Local Wind Forcing Research Articles

Wind tunnel study of wind effects on 90° helical and square tall buildings: A comparative study

Wind effects on a square model and a 90° helical model are investigated by both pressure measurement and aero-elastic model tests in a boundary layer wind tunnel. Based on the pressure measurement results, local wind force coefficients, base moment coefficients, vertical correlation coefficients and power spectral densities for the two models were analyzed and discussed in detail. The aerodynamic damping ratios were calculated by the random decrement technique (RDT) on the basis of the wind-induced acceleration obtained from the aero-elastic tests. Comparisons of wind loads and wind-induced accelerations were made for the square and helical models. The results shown that the aerodynamic damping should not be ignored, especially for across-wind direction. Moreover, the helical model has been proved to yield a better behavior in aerodynamic performance than square model. This study aims to provide useful information for wind-resistant design of helical building.

Interannual Variability in Sea Surface Height at Southern Midlatitudes of the Indian Ocean

AbstractThis study examines interannual variability in sea surface height (SSH) at southern midlatitudes of the Indian Ocean (10°–35°S). Our focus is on the relative role of local wind forcing and remote forcing from the equatorial Pacific Ocean. We use satellite altimetry measurements, an atmospheric reanalysis, and a one-dimensional wave model tuned to simulate observed SSH anomalies. The model solution is decomposed into the part driven by local winds and that driven by SSH variability radiated from the western coast of Australia. Results show that variability radiated from the Australian coast is larger in amplitude than variability driven by local winds in the central and eastern parts of the south Indian Ocean at midlatitudes (between 19° and 33°S), whereas the influence from eastern boundary forcing is confined to the eastern basin at lower latitudes (10° and 17°S). The relative importance of eastern boundary forcing at midlatitudes is due to the weakness of wind stress curl anomalies in the interior of the south Indian Ocean. Our analysis further suggests that SSH variability along the west coast of Australia originates from remote wind forcing in the tropical Pacific, as is pointed out by previous studies. The zonal gradient of SSH between the western and eastern parts of the south Indian Ocean is also mostly controlled by variability radiated from the Australian coast, indicating that interannual variability in meridional geostrophic transport is driven principally by Pacific winds.

An integrated framework of input determination for ensemble forecasts of monthly estuarine saltwater intrusion

Mid- and long-term saltwater intrusion forecasts for estuaries are challenging due to a wide range of dynamic interactions and the limited amount of available data. This study proposes a tailor-made method for input determination for ensemble forecasts of monthly estuarine saltwater intrusion. The proposed method is based on determining the initial set of candidates by the combined use of Pearson’s Coefficient (r) and Maximal Information Coefficient (MIC); and afterwards reducing the dimension of the input data sets by Principal Component Analysis (PCA). The current study uses Bayesian Model Averaging (BMA) method to combine the forecasting results of Random Forest (RF), Support Vector Machine (SVM) and Elman Neural Network (ENN) models to create an integrated forecast. The proposed modeling approach was tested and compared with seven alternative procedures to forecast the monthly saltwater intrusion at the Pearl River Delta (PRD). The results indicated that: (a) the monthly dynamics of saltwater intrusion are more sensitive to the long-term solar activities than the local wind force; (b) the valuable non-linear signals hidden in the related time series could be identified by the combined use of r and MIC; (c) dynamic statistics related to low runoff, high antecedent chlorinity, strong tidal force, and strong wind force are preferable over the monthly average values as model inputs; and (d) the proposed method achieved highest forecast accuracy with Nash-Sutcliffe coefficient (NSE) of 0.78. This study provides insights to the input determination for data-driven models of complex estuarine saltwater intrusion.

Sea Surface Salinity Estimation and Spatial-Temporal Heterogeneity Analysis in the Gulf of Mexico

As an important parameter to characterize physical and biogeochemical processes, sea surface salinity (SSS) has received extensive attention. Cubist is a data mining model, which can be well-suited to estimate and analyze SSS in the Gulf of Mexico (GOM) because it can reflect the SSS internal heterogeneity in the GOM—overall circular distribution, and the seasonality related to temperature and river discharge changes. Using remote sensing reflectance (Rrs) at 412, 443, 488 (490), 555, and 667 (670) nm and sea surface temperature (SST), a cubist model was developed to estimate SSS with high accuracy with the overall performance demonstrates a root mean square error (RMSE) of 0.27 psu and correlation coefficient of 0.97 of R2. The model divides the GOM area according to model rules into four sub-regions, which include estuary, nearshore, and open sea, reflecting the gradient distribution of SSS. The division of sub-regions and seasonal changes can be explained by the distribution of water bodies, river discharges, and local wind forces since it is obvious that the estuary region reaches the largest low-value area and spreads eastward with the monsoon in the spring when the river flow increases to the highest value. While the east to west wind in the non-summer monsoon period guides the plume westward, and the lowest river discharge in winter corresponds to the smallest low value area. After comparison with other statistical models, the cubist model showed satisfactory results in independent verification of cruise data, proving the estimation capability under different geographical conditions (such as estuaries and open seas) and seasons. Therefore, considering high accuracy and heterogeneity mining, the cubist-based model is an ideal method for coastal SSS estimation and spatial-temporal heterogeneity analysis, and can provide ideas for model construction for coastal areas with similar geographic environments.

The sea level variability and its projections over the Indo‐Pacific Ocean in CMIP5 models

The present study examines the representation of interannual and decadal variability of sea level over the Indo-Pacific region in the Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations and future projections. The thermocline ridge region of the Indian Ocean (TRIO) shows strong sea level variability on both the interannual and decadal timescales in the CMIP5 historical simulations. Meanwhile, prominent interannual variability is seen in the tropical western Pacific and central to eastern equatorial Pacific, whereas, the decadal sea level variability is dominant in the North Pacific and western Pacific regions. CMIP5 models project similar spatial patterns of sea level variability over the Indo-Pacific region for the twenty-first century under the mid-range and high future emission scenarios, although with some differences in the amplitude and inter-model disagreement. In the interannual timescale, there is a regional contrast in the projected future variability between the Pacific (with increase in variability with increasing greenhouse gas emissions) and TRIO (with decrease of ~ 6 % variability), which is evident in both RCP4.5 and RCP8.5 future emission scenarios with inter-model consensus. On the other hand, increase in decadal sea level variability (~ 6 % and ~ 8 % in RCP4.5 and RCP8.5 scenarios respectively) is projected over the North Pacific region (north of 30°N). In the Indian Ocean, increase in decadal sea level variability is projected over the northwestern region including Arabian Sea, though it is not a region of large decadal variability (in the reanalysis and historical simulations). Analysis revealed that the local wind forcing (wind stress curl) plays a dominant role in the sea level variability over the Indo-Pacific region on both interannual and decadal timescales.

Revising the Effects of Local and Remote Atmospheric Forcing on the Atlantic Jet and Western Alboran Gyre Dynamics

AbstractThe main objective of the present study is to gain insight into the response of the Atlantic Jet (AJ) to the atmospheric forcing, in order to understand the hydrodynamic processes behind its disconnection from the Western Alboran Gyre (WAG) and the eastward displacement of the latter. An analysis was carried out based on observations of high frequency radar (HFR) derived currents and sea level, numerical model simulations and satellite imagery, with particular focus on the events of intense and endured easterlies. Among the main factors that characterize the atmospheric forcing linked to intense and persistent easterly wind events, the conjugation of the direct local wind forcing effect on the Atlantic Inflow (AI) on the eastern side of the Strait of Gibraltar and the response of the coastal waters in the northwestern (NW) corner of the Alboran Sea to the wind forcing over the Alboran Sea are of particular interest. In addition, the effect of the elevation surges in the Strait coming from the western Mediterranean Sea on reducing the AI intensity must be taken into account. These surges appear in coincidence with the establishment of persistent and intense easterlies over the Alboran Sea. The alteration of the AJ‐WAG system in response to this forcing causes important changes in the phytoplankton distributions in the Alboran Sea. After the events of persistent and intense easterlies the phytoplankton accumulations in the NW Alboran Sea are displaced offshore and the phytoplankton transported with the AJ is injected toward the central zone of the WAG.

Springtime Upwelling and Its Formation Mechanism in Coastal Waters of Manaung Island, Myanmar

Multisource satellite remote sensing data and the World Ocean Atlas 2018 (WOA18) temperature and salinity dataset have been used to analyze the spatial distribution, variability and possible forcing mechanisms of the upwelling off Manaung Island, Myanmar. Signals of upwelling exist off the coasts of Manaung Island, in western Myanmar during spring. It appears in February, reaches its peak in March and decays in May. Low-temperature (<28.3 °C) and high-salinity (>31.8 psu) water at the surface of this upwelling zone is caused by the upwelling of seawater from a depth below 100 m. The impact of the upwelling on temperature is more significant in the subsurface layer than that in the surface layer. In contrast, the impact of the upwelling on salinity in the surface layer is more significant. Further research reveals that the remote forcing from the equator predominantly induces the evolution of the upwelling, while the local wind forcing also contributes to strengthen the intensity of the upwelling during spring.

Effects of corner modifications on wind loads and local pressures on walls of tall buildings

In this study, the aerodynamic characteristics of tall buildings with corner modifications (e.g., local wind force coefficients, mean pressure distributions, normalized power spectrum density, and extreme local pressure) were examined. Wind tunnel experiments were conducted to measure the wind pressures on building models with different heights and recessed corners of different ratios. At a wind direction of a = 0° (i.e., wind blowing on the front of a building), corner modifications effectively reduced wind forces in all cases. Specifically, small corner modification ratios reduced wind forces more effectively than their larger counterparts. However, corner modifications resulted in extreme local pressure on building surfaces. In addition, for small corner modification ratios, the probability of extreme local pressure occurring at a = 0° was high. This probability was also high for large corner modification ratios at a = 15° (i.e., wind blowing slightly obliquely on the front of a building) because wind blowing obliquely creates substantial vortex shedding on one side surface and extreme negative pressure over one building side surface. Results of computational fluid dynamic modeling were adopted to determine details of the aerodynamic characteristics of tall buildings with corner modifications.

Description on variability of shelf-edge hydrography and current structure of the South Eastern Arabian Sea during summer and winter monsoons.

The paper describes the hydrography and vertical current structure along the shelf edge of South East Arabian Sea (SEAS) during summer and winter monsoons based on current profiles from moving Acoustic Doppler Current Profiler (ADCP). During summer monsoon, SEAS was characterized by upwelling with low saline water at the surface along the southern sector (8° N to 11° N). During winter, thermal structure was vertically homogeneous in the upper 80 m, and intrusion of low saline Bay of Bengal waters were found up to 14° N. In the southern sector, turbidity was more than the northern sector during winter and summer seasons. ADCP-derived current profiles during summer along 200-m isobath show dominant northward flow in the south, and southeasterly in the north as part of the West India Coastal Current (WICC). A comparison between ADCP current profiles and Ekman currents during summer indicates dominance of remote forcing (coastal Kelvin waves) over the local wind forcing in the 8-9° N sector whereas a combined influence of both remote forcing and wind in the 9-15° N sector. During winter, the direction of surface current reversed and was poleward generally except at the southern sector (7-8° N) where the flow was southwestward. Sector-wise comparison of ADCP and Ekman current showed less influence of wind on current fields throughout the sector except at south; wind has a major role in the current generation, whereas along the 8-15° N sector, the remote forcing dominates over the wind.

Interannual Variability of Winter Sea Levels Induced by Local Wind Stress in the Northeast Asian Marginal Seas: 1993–2017

The interannual variability of winter sea levels averaged over the northeast Asian marginal seas, consisting of the Yellow Sea, East China Sea, and the East Sea (ES), was investigated. The spatial-mean sea level in winter observed using satellite altimetry shows significant interannual variations with a long-term rising trend of 3.88 mm y−1 during 1993–2017, with relatively high (Period H) and low (Period L) sea level anomalies. These anomalies correlate with the patterns of the East Asian winter monsoon at interannual timescales. The atmospheric pressure difference between the Sea of Okhotsk (SO) and ES around the Soya Strait is large during Period H. Ekman transport increases due to enhanced southeastward wind stress and results in a horizontal mass convergence that yields positive sea level anomalies during Period H. In contrast, the wind-induced transport is enhanced in the southern ES rather than in the southern SO resulting in horizontal mass divergence and negative anomalies in the spatial-mean winter sea level during Period L. Our results highlight the important roles of local wind forcing and Ekman dynamics in inducing interannual winter sea level variability in the region indicating the high predictive ability of atmospheric pressure anomalies around the Soya Strait.

On the interaction between partially-reflected waves and an opposing wind

Air-sea interaction is a relevant topic with still many open questions, also in consideration of the numerous and complex scenarios that occur in nature. In this manuscript we report an experimental study of partially-reflected regular (long) waves under the action of an opposite wind, which generates short wind-waves (wind generated water waves, WGW). Wind blows against the incident wave, and therefore it is in the same direction as the reflected wave. Regular waves are generated by a paddle and are defined as mechanically generated water waves (MGW). MGW and WGW are separated by a spectral filtering technique, and free surface statistics of wave height, period and steepness are studied. The spatial variation induced by partial reflection of MGW (generated by paddles) is experimentally observed and theoretically described, and the effects of wind on reflection parameters are observed and examined. A phase-conditional analysis of wind-waves statistics, separating crest and trough, and upwind and downwind sides, reveals the importance of the local wind forcing, and captures features of the wind-waves modulated by the longer waves which are not detected with standard statistical analyses. The phase celerity of the wind-waves is analysed including the effects of the wind-induced shear current and the non-inertial effects of the MGW; to the best of our knowledge, this last effect experimental analysis is novel.The study is relevant to characterize the wave field in front of coastal structures and beaches under swell and wind waves (co-linear in the same, and in the opposite direction) conditions, affecting run-up, overtopping and mean quantities modulated by the dynamics of group of waves.

Characteristics of local wind forces on the edges of signboards installed on the rooftops of buildings

Many signboards are installed on the rooftops of low- and middle-rise buildings in Japan, which are often damaged by strong winds. However, there are no specifications of wind force coefficients for such signboards in the Japanese building codes and standards. In the present study, wind tunnel experiments were conducted to collect data on wind force coefficients under various conditions, including building shapes, as well as signboard shapes and locations. The distributions of the maximum peak wind force coefficients were obtained at various wind angles. Based on the results, the variation of the maximum peak wind force coefficient with wind angle was investigated. It was found that very large wind forces occurred near the edges of the signboard at oblique wind angles ranging from 30° to 60° with respect to the outside surface of the signboard. The present study focuses on such conditions that cause large wind forces on signboards. The results indicate that the building shape significantly affects the maximum peak wind force coefficient.

Influence of coastal upwelling on micro-phytoplankton variability at Valparaíso Bay (~33ºS), Central Chile

In this work 10 years of data (1986-1996) from a fixed station located in the northern part of Valparaíso Bay (33º00’S; 71º35’W) were analysed to study the influence of coastal upwelling activity on the temporal variation of micro-phytoplankton (20-200 μm) and their relationship with oceanographic conditions. The upwelling activity at the bay was associated to semi-annual wind regime with an intensification of upwelling-favourable S-SW winds from September to March followed by a decrease and the occurrence of downwelling events from April to August. Oceanographic conditions showed the ascent of cold, nutrient-rich salty water in spring (September-November). However, during summertime under highest upwelling index, thermal stratification conditions were registered. This stratification might be associated to either the solar radiation or the presence of an upwelling shadow area in the bay. The upwelling period had the highest micro-phytoplankton abundance mainly dominated by diatoms. This period was associated with an increase in biomass and richness in the bay. Meanwhile during non-upwelling period —under homogenous conditions of temperature, salinity and nutrients— an increase in diversity (but low abundance and richness) associated to dinoflagellates and silicoflagellates was noted. Therefore, the results suggest the presence of a bi-modal regime of micro-phytoplankton in the bay in response to changes in oceanographic conditions related to local wind forcing and mixing/stratification.

Genesis of the Antarctic Slope Current in West Antarctica

AbstractThe stability of the West Antarctic Ice Sheet (WAIS) depends on ocean heat transport toward its base and remains a source of uncertainty in sea level rise prediction. The Antarctic Slope Current (ASC), a major boundary current of the ocean's global circulation, serves as a dynamic gateway for heat transport toward Antarctica. Here, we use observations collected from the Bellingshausen Sea to propose a mechanistic explanation for the initiation of the westward‐flowing ASC. Waters modified throughout the Bellingshausen Sea by ocean‐sea‐ice and ocean‐ice‐shelf interactions are exported to the continental slope in a narrow, topographically steered western boundary current. This focused outflow produces a localized front at the shelf break that supports the emerging ASC. This mechanism emphasizes the importance of buoyancy forcing, integrated over the continental shelf, as opposed to local wind forcing, in the generation mechanism and suggests the potential for remote control of melt rates of WAIS' largest ice shelves.

Freshwater transport between the Kara, Laptev, and East-Siberian seas

The Kara and Laptev seas receive about one half of total freshwater runoff to the Arctic Ocean from the Ob, Yenisei, and Lena rivers. Discharges of these large rivers form freshened surface water masses over wide areas in these seas. These water masses, i.e., the Ob-Yenisei and Lena river plumes, generate an eastward buoyancy boundary current that accounts for the large-scale zonal freshwater transport along the Siberian part in the Arctic Ocean. In this study we investigate spreading of the Ob-Yenisei plume from the Kara Sea to the Laptev Sea through the Vilkitsky Strait and of the Lena plume from the Laptev Sea to the East-Siberian Sea through the Laptev and Sannikov straits during ice-free season. Large horizontal density gradient between freshened plume water and salty ambient sea water is the main driver of these processes, however, their intensity strongly depends on local wind forcing. The Ob-Yenisei plume is spreading to the Laptev Sea in a narrow alongshore current which is induced by strong and long-term southwesterly winds. Under other wind forcing the plume does not reach the Vilkitsky Strait. The Lena plume is almost constantly spreading to the East-Siberian Sea as a large-scale surface water mass which intensity is governed by eastward Ekman transport and is prone to large synoptic variability.

Control of the Oceanic Heat Content of the Getz‐Dotson Trough, Antarctica, by the Amundsen Sea Low

AbstractThe changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz‐Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low‐pressure system, which is modulated by large‐scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz‐Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL.

Unprecedented Surface Chlorophyll Blooms in the Southeastern Arabian Sea During an Extreme Negative Indian Ocean Dipole

AbstractObservations of ocean color in the southeastern Arabian Sea (SEAS) during the late boreal summer and fall of 2016 reveal the highest surface chlorophyll concentrations (>1.0 mg m−3) ever reported in the region in the past two decades of the satellite era. Anomalous chlorophyll blooms were triggered by two cold‐core eddies, along the periphery of the coastal upwelling zone. Observational evidence suggest that the eddy formation and propagation can be attributed to the local wind forcing and the remote effects originating around Sri Lanka and the southern tip of India (STI) during the extreme negative Indian Ocean Dipole (IOD) of 2016. The eddy‐induced anomalous shoaling of thermocline indicates strong upwelling of subsurface nutrients and chlorophyll and enhanced light availability for phytoplankton growth favoring the observed biological response. The study signifies the unprecedented impacts of extreme climatic events on oceanic chlorophyll and the need for better understanding of the associated biophysical interactions.

Hydrographic and dynamical characterisation of the Bagnoli-Coroglio Bay (Gulf of Naples, Tyrrhenian Sea)

ABSTRACT Hydrographic and current measurements collected within the ABBaCo project during 3 basin-scale surveys in two different seasons and high-resolution regional ocean modelling data provide, for the first time, an assessment of the physical and dynamical oceanography inside the small but important Bagnoli-Coroglio Bay within the Gulf of Naples (Tyrrhenian Sea). Temperature and salinity distributions indicate the typical seasonal differences of the Southern Tyrrhenian Sea with warmer temperature and a more stratified water column in June compared to October and December. Current measurements, along with realistic ocean circulation modelling, show a predominant barotropic anticyclonic circulation over the bay during Autumn, which appears to be generated mainly by the local wind stress curl. Sensitivity numerical experiments confirm that the variability of the local wind forcing plays a major role in shaping the dynamics inside the bay.

Occurrence and seasonal variability of Dense Shelf Water Cascades along Australian continental shelves

Transport of water between the coast and the deeper ocean, across the continental shelf, is an important process for the distribution of biota, nutrients, suspended and dissolved material on the shelf. Presence of denser water on the inner continental shelf results in a cross-shelf density gradient that drives a gravitational circulation with offshore transport of denser water along the sea bed that is defined as Dense Shelf Water Cascade (DSWC). Analysis of field data, collected from multiple ocean glider data missions around Australia, confirmed that under a range of wind and tidal conditions, DSWC was a regular occurrence during autumn and winter months over a coastline spanning > 10,000 km. It is shown that even in the presence of relatively high wind- and tidal-induced vertical mixing, DSWCs were present due to the strength of the cross-shelf density gradient. The occurrence of DSWC around Australia is unique with continental scale forcing through air-sea fluxes that overcome local wind and tidal forcing. It is shown that DSWC acts as a conduit to transport suspended material across the continental shelf and is a critical process that influences water quality on the inner continental shelf.

Observations of enhanced internal waves in an area of strong mesoscale variability in the southwestern East Sea (Japan Sea)

Oceanic internal waves near the local inertial frequency or near-inertial internal waves and internal waves of tidal origin or internal tides are two types of low-frequency internal waves. Their interactions, interaction with background field, and resulting internal waves at higher frequencies beyond the near-inertial and tidal frequencies have rarely been reported despite its importance on ocean mixing and circulation of energy and materials. Here, we present five episodic enhancements of the high-frequency or continuum frequency waves (CFWs) observed in the southwestern East Sea (Japan Sea) and discuss causes for the enhanced CFWs in relation to near-inertial waves (NIWs), semidiurnal internal tides (SDITs), mesoscale flow fields, and their interactions. The NIWs were amplified due to local surface wind forcing, significantly interacting with mesoscale strain via wave capture. The SDITs were generated in a remote place and propagated into the observational site, largely depending on the mesoscale fields. The observational results suggest that the five episodes of CFWs are results of enhanced NIWs or SDITs, or their wave-wave interaction, rather than locally generated lee-waves. Our study suggests the significant impact of mesoscale circulation on the variability of internal waves from near-inertial to buoyancy frequencies through multiple pathways.