Analysis of AVHRR, CZCS and historical in situ data off the Oregon Coast

The original scientific objectives of this grant were to: (1) characterize the seasonal cycles and interannual variability for phytoplankton concentrations and sea surface temperature (SST) in the California Current using satellite data; and (2) to explore the spatial and temporal relationship between these variables and surface wind forcing. An additional methodological objective was to develop statistical methods for forming mean fields, which minimize the effects of random data gaps and errors in the irregularly sampled CZCS (Coastal Zone Color Scanner) and AVHRR (Advanced Very High Resolution Radiometer) satellite data. A final task was to evaluate the level of uncertainty in the wind fields used for the statistical analysis. Funding in the first year included part of the cost of an image processing system to enable this and other projects to process and analyze satellite data. This report consists of summaries of the major projects carried out with all or partial support from this grant. The appendices include a list of papers and professional presentations supported by the grant, as well as reprints of the major papers and reports

The Impact of SST Specification on ECMWF Surface Wind Stress Fields in the Eastern Tropical Pacific

The impact of SST specification on low-level winds in the operational ECMWF numerical weather prediction model is investigated in the eastern tropical Pacific from comparisons of ECMWF wind stress fields with QuikSCAT satellite scatterometer observations of wind stress during the August–December cold seasons of 2000 and 2001. These two time periods bracket the 9 May 2001 change from the Reynolds SST analyses to the Real-Time Global SST (RTG_SST) analyses as the ocean boundary condition in the ECMWF model. The ocean–atmosphere interaction in the eastern tropical Pacific that is clearly evident in QuikSCAT wind stress divergence and curl fields is also evident in the ECMWF winds, but is more than twice as strong in the 2001 cold season as in the 2000 cold season, due primarily to the improved spatial and temporal resolution of the RTG_SST analyses compared with the Reynolds SST analyses. While a significant improvement compared with 2000, the response of the 2001 ECMWF wind stress field to SST is only about half as strong as the coupling inferred from QuikSCAT data and satellite observations of SST from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). It is concluded that the underrepresentation of the ocean–atmosphere coupling is attributable partly to underrepresentation of SST gradients in the RTG_SST fields and partly to inadequacies of the ECWMF model. The latter may be due to errors in the parameterization of boundary layer processes or to insufficient horizontal or vertical resolution in the model

Seasonal variability of alongshore geostrophic velocity off central California

Seasonal variability of alongshore geostrophic velocity relative to 500 dbar is examined from 23 years of hydrographic data along two sections off central California (one off Point Sur and the other off Point Conception). The seasonal cycles are determined by least square fits of the gappy data records to harmonics with annual and semiannual periods. Attention is focused on variability over the outer continental slope. Geostrophic flow in the upper 100 m along both sections is coherent and predominantly annual with equatorward flow From February to September and poleward flow from October to January. The flow deeper than 100 m is distinctly different along the two sections. The nearshore deep flow is predominantly semiannual off Point Conception (notably different from surface flow) and poleward all year with maxima in December and June. This semiannual variability at depth is a prevalent feature of the California Current system. The nearshore deep flow off Point Sur is unusual in that it is predominantly annual with maximum poleward flow in December and weak equatorward flow from March to May. The seasonal variations over the continental slope compare favorably with existing models of eastern boundary current systems in two respects: (1) The surface flow leads the predominantly annual wind forcing in this region by about one month; and (2) the deep poleward counterfiow is coherent with the local poleward barotropic pressure gradient at both locations, with a phase lag of approximately 2 months. However, contrary to the results of existing dynamical models, the semiannual poleward pressure gradient and undercurrent off Point Conception appear to be unrelated to the wind forcing, which is predominantly annual in this region. These semiannual variations are highly coupled to the semiannual flow throughout the water column inside the Southern California Bight, suggesting that they may be topographically generated.Copyrighted by American Geophysical Union

Central California Coastal Circulation Study : drifter observations, February, July, October 1984 and January 1985

Surface drifters were depldyed and tracked over the continental shelf and upper continental slope off central California between Point Conception and Point Sur. The drifters were deployed and tracked from aircraft by Aero-Marine Surveys, Inc. under subcontract to Raytheon Service Company as part of the Central California Coastal Circulation Study sponsored by the Minerals Management Service. The objectives of this 18-month field program were to obtain a set of observations of the ocean water mass and velocity fields and develop a detailed description of these fields and their seasonal and shorter period variations. The ultimate goal is to assess the impact of exploitation of offshore oil and gas resources of the outer continental shelf region. This data report contains maps of drifter trajectories for a total of 78 drifters deployed and tracked during four separate months (February, July and October 1984, and January 1985). Also included are time series plots of winds during the times of deployment

Comment on "seasonal variation in wind speed and sea state from global satellite measurements"

In a recent paper, Sandwell and Agreen [1984; hereafter SA] presented figures of global seasonal wind speed and sea state as measured by the GEOS 3 satellite altimeter. Since that time, Chelton and McCabe [1985; hereafter CM] have found that problems exist in the algorithms used to retrieve wind speed from altimeter measurements of radar backscatter. These problems were discovered too late to be of use in the analysis of SA. However, because they have a significant impact on the accuracy of wind speed estimation from altimeters, it is important that they he pointed out now so that the results of SA are not misused. Although the results presented here do not alter many of the conclusions of SA in a qualitative sense, they do become important for any quantitative interpretation of the seasonal winds presented by SA. In addition, the data distribution maps presented here (Figures 3a-3i) are useful for pointing out limitations in other applications of GEOS 3 data (e.g., use of the altimeter sea level measurements to study surface geostrophic currents)

The sea state bias in altimeter estimates of sea level from collinear analysis of TOPEX data

The wind speed and significant wave height (H1/3) dependencies of the sea state is in altimeter estimates of sea level, expressed in the form ∆hSSB=bH1/3, are examined from least squares analysis of 21 cycles of collinear TOPEX data. The bias coefficient b is found to increase in magnitude with increasing wind speed up to about 12 m s–1 and decrease monotonically in magnitude with increasing H1/3. A parameterization of b as a quadratic function of wind speed only, as in the formulation used to produce the TOPEX geophysical data records (GDRs), is significantly better than a parameterization purely in terms of H1/3. However, a four-parameter combined wind speed and wave height formulation for b (quadratic in wind speed plus linear in H1/3) significantly improves the accuracy of the sea state bias correction. The GDR formulation in terms of wind speed only should therefore be expanded to account for a wave height dependence of b. An attempt to quantify the accuracy of the sea state bias correction ∆hSSB concludes that the uncertainty is a disconcertingly large 1% of H1/3

The effects of uncorrelated measurement noise on SWOT estimates of sea-surface height, velocity and vorticity

Author Posting. © American Meteorological Society, 2022. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of the Atmospheric and Oceanic Technology 39(7), (2022): 1053–1083, https://doi.org/10.1175/jtech-d-21-0167.1.The Ka-band Radar Interferometer (KaRIn) on the Surface Water and Ocean Topography (SWOT) satellite will revolutionize satellite altimetry by measuring sea surface height (SSH) with unprecedented accuracy and resolution across two 50-km swaths separated by a 20-km gap. The original plan to provide an SSH product with a footprint diameter of 1 km has changed to providing two SSH data products with footprint diameters of 0.5 and 2 km. The swath-averaged standard deviations and wavenumber spectra of the uncorrelated measurement errors for these footprints are derived from the SWOT science requirements that are expressed in terms of the wavenumber spectrum of SSH after smoothing with a filter cutoff wavelength of 15 km. The availability of two-dimensional fields of SSH within the measurement swaths will provide the first spaceborne estimates of instantaneous surface velocity and vorticity through the geostrophic equations. The swath-averaged standard deviations of the noise in estimates of velocity and vorticity derived by propagation of the uncorrelated SSH measurement noise through the finite difference approximations of the derivatives are shown to be too large for the SWOT data products to be used directly in most applications, even for the coarsest footprint diameter of 2 km. It is shown from wavenumber spectra and maps constructed from simulated SWOT data that additional smoothing will be required for most applications of SWOT estimates of velocity and vorticity. Equations are presented for the swath-averaged standard deviations and wavenumber spectra of residual noise in SSH and geostrophically computed velocity and vorticity after isotropic two-dimensional smoothing for any user-defined smoother and filter cutoff wavelength of the smoothing.This research was supported by NASA Grant NNX16AH76G

Large-scale interannual physical and biological interaction in the California Current

Thirty years of temperature, salinity, steric height and zooplankton data are examined to explore the potential causes of large-scale biological variability in the California Current. The physical and biological properties are all found to be dominated by a pronounced interannual signal with very large spatial scale...

Satellite observations of chlorophyll, phytoplankton biomass, and Ekman pumping in nonlinear mesoscale eddies

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 6349–6370, doi:10.1002/2013JC009027.Nonlinear mesoscale eddies can influence biogeochemical cycles in the upper ocean through vertical and horizontal advection of nutrients and marine organisms. The relative importance of these two processes depends on the polarity of an eddy (cyclones versus anticyclones) and the initial biological conditions of the fluid trapped in the core of the eddy at the time of formation. Eddies originating in the eastern South Indian Ocean are unique in that anticyclones, typically associated with downwelling, contain elevated levels of chlorophyll-a, enhanced primary production and phytoplankton communities generally associated with nutrient-replete environments. From analysis of 9 years of concurrent satellite measurements of sea surface height, chlorophyll, phytoplankton carbon, and surface stress, we present observations that suggest eddy-induced Ekman upwelling as a mechanism that is at least partly responsible for sustaining positive phytoplankton anomalies in anticyclones of the South Indian Ocean. The biological response to this eddy-induced Ekman upwelling is evident only during the Austral winter. During the Austral summer, the biological response to eddy-induced Ekman pumping occurs deep in the euphotic zone, beyond the reach of satellite observations of ocean color.This work was funded by NASA grants NNX08AI80G, NNX08AR37G, NNX10AO98G, and NNX13AD78G.2014-06-0

Observing large-scale temporal variablity of ocean currents by satellite altimetry : with application to the Antarctic circumpolar current

A new method is developed for studying large-scale temporal variability of ocean currents from satellite altimetric sea level measurements at intersections (crossovers) of ascending and descending orbit ground tracks. Using this method, sea level time series can be constructed from crossover sea level differences in small sample areas where altimetric crossovers are clustered. The method is applied to Seasat altimeter data to study the temporal evolution of the Antarctic Circumpolar Current (ACC) over the 3-month Seasat mission (July-October 1978). The results reveal a generally eastward acceleration of the ACC around the Southern Ocean with meridional disturbances which appear to be associated with bottom topographic features. This is the first direct observational evidence for large-scale coherence in the temporal variability of the ACC. It demonstrates the great potential of satellite altimetry for synoptic observation of temporal variability of the world ocean circulation