基于WRF模式的青藏高原斜坡和平台加热影响亚洲夏季风的模拟研究

青藏高原大地形的热力强迫作用对亚洲夏季风的形成和发展具有重要的影响.本文利用较高分辨率的WRF区域模式,探讨了高原不同区域(斜坡和平台)的地形加热分别对南亚夏季风和东亚夏季风的影响.结果表明:高原南部喜马拉雅山脉的斜坡地形加热对其周围局地的环流形势和降水影响十分明显,是南亚夏季风北支分量形成和维持的主导因子,也是斜坡上气流爬坡和降水发生的必要条件.斜坡加热对东亚夏季风也有明显的增强作用,它不仅加强了中国东部低空西南季风环流,还会造成北部南下的异常干冷空气的响应.斜坡上的地形加热作用也是对流层高层暖中心位置维持在斜坡上空的一个重要原因.而高原平台加热对季风环流和降水的影响虽然没有喜马拉雅山脉斜坡加热那么显著,但是对南亚夏季风的影响范围更广,对经向哈得来环流影响更明显,能够调控高原以外更远处热带洋面上的西南季风环流.通过比较高原不同区域地形加热条件下的多种季风指数,进一步表明了高原地形加热对南亚和东亚夏季风均有增强作用,但是高原不同区域的地形加热对两类夏季风子系统又会产生不一样的影响.     

Flow patterns at the Chesapeake Bay entrance

The spatial and temporal variability of water entering and leaving the Chesapeake Bay estuary was determined with a spatial resolution of 75 m. The four cruises during which the observations were made took place under different conditions of freshwater discharge, tidal phase, and wind forcing. The tidal variability of the flows was dominated by the semidiurnal constituents that displayed greatest amplitudes and phase lags near the surface and in the channels that lie at the north and south sides of the entrance. The subtidal variability of the flows was classified into two general scenarios. The first scenario occurred during variable or persistently non-southwesterly winds. Under these conditions there was surface outflow and bottom inflow in the two channels, inflow over the shoal between the two channels, and possible anticyclonic gyre formation over the shoal. The flow pattern in the channels was produced by gravitational circulation and wind forcing. Over the shoal it was caused by tidal rectification and wind forcing. The second scenario occurred during persistently southwesterly winds. The anticyclonic gyre over the shoal vanished suggesting that wind forcing dominated the tidal rectification mechanism over the shoal, while gravitational circulation and wind forcing continued to cause the flows in the channels. In both scenarios, most of the volume exchange took place in the channels.     

Two types of summertime heating over Asian large-scale orography and excitation of potential-vorticity forcing II. Sensible heating over Tibetan-Iranian Plateau

Based on analysis and simulation, the interaction of thermal forcing between the Tibetan Plateau (TP) and Iranian Plateau (IP) in summer is investigated. Associated influences on water vapor transport in the Asian subtropical monsoon region and the formation of a cold center in the lower stratosphere over Eurasia are also investigated. Results show that surface sensible heating (SH) over the two plateaus not only have mutual influences but also feedback to each other. SH over the IP can reduce the SH and increase the LH over the TP, whereas the SH over the TP can increase surface heating over the IP, thereby reaching quasi-equilibrium among the SH and LH over the TP, IP SH and atmosphere vertical motion. Therefore, the so-called Tibetan-Iranian Plateau coupling system (TIPS) is constructed, which influences atmosphere circulation. In the TIPS system, interaction between surface SH and LH over the TP plays a leading role. SH of the IP and TP influences on other regions not only have superimposed effects but also mutually offset. Accounting for contributions to the convergence of water vapor transport in the Asian subtropical monsoon region, TP SH contributes more than twice that of the IP. The combined influence of SH over TP and IP represents the major contribution to the convergence of water vapor transport in that region. In addition, the heating effect of TIPS increases the upper tropospheric temperature maximum and lifts the tropopause, cooling the lower stratosphere. Combined with large-scale thermal forcing of the Eurasian continent, the TIPS produces a strong anticyclonic circulation and the South Asian High that warms the upper troposphere and cools the lower stratosphere, thereby affecting regional and global weather and climate.     

Wind effects on the lateral structure of density-driven circulation in Chesapeake Bay

The response of the density-driven circulation in the Chesapeake Bay to wind forcing was studied with numerical experiments. A model of the bay with realistic bathymetry was first applied to produce the density-driven flow under average river discharge and tidal forcing. Subsequently, four spatially uniform wind fields (northeasterly, northwesterly, southwesterly, and southeasterly) were imposed to examine the resulting cross-estuary structure of salinity and flow fields. In general, northeasterly and northwesterly winds intensified the density-driven circulation in the upper and middle reaches of the bay, whereas southeasterly and southwesterly winds weakened it. The response was different in the lower bay, where downwind flow from the upper and middle reaches of the bay competed with onshore/offshore coastal flows. Wind remote effects were dominant, over local effects, on volume transports through the bay entrance. However, local effects were more influential in establishing the sea-level slopes that drove subtidal flows and salinity fields in most of the bay. The effect of vertical stratification on wind-induced flows was also investigated by switching it off. The absence of stratification allowed development of Ekman layers that reached depths of the same order as the water depth. Consequently, bathymetric effects became influential on the homogeneous flow structure causing the wind-induced flow inside the bay to show a marked transverse structure: downwind over the shallow areas and upwind in the channels. In the presence of stratification, Ekman layers became shallower and the wind-induced currents showed weaker transverse structure than those that developed in the absence of stratification. In essence, the wind-driven flows were horizontally sheared under weak stratification and vertically sheared under stratified conditions.     

Suspended sediment fluxes at an intertidal flat: The shifting influence of wave,wind, tidal,and freshwater forcing

Using in situ, continuous, high frequency (8–16 Hz) measurements of velocity, suspended sediment concentration (SSC), and salinity, we investigate the factors affecting near-bed sediment flux during and after a meteorological event (cold front) on an intertidal flat in central San Francisco Bay. Hydrodynamic forcing occurs over many frequency bands including wind wave, ocean swell, seiching (500–1000 s), tidal, and infra-tidal frequencies, and varies greatly over the time scale of hours and days. Sediment fluxes occur primarily due to variations in flow and SSC at three different scales: residual (tidally averaged), tidal, and seiching. During the meteorological event, sediment fluxes are dominated by increases in tidally averaged SSC and flow. Runoff and wind-induced circulation contribute to an order of magnitude increase in tidally averaged offshore flow, while waves and seiching motions from wind forcing cause an order of magnitude increase in tidally averaged SSC. Sediment fluxes during calm periods are dominated by asymmetries in SSC over a tidal cycle. Freshwater forcing produces sharp salinity fronts which trap sediment and sweep by the sensors over short (∼30 min) time scales, and occur primarily during the flood. The resulting flood dominance in SSC is magnified or reversed by variations in wind forcing between the flood and ebb. Long-term records show that more than half of wind events (sustained speeds of greater than 5 m/s) occur for 3 h or less, suggesting that asymmetric wind forcing over a tidal cycle commonly occurs. Seiching associated with wind and its variation produces onshore sediment transport. Overall, the changing hydrodynamic and meteorological forcing influence sediment flux at both short (minutes) and long (days) time scales.     

Relative importance of wind and buoyancy forcing for interdecadal regime shifts in the Pacific Ocean

Interdecadal variations in the Northern Hemisphere and the North Pacific have been documented in many studies[1 4]. The connection between the subtropical North Pacific and the tropics is regarded as the most important process triggering and maintaining t…     

Seasonal variation of upper layer circulation in the northern part of the East/Japan Sea

Seasonal variation of upper layer circulation in the northern part of the East/Japan Sea and its mechanism were investigated using empirical orthogonal function (EOF) analysis with satellite sea surface heights over the northern East/Japan Sea and a three-dimensional circulation model. The spatial structure and temporal variation of first EOF mode, which explains about 64% of the total variance, indicate that a large cyclonic circulation in the northern East/Japan Sea shows a semi-annual variation with maximum strength in summer and winter. According to numerical model result, the Liman Cold Current, accepted as a major current in the northern East/Japan Sea, is well mixed vertically by the winter monsoon and the current in the upper layer has a relatively deep structure, with a maximum westward speed of about 20 cm/s in winter. On the other hand, in summer the current has a stronger baroclinic structure of velocity than in winter. Numerical experiments showed that in summer the temporal variation of upper layer circulation is controlled by thermal forcing, such as sea surface heat flux and inflow of heat transport into the East/Japan Sea through the Korea/Tsushima Strait. Moreover, the cyclonic circulation in the upper layer of the northern East/Japan Sea is also generated and strengthened by the positive wind stress curl occupying most of the East/Japan Sea during the winter. The seasonal variation of each forcing that drives the circulation is responsible for the strength or weakness of the upper layer circulation in the northern East/Japan Sea. The contribution of each forcing to the seasonal variation of the upper layer circulation is examined through sensitivity experiments. According to these numerical experiments, the upper layer circulation in the northern East/Japan Sea is strengthened twice a year, in winter and summer, and this semi-annual variation is determined by a combination of wind (winter) and thermal (summer) forcing.     

Coastal and shelf circulation in the vicinity of Camamu Bay (14°S), Eastern Brazilian Shelf

The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002-2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season.     

Can intra-seasonal wind stress forcing strongly affect spring predictability barrier for ENSO in Zebiak–Cane model?

The influence of the uncertainties of intra-seasonal wind stress forcing on Spring Predictability Barrier (SPB) in El Niño–Southern Oscillation (ENSO) prediction is studied with the Zebiak–Cane model and observational wind data which are analyzed with Continuous Wavelet Transform (CWT) and utilized to extract intra-seasonal wind stress signals as external forcing. The observational intra-seasonal wind stress forcing are joined into Zebiak–Cane model to get the Zebiak–Cane-add model and subsequently with the Conditional Nonlinear Optimal Perturbation (CNOP) method, the evolutions of the optimal initial errors (i.e., CNOPs), model errors caused by intra-seasonal wind stress uncertainties, and their joint errors based on ENSO events are calculated. By investigating their error growth rates and prediction errors of Niño-3 indices, the effect of observational intra-seasonal wind stress forcing on seasonal error growth of ENSO is explored and the impact of initial error and model error on ENSO predictability is compared quantitatively. The results show that the model errors led by observational intra-seasonal wind stress forcing could scarcely cause a significant SPB whereas the initial errors and their joint errors can do; hence, the initial errors are most likely the main error source of SPB. In fact, this result emphasizes the primary influence of initial errors on ENSO predictability and lays the basis of adaptive data assimilation for ENSO forecast.     

利用COSMIC RO数据分析青藏高原平流层重力波活动特征

本文利用2006年5月至2013年4月COSMIC干温廓线数据,提取了青藏高原地区大气重力波势能,以此研究了青藏高原大气重力波势能的分布频率模型和大气重力波活动的时空变化特征,并进一步分析了高原大气重力波活动与高原地形、风速和高原大陆热辐射之间的相关性.青藏高原地区大气重力波势能的分布频率服从对数生长分布;青藏高原地区大气重力波在16~18km和28~31km高度较活跃,而在20~26km高度较平静;高原大陆边缘各季节重力波活动均较活跃,而高原大陆上空大气重力波活动呈明显季节性变化,其在冬春季节较活跃,在夏秋季节较平静;2010年冬季青藏高原大气重力波活动异常平静;各季节整个高原上空大气重力波活跃度有随大气高度升高而降低的趋势,高原上低层大气重力波向高层传播会发生耗散作用.地形与风速是影响青藏高原大气重力波活动的重要因素.地形主要影响平流层底部的重力波活动;纬向风比经向风对该地区平流层大气重力波活动的影响大,纬向风总体上会促进高原大气重力波活动.青藏高原大陆热辐射对高原大气的加热作用是导致青藏高原大气重力波活动呈季节性变化的重要因素.     

Development and evaluation of a storm surge warning system in Taiwan

An operational storm surge forecasting system aimed at providing warning information for storm surges has been developed and evaluated using four typhoon events. The warning system triggered by typhoon forecasts from Taiwan Cooperative Precipitation Ensemble Forecast Experiment (TAPEX) has been executed with two storm surge forecasting scenarios with and without tides. Three numerical experiments applying different meteorological inputs have been designed to assess the impact of typhoon forcing on storm surges. One uses synthetic wind fields, and the others use realistic wind fields with and without adjustments to the initial wind fields for the background circulation. Local observations from Central Weather Bureau (CWB) weather stations and tide gauge stations are used to evaluate the wind fields and storm surges from our numerical experiments. The comparison results show that the accuracy of the storm surge forecast is dominated by the track, the intensity, and the driving flow of a typhoon. When the structure of a typhoon is disturbed by Taiwan’s topography, using meteorological inputs from real wind fields can result in a better typhoon simulation than using inputs from synthetic wind fields. The driving flow also determines the impact of topography on typhoon movement. For quickly moving typhoons, storm forcing from TAPEX is reliable when a typhoon is strong enough to be relatively unaffected by environmental flows; otherwise, storm forcing from a sophisticated typhoon initialization scheme that better simulates the typhoon and environmental flows results in a more accurate prediction of storm surges. Therefore, when a typhoon moves slowly and interacts more with the topography and environmental flows, incorporating realistic wind fields with adjustments to the initial wind fields for the background circulation in the warning system will obtain better predictions for a typhoon and its resultant storm surges.     

内日球太阳风的激波加热

利用HeliosA,B飞船1974年至1980年的太阳风探测资料,分析了不同速度间隔太阳风质子温度径向变化指数在太阳不同活动期的变化,以及不同太阳活动期间内日球行星际激波强度分布的变化．结果指出,在0．3-1．0AU区间行星际激波可能是太阳风加热的一个重要因素,这一因素在太阳活动高年可能起着主要的作用．激波MHD数值模拟也从量的方面表明激波加热太阳风的有效性．     

Seasonal variability of SST fronts and winds on the southeastern continental shelf of Brazil

Fourteen years (September 2002 to August 2016) of high-resolution satellite observations of sea surface temperature (SST) data are used to describe the frontal pattern and frontogenesis on the southeastern continental shelf of Brazil. The daily SST fronts are obtained using an edge-detection algorithm, and the monthly frontal probability (FP) is subsequently calculated. High SST FPs are mainly distributed along the coast and decrease with distance from the coastline. The results from empirical orthogonal function (EOF) decompositions reveal strong seasonal variability of the coastal SST FP with maximum (minimum) in the astral summer (winter). Wind plays an important role in driving the frontal activities, and high FPs are accompanied by strong alongshore wind stress and wind stress curl. This is particularly true during the summer, when the total transport induced by the alongshore component of upwelling-favorable winds and the wind stress curl reaches the annual maximum. The fronts are influenced by multiple factors other than wind forcing, such as the orientation of the coastline, the seafloor topography, and the meandering of the Brazil Current. As a result, there is a slight difference between the seasonality of the SST fronts and the wind, and their relationship was varying with spatial locations. The impact of the air-sea interaction is further investigated in the frontal zone, and large coupling coefficients are found between the crosswind (downwind) SST gradients and the wind stress curl (divergence). The analysis of the SST fronts and wind leads to a better understanding of the dynamics and frontogenesis off the southeastern continental shelf of Brazil, and the results can be used to further understand the air-sea coupling process at regional level.

     

Wind forcing of the Ganga-Brahmaputra river plume

Ocean Dynamics - Response of Ganga-Brahmaputra river plume to wind forcing in the Bay of Bengal is studied using a numerical ocean circulation model. Four different wind forcing scenarios, namely,...     

Dispersion in the Yucatan coastal zone: Implications for red tide events

The mechanisms governing dispersion processes in the northern Yucatan coast are investigated using a barotropic numerical model of coastal circulation, which includes wind-generated and large scale currents (i.e. Yucatan Current). This work provides the foundations for studying the dispersion of harmful algal blooms (HABs) in the area. Modelling experiments include effects of climatic wind (from long term monthly mean NCEP reanalysis), short term wind events (from in situ point measurements), and Yucatan Current (YC) characteristics. Its magnitude was approximated from published reports, and its trajectory from geostrophic current fields derived from altimeter data. These provided a range of real and climatic conditions to study the routes in which phytoplankton blooms may travel. The 2-D model results show that a synthetic and conservative bloom seeded in the Cabo Catoche (CC) region (where it usually grows), moves along the coast to the west up to San Felipe (SF), where it can either move offshore, or carry on travelling westwards. The transport to the west up to SF is greatly influenced by the trajectory, intensity and proximity of the YC jet to the peninsula, which enhances the westward circulation in the Yucatan Shelf. Numerical experiments show that patch dispersion is consistently to the west even under the influence of northerly winds. When the YC flows westward towards the Campeche Bank, momentum transfer caused by the YC jet dominates the dispersion processes over wind stress. On the other hand, when it flows closer to Cuba, the local processes (i.e. wind and bathymetry) become dominant. Coastal orientation and the Coriolis force may be responsible for driving the patch offshore at SF if external forcing decreases.     

The effects of river discharge and seasonal winds on the shelf off southeastern South America

The Río de la Plata waters form a low salinity tongue that affects the circulation, stratification and the distributions of nutrients and biological species over a wide extent of the adjacent continental shelf. The plume of coastal waters presents a seasonal meridional displacement reaching lower latitudes (28°S) during austral winter and 32°S during summer. Historical data suggests that the wind causes the alongshore shift, with southwesterly (SW) winds forcing the plume to lower latitudes in winter while summer dominant northeasterly (NE) winds force its southward retreat. To establish the connection between wind and outflow variations on the distribution of the coastal waters, we conducted two quasi-synoptic surveys in the region of Plata influence on the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. We observed that: (A) SW winds dominating in winter force the northward spreading of the plume to low latitudes even during low river discharge periods; (B) NE winds displace the plume southward and spread the low salinity waters offshore over the entire width of the continental shelf east of the Plata estuary. The southward retreat of the plume in summer leads to a volume decrease of low salinity waters over the shelf. This volume is compensated by an increase of Tropical waters, which dominate the northern shelf. The subsurface transition between Subantarctic and Subtropical Shelf Waters, the Subtropical Shelf Front, and the subsurface water mass distribution, however, present minor seasonal variations. Along shore winds also influence the dynamics and water mass variations along the continental shelf area. In areas under the influence of river discharge, Subtropical Shelf Waters are kept away from the coastal region. When low salinity waters retreat southward, NE winds induce a coastal upwelling system near Santa Marta Cape. In summer, solar radiation promotes the establishment of a strong thermocline that increases buoyancy and further enhances the offshore displacement of low salinity waters under the action of NE winds.     

A two time-scale model of stratified shelf currents

Stratification is incorporated into an unsteady model of shelf currents by splitting the dynamic response of the flow into two parts, each with its own time scale. The barotropic part of the response is independent of depth and varies rapidly on a short time scale, whereas the baroclinic part depends on depth and changes slowly with time on a long time scale.The three-dimensional model has a continental shelf sloping down from an eastern boundary to the deep ocean. The equations for the barotropic component of the pressure field contain forcing by the wind stress and feedback from the baroclinic field. An integral of the heat equation over the long time scale determines the slow changes in the temperature field and hence in the baroclinic component of the velocity distribution.The temperature field is specified at the start of the numerical calculation. Its subsequent development is controlled by the numerical procedure. It is found that significant changes in the temperature field require a long period of upwelling favourable winds, whereas the longshore currents react more quickly to changes in the wind stress.     

A climatic dataset of ocean vertical turbulent mixing coefficient based on real energy sources

Using data on wind stress, significant height of combined wind waves and swell, potential temperature, salinity and seawater velocity, as well as objectively-analyzed in situ temperature and salinity, we established a global ocean dataset of calculated wind- and tide-induced vertical turbulent mixing coefficients. We then examined energy conservation of ocean vertical mixing from the point of view of ocean wind energy inputs, gravitational potential energy change due to mixing (with and without artificially limiting themixing coefficient), and K-theory vertical turbulent parameterization schemes regardless of energy inputs. Our research showed that calculating the mixing coefficient with average data and artificial limiting the mixing coefficient can cause a remarkable lack of energy conservation, with energy losses of up to 90% and changes in the energy oscillation period. The data also show that wind can introduce a huge amount of energy into the upper layers of the Southern Ocean, and that tidesdo so in regions around underwater mountains. We argue that it is necessary to take wind and tidal energy inputs into account forlong-term ocean climate numerical simulations. We believe that using this ocean vertical turbulent mixing coefficient climatic dataset is a fast and efficient method to maintain the ocean energy balance in ocean modeling research.     

Response of large-scale coastal basins to wind forcing: influence of topography

Because wind is one of the main forcings in storm surge, we present an idealised process-based model to study the influence of topographic variations on the frequency response of large-scale coastal basins subject to time-periodic wind forcing. Coastal basins are represented by a semi-enclosed rectangular inner region forced by wind. It is connected to an outer region (represented as an infinitely long channel) without wind forcing, which allows waves to freely propagate outward. The model solves the three-dimensional linearised shallow water equations on the f plane, forced by a spatially uniform wind field that has an arbitrary angle with respect to the along-basin direction. Turbulence is represented using a spatially uniform vertical eddy viscosity, combined with a partial slip condition at the bed. The surface elevation amplitudes, and hence the vertical profiles of the velocity, are obtained using the finite element method (FEM), extended to account for the connection to the outer region. The results are then evaluated in terms of the elevation amplitude averaged over the basin’s landward end, as a function of the wind forcing frequency. In general, the results point out that adding topographic elements in the inner region (such as a topographic step, a linearly sloping bed or a parabolic cross-basin profile), causes the resonance peaks to shift in the frequency domain, through their effect on local wave speed. The Coriolis effect causes the resonance peaks associated with cross-basin modes (which without rotation only appear in the response to cross-basin wind) to emerge also in the response to along-basin wind and vice versa.     

初值和海温强迫对延伸期可预报性时空分布的影响

利用全球谱模式T106L19和增长模繁殖法,分别在气候海温和预测海温强迫下,进行了动力延伸集合预报试验.基于方差分析思想,利用集合预报结果,定义和计算了初值影响指数、海温强迫影响指数、潜在可预报性指数以及波动活动指数.通过分析四个指数,揭示了初值和海温强迫对延伸期可预报性时空分布以及潜在可预报性的影响,并探讨了其影响机理.结果表明：初值影响指数分布具有地域和季节的差异,初值的影响在中高纬度地区大于热带地区;相同季节,海温强迫影响指数分布与初值影响指数分布相似;潜在可预报性指数呈带状分布,大值集中在热带地区,且在低纬度地区,高层的潜在可预报性大于低层;初值和海温强迫对延伸期可预报性时空分布的影响,依赖于大气环流形势,初值和海温强迫影响的显著区正是大气长波的活跃区和西风急流区,急流区的强风切变为长波活动提供了斜压不稳定能量,而长波的发展调控着初值和海温强迫的影响,这说明延伸期的可预报性具有明显的流依赖性,大气外强迫的作用也与大气内部的动力过程密切相关.