Strong Onshore Winds Research Articles

Climatology of sea breeze over Jiangsu coast: An overview from the ERA5 reanalysis

AbstractThe sea breeze characteristics for 30 years of summer (1981–2010) in the coastal region of Jiangsu Province were analysed in this study, using the four‐step filter method based on ECMWF Fifth‐Generation Reanalysis (ERA5) data. The results showed that the filtering method reasonably identified three types of sea breeze. Additionally, approximately 23% of the total summer days remained; the corkscrew type dominates the three types of sea breeze; and the backdoor sea breeze was the least common. Except for the type of backdoor sea breeze, the other two types had relatively fixed dominant weather types on three coastlines. The synoptic weather patterns in the 500‐hPa level corresponding to each sea breeze type were different. Further, the details of the diurnal cycle of lower tropospheric circulation indicated the strength of the onshore wind and the location of the large potential temperature gradient at noon being influenced by sea breeze type and coastal location. The wind strength for the three types of sea breeze on the middle coastline was stronger than that on the other two coastlines. On the three coastlines, strong onshore winds were concentrated in areas near the coastline, where they reached their maximum strengths around noon. On the southern coastline, strong onshore winds moved further inland.

Некоторые оценки тяжелых ледовых условий (сжатий) в Печорском море по данным наблюдений и моделирования (моделирование и анализ)

The work continues the study focused on the analysis of the severe ice conditions’ formation in the Pechora Sea during navigation in 2003 and 2018 (published in vol.12, no.4, 2022). Using a numerical model, the authors reproduce the dynamics and some elements of the morphometry of the ice cover during the indicated ice seasons. Thus, they reveal that the significant ice pressure (100—150 kPa and more) is caused by the strong on-shore wind, while the ice pressure related to the tidal phenomena have the predominant value at the weak winds. The tidal ice pressure occurs permanently and regularly, and varies mainly within the range of 10—50 kPa. The contribution of ice hummocks to the total ice volume becomes significant when strong ice pressure happens at the background of high ice thickness (close to the seasonal maximum, i.e. more than 60—70 cm). With small ice thicknesses, even strong ice pressure, as a rule, does not lead to a significant increase in the equivalent thickness of hummocks.

Principal Modes of Diurnal Cycle of Rainfall over South China during the Presummer Rainy Season

Abstract The principal modes of the diurnal cycle of rainfall (DCR) over South China during the presummer rainy season are examined using 23-yr satellite observations and reanalysis data. Three distinctly different DCR modes are identified via empirical orthogonal function analysis, that is, the early-afternoon precipitation (EAP) mode, the late-afternoon precipitation (LAP) mode, and the morning precipitation (MP) mode. Under the EAP mode, the rainfall starts to increase from midnight and reaches its peak in the early afternoon. The nocturnal to morning rainfall generally concentrates on the northeastern Pearl River delta (PRD) and along the coastline. The coastal rainfall is initiated from the convergence zone induced by the strong onshore wind and is further enhanced via the establishment of a land breeze in the early morning. The northeastern PRD center is mainly attributed to the windward mechanical lifting associated with the strong low-level wind. The afternoon rainfall is pronounced over inland areas and exhibits significantly regional diversity. The eastern inland rainfall develops from the early-morning rainfall over the northeastern PRD, whereas the eastward-propagating rain belts associated with frontal activities are responsible for the formation of western inland rainfall. The LAP mode features a late-afternoon peak, which is triggered and developed locally with favorable thermal–dynamic conditions over western inland South China. The MP mode exhibits a single early-morning peak. Nocturnal to morning rainfall is prominent on the northeastern PRD and near-offshore region. The near-offshore rainfall is basically induced by the convergence between the onshore wind and land breeze in the early morning, which further propagates far offshore in the morning due to effects of gravity waves.

Inorganic geochemistry of saline substrates associated with coastal turf, Otago Peninsula, New Zealand

ABSTRACT Halophytic plant communities have developed on exposed coastal landforms to form low-stature turf in several locations around New Zealand. We quantify the inorganic geochemistry, mineralogy and simulated ephemeral surface water composition of saline soil-free bare ground at a well-preserved site at Maori Head on Otago Peninsula, where halophytes have colonised from adjacent coastal turf. The community at this site is dominated by Leptinella dioica, Samolus repens, Disphyma australe and Selliera radicans. Maori Head substrates are dominated by clay-altered basalt colluvium and show a general trend from high electrical conductivity (EC > 10 mS/cm) and weakly acid pH (< 6) on bare ground to low EC (< 1 mS cm−1) and higher pH (> 7) in turf and adjacent pasture soils. High salinities on dry substrates and in simulated ephemeral surface waters result from surface evaporation of airborne sea spray and contain halite (NaCl) with some gypsum (CaSO4.2H2O). Chemical interactions with the substrate are minor. The high surface salinities were maintained despite a major winter rain event with strong onshore winds. However, surface salinity is frequently leached by rain and shallow groundwater, whereas inland saline sites (pH 8–11) have substantial salt accumulations driven by lower rainfall, high evaporation and chemical interaction with Otago Schist-derived substrates.

Compound effects of rain, storm surge, and river discharge on coastal flooding during Hurricane Irene and Tropical Storm Lee (2011) in the Mid-Atlantic region: coupled atmosphere-wave-ocean model simulation and observations

Coastal flooding from landfalling tropical cyclones (TCs) is a major hazard with increasing severity in a warming climate and rising seas. It is difficult to predict because of highly complex compound effects of TC induced heavy rainfall, storm surge, and river discharge. This can be further exacerbated by sequential TCs such as Hurricane Irene and Tropical Storm Lee in late August to mid-September 2011, which caused major coastal flooding in the Mid-Atlantic region. This study focuses on better understanding and improving prediction of the compound effects of rain, storm surge, and river discharge using a high-resolution coupled atmosphere-wave-ocean model, namely the Unified Wave INterface–Coupled Model (UWIN-CM) and observations from NDBC buoys, NOAA tide gauges, and USGS estuary sites. UWIN-CM effectively captured the storm track and intensity, surface winds, surface waves, and ocean surface evolution associated with the two storms, compared with the observations. Compound effects of wind, rain, storm surge, and river-stream discharge on coastal flooding are investigated. The storm surge from Hurricane Irene was observed along the coasts Maryland, New Jersey, and New York, Delaware Bay, the lower reaches of the Delaware River, and in lower Chesapeake Bay. Strong onshore wind pushes water upstream, which has the highest compound effects on coastal flooding. Heavy rain and river-stream discharge into the coastal zone contributes mainly to locations upstream away from the open bay water. A new, indirect machine learning method of estimating the spatial extent of coastal flooding using simulated coastal sea surface height is shown.

Numerical evaluation of influences of onshore wind on overtopping characteristics of coastal seawall under solitary wave

To protect the coastal communities and infrastructures from the devastating damages caused by the extreme ocean surface waves during the severe weather conditions, many coastal defense structures, such as seawall, have been widely constructed along the coastline. In fact, huge ocean surface waves and strong onshore winds coexist in the real harsh coastal environment. Hence, it is of significant necessity to accurately evaluate the overtopping characteristics of coastal defense structures under the combined impact of waves and onshore wind. In this paper, overtopping characteristics of the coastal seawalls under the combined impact of onshore wind and solitary wave are numerically studied. Influences of some main factors, such as onshore wind speed, water depth, wave height, beach slope, and seawall slope, are discussed in detail. The results show that the presence of onshore wind can greatly affect the runup and the amount of overtopping water of seawall under the action of solitary wave. It is hoped that the results of this paper will further deepen our understanding of the overtopping characteristics of seawalls during the severe weather conditions.

Pelagic seabirds reduce risk by flying into the eye of the storm

Cyclones can cause mass mortality of seabirds, sometimes wrecking thousands of individuals. The few studies to track pelagic seabirds during cyclones show they tend to circumnavigate the strongest winds. We tracked adult shearwaters in the Sea of Japan over 11 y and found that the response to cyclones varied according to the wind speed and direction. In strong winds, birds that were sandwiched between the storm and mainland Japan flew away from land and toward the eye of the storm, flying within ≤30 km of the eye and tracking it for up to 8 h. This exposed shearwaters to some of the highest wind speeds near the eye wall (≤21 m s-1) but enabled them to avoid strong onshore winds in the storm's wake. Extreme winds may therefore become a threat when an inability to compensate for drift could lead to forced landings and collisions. Birds may need to know where land is in order to avoid it. This provides additional selective pressure for a map sense and could explain why juvenile shearwaters, which lack a map sense, instead navigating using a compass heading, are susceptible to being wrecked. We suggest that the ability to respond to storms is influenced by both flight and navigational capacities. This may become increasingly pertinent due to changes in extreme weather patterns.

Citizen Scientists Record Significant Range Extensions for Tropical Sea Slug Species in Subtropical Eastern Australia

The Sea Slug Census program in Australia engages with citizen scientists to record the diversity and distribution of sea slugs across multiple locations. The program has consistently recorded shifts in distribution patterns but a recent, nine-day census in subtropical eastern Australia recorded unprecedented range extensions of tropical species. Seven species (six chromodorids and one polycerid) were found further south of their previously known distribution with Hypselodoris bertschi being recorded for the first time in Australia. These observations suggested the recent transport of larvae via the East Australian Current with recruitment to coastal sites possibly promoted by a protracted period of strong onshore winds associated with the 2021/22 La Niña in the western Pacific. With the increasing frequency of poleward range extensions of marine taxa, citizen science programs such as the Sea Slug Census provide the opportunity to substantially increase monitoring efforts. Linking with iNaturalist strengthens the value of the observations through online peer review to confirm species identities as well as the incorporation of substantiated (Research Grade) records into international biodiversity databases such as GBIF.

Characteristics of rainstorm in Fujian induced by typhoon passing through Taiwan Island

Based on the typhoon track and intensity data and the precipitation data of typhoon in China during 1961–2020, the overall characteristics of the rainstorm in Fujian caused by typhoon passing though Taiwan Island were studied. More than 80 percent of typhoons passing though the Taiwan Island can bring heavy rain to Fujian. There are 1.5 events of typhoon rainstorm in Fujian every year, and the average annual impact days are 3.0. In terms of spatial distribution, the frequency and intensity of cross-island typhoon rainstorm decrease rapidly from the coastal areas of Fujian to the inland areas, and Zherong, Changle and Jiu xianshan stations in the coastal areas are the high value centers. The typhoon paths of cross-island typhoon rainstorm in Fujian are mainly divided into three categories: landing-Fujian type (including landing-Fujian northeast turning, landing-Fujian middle northbound and landing-Fujian south westbound), landing-Guangdong and Zhejiang type and offshore turning type, among which landing-Fujian type typhoon has the most significant influence(only the landing-Fujian type appears the rainstorm of ≥50 mm·(24 h)−1), and the rainstorm intensity, influence range and asymmetrical structure of the rainstorm are the strongest, the most extensive and the most significant in the landing-Fujian middle northbound path. Based on the NCEP reanalysis data, the comparative analysis of the environmental fields causing the difference of precipitation intensity between the two typhoons landing-Fujian middle northbound and landing-Fujian south westbound shows that: To the landing-Fujian middle northbound track, strong wind speed area on the north side of the typhoon center leads to strong onshore winds, in the role of mountain terrain, piedmont has better convergence and very strong deep vertical upward movement, with better moisture conditions, it can send low high-energy water vapor to the middle, the precipitation dynamics and water vapor conditions are significantly stronger than the landing-Fujian south westbound track, resulting in more typhoon heavy rain.

极端天气陆地河流面源污染对珠江河口叶绿素分布的影响——评价方法构建与应用

PDF HTML阅读 XML下载 导出引用 引用提醒 极端天气陆地河流面源污染对珠江河口叶绿素分布的影响——评价方法构建与应用 DOI: 10.5846/stxb202103130681 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 南方海洋科学与工程广东省实验室（广州）人才团队引进重大专项（GML2019ZD0303）；热带海洋环境国家重点实验室自主研究项目（LTOZZ2103）；国家自然科学基金面上项目（42077057，41876136） Impacts of non-point source pollutants from river on chlorophyll distribution in the Pearl River Estuary under the extreme weather conditions: the framework and application Author: Affiliation: Fund Project: 100 Talents Program of the Chinese Academy of Sciences，The National Natural Science Foundation of China (General Program, Key Program, Major Research Plan), Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) talent team introduced major special projects 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:近十年来，随着我国粤港澳大湾区的建设，珠江流域人口增长十分迅速，食品和能源需求急剧增加，导致流域上游省份农业用地面积增加，下游省份城市化进程加速，随之导致含农业化肥的农田灌溉用水和生活污水排放大幅度增加。流域的面源污染物进入珠江河口，导致河口海域赤潮发生频率明显增高。此外，全球变暖加剧了南海和西太台风等极端天气事件的发生频率，大量的台风在珠江口附近登陆，对人类活动产生的营养盐在海水中再分布产生较大影响。现场及卫星观测表明，台风等极端天气影响浮游植物时空分布的4个主要影响因子为：海水的垂直混合、流域的降水增加、河流淡水及营养盐的大量输入及风场变化。但是，尚未对这些因子如何影响叶绿素分布进行过清晰阐释。研究构建了粤港澳大湾区陆海气集成模拟系统，一方面通过控制实验来对模式敏感性进行测试，另一方面以台风"天鸽"过境珠江口为例，来对台风过境期间海表叶绿素时空变化机理进行探究。结果表明，陆地模式径流分辨率的提高对模拟结果有很大影响，台风"天鸽"过境时，向岸的风抑制了珠江冲淡水排出，促进外海水涌入，提高海表盐度，降低叶绿素浓度。而台风所致的陆地降水在过境后四天（8月27日）通过径流输出至河口，此次剧烈的增水向珠江口输入大量的营养盐，促进浮游植物持续生长，使叶绿素浓度在台风过境两周（9月1日）后仍在增加。研究所构建的集成模拟系统可适于其它受河口面源污染影响的海湾及近海系统。为未来实现赤潮、缺氧等生态灾害预报提供可能性。 Abstract:Over the last 10 years, progressive development of the Guangdong-Hong Kong-Macau Greater Bay area in our country has caused rapid population growth within the Pearl River watershed. In the meantime, demand for food and energy has increased dramatically. These result an expansion of agriculture land in the upper reaches of the Pearl River catchment and an increase in the rate of urbanization in the downstream area. As a result, groundwater and surface runoff entering the Pearl River up-stream now contains higher levels of fertilizer than previous, whilst in urban areas sewage and effluent production has increased considerably. A side effect of these changes is that the amount of pollution entering the estuary in a non-point source manner has increased, leading to widespread eutrophication there. Besides, global warming increased typhoons from South China Sea and Western Pacific making landfall near the Pearl River estuary, resulting in the enhanced redistribution of anthropogenic nutrients into the oceanic water. Both in-situ and satellite have been used to investigate the impact these typhoons have on Pearl River phytoplankton dynamics. They identified four factors impacting the redistribution of phytoplankton include the vertical mixing of oceanic water, high precipitation in the watershed, the large freshwater discharge and nutrients loading, and the variation of wind fields. To date, there have been no studies address how these four factors work together to influence the distribution of chlorophyll in the Pearl River estuary. In this research, we built a land-ocean-atmosphere modeling system for the China Great Bay Area. We conducted numerous controlled experiments to examine the sensitivity of the system to multiple forcing conditions. We also investigated the effect of each controlling mechanism has on phytoplankton dynamics, using the incidence of Hurricane Hato's arrival in the area as an opportunity to collect first-hand, real-time observations. Our results show that the temporal resolution of river runoff data used in the modeling system has a profound impact on the simulation results. Using daily river runoff data from land surface models produced very different results from those using monthly data. As Hato passed by, strong onshore winds inhibited the spread of freshwater from the Pearl River, increasing the influx of seawater which in turn increased sea surface salinity and reduced the concentration of surface chlorophyll. Four days (August 27th) after Hato had passed by, water that had fallen on land as precipitation during the typhoon entered the Estuary as runoff. The dramatic increase of river discharge coincident with this event enhanced the level of nutrient loading and caused dramatic phytoplankton growth. Chlorophyll levels were still increasing two weeks after the typhoon Hato passing by (September 1st). The land-ocean-atmosphere modeling system built for the Pearl River Estuary in our research could be adapt to other estuarine and coastal regions and would ultimately help predict algal blooms, hypoxia within the aquatic environment, and other ecological hazards in the future. 参考文献 相似文献 引证文献

Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System

The holopelagic macroalgae sargassum has proliferated across the tropical Atlantic since 2011, of consequence for coastal populations from West Africa to the Caribbean with limited early warning of major beaching events. As part of an interdisciplinary project, ‘Teleconnected SARgassum risks across the Atlantic: building capacity for TRansformational Adaptation in the Caribbean and West Africa’ (SARTRAC), an ensemble forecast system, SARTRAC-EFS, is providing seasonal predictions of sargassum drift. An eddy-resolving ocean model hindcast provides the winds and currents necessary to generate ensemble members. Ensemble forecasts are then obtained for different combinations of ‘windage’, the fractional influence of winds on sargassum mats, and in situ rates of growth, mortality, and sinking. Forecasts for north and south of Jamaica are evaluated with satellite-observed distributions, associated with beaching events in specific years of heavy inundation, 2015 and 2018-20. These seasonal forecasts are evaluated, on lead times of up to 180 days. Forecasts are subject to leading modes of tropical climate variability, in particular the Atlantic Meridional Mode (AMM). More accurate forecasts for a given year are obtained with ensemble members from hindcast years with a similar spring AMM-index. This is most clearly evident during negative AMM phases in spring of 2015 and 2018, when positive sea surface temperature anomalies and anomalously weak trade winds were established across the northern tropics. On this evidence, SARTRAC-EFS is potentially useful in providing early warning of high sargassum prevalence. Extended to sargassum drift off West Africa, extensive cloud cover limits availability of the satellite data needed for full application and evaluation of SARTRAC-EFS in this region, although experimental forecasts off the coast of Ghana are found highly sensitive to the windage that is associated with strong onshore winds during boreal summer. Alongside other forecast systems, SARTRAC-EFS is providing useful early warnings of sargassum inundation at seasonal timescale.

Species-specific traits and the environment drive ichthyoplankton fluxes between an intermittently closed-open lagoon and adjacent coastal waters

This study evaluated the connectivity between an intermittently closed-open lagoon (LR) and adjacent coastal waters (CW), and the influence of that connection on ichthyoplankton assemblages. We performed field surveys along a year in LR and CW to capture different sandbar states (open/closed) and lagoon-sea connectivity processes (e.g. lagoon discharges, marine intrusion, overwashing). Results showed differences in environmental conditions and ichthyoplankton communities between LR and CW, and also evidenced different mechanisms of exchange of water and ichthyoplankton. At least three estuarine species were found to spawn and use LR as a nursery ground. Larval stages of six marine species were found at CW. Environmental factors (e.g. wind and runoff) and species-specific traits (e.g. life cycle strategy, spawning season) determined ichthyoplankton exchange vs. retention. Marine estuarine-related species entered LR mostly during summer-autumn, at their peak abundance in CW. Sandbar closure did not impede the entrance of these species. Overwashing events (closed bar) and light onshore winds during the open-sandbar period were the most important process that promoted the transport of these larvae into LR. Conversely, there was no evidence of entrance of marine estuarine-independent larvae to LR. Eggs and larvae of estuarine species that spawned within LR were effectively retained there, except when heavy rains led to severe lagoon discharges. Similarly, unusual strong onshore winds advected eggs from CW to LR, possibly reducing their survival. Generally, eggs were more vulnerable to drift than larvae. Many of the estuarine larvae that drifted out of the lagoon seemed to survive, suggesting a loss of individuals from the lagoon population but a potential contribution to the genetic flux among coastal populations.

Stranded in the high tide line: Spatial and temporal variability of beached microplastics in a semi-enclosed embayment (Arcachon, France)

Coastal environments are a predominant ultimate destination of marine debris, becoming a key focus of studies assessing microplastic (MP) contamination. Here, we described the visible fraction of MP (from 0.5 to 5 mm) that washed up during the high tide at different sites of a semi-enclosed mesotidal bay and investigated the main abiotic factors driving MP beaching. Three contrasted beaches of the Arcachon Bay (SW France) were monitored on a monthly basis during 2019. Samplings were made along a 100 m longitudinal transect at the high-water strandline (4 quadrats of 0.25m2) and at an intermediate tidal range. Each sampled particle was characterized by morphometric data (e.g. size, shape, color, roughness) and polymer identification was performed by ATR-FTIR technique. Results show that MP concentration was higher on the beach located at the mouth of the bay (36.0 ± 39.2 MP.m-2) than at the back and the outside of the bay (respectively 2.7 ± 4.4 and 1.7 ± 2.4 MP.m-2). This may be related to the strong currents at the entry of the embayment and the beach orientation, exposed to predominant winds. Beached MP were mainly pre-production pellets and fragments as they represented respectively 49% and 39% of all analyzed shapes. Polymers with low density were particularly abundant. Polyethylene represented 69% of all the particles while polypropylene accounted for 17% and polystyrene for 10%. We also observed that MP were mostly washed up when wind, waves and river flow were more intense. Analysis suggest that wind direction and speed are key factors influencing beaching as strong onshore wind enhance this process.

Daily accumulation rates of marine litter on the shores of Rapa Nui (Easter Island) in the South Pacific Ocean

Large amounts of floating litter accumulate in the subtropical gyres, and the shorelines of oceanic islands in the vicinity of these accumulation zones receive large amounts of litter. The dynamics of marine litter arrival were evaluated on a small pocket beach on Rapa Nui (Easter Island) between November 2016 and June 2017 over a total time period of 190 days. Plastics dominated the stranded litter, with the majority of the items showing indications to come from the high seas fisheries operating in the South Pacific. Litter accumulation rates varied between 0 and 1.90 items m−1 day−1, and were highest following days of strong winds from northern directions. The results indicate that the shores of Rapa Nui are exposed to continuous input of plastic litter from the contaminated waters of the subtropical gyre, which is intensified during strong onshore winds.

Influence of Meteorological Conditions and Aerosol Properties on the COVID-19 Contamination of the Population in Coastal and Continental Areas in France: Study of Offshore and Onshore Winds

Human behaviors probably represent the most important causes of the SARS-Cov-2 virus propagation. However, the role of virus transport by aerosols—and therefore the influence of atmospheric conditions (temperature, humidity, type and concentration of aerosols)—on the spread of the epidemic remains an open and still debated question. This work aims to study whether or not the meteorological conditions related to the different aerosol properties in continental and coastal urbanized areas might influence the atmospheric transport of the SARS-Cov-2 virus. Our analysis focuses on the lockdown period to reduce the differences in the social behavior and highlight those of the weather conditions. As an example, we investigated the contamination cases during March 2020 in two specific French areas located in both continental and coastal areas with regard to the meteorological conditions and the corresponding aerosol properties, the optical depth (AOD) and the Angstrom exponent provided by the AERONET network. The results show that the analysis of aerosol ground-based data can be of interest to assess a virus survey. We found that moderate to strong onshore winds occurring in coastal regions and inducing humid environment and large sea-spray production episodes coincides with smaller COVID-19 contamination rates. We assume that the coagulation of SARS-Cov-2 viral particles with hygroscopic salty sea-spray aerosols might tend to inhibit its viral infectivity via possible reaction with NaCl, especially in high relative humidity environments typical of maritime sites.

Variations in Friction Velocity with Wind Speed and Height for Moderate-to-Strong Onshore Winds Based on Measurements from a Coastal Tower

AbstractVariations in friction velocity with wind speed and height are studied under moderate (≥9 m s−1)-to-strong onshore wind conditions caused by three landfalling typhoons. Wind data are from a coastal 100-m tower equipped with 20-Hz ultrasonic anemometers at three heights. Results show that wind direction affects variations in friction velocity with wind speed. A leveling off or decrease in friction velocity occurs at a critical wind speed of ~20 m s−1under strong onshore wind conditions. Friction velocity does not always decrease with height in the surface layer under typhoon conditions. Thus, height-based corrections on friction velocities using the model from Anctil and Donelan may not be reliable. Surface-layer heights predicted by the model that are based on Ekman dynamics are verified by comparing with those determined by a proposed method that is based on the idea of mean boundary layer using wind-profile data from one of the landfalling typhoons. Friction velocity at the top of the surface layer is then estimated. Results show that friction velocity decreases by about 20% from its surface value and agrees well with previous results of Tennekes.

Coastal Ocean Response and Its Feedback to Typhoon Hato (2017) Over the South China Sea: A Numerical Study

AbstractThe coastal ocean response and feedback to Typhoon Hato (2017) were studied based on high‐resolution numerical simulations using both a coupled and an uncoupled cloud‐resolving model. As a category 3 landfalling typhoon that moved west‐northwestward across the northern South China Sea, Hato (2017) rapidly intensified prior to its landfall and induced significant impacts on the coastal water column, causing warm and cold patches in sea surface temperature (SST) over the continental shelf to the right of the track. This feature was well captured in an air‐sea coupled model experiment. The coastal SST warming was found to be related to a two‐layer oceanic circulation across the continental shelf forced by the onshore surface wind stress to the right of the storm track. The associated onshore surface currents imposed a warm temperature advection and downwelling, leading to the SST warming in the inner sea shelf, as diagnosed from an ocean temperature budget analysis. A sensitivity experiment, in which the typhoon vortex was removed from the initial conditions, further confirmed that it was the strong onshore wind stress to the right of the storm track that forced the onshore surface currents and the SST warming in the inner sea shelf. Results from an atmosphere‐only model experiment with the typhoon‐forced coastal warm SST anomalies removed demonstrate that the typhoon‐induced coastal warm SST anomalies contributed partly to the rapid intensification of Typhoon Hato prior to its landfall over South China and also slowed down the weakening of Hato at and shortly after its landfall.

Wind effects on dense shelf water cascades in south-west Australia

Evaporation and cooling during autumn and winter in south-western Australia results in denser water near the coast and a cross-shelf density gradient. These conditions drive a gravitational circulation, with offshore transport of higher density water along the seabed, defined as dense shelf water cascades (DSWCs). A field experiment with data collected from multiple platforms is described to identify the temporal variability and controlling mechanisms of DSWCs during the austral autumn and winter (May–June 2015) when DSWCs were most prevalent. Data analysis confirmed that both the wind intensity and direction were critical parameters to: (1) inhibit DSWC formation through vertical mixing; and, (2) enhance DSWCs, depending on the wind direction. An intense and persistent DSWC was initiated through the passage of a winter cold front with strong onshore winds. Here, onshore winds caused downwelling, resulting in onshore movement of surface water and offshore movement in the bottom layer, enhancing the cross-shelf density gradient and the cascade. Vertical mixing of the water column and shutdown of the DSWC occurred when the winds were strong and had an offshore component or were upwelling-favourable, which resulted in the advection of higher density water offshore at the surface and vertical mixing.

Interdecadal Foredune Changes along the Southeast Australian Coastline: 1942–2014

Foredunes are important features within coastal landscapes, yet there are relatively few medium to long-term studies on how they evolve and change over time. This study of Australia’s New South Wales (NSW) foredunes has used 70 years of aerial photographs (or photogrammetry) and recent Light Detection and Ranging (LiDAR) datasets to assess multi-decadal fluctuations in foredune morphology. It was shown that over the past 70 years NSW foredunes have exhibited considerable spatial variation, ranging from accretion/aggradation to recession. Those sites that accreted predominantly extended seaward as new incipient dunes, gaining a maximum of 235 m3 m−1 in sand volume over the study period (for the entire dune system). These sites were commonly found in the north of the state, within closed sediment compartments, and with strong onshore (and alongshore) wind climates present (increasing the potential for aeolian sand transport). Stable foredunes were those that remained within +/− 50 m3 m−1 of their initial volume and managed to recover from the various storm impacts over the study period. The majority of these sites were found within the central to southern half of the state, behind embayed beaches, and within leaky sediment compartments, or those that have estuarine sinks. Finally, those foredunes in recession have retreated landwards and/or have reduced in height or width, and lost up to 437 m3 m−1 of sand volume over the study period. There was no clear spatial trend for these sites; however, generally they were found in compartments that had unusual orientations, had disruptions in longshore drift/cross shore sand delivery (i.e., rocky reefs), or were being impacted by humans (i.e., the installation of river training walls, sand bypassing systems, or coastal management programs). This study has shown that NSW foredunes have undergone substantial recent changes and, by understanding their past history, will provide better insight into how they can be managed into the future.

Effects of Wind Direction on Variations in Friction Velocity With Wind Speed Under Conditions of Strong Onshore Wind

AbstractThe effects of wind direction on variations in friction velocity with wind speed are studied under moderate (≥9 m/s) to strong (≥22 m/s) onshore wind conditions using 20‐Hz ultrasonic wind data from a coastal tower at three different heights. The effects of different averaging time intervals of 20, 10, 2, and 1 min on the variations are also investigated. Three typhoons passed by the tower during the 150 hr of observations. Regardless of wind direction, friction velocity increases with increasing wind speed, and linear regression shows that the rate of increase is ~50% less than previously reported. However, a leveling‐off or decrease in friction velocity with increasing wind speed is found under strong wind conditions using a bin‐averaged method. Wind direction affects the variations in friction velocity with wind speed. Friction velocity increases with increasing wind speed at a similar growth rate to that of previously published results, with a leveling‐off or decrease at wind speeds higher than 22 m/s when the wind blows roughly normal to the shoreline. Form drag induced by sea waves with longer wavelengths is suggested to be responsible for the effects of wind direction on the variations in friction velocity with wind speed. An averaging time interval of 1 min yields representative variations in friction velocity with wind speed using the observations described in this study.