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D. E. Harrison

Abstract

Oceanic gyres defined by the mean zero wind-stress curl lines have been the focus of wind-driven ocean circulation theory since its beginnings. In the face of single-signed vorticity input from the curl of the wind stress over a gyre, a mechanism to balance the wind vorticity input is required if an equilibrium is to be established. Traditional models have tended to restrict their allowable physics so that a dissipation mechanism is required for equilibrium. However, dissipation is not necessary, in principle, for equilibrium for a general fluid system. In fact, it has recently been shown that lateral eddy vorticity transport between gyres can provide an important part of the vorticity tendency required for equilibration in model oceanic systems. This note examines the possibility that in the North Atlantic subtropical gyre this process also might be important. After a brief review of the equilibration mechanisms possible in a primitive equation fluid, attention is focussed on estimates of the eddy lateral transport of relative vorticity in the North Atlantic. It appears that there could be sufficient eddy transport across the Gulf Stream to balance the wind vorticity input over the gyre. Equilibrium might thus be possible, without any vorticity dissipation mechanism or without invoking higher order dynamical processes. If this mechanism is important in the ocean there should be interesting effects on the subpolar gyres, details of which depend on the circumstances surrounding the vorticity exchange process.

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D. E. Harrison

Abstract

Several experiments using an ocean general circulation model have been carried out in order to explore the degree to which the oceanic waveguide response during the 1982–83 ENSO event was 1ocally and remotely forced. Experiments in which the chosen monthly mean surface stress field was imposed only within three degrees of the equator (3°N/S) and within seven degrees of the equator (7°N/S) reveal that the 7°N/S winds reproduce the equatorial results of the full winds case to within differences small compared to the variability of interest The 3°N/S winds case reproduce equatorial dynamic height acceptably, but introduces errors in SST and upper-ocean currents that approach the ENSO signal. A 7°N–S experiment in which the meridional stress is set to zero (NOYST) snows that meridional stress plays a nontrivial, but not dominant role, in the 1982–83 model behavior; errors generally are comparable to those of the 3°N/S case. A final experiment, in which the 1982–83 winds were imposed west of the dateline and climatological winds were imposed east of 170°W (WPAC), illustrates the extent to which the central and eastern Pacific were forced by winds in the western Pacific. While there is nontrivial remote forcing, the locally forced variability is roughly twice as great.

Implications for coupled ocean-atmosphere modeling and for design of future surface wind stress monitoring arrays for ENSO prediction are considered.

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D. E. Harrison

Abstract

Multidecadel time series of surface winds from central tropical Pacific islands are used to compute trends in the trade winds between the end of WWII and 1985. Over this period, averaged over the whole region, there is no statistically significant trend in speed or zonal or meridional wind (or pseudostress). However, there is some tendency, within a few degrees of the equator, toward weakening of the easterlies and increased meridional flow toward the equator. Anomalous conditions subsequent to the 1972–73 ENSO event make a considerable contribution to the long-term trends. The period 1974–80 has been noted previously to have been anomalous, and trends over that period are sharply greater than those over the longer records.

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D. E. Harrison

Abstract

A method for evaluating the utility of a given parameterization field in a spatially inhomogeneous circulation is described. This method is used to examine the mean field diffusion parameterization of the heat, momentum and vorticity time deviation eddy terms from a mesoscale resolution numerical ocean circulation experiment. The diffusion model does not satisfactorily describe the eddy terms for any field throughout any of the different regions of the model flow.

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D. E. Harrison

Abstract

The monthly mean surface wind changes during recent ENSO events, as observed from 11 islands in the tropical Pacific, are described. Two different composite ENSO wind fields are evaluated and compared. The month-to-month wind changes during each event are also discussed.

The wind changes for each event between 1953 and 1980 except 1969 show several common features:

(i) Westerly anomalies appear first west of the date line and then at the date line sometime in summer (0) to fall (0), then intensify over the following several months. The anomalies are confined to within ±3° of the equator during this stage.

(ii) In either November (0), December (0), or January (+1) there is an abrupt southward shift of the narrow band of westerly anomalies, so that the maximum anomaly is then at ∼5°S latitude at the date line, and nearly normal conditions prevail north of the equator.

(iii) Westerly anomalies are gone or greatly reduced one to two months after the southward shift.

The event-to-event variations are considerable, particularly prior to July (0) and after February (+1), so that composites show much reduced anomaly amplitude and much smaller month-to-month anomaly changes than are typical of any given event. The large amplitude months of the composites show similarities with a composite by Rasmusson and Carpenter, but a number of significant differences are identified. These findings, and their relationship to existing simple ideas concerning tropical Pacific coupled ocean-atmosphere interactions, are discussed.

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D. E. Harrison

Abstract

The importance of mesoscale eddies in the basin energy budgets of closed-basin numerical model oceanic systems that attempt to resolve such motions varies greatly from calculation to calculation. In existing calculations, eddy importance has been found to depend strongly on the dissipation mechanism(s) selected. These energy budget results can be understood by examination of how eddy and mean flow kinetic energy are dissipated in the long-time mean in the different regions of the model flow. Scale analysis arguments are presented, assuming that the characteristics of the flows satisfy certain mild quasi-oceanic constraints, to investigate these dissipation terms. From these scale. estimates it appears that many of the model ocean results can he understood in terms of a nondimensional parameter that measures the relative importance of horizontal and bottom friction dissipation. When horizontal friction dissipation dominates, eddies can only be of modest importance in basin energy budgets, but when bottom friction dissipation dominates, eddies generally must be important. This follows simply from the assumed flow characteristics. The implications of these results on the interpretation of present modeling results are described.

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D. E. Harrison

Abstract

The question of the importance of mesoscale motions in the long time averaged ocean circulation is examined from the viewpoint offered by Eulerian scale estimates of the magnitudes of the explicit eddy and largest inviscid mean flow terms in the mean heat, momentum and vorticity equations. Comparisons of these estimates reveal the quantities that must be known to obtain reliable estimates of the importance of eddy terms in the mean balances. Using historical information and long time series of data from the western North Atlantic, two distinct regimes (“near field” and “mid-ocean”) are identified for this ocean region and the appropriate term comparisons are made for each regime. From estimates of the reliability of the ocean values used in these comparisons the robustness of the comparisons is examined. The momentum and vorticity equation estimates suggest that terms based on the eddy Reynolds stress can generally be neglected compared to terms involving f 0 and β in both the near field of the Gulf Stream and the mid-ocean. In the near field, mean advective terms appear to be at least as important as the eddy terms, but the eddy terms dominate these advective ones in the mid-ocean. The heat equation comparisons suggest that the eddy term is comparable to the mean horizontal advection of heat in the mid-ocean but is of somewhat reduced importance in the near field. Some remarks on the generality of results from numerical ocean models that contain mesoscale motions to the question of eddy importance in the ocean are offered.

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D. E. Harrison

Abstract

Using a version of the global surface marine observation historical data set, a new 1° spatial resolution global ocean surface wind stress climatology has been evaluated using the Large and Pond surface drag coefficient formulation. These new results are compared, after spatial smoothing, with those of Hellerman and Rosenstein, who used a different drag coefficient form. It is found that the new stresses are almost everywhere smaller than those of Hellerman and Rosenstein, often by 20%–30%, which is greater than the formal error estimates from their calculations. The stress differences show large-scale spatial structure, as would he expected given the spatial variation of the surface stability parameter and the known different wind variability regions. Basin zonally averaged Ekman transports are computed to provide perspective on the significance of the stress differences; annual mean differences can exceed 10 Sv (Sv = 106 m3 s−1) equatorward of 20° lat, but are smaller poleward. Wind stress curl and Sverdrup transport calculations provide a different perspective on the differences; particularly noticeable differences are found in the regions of the Gulf Stream and Kuroshio separation. Large annual variations in midlatitude wind stress curl suggest that study of the forced response at annual periods should be of interest.

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D. E. Harrison

Abstract

Several primitive-equation ocean general circulation model experiments have been carried out in order to explore the sensitivity of equatorial sea surface temperature (SST) results to uncertainty in the net surface heal flux (Q) imposed at the surface. Both climatological seasonal cycle experiments and hindcasts of the 1982/33 ENSO event are considered. It is found that regions of light winds, which typically reach values of SST in excess of 31°C using this ocean model and past Q parameterizations, attain more realistic SST values of 29°–30°C when Q is reduced by as little as 10 W m−2. Sensitivity in this regime is about 0.1–0.2°C (W m−2)−1 for low-frequency SST changes. In regions of easterly winds with their associated upwelling, horizontal advection, and stronger mixing, changes of Q in excess of 50 W m−2 produce SST changes typically of 0.7°C, for a sensitivity of about 0.02°C (W m−2)−1. These results apply equally well to the ENSO hindcasts and the seasonal cycle studies. The reasons for the large variation in sensitivity and the very large sensitivity under light winds are described. To the extent that these results are representative of oceanic conditions, very accurate Q information will be required for studies of the low-frequency variability of SST in light wind regions like the western Pacific; much less accurate fluxes appear needed for studies of comparable variability in upwelling regions.

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Mark Carson and D. E. Harrison

Abstract

Regional interdecadal variability, on subbasin to basin scales, is shown to be a robust feature of the post–World War II (WWII) historical temperature record, even after a recently proposed bias correction to XBT fall rates is applied. This study shows that the previously reported strong regional variability is generally unaffected by this correction, even though the interdecadal variability in the most recently published estimates of global ocean heat content is much reduced after a correction is applied. Following methods used in previous trend analysis work, estimates of interdecadal trends are calculated for individual regions of the global ocean where there are sufficient data. Spatial maps of temperature trends for the surface and three subsurface depths (50, 100, and 300 m) are presented, with both bias-corrected and uncorrected data trends at 100 and 300 m shown for comparison. In the upper two depths and at the surface, interdecadal variability is shown to be present and strong in most of the analysis regions. At 100 m, the differences between trends based on bias-corrected versus uncorrected data are small, and barely distinguishable for much of the ocean analyzed. There are more differences at 300 m between the two data treatments, but large-scale patterns are still present in the bias-corrected trends, especially where the trends are stronger.

Given the sampling issues discussed in previous works, the presence of strong interdecadal variability on smaller scales raises concerns that global interdecadal variability in the historical record still may not be properly resolved.

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