Wind Stress Values Research Articles

Impact of tropical cyclone Biparjoy on oceanic parameters in the Arabian Sea

Climate change is expected to intensify tropical cyclones (TCs), requiring a deeper understanding of their ecosystem impacts. This study investigated TC Biparjoy impact on parameters from June 6 to 19, 2023, using satellite and vertical profiles. Initially, Chlorophyll-a levels remained steady but surged above 4 mg/m3 after the high-intensity phase, indicating increased phytoplankton biomass. Sea surface temperature (SST) initially exceeded 32 °C, favoring cyclone intensification, but dropped below 26 °C post-high-intensity phase due to mixing and upwelling of cooler waters. The SST gradient exceeded 0.15 °C/km post-cyclone. Elevated sea surface height around 0.5 m offshore and over 1 m along the coast was recorded. Wind stress values exceeding 0.4 N/m3 were observed during the high-intensity phase. Vertical profile showed uplifted low-temperature, nutrient-rich waters, enhancing phytoplankton productivity, supported by increased nitrate (>10 mmol/m3) and phosphate (>1.2 mmol/m3) levels. Additionally, slight increase in DIC, and pH during the cyclone period suggested changes in biogeochemical processes.

Extreme air–sea turbulent fluxes during tropical cyclone Barijat observed by a newly designed drifting buoy

Using in situ observations collected by a drifting air-sea interface buoy (DrIB) in the northern South China Sea from August 30 to September 13, 2018, the extreme air-sea turbulent fluxes that occurred from September 8 to 13 during tropical cyclone (TC) Barijat were investigated. The most striking features were substantial increases in momentum and heat fluxes, with maximum increases of 10.8 m s-1 in the wind speed (WS), 0.73 N m-2 in the wind stress, 68.1 W m-2 in the sensible heat fluxes (SH) and 258.8 W m-2 in the latent heat fluxes (LH). The maximum WS, wind stress, SH and LH values amounted to 15.3 m s-1, 0.8 N m-2, 70.9 W m-2 and 329.9 W m-2, respectively. Using these new DrIB observations, the performance of two state-of-the-art, high-resolution reanalysis products, ERA5 and MERRA2, was assessed. The consistency of the observed values with ERA5 was slightly better than with MERRA2, reflected in higher correlations but both products underestimated the WS during TC conditions. In calm weather conditions, the turbulent heat fluxes were overestimated, because they simulated a too dry and cold atmospheric state, enhancing the air-sea differences in temperature and humidity. Considering that an accurate representation of the air-sea turbulent and momentum fluxes is essential for understanding and predicting ocean and atmospheric variability, our findings indicate that more high-quality temperature and relative humidity observations are required to evaluate and improve existing reanalysis products.

Evaluation of Nearshore QuikSCAT 4.1 and ERA-5 Wind Stress and Wind Stress Curl Fields over Eastern Boundary Currents

Fields of coastal wind stress and wind stress curl in the 10–100 km next to the land control the processes of upwelling and downwelling of nutrients and water properties that are vital to highly productive coastal marine ecosystems. Here we ask the question: Do the present surface wind stress products from a satellite-borne scatterometer (QuikSCAT) and an atmospheric reanalysis model (ERA-5) systematically overestimate the magnitude of wind speed and stress in the 10–50 km next to the coast? We compare QuikSCAT wind speed retrievals to the relatively unused wind speed retrievals from satellite altimeters, which are able to approach closer to the coast than scatterometers without land reflections, due to their smaller radar footprints. Altimeter data on tracks approaching and crossing the coast indicate that the increases in coastal QuikSCAT wind speed values and ERA-5 coastal wind stress values are unrealistic. For analyses of wind speed and stress requiring high accuracy, especially those involving wind stress curl, we suggest considering individual Level 2B scatterometer wind retrievals as suspect at distances of 10 km and less from the coast, along with use of the Poor Coastal Processing flag. We found that similar increases in wind stress values next to the coast in gridded ERA-5 fields are not due to errors in the model physics or wind speeds. They are created during the interpolation of wind stress from the original model grid to a regular rectangular grid. We recommend that researchers who are analyzing wind stress and wind stress curl should calculate wind stress themselves from the gridded ERA-5 vector wind speed fields, rather than using the interpolated model wind stress or curl fields.

Thorpe Turbulence Scaling in Nighttime Convective Surface Layers in the North Indian Ocean

AbstractWe use profiles from a Lagrangian float in the north Indian Ocean to explore the usefulness of Thorpe analysis methods to measure vertical scales and dissipation rates in the ocean surface boundary layer. An rms Thorpe length scale LT and an energy dissipation rate εT were computed by resorting the measured density profiles. These are compared to the mixed layer depth (MLD) computed with different density thresholds, the Monin–Obukhov (MO) length LMO computed from the ERA5 reanalysis values of wind stress, and buoyancy flux B0 and dissipation rates ε from historical microstructure data. The Thorpe length scale LT is found to accurately match MLD for small (<0.005 kg m−3) density thresholds, but not for larger thresholds, because these do not detect the warm diurnal layers. We use ξ = LT/|LMO| to classify the boundary layer turbulence during nighttime convection. In our data, 90% of points from the Bay of Bengal (Arabian Sea) satisfy ξ < 1 (1 < ξ <10), indicating that wind forcing is (both wind forcing and convection are) driving the turbulence. Over the measured range of ξ, εT decreases with decreasing ξ, i.e., more wind forcing, while ε increases, clearly showing that ε/εT decreases with increasing ξ. This is explained by a new scaling for ξ ≪ 1, εT = 1.15B0ξ0.5 compared to the historical scaling ε = 0.64B0 + 1.76ξ−1. For ξ ≪ 1 we expect ε = εT. Similar calculations may be possible using routine Argo float and ship data, allowing more detailed global measurements of εT, thereby providing large-scale tests of turbulence scaling in boundary layers.

Variability of Pacific herring (Clupea pallasii) spawn abundance under climate change off the West Coast of Canada over the past six decades

Abstract Pacific herring (Clupea pallasii) is an ecologically- and commercially-important species in the northeast Pacific Ocean. Detailed records of herring spawn off the coast of British Columbia (BC), Canada, collected annually since 1928, indicate that spawn distribution has gone through systematic changes over time. The cessation of spawning in some areas has led to concerns that commercial herring fisheries have resulted in serial depletion of local spawning. However, previous studies suggested that changes in spawn distribution may not necessarily be fisheries induced. Hitherto, there have been no satisfactory explanations for these changes. In this study, we developed boosted regression tree (BRT) models for four major spawning regions in BC (the Prince Rupert District, Haida Gwaii, the Central Coast, and the West Coast of Vancouver Island) to explore the nonlinear relationships between herring spawn abundance and environmental variables, including two indices of Pacific Ocean basin-scale variability (the PDO: Pacific Decadal Oscillation; and the NPGO: North Pacific Gyre Oscillation) and regional scale wind-stress off the BC coast. We find a “bowl-shaped” relationship between herring spawn abundance and two environmental variables (the PDO and NPGO) across all spawning regions and a “dome-shaped” relationship between spawn abundance and upwelling-favorable wind-stress, the latter indicating that there is an optimal range of wind stress values that favor herring spawn abundance. Results also suggest that, in Haida Gwaii, herring spawn abundance has been primarily governed by the NPGO and downwelling-favorable alongshore wind-stress; in contrast, the PDO and upwelling-favorable alongshore wind-stress are top contributors to spawn fluctuations in all other regions. These results help facilitate our understanding of how changes in physical environmental conditions affect herring spawn abundance as well as subsequent population dynamics.

Settlement and post-settlement survival rates of the white seabream (Diplodus sargus) in the western Mediterranean Sea.

Survival during the settlement window is a limiting variable for recruitment. The survival is believed to be strongly determined by biological interactions and sea conditions, however it has been poorly investigated. We examined the settlement patterns related to relevant biotic and abiotic factors (i.e. Density-dependence, wind stress, wave height and coastal current velocity) potentially determining post-settler survival rates of a coastal necto-benthic fish of wide distribution in the Mediterranean and eastern Atlantic, the white seabream (Diplodus sargus). An observational study of the demography of juveniles of this species was carried out at six coves in Menorca Island (Balearic Islands, western Mediterranean). Three of the coves were located in the northern and wind exposed coast, at the Northeast (NE) side; while the other three were found along the southern and sheltered coast, at the Southwest (SW) side of the island. The settlement period extended from early May to late June and maximum juvenile densities at the sampling sites varied between 5 and 11 ind. m-1 with maximum values observed in late May simultaneously occurring in the two coasts. Our analysis of juvenile survival, based on the interpretation of the observed patters using an individual based model (IBM), revealed two stages in the size-mortality relationships. An initial density-dependent stage was observed for juveniles up to 20 mm TL, followed by a density independent stage when other factors dominated the survival at sizes > 20 mm TL. No significant environmental effects were observed for the small size class (<20mm TL). Different significant environmental effects affecting NE and SW coves were observed for the medium (20-30mm TL) and large (>30mm TL) size class. In the NE, the wind stress consistently affected the density of fish of 20–30 mm and >30 mm TL with a dome-shape effect with higher densities at intermediate values of wind stress and negative effect at the extremes. The best models applied in the SW coves showed a significant non-linear negative effect on fish density that was also consistent for both groups 20–30 mm and >30 mm TL. Higher densities were observed at low values of wave height in the two groups. Because of these variations, the number of juveniles present at the end of the period was unrelated to their initial density and average survival varied among locations. In consequence, recruitment was (1) primarily limited by denso-dependient procedures at settlement stage, and (2) by sea conditions at post-settlement, where extreme wave conditions depleted juveniles. Accordingly, regional hydrodynamic conditions during the settlement season produced significant impacts on the juvenile densities depending on their size and with contrasted effects in respectto cove orientation. The similar strength in larval supply between coves, in addition to the similar mean phenology for settlers in the north and south of the Island, suggests that all fish may come from the same parental reproductive pool. These factors should be taken into account when assessing relationships between settlers, recruits and adults of white seabream.

Hydrodynamic modeling of floods in Saint Petersburg considering the operating dam

The Baltic Sea level is computed from the BALT-P three-dimensional hydrodynamic model at the grid with the horizontal resolution of 2 nautical miles, and the computation for the top of the Gulf of Finland is provided at the grid with the spatial resolution of 90 m. The sea motion is induced by the air pressure gradients and wind stress values from the HIRLAM model forecast specified on the sea surface. The sea level is calculated for three floods in Saint Petersburg with account that each of eight spillways of the dam closed and opened not instantly but during the prescribed period of time. It is revealed that the BALT-P model simulates the sea level well during three floods in Saint Petersburg considering the operating dam. The model can be used for the sea level forecasting in Saint Petersburg based on the HIRLAM atmospheric model data with the lead time of 60 hours.

Modelling Three-Dimensional Winds on a NW Pacific Region of Mexico Using Boundary-Fitted Grids

In this study, the momentum equations describing an atmospheric flow over a NW Pacific region of Mexico are solved numerically. In order to capture the complex flow-topography interactions with detail, a combination of a numerical wind model in full 3D curvilinear coordinates, along with a high resolution boundary-fitted grid is used. Boundary conditions were obtained from ten years (2002-2012) of measured offshore wind data. Prevailing winds from April to September during that period of observations were selected for the simulations. For the cases analyzed, it was found that at the points of the study region (PSS, PSM, PM), wind speed increased about 10% to 20% of its offshore values, while inland they decreased about 86% to 96%. This spatial behavior agreed very well with the observed local winds. A coastal jet (CJ), 35 km long with speeds about 1.5 - 2 m/s, emanating from PSS was found for NNW winds. Modeled winds were also used to compute wind stresses, wind stress curl, and CUI fields. Wind stress values agreed very well to those reported in the literature. High values of wind stress curl, and CUI were found at the lee of the points (PSS, PSM, PM). Indirect estimations of sea surface currents were about 15 - 20 cm/s offshore and 5 - 10 cm/s at the coast.

Using an ensemble data set of turbulent air‐sea fluxes to evaluate the IPSL climate model in tropical regions

AbstractOcean‐atmosphere interactions represent a key component of the hydrological cycle in tropical regions and their variability has profound influences on low‐latitude climate. In order to evaluate how climate models represent these fluxes while taking into account the observational uncertainties, we assemble a comprehensive database of 14 climatological surface flux products, including in situ‐based, satellite, hybrid, and reanalysis data sets. We find that the large observational uncertainties are reflected in the climatological magnitudes, as well as in the spatial patterns and seasonal variations and that, for the most part, they do not carry specific signatures of product type. This data ensemble allows us to draw several conclusions on the current representation of the intertropical turbulent air‐sea fluxes in the atmospheric component of the Intitut Pierre Simon Laplace‐Coupled Model 5A, when forced by observed sea surface temperatures. Despite significantly underestimated near‐surface wind speeds over the entire tropical oceans domain, the atmospheric model produces generally well represented zonal and meridional wind stress values, and only weak biases in the spatial patterns and seasonality. The simulated latent heat flux develops a bias pattern matching that of the wind speed, but with no systematic underestimation. Compared to the same reference, the sensible heat flux is overestimated over the entire region of interest, in response to a significant overestimation of the sea‐air temperature contrast. The observational ensemble and analyses presented in this paper offer a good framework for large‐scale model surface flux evaluation.

Variations of surface boundary layer parameters associated with Cyclone Gonu over the Arabian Sea using QuikSCAT data

This study attempted to quantify the variations of the surface marine atmospheric boundary layer (MABL) parameters associated with the tropical Cyclone Gonu formed over the Arabian Sea during 30 May–7 June 2007 (just after the monsoon onset). These characteristics were evaluated in terms of surface wind, drag coefficient, wind stress, horizontal divergence, and frictional velocity using 0.5° × 0.5° resolution Quick Scatterometer (QuikSCAT) wind products. The variation of these different surface boundary layer parameters was studied for three defined cyclone life stages: prior to the formation, during, and after the cyclone passage. Drastic variations of the MABL parameters during the passage of the cyclone were observed. The wind strength increased from 12 to 22 m s−1 in association with different stages of Gonu. Frictional velocity increased from a value of 0.1–0.6 m s−1 during the formative stage of the system to a high value of 0.3–1.4 m s−1 during the mature stage. Drag coefficient varied from 1.5 × 10−3 to 2.5 × 10−3 during the occurrence of Gonu. Wind stress values varied from 0.4 to 1.1 N m−2. Wind stress curl values varied from 10 × 10−7 to 45 × 10−7 N m−3. Generally, convergent winds prevailed with the numerical value of divergence varying from 0 to –4 × 10−5 s−1. Maximum variations of the wind parameters were found in the wall cloud region of the cyclone. The parameters returned to normally observed values in 1–3 days after the cyclone passage.

Finding a proxy for wind stress over the coastal ocean

Wind stress forcing is a critical driver of oceanographic processes. In the absence of over-ocean wind measurements, re-analysis products and over-land measurements are often used. The present paper compares a unique wind-data series from an ocean mooring with two re-analysis products and data from six over-land sites in Sydney, Australia, to determine whether these data can infer over-ocean wind conditions. Four oceanographic moorings are located here; however, over-ocean meteorological observations are no longer available. Correlations between over-ocean and over-land sites wind stress were &gt;0.8, whereas for re-analysis products, correlations ranged from 0.28 to 0.72. Re-analysis products were unable to resolve variability at the over-ocean site with periods shorter than 2 days, indicating that they are not appropriate wind proxies for the coastal ocean. Somewhat counter-intuitively, our results showed that proximity of over-land sites to the over-ocean site does not necessarily imply the highest correlation or the lowest error and careful site selection is required. An upwelling study showed that wind observations from selected over-land sites can accurately represent isotherm uplift in the coastal ocean. Threshold wind stress values that uplift isotherms at these over-land sites are provided, along with a recommendation for the use of the over-land site that best represents over-ocean wind conditions in this region.

Drag of the Water Surface at Very Short Fetches: Observations and Modeling

AbstractThe specific properties of the turbulent wind stress and the related wind wave field are investigated in a dedicated laboratory experiment for a wide range of wind speeds and fetches, and the results are analyzed using the wind-over-waves coupling model. Compared to long-fetch ocean wave fields, wind wave fields observed at very short fetches are characterized by higher significant dominant wave steepness but a much smaller macroscale wave breaking rate. The surface drag dependence on fetch and wind then closely follows the dominant wave steepness dependence. It is found that the dimensionless roughness length z*0 varies not only with wind forcing (or inverse wave age) but also with fetch. At a fixed fetch, when gravity waves develop, z*0 decreases with wind forcing according to a −1/2 power law. Taking into account the peculiarities of laboratory wave fields, the WOWC model predicts the measured wind stress values rather well. The relative contributions to surface drag of the equilibrium-range wave-induced stress and the airflow separation stress due to wave breaking remain small, even at high wind speeds. At moderate to strong winds, the form drag resulting from dominant waves represents the major wind stress component.

Comparison of surface wind stress characteristics over the Tropical Atlantic (10°N–40°S) in fields derived from the UWM/COADS, NCEP/NCAR and QuikSCAT datasets

A comparison of monthly wind stress derived from winds of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis and UWM/COADS (The University of Wisconsin-Milwaukee/Comprehensive Ocean-Atmosphere Data Set) dataset (1950–1993), and of NCEP/NCAR reanalysis and satellite-based QuikSCAT dataset (2000–2006), is made over the South Atlantic (10°N–40°S). On a mean seasonal scale, the comparison shows that these three wind stress datasets have qualitatively similar patterns. Quantitatively, in general, from about the equator to 20°S in the mid-Atlantic the wind stress values are stronger in NCEP/NCAR data than those in UWM/COADS data. On the other hand, in the Intertropical Convergence Zone (ITCZ) area the wind stress values in NCEP/NCAR data are slightly weaker than those in UWM/COADS data. In the South Atlantic, between 20° S–40°S, the QuikSCAT dataset presents complex circulation structures which are not present in NCEP/NCAR and UWM/COADS data. The wind stress is used in a numerical ocean model to simulate ocean currents, which are compared to a drifting-buoy observed climatology. The modeled South Equatorial Current agrees better with observations between March–May and June–August. Between December–February, the South Equatorial Current from UWM/COADS and QuikSCAT experiments is stronger and more developed than that from NCEP/NCAR experiment. The Brazil Current, in turn, is better represented in the QuikSCAT experiment. Comparison of the annual migration of ITCZ at 20° and 30°W in UWM/COADS and NCEP/NCAR data sources show that the southernmost position of ITCZ at 30°W in February, March and April coincides with the rainy season in NE Brazil, while the northernmost position of ITCZ at 20°W in August coincides with the maximum rainfall of Northwest Africa.

Tidal and Meteorological Forcing of Suspended Sediment Flux in a Muddy Mesotidal Estuary

Although the supply and fate of suspended sediment is of fundamental importance to the functioning and morphological evolution of muddy estuaries, reliable sediment budgets have been established in only a few cases. Especially for smaller estuaries, inadequate bathymetric surveys and a lack of intertidal sedimentation data often preclude estimation of the sediment budget from morphological change, while instrument-derived residual fluxes typically lie well within the errors associated with measurement of much larger gross tidal transports. Given suitably long-term records of continuously monitored suspended sediment concentration (SSC), however, analysis of the major scales of variation in sediment transport and their relation to hydrodynamic and meteorological forcing permits qualitative testing of hypotheses suggested by directly measured residual fluxes. This paper analyzes data from a 1-year acoustic Doppler profiler deployment in the Blyth estuary, a muddy mesotidal barrier-enclosed system on the UK east coast. Flux calculations indicate a small sediment import equivalent to just 1.5% of the gross flood tide transport. Little confidence can be assigned to either the magnitude or direction of such a small residual when considered in isolation. However, the inference that the sediment regime is finely balanced is qualitatively supported by the close similarity between flood-tide and ebb-tide SSC values. Singular spectrum analysis of the SSC time series shows the expectedly large contributions to the variance in SSC at intratidal and subtidal (semimonthly and monthly) scales but also picks out intermittent variability that is initially attributed to a combination of non-tidal surge and wind stress forcing. Closer examination of the data through cross-correlograms and event-scale analysis indicates that local meteorological forcing is the major factor. Acting through the resuspension of intertidal mudflat sediments at times of strong westerlies, meteorological forcing is directly implicated in episodic sediment export from the estuary. Thresholding of tide-averaged fluxes using a range of critical wind stress values further indicates that ‘tide-dominated’ (i.e., low wind stress) and ‘wave-dominated’ (high wind stress) conditions are associated with sediment import and export. Sediment balance is potentially sensitive to the frequency of high wind stress events, since the associated sediment exports are several times larger than the average import under calm conditions. Intermittent meteorological forcing may thus exert an important control on the sedimentary balance of otherwise tidally dominated muddy estuarine systems, and the role of wind climate should not be overlooked in studies of estuary response to environmental change.

Sea Surface Winds over the Mediterranean Basin from Satellite Data (2000–04): Meso- and Local-Scale Features on Annual and Seasonal Time Scales

Abstract This paper investigates the mean spatial features of the winds in the Mediterranean and Black Seas using the wind fields observed by the SeaWinds scatterometer. Five years (2000–04) of data have been analyzed on annual and seasonal basis, with particular attention paid to the meso- and local scales. The fields show the existence of structured regional wind systems—in particular, the mistral in the western Mediterranean and the etesians in the Levantine Basin, which are characterized, respectively, by high wind variability and moderate steadiness and by low wind variability and high steadiness. Estimated seasonal mean wind stress τ fields show that the values falling in the top range 0.15 &amp;lt; τ &amp;lt; 0.20 N m−2 affect a large portion of the Mediterranean Basin in winter, in the belt extending from the Gulf of Lion up to the Levantine Basin and the northern Black Sea. In the other seasons, only few regions experience such high values of τ. The analysis of the wind vorticity shows and quantifies the main cyclonic and anticyclonic circulations, and the study of the joint features of wind stress and vorticity has identified the strongest and most persisting local-scale wind circulations produced by the interaction between the wind flow and the orography. They occur at the lee side of Sardinia–Corse and Crete–Rhodos Islands and persist in all seasons, with some seasonal variation in strength and extent. The areas affected by the orographic disturbances are characterized by high values of wind stress and by a structure of vorticity showing alternating areas of cyclonic and anticyclonic circulations, whose strength is comparable to those of the regional-scale cyclones.

A Modeling Study of Eulerian and Lagrangian Aspects of Shelf Circulation off Duck, North Carolina

The effects of wind-forced upwelling and downwelling on the continental shelf off Duck, North Carolina, are studied through experiments with a two-dimensional numerical primitive equation model. Moored and shipboard measurements obtained during August‐November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study (ISS) are used for model‐data comparisons. The model is initialized with realistic stratification and forced with observed wind and heat flux data. Both strongly stratified and weakly stratified conditions, found during August and October, respectively, are studied. August is characterized by fluctuating alongshelf wind direction, and October is dominated by downwelling-favorable winds. The across-shelf momentum balance is primarily geostrophic on the continental shelf. The alongshelf momentum balance is mainly between the Coriolis force and vertical diffusion with additional contributions from the local acceleration and nonlinear advection terms. The model solutions are utilized to acquire detailed information on the time- and space-dependent variability of the across-shelf circulation and transport and to investigate the dependence of this circulation on the seasonal change in stratification. When the stratification breaks down, as in October, the across-shelf transport is reduced significantly in comparison with the theoretical Ekman transport for large wind stress values. The paths of individual model water parcels are traced using two methods: calculation of Lagrangian trajectories and time evolution of three Lagrangian label fields. The August period produces complex Lagrangian dynamics because of the switching between upwelling and downwelling winds. The October period illustrates a mean downwelling response that advects parcels across and along the shelf and vertically.

Inter-annual variability in larval supply to populations of three invertebrate taxa in the northern California Current

We investigated sources of inter-annual variability in larval supply to crab and sea urchin populations at Bodega Head and Point Reyes in northern California. During the spring and summer upwelling seasons of the years 1992 through 1997 we monitored the weekly settlement rates of nine species of crabs and two species of sea urchins. As observed in previous studies, daily values of alongshore windstress, temperature and salinity provided evidence for the poleward flow of relatively warm, low salinity water from south of Point Reyes, an apparent retention zone, during upwelling relaxation events. In years dominated by these events (1992, 1993, 1995 and 1996) we observed that alongshore windstress, temperature and salinity were coherent and temperature was significantly correlated with cancrid crab settlement. During these years the magnitude of cancrid crab settlement and the fraction of cancrid crabs relative to other crab species settling were high. Over four years of concurrent sampling there was consistently greater cancrid crab settlement at the Point Reyes site, within the retention zone, than at Bodega Head. Settlement of non-cancrid crabs (porcellanids, grapsids, pagurids and majids) was not as closely linked to intra-annual patterns of upwelling and relaxation, possibly due to the shorter seasonal availability of larvae allowing for the influence of fewer relaxation events. Settlement of this group among years was positively correlated with environmental indicators of strong seasonal upwelling; high salinity, Bakun upwelling index and low temperature. Sea urchin settlement events were observed in June and July of 1992, 1994 and 1997 during warming periods when salinity and temperature were increasing and alongshore windstress was low. Across the six years of the study, we found that cancrid crab larvae had a more even seasonal availability than larvae of non-cancrid species, which settled in greatest numbers during the early portion of the upwelling season. Sea urchins settled in greatest numbers during the later part of the upwelling season. Together these patterns demonstrate the taxon-specific way that inter-annual variability in larval supply is forced by the coincidence of larval availability with favorable physical transport mechanisms.

Interpreting wind-driven Southern Ocean variability in a stochastic framework

A stochastic model is derived from wind stress and bottom pressure gauge data to examine the response of the Antarctic Circumpolar Current (ACC) transport to wind stress forcing. A general method is used to estimate the drift and diffusion coefe cients of a continuous stationary Markovian system. As a e rst approximation, the response of the ACC to wind stress forcing can be described by a multivariate Ornstein-Uhlenbeck process: Gaussian red noise wind stress drives the evolution of the ACC transport, which is damped by a linear drag term. The model indicates that about 30( 610)% of ACC variability is directly driven by the wind stress. This stochastic model can serve as a null hypothesis for studies of wind driven ACC variability. A more accurate stochastic description of the wind stress over the Southern Ocean requires a multiplicative noise component. The variability of the wind stress increases approximately linearly with increasing wind stress values. A multiplicative stochastic process generates a power-law distribution rather than a Gaussian distribution. A simple stochastic model shows that non-Gaussian forcing could have a signie cant impact on the velocity (or transport) probability density functions (PDFs) of the wind-driven circulation. The net oceanic damping determines whether the distribution of the oceanic e ow is Gaussian (small damping) or resembles the distribution of the atmospheric forcing (large damping).

Laboratory Studies Of Wind Stress Over Surface Waves

Simultaneous laboratory observations of wind speed, wind stress, and surfacewind-wave spectra are made under a variety of wind forcing patterns using cleanwater as well as water containing an artificial surfactant. Under typical experimentalconditions, more than half of the total stress is supported by the wave-induced stressrather than by the surface viscous stress. When the surfactant reduces the shortwind-wave spectra, the wind stress also decreases by as much as 20–30% at agiven wind speed. When the wind forcing is modulated in time, the wind stresstends to be higher under decreasing wind than under increasing wind at a givenwind speed, mainly because the response of short wind-wave spectra to varyingwind forcing is delayed in time. These examples clearly demonstrate that therelationship between the wind speed and the wind stress can be significantlymodified if the surface wave field is not in equilibrium with the wind forcing. Next, we examine whether the wind stress is estimated accurately if the wave-inducedstress by all surface wave components is explicitly evaluated by linear superpositionand is added to the surface viscous stress. It is assumed that the surface viscous stressis uniquely related to the wind speed, and that the wind input rate is determined by thelocal, reduced turbulent stress rather than the total stress. Our wind stress estimatesincluding the wave contributions agree well with observed wind stress values, evenif the surface wave field is away from its equilibrium with the wind in the presenceof surface films and/or under time-transient wind forcing. These observations stronglysuggest that the wind stress is accurately evaluated as a sum of the wave-induced stressand the surface viscous stress. At very high winds, our stress estimates tend to be lowerthan the observations. We suspect that this is because of the enhancement of wind stressover very steep (or breaking) short wind-waves.

Assimilation of satellite altimeter data in a primitive-equation model of the Azores–Madeira region

Abstract The aim of this study is to implement satellite altimetric assimilation into a high-resolution primitive-equation ocean model and check the validity and sensitivity of the results. Beyond this paper, the remote objective is to get a dynamical tool capable of simulating the surface ocean processes linked to the air–sea interactions as well as to perform mesoscale ocean forecasting. For computational cost and practical reasons, this study takes place in a 1000 by 1000 sq km open domain of the Canary basin. The assimilation experiments are carried out with the combined TOPEX/POSEIDON and ERS-1 data sets between June 1993 and December 1993. The space–time domain overlaps with in situ data collected during the SEMAPHORE experiment and thus enables an objective validation of the results. A special boundary treatment is applied to the model by creating a surrounding recirculating area separated from the interior by a buffer zone. The altimetric assimilation is done by implementing a reduced-order optimal interpolation algorithm with a special vertical projection of the surface model/data misfits. We perform a first experiment with a vertical projection onto an isopycnal EOF representing the Azores Current vertical variability. An objective validation of the model's velocities with Lagrangian float data shows good results (the correlation is 0.715 at 150 dbar). The question of the sensitivity to the vertical projection is addressed by performing similar experiments using a method for lifting/lowering of the water column, and using an EOF in Z -coordinates. Some comparisons with in situ temperature data do not show any significant difference between the three projections, after five months of assimilation. However, in order to preserve the large-scale water characteristics, we felt that the isopycnal projection was a more physically consistent choice. Then, the complementary character of the two satellites is assessed with two additional experiments which use each altimeter data sets separately. There is an evidence of the benefit of combining the two data sets. Otherwise, an experiment assimilating long-wavelength bias-corrected CLS altimetric maps every 10 days exhibits the best correlation scores and emphasizes the importance of reducing the orbit error and biases in the altimetric data sets. The surface layers of the model are forced using realistic daily wind stress values computed from ECMWF analyses. Although we resolve small space and time scales, in our limited domain the wind stress does not significantly influence the quality of the results obtained with the altimetric assimilation. Finally, the relative effects of the data selection procedure and of the integration times (cycle lengths) is explored by performing data window experiments. A value of 10 days seems to be the most satisfactory cycle length.