Dudley B. Chelton

Oregon State University, Corvallis, Oregon, United States

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Publications (106)345.99 Total impact

  • R. M. Samelson, M. G. Schlax, D. B. Chelton
    02/2014; 44(3).
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    ABSTRACT: [1] 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.
    Journal of Geophysical Research: Oceans. 11/2013;
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    ABSTRACT: Eighteen years of weekly SLA merged maps in the Western Mediterranean are analyzed using the new method proposed by Chelton et al. (2011) to identify and track mesoscale eddies. The method has been adapted to take into account the specificity of the Mediterranean basin. Results are similar to the global ocean results with a radius smaller due to a smaller Rossby radius. The areas of intense rotational speed and amplitude of eddies are similar to the areas of intense eddy kinetic energy calculated from altimetry sea level anomalies. Eddies propagation speed shows a wide range of values without a clear preferred direction. Nevertheless, eddies seems to propagate following the main currents. Temporal analysis of the number of eddies per day is made focusing on the annual and semi-annual variability. This annual and semi-annual cycle is analyzed using a regional model of the Mediterranean Sea and studying the interaction with atmospheric forcings.
    European Geophysical Union Assembly, Vienna; 04/2013
  • Journal of Climate 04/2013; 26(8):2514-2533. · 4.36 Impact Factor
  • Larry W. O'Neill, Dudley B. Chelton, Steven K. Esbensen
    Journal of Climate 09/2012; 25(17):5916-5942. · 4.36 Impact Factor
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    ABSTRACT: This paper examines the effect of "stencil width" on surface ocean geostrophic velocity and vorticity estimated from differentiating gridded satellite altimeter sea surface height products. In oceanographic applications, the value of the first derivative at a central grid point is generally obtained by differencing the sea surface heights at adjacent grid points. This is called a "three-point stencil centered difference". Here the stencil width is increased from three to five, seven, and nine points, using well-known formulae from the numerical analysis literature. The discrepancies between velocities computed with successive stencils decreases with increasing stencil width, suggesting that wide stencil results are more reliable. Significant speed-dependent biases (up to 10-20%) are found between results computed from three-point stencils versus those computed from wider stencils. The geostrophic velocity, and the variance of geostrophic velocity, are underestimated with thin stencils. Similar results are seen in geostrophic velocities computed from high-resolution model output. In contrast to the case when three-point stencils are used, wider stencils yield estimates of the anisotropy of velocity variance that are insensitive to the differences in grid spacing between two widely used altimeter products. Three-point stencils yield incorrect anisotropies on the 1/4° anisotropic AVISO grid; we recommend the use of 7-point stencils. Despite the demonstrated inadequacies of the three-point stencils, the conclusions of earlier studies based on them, that the zonally averaged midlatitude eddy kinetic energy field is nearly isotropic, are found to pertain also with wider stencils. Finally, the paper also examines the strengths and limitations of applying noise-suppressing differentiators, versus classic centered differences, to altimeter data.
    Journal of Geophysical Research (Oceans). 03/2012; 117(C3):3029-.
  • P. Gaube, D. B. Chelton, L. W. O'Neill
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    ABSTRACT: Numerous past studies have discussed the biological importance of upwelling of nutrients into the interiors of nonlinear eddies. Such upwelling can occur during the transient stages of formation of cyclones from shoaling of the thermocline. In their mature stages, upwelling can occur from Ekman pumping driven by eddy-induced wind stress curl. Previous investigations of ocean-atmosphere interaction in regions of persistent sea-surface temperature (SST) frontal features have shown that the wind field is locally stronger over warm water and weaker over cold water. Spatial variability of the SST field thus results in a wind stress curl and an associated Ekman pumping in regions of crosswind temperature gradients. It can therefore be anticipated that any SST anomalies associated with eddies can generate Ekman pumping in the eddy interiors. Another mechanism for eddy-induced Ekman pumping is the curl of the stress on the sea surface that arises from the difference between the surface wind velocity and the surface ocean velocity. While SST-induced Ekman upwelling can occur over eddies of either polarity surface current effects on Ekman upwelling occur only over anticyclonic eddies The objective of this study is to determine the spatial structures and relative magnitudes of the two mechanisms for eddy-induced Ekman pumping within the interiors of mesoscale eddies. This is achieved by collocating satellite-based measurements of SST, surface winds and wind stress curl to the interiors of eddies identified and tracked with an automated procedure applied to the sea-surface height (SSH) fields in the Reference Series constructed by AVISO from the combined measurements by two simultaneously operating altimeters. It is shown that, on average, the wind stress curl from eddy-induced surface currents is largest at the eddy center, resulting in Ekman pumping velocities of order 10 cm day-1. While this surface current-induced Ekman pumping depends only weakly on the wind direction, Ekman pumping from the wind stress curl associated with eddy-induced SST variations depends strongly on both the magnitudes of the SST anomalies and on the ambient wind direction. In some regions within the eddy interiors, the SST-induced wind stress curl acts to reduce the magnitude of the Ekman pumping from surface currents while in other regions it acts to reinforce the current induced Ekman pumping. In midlatitude regions, the Ekman pumping from eddy-induced SST variations is shown to be nearly an order of magnitude smaller than that associated with eddy-induced surface currents. Eddy-induced SST effects on Ekman pumping become more comparable to surface current effects at high latitudes, especially over the Antarctic Circumpolar Current
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Oceanic Rossby waves have been widely invoked as a mechanism for large-scale variability of chlorophyll (CHL) observed from satellites. High-resolution satellite altimeter measurements have recently revealed that sea-surface height (SSH) features previously interpreted as linear Rossby waves are nonlinear mesoscale coherent structures (referred to here as eddies). We analyze 10 years of measurements of these SSH fields and concurrent satellite measurements of upper-ocean CHL to show that these eddies exert a strong influence on the CHL field, thus requiring reassessment of the mechanism for the observed covariability of SSH and CHL. On time scales longer than 2 to 3 weeks, the dominant mechanism is shown to be eddy-induced horizontal advection of CHL by the rotational velocities of the eddies.
    Science 09/2011; 334(6054):328-32. · 31.20 Impact Factor
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    Jeffrey J. Early, R. ~M. Samelson, Dudley B. Chelton
    Journal of Physical Oceanography 01/2011; · 3.18 Impact Factor
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    ABSTRACT: Eddy detection and tracking algorithms are applied to both satellite altimetry and a high-resolution (dx = 5 km) climatological model solution of the U.S. West Coast to study the properties of surface and undercurrent eddies in the California Current System. Eddy properties show remarkable similarity in space and time, and even somewhat in polarity. Summer and fall are the most active seasons for undercurrent eddy generation, while there is less seasonal variation at surface. Most of the eddies have radii in the range of 25–100 km, sea level anomaly amplitudes of 1–4 cm, and vorticity normalized by f amplitudes of 0.025–0.2. Many of the eddies formed near the coast travel considerable distance westward with speeds about 2 km/day, consistent with the β effect. Anticyclones and cyclones show equatorward and poleward displacements, respectively. Long-lived surface eddies show a cyclonic dominance. The subsurface California Undercurrent generates more long-lived anticyclones than cyc
    Journal of Geophysical Research 01/2011; 116(C8). · 3.17 Impact Factor
  • Dudley B. Chelton, Schlax, Roger M. Michael G. Samelson
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    ABSTRACT: Sixteen years of sea-surface height (SSH) fields constructed by merging the measurements from two simultaneously operating altimeters are analyzed to investigate mesoscale variability in the global ocean. The prevalence of coherent mesoscale features (referred to here as "eddies") with radius scales of O(100 km) is readily apparent in these high-resolution SSH fields. An automated procedure for identifying and tracking mesoscale features based on their SSH signatures yields 35,891 eddies with lifetimes =>16 weeks. These long-lived eddies, comprising approximately 1.15 million individual eddy observations, have an average lifetime of 32 weeks and an average propagation distance of 550 km. Their mean amplitude and a speed-based radius scale as defined by the automated procedure are 8 cm and 90 km, respectively. The tracked eddies are found to originate nearly everywhere in the World Ocean, consistent with previous conclusions that virtually all of the World Ocean is baroclinically unstable. Overall, there is a slight preference for cyclonic eddies. However, there is a preference for the eddies with long lifetimes and large propagation distances to be anticyclonic. In the southern hemisphere, the distributions of the amplitudes and rotational speeds of eddies are more skewed toward large values for cyclonic eddies than for anticyclonic eddies. As a result, eddies with amplitudes >10 cm and rotational speeds >20 cm s -1 are preferentially cyclonic in the southern hemisphere. By contrast, there is a slight preference for anticyclonic eddies for nearly all amplitudes and rotational speeds in the northern hemisphere. On average, there is no evidence of anisotropy of these eddies. Their average shape is well represented as Gaussian within the central 2/3 of the eddy, but the implied radius of maximum rotational speed is 64% smaller than the observed radius of maximum speed. In part because of this mismatch between the radii of maximum axial speed in the observations and the Gaussian approximation, a case is made that a quadratic function that is a very close approximation of the mode profile of the eddy (i.e., the most frequently occurring value at each radius) is a better representation of the composite shape of the eddies. This would imply that the relative vorticity is nearly constant within the interiors of most eddies, i.e., the fluid motion consists approximately of solid-body rotation. Perhaps the most significant conclusion of this study is that essentially all of the observed mesoscale features outside of the tropical band 20°S-20°N are nonlinear by the metric U / c , where U is the maximum circum-average geostrophic speed within the eddy interior and c is the translation speed of the eddy. A value of U / c > 1 implies that there is trapped fluid within the eddy interior. Many of the extratropical eddies are highly nonlinear, with 48% having U / c > 5 and 21% having U / c > 10. Even in the tropics, approximately 90% of the observed mesoscale features are nonlinear by this measure. Two other nondimensional parameters also indicate strong degrees of nonlinearity in the tracked eddies. The distributions of all three measures of nonlinearity are more skewed toward large values for cyclonic eddies than for anticyclonic eddies in the southern hemisphere extratropics but the opposite is found in the northern hemisphere extratropics. There is thus a preference for highly nonlinear extratropical eddies to be cyclonic in the southern hemisphere but anticyclonic in the northern hemisphere. Further evidence in support of the interpretation of the observed features as nonlinear eddies is the fact that they propagate nearly due west with small opposing meridional deflections of cyclones and anticyclones (poleward and equatorward, respectively) and with propagation speeds that are nearly equal to the long baroclinic Rossby wave phase speed. These characteristics are consistent with theoretical expectations for large, nonlinear eddies. While there is no apparent dependence of propagation speed on eddy polarity, the eddy speeds relative to the local long Rossby wave phase speeds are found to be about 20% faster in the southern hemisphere than in the northern hemisphere. The distributions of the propagation directions of cyclones and anticyclones are essentially the same, except mirrored about a central azimuth angle of about 1.5° equatorward. This small, but we believe statistically significant, equatorward rotation of the central azimuth may be evidence of the effects of ambient currents (meridional advection or the effects of vertical shear on the potential vorticity gradient vector) on the propagation directions of the eddies. While the results presented here are persuasive evidence that most of the observed westward-propagating SSH variability consists of isolated nonlinear mesoscale eddies, it is shown that the eddy propagation speeds are about 25% slower than the westward propagation speeds of features in the SSH field that have scales larger than those of the tracked eddies. This scale dependence of the propagation speed may be evidence for the existence of dispersion and the presence of features that obey linear Rossby wave dynamics and have larger scales and faster propagation speeds than the nonlinear eddies. The amplitudes of these larger-scale signals are evidently smaller than those of the mesoscale eddy field since they are not easily isolated from the energetic nonlinear eddies.
    Progress In Oceanography 01/2011; · 3.71 Impact Factor
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    Richard W. Reynolds, Dudley B. Chelton
    Journal of Climate 01/2010; 23(13):3545-3562. · 4.36 Impact Factor
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    ABSTRACT: Most of the kinetic energy of ocean circulation is contained in ubiquitous mesoscale eddies. Their prominent signatures in sea surface height have rendered satellite altimetry highly effective in observing global ocean eddies. Our knowledge of ocean eddy dynamics has grown by leaps and bounds since the advent of satellite altimetry in the early 1980s. A satellite’s fast sampling allows a broad view of the global distribution of eddy variability and its spatial structures. Since the early 1990s, the combination of data available from two simultaneous flying altimeters has resulted in a time-series record of global maps of ocean eddies. Despite the moderate resolution, these maps provide an opportunity to study the temporal and spatial variability of the surface signatures of eddies at a level of detail previously unavailable. A global census of eddies has been constructed to assess their population, polarity, intensity, and nonlinearity. The velocity and pattern of eddy propagation, as well as eddy transports of heat and salt, have been mapped globally. For the first time, the cascade of eddy energy through various scales has been computed from observations, providing evidence for the theory of ocean turbulence. Notwithstanding the tremendous progress made using existing observations, their limited resolution has prevented study of variability at wavelengths shorter than 100 km, where important eddy processes take place, ranging from energy dissipation to mixing and transport of water properties that are critical to understanding the ocean’s roles in Earth’s climate. The technology of radar interferometry promises to allow wide-swath measurement of sea surface height at a resolution that will resolve eddy structures down to 10 km. This approach holds the potential to meet the challenge of extending the observations to submesoscales and to set a standard for future altimetric measurement of the ocean.
    Oceanography. 01/2010;
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    Proceedings of the “OceanObs’09: sustained ocean observations and information for society” conference, vol. 2.Proceedings of the “OceanObs’09: sustained ocean observations and information for society” conference, vol. 2.; 01/2010
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    Dudley B. Chelton, Xie Shang-Ping
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    ABSTRACT: Satellite observations have revealed a remarkably strong positive correlation between sea surface temperature (SST) and surface winds on oceanic mesoscales of 10–1000 km. Although SST influence on the atmosphere had previously been identified from several in situ observational studies, its widespread existence in regions of strong SST gradients throughout the world’s ocean and the detailed structure of the surface wind response to SST have only become evident over the past decade from simultaneous satellite measurements of SST and surface winds. This has stimulated considerable scientific interest in the implications of this air-sea interaction to large-scale and mesoscale circulation of the atmosphere and ocean. Convergence and divergence of surface winds in regions of spatially varying SST generate vertical motion that can penetrate deep into the atmosphere. Spatial variability of the SST field also results in a curl of the wind stress and associated upwelling and downwelling that feeds back on the ocean and alters SST itself. Significant progress has been made toward understanding the two-way coupling between the ocean and atmosphere but many exciting research opportunities remain. In addition to regional and global modeling, future research on coupled ocean-atmosphere interaction will continue to be guided by satellite observations. In particular, high-resolution measurements in the vicinity of narrow, intense SST fronts and immediately adjacent to land provided by the next-generation scatterometer will open up new areas of research that cannot be addressed from presently available data sets.
    Oceanography. 01/2010;
  • Journal of Climate 01/2010; 23(3):559-. · 4.36 Impact Factor
  • Journal of Climate 01/2010; 23(2). · 4.36 Impact Factor
  • Journal of Climate 01/2010; · 4.36 Impact Factor
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    ABSTRACT: Abstract Observations primarily from satellites have shown a statistical relationship be- tween the surface wind stress and underlying sea-surface temperature (SST) on intermediate space and time scales, in many regions inclusive of eastern-boundary upwelling current systems. In this paper we utilize this empirical SST-wind stress relationship to provide a simple representation of mesoscale,air-sea coupling for an oceanic model forced by surface winds, viz., the Regional Oceanic Model Sys- tem (ROMS). We apply this model formulation to an idealized upwelling problem with prevailing equatorward winds to determine the coupling consequences on flow, SST, stratification and wind evolutions. The initially uniform wind field adjusts through coupling to a cross-shore profile with weaker nearshore winds, similar to realistic ones. The modified wind stress weakens,the nearshore upwelling circu- lation and increases SST in the coastal zone. The SST-induced wind stress curl strengthens offshore upwelling through Ekman suction. The total curl-driven up- welling exceeds the coastal upwelling. The SST-induced changes in the nearshore wind stress field also strengthen and broaden the poleward undercurrent. The cou- pling also shows significant impact on the developing mesoscale eddies by dam- aging cyclonic eddies more than anticyclonic eddies, leading to dominance by the latter. Dynamically this is a consequence,of cyclones with stronger SST gradients that induce stronger wind perturbations in this particular upwelling problem,and that are generally more susceptible to disruption than anticyclones at finite Rossby number. The net effect is a weakening,of eddy kinetic energy.
    Journal of Physical Oceanography 01/2009; 39(11). · 3.18 Impact Factor
  • Journal of Climate 01/2009; 22. · 4.36 Impact Factor