Donna L. Witter

Kent State University

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Geosat altimeter observations of the surface circulation of the Southern Ocean

Journal of Geophysical Research Atmospheres (1990)

Dudley B. Chelton Michael G. Schlax Donna L. Witter James G. Richman

The variability of sea level and surface geostrophic currents in the Southern Ocean is investigated from the first 26 months of unclassified Geosat altimeter data (November 1986 to December 1988). Because of problems unique to Geosat, it has been necessary to develop new techniques for analyzing the height data. These techniques are presented here, and the processed Geosat data are used to examine the relation between mesoscale variability and the mean circulation (as determined from historical hydrographic data). The two are shown to be significantly correlated, implicating the importance of hydrodynamic instabilities in the Antarctic Circumpolar Current. The geographical patterns of both the mean flow and the mesoscale variability are shown to be controlled by the bathymetry. An efficient objective analysis algorithm is introduced for generating smoothed fields from observations randomly distributed in time and two space dimensions. The algorithm is applied to the 26 months of Geosat data, and the smoothed fields are used to investigate the large‐scale, low‐frequency variability of sea level and surface geostrophic velocity in the Southern Ocean. Approximately 33% of the variance is accounted for by the first three empirical orthogonal functions (EOFs) of sea level variability. These three modes describe variability over seasonal time scales and separate into an annual cycle (mode 1), a semiannual cycle (mode 2) and a mode which describes year‐to‐year variability in the seasonal cycles for 1987 and 1988 (mode 3). The complexity of the spatial patterns of the second‐ and higher‐order modes and the small percentages of variance accounted for by the first three modes (15%, 10% and 8%, respectively) reflect the generally regional, as opposed to coherent circumpolar, nature of sea level variability in the Southern Ocean. The inherent weak zonal coherence of the variability is further emphasized by EOF analysis separately within each basin of the Southern Ocean.

Empirical orthogonal functions

Sea-surface height

Mode (computer interface)

Geostrophic current

Annual cycle

10.1029/jc095ic10p17877

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Citations (247)

Energetics of Unstable Waves in the Antarctic Circumpolar Current

AGUFM (2002)

Donna L. Witter

Energetics

Circumpolar star

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Eddy Kinematics During the Agulhas-South Atlantic Thermohaline Transport Experiment (ASTTEX): Sea Level Variability Along the ASTTEX Mooring Line and Beyond

AGUFM (2006)

Donna L. Witter

Mooring

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Unstable jet flow along zonal ridge topography

Donna L. Witter

Zonal flow (plasma)

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Citations (1)

Signal and noise in tropical Pacific sea level height analyses

Alexey Kaplan Cane D. Chen Donna L. Witter Robert E. Cheney

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Citations (1)

Interannual variability of South Atlantic circulation from 4 years of TOPEX/POSEIDON satellite altimeter observations

Journal of Geophysical Research Atmospheres (1999)

Donna L. Witter Arnold L. Gordon

Variability of large‐scale and regional South Atlantic circulation is investigated using TOPEX/POSEIDON sea level observations. Interannual variations are identified from empirical orthogonal functions of gridded sea level fields, year‐to‐year fluctuations of root‐mean‐square sea level variability, and variability of Agulhas eddies evaluated from the along‐track data. Two modes of variability are identified. A basin‐scale mode indicates that sea level in the eastern South Atlantic underwent a transition from a state of high sea level and enhanced gyre‐scale geostrophic circulation in 1993 and 1994, to a state of lower sea level and more sluggish circulation in 1996. The dominant mode of basin‐scale zonal wind has the same temporal signature, suggesting a link between the observed variation of gyre‐scale circulation and the regional wind forcing. Time variations of this mode also coincide with a transition from a broad Agulhas eddy corridor observed in 1993 and 1994 to a narrower corridor observed in 1996. The input of salt and vorticity to the South Atlantic subtropical gyre via Agulhas eddies may therefore be partially controlled by interannual variations of the wind‐forced, large‐scale circulation. A second mode isolates interannual variations in the Brazil‐Malvinas Confluence region. During 1993, eddy variability along the Brazil Current extension was relatively strong and variability along the continental slope was weak. The opposite pattern was observed in 1995. These variations may be related to interannual variations of the latitude of the confluence. While variations associated with both modes are smaller than those observed on seasonal timescales, these interannual variations contribute significantly to the total South Atlantic variability.

Eddy

Empirical orthogonal functions

Anticyclone

Sea-surface height

Oceanic basin

Gulf Stream

10.1029/1999jc900023

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Citations (40)

Sea Surface Height Variability in the eastern South Atlantic from Satellite and in situ Measurements

AGU Fall Meeting Abstracts (2011)

S. Baker‐Yeboah D. Randolph Watts Deirdre A. Byrne Donna L. Witter

Sea-surface height

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Citations (1)

Assessing the Application of SeaWiFS Ocean Color Algorithms to Lake Erie

AGU Fall Meeting Abstracts (2008)

Donna L. Witter Joseph D. Ortiz Sarah Palm Robert T. Heath Judith W. Budd

The feasibility of satellite-based monitoring of phytoplankton chlorophyll a concentrations in Lake Erie is assessed by applying globally calibrated, ocean-derived color algorithms to spatially and temporally collocated measurements of SeaWiFS remote sensing reflectance. Satellite-based chlorophyll a retrievals were compared with fluorescence-based measurements of chlorophyll a from 68 field samples collected across the lake between 1998 and 2002. Twelve ocean-derived color algorithms, one regional algorithm derived for the Baltic Sea's Case 2 waters, and a set of regional algorithms developed for the western, central and eastern basins of Lake Erie were considered. While none of the ocean-derived algorithms performed adequately, the outlook for the success of regionally calibrated and validated algorithms, with forms similar to the ocean-derived algorithms, is promising over the eastern basin and possibly the central basin of the lake. In the western basin, each of the regional algorithms considered performed poorly, indicating that alternative approaches to algorithm development, or to satellite data screening and analysis procedures will be needed.

SeaWiFS

Ocean color

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Signal and noise in tropical Pacific sea level height fields from satellite altimetry and ocean models

EAEJA (2003)

Alexey Kaplan Mark A. Cane D. Chen Donna L. Witter Robert E. Cheney

Satellite Altimetry

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Multivariate approach to estimate colour producing agents in Case 2 waters using first-derivative spectrophotometer data

Geocarto International (2012)

Khalid A. Ali Donna L. Witter Joseph D. Ortiz

Abstract The complex composition and distribution of colour producing agents (CPAs) in turbid aquatic environments such as the Western Basin of Lake Erie (WBLE) presents a challenge to the application of remote sensing data for differentiating among in-water constituents and estimating their concentrations independently. In this study, multivariate procedures are applied to lab-based spectrophotometer data to estimate the concentration of chlorophyll-a and suspended matters in the WBLE. Principal Component Analysis of first-derivative transformed hyper-spectral data from the spectrophotometer extracted three significant spectral components for each cruise, explaining up to 88% of the spectral variability. Spectral matching using reference spectra indicated that two of the extracted patterns represent signatures of in-water constituents that govern the optical properties of the WBLE, namely, cyanobacteria and diatoms associated with green algae. The spectrophotometer data clearly revealed known spectral features associated with phytoplankton, such as the absorption minima near 550 and 700 nm, which can be attributed to the minimum of absorption and fluorescence of chlorophyll-a, respectively. The method also extracted the absorption peaks due to chlorophyll-a, near 670 nm, and due to phycocyanin, near 620 nm. Principal component regression of chlorophyll-a on the PC scores indicated that 63.4% of variation of chlorophyll-a in the WBLE can be explained by two components. Factors 2 and 3 explain 60% of the joint spatiotemporal variability of suspended matters in the WBLE. The results illustrate the potential of multivariate technique applied to remote sensing data in isolating the patterns that represent constituents in turbid Case 2 waters. Keywords: chlorophyll-aremote sensingLake EriePCA Acknowledgements The authors thank M. Thomas, Captain of the Stone Laboratory research vessels, Stone Laboratories, and Ohio SeaGrant for access and funds required to collect the samples.

Maxima and minima

10.1080/10106049.2012.743601

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Citations (21)