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Coincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1.

Knowledge of horizontal relative dispersion in nearshore oceans is important for many applications including the transport and fate of pollutants and the dynamics of nearshore ecosystems. Two-particle dispersion statistics are calculated from millions of synthetic particle trajectories from high-resolution numerical simulations of the Southern California Bight. The model horizontal resolution of 250 m allows the investigation of the two-particle dispersion, with an initial pair separation of 500 m. The relative dispersion is characterized with respect to the coastal geometry, bathymetry, eddy kinetic energy, and the relative magnitudes of strain and vorticity. Dispersion is dominated by the submesoscale, not by tides. In general, headlands are more energetic and dispersive than bays. Relative diffusivity estimates are smaller and more anisotropic close to shore. Farther from shore, the relative diffusivity increases and becomes less anisotropic, approaching isotropy ~10 km from the coast. The degree of anisotropy of the relative diffusivity is qualitatively consistent with that for eddy kinetic energy. The total relative diffusivity as a function of pair separation distance R is on average proportional to R5/4. Additional Lagrangian experiments at higher horizontal numerical resolution confirmed the robustness of these results. Structures of large vorticity are preferably elongated and aligned with the coastline nearshore, which may limit cross-shelf dispersion. The results provide useful information for the design of subgrid-scale mixing parameterizations as well as quantifying the transport and dispersal of dissolved pollutants and biological propagules.

Oceanographic observations made during the Sound Predictions 2009 field experiment in Prince William Sound, Alaska, have documented rapid changes in the upper water column (0–40 m) circulation. An assortment of drifting buoys, sampling four different depths, and HF radar surface current mapping, revealed three modes of circulation: anticyclonic, open cyclonic, and closed cyclonic. Each mode was observed at least once within an 18-day window, and the transition between them took as little as a day. Time-resolved hydrographic measurements show that the mass field was variable, but generally arranged such that the surface geostrophic flow should be in a closed-core cyclonic eddy configuration. Observations show that the mass field was likely influenced by relatively low salinity water flowing into Prince William Sound from the shelf, and from local freshwater input. We quantitatively examine why a closed-core circulation was not always observed by focusing on the transition between the closed and open cyclonic flow patterns. The western region of the central sound is a key area for this transition. Here, the high-frequency radar revealed that the closed circulation was established when the net flow shifted direction from northward to southward. A detailed comparison of the meridional geostrophic and wind-driven flows, using measured winds and hydrographic data from CTD profiles and two autonomous vehicles, shows that the geostrophic flow was mostly southward while the wind-driven flow was mostly northward. A net southward flow can be caused by a decrease in the northward wind-driven flow or an increase in the southward geostrophic flow.

This study explores Eulerian and Lagrangian circulation during weak winds at two inner-shelf locations off the Southern California coast where the shoreline, shelf, wind, and wave characteristics differ from those in previous studies. In agreement with recent observational studies, wave-driven Eulerian offshore flow just outside the surf zone, referred to as undertow, is a substantial component of the net cross-shore circulation during periods of weak winds. Drifter observations show onshore surface flow, likely due to light onshore winds, and a consistent decrease in onshore velocity of roughly 4 cm s−1 within a few hundred meters of the surf zone. Undertow is examined as a possible explanation for the observed Lagrangian decelerations. Model results suggest that, even when waves are small, undertow can decrease the velocity of shoreward-moving drifters by >2 cm s−1, roughly half the observed deceleration. The coastal boundary condition also has the potential to contribute to the observed decelerations. Subtracting predicted Stokes drift velocities from the Lagrangian drifter observations improves the agreement between the drifter observations and coincident Eulerian ADCP observations.

The oceanic near-surface temperature profile must be accurately characterized to enable precise determination of air–sea heat exchange and satellite retrievals of sea surface temperature. An improved solar transmission parameterization is integrated into existing models for the oceanic warm layer and cool skin within the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) bulk flux model to improve the accuracy of predictions of the temperature profile and corresponding heat flux components. Application of the revised bulk flux model to data from 12 diverse cruises demonstrates that the improved parameterization results in significant changes to the predicted cool-skin effect and latent heat fluxes at low wind speeds with high solar radiation due to reduced absorption of solar radiation just below the surface. Daytime skin-layer cooling is predicted to increase by 0.03 K on average but by more than 0.25 K for winds below 1 m s−1 and surface irradiance exceeding 900 W m−2. Predicted changes to the warm-layer correction were smaller but exceeded 0.1 K below 1 m s−1. Average latent and sensible heat fluxes changed by 1 W m−2, but the latent flux decreased by 5 W m−2 near winds of 0.5 m s−1 and surface irradiance of 950 W m−2. Comparison with direct observations of skin-layer cooling demonstrated, in particular, that use of the improved solar transmission model resulted in the reduction of previous systematic overestimates of diurnal skin-layer warming. Similar results can be achieved using a simplified treatment of solar absorption with an appropriate estimate of the fraction of incident solar radiation absorbed within the skin layer.

A drifter for observing small spatial and temporal scales of motion in the coastal zone is presented. The drifter uses GPS to determine its position, and the Mobitex terrestrial cellular communications system to transmit the position data in near–real time. This configuration allows position data with order meter accuracy to be sampled every few minutes and transmitted inexpensively. Near-real-time transmission of highly accurate position data enables the drifters to be retrieved and redeployed, further increasing economy. Drifter slip measurements indicate that the drifter follows water to within ∼1–2 cm s−1 during light wind periods. Slip values >1 cm s−1 are aligned with the direction of surface wave propagation and are 180° out of phase, so that the drifter “walks” down waves. Nearly 200 drifter tracks collected off the Santa Barbara, California, coast show comparisons with high-frequency (HF) radar observations of near-surface currents that improve by roughly 50% when the average drifter values are computed from more than 25 observations within a 2-km square HF radar bin. The improvement is the result of drifter resolution of subgrid-scale eddies that are included in time–space-averaged HF radar fields. The average eddy kinetic energy on 2-km space and hour time scales is 25 cm2 s−2, when computed for bins with more than 25 drifter observations. Comparisons with trajectories that are computed from HF radar data show mean separation velocities of 5 and 9 cm s−1 in the along- and across-shore directions, respectively. The drifters resolve scales of motion that are not present in HF radar fields, and are thus complementary to HF radar in coastal ocean observing systems.

A computationally simple, double exponential, chlorophyll-dependent solar transmission parameterization for ocean general circulation models used in climate studies is presented. The transmission parameterization comes from empirical fits to a set of in-water solar flux profiles calculated with an atmosphere–ocean radiative transfer model system, run with chlorophyll concentration values over the range observed in oligotrophic, open ocean waters. Transmission parameters are available from a lookup table, or can be written as logarithmic and square root functions of chlorophyll concentration, available globally from remotely sensed ocean color data. The rms and maximum errors introduced by curve fitting are less than 3 × 10−3 and 1.5 × 10−2, respectively. Error associated with neglect of second-order cloud and solar zenith angle influences is mostly a few percent. An extension to account for second-order processes in cases where they are large (>10%) is given. The double exponential form enables solar transmission to be resolved at depths beyond 2 m. Only the first exponential term need be considered to accurately determine transmission at depths greater than 8 m. The transmission parameterization is validated with in situ optical and biological data collected in the eastern equatorial Pacific during the Eastern Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) field program, and in the western equatorial Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The rms (maximum) errors between parameterized transmission and the mean transmission profile computed from in situ values are 0.5 (1.5) and 1.9 (6.6) W m−2, for the eastern and western equatorial Pacific regions, respectively. For comparison, rms (maximum) errors between transmission from a commonly used Jerlov water type–based parameterization and mean measured values are 7.3 (26.7) and 5.0 (8.8) W m−2 for the eastern and western Pacific, respectively (both cases assume a climatological surface flux of 200 W m−2). Proper use of the solar transmission parameterization should increase the accuracy of modeled SST and upper ocean stratification. The parameterization allows ocean radiant heating in climate models to be discussed in terms of chlorophyll concentration, the physical parameter on which solar transmission most heavily depends.

Surface current data from drifting buoys and remotely sensed wind data recorded over the continental shelf in the northeastern Gulf of Mexico during the passage of tropical storm Josephine in October 1996 are examined. Drifter data show the existence of a strong surface jet (velocities reaching 1 m s −1 ) that moves up the west Florida shelf and westward along the Louisiana–Texas shelf, and lasts for nearly 1 week. The coastal jet occurs during an intense synoptic scale wind event where wind speeds reach 15 m s −1 . A simple force balance and statistical analysis are performed to assess the role of strong wind forcing. The primary balance shows an Ekman-type current. The role of local acceleration is greatest when winds are directed along bathymetry. A simple two-dimensional strongly forced shelf response model developed from the linear steady-state momentum equations also indicates larger along-shore currents due to both Ekman-type forcing by cross-shore winds and a cross-shore pressure gradient arising from conservation of mass. Model parameters fit empirically are within 15% of theoretical values. The simple model explains 30% and 46% of the variance in the observed along-shore and cross-shore surface currents, respectively.

Sea surface height anomaly data from satellite are continuously available for the entire Gulf of Mexico. Surface current velocities derived from these remotely sensed data are compared with surface velocities from drifting buoys. The comparison shows that satellite altimetry does an excellent job resolving gulf eddies over the shelf rise (depths between ---200 and 2000 m) if the proper length scale is used. Correlations between altimeter- and drifter-derived velocities are statistically significant (r > 0.5) when the surface slope is computed over 125 km, indicating that remotely sensed sea surface height anomaly data can be used to aid the understanding of circulation over the shelf rise. Velocity variance over the shelf rise from the altimetry data shows regions of pronounced eddy energy south of the Mississippi outflow, south of the Texas-Louisiana shelf, and in the northwest and northeast corners of the gulf. These are the same locations where surface drifters are most likely to cross the shelf rise, suggesting gulf eddies promote cross-shore flows. This is clearly exemplified with both warm and cold eddies. Finally, the contribution of gulf eddies and wind stress to changes in the mean circulation are compared. Results indicate that the eddy-generated vorticity flux to the mean flow is greater than the contribution from the surface wind stress curl, especially in the region of the Loop current and along the shelf rise base in the western gulf. Future modeling efforts must not neglect the role of eddies in driving gulf circulation over the shelf rise.

Accurate determination of sea surface temperature (SST) is critical to the success of coupled ocean‐atmosphere models and the understanding of global climate. To accurately predict SST, both the quantity of solar radiation incident at the sea surface and its divergence, or transmission, within the water column must be known. Net irradiance profiles modeled with a radiative transfer model are used to develop an empirical solar transmission parameterization that depends on upper ocean chlorophyll concentration, cloud amount, and solar zenith angle. These factors explain nearly all of the variations in solar transmission. The parameterization is developed by expressing each of the modeled irradiance profiles as a sum of four exponential terms. The fit parameters are then written as linear combinations of chlorophyll concentration and cloud amount under cloudy skies, and chlorophyll concentration and solar zenith angle during clear-sky periods. Model validation gives a climatological rms error profile that is less tha n4Wm 22 throughout the water column (when normalized to a surface irradiance of 200 W m22). Compared with existing solar transmission parameterizations this is a significant improvement in model skill. The two-equation solar transmission parameterization is incorporated into the TOGA COARE bulk flux model to quantify its effects on SST and subsequent rates of air‐sea heat exchange during a low wind, high insolation period. The improved solar transmission parameterization gives a mean 12 W m22 reduction in the quantity of solar radiation attenuated within the top few meters of the ocean compared with the transmission parameterization originally used. This results in instantaneous differences in SST and the net air‐sea heat flux that often reach 0.28 Ca nd 5Wm 22, respectively.

Radiative transfer calculations are used to quantify the effects of physical and biological processes on variations in the transmission of solar radiation through the upper ocean. Results indicate that net irradiance at 10 cm and 5 m can vary by 23 and 34 W m−2, respectively, due to changes in the chlorophyll concentration, cloud amount, and solar zenith angle (when normalized to a climatological surface irradiance of 200 W m−2). Chlorophyll influences solar attenuation in the visible wavebands, and thus has little effect on transmission within the uppermost meter where the quantity of near-infrared energy is substantial. Beneath the top few meters, a chlorophyll increase from 0.03 to 0.3 mg m−3 can result in a solar flux decrease of more than 10 W m−2. Clouds alter the spectral composition of the incident irradiance by preferentially attenuating in the near-infrared region, and serve to increase solar transmission in the upper few meters as a greater portion of the irradiance exists in the deep-...

Optical, physical, and biological data collected in the western Pacific warm waterpool (WWP) as part of the Tropical Ocean-Global Atmosphere-Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE) are used to assess variations in the transmission of solar radiation through the water column, to investigate factors that regulate in-water solar transmission, and to examine the sensitivity of upper ocean thermal processes to solar transmission parameterizations. Solar transmission within the upper ocean mixed layer, below 10 m, can be accurately parameterized (within 10%) by using a single exponential profile. Variations in the in-water transmission profile are explained primarily by estimates of the upper ocean chlorophyll concentration (r2 = 0.83). Mixed layer chlorophyll concentration influences attenuation of the in-water light field. Clouds play a secondary role by altering the shape of the incident solar spectrum. A comparison of solar transmission parameterizations indicates that use of a Jerlov type parameterization for the WWP can lead to a 15 W m−2 error in the solar flux at 30 m (based on a climatological surface irradiance of 220 W m−2). Application of a one-dimensional ocean mixed layer model gives a mean sea surface temperature (SST) error of 0.15°C when the model is forced with a typical Jerlov type solar transmission profile. Instantaneous SST differences nearly reach 1.0°C. An increase in solar flux divergence for a mixed layer results in enhanced stratification and mixed layer shoaling. As mixed layer depth decreases, the quantity of solar radiation penetrating beyond the mixed layer increases, resulting in a destabilization of thermocline waters and a mixed layer depth increase. This feedback mechanism helps regulate both mixed layer depth and the solar irradiance lost to the mixed layer through penetration.

A hybrid parameterization for the determination of in-water solar fluxes is developed and applied to compute the flux of solar radiation that penetrates beyond the upper-ocean mixed layer into permanent pycnocline waters on global space and climatological timescales. The net flux of solar radiation at depth is modeled using values of the solar flux incident at the sea surface, derived from the International Satellite Cloud Climatology Project dataset, and in-water attenuation coefficients, determined using upper ocean chlorophyll concentration supplied by Coastal Zone Color Scanner imagery. Solar radiation penetration can be a significant term (20 W m−2) in the mixed layer heat budget for tropical regions. In mid- and high-latitude regions, the annual solar flux entering permanent pycnocline waters is small (

[1] Relative dispersion statistics and related Lagrangian parameters, not well observed in coastal regions, are obtained from in situ surface drifter observations and presented in the context of Lagrangian stochastic models. Clusters of GPS tracked surface drifters, with initial horizontal spacing of 5–10 m, were repetitively deployed in the Santa Barbara Channel from July 2004 to June 2005. The drifters sampled their position every 10 min for 1–2 days. Mean square pair separation distance, or relative dispersion, increases approximately exponentially in time during the first ∼5 h of sampling (e-folding time of 0.9 h). Thereafter, the dispersion increase is approximately quadratic in time. Large error bars on the observed mean dispersion, and higher-order Lagrangian statistics that are not clearly supportive of the aforementioned dispersion curves, indicate uncertainty. The mean square relative (separation) velocity shows near-linear growth with pair separation distance, extending from 0.3 to 85 cm2 s−2over length scales from ∼8 m to 2.2 km. The observed length scale dependency in square relative velocity is investigated in a Lagrangian stochastic model (LSM) for a cloud of particles. Modeled dispersion agrees with observations only when the velocity scale for the sub-grid scale random normal deviate in the LSM (typically a constant) is length scale dependent, and takes into consideration the observed scaling. Occasional large (>25 cm s−1) discrepancies in grid-scale velocities between drifters and HF radar cause general disagreement in distributions of ending positions of LSM trajectories when compared with Lagrangian observations.

[1] A purely Lagrangian assessment of dispersion from modeled surface current trajectories in the coastal ocean is presented. Modeled trajectories come from ROMS simulations for the Southern California Bight during the 1996 through 1999 period. Data are from surface current trajectories collected primarily in the Santa Barbara Channel with CODE style drifters. Distributions of particle positions from trajectories emanating from launch locations within 10 kilometers of the coast throughout the Santa Barbara Channel that advect for one through four days (Lagrangian PDFs) are evaluated descriptively and quantitatively. The two dimensional Kolmogorov-Smirnov (K-S) statistical test for comparing discrete sampled data with a known probability distribution is the quantitative basis. In general, dispersion distributions from observations are similar to Lagrangian PDFs computed from modeled trajectories and the K-S statistic quantifies this accordingly. A few specific regions of poor model-data agreement are indicated and discussed. The purely Lagrangian assessment, elucidates an improved understanding of model performance and ocean circulation beyond that offered in a Eulerian sense, and is necessary when modeled trajectories are utilized for applied oceanographic and marine ecology problems.

Oceanic ecosystems altered by interdecadal climate variability may provide a feedback to the physical climate by phytoplankton affecting heat fluxes into the upper ocean and dimethylsulfide fluxes into the atmosphere

Recent optical, physical, and biological oceanographic observations are used to assess the magnitude and variability of the penetrating flux of solar radiation through the mixed layer of the warm water pool (WWP) of the western equatorial Pacific Ocean. Typical values for the penetrative solar flux at the climatological mean mixed layer depth for the WWP (30 m) are approx. 23 W/sq m and are a large fraction of the climatological mean net air-sea heat flux (approx. 40 W/sq m). The penetrating solar flux can vary significantly on synoptic timescales. Following a sustained westerly wind burst in situ solar fluxes were reduced in response to a near tripling of mixed layer phytoplankton pigment concentrations. This results in a reduction in the penetrative flux at depth (5.6 W/sq m at 30 m) and corresponds to a biogeochemically mediated increase in the mixed layer radiant heating rate of 0.13 C per month. These observations demonstrate a significant role of biogeochemical processes on WWP thermal climate. We speculate that this biogeochemically mediated feedback process may play an important role in enhancing the rate at which the WWP climate system returns to normal conditions following a westerly wind burst event.