Remote Wind Forcing Research Articles

Subtidal temperature variability in stratified shelf water off the mid-east coast of Korea

A wind-forced coastal trapped wave (CTW) model is applied to examine subtidal (period of a few days to weeks) temperature variability in stratified waters off the mid-east coast of Korea, and mechanisms underlying the temporal structure modified from the model prediction are discussed. Model-predicted, depth-averaged temperature is significantly correlated with observed temperature (TOBS) and the correlation coefficients increase as including remote wind forcing. Modeled phase speeds (4.6–5.9 and 9.5–10.3m/s off northern and southern coasts) are consistent with speeds of sea level propagation observed along the coast (6.1 and 10.4m/s respectively). Our results suggest that the wind-forced CTW model reasonably reproduces most (though not all) subtidal temperature variability off the coast within a semi-enclosed deep basin as were often cases for open-sea shelves. However, TOBS is hardly explained solely by the model particularly when horizontal nonlinear advection becomes significant. Abrupt changes in water properties caused by sudden downwelling and successive amplification of diurnal oscillations are suggested as (nonlinear and/or turbulent) mechanisms modifying the temporal structure of temperature beyond the CTW dynamics.

Nineteen-year changes in surface salinity in the Southern Ocean south of Australia

A 19-year time series of underway sea surface salinity (SSS) data from the SURVOSTRAL line between Hobart, Australia and Adelie Land, Antarctica has revealed the distinct patterns of seasonal and interannual variability in two key zones of the Southern Ocean: in the Subantarctic Zone and in the Antarctic Zone. Both regions show a weak seasonal freshening of the surface waters over the summer months, with surface waters becoming shallower, warmer and fresher during the summer heating cycle.In the region north of the Subantarctic Front, interannual variations in the summer SSS signature are linked to the latitudinal movements of the Subtropical Front. When this front shifts southward, more high salinity subtropical waters are brought into the domain. Rather than responding to local wind stress forcing, the interannual SSS variability is strongly linked to southward flow from eastern Tasmania (the Tasman outflow), whose decadal variability responds with a 2–3year delay to remote wind forcing in the South Pacific (Hill et al., 2008). Over this period, variations in the local surface freshwater forcing make a minor contribution to the SURVOSTRAL SSS signature. There appears to be a regime shift in the surface forcing and the SSS response, before and after the large perturbation in 2001–2002.In the Antarctic Zone, the summer SSS signature shows distinct biannual variations that are correlated with upstream sea-ice coverage in the preceding spring. There is no significant correlation between local precipitation changes and the SURVOSTRAL SSS time series. Rather, the integrated effects of upstream sea-ice melt dominate the interannual variations in SSS at the SURVOSTRAL line. Finally, a simple 2D Lagrangian particle analysis shows that in some years the surface waters circulate for a longer time in the northern part of the domain, where their freshwater content may be reduced by mixing with the surrounding waters.

On the relationship between stratification and primary productivity in the North Atlantic

[1] Recent observational studies linking variability in global ocean productivity with upper ocean warming are based on the paradigm that warming produces a more stable water column which, in turn, inhibits primary productivity for a large fraction of the global ocean, namely the tropics and subtropics. Though seemingly straightforward, this paradigm relies on the assumption that an increase in the stratification of the upper ocean water column decreases the vertical mixing or overturning of the surface waters such that the supply of nutrients to the euphotic zone is reduced, and so too the primary productivity. Here we show, using observational data in the North Atlantic subtropical gyre, that while upper ocean stratification and primary productivity are strongly linked on seasonal time scales, they have at most a weak correlative relationship on interannual time scales over the modern observational record. We suggest that interannual variability in ocean biomass and primary productivity depends on a host of variables that are not easily predicted from the expected temperature response to climate variability and change. These variables include the strength of local and remote wind and buoyancy forcing and the surface or subsurface advective supply of nutrients.

Adjoint Sensitivity of the Niño-3 Surface Temperature to Wind Forcing

Abstract The evolution of sea surface temperature (SST) over the eastern equatorial Pacific plays a significant role in the intense tropical air–sea interaction there and is of central importance to the El Niño–Southern Oscillation (ENSO) phenomenon. Effects of atmospheric fields (especially wind stress) and ocean state on the eastern equatorial Pacific SST variations are investigated using the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint model, which can calculate the sensitivities of a cost function (in this case the averaged 0–30-m temperature in the Niño-3 region during an ENSO event peak) to previous atmospheric forcing fields and ocean state going backward in time. The sensitivity of the Niño-3 surface temperature to monthly zonal wind stress in preceding months can be understood by invoking mixed layer heat balance, ocean dynamics, and especially linear equatorial wave dynamics. The maximum positive sensitivity of the Niño-3 surface temperature to local wind forcing usually happens ~1–2 months before the peak of the ENSO event and is hypothesized to be associated with the Ekman pumping mechanism. In model experiments, its magnitude is closely related to the subsurface vertical temperature gradient, exhibiting strong event-to-event differences with strong (weak) positive sensitivity during La Niña (strong El Niño) events. The adjoint sensitivity to remote wind forcing in the central and western equatorial Pacific is consistent with the standard hypothesis that the remote wind forcing affects the Niño-3 surface temperature indirectly by exciting equatorial Kelvin and Rossby waves and modulating thermocline depth in the Niño-3 region. The current adjoint sensitivity study is consistent with a previous regression-based sensitivity study derived from perturbation experiments. Finally, implication for ENSO monitoring and prediction is also discussed.

Coastal and shelf circulation in the vicinity of Camamu Bay (14°S), Eastern Brazilian Shelf

The Camamu Bay (CMB) is located on the narrowest shelf along the South American coastline and close to the formation of two major Western Boundary Currents (WBC), the Brazil/North Brazil Current (BC/NBC). These WBC flow close to the shelf break/slope region and are expected to interact with the shelf currents due to the narrowness of the shelf. The shelf circulation is investigated in terms of current variability based on an original data set covering the 2002–2003 austral summer and the 2003 austral autumn. The Results show that the currents at the shelf are mainly wind driven, experiencing a complete reversal between seasons due to a similar change in the wind field. Currents at the inner-shelf have a polarized nature, with the alongshore velocity mostly driven by forcings at the sub-inertial frequency band and the cross-shore velocity mainly supra-inertially forced, with the tidal currents playing an important role at this direction. The contribution of the forcing mechanisms at the mid-shelf changes between seasons. During the summer, forcings in the two frequency bands are important to drive the currents with a similar contribution of the tidal currents. On the other hand, during the autumn season, the alongshore velocity is mostly driven by sub-inertial forcings and tidally driven currents still remain important in both directions. Moreover, during the autumn when the stratification is weaker, the response of the shelf currents to the wind forcing presents a barotropic signature. The meso-scale processes related to the WBC flowing at the shelf/slope region also affect the circulation within the shelf, which contribute to cause significant current reversals during the autumn season. Currents at the shelf-estuary connection are clearly supra-inertially forced with the tidal currents playing a key role in the generation of the along-channel velocities. The sub-inertial forcings at this location act mainly to drive the weak ebb currents which were highly correlated with both local and remote wind forcing during the summer season.

Remote and local forcing of a coastal lagoon: The Virginia Coast Reserve

The effects of local and remote wind forcing of water level heights in the Virginia Coast Reserve (VCR) are examined in order to determine the significant forces governing estuarine motions over subtidal time scales. Recent (1996–2008) data from tide and wind stations in the lagoon, a tide station to the north at Sandy Hook, NJ, and one offshore wind station at the Chesapeake Light Tower are examined. Sea surface height spectrum calculations reveal significant diurnal and semidiurnal tidal effects along with subtidal variations, but a suppressed inertial signal. Sea-surface heights (SSH) with 2–5 day periods at Wachapreague, VA are coherent with those at Sandy Hook and lag them in time, suggesting that southward-propagating continental shelf waves provide subtidal variability within the lagoon. The coherence between lagoon winds and sea surface height, as well as between winds and cross-lagoon sea height gradient, were significant at a relatively small number of frequency and wind direction combinations. The frequencies at which this wind forcing occurs are the tidal and subtidal bands present to the north, so that lagoon winds selectively augment existing SSH signals, but do not generate them. The impact of the wind direction is closely related to the geometry of the lagoon and bounding landmasses. The effect of wind stress is also constrained by geometry in affecting the cross-lagoon water height gradient. Water levels at subtidal frequencies are likely forced by a combination of local wind forcing, remote wind forcing and oceanic forcing modified by the complex topography of the lagoon, shelf, and barrier islands.

Rapid response of the East Australian Current to remote wind forcing: The role of barotropic-baroclinic interactions

The strength of the East Australian Current (EAC) is observed to vary in response to changes in basin-scale winds in the South Pacific, with a time lag of three years. First mode baroclinic Rossby waves would take 10-15 years to reach the western boundary from the center of the South Pacific, so a faster mechanism is needed to explain this link. We use an ocean general circulation model forced with idealized anomalies of wind stress curl to examine the mechanism responsible for the rapid response of the EAC. A curl perturbation in the central South Pacific produces baroclinic and barotropic Rossby waves. The barotropic waves propagate quickly to the western boundary at New Zealand (NZ), where they scatter into a coastal Kelvin wave that travels anti-cyclonically around the coast of NZ. In the Tasman Sea, the height anomaly associated with the Kelvin wave spawns first mode baroclinic waves that take about three years to propagate across the Tasman Sea to influence the EAC. The model suggests that the rapid response of the EAC to changes in wind forcing can be explained by a combination of barotropic and baroclinic Rossby waves with conversion between modes facilitated by topography.

The role of meteorological forcing on the St. Johns River (Northeastern Florida)

Water surface elevations in the St. Johns River (Northeastern Florida) are simulated over a 122-day time period spanning June 1–September 30, 2005, which relates to a particularly active hurricane season for the Atlantic basin, and includes Hurricane Ophelia that significantly impacted the St. Johns River. The hydrodynamic model employed for calculating two-dimensional flows is the ADCIRC (Advanced Circulation Model for Oceanic, Coastal, and Estuarine Waters) numerical code. The region of interest is modeled using three variations of an unstructured, finite element mesh: (1) a large-scale computational domain that hones in on the St. Johns River from the Western North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea; (2) a shelf-based subset of the large domain; (3) an inlet-based subset of the large domain. Numerical experiments are then conducted in order to examine the relative importance of three long-wave forcing mechanisms for the St. Johns River: (1) astronomic tides; (2) freshwater river inflows; (3) winds and pressure variations. Two major findings result from the various modeling approaches considered in this study, and are applicable in general (e.g., over the entire 122-day time period) and even more so for extreme storm events (e.g., Hurricane Ophelia): (1) meteorological forcing for the St. Johns River is equal to or greater than that of astronomic tides and generally supersedes the impact of freshwater river inflows, while pressure variations provide minimal impact; (2) water surface elevations in the St. Johns River are dependent upon the remote effects caused by winds occurring in the deep ocean, in addition to local wind effects. During periods of calm weather through the 122-day time period, water surface elevations in the St. Johns River were generally tidal in response, with amplitudes exceeding 1 m at the mouth and diminishing to less than 10 cm 150 km upriver. Considering an extreme storm event, the timing of Hurricane Ophelia occurred during the neap phase of the tidal cycle and at the mouth of the St. Johns River, the wind-driven storm surge was near equal to the tidal component, each contributing about 0.5 m to the overall water surface elevation. However, 150 km upriver, meteorological forcing dominated, as over 90% of the total water surface elevation was driven by winds and pressures. The simulation results replicate these behaviors well. As a supplement, it is shown that applying a hydrograph boundary condition, generated by a large domain, to a localized domain is highly beneficial towards accounting for the remote wind forcing.

Pacific Decadal Variability in the View of Linear Equatorial Wave Theory*

Abstract It has recently been proposed, within the framework of the linear shallow-water equations, that tropical Pacific decadal variability (PDV) can be accounted for by basin modes with eigenperiods of 10 to 20 yr, amplifying a midlatitude wind forcing with an essentially white spectrum. Here the authors use a different formalism of linear equatorial wave theory. The Green’s function is computed for the wind-forced response of a linear equatorial shallow-water ocean and use the earlier results of Cane and Moore to obtain a compact, closed form expression for the motion of the equatorial thermocline, which applies to all frequencies lower than seasonal. This expression is new and allows a systematic comparison of the effect of low- and high-latitude winds on the equatorial thermocline. At very low frequencies (decadal time scales), the planetary geostrophic solution used by Cessi and Louazel is recovered, as well as the equatorial wave solution of Liu, and a formal explanation for this convergence is given. Nonetheless, this more general solution leads one to a different interpretation of the results. In contrast to the aforementioned studies, the authors find that the equatorial thermocline is inherently more sensitive to local than to remote wind forcing and that planetary Rossby modes only weakly alter the spectral characteristics of the response. Tropical winds are able to generate a strong equatorial response with periods of 10 to 20 yr, while midlatitude winds can only do so for periods longer than about 50 yr. The results suggest that ocean basin modes are an unlikely explanation of decadal fluctuations in tropical Pacific sea surface temperature.

Observational evidence for remote forcing of the West India Coastal Current

Circulation in the north Indian Ocean is influenced by both local and remote wind forcing. So far, however, determining the contribution of these two forcing mechanisms at a location has been possible only in numerical experiments. Here we separate remote and local forcing in observations. Using field measurements (current, sea level, and wind) for a month during March–April 2003 off Goa in the near‐coast regime of the West India Coastal Current (WICC), we show that the current was driven by local winds only at periods less than ∼10 days, with remote forcing contributing at longer periods. The high‐passed (HP; period less than 10 days) component of the along‐shore current was strongly correlated with the HP component of the along‐shore wind, the current lagging the wind by half a day. The low‐passed (LP) components of the wind and current were not correlated: the former was unidirectional, but the latter reversed during the period of observation. The relationship between the HP wind and current was used to estimate the locally forced LP current, permitting an estimate of the remote current, the LP residual. This separation of locally forced and remotely forced currents showed that remote forcing contributed as much as local forcing to the WICC. The local current behaved like a classical eastern boundary current forced by local winds. The reversal in the remote current was due to winds 700 km farther south along the coast; frictional damping had an impact only at periods less than 10 days, there being no remotely forced HP current.

Mechanisms Governing Interannual Variability of Upper-Ocean Temperature in a Global Ocean Hindcast Simulation

Abstract The interannual variability in upper-ocean (0–400 m) temperature and governing mechanisms for the period 1968–97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño–Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air–sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.

Wind-driven barotropic circulation off Newfoundland and Labrador

Three-dimensional (3-D) finite element models are used to investigate barotropic wind-driven circulation over the Newfoundland and Labrador Shelf, including its 3-D structure and seasonal and interannual variability, roles of regional vs. remote wind forcing, and importance of the wind-driven circulation. Long-term monthly mean and individual seasonal-mean wind-driven circulation is computed with wind stresses from the NCEP–NCAR reanalysis data (1990–1999) prescribed at the sea surface and corresponding large-scale remote forcing specified at the open boundary determined from a North Atlantic model. The shelf model solutions are discussed for annual-mean circulation features, for seasonal and interannual variations and for relative importance of regional to remote forcing. The model solutions show a shelf-edge current and an inshore current, both of which directed equatorward. The cross-shelf flows and anticyclonic circulation over shallow banks are evident as a consequence of topographic steering. There are prominent seasonal variations in the two branches of the Labrador Current, strongest in December and weakest in July. The interannual changes are also significant in the 1990s, with larger winter transport in the early 1990s and smallest winter transport in 1996. The model shelf-edge Labrador Current is predominated by the remote forcing, while the regional wind forcing plays a significant role in the inshore Current. The model transport for the primary shelf-scale flows is examined against previous model results and in situ observations. Importance of wind-driven currents in the overall circulation is evaluated, which indicates that the wind forcing contributes significantly to the inshore branch of the Labrador Current and less to the shelf-edge branch.

Observations of divergence and upwelling around Point Loma, California

Historical records of near‐surface water temperatures in the Southern Californian Bight often show a preferential cooling in the lee of headlands such as Point Dume, Palos Verdes, and Point Loma. At times, this cooler water is associated with an increase in chlorophyll‐a as is evident in satellite images of ocean color from the region. Here we combine hydrographic data from a 1 day cruise aboard the RV Roger Revelle (a precursor to the 0304 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise) with high‐frequency (HF) radar (CODAR) measurements, satellite images, and long‐term thermistor records of near‐surface temperature to identify a small‐scale, isolated upwelling in the lee of Point Loma (32.5°N). Associated with the more saline water downstream of the headland are higher nutrient concentrations, an increase in chlorophyll‐a concentration, and a bloom of chain‐forming diatoms, indicative of a mature upwelling system. It is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the Point Loma headland. Time series of surface vorticity calculated from HF radar measurements of sea surface velocity show that as the flow separates from the headland, relative vorticity increases offshore of the cape. Inshore, the time series of divergence/convergence shows a tendency toward divergence at the surface, indicating a preferential upwelling which appears to raise the thermocline, thus resulting in a flux of cold nutrient‐rich water to the surface. In the presence of high nutrients and light, photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents.

Local versus remote wind forcing of the equatorial Pacific surface temperature in July 2003

After the moderate El Niño of 2002 the tropical Pacific seemed poised to enter a La Niña phase. Wind anomalies during May and June 2003, however, led to a sudden termination of this event. Counter to earlier predictions, forecasts from July 2003 correctly predicted normal conditions for the remainder of the year. In this paper the causes of this abrupt turn of events are analyzed. It is shown that wind anomalies in the eastern tropical Pacific played a major role in terminating the incipient cold ENSO phase. Although westerly wind bursts in the western Pacific explained most of the ocean subsurface behavior to the west of 110°W, their impact on SST was confined to the central equatorial Pacific.

Local wind forcing of the Monterey Bay area inner shelf

Wind forcing and the seasonal cycles of temperature and currents were investigated on the inner shelf of the Monterey Bay area of the California coast for 460 days, from June 2001 to September 2002. Temperature measurements spanned an approximate 100 km stretch of coastline from a bluff just north of Monterey Bay south to Point Sur. Inner shelf currents were measured at two sites near the bay's northern shore. Seasonal temperature variations were consistent with previous observations from the central California shelf. During the spring, summer and fall, a seasonal mean alongshore current was observed flowing northwestward in the northern bay, in direct opposition to a southeastward wind stress. A barotropic alongshore pressure gradient, potentially driving the northwestward flow, was needed to balance the alongshore momentum equation. With the exception of the winter season, vertical profiles of mean cross-shore currents were consistent with two-dimensional upwelling and existing observations from upwelling regions with poleward subsurface flow. At periods of 15–60 days, temperature fluctuations were coherent both throughout the domain and with the regional wind field. Remote wind forcing was minimal. During the spring upwelling season, alongshore currents and temperatures in the northern bay were most coherent with winds measured at a nearby land meteorological station. This wind site showed relatively low correlations to offshore buoy wind stations, indicating localized wind effects are important to the circulation along this stretch of Monterey Bay's inner shelf.

Sensitivity of Basinwide Meridional Overturning to Diapycnal Diffusion and Remote Wind Forcing in an Idealized Atlantic–Southern Ocean Geometry

Recent numerical experiments indicate that the rate of meridional overturning associated with North Atlantic Deep Water is partially controlled by wind stress in the Southern Ocean, where the zonal periodicity of the domain alters the nature of the flow. Here, the authors solve the cubic scale relationship of Gnanadesikan to find a simple expression for meridional overturning that is used to clarify the relative strength of the wind-forced component. The predicted overturning is compared with coarse-resolution numerical experiments with an idealized Atlantic Ocean–Southern Ocean geometry. The scaling accurately predicts the sensitivity to forcing for experiments with a level model employing isopycnal diffusion of temperature, salinity, and “layer thickness.” A layer model produces similar results, increasing confidence in the numerics of both models. Level model experiments with horizontal diffusivity have similar qualitative behavior but somewhat different sensitivity to forcing. The paper highlights the difference in meridional overturning induced by changes in wind stress or vertical diffusivity. Strengthening the Southern Ocean wind stress induces a circulation anomaly in which most of the water is subducted in the Ekman layer of the wind perturbation region, follows isopycnals down into the thermocline, and changes density again when the isopycnals near the surface in the Northern Hemisphere. Approximating the circulation anomaly by this subduction route allows for a surprisingly accurate prediction of the resulting heat transport anomaly, based on the surface temperature distribution. Some of the induced flow follows a second, near-surface northward route through low-latitude water that is lighter than the subducted flow. Overturning anomalies far from the wind stress perturbations are not completely determined by wind stress in the zonally periodic Southern Ocean: wind stress outside the periodic region strongly influences the transport of heat across the equator primarily by changing the temperature of the flow across the equator.

A semiannual Indian Ocean forced Kelvin wave observed in the Indonesian seas in May 1997

Recent observations within the Indonesian exit passages and internal seas highly resolve the arrival and passage of a semiannual Kelvin wave. In mid‐May 1997, surface and subsurface currents were to the southeast at a mooring located south of Java in the South Java Current, while local wind forcing was northwestward. Subsequent northward fluctuations in the geostrophic current through Lombok Strait and in observed currents from two moorings located in Makassar Strait are commensurate with the speed and passage of a Kelvin wave through the region. The Kelvin wave was due to westerly wind forcing in the remote equatorial Indian Ocean during the semiannual April/May monsoon transition period. This was confirmed through a simple remote wind‐forced analytical Kelvin wave model of velocity at the South Java Current mooring location and sea level in Lombok Strait and also in the numerical general circulation model of Murtugudde et al. [1998]. Warm temperature anomalies measured at the south Java mooring and within Makassar Strait are associated with the passage of the Kelvin wave. Salinity anomalies measured at the south Java mooring are consistent with an Indian Ocean source. The observed passage of the Kelvin wave during May 1997 unambiguously demonstrates for the first time that equatorial Indian Ocean remote wind forcing may on occasions influence the internal Indonesian seas

The California Current system in the Southern California Bight and the Santa Barbara Channel

Recent observations in the Santa Barbara Channel and over the California Cooperative Oceanic Fisheries Investigations (CalCOFI) southern California grid are used to examine seasonal circulation patterns in and near the Southern California Bight, defined as the region east of the Santa Rosa Ridge and including the Santa Barbara Channel. Poleward flow relative to 500 m is found throughout the bight, in all seasons except for spring and all subregions except the western part of the Santa Barbara Channel. In spring there is equatorward flow throughout the bight at all depths to 500 m, though it tends to be surface or midcolumn intensified. Equatorward flow offshore of the bight, present in all seasons, narrows and accelerates into a jet‐like feature and simultaneously moves close to the Santa Rosa Ridge in summer. Current meter data from the eastern entrance to the Santa Barbara Channel are consistent with the seasonal results from the CalCOFI data and further show an upward propagation of phase at annual period, with a phase speed of 1 to 2 m d−1. Comparison with the CalCOFI ship winds suggests that poleward flow in the bight may be the result of positive wind stress curl, through a Sverdrup balance, though the observed transport is only about 75% of that expected from the curl amplitude. Equatorward flow in the bight may be the result of coastal upwelling, though the transport is larger than expected from the relatively weak winds in the bight, and the offshore extent of the equatorward flow is about 10 Rossby radii, farther than might be expected for linearized coastal upwelling. Equatorward flow outside the bight cannot be explained as a Sverdrup flow because the wind stress curl is positive throughout most of the region. Upward propagation of phase and downward propagation of energy may be indicative of forcing by remote wind equatorward of the bight. Given the complexity of possible forcing mechanisms for circulation in and near the bight, a model that includes effects of remote wind forcing, Ekman pumping, and topography is needed to explain the observations in a satisfactory way.

Effects of subsurface ocean dynamics on instability waves in the tropical Pacific

Tropical instability waves in a primitive equation model of the tropical Pacific Ocean, forced with analyzed wind stresses updated daily, show unexpectedly close phase correspondence with observation through the latter half of 1992. This suggests that these waves are not pure instabilities developing from infinitesimal disturbances, but that their phases and phase speeds are at least partially determined by the wind stress forcing. To quantify and explain this observation, we perfomed several numerical experiments, which indicate that remotely forced Rossby waves can influence both the phase and phase speed of tropical instability waves. We suggest that a remote wind forcing determines the high model/observation phase correspondence of tropical instability waves through a relatively realistic simulation of equatorial Kelvin and Rossby wave activity.

Subtidal Flow Patterns in Western Florida Bay

Recent observations using moored current meters, shipboard ADCP transects, salinity mapping and drifters have been used to study the residual circulation including wind drift in western Florida Bay.Rapid, nearly synoptic surveys of salinity over a large area was an effective tracer-mapping technique, when salinity gradients were sufficiently strong, and provided qualitative information on Lagrangian water motion for the entire study area. The salinity maps indicated a general south-eastward advection, which was only subordinate to tidal mixing in a narrow zone adjacent to the Florida Keys.Drifter data collected simultaneously, allowed quantitative estimates to be added to the transport pattern suggested by salinity maps. The selectively deployed drifters yielded estimates of total drift velocities. In addition, moored current meters and shipboard current profiling were used to determine the distribution of flow across the mouth of the bay facing the Gulf of Mexico and the transport through Long Key Channel, a major connection between the bay and the Atlantic Ocean.Analysis showed that from 64 to over 92% of the drifter trajectory variances could be explained by the combination of a local wind drift, expressed in terms of a wind drift factor multiplied by the surface shear velocity, and an ambient current. For a 1m high drifter deployed at the surface of the water column, the wind drift factor was found to be approximately 0·125m, making the drift speed roughly equal to 0·45% of wind speed. The mean drifter speeds were linearly proportional to mean transport estimates derived from the current meter observations in Long Key Channel, enhancing confidence in both data sets.The total south-eastward directed residual current varied between 100 and 5000m day−1and was weaker in summer than in winter, when southward winds associated with periodic passage of cold fronts boost the residual flow. The estimated contribution from local wind drift varied between 500m day−1in summer to 1000m day−1in winter. The remaining contribution to the observed Lagrangian residual circulation in western Florida Bay is caused by other forcing, including tidal rectification, remote wind forcing and large-scale current systems (the Gulf Stream and Florida Current systems).