Onshore Winds Research Articles

Direct onshore wind predicts daily swimmer's itch (avian schistosome) incidence at a Michigan beach.

Swimmer's itch (SI) is a painful rash caused by skin penetration by free-swimming infectious cercariae of avian schistosomes, snail-borne helminth parasites related to the causative agents of human schistosomiasis. The goal of this study was to determine if commonly collected environmental data could be used to predict daily fluctuations in SI incidence at an inland beach in northwestern Michigan. Lifeguards collected daily data over four summers, including the number of self-reported SI cases, total swimmers, water temperature, wind speed and wind direction. Mixed-effects binomial regression revealed that wind direction, wind speed and time of day were the best predictors of daily SI risk. Swimmers entering the water in the morning or on days with direct onshore wind perpendicular to the shoreline had the greatest SI risk. However, there was a negative effect of wind speed after accounting for direction, where SI risk was greatest on days with a gentle breeze originating directly offshore. These results suggest that at this beach, direct onshore winds generate a surface-water current that causes SI cercariae to aggregate in the shallow waters used by swimmers. Data are needed from additional sites to confirm whether the onshore wind is a generally important driver of SI incidence.

An Assessment of the Seasonal Rainfall and Its Societal Implications in Zanzibar Islands during the Season of October to December, 2019

The assessment of the performance of the October to December (OND), 2019 rainfall season in Zanzibar (Unguja and Pemba) with reference to local downscaled Tanzania Meteorological Authority (TMA) forecast, and regional (Intergovernmental Authority on Development Climate Prediction and Application Center (IGAD-ICPAC) weather forecasts were assessed by comparing the long term average of OND rainfall data and previous OND rainfall seasons of 2016, 2017 and 2018 as well as extreme positive Indian Ocean Dipole (IOD) during OND seasons of 1961, 1994, 1997, 2006 and 2019 for Zanzibar. The study assessed zonal (u) and meridional (v) winds at 850 and 200 mb, monthly and dekadal sea surface temperature (SST); the Madden Julien Oscillations (MJO) forecast reports and the ocean heat content data. Both gridded and observed datasets were processed into dekadal, monthly and seasonal averages and then analysed. The results revealed that, based on the observations, above normal rainfall of 936 and 908 mm were reported at stations of Kisauni (Unguja) and Karume airport (Pemba) during 2019 OND season. This amount was the first and second ever recorded for the extreme positive IOD during OND seasons of 1961, 1994, 1997, 2006 and 2019, and also the first for the previous higher OND rainfall seasons of 2016, 2017 and 2018 which was highly variable. Moreover, these values were second ever-recorded highest OND rainfall since 1916 to 2019 where the first one was observed in 1961. Furthermore, the results revealed that 2019 OND seasonal rainfall had the highest amount of contribution based on historical climatology. For instance, the 2019 OND rainfall for Kisauni, Pemba airport and Matangatuani contributed to 198%, 303% and 231% of the long term (1987-2016) mean OND rainfall in Zanzibar. Indeed, the results show that the presence of the MJO during OND and the enhanced positive IOD was among the causes for the observed wetness of the 2019 OND in Zanzibar and most parts of the country. Moreover, the dominant easterly, southeasterly and northwesterly onshore winds during 2019 OND also contributed to heavy rainfall. The monthly rainfall variability among stations had the highest amount of rainfall which ranges from 400 to 500 mm which was observed during October in Kisauni and Karume airport, while the lowest amount ranging from 150 to 180 mm was observed during November in Matangatuani and the surrounding stations. Based on the comparison of the forecasted reports issued by ICPAC and TMA, the results revealed that irrespective of not considering the likelihood of occurrence of MJO and strong positive IOD both forecasts has performed well with that of ICPAC being leading. Conclusively, since the 2019 OND season has been uniquely characterized by the presence of MJO and IOD polarities it would be worthful to consider the two as input parameters during the OND rainfall forecast over the region.

Coastal ridge constructive processes at a multi‐decadal scale in Barreta Island (southern Portugal)

AbstractMultiple ridges across prograding coasts may display variable geometries, commonly expressed through varying elevations. Changes in ridge elevation have been traditionally related to the occurrence of fluctuating progradation rates, which might, in turn, be driven by shifting environmental conditions.Here, we explore the geometry and growth mechanisms of multiple ridges, generated at Barreta Island (Ria Formosa, southern Portugal), as a consequence of the rapid progradation of the island over the last 70 years, following the artificial fixation of the downdrift Faro‐Olhão inlet with jetties in 1955. The variability in the morphology of these features was analysed in combination with available wind and wave data, in order to better distinguish growth mechanisms and understand the main parameters determining the final geometry of the observed ridges.The results suggest that (1) most of the identified ridges fall in the beach ridge classification, as they have been mostly built by marine processes, and (2) the parameters derived from, or closely related to wave climate variability (e.g. progradation rates, storm occurrence) can jointly explain most of the observed morphological changes, while aeolian processes played a secondary role. Indeed, ridge geometry appears mainly controlled by progradation rates, with higher ridges associated with lower progradation rates. Progradation rate, in turn, is mostly related to longshore wave power, storminess, and the occurrence storm groups. Yet, the final configuration of ridges can also be affected by runup levels and onshore winds. Therefore, establishing the relation between ridge geometry and wave climate is not a straightforward task, because of the complex processes and interactions that control coastal morphodynamics. © 2019 John Wiley & Sons, Ltd.

Sandy coast erosion under the conditions of a storm surge combined with a spring tide

By comparing the changes of beaches and features in the southeastern coast of Xiamen Island before and after Typhoon No. 9914, the beach cycle and coast erosion processes under the condition of superimposed the spring tide of storm surge were discussed. The study showed that during Typhoon No. 9914, the coastal water level rose to 4.04m above the main sea level (MSL). The storm surge elevation was 1.43m. The stormy waves with 22m/s onshore winds brought overwash onto Huandao Road, up to the main sea level of more than 8.5m, depositing about 8-10cm of beach sands on the road. It caused the beach berm to disappear completely. The sand prism lying in the high tidal zone was scoured. Part of the sand of the prism was washed upward away from the beach, part of which was brought to the low tide zone. The bare shoreface without sand prism was scoured directly by waves and currents. Although the sandy sediment which was brought to the deeper zone by the storm can come back to the shoreface, but the overwash sand could not be returned after the storm, and the prime failed to return to its original condition for a long time, eventually leading to beach erosion and coast retreat.

Intraseasonal Variability of the Diurnal Cycle of Precipitation in the Philippines

AbstractPrecipitation in the region surrounding the South China Sea over land and coastal waters exhibits a strong diurnal cycle associated with a land–sea temperature contrast that drives a sea-breeze circulation. The boreal summer intraseasonal oscillation (BSISO) is an important modulator of diurnal precipitation patterns, an understanding of which is a primary goal of the field campaign Propagation of Intraseasonal Tropical Oscillations (PISTON). Using 21 years of CMORPH precipitation for Luzon Island in the northern Philippines, it is shown that the diurnal cycle amplitude is generally maximized over land roughly 1 week before the arrival of the broader oceanic convective envelope associated with the BSISO. A strong diurnal cycle in coastal waters is observed in the transition from the inactive to active phase, associated with offshore propagation of the diurnal cycle. The diurnal cycle amplitude is in phase with daily mean precipitation over Mindanao but is nearly out of phase over Luzon. The BSISO influence on the diurnal cycle on the eastern side of topography is nearly opposite to that on the western side. Using wind, moisture, and radiation products from the ERA5 reanalysis, it is proposed that the enhanced diurnal cycle west of the mountains during BSISO suppressed phases is related to increased insolation and weaker prevailing onshore winds that promote a stronger sea-breeze circulation when compared with the May–October mean state. Offshore propagation is suppressed until ambient midlevel moisture increases over the surrounding oceans during the transition to the active BSISO phase. In BSISO enhanced phases, strong low-level winds and increased cloudiness suppress the sea-breeze circulation.

Fecal Source Tracking in A Wastewater Treatment and Reclamation System Using Multiple Waterborne Gastroenteritis Viruses.

Gastroenteritis viruses in wastewater reclamation systems can pose a major threat to public health. In this study, multiple gastroenteritis viruses were detected from wastewater to estimate the viral contamination sources in a wastewater treatment and reclamation system installed in a suburb of Xi’an city, China. Reverse transcription plus nested or semi-nested PCR, followed by sequencing and phylogenetic analysis, were used for detection and genotyping of noroviruses and rotaviruses. As a result, 91.7% (22/24) of raw sewage samples, 70.8% (17/24) of the wastewater samples treated by anaerobic/anoxic/oxic (A2O) process and 62.5% (15/24) of lake water samples were positive for at least one of target gastroenteritis viruses while all samples collected from membrane bioreactor effluent after free chlorine disinfection were negative. Sequence analyses of the PCR products revealed that epidemiologically minor strains of norovirus GI (GI/14) and GII (GII/13) were frequently detected in the system. Considering virus concentration in the disinfected MBR effluent which is used as the source of lake water is below the detection limit, these results indicate that artificial lake may be contaminated from sources other than the wastewater reclamation system, which may include aerosols, and there is a possible norovirus infection risk by exposure through reclaimed water usage and by onshore winds transporting aerosols containing norovirus.

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.

Inversion of Wind-Stress Drag Coefficient in Simulating Storm Surges by Means of Regularization Technique.

In this study, water levels observed at tide stations in the Bohai Sea, Yellow Sea, and East China Sea during Typhoons 7203 and 8509 were assimilated into a numerical assimilation storm surge model combined with regularization technique to study the wind-stress drag coefficient. The Tikhonov regularization technique with different regularization parameters was tested during the assimilation. Using the regularization technique, the storm surge elevations were successfully simulated in the whole sea areas during Typhoons 7203 and 8509. The storm surge elevations calculated with the regularization technique and the elevations calculated with independent point method were separately compared with the observed data. Comparison results demonstrated that the former was closer to the observed data. The regularization technique had the best performance when the regularization parameter was 100. The spatial distribution of the inverted drag coefficient, storm surge elevations, and the wind fields during both typhoons were presented. Simulated results indicated that the change of drag coefficient is more significant in the coastal regions of the Bohai Sea and north of the Yellow Sea. Further analysis showed that the rising water elevation in the Bohai Sea is mostly attributed to the influence of onshore winds, and the negative storm surge in the South Yellow Sea is mainly caused by offshore winds.

Delivery of zooplankton to the surf zone during strong internal tidal forcing and onshore winds in Baja California

Author(s): Fernandez-Aldecoa, RG; Ladah, LB; Morgan, SG; Dibble, CD; Solana-Arellano, E; Filonov, A | Abstract: Various physical mechanisms are implicated in the transport of zooplankton to the outer edge of the surf zone, which is the final barrier before reaching the adult habitat of many meroplanktonic organisms. To explore these physical mechanisms, we measured the abundance of zooplankton in the surf zone hourly for 3 consecutive days during strong internal tidal forcing while concurrently measuring winds, currents, and seawater temperature. Strong temperature changes in the water column that were associated with internal tidal bores, as well as onshore coastal winds, coincided with peaks in abundance of barnacle cyprids, gastropods, and bryozoan larvae in the surf zone. This study supports the hypothesis that both internal tidal bores and onshore winds can accumulate zooplankton nearshore, and that these transport mechanisms may act in concert.

Influence of Synoptic Pattern and Low‐Level Wind Speed on Intensity and Diurnal Variations of Orographic Convection in Summer Over Pearl River Delta, South China

AbstractBased on 5 years of operational Doppler radar data, the influences of atmospheric synoptic patterns and low‐level prevailing wind speed on the intensity and diurnal variations of summer orographic convection occurrence frequencies over Pearl River Delta, South China, have been investigated. Results show that the inland orographic convection and rainfall generally occurs under synoptic pattern characterized with the prevailing southwesterly wind within the lower troposphere. The summer orographic convection over the mountains in northeastern Pearl River Delta is not only controlled by the orographic thermal conditions but also the dynamic forcing with the increase of wind speed. Owing to the strong windward mechanical lifting and moisture transport associated with the strong ambient onshore winds, the number of convection occurrences characterized by a dominant diurnal afternoon peak occurs much more frequently in the high‐wind speed days. While due to the weak orographic mechanical lifting and moisture supply in the low‐wind speed days, the number of convection occurrences in the afternoon decreases considerably and two comparable peaks occur in the afternoon and early morning. The nighttime peak in the low‐wind speed days is mainly attributed to the nocturnal acceleration of the low‐level southwesterly wind associated with the inertial oscillation and the corresponded enhanced windward lifting effects.

Characteristics of land and sea breezes along the Guinea Coast of West Africa

This study utilized 34-years hourly data of wind speed and direction, precipitation, and air temperature over land to investigate the climatology of land and sea breezes (LSB) at seven coastline stations along the Guinean Coast of West Africa. Daily sea surface temperature from the adjacent Atlantic Ocean and Tropical Rainfall Measurement Mission daily precipitation data was used. Ten years reanalysis data of ERA-Interim, Climate Forecast System Reanalysis version 2 (CFSRv2), and National Centers for Environmental Prediction (NCEP2) were also used for comparison with the observations. Adopted criteria were based on both surface and upper air information to identify LSB days. The results show that LSB is a seasonal phenomenon but most frequent in winter. All reanalyses underestimated the monthly occurrence and seasonal cycle of LSB, although CFSRv2 is better in reproducing the observations than ERA-Interim and NCEP2. The wind roses revealed the existence of night/early morning offshore winds. Except in summer when the southwesterly monsoon winds prevail all day, onshore winds are observed from about 1100 local solar time (LST), with a mean cessation time at 0100 LST, giving a mean duration of about 14 h. However, in Abidjan, they occurred at all hours. The vertical plots confirmed the wind direction reversals for LSB days and give an indication of its depth and speed in different months. It has been found that LSB height is influenced by the West African monsoon as intertropical discontinuity advances northwards from January. This was also seen in LSB increased speed from December to August.

Spatial distribution of foreshore litter on the northwest European continental shelf

Foreshore litter data from a UK citizen-science programme, combined with OSPAR data, were analysed for possible spatial patterns around Scottish and North Sea coastlines. Loading distributions were positively skewed, and statistics commensurate with such distributions were used. When considering coast type, litter loadings on Scottish harbour and river foreshores were influenced by local litter sources. When considering exposure to the predominant westerly winds over Scotland, litter loadings on the west coast (i.e. predominant onshore winds) were greater on foreshores on open coasts compared to those within embayments. The opposite was true for the Scottish east coast (i.e. predominantly offshore winds). The north east coast of the UK appeared to have an organised pattern of plastic litter loading, increasing in magnitude (median) and spread (inter quartile range) in the direction of the coastal flow. Four other coastal segments with similar patterns were suggested from the west of Scotland to Denmark.

Ancient landscapes of the Namib Desert harbor high levels of genetic variability and deeply divergent lineages for Collembola.

AimTo assess spatial patterns of genetic and species‐level diversity for Namib Desert Collembola using mitochondrial DNA cytochrome c oxidase subunit I (COI) gene sequences.LocationNamib Desert gravel plains.TaxonCollembola (springtails).MethodsA total of 77 soil samples were collected along NE‐SW (60 km) and E‐W (160 km) transects from within a 4,000 km2 area of the Namib Desert gravel plains. We extracted 434 springtails from the 37 samples which contained Collembola and sequenced them at the COI gene locus. In the absence of specific taxonomic keys and previous genetic data for these taxa, we used Generalized Mixed Yule Coalescent (GMYC) analyses to provide putative species‐level designations.ResultsWe obtained 341 successful COI sequences, 175 of which were unique haplotypes. GMYC analyses identified 30 putative species, with up to 28% sequence divergence (uncorrected p‐distance). The distribution of genetic variants was disjunct, with 97% of haplotypes and 70% of “GMYC species” found only at single sites.Main conclusionsDispersal events, although rare, may be facilitated by environmental events such as prevailing onshore winds or occasional flow of rainwater to the coast. We conclude that the high genetic diversity we observed is the result of ancient springtail lineages, patchy distribution of suitable habitats, and limited dispersal (gene flow) among habitable locations.

Investigation of two pack ice besetting events on the Umiak I and development of a probabilistic prediction model

Pack ice besetting events are defined as incidents in which a typical ice-class vessel encounters dynamic sea ice conditions which are beyond the ship's icebreaking and maneuverability capacities, and the vessel becomes stuck in the pack ice until such conditions subside. Besetting events can cause severe disruption to a vessel's transit schedule, leading to increased operational downtime, and ship crew and fuel costs. Hence, predicting the probability of occurrence of these events can help vessel operators plan in advance accordingly, possibly rerouting the vessel, and saving operational costs. In this paper, two pack ice besetting events experienced by the Umiak I, an icebreaking bulk carrier owned and operated by Fednav Limited which runs nickel ore from the mine at Voisey's Bay, Labrador to Québec City, are analyzed in terms of the wind, surface ocean current, and sea ice conditions which caused them. The particular metocean and pack ice conditions associated with the occurrence of these events are used to develop a framework for a model to predict the future occurrence of such events for the Umiak I and quantify probabilities of besetting event occurrence linked to specific sets of coincident metocean and ice conditions. The model will serve as a tool for predicting besetting event probabilities during Umiak I navigations, as well as in hindcast shipping feasibility studies, to determine the likelihood of besetting for vessels with Umiak I capabilities in regions of new operations. The factors considered in this study are the vessel distance to the nearest coastline, regional wind and surface current speed, direction relative to the nearest coastline, divergence, and vorticity. The ice factors considered are the total concentration, mean thickness, and floe size (this represents a mean floe diameter). The datasets analyzed in this paper suggest the primary causes of the two besetting events experienced by the Umiak I in 2012 and 2013 were the relatively large ice floes encountered by the vessel (the majority of the floes during the besetting events were greater than 6 km in diameter), in combination with a convergent regional wind field and onshore winds and currents.

Evaluation of targeted copper sulfate (CuSO4) application for controlling swimmer's itch at a freshwater recreation site in Michigan.

Swimmer's itch has historically been controlled by applying copper sulfate (CuSO4) to lakes as a way to eliminate snails that serve as the intermediate hosts for swimmer's itch-causing parasites. CuSO4 is still sometimes applied specifically to areas of lakes where swimmer's itch severity is high. It is unclear whether targeted application of chemical molluscicides like CuSO4 is effective for controlling swimmer's itch. Previous research has found that the larval stage of the parasites responsible for swimmer's itch are released from infected snails and are concentrated by onshore and alongshore winds, and thus, may not be affected by such focused applications. In this study, we evaluated the impact of targeted CuSO4 application to a specific recreational swimming area in a lake in Michigan. We measured the effect on snail populations, as well as on the presence/abundance of swimmer's itch-causing parasites using qPCR. Ultimately, while CuSO4 was confirmed to significantly reduce populations of snails within the treatment area, it was found to have no significant impact on swimmer's itch-causing parasites in the water, likely due to the free-swimming larval stages (cercariae) moving into the treatment area from surrounding regions.

Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf.

Landscape Evolution of a Fluvial Sediment-Rich Avicennia marina Mangrove Forest: Insights from Seasonal and Inter-annual Surface-Elevation Dynamics

Mangrove forests are vulnerable to accelerated sea-level rise associated with climate warming because they occupy a relatively narrow zone on the mid-to-upper-intertidal flats. The fate of these ecosystems largely depends on their capacity to accrete sediment at a rate sufficient to maintain their elevation relative to sea level. We investigated the role of biophysical processes and feedbacks controlling surface-elevation dynamics in a fluvial sediment-rich Avicennia marina mangrove forest (New Zealand) at seasonal-to-inter-annual timescales (over 9 years) using the Rod Surface Elevation Table method. We found that sediment accretion in the forest was not measurably enhanced by episodic and short-lived storm discharges from rivers nor by elevated sea levels during storms. Critically, the coupling of frequent onshore winds and resulting resuspension of intertidal muds, with the fortnightly cycle of spring tide inundation, controlled sediment delivery and resulting accretion rates of 13 to 47 mm y−1. In turn, net surface-elevation trends of 0 to 28 mm y−1 were dominated by the physical processes of sediment accretion and shallow subsidence due to seasonal desiccation and resulting compaction of the infrequently inundated forest platform (4 to 16 mm y−1). Our data suggest that monthly and seasonal variation in tidally controlled hydroperiod and sediment delivery rather than episodic storm events are important for the maintenance of mangrove elevation within the intertidal zone.

Multi-decadal variability in sandy beach area and the role of climate forcing

Sandy beaches comprise 31% of the ice free coasts of the world, providing a variety of ecosystem services essential to support human well-being, particularly protection against erosion and extreme climatic events. These highly dynamic environments are controlled principally by wave energy, tides and grain size, generating complex morphodynamic patterns that translate into strong area variation. Estimates of beach area and its variability are prime inputs for coastal management, a challenge that requires data representing a wide spatiotemporal scale. In this paper, a Landsat-based semi-automated methodology was developed to reconstruct the area of 21 sandy beaches of the Montevideo (Uruguay) coast from 1984 to 2016. This long-term information was also used to discern erosion-accretion cycles by means of wavelet analysis, and to explore the role of climatic forcing on these cycles through linear mixed models. A random forest classification algorithm was applied to Landsat images in order to estimate beach area. Long-term trends described a 27-year cycle with well-delimited quasi-decadal erosion and accretion phases related to climatic configurations. The beach area was negatively affected by an increase in sea level and climatic conditions of the previous year, being positively correlated with sea surface temperature anomalies and offshore winds (which favored accretion) and negatively correlated with onshore winds and intense El Niño Southern Oscillation (ENSO) events (favoring erosion). Our findings, together with the predicted increase in the occurrence and intensity of storms and extreme ENSO events, constitute a worrying scenario, as erosion in this populated coastal zone could have negative social, economic and ecological repercussions. The methodology developed here was useful to detect long-term changes in beach area and is entirely based on open-access information. Therefore, it is potentially applicable at any location on the planet. This approach is also useful to counteract the scarcity of long-term information that has precluded robust assessments of climate change effects on coastal zones.

Spreading dynamics of small river plumes off the northeastern coast of the Black Sea observed by Landsat 8 and Sentinel-2

We use near simultaneous ocean color satellite imagery from NASA's Landsat 8 and ESA's Sentinel-2 missions to reconstruct surface currents along the northeastern shore of the Black Sea and study the spread of a small river plume formed in this area. Several times a year Landsat 8 and Sentinel-2 satellites both pass over the study area within a time interval of <10 min. Analysis of near simultaneous ocean color composites obtained during these periods provides an opportunity for accurate reconstruction of surface currents. It is especially efficient for detecting motion of frontal zones associated with river plumes which are visible in optical satellite imagery. Using an optical flow algorithm we reconstructed surface currents associated with the motion of a small plume formed by the Mzymta River in response to different wind forcing conditions. We show that the dynamics of the Mzymta plume are significantly different to those of large plumes. First, a near-field jet of the small plume is abruptly decelerated by friction with the subjacent ocean and does not form a recirculating bulge even under low wind forcing conditions. As a result, freshwater discharge does not accumulate near the river mouth and is transported to the far-field part of the plume. Second, under certain external forcing conditions the angle between wind direction and Ekman transport within the plume takes anomalously large values of 60–80°. As a result, onshore winds cause upstream accumulation of the river plume, while offshore/downwelling and upwelling winds result in downstream and offshore transport of freshwater discharge, respectively.

Wind-induced hydrodynamic changes impact on sediment resuspension for large, shallow Lake Taihu, China

The internal sediment release is a key factor controlling eutrophication processes in large, shallow lakes. Sediment resuspension is associated with the wave and current induced shear stress in large, shallow lakes. The current study investigated the wind field impacts on sediment resuspension from the bottom at Meiliang Bay of large, shallow Lake Taihu. The impacts of the wind field on the wave, current, and wave-current combined shear stresses were calculated. The critical wind speed range was 4–6 m/s after which wave and current shear stress started to increase abruptly, and onshore wind directions were found to be mainly responsible for greater shear stress at the bottom of Lake Taihu. A second order polynomial fitting correlation was found between wave (R2 0.4756) and current (R2 0.4466) shear stresses with wind speed. Wave shear stress accounted for 92.5% of the total shear stress at Meiliang Bay. The critical wave shear stress and critical total shear stress were 0.13 N/m2 for sediment resuspension whereas the current shear stress was 0.019 N/m2 after which suspended sediment concentrations (SSC) increased abruptly. A second order polynomial fitting correlation was found between wave (R2 0.739), current (R2 0.6264), and total shear stress (R2 0.7394) with SSC concentrations at Meiliang Bay of Lake Taihu. The sediment resuspension rate was 120 to 738 g/m2/d during 4–6 m/s onshore winds while offshore winds contributed ≥ 200 g/m2/d. The study results reveal the driving mechanism for understanding the role of the wind field in sediment resuspension while considering wind speed and direction as control parameters to define wave and current shear stresses.