Showing papers on "Wind stress published in 2002"

STOIC: a study of coupled model climatology and variability in tropical ocean regions

Abstract: We describe the behaviour of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose: nevertheless several common features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with interannual Nino3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress.

Modeling the Martian dust cycle, 1. Representations of dust transport processes

Abstract: [1] A dust transport scheme has been developed for a general circulation model of the Martian atmosphere. This enables radiatively active dust transport, with the atmospheric state responding to changes in the dust distribution via atmospheric heating, as well as dust transport being determined by atmospheric conditions. The scheme includes dust lifting, advection by model winds, atmospheric mixing, and gravitational sedimentation. Parameterizations of lifting initiated by (1) near-surface wind stress and (2) convective vortices known as dust devils are considered. Two parameterizations are defined for each mechanism and are first investigated offline using data previously output from the non-dust-transporting model. The threshold-insensitive parameterizations predict some lifting over most regions, varying smoothly in space and time. The threshold-sensitive parameterizations predict lifting only during extreme atmospheric conditions (such as exceptionally strong winds), so lifting is rarer and more confined to specific regions and times. Wind stress lifting is predicted to peak during southern summer, largely between latitudes 15° and 35°S, with maxima also in regions of strong slope winds or thermal contrast flows. These areas are consistent with observed storm onset regions and dark streak surface features. Dust devil lifting is also predicted to peak during southern summer, with a moderate peak during northern summer. The greatest dust devil lifting occurs in early afternoon, particularly in the Noachis, Arcadia/Amazonis, Sirenum, and Thaumasia regions. Radiatively active dust transport experiments reveal strong positive feedbacks on lifting by near-surface wind stress and negative feedbacks on lifting by dust devils.

The Inner Shelf Response to Wind-Driven Upwelling and Downwelling*

Abstract: A two-dimensional numerical model is used to study the response to upwelling- and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as t because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratif...

Interannual variability in the tropical Indian Ocean

Abstract: [1] The interannual variability in the tropical Indian Ocean is examined using 41-year (1958–1998) seasonal anomalies of the upper-ocean heat content (HCA), sea surface temperature (SSTA), and surface wind stress. Precipitation anomalies from a shorter period (1979–1998) have also been analyzed. This analysis demonstrates that a coupled ocean–atmosphere interannual oscillation with a period ranging from 2 to 5 years is the major variability in the tropical Indian Ocean. At the peak phase, anomalous equatorial zonal winds over the central and the eastern ocean and anomalous trade winds to the south induce zonal SSTA and HCA gradients near the equator and an east–west shift of the convection. This interannual oscillation is the dominant signal from the boreal autumn to the next spring. The westward propagating HCA causes a phase delay between the peaks of the surface cooling near the eastern coast and the warming near the western coast near the equator. During its propagation, the southern HCA branch is strengthened by the anomalous wind curl of the equatorial and southeast trade wind anomalies over the southern ocean. As a result, the southern HCA is maintained near the western coast for a much longer period. This Indian Ocean oscillation is significantly correlated with the El Nino/Southern Oscillation (ENSO) variability in the Pacific Ocean.

Influence of wind stress and ambient flow on a high discharge river plume

Abstract: [1] The response of a high discharge river plume to an alongshore ambient flow and wind forcing is studied with a three-dimensional numerical model The study extends prior model studies of plumes by including (1) a very large volume discharge (14,000 m3 s−1, about twice the maximum used in other models), (2) ambient flow in a direction opposite to that of the propagation of coastally trapped waves, and (3) a sequence of wind direction reversals The magnitude of the ambient flow, wind stress, estuary width, and river outflow are based on typical values for the Columbia River on the Washington coast The model results challenge two longstanding notions about the Columbia plume; first, that plume orientation is in a relatively stable southwest position in summer For example, with average discharge conditions (7000 m3 s−1) a summertime downwelling event can erode the southwestward plume and advect it to the north of the river mouth over several days Second, the plume is not always unidirectional; branches can occur both upstream and downstream of the river mouth simultaneously The model also provides an explanation for the observation that the plume rarely tends southward during the winter season; in contrast to summer conditions the rotational tendency of the plume and the ambient flow are in the same direction, so that wind stress must be be significant (>14 dynes cm−2 for at least 2 days) to reverse the plume direction Distinct anticyclonic freshwater pools form in modeled plumes both north and south of the river mouth under steady forcing conditions when ambient flow is present The scale of modeled pools is consistent with features observed in the Columbia plume

Global estimates of the wind‐induced energy flux to inertial motions in the surface mixed layer

Abstract: [1] It is believed that large-scale oceanic thermohaline circulation is strongly influenced by small-scale ocean mixing processes. In order to satisfy the large-scale advective-diffusive balance of the meridional overturning circulation, about 2.1 TW (1 TW = 1012 W) of power is required for the ocean mixing processes. This power is thought to be supplied by internal tides as well as wind stress fluctuations. Although the global energy flux from internal tides is estimated to be about 0.9 TW based on astronomical measurements, direct estimate of the global energy flux from wind stress fluctuations has not been made so far. In this study, using a simple numerical model, we estimate the wind-induced global energy flux to inertial motions in the surface mixed layer. The calculated results show that the estimated global energy flux falls short of the required value.

Mesoscale patterns in the Cape São Vicente (Iberian Peninsula) upwelling region

Abstract: [1] The coastal upwelling region near Cape Sao Vicente, the southwestern tip of the Iberian Peninsula where the southern zonal coast meets the meridional western coast, was studied using over 1200 advanced very high resolution radiometer (AVHRR) satellite images of sea surface temperature and time series of sea level height, wind velocities, and nearshore sea surface temperature recorded at coastal sites within 200 km of the cape. Summer upwelling is more intense and persistent off the western coast than off the southern coast, where a recurrent warm coastal countercurrent flows westward, and at times turns northward along the western coast after reaching the cape. In this region the equatorward current jet of cold water upwelled off the western coast is no longer bounded by a coast. Three preferred directions for the spreading of this water are identified. The most persistent is eastward along the southern shelf break and slope, possibly merging with waters previously upwelled locally, which becomes separated from shore by the coastal progression of the warmer counterflow. The second preferred direction results in the southward development of a cold filament feature fed by cold waters upwelled farther north and represents the southernmost extent of the intense coastal upwelling jet, which overshoots the cape. The least frequent feature to develop is a cold filament that grows westward at the latitude of the cape, appearing to result from the meandering of the equatorward jet. The coastal countercurrent is seen to interact with the equatorward jet at times of relaxation, not only by separating the cold upwelled water from the coast but, when it is energetic enough, breaking westward offshore through the equatorward cold flow and separating the eastward and southward cold features from the upcoast cold waters. Empirical evidence shows the presence of an alongshore pressure gradient, stronger in summer, driving the coastal progression of the warm counterflow. Wind forcing plays an important role in the circulation by augmenting or diminishing the effect of the preexisting alongshore pressure gradients. The extent of the progression of the warm coastal countercurrent along the southern and western coast is dictated by the the strength of the upwelling favorable wind stress, which is able to balance and reverse the alongshore flow, at least in the upper layers.

Ekman Transport Dominates Local Air–Sea Fluxes in Driving Variability of Subantarctic Mode Water

Abstract: Subantarctic Mode Water (SAMW) is formed by deep convection in winter on the equatorward side of the Antarctic Circumpolar Current. Observations south of Australia show that the SAMW temperature (T) and salinity (S) vary significantly from year to year. The magnitude and density-compensating nature of the temperature and salinity changes cannot be explained by variations in air-sea exchange of heat and freshwater in the subantarctic zone where SAMW is formed. Rather, the T and S variability reflects variations in the equatorward Ekman transport of cool, low salinity water across the subantarctic front. Experiments with a coupled climate model suggest that the observations south of Australia are typical of the subantarctic zone. The model changes in SAMW properties are correlated significantly (at 99% level) with changes in wind stress and northward Ekman transport of cool low- salinity water. In contrast, air-sea heat flux anomalies are mostly a response to changes in SST, and anomalies in precipitation minus evaporation in the subantarctic zone are too small to account for the model SAMW salinity variations. Mode waters provide significant reservoirs of heat and freshwater that extend below the depth of the seasonal thermocline and, hence, can persist from year to year. The fact that wind stress variations can drive changes in mode water properties therefore has implications for climate variability.

New evidence for a relation between wind stress and wave age from measurements during ASGAMAGE

Abstract: Data from the 1996 ASGAMAGE experiment, performed in the southern North Sea at research platform Meetpost Noordwijk (MPN), are analysed for the parameters affecting the momentum flux. The stress turns out to be quadratically related to the 10-m wind speed and linearly to the wind speed at a wavelength related level. The Charnock parameter (dimensionless roughness length) shows a pronounced correlation with wave age. This implies, due to a coupling between wave age and the steepness of the waves, a connection between the stress and the steepness. We find that our North Sea results are consistent withopen ocean observations. For a given wind speed the mean stress at MPN turns out to be higher because the wave age there is in general lower. We define and give an expression for a drag coefficient at a wavelength related level that can be calculated straightforwardly from the wave age and then reduced to a standard level.

El Niño and La Niña sea surface temperature anomalies: Asymmetry characteristics associated with their wind stress anomalies

Abstract: [1] The asymmetric nature of El Nino and La Nina sea surface temperature (SST) anomalies is investigated by the use of National Centers for Environmental Prediction reanalysis data and various ocean and atmosphere models. It is demonstrated that the relatively weak SST anomalies during La Nina compared with those of El Nino are related to the westward shift of wind stress anomalies by 10°–15°. The asymmetric characteristics of atmospheric responses are confirmed by the general circulation model experiments with the two different SST anomalies, which have equal amplitude but are of opposite sign from each other. The experiments with an intermediate ocean model and a hybrid coupled model clearly show that the SST anomalies over the equatorial Pacific become weaker as the zonal wind stress shifts to the west. Not only the amplitude but also the oscillation timescale of the SST anomaly is shown to be sensitive to the location of wind stress anomalies. The duration of La Nina, which is rather shorter than that of El Nino, is also related to the longitudinal displacement of the wind stress anomaly.

Sources of Eddy Kinetic Energy in the Labrador Sea

Abstract: Experiments with a suite of North Atlantic general circulation models are used to examine the sources of eddy kinetic energy (EKE) in the Labrador Sea. A high-resolution model version (112°) quantitatively reproduces the observed signature. A particular feature of the EKE in the Labrador Sea is its pronounced seasonal cycle, with a maximum intensity in early winter, as already found in earlier studies based on altimeter data. In contrast to a previously advanced hypothesis, the seasonally varying eddy field is not related to a forcing by high-frequency wind variations but can be explained by a seasonally modulated instability of the West Greenland Current (WGC). The main source of EKE in the Labrador Sea is an energy transfer due to Reynolds interaction work (barotropic instability) in a confined region near Cape Desolation where the WGC adjusts to a change in the topographic slope: Geostrophic contours tend to converge upstream of Cape Desolation, such that the topographically guided WGC narrows as well and becomes barotropically unstable. The eddies spawned from the WGC instability area, dominating the EKE in the interior Labrador Sea, are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column, while the few cyclones originating as well from the instability area show a more depth-independent structure. Companion experiments with a ⅓° model exhibit the strength of the WGC, influenced by either changes in the wind stress or heat flux forcing, as a leading factor determining seasonal to interannual changes of EKE in the Labrador Sea

Relative role of dynamic and thermodynamic processes in the development of the Indian Ocean dipole: An OGCM diagnosis

Abstract: [1] The relative role of oceanic dynamics and surface heat fluxes in the initiation and development of the Indian Ocean dipole was investigated by analyzing results from an oceanic general circulation model. The model was forced by observed surface wind stress and heat flux fields for 1958–1997. The results show that it was capable of reproducing observed dipole events over the tropical Indian Ocean. The diagnosis of the mixed-layer heat budget indicates that the SST anomaly (SSTA) in the east pole is primarily induced by anomalous surface latent heat flux and vertical temperature advection, whereas in the west pole it is mainly caused by meridional and vertical temperature advection anomalies. In both regions shortwave radiation anomalies tend to damp the SSTA. The ocean Rossby waves are essential in linking the anomalous wind and SST off Sumatra and subsurface temperature variations in southwest Indian Ocean.

A modeling study of shelf circulation off northern California in the region of the Coastal Ocean Dynamics Experiment: Response to relaxation of upwelling winds

Abstract: [1] A two-part modeling study of the wind-forced flow on the continental shelf off northern California in the region (37°–40°N) of the Coastal Ocean Dynamics Experiment (CODE) is pursued. This paper involves a process-oriented study with idealized wind stress forcing. Gan and Allen [2002] involves forcing with observed winds and heat flux for April–May 1982 and comparison of model results with CODE observations. A characteristic, but previously unexplained, response observed during CODE following the weakening, or relaxation, of southward upwelling favorable winds is the time-dependent development of northward currents over the inner shelf next to the coast. The presence of northward winds is not necessary for this occurrence. The objective in this paper is to investigate the dynamics of the shelf flow response to upwelling wind relaxation events under idealized conditions. In the basic case experiment a spatially uniform upwelling favorable southward wind stress of 0.1 Pa is applied to the ocean initially at rest. The stress is held constant for 10 days and then decreased linearly to zero over 3 days. In response to the southward wind stress, southward alongshore currents develop on the shelf accompanied by upwelling of cold, dense water near the coast. Considerable spatial variability in the shelf flow, clearly related to the alongshore variations in coastline and bottom topography, is found. The alongshore currents tend to separate from the coast south of capes, and the coldest surface water is found at those locations. As the winds decrease, northward currents, similar to those observed, develop on the inner shelf next to the coast at many alongshore locations. Examination of the alongshore momentum balances shows that the northward currents are forced by a northward pressure gradient force associated with negative alongshore pressure gradients. These pressure gradients are set up by the interaction of the wind-forced flow with the alongshore variations in shelf topography. In general, negative alongshore pressure gradients, intensified off Pt. Reyes and Pt. Arena by the gradient wind balance, are found south of capes. The negative pressure gradients geostrophically balance onshore flow at depth, and upwelling is strengthened in these locations. North of capes, positive pressure gradients that are primarily in balance with nonlinear advective effects are found. After the winds cease the forced across-shelf circulation weakens, and the resulting unbalanced negative pressure gradients south of the capes accelerate the alongshore currents northward. Processes with similar dynamics are found embedded in the more complex coastal ocean response to observed time varying winds by Gan and Allen [2002].

Analyses and simulations of the upper ocean's response to Hurricane Felix at the Bermuda Testbed Mooring site: 13–23 August 1995

Abstract: causedalargeincreaseinkineticenergyatnear-inertialfrequencies,internalgravitywavesin the thermocline, and inertial pumping, mixed layer deepening, and significant vertical redistribution of heat, with cooling of the upper 30 m and warming at depths of 30–70 m. The temperature evolution was simulated using four one-dimensional mixed layer models: Price-Weller-Pinkel (PWP), K Profile Parameterization (KPP), Mellor-Yamada 2.5 (MY), and a modified version of MY2.5 (MY2). The primary differences in the model results were in their simulations of temperature evolution. In particular, when forced using a drag coefficient thathadalinear dependenceonwindspeed,the KPPmodelpredicted seasurface cooling, mixed layer currents, and the maximum depth of cooling closer to the observations than any of the other models. This was shown to be partly because of a special parameterization for gradient Richardson number (RgKPP) shear instability mixing in responsetoresolvedshearintheinterior.TheMY2modelpredictedmoreseasurfacecooling and greater depth penetration of kinetic energy than the MY model. In the MY2 model the dissipation rate of turbulent kinetic energy is parameterized as a function of a locally defined Richardson number (RgMY2) allowing for a reduction in dissipation rate for stable Richardsonnumbers(RgMY2)wheninternalgravitywavesarelikelytobepresent.Sensitivity simulations with the PWP model, which has specifically defined mixing procedures, show that most of the heat lost from the upper layer was due to entrainment (parameterized as a function of bulk Richardson number RbPWP), with the remainder due to local Richardson number (RgPWP) instabilities. With the exception of the MY model the models predicted reasonable estimates of the north and east current components during and after the hurricane passage at 25 and 45 m. Although the results emphasize differences between the modeled responsestoagivenwindstress,currentcontroversyovertheformulationofwindstressfrom wind speed measurements (including possible sea state and wave age and sheltering effects) cautions against using our results for assessing model skill. In particular, sensitivity studies show that MY2 simulations of the temperature evolution are excellent when the wind stress is increased, albeit with currents that are larger than observed. Sensitivity experiments also indicate that preexisting inertial motion modulated the amplitude of poststormcurrents,butthattherewasprobablynotasignificantresonantresponsebecauseof clockwise wind rotation for our study site. INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 4572 Oceanography: Physical: Upper ocean processes; 4255 Oceanography: General: Numericalmodeling;4544Oceanography:Physical:Internalandinertialwaves;KEYWORDS:hurricane,tropical cyclone, mixed layer modeling, upper ocean processes, inertial currents, ocean storms

A northern boundary current along Australia's southern shelves: The Flinders Current

Abstract: [1] South of Australia, the monthly mean wind stress curl is positive during both summer and winter and leads to Ekman pumping and downwelling throughout the region. Sverdrup dynamics indicates that this downwelling should lead to a northward transport of around 5–10 Sv (1 Sv = 10 m3 s−1). Classical arguments for western boundary currents are adapted to show that this transport should be deflected into an upwelling favorable boundary current that flows from east to west along Australia's southern shelves: the Flinders Current. Support for this proposition is obtained from results of the Ocean Circulation and Climate Advanced Modelling project (OCCAM), Sverdrup transports, and limited observations for the region. In addition, the OCCAM results show that the northward transport leads to upwelling at depths below 400 m and that the Flinders Current (1) intensifies from Victoria to Western Australia (transport ∼8 Sv with speeds up to 15 cm s−1), (2) can extend from the surface to depths of 800 m, and (3) is found during both summer and winter. During winter the winds are downwelling favorable and lead to a coastal current that flows from west to east and opposes the Flinders Current. Further support for the origin and nature of the Flinders Current is obtained from simple numerical experiments made using a rectangular domain and idealized representations of the summer and winter wind stress.

Forcing Mechanisms of Sea Level Interannual Variability in the Bay of Bengal

Abstract: A nonlinear, 4‰-layer reduced-gravity ocean model with active thermodynamics and mixed layer physics is used to investigate the causes of sea level interannual variability in the Bay of Bengal, which may contribute to flooding and cholera outbreaks in Bangladesh. Forcing by NCEP‐NCAR reanalysis fields from 1958 to 1998 yields realistic solutions in the Indian Ocean basin north of 298S. Controlled experiments elucidate the roles of the following forcing mechanisms: interannual variability of the Bay of Bengal wind, equatorial wind, river discharges into the bay, and surface buoyancy flux including precipitation minus evaporation (heat fluxes 1 P 2 E). Sea level changes in the bay result largely from wind variability, with a typical amplitude of 10 cm and occasionally 10‐25 cm at an interannual timescale. Near the eastern and northern boundaries, sea level anomalies (SLAs) are predominantly caused by equatorial wind variability, which generates coastal Kelvin waves that propagate into the bay along the eastern boundary. Near the western boundary the bay wind has a comparable influence as the equatorial wind, especially during the southwest monsoon season, owing to the counterclockwise propagation of coastal Kelvin waves forced by the large-scale alongshore wind stress in the bay. In the bay interior, SLAs are dominated by the equatorial wind forcing in the central bay, result almost equally from the equatorial and the bay wind in the southwestern bay, and are dominated by the bay wind forcing in the southwestern bay during the southwest monsoon. The westward intensification of the bay wind influence is associated with the westward propagation of Rossby waves forced by the large-scale wind curl in the interior bay. The effect of heat fluxes 1 P 2 E is generally small. Influence of interannual variability of river discharges is negligible. SLAs caused by the equatorial wind at the equator and that caused by the bay wind along the northern and western boundaries as well as in the southwestern bay are significantly correlated, reflecting the anomalous wind pattern associated with the dipole mode event in the tropical Indian Ocean. Given the dominance of equatorial wind forcing near the northern bay boundary, SLAs (or alternatively westerly wind anomalies) in the equatorial ocean may serve as a potential index for predicting Bangladesh flooding and cholera.

An Oceanic General Circulation Model (OGCM) investigation of the Red Sea circulation, 1. Exchange between the Red Sea and the Indian Ocean

Abstract: [1] The mechanisms involved in the seasonal exchange between the Red Sea and the Indian Ocean are studied using an Oceanic General Circulation Model (OGCM), namely the Miami Isopycnic Coordinate Ocean Model (MICOM). The model reproduces the basic characteristics of the seasonal circulation observed in the area of the strait of Bab el Mandeb. There is good agreement between model results and available observations on the strength of the exchange and the characteristics of the water masses involved, as well as the seasonal flow pattern. During winter, this flow consists of a typical inverse estuarine circulation, while during summer, the surface flow reverses, there is an intermediate inflow of relatively cold and fresh water, and the hypersaline outflow at the bottom of the strait is significantly reduced. Additional experiments with different atmospheric forcing (seasonal winds, seasonal thermohaline air–sea fluxes, or combinations) were performed in order to assess the role of the atmospheric forcing fields in the exchange flow at Bab el Mandeb. The results of both the wind- and thermohaline-driven experiments exhibit a strong seasonality at the area of the strait, which is in phase with the observations. However, it is the combination of both the seasonal pattern of the wind stress and the seasonal thermohaline forcing that can reproduce the observed seasonal variability at the strait. The importance of the seasonal cycle of the thermohaline forcing on the exchange flow pattern is also emphasized by these results. In the experiment where the thermohaline forcing is represented by its annual mean, the strength of the exchange is reduced almost by half.

Low-frequency variability of the exchanged flows through the Strait of Gibraltar during CANIGO

Abstract: Time series of the exchanged flows through the Strait of Gibraltar at the eastern section have been estimated from current-meter observations taken between October 1995 and May 1998 within the Canary Islands Azores Gibraltar Observations (CANIGO) project. The inflow exhibits a clear annual signal that peaks in late summer simultaneously with a deepening of the interface. The cycle seems to be driven by the seasonal signal of the density contrast between the surface Atlantic water that forms the inflow and the deep Mediterranean water of the outflow. The outflow and the depth of the interface have predominant semiannual signals and a smaller annual one whose phase agrees with that of the density contrast as well. Local wind stress and atmospheric pressure difference between the Atlantic and the Western Mediterranean to less extent have clear semiannual signal, so that the possibility that the semiannual cycle of the outflow and of the depth of the interface are forced by them was analyzed. The composite Froude number in this section is well below the critical value, suggesting submaximal exchange. Therefore, the conditions in the Alboran basin influence the exchange and some evidence that the size and location of the Western Alboran Gyre contribute to the observed signals, both annual and semiannual, is provided. r 2002 Elsevier Science Ltd. All rights reserved.

Effects of Tropospheric Wind Shear on the Spectrum of Convectively Generated Gravity Waves

Abstract: The authors examine the effects of tropospheric wind shear on the phase speed spectrum of gravity waves generated by tropical convection. A two-dimensional cloud-resolving model is used to perform numerous squall line simulations with the vertical shear of the horizontal wind varied in three layers of the troposphere. Several simplified simulations using prescribed heating are also performed to elucidate the interactions of wind shear with thermal forcing. It is found that the dominant phase speed range of convectively generated stratospheric gravity waves is primarily determined by the vertical scale of the tropospheric heating and is then modified by the tropospheric wind. The gravity wave spectrum is especially sensitive to shear in the upper troposphere. Through a mechanism similar to critical level filtering, such shear acts to reduce the momentum flux of waves propagating in the same direction as the storm-relative mean wind. Through interaction with convective turrets, shear in the upper t...

Impacts of the Indian Ocean on the ENSO cycle

Abstract: This study examines the impacts of the Indian Ocean on the ENSO (El Nino-Southern Oscillation) cycle by performing experiments with a coupled atmosphere-ocean general circulation model (CGCM). In one of the experiments, the ocean model domain includes only the tropical Pacific Ocean (the Pacific Run). In the other experiment, the ocean model domain includes both the Indian and tropical Pacific Oceans (the Indo-Pacific Run). The experiment results show that the CGCM simulation of ENSO including both the Indian and tropical Pacific Oceans tends to be more realistic than that including the tropical Pacific Ocean only. In particular, the Indo-Pacific Run produces ENSO events with larger amplitude and greater variability on decadal time scales. The interactive Indian Ocean also affects the surface heat flux anomalies in the Indian Ocean during the ENSO cycle and surface wind stress anomalies in both the tropical Indian and Pacific Oceans. There are indications that both surface heat flux and wind stress are actively forcing a portion of the interannual variability in the Indian Ocean during the ENSO cycle.

Wind Stress Forcing of the Ocean in the SOC Climatology: Comparisons with the NCEP NCAR, ECMWF, UWM/COADS, and Hellerman and Rosenstein Datasets

Abstract: Results from an analysis of the Southampton Oceanography Centre (SOC) global wind stress climatology, which is based on in situ reports for the period 1980-93, are presented. The accuracy of the SOC stresses has been assessed at several locations by comparison of individual monthly means with measurements from Woods Hole Oceanographic Institution research buoy deployments. For the subduction buoy array, situated in the subtropical North Atlantic, the random error in the SOC individual monthly mean wind stress ranges from 0.004 to 0.008 N m-2, which corresponds to between 5% and 10% of the mean stress depending on which buoy is considered. The large-scale characteristics of the SOC fields are compared with those of the NCEP-NCAR and ECMWF atmospheric model reanalyses, and the in situ observation based on the University of Wisconsin-Milwaukee/Comprehensive Ocean–Atmosphere Dataset (UWM/COADS) and Hellerman and Rosenstein (HR) climatologies. The NCEP-NCAR fields show noticeably weaker wind stress forcing in the Tropics than SOC, while ECMWF and UWM/COADS are in good agreement. From the Tropics to the midlatitudes, the HR stresses tend to be stronger than SOC and the other recent climatologies. At higher latitudes, differences in the spatial structure of the Northern Hemisphere subpolar gyres in SOC and HR are found that are consistent with variations in the state of the North Atlantic and North Pacific Oscillations within the periods on which the climatologies are based. A detailed comparison of the wind-driven response of the ocean is presented for SOC and HR. The North Atlantic subpolar gyre is more intense in SOC than HR and this leads to a doubling in the strength of the Ekman suction. January mean upwelling velocities in this region deduced from the two datasets are 18.9 and 8.6 m month-1, respectively. In the North Pacific a single large-scale subpolar gyre is evident in SOC compared with two smaller gyres in HR. Seasonal to interannual variability in the wind-driven ocean response is quantified using an extended version of the SOC dataset covering the period 1980–97. Significant variability in the Ekman transport across several latitudes that correspond to WOCE hydrographic sections is observed and related to the major atmospheric pressure oscillations

High Frequency Response of Wind-driven Currents Measured By Drifting Buoys and Altimetry Over The World Ocean

Abstract: [1] We investigate the wind-driven current response over the world ocean using SVP drifting buoys, ERS-1 and ERS-2, and TOPEX altimetry data and ECMWF wind stress fields. Wind-driven ageostrophic currents are estimated from drifting buoys by removing altimetry deduced geostrophic currents from observed drifting velocities. Removing the geostrophic signal enhances the coherence with wind stress at periods longer than 10 days. For superinertial periods lower than 20 days, a two-parameter (i.e., angle and amplitude) model is fitted to the isolated ageostrophic signal to study the upper ocean response to surface wind stress in that frequency band. The model, which can explain up to 30% of the variance in some areas, indicates that the currents spiral to the right (left) of the wind with depth in the Northern (Southern) Hemisphere, in agreement with the Ekman theory. The absolute value of the angle parameter is found to exhibit significant spatial and seasonal variability. The vertical eddy viscosity and the Ekman layer depth as deduced from Ekman theory are compared to previous results. Their seasonal variability is also studied. Model results are then used to compute the mean and standard deviation of ageostrophic currents in 5° boxes over the world ocean. The equatorial divergence and the subpolar and subtropical convergence zones are well reproduced.

Heat Budget in the Kuroshio Extension Region: 1993–99

Abstract: Processes responsible for the seasonal and interannual variations of the sea surface temperature as well as of the heat content of the upper ocean (0–400 m) in the Kuroshio Extension region are examined from a 3D advection–diffusion model in finite elements, with an embedded bulk mixed layer. The geostrophic velocity is specified externally from TOPEX/Poseidon altimeter data, and Ekman velocity is specified from NCEP wind stress. The thermal field from the model shows good agreement with observations. While both atmospheric and oceanic processes are required to explain observed nonseasonal SST changes, the interannual heat storage rate is dominated by horizontal advection. In particular, the transition between an elongated and a contracted state of the Kuroshio caused by geostrophic advection has a clear signature in the SST. There is an indication that this process is accompanied by consistent changes in nonseasonal entrainment: when the Kuroshio is in an elongated state and warmer waters are pr...

Modeling the dynamics of sediment transport and resuspension in the northern Adriatic Sea

Abstract: [1] A coupled Adriatic Sea General Circulation and sediment transport model was used to study the dynamics of coarse and fine sediment transport and resuspension in the Northern Adriatic Sea. The sediment sizes of coarse (>50 μm) and fine (<50 μm) materials were sorted by their settling velocities. The bottom boundary layer (BBL) was discretized by a vertical sigma coordinate system with high resolution, and the wave-current interaction mechanism was considered. The sediment distributions and fluxes under various forcing conditions such as the Po River plume, the Bora and Scirocco wind stress and the surface waves were studied by process oriented numerical simulations. The conclusions are that maximum northward sediment transport occurs under the forcing by the Po River plume with the Scirocco wave resuspension. The largest southward sediment transport was due to the combined effect of the Po River plume and the Bora wind-forcing under the Bora wave conditions. A realistic forcing numerical experiment was also conducted for November 1994 when the full range of forcing functions were experienced by the region. This study shows that wave-driven sediment resuspension is an important resuspension mechanism in the shallow coastal areas of the Northern Adriatic Sea, and contributes significantly to the complexity of the sediment distribution and flux features in the region.

Variation in the position of the upwelling front on the Oregon shelf

Abstract: [1] As part of an experiment to study wind-driven coastal circulation, 17 hydrographic surveys of the middle to inner shelf region off the coast of Newport, OR (44.65°N, from roughly the 90 m isobath to the 10 m isobath) were performed during Summer 1999 with a small, towed, undulating vehicle. The cross-shelf survey data were combined with data from several other surveys at the same latitude to study the relationship between upwelling intensity and wind stress field. A measure of upwelling intensity based on the position of the permanent pycnocline is developed. This measure is designed so as to be insensitive to density-modifying surface processes such as heating, cooling, buoyancy plumes, and wind mixing. It is highly correlated with an upwelling index formed by taking an exponentially weighted running mean of the alongshore wind stress. This analysis suggests that the front relaxes to a dynamic (geostrophic) equilibrium on a timescale of roughly 8 days, consistent with a similar analysis of moored hydrographic observations. This relationship allows the amount of time the pycnocline is outcropped to be estimated and could be used with historical wind records to better quantify interannual cycles in upwelling.

Wind forcing in the equilibrium range of wind-wave spectra

Abstract: A new analytical model is developed for the equilibrium range of the spectrum of wind-forced ocean surface gravity waves. We first show that the existing model of Phillips (1985) does not satisfy overall momentum conservation at high winds. This constraint is satisfied by applying recent understanding of the wind forcing of waves. Waves exert a drag on the air flow so that they support a fraction of the applied wind stress, which thus leaves a smaller turbulent stress near the surface to force growth of shorter wavelength waves. Formulation of the momentum budget accounting for this sheltering constrains the overall conservation of momentum and leads to a local turbulent stress that reduces as the wavenumber increases. This local turbulent stress then forces wind-induced wave growth. Following Phillips (1985), the wind sea is taken to be a superposition of linear waves, and equilibrium is maintained by a balance between the three sources and sinks of wave action. These assumptions lead to analytical formulae for the local turbulent stress and the degree of saturation, B(k), of waves in the equilibrium range. We identify a sheltering wavenumber, k s , over which the local turbulent stress is significantly reduced by longer waves. At low wavenumbers or at low winds, when k « k s , the sheltering is weak and B(k) has a similar form to the model of Phillips (1985). At higher wavenumbers or at higher winds, k >> k s , B(k) makes a transition to being proportional to k°. The additional constraint of conservation of momentum also yields a formula for the coefficient that appears in the solution for B(k). The spectra for mature seas are calculated from the model and are shown to agree with field observations. In particular, our model predicts more realistic spectral levels toward the high wavenumber limit compared to the previous model of Phillips (1985). We suggest that the model may explain the overshoot phenomena observed in the spectral energy levels as the fetch increases.

Solar Wind Variations Related to Fluctuations of the North Atlantic Oscillation

Abstract: A study on a possible solar wind interaction with the North AtlanticOscillation (NAO) is performed. Results are presented suggesting arelationship between the NAO index and the electric field strength E ofthe solar wind. A possible scenario for the suggested interaction isthat an electromagnetic disturbance is generated by the solar wind inthe global electric circuit of the ionosphere. This disturbance is thendynamically propagating downward through the atmosphere and subsequentlyinfluencing the large-scale pressure system in the North Atlanticregion. A relationship is also evident on longer time-scales when usingthe group sunspot number as a proxy for the solar wind. (Art. No. 1718)