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D. G. Wright

Abstract

Low-frequency motions (≲0.25 cpd) have recently been observed in Juan de Fuca Strait. The three-layer model developed in Part I of this paper is used to show that some of this activity may be due to an instability (baroclinic) of the mean current to low-frequency quasi-geostrophic disturbances.

Satellite infrared imagery and hydrographic maps show eddies in the deep ocean just beyond the continental slope in the northeast Pacific. The eddies are aligned in the north-south direction paralleling the continental slope region and have a wavelength of ∼100 km. A modification of the three-layer model derived in Part I is used to study the stability of the current system in this area. It is found that for typical vertical and horizontal shears associated with this current system the most unstable waves have properties in agreement with observations.

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D. G. Wright

Abstract

A three-layer model is used to study the stability of large-scale oceanic zonal flows over topography. The mean density profile employed has upper and lower layers of constant densities ρ1 * and ρ3 *, respectively (ρ1 *3 *), and a middle layer whose density varies linearly from ρ1 * to ρ3 *. The model developed here includes vertical and horizontal shear of zonal flow in a channel as well as the effects of β and cross-channel variations in topography. In this paper (Part I) the effects of density stratification, curvature in the mean velocity profile, β, constant slope topography and layer thicknesses are studied. The following general conclusions with regard to the stability of the flow are made:

• Curvature in the mean velocity profile has a strong destabilizing influence.

• Density stratification stabilizes.

• The β-effect stabilizes.

• Topography stabilizes one of two possible classes of instability (a bottom intensified instability).

• Increasing either H 1, or H 3 relative to H 2, stabilizes.

The model is compared with two-layer models and results clearly indicate the importance of having at least three 1ayers when curvature of the mean velocity profile is present or when H 2 is significant.

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J. R. N. Lazier and D. G. Wright

Abstract

Analyses of CTD and current meter data obtained between 1978 and 1988 off southern Labrador reveal two distinct regimes in the Labrador Current. The first lies over the shelf and upper slope and is the traditional Labrador Current transporting cold low-salinity water south from Baffin Bay and Hudson Strait. The main branch of this flow, located over the shelf break, exhibits an annual variation in speed with a minimum in March–April and a maximum in October. Historical temperature and salinity data suggest this variation is related to the fact that the annual variation in steric height is 0.1 m greater over the continental shelf than over the deep ocean. The greater amplitude over the shelf is due to the large salinity variations induced by the additional freshwater in spring and summer, which is largely confined to the waters over the shelf. The effect of the annual variation in salinity is also examined through diagnostic estimates of the Joint Effect of Baroclinicity and Relief (JEBAR). These reinforce the contention that there is an annual variation in both speed and transport of the flow over the shelf break in response to the variation in the freshwater transport from the north. The second current regime, referred to in this analysis as the deep Labrador Current, lies over the lower continental slope, seaward of the flow over the shelf break. It is a more barotropic flow than the shelf break current and exhibits a different annual cycle. The flow minimum appears in summer rather than spring, and the maximum is in winter rather than fall. This observation is consistent with earlier estimates of the Sverdrup transport in the subpolar gyre.

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David Brickman, D. G. Wright, and William Hyde

Abstract

A low-order, basin-averaged, coupled atmosphere–ocean paleoclimate model is developed and the results from a 3.2-Myr model paleointegration described. A three-basin version of the Wright–Stocker ocean model is used to compute the thermohaline circulation component of the climate system, with a six-basin energy balance atmosphere coupled to the ocean and land surfaces.

As expected, amplitude spectra of the paleo-integration results show that the annually averaged global air temperature (T atm) closely follows the net radiation (Q N), with power in the obliquity (40 kyr) and eccentricity bands (100 kyr and 400 kyr). However, there are also some unexpected results: the globally and annually averaged ocean temperature (T ocean) is negatively correlated with T atm in the obliquity band, T ocean shows significant energy in the precessional band (20 kyr), and the response of T ocean to Q N variations is suppressed in the eccentricity band.

Physical explanations for the above results are presented and supported by a simple box climate model. This model helps to isolate and clarify the mechanism by which the ocean temperature varies significantly at precessional periods while the atmospheric temperature does not. The same model also illustrates the cause of the 40 kyr atmosphere–ocean temperature anticorrelation. Model integrations and analysis confirm that convection serves to rectify the zero annual-mean precessional forcing, resulting in 20 kyr energy in the ocean, which shows up only weakly in the atmosphere. The 40 kyr anticorrelation is the result of the latitudinal distribution of net radiation at obliquity periods, and thus can be reproduced only by a climate model with horizontal resolution. Ocean convection plays a critical role in determining both the 20 kyr and 40 kyr responses.

The suppressed response of the ocean in the eccentricity band is attributed to a combination of two effects. First, the larger albedo at high latitudes results in reduced variation of the air–sea heat flux at high latitudes so that variations in convection, and hence in deep water temperatures, are also reduced. Second, the nonlinearity of the equation of state for seawater contributes latitudinal variations in ocean densities, which result in changes in the overturning circulation, which further suppress the ocean temperature variations in the eccentricity band.

The implications of the authors’ results for interpretations of the paleoclimate record are discussed.

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G. W. Reuter, C. J. Wright, and D. Eyre

Abstract

The collection equation is solved using a probabilistic collection kernel that includes the effects of overlapping turbulent eddies. The numerical results show that turbulence contributes to the collection process, and as the turbulence increase, so does the broadening of the drop spectrum. But even for very intense turbulence, the droplet growth rate remains fairly slow.

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K. R. Thompson, R. F. Marsden, and D. G. Wright

Abstract

A simple, approximate formula for mean wind stress is given in terms of the mean and variance of the wind fluctuations over the averaging period. The formula is nonlinear with respect to the mean wind speed.

The formula is tested using 3 h wind observations from eight North Atlantic Ocean Weather Ships. Mean wind stress is calculated 1) by vector averaging the 3 h wind stresses and 2) by applying the approximate formula. For an averaging period of 4 months the two methods agree to within ±0.025 Pa, 95% of the time. For an averaging period of 1 month the approximate formula slightly overestimates the stress. This is due to skewness in the probability density function of the observed 3 h wind fluctuations. An expression for the modification of the mean stress due to skewness is given.

A straightforward method is described for the estimation of vector mean wind and variance fields, and thus mean stress fields, over the open ocean. To cheek the method, the long-term stress field of the North Atlantic, and the seasonal Sverdrup transport across 31°N, are computed and compared with the values given by Willebrand, and Bunker and Leetma. Good agreement is obtained. The zonally integrated Sverdrup transport across 45°N is also calculated and shown to exhibit significant interannual fluctuations.

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C. W. Wright, E. J. Walsh, D. Vandemark, W. B. Krabill, A. W. Garcia, S. H. Houston, M. D. Powell, P. G. Black, and F. D. Marks

Abstract

The sea surface directional wave spectrum was measured for the first time in all quadrants of a hurricane's inner core over open water. The NASA airborne scanning radar altimeter (SRA) carried aboard one of the NOAA WP-3D hurricane research aircraft at 1.5-km height acquired the open-ocean data on 24 August 1998 when Bonnie, a large hurricane with 1-min sustained surface winds of nearly 50 m s−1, was about 400 km east of Abaco Island, Bahamas. The NOAA aircraft spent more than five hours within 180 km of the eye and made five eye penetrations. Grayscale coded images of Hurricane Bonnie wave topography include individual waves as high as 19 m peak to trough. The dominant waves generally propagated at significant angles to the downwind direction. At some positions, three different wave fields of comparable energy crossed each other. Partitioning the SRA directional wave spectra enabled determination of the characteristics of the various components of the hurricane wave field and mapping of their spatial variation. A simple model was developed to predict the dominant wave propagation direction.

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E. J. Walsh, C. W. Wright, D. Vandemark, W. B. Krabill, A. W. Garcia, S. H. Houston, S. T. Murillo, M. D. Powell, P. G. Black, and F. D. Marks Jr.

Abstract

The NASA Scanning Radar Altimeter (SRA) flew aboard one of the NOAA WP-3D hurricane research aircraft to document the sea surface directional wave spectrum in the region between Charleston, South Carolina, and Cape Hatteras, North Carolina, as Hurricane Bonnie was making landfall near Wilmington, North Carolina, on 26 August 1998. Two days earlier, the SRA had documented the hurricane wave field spatial variation in open water when Bonnie was 400 km east of Abaco Island, Bahamas. Bonnie was similar in size during the two flights. The maximum wind speed was lower during the landfall flight (39 m s−1) than it had been during the first flight (46 m s−1). Also, Bonnie was moving faster prior to landfall (9.5 m s−1) than when it was encountered in the open ocean (5 m s−1). The open ocean wave height spatial variation indicated that Hurricane Bonnie would have produced waves of 10 m height on the shore northeast of Wilmington had it not been for the continental shelf. The gradual shoaling distributed the wave energy dissipation process across the shelf so that the wavelength and wave height were reduced gradually as the shore was approached. The wave height 5 km from shore was about 4 m.

Despite the dramatic differences in wave height caused by shoaling and the differences in the wind field and forward speed of the hurricane, there was a remarkable agreement in the wave propagation directions for the various wave components on the two days. This suggests that, in spite of its complexity, the directional wave field in the vicinity of a hurricane may be well behaved and lend itself to be modeled by a few parameters, such as the maximum wind speed, the radii of the maximum and gale force winds, and the recent movement of the storm.

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C. W. Wright, E. J. Walsh, W. B. Krabill, W. A. Shaffer, S. R. Baig, M. Peng, L. J. Pietrafesa, A. W. Garcia, F. D. Marks Jr., P. G. Black, J. Sonntag, and B. D. Beckley

Abstract

Over the years, hurricane track forecasts and storm surge models, as well the digital terrain and bathymetry data they depend on, have improved significantly. Strides have also been made in the knowledge of the detailed variation of the surface wind field driving the surge. The area of least improvement has been in obtaining data on the temporal/spatial evolution of the mound of water that the hurricane wind and waves push against the shore to evaluate the performance of the numerical models. Tide gauges in the vicinity of the landfall are frequently destroyed by the surge. Survey crews dispatched after the event provide no temporal information and only indirect indications of the maximum water level over land. The landfall of Hurricane Bonnie on 26 August 1998, with a surge less than 2 m, provided an excellent opportunity to demonstrate the potential benefits of direct airborne measurement of the temporal/spatial evolution of the water level over a large area. Despite a 160-m variation in aircraft altitude, an 11.5-m variation in the elevation of the mean sea surface relative to the ellipsoid over the flight track, and the tidal variation over the 5-h data acquisition interval, a survey-quality global positioning system (GPS) aircraft trajectory allowed the NASA scanning radar altimeter carried by a NOAA hurricane research aircraft to demonstrate that an airborne wide-swath radar altimeter could produce targeted measurements of storm surge that would provide an absolute standard for assessing the accuracy of numerical storm surge models.

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J. W. Hurrell, M. Visbeck, A. Busalacchi, R. A. Clarke, T. L. Delworth, R. R. Dickson, W. E. Johns, K. P. Koltermann, Y. Kushnir, D. Marshall, C. Mauritzen, M. S. McCartney, A. Piola, C. Reason, G. Reverdin, F. Schott, R. Sutton, I. Wainer, and D. Wright

Abstract

Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

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