J. Carter Ohlmann

Solar radiation, phytoplankton pigments and the radiant heating of the equatorial Pacific warm pool

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.

Ocean Radiant Heating. Part II: Parameterizing Solar Radiation Transmission through the Upper Ocean

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.

Eddy energy and shelf interactions in the Gulf of Mexico

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.

Ocean Radiant Heating in Climate Models

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 3 1023 and 1.5 3 1022, 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 m22, 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 m22 for the eastern and western Pacific, respectively (both cases assume a climatological surface flux of 200 W m 22). 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.

Potential Feedbacks Between Pacific Ocean Ecosystems and Interdecadal Climate Variations.

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

Ocean Radiant Heating. Part I: Optical Influences

Abstract 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-...

GPS-Cellular Drifter Technology for Coastal Ocean Observing Systems

Abstract 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 val...

Radiant heating of the western equatorial Pacific during TOGA‐COARE

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.

Cloud Color and Ocean Radiant Heating

It is well recognized that clouds regulate the flux of solar radiation reaching the sea surface. Clouds also affect the spectral distribution of incident irradiance. Observations of spectral and total incident solar irradiance made from the western equatorial Pacific Ocean are used to investigate the ‘‘color’’ of clouds and to evaluate its role in upper-ocean radiant heating. Under a cloudy sky, values of the near-ultraviolet to green spectral irradiance are a significantly larger fraction of their clear-sky flux than are corresponding clear-sky fractions calculated for the total solar flux. For example, when the total solar flux is reduced by clouds to one-half of that for a clear sky, the near-ultraviolet spectral flux is only reduced ;35% from its clear-sky value. An empirical parameterization of the spectral cloud index is developed from field observations and is verified using a planeparallel, cloudy-sky radiative transfer model. The implications of cloud color on the determination of ocean radiant heating rates and solar radiation transmission are assessed using both model results and field determinations. The radiant heating rate of the upper 10 cm of the ocean (normalized to the climatological incident solar flux) may be reduced by a factor of 2 in the presence of clouds. This occurs because the near-infrared wavelengths of solar radiation, which are preferentially attenuated by clouds, are absorbed within the upper 10 cm or so of the ocean while the near-ultraviolet and blue spectral bands propagate farther within the water column. The transmission of the solar radiative flux to depth is found to increase under a cloudy sky. The results of this study strongly indicate that clouds must be included in the specification of ocean radiant heating rates for air‐sea interaction studies.

Simulations of Nearshore Particle-Pair Dispersion in Southern California

AbstractKnowledge 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 degr...