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Ralph F. Milliff and Jan Morzel

Abstract

The time-average wind stress curl field for the global ocean is computed from the wind retrievals of the NASA Scatterometer (NSCAT) mission spanning the period 1 October 1996–29 June 1997. Particular attention is paid to large-amplitude, small-scale “patchiness” in the average wind stress curl over the major ocean basins, and to long and narrow wind stress curl features that occur along ocean eastern boundary regions. The 9-month-average wind stress curl field from NSCAT is examined at 0.5°, 1°, and on a Gaussian grid consistent with T62 truncation in a spectral forecast model. The latter field is compared with the average wind stress curl field from NCEP analyses for the same period. Artifacts in the NCEP average overlap the regions of boundary wind stress curl extrema in the high-resolution averages from NSCAT. The artifacts are attributed to the effects of spectral truncation and tall near-coastal topography in the NCEP forecast model. Possible explanations are discussed for the boundary wind stress curl features that are most pronounced in the NSCAT data with 0.5° resolution. Patchiness in the average wind stress curl fields from NSCAT is considered in the context of aliases introduced by the complex spatiotemporal sampling pattern, and given the intermittency and large gradients that characterize the true wind stress curl field over the ocean. Implications from this rather detailed study of a single broad-swath, active scatterometer system are timely in light of recent plans for international cooperation in providing more than a decade of near-mesoscale resolution of the global surface vector wind field.

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Ralph F. Milliff and James C. McWilliams

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The boundary pressure adjustment process on an ocean basin scale is elucidated in two sets of numerical experiments. First, an initial-value problem is posed in a primitive equation shallow-water model that leads to significant changes in the pressure averaged along the boundary in a closed rectangular ocean basin. These results are compared with the analogous problem in a shallow-water quasigeostrophic model where the boundary pressure adjustment is parameterized by a consistency constraint that closes the mass, circulation, and energy balances in quasigeostrophy. There is very good agreement in the evolution of the boundary-average pressure and qualitative agreement in the evolution of the balanced motions in the interior. Second, idealized Kelvin wave experiments are posed in the primitive equation system on an f plane, a β plane, and a β plane in a domain of doubled dimensions. For β≠0, a scattering process is evident as the initial Kelvin wave transits the first meridional boundary it encounters. Two distinct scattering products are observed. In one there is a mass flux out of the coastal waveguide into the balanced interior motions that occurs on a time scale comparable to the basin circuit lime for the initial Kelvin wave. The second scattering product occurs in the wake of the Kelvin wave within the waveguide, forming a basin-scale coastal current. The relevant time scale for the waveguide scattering product is comparable to the time required to equilibrate the mass anomaly imposed in the waveguide by the Kelvin wave initial condition. The experiments demonstrate a coupling between short time scale motions of the coastal waveguide and longer time scale motions on the ocean interior. Implications of these processes are assessed for both these model problems and more general problems of transient ocean dynamics.

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Ralph F. Milliff and Allan R. Robinson

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A coastal quasigeostrophic (QG) methodology is extended in an exploration of the mesoscale variability of the Rhodes Gyre region of the Eastern Mediterranean Sea, given multiscale data taken as part of the POEM program. Standard objective analysis procedures are modified to include a coastal constraint that prohibits geostrophic flow normal to the coastline and imposes a characteristic nearshore variability in the coastal portion of the domain. Objectively analyzed fields are dynamically adjusted in short QG model integrations and then interpreted for the structure and characteristics of the subbasin scale flow system and the mesoscale. Two-week simulations are then performed to explore the stability of the subbasin scale flow, the dynamical balances, and the variabilities and interactions of the mesoscale. A matrix of simulation experiments is described to test the sensitivities to two variations in initial conditions, and three parameterizations of the bottom topography. An energy and vorticity analysis (EVA) is used to study balance of terms and dynamical processes in the central simulation.

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Ralph F. Milliff and Peter A. Stamus

Abstract

This study reports on the operational utility of ocean surface vector wind (SVW) data from Quick Scatterometer (QuikSCAT) observations in the National Oceanic and Atmospheric Administration (NOAA) National Weather Service (NWS) Weather Forecast Offices (WFOs) covering the coastal United States, including island states and territories. Thirty-three U.S. coastal WFOs were surveyed, and 16 WFO site visits were conducted, from late summer 2005 to the 2005/06 winter season, in order to quantify the impact of QuikSCAT SVW data on forecasts and warnings, with a particular focus on operations affecting marine users. Details of the survey design and site visit strategies are described. Survey results are quantified and site visit impressions are discussed. Key findings include (i) QuikSCAT data supplement primary datasets and numerical weather prediction fields, in the manual production of local public (weather) and marine forecasts and warnings; (ii) operational utility of satellite SVW data would be enhanced by SVW retrievals of finer temporal resolution, closer to the coasts; and (iii) rain flags in the SVW data have little impact on utility for WFO operations.

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Ralph F. Milliff and Roland A. Madden

Abstract

Eastward phase propagation, at speed faster than 30 m s−1, of a signal in the equatorial troposphere of the Eastern Pacific is detected, first in historical meteorological observations and then in more recent data. A first baroclinic mode vertical structure is identified with this signal in separate analyses based on linear theory and complex empirical orthogonal functions, respectively. This rapid, eastward signal is conceptualized as a far-field dispersion product of strong convection associated with the intraseasonal tropical oscillation in the Indian Ocean and Western Pacific.

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Christopher K. Wikle, L. Mark Berliner, and Ralph F. Milliff

Abstract

Boundary value problems are ubiquitous in the atmospheric and ocean sciences. Typical settings include bounded, partially bounded, global, and limited area domains, discretized for applications of numerical models of the relevant fluid equations. Often, limited area models are constructed to interpret intensive datasets collected over a specific region, from a variety of observational platforms. These data are noisy and they typically do not span the domain of interest uniformly in space and time. Traditional numerical procedures cannot easily account for these uncertainties. A hierarchical Bayesian modeling framework is developed for solving boundary value problems in such settings. By allowing the boundary process to be stochastic, and conditioning the interior process on this boundary, one can account for the uncertainties in the boundary process in a reasonable fashion. In the presence of data and all its uncertainties, this idea can be related through Bayes' theorem to produce distributions of the interior process given the observational data. The method is illustrated with an example of obtaining atmospheric streamfunction fields in the Labrador Sea region, given scatterometer-derived observations of the surface wind field.

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Kettyah C. Chhak, Andrew M. Moore, and Ralph F. Milliff

Abstract

At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

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Christopher K. Wikle, Ralph F. Milliff, and William G. Large

Abstract

Near-surface wind spectra are considered from three very different data sources, covering a range of spatial scales from 100 to 103 km. The data were observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment intensive observation period spanning November 1992 to February 1993. Spectra are examined from low-resolution yet spatially and temporally complete National Centers for Environmental Prediction reanalysis wind fields, moderate resolution satellite-based ERS-1 scatterometer winds, and high-resolution aircraft observations from the National Center for Atmospheric Research Electra. Combined spectra (kinetic energy vs wavenumber k) from these data demonstrate a power–law relation over the entire range in spatial scales, with a best-fit slope very near k −5/3. Energy spectra for subsets of the data support spectral slopes of k −5/3 and k −2, but there is little evidence for a slope of k −3.

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Toshio M. Chin, Ralph F. Milliff, and William G. Large

Abstract

A numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields. To capture the prevailing wavenumber characteristics (power-law dependence) of sea surface wind vector components, wavelet coefficients are computed from the scatterometer measurements along the satellite tracks. The statistics of the wavelet coefficients are then used to simulate high-resolution wind components over the off-track regions where scatterometer data are not available. Using this technique, daily wind fields with controlled spectral features have been produced by combining the low-wavenumber wind fields from ECMWF analyses with the high-wavenumber measurements from the ERS-1 scatterometer. The resulting surface wind fields thus reflect nearly all available measurements affecting surface wind, including the synoptic surface pressure. The new surface wind forces a basin-scale quasigeostrophic ocean model such that the average circulation and energetics are consistent with the previous studies, in which purely synthetic high-wavenumber wind forcing was used.

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Kettyah C. Chhak, Andrew M. Moore, Ralph F. Milliff, Grant Branstator, William R. Holland, and Michael Fisher

Abstract

As discussed in Part I of this study, the magnitude of the stochastic component of wind stress forcing is comparable to that of the seasonal cycle and thus will likely have a significant influence on the ocean circulation. By forcing a quasigeostrophic model of the North Atlantic Ocean circulation with stochastic wind stress curl data from the NCAR CCM3, it was found in Part I that much of the stochastically induced variability in the ocean circulation is confined to the western boundary region and some major topographic features even though the stochastic forcing is basinwide. This can be attributed to effects of bathymetry and vorticity gradients in the basic state on the system eigenmodes. Using generalized stability theory (GST), it was found in Part I that transient growth due to the linear interference of nonnormal eigenmodes enhances the stochastically induced variance. In the present study, the GST analysis of Part I is extended and it is found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system. These eigenmodes are confined primarily to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically induced perturbations, and it is this process that maintains the variance of the stochastically induced circulations. Analysis of the large-scale circulation also reveals that the system possesses a large number of degrees of freedom, which has significant implications for ocean prediction. Sensitivity studies show that the results and conclusions of this study are insensitive and robust to variations in model parameters and model configuration.

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