Showing papers on "Wind stress published in 2007"

Delayed upwelling alters nearshore coastal ocean ecosystems in the northern California current

Abstract: Wind-driven coastal ocean upwelling supplies nutrients to the euphotic zone near the coast. Nutrients fuel the growth of phytoplankton, the base of a very productive coastal marine ecosystem [Pauly D, Christensen V (1995) Nature 374:255–257]. Because nutrient supply and phytoplankton biomass in shelf waters are highly sensitive to variation in upwelling-driven circulation, shifts in the timing and strength of upwelling may alter basic nutrient and carbon fluxes through marine food webs. We show how a 1-month delay in the 2005 spring transition to upwelling-favorable wind stress in the northern California Current Large Marine Ecosystem resulted in numerous anomalies: warm water, low nutrient levels, low primary productivity, and an unprecedented low recruitment of rocky intertidal organisms. The delay was associated with 20- to 40-day wind oscillations accompanying a southward shift of the jet stream. Early in the upwelling season (May–July) off Oregon, the cumulative upwelling-favorable wind stress was the lowest in 20 years, nearshore surface waters averaged 2°C warmer than normal, surf-zone chlorophyll-a and nutrients were 50% and 30% less than normal, respectively, and densities of recruits of mussels and barnacles were reduced by 83% and 66%, respectively. Delayed early-season upwelling and stronger late-season upwelling are consistent with predictions of the influence of global warming on coastal upwelling regions.

Tropical Cyclone Destructive Potential by Integrated Kinetic Energy

Abstract: Tropical cyclone damage potential, as currently defined by the Saffir-Simpson scale and the maximum sustained surface wind speed in the storm, fails to consider the area impact of winds likely to force surge and waves or cause particular levels of damage. Integrated kinetic energy represents a framework that captures the physical process of ocean surface stress forcing waves and surge while also taking into account structural wind loading and the spatial coverage of the wind. Integrated kinetic energy was computed from gridded, objectively analyzed surface wind fields of 23 hurricanes representing large and small storms. A wind destructive potential rating was constructed by weighting wind speed threshold contributions to the integrated kinetic energy, based on observed damage in Hurricanes Andrew, Hugo, and Opal. A combined storm surge and wave destructive potential rating was assigned according to the integrated kinetic energy contributed by winds greater than tropical storm force. The ratings are based...

Observed Flow Compensation Associated with the MOC at 26.5°N in the Atlantic

Abstract: The Atlantic meridional overturning circulation (MOC), which provides one-quarter of the global meridional heat transport, is composed of a number of separate flow components. How changes in the strength of each of those components may affect that of the others has been unclear because of a lack of adequate data. We continuously observed the MOC at 26.5°N for 1 year using end-point measurements of density, bottom pressure, and ocean currents; cable measurements across the Straits of Florida; and wind stress. The different transport components largely compensate for each other, thus confirming the validity of our monitoring approach. The MOC varied over the period of observation by ±5.7 × 106 cubic meters per second, with density-inferred and wind-driven transports contributing equally to it. We find evidence for depth-independent compensation for the wind-driven surface flow.

Turbulent Fluxes in the Hurricane Boundary Layer. Part I: Momentum Flux

Abstract: An important outcome from the ONR-sponsored Coupled Boundary Layer Air–Sea Transfer (CBLAST) Hurricane Program is the first-ever direct measurements of momentum flux from within hurricane boundary layers. In 2003, a specially instrumented NOAA P3 aircraft obtained measurements suitable for computing surface wind stress and ultimately estimating drag coefficients in regions with surface wind between 18 and 30 m s−1. Analyses of data are presented from 48 flux legs flown within 400 m of the surface in two storms. Results suggest a roll-off in the drag coefficient at higher wind speeds, in qualitative agreement with laboratory and modeling studies and inferences of drag coefficients using a log-profile method. However, the amount of roll-off and the wind speed at which the roll-off occurs remains uncertain, underscoring the need for additional measurements.

The Coupled Boundary Layers and Air–Sea Transfer Experiment in Low Winds

Abstract: The Office of Naval Research's Coupled Boundary Layers and Air–Sea Transfer (CBLAST) program is being conducted to investigate the processes that couple the marine boundary layers and govern the exchange of heat, mass, and momentum across the air–sea interface. CBLAST-LOW was designed to investigate these processes at the low-wind extreme where the processes are often driven or strongly modulated by buoyant forcing. The focus was on conditions ranging from negligible wind stress, where buoyant forcing dominates, up to wind speeds where wave breaking and Langmuir circulations play a significant role in the exchange processes. The field program provided observations from a suite of platforms deployed in the coastal ocean south of Martha's Vineyard. Highlights from the measurement campaigns include direct measurement of the momentum and heat fluxes on both sides of the air–sea interface using a specially constructed Air–Sea Interaction Tower (ASIT), and quantification of regional oceanic variability over sca...

February 2003 marine atmospheric conditions and the bora over the northern Adriatic

Abstract: [1] A winter oceanographic field experiment provided an opportunity to examine the atmospheric marine conditions over the northern Adriatic. Mean February winds are from a northeasterly direction over most of the Adriatic and a more northerly direction along the western coast. Wind speeds are fastest in jets over the NE coast during bora events and weakest in the mid-northwestern Adriatic. Diurnal air temperature cycles are smallest on the NE coast and largest in the midwestern Adriatic. The maximum sea-air difference is +10°C on the eastern coast and near zero on the midwestern Adriatic. Boras are northeasterly (from) wind events that sweep off Croatia and Slovenia, bringing slightly colder and drier air over the northern Adriatic. The main bora season is December to March. Winter 2002–2003 was normal for bora events. Synoptic-scale temporal variations are correlated over the northern Adriatic. Fastest Bora winds and highest wind stress over the northern Adriatic is concentrated in four topographically controlled jets. The strongest is the Senj Jet, while the Trieste Jet extends across the entire northern Adriatic. Between each two jets is a weak wind zone. The greatest mean net heat loss is in bora jets in the NE Adriatic, where it was −438 W m−2 and is weakest in the midwestern northern Adriatic, where it was near zero. Wind stress is concentrated over the NE half of Adriatic in four bora jets, while wind stress is weak in the NW Adriatic. There is significant variation in wind stress mean and standard deviation structure over the northern Adriatic with each bora event.

Summertime Coupling between Sea Surface Temperature and Wind Stress in the California Current System

Abstract: Satellite observations of wind stress and sea surface temperature (SST) are analyzed to investigate ocean–atmosphere interaction in the California Current System (CCS). As in regions of strong SST fronts elsewhere in the World Ocean, SST in the CCS region is positively correlated with surface wind stress when SST fronts are strong, which occurs during the summertime in the CCS region. This ocean influence on the atmosphere is apparently due to SST modification of stability and mixing in the atmospheric boundary layer and is most clearly manifest in the derivative wind stress fields: wind stress curl and divergence are linearly related to, respectively, the crosswind and downwind components of the local SST gradient. The dynamic range of the Ekman upwelling velocities associated with the summertime SST-induced perturbations of the wind stress curl is larger than that of the upwelling velocities associated with the mean summertime wind stress curl. This suggests significant feedback effects on the ocean, which likely modify the SST distribution that perturbed the wind stress curl field. The atmosphere and ocean off the west coast of North America must therefore be considered a fully coupled system. It is shown that the observed summertime ocean– atmosphere interaction is poorly represented in the NOAA North American Mesoscale Model (formerly called the Eta Model). This is due, at least in part, to the poor resolution and accuracy of the SST boundary condition used in the model. The sparse distribution of meteorological observations available over the CCS for data assimilation may also contribute to the poor model performance.

A Physics-Based Parameterization of Air–Sea Momentum Flux at High Wind Speeds and Its Impact on Hurricane Intensity Predictions

Abstract: A new bulk parameterization of the air–sea momentum flux at high wind speeds is proposed based on coupled wave–wind model simulations for 10 tropical cyclones that occurred in the Atlantic Ocean during 1998–2003. The new parameterization describes how the roughness length increases linearly with wind speed and the neutral drag coefficient tends to level off at high wind speeds. The proposed parameterization is then tested on real hurricanes using the operational Geophysical Fluid Dynamics Laboratory (GFDL) coupled hurricane–ocean prediction model. The impact of the new parameterization on the hurricane prediction is mainly found in increased maximum surface wind speeds, while it does not appreciably affect the hurricane central pressure prediction. This helps to improve the GFDL model–predicted wind–pressure relationship in strong hurricanes. Attempts are made to provide physical explanations as to why the reduced drag coefficient affects surface wind speeds but not the central pressure in hurric...

North Pacific decadal variability in the Community Climate System Model version 2

Abstract: North Pacific decadal oceanic and atmospheric variability is examined from a 650-yr control integration of the Community Climate System Model version 2. The dominant pattern of winter sea surface temperature (SST) variability is similar to the observed “Pacific decadal oscillation,” with maximum amplitude along the Kuroshio Extension. SST anomalies in this region exhibit significant spectral peaks at approximately 16 and 40 yr. Lateral geostrophic heat flux divergence, caused by a meridional shift of the Kuroshio Extension forced by basin-scale wind stress curl anomalies 3–5 yr earlier, is responsible for the decadal SST variability; local surface heat flux and Ekman heat flux divergence act as a damping and positive feedback, respectively. A simple linear Rossby wave model is invoked to explicitly demonstrate the link between the wind stress curl forcing and decadal variability in the Kuroshio Extension. The Rossby wave model not only successfully reproduces the two decadal spectral peaks, but a...

Highly resolved observations and simulations of the ocean response to a hurricane

Abstract: [1] An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.

The Scripps Coupled Ocean–Atmosphere Regional (SCOAR) Model, with Applications in the Eastern Pacific Sector

Abstract: A regional coupled ocean–atmosphere model is introduced. It is designed to admit the air–sea feedbacks arising in the presence of an oceanic mesoscale eddy field. It consists of the Regional Ocean Modeling System (ROMS) and the Regional Spectral Model (RSM). Large-scale forcing is provided by NCEP/DOE reanalysis fields, which have physics consistent with the RSM. Coupling allows the sea surface temperature (SST) to influence the stability of the atmospheric boundary layer and, hence, the surface wind stress and heat flux fields. The system is denominated the Scripps Coupled Ocean–Atmosphere Regional (SCOAR) Model. The model is tested in three scenarios in the eastern Pacific Ocean sector: tropical instability waves of the eastern tropical Pacific, mesoscale eddies and fronts of the California Current System, and gap winds of the Central American coast. Recent observational evidence suggests air–sea interactions involving the oceanic mesoscale in these three regions. Evolving SST fronts are shown ...

Midlatitude Wind Stress–Sea Surface Temperature Coupling in the Vicinity of Oceanic Fronts

Abstract: The influences of strong gradients in sea surface temperature on near-surface cross-front winds are explored in a series of idealized numerical modeling experiments. The atmospheric model is the Naval Research Laboratory Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model, which is fully coupled to the Regional Ocean Modeling System (ROMS) ocean model. A series of idealized, two-dimensional model calculations is carried out in which the wind blows from the warm-to-cold side or the cold-to-warm side of an initially prescribed ocean front. The evolution of the near-surface winds, boundary layer, and thermal structure is described, and the balances in the momentum equation are diagnosed. The changes in surface winds across the front are consistent with previous models and observations, showing a strong positive correlation with the sea surface temperature and boundary layer thickness. The coupling arises mainly as a result of changes in the flux Richardson number across the front, and the strength of the coupling coefficient grows quadratically with the strength of the cross-front geostrophic wind. The acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced Coriolis force in the vicinity of the front. Much of the loss/gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration/ deceleration of the flow parallel to the front via the Coriolis term. This mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient, and is active for flows off the equator with sufficiently strong winds. Although the main focus of this work is on the midlatitude, strong wind regime, calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross-front momentum budget, consistent with previous work.

Wind work in a model of the northwest Atlantic Ocean

Abstract: The work done by the wind over the northwest Atlantic Ocean is examined using a realistic high-resolution ocean model driven by synoptic wind forcing. Two model runs are conducted with the difference only in the way the wind stress is calculated. Our results show that the effect of including ocean surface currents in the wind stress formulation is to reduce the total wind work integrated over the model domain by about 17%. The reduction is caused by a sink term in the wind work calculation associated with the presence of ocean currents. In addition, the modelled eddy kinetic energy decreases by about 10%, in response to direct mechanical damping by the surface stress. A simple scaling argument shows that the latter can be expected to be more important than bottom friction in the energy budget.

Ocean–Atmosphere Interaction over Agulhas Extension Meanders

Abstract: Many years of high-resolution measurements by a number of space-based sensors and from Lagrangian drifters became available recently and are used to examine the persistent atmospheric imprints of the semi-permanent meanders of the Agulhas Extension Current (AEC), where strong surface current and temperature gradients are found. The sea surface temperature (SST) measured by the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and the chlorophyll concentration measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) support the identification of the meanders and related ocean circulation by the drifters. The collocation of high and low magnitudes of equivalent neutral wind (ENW) measured by Quick Scatterometer (QuikSCAT), which is uniquely related to surface stress by definition, illustrates not only the stability dependence of turbulent mixing but also the unique stress measuring capability of the scatterometer. The observed rotation of ENW in opposition to the rotation of the surface current clearly demonstrates that the scatterometer measures stress rather than winds. The clear differences between the distributions of wind and stress and the possible inadequacy of turbulent parameterization affirm the need of surface stress vector measurements, which were not available before the scatterometers. The opposite sign of the stress vorticity to current vorticity implies that the atmosphere spins down the current rotation through momentum transport. Coincident high SST and ENW over the southern extension of the meander enhance evaporation and latent heat flux, which cools the ocean. The atmosphere is found to provide negative feedback to ocean current and temperature gradients. Distribution of ENW convergence implies ascending motion on the downwind side of local SST maxima and descending air on the upwind side and acceleration of surface wind stress over warm water (deceleration over cool water); the convection may escalate the contrast of ENW over warm and cool water set up by the dependence of turbulent mixing on stability; this relation exerts a positive feedback to the ENW-SST relation. The temperature sounding measured by the Atmospheric Infrared Sounder(AIRS) is consistent with the spatial coherence between the cloud-top temperature provided by the International Satellite Cloud Climatology Project (ISCCP) and SST. Thus ocean mesoscale SST anomalies associated with the persistent meanders may have a long-term effect well above the midlatitude atmospheric boundary layer, an observation not addressed in the past.

Observed Oceanic Response over the Upper Continental Slope and Outer Shelf during Hurricane Ivan

Abstract: Hurricane Ivan passed directly over an array of 14 acoustic Doppler current profilers deployed along the outer continental shelf and upper slope in the northeastern Gulf of Mexico. Currents in excess of 200 cm s−1 were generated during this hurricane. Shelf currents followed Ekman dynamics with overlapping surface and bottom layers during Ivan’s approach and transitioned to a dominant surface boundary layer as the wind stress peaked. Slope currents at the onset of Ivan were wind driven near the surface, but deeper in the water column they were dominated during and after the passage of Ivan by subinertial waves with a period of 2–5 days that had several characteristics of topographic Rossby waves. Currents on the slope at 50 m and greater depths commonly exceeded 50 cm s−1. Surprisingly, the strongest currents were present to the left of the storm track on the shelf while more energetic currents were to the right of the hurricane path on the slope during the forced stage. Near-inertial motion last...

Effects of the 2003 European heatwave on the Central Mediterranean Sea: surface fluxes and the dynamical response

Abstract: . The effects of the 2003 European heatwave on the sea surface layer of the Central Mediterranean were studied using a regional 3-D ocean model. The model was used to simulate the period 2000 to 2004 and its performance was validated using remotely-sensed and in situ data. Analysis of the results focused on changes in the Sea Surface Temperature (SST) and on changes to the surface and sub-surface current field. This permitted us to identify and quantify the anomalies of atmospheric and sea surface parameters that accompanied the heatwave. The dominant annual cycle in each variable was first removed and a wavelet analysis then used to locate anomalies in the time-frequency domain. We found that the excess heating affecting the sea surface in the summer of 2003 was related to a significant increase in air temperature, a decrease in wind stress and reduction of all components of the upward heat flux. The monthly averages of the model SST were found to be in good agreement with remotely-sensed data during the period studied, although the ocean model tended to underestimate extreme events. The spatial distribution of SST anomalies as well as their time-frequency location was similar for both the remotely-sensed and model temperatures. We also found, on the basis of the period of the observed anomaly, that the event was not limited to the few summer months of 2003 but was part of a longer phenomenon. Both the model results and experimental data suggest the anomalous heating mainly affected the top 15 m of ocean and was associated with strong surface stratification and low mixing. The skill of the model to reproduce the sub-surface hydrographic features during the heatwave was checked by comparison with temperature and salinity measurements. This showed that the model was generally in good agreement with observations. The model and observations showed that the anomalous warming also modified the currents in the region, most noticeably the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). The AIS was reduced in intensity and showed less meandering, mainly due to the reduced density gradient and low winds, while the ATC was enhanced in strength, the two currents appearing to modulate each other in order to conserve the total transport of Modified Atlantic Water.

Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

Abstract: New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.

Sediment-transport modeling on Southern Californian shelves: A ROMS case study

Abstract: Suspended sediment-transport processes in Santa Monica and San Pedro Bay are analyzed using the sediment-transport capabilities of the Regional Oceanic Modeling System ( roms ). A one-month simulation for December 2001 has been carried out with a set of nested domains. The model inputs include tides, winds, surface waves, and idealized initial sediment conditions for sand and non-cohesive silt. Apart from the control run, the sensitivity of the results to surface waves, ripple roughness and bed armoring has been analyzed. From the control experiment, the horizontal transport of sand turns out to be limited to within a few km of the nearshore erosion zones. During high wave events, silt is transported over further distances and also partly offshelf in distinct plumes. The effectiveness of horizontal silt transport depends strongly on vertical mixing due to both surface wind stress and wave-enhanced bottom stress. High wave events coincident with strong winds (hence strong vertical mixing) are the most optimal conditions for sediment-transport. Excluding wave effects in the simulation shows that surface waves are the dominant factor in resuspending bed material on the Southern Californian shelves. The sensitivity experiments also show that the direct influence of additional ripple roughness on erosion and deposition is relatively weak. Switching off bed armoring locally results in increases of near-bottom concentrations by a factor of 20 for silt and a factor of 5 for sand as well as stronger spatial gradients in grain size.

Quantifying the Dependence of Westerly Wind Bursts on the Large-Scale Tropical Pacific SST

Abstract: The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Nino event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inheren...

Bora event variability and the role of air‐sea feedback

Abstract: 1) was conducted to improve forecast momentum and heat flux fields, and to evaluate surface flux field differences for two consecutive bora events during February 2003. (COAMPS 1 is a registered trademark of the Naval Research Laboratory.) The strength, mean positions and extensions of the bora jets, and the atmospheric conditions driving them varied considerably between the two events. Bora 1 had 62% stronger heat flux and 51% larger momentum flux than bora 2. The latter displayed much greater diurnal variability characterized by inertial oscillations and the early morning strengthening of a west Adriatic barrier jet, beneath which a stronger west Adriatic ocean current developed. Elsewhere, surface ocean current differences between the two events were directly related to differences in wind stress curl generated by the position and strength of the individual bora jets. The mean heat flux bias was reduced by 72%, and heat flux RMSE reduced by 30% on average at four instrumented over-water sites in the two-way coupled simulation relative to the uncoupled control. Largest reductions in wind stress were found in the bora jets, while the biggest reductions in heat flux were found along the north and west coasts of the Adriatic. In bora 2, SST gradients impacted the wind stress curl along the north and west coasts, and in bora 1 wind stress curl was sensitive to the Istrian front position and strength. The two-way coupled simulation produced diminished surface current speeds of 12% over the northern Adriatic during both bora compared with a one-way coupled simulation.

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 < τ < 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 ...

Influence of energetic wind and waves on gas transfer in a large wind–wave tunnel facility

Abstract: Air–water gas exchange experiments were carried out in a large wind wave tunnel in Marseille, France, to investigate gas transfer processes under energetic wind and wave fields, where macroscale breaking waves create bubble plumes (white caps) and turbulence on the water surface. We measured the gas transfer velocities of N2O, DMS, He, SF6, CH3Br, and total air. Their diffusivity and solubility span a large range, allowing us to investigate gas transfer mechanisms under a variety of physical conditions. We observed that the gas transfer velocities varied with friction velocity in a linear manner. Gas transfer in the presence of pure wind waves is generally consistent with the surface renewal model, as the gas transfer velocity has a strong dependence on diffusivity with an exponent of 0.53(±0.02). Contrary to expectations, the bubble plumes generated by breaking waves contributed relatively little in our pure wind wave experiments. Superposition of mechanically generated waves onto the wind waves in the high wind regime attenuated DMS gas transfer (as a function of friction velocity) across the air–water interface by ~20% compared with gas transfer under pure wind waves, implying suppression of gas transfer directly across the sheared water surface. Greater transfer of less soluble gases may result from bubble-mediated gas transfer.

Wind profiles, momentum fluxes and roughness lengths at Cabauw revisited

Abstract: We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (< 40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.

Reconstructing the Meridional Overturning Circulation from Boundary Densities and the Zonal Wind Stress

Abstract: Numerical models are used to test whether the meridional overturning circulation (MOC) can be reconstructed from boundary densities and the wind stress. In idealized model setups without topography the strength as well as the temporal and spatial variability of the MOC cell can largely be reproduced from boundary densities and the zonal wind stress. With added slopes along the meridional boundaries, most of the depth-averaged flow is missed and neither strength nor spatial structure of the MOC is well reproduced. However, the temporal evolution of both MOC and its estimate are similar. In an eddy-permitting model with realistic bottom topography the contribution of the depth-averaged meridional flow to the MOC is captured at some places while it is missed at others. Nevertheless, boundary densities and the zonal wind stress allow the leading modes of the temporal and spatial MOC variability to be reproduced. On seasonal time scales most of the MOC variability is due to the wind stress but changes in the boundary density affect the MOC as well. On interannual time scales the MOC variability largely reflects changes in the boundary density. Generally, the MOC reconstructions are accurate when bottom velocities are small, an assumption made in the reconstruction approach. The results are relevant for estimates of both the modern and the past MOC. In the real ocean, boundary densities can be obtained from measurements of temperature, conductivity, and pressure in the water column, whereas past seawater densities have left their imprint in sea sediments.

Numerical simulation of air-sea coupling during coastal upwelling

Abstract: Air–sea coupling during coastal upwelling was examined through idealized three-dimensional numerical simulations with a coupled atmosphere–ocean mesoscale model. Geometry, topography, and initial and boundary conditions were chosen to be representative of summertime coastal conditions off the Oregon coast. Over the 72-h simulations, sea surface temperatures were reduced several degrees near the coast by a wind-driven upwelling of cold water that developed within 10–20 km off the coast. In this region, the interaction of the atmospheric boundary layer with the cold upwelled water resulted in the formation of an internal boundary layer below 100-m altitude in the inversion-capped boundary layer and a reduction of the wind stress in the coupled model to half the offshore value. Surface heat fluxes were also modified by the coupling. The simulated modification of the atmospheric boundary layer by ocean upwelling was consistent with recent moored and aircraft observations of the lower atmosphere off the Oregon coast during the upwelling season. For these 72-h simulations, comparisons of coupled and uncoupled model results showed that the coupling caused measurable differences in the upwelling circulation within 20 km off the coast. The coastal Ekman transport divergence was distributed over a wider offshore extent and a thinner ocean surface boundary layer, with consistently smaller offshore and depth-integrated alongshore transport formed in the upwelling region, in the coupled case relative to the uncoupled case. The results indicate that accurate models of coastal upwelling processes can require representations of ocean–atmosphere interactions on short temporal and horizontal scales.

Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden‐Julian Oscillation versus submonthly forcing and processes

Abstract: [ 1] Intraseasonal variability of Indian Ocean sea surface temperature (SST) during boreal winter is investigated by analyzing available data and a suite of solutions to an ocean general circulation model for 1998 - 2004. This period covers the QuikSCAT and Tropical Rainfall Measuring Mission (TRMM) observations. Impacts of the 30 - 90 day and 10 - 30 day atmospheric intraseasonal oscillations (ISOs) are examined separately, with the former dominated by the Madden-Julian Oscillation (MJO) and the latter dominated by convectively coupled Rossby and Kelvin waves. The maximum variation of intraseasonal SST occurs at 10 degrees S - 2 degrees S in the wintertime Intertropical Convergence Zone (ITCZ), where the mixed layer is thin and intraseasonal wind speed reaches its maximum. The observed maximum warming ( cooling) averaged over ( 60 degrees E - 85 degrees E, 10 degrees S - 3 degrees S) is 1.13 degrees C ( - 0.97 degrees C) for the period of interest, with a standard deviation of 0.39 degrees C in winter. This SST change is forced predominantly by the MJO. While the MJO causes a basin-wide cooling ( warming) in the ITCZ region, submonthly ISOs cause a more complex SST structure that propagates southwestward in the western-central basin and southeastward in the eastern ocean. On both the MJO and submonthly timescales, winds are the deterministic factor for the SST variability. Short-wave radiation generally plays a secondary role, and effects of precipitation are negligible. The dominant role of winds results roughly equally from wind speed and stress forcing. Wind speed affects SST by altering turbulent heat fluxes and entrainment cooling. Wind stress affects SST via several local and remote oceanic processes.

Connection between the decadal variability in the Southern Ocean circulation and the Southern Annular Mode

Abstract: Previous studies demonstrated the remarkable upward trend of the Southern Annular Mode ( SAM) and Southern Ocean wind stress in association with anthropogenic forcing. An oceanic reanalysis data set is used to investigate the response of the circulation in the Southern Ocean to the decadal variability of SAM. Our results indicate the strengthening and the poleward shift of the northward Ekman velocity as well as the Ekman pumping rate, which led to a corresponding strengthening trend in the Deacon Cell. This strengthening, in turn, intensified the meridional density gradient and the tilting of the isopycnal surfaces. On the interannual time scale, the Antarctic Circumpolar Currents ( ACC) transport exhibits a positive correlation with SAM index as seen separately in observations. However, there is no significant trend in the total transport of ACC. Possible reasons are discussed.