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Robert A. Weller

Abstract

Oscillations with near-inertial frequencies were an energetic component of the upper ocean velocity field observed at each of two moorings separated by 44 km during the Joint Air Sea INteraction (JASIN) experiment during the late summer of 1978. At each mooring the amplitude of the inertial motion was highest in the mixed layer, where it was nearly depth-independent. Previous work (Pollard and Millard, 1970; Pollard, 1980) had found that the amplitude and phase of inertial motion in the mixed layer was related to the local wind stress. In this case, the loacal winds, measured at each mooring, were coherent; but the time series of mixed-layer, near-inertial motion at one mooring bore little resemblance to that at the other mooring. During JASIN 1978 the differences in the inertial response at the two moorings coincided with differences in the quasi-geostrophic flow field in the vicinity of the two moorings. Inclusion of the horizontal gradients of the quasi-geostrophic flow in model equations provides a source of damping and frequency shifting. Divergence in the quasi-geostrophic flow damps the inertial motion. Vorticity in the quasi-geostrophic flow shifts the frequency of the inertial response to either above or below the local inertial frequency. Upward flow was observed at one mooring as the boundary between two adjacent eddies passed the mooring site, and that observed vertical velocity was used to estimate divergence. Using that divergence, the model equation Save a prediction similar to the inertial response observed at that mooring; both observation and prediction had lower amplitude than anticipated from the observed wind stress.

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Robert A. Weller

Abstract

Observations of the horizontal velocity field in the upper 150 m, made from the Research Platform FLIP as it drifted of the coast of Baja California, were used to examine the velocity of the velocity field and its relation to local wind forcing. At subinertial frequencies a spatially varying flow field associated with the California Current System was encountered. In addition, there was low frequency near-surface flow to the right of the wind stress that decayed with depth. At near-inertial frequencies victory motion with an amplitude of up to 40 cm s−1 was observed. Most of the energy in the near-inertial frequency band was associated with modes with vertical wavelengths large compared to the thickness of the mixed layer. The local wind alone had neither the strength nor the variability needed to directly produce the observed inertial period variability. It is suggested that FLIP encountered regions in which the shear of the quasi-geostrophic flow resulted in localized intensifications of near-inertial motion.

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Robert A. Weller

Abstract

Time series of surface meteorology and air–sea exchanges of heat, freshwater, and momentum collected from a long-term surface mooring located 1600 km west of the coast of northern Chile are analyzed. The observations, spanning 2000–10, have been withheld from assimilation into numerical weather prediction models. As such, they provide a unique in situ record of atmosphere–ocean coupling in a trade wind region characterized by persistent stratocumulus clouds. The annual cycle is described, as is the interannual variability. Annual variability in the air–sea heat flux is dominated by the annual cycle in net shortwave radiation. In austral summer, the ocean is heated; the 9-yr mean annual heating of the ocean is 38 W m−2. Ocean cooling is seen in 2006–08, coincident with La Niña events. Over the full record, significant trends were found. Increases in wind speed, wind stress, and latent heat flux over 9 yr were 0.8 m s−1, 0.022 N m−2, and 20 W m−2 or 13%, 29%, and 20% of the respective 9-yr means. The decrease in the annual mean net heat flux was 39 W m−2 or 104% of the mean. These changes were found to be largely associated with spring and fall. If this change persists, the annual mean net air–sea heat flux will change sign by 2016, when the magnitude of the wind stress will have increased by close to 60%.

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Hampton N. Shirer and Robert Wells

Abstract

Maximally truncated spectral models have been used recently by fluid and atmospheric dynamicists to study nonlinear behavior of the governing partial differential system. However, too few external control parameters may be available in the truncated model to describe adequately the steady states near singular parameter values at which two or more stationary solutions meet. These missing parameters correspond in many cases to small but significant physical effects whose inclusion may be critically important for the model results to be realistic. We apply to truncated spectral models a recently developed contact catastrophe method that allows determination of the crucial physical effects that govern the steady states of a fluid system.

Spectral systems of three different fluid flow models of interest in atmospheric science are considered. Two parameters are necessary for modeling Rayleigh-Bénard convection. One represents the magnitude of the horizontal component, the other the magnitude of the vertical component of the externally imposed heating. Four parameters are required for modeling axisymmetric flow in either a rotating annulus or the atmosphere if the Prandtl number σ and the aspect ratio a are related by σa < 1. These are the horizontal and vertical components of the external heating, the Coriolis parameter, and either the inclination angle of the vessel (annulus) or the Newtonian heating rate (atmosphere). Four parameters are essential for modeling quasi-geostrophic flow in a channel. They are the three Fourier coefficients of the Newtonian heating rate and the amplitude of a superimposed time-independent zonal current.

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Keir Colbo and Robert A. Weller

Abstract

The accuracies of the meteorological sensors (air temperature, relative humidity, barometric pressure, near-surface temperature, longwave and shortwave radiation, and wind speed and direction) that compose the Improved Meteorological (IMET) system used on buoys at long-term ocean time series sites known as ocean reference stations (ORS) are analyzed to determine their absolute error characteristics. The predicted errors are compared to in situ measurement discrepancies and other observations (direct flux shipboard sensors) to confirm the predictions. The meteorological errors are then propagated through bulk flux formulas and the Coupled Ocean–Atmosphere Response Experiment (COARE) algorithm to give predicted errors for the heat flux components, the freshwater flux, and the momentum flux. Absolute errors are presented for three frequency bands [instantaneous (1-min sampling), diurnal, and annual]. The absolute uncertainty in the annually averaged net heat flux is found to be 8 W m−2 for conditions similar to the current ORS deployments in the subtropics.

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Lisan Yu and Robert A. Weller

A 25-yr (1981–2005) time series of daily latent and sensible heat fluxes over the global ice-free oceans has been produced by synthesizing surface meteorology obtained from satellite remote sensing and atmospheric model reanalyses outputs. The project, named Objectively Analyzed Air–Sea Fluxes (OAFlux), was developed from an initial study of the Atlantic Ocean that demonstrated that such data synthesis improves daily flux estimates over the basin scale. This paper introduces the 25-yr heat flux analysis and documents variability of the global ocean heat flux fields on seasonal, interannual, decadal, and longer time scales suggested by the new dataset.

The study showed that, among all the climate signals investigated, the most striking is a long-term increase in latent heat flux that dominates the data record. The globally averaged latent heat flux increased by roughly 9 W m−2 between the low in 1981 and the peak in 2002, which amounted to about a 10% increase in the mean value over the 25-yr period. Positive linear trends appeared on a global scale, and were most significant over the tropical Indian and western Pacific warm pool and the boundary current regions. The increase in latent heat flux was in concert with the rise of sea surface temperature, suggesting a response of the atmosphere to oceanic forcing.

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Anand Gnanadesikan and Robert A. Weller

Abstract

The physical processes responsible for maintaining the mixed layer am examined by considering the velocity structure. The low-frequency Ekman response in the interior of unstratified mixed layers is much less sheared than is predicted using eddy viscosity models that reproduce the temperature structure. However, the response is more sheared than predicted by models that parameterize the mixed layer as a slab. An explanation is sought by considering the effect of an infinite train of surface gravity waves on the mean Ekman spiral. For some realistic conditions, the Ekman spiral predicted by assuming small-scale diffusion alone is strongly unstable to Langmuir cells driven by wave-current interaction. In the Northern Hemisphere, these cells are oriented to the right of the wind, the result of a balance between maximizing the wave-current forcing, maximizing the efficiency of this forcing in producing cells, and minimizing the crosscell shear. The cells are capable of replacing small-scale turbulent diffusion as the principal transport mechanism within the mixed layer. Finite-difference code runs that include infinite-length trains of surface gravity waves qualitatively explain the reduction in shear within the mixed layer relative to that predicted by small-scale mixing. However, the theory also predicts an Eulerian return flow balancing the Stokes drift that has not been observed.

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Jerome Smith, Robert Pinkel, and Robert A. Weller

Abstract

During October–November 1983, a MIxed Layer Dynamics EXperiment (MILDEX) took place off the coast of California. As a part of MILDEX, a number of sensors were deployed from the Research Platform FLIP in an effort to monitor the flow structure in the near-surface mixed layer.

Profiling current meters (VMCMs) measured velocities down to 150 m on an hourly basis. Other VMVMs were set at fixed depths. Depths of interest were examined using a package which measured three-component velocities and displayed them in FLIP's laboratory in real time. The current meters often observed sequences of downwind-directed jets, with maximum velocities exceeding 25 cm s−1. Downwelling velocities of equal magnitude were also observed. The strongest currents were found 10 to 35 m below the surface, at mid-depth in the mixed layer. Surface convergences associated with these jets were visualized by scattering tracers (computer cards) on the sea surface. These features suggest Langmuir circulation.

Six Doppler sonars were mounted on FLIP's hull. One was directed such that the beam grazed the sea surface. Surface velocities were measured in the crosswind direction over 600 to 1400 m from FLIP. During a period of strong Langmuir circulation, the sonar detected crosswind surface convergences at scales up to about 3 times the mixed layer depth. This ratio of maximum spacing to mixed layer depth stayed roughly constant as the mixed layer depth varied from 40 to 60 m. Peak convergences were of order ±0.003 s−1 at the largest scales. The features moved with the mixed layer, and persisted for up to 2 hours each. FLIP moved about 2 km straight downwind relative to the mixed layer in this time; thus, the features were about 2 km long (or more), and roughly parallel to the wind. This scenario is supported by data from a second sonar which measured the downwind component of the lower mixed-layer flows out to about 800 m downwind of FLIP.

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Lisan Yu, Robert A. Weller, and Bomin Sun

Abstract

A new daily latent and sensible flux product developed at the Woods Hole Oceanographic Institution (WHOI) with 1° × 1° resolution for the Atlantic Ocean (65°S–65°N) for the period from 1988 to 1999 was presented. The flux product was developed by using a variational objective analysis approach to obtain best estimates of the flux-related basic surface meteorological variables (e.g., wind speed, air humidity, air temperature, and sea surface temperature) through synthesizing satellite data and outputs of numerical weather prediction (NWP) models at the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF). The state-of-the-art bulk flux algorithm 2.6a, developed from the field experiments of the Coupled Ocean–Atmosphere Response Experiment (COARE), was applied to compute the flux fields.

The study focused on analyzing the mean field properties of the WHOI daily latent and sensible heat fluxes and their comparisons with the ship-based climatology from the Southampton Oceanography Centre (SOC) and NWP outputs. It is found that the WHOI yearly mean fluxes are consistent with the SOC climatology in both structure and amplitude, but the WHOI yearly mean basic variables are not always consistent with SOC; the better agreement in the fluxes is due to the effects of error compensation during variable combinations. Both ECMWF and NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP) Reanalysis-2 (NCEP2) model data have larger turbulent heat loss (∼20 W m−2) than the WHOI product. Nevertheless, the WHOI fluxes agree well with the NCEP-2 Reanalysis fluxes in structure and the trend of year-to-year variations, but not with the ECMWF operational outputs; the latter have a few abrupt changes coinciding with the modifications in the model forecast–analysis system. The degree of impact of the model changes on the basic variables is not as dramatic, a factor that justifies the inclusion of the basic variables, not the fluxes, from the ECMWF operational model in the synthesis. The flux algorithms of the two NWP models give a larger latent and sensible heat loss. Recalculating the NWP fluxes using the COARE algorithm considerably reduces the strength but does not replicate the WHOI results. The present analysis could not quantify the degree of improvement in the mean aspect of the WHOI daily flux fields as accurate basinwide verification data are lacking.

This study is the first to demonstrate that the synthesis approach is a useful tool for combining the NWP and satellite data sources and improving the mean representativeness of daily basic variable fields and, hence, the daily flux fields. It is anticipated that such an approach may become increasingly relied upon in the preparation of future high-quality flux products.

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Lisan Yu, Xiangze Jin, and Robert A. Weller

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The present study used a new net surface heat flux (Q net) product obtained from the Objective Analyzed Air–Sea Fluxes (OAFlux) project and the International Satellite Cloud Climatology Project (ISCCP) to examine two specific issues—one is to which degree Q net controls seasonal variations of sea surface temperature (SST) in the tropical Atlantic Ocean (20°S–20°N, east of 60°W), and the other is whether the physical relation can serve as a measure to evaluate the physical representation of a heat flux product. To better address the two issues, the study included the analysis of three additional heat flux products: the Southampton Oceanographic Centre (SOC) heat flux analysis based on ship reports, and the model fluxes from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The study also uses the monthly subsurface temperature fields from the World Ocean Atlas to help analyze the seasonal changes of the mixed layer depth (h MLD).

The study showed that the tropical Atlantic sector could be divided into two regimes based on the influence level of Q net. SST variability poleward of 5°S and 10°N is dominated by the annual cycle of Q net. In these regions the warming (cooling) of the sea surface is highly correlated with the increased (decreased) Q net confined in a relatively shallow (deep) h MLD. The seasonal evolution of SST variability is well predicted by simply relating the local Q net with a variable h MLD. On the other hand, the influence of Q net diminishes in the deep Tropics within 5°S and 10°N and ocean dynamic processes play a dominant role. The dynamics-induced changes in SST are most evident along the two belts, one of which is located on the equator and the other off the equator at about 3°N in the west, which tilts to about 10°N near the northwestern African coast.

The study also showed that if the degree of consistency between the correlation relationships of Q net, h MLD, and SST variability serves as a measure of the quality of the Q net product, then the Q net from OAFlux + ISCCP and ERA-40 are most physically representative, followed by SOC. The NCEP–NCAR Q net is least representative. It should be noted that the Q net from OAFlux + ISCCP and ERA-40 have a quite different annual mean pattern. OAFlux + ISCCP agrees with SOC in that the tropical Atlantic sector gains heat from the atmosphere on the annual mean basis, where the ERA-40 and the NCEP–NCAR model reanalyses indicate that positive Q net occurs only in the narrow equatorial band and in the eastern portion of the tropical basin. Nevertheless, seasonal variances of the Q net from OAFlux + ISCCP and ERA-40 are very similar once the respective mean is removed, which explains why the two agree with each other in accounting for the seasonal variability of SST.

In summary, the study suggests that an accurate estimation of surface heat flux is crucially important for understanding and predicting SST fluctuations in the tropical Atlantic Ocean. It also suggests that future emphasis on improving the surface heat flux estimation should be placed more on reducing the mean bias.

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