利用AIRS观测资料分析了平流层30~40km高度上的大气波动,得到了与数值模拟结果相一致的半圆弧状波动.对比结果也验证了WRF对台风诱发平流层重力波的波动形态、传播方向、不同时刻扰动强度的变化以及影响范围的模拟效果.此外,也揭示了多资料的结合对比有助于更加全面地了解台风诱发平流层重力波的波动特征.