Search Results

You are looking at 1 - 10 of 39 items for :

  • Author or Editor: D. E. Harrison x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
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.

Full access
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.

Full access
Mark Carson and D. E. Harrison

Abstract

There is great interest in World Ocean temperature trends, yet the historical global ocean database has very uneven coverage in space and time. Previous work on 50-yr upper ocean temperature trends from the NOAA ocean data archive is extended here. Trends at depths from 50 to 1000 m are examined, based on observations gridded over larger regions than in the earlier study. Despite the use of larger grid boxes, most of the ocean does not have significant 50-yr trends at the 90% confidence level (CL). In fact only 30% of the ocean at 50 m has 90% CL trends, and the percentage decreases significantly with increasing depth. As noted in the previous study, there is much spatial structure in 50-yr trends, with areas of strong warming and strong cooling. These trend results are compared with trends calculated from data interpolated to standard levels and from a highly horizontally interpolated version of the dataset that has been used in previous heat content trend studies. The regional trend results can differ substantially, even in the areas with statistically significant trends. Trends based on the more interpolated analyses show more warming. Together with major temporal and spatial sampling limitations, the previously described strong interdecadal and spatial variability of trends makes it very difficult to formally estimate uncertainty in World Ocean averages, but these results suggest that upper ocean heat content integrals and integral trends may be substantially more uncertain than has yet been acknowledged. Further exploration of uncertainties is needed.

Full access
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.

Full access
D. E. Harrison and D. S. Luther

Abstract

Multidecadal time series of surface wind observations from tropical Pacific islands have been examined in order to investigate the space and time scales of variability. Climatological monthly means and variances are compared with comparable means and variances derived from ship observations; usually the means agree to within ∼1 m s−1 in speed and ∼10 degrees in direction. Annual and semiannual cycles differ in detail. Island zonal wind variances are often significantly larger, by up to 10 m2 s−2 near the equator between September and December; because of the spatial coherence of the island results, these discrepancies are believed to result from the poor high-frequency sampling typical of ship data. A substantial near-equatorial zonal wind variance maximum is shown to be related to ENSO period variability; excluding ENSO time periods leaves a relatively spatially uniform variance of ∼5 m2 s−2 over a broad region.

The frequency distribution of variance, derived from daily-averaged data, exhibits considerable geographical variation. Within a few degrees of the equator the most energetic zonal wind variability is found in a broad band extending from about 3- to 60-day periods, with maximum at about 10 days; there is also significant interannual power in records located west of 170°W. There is occasionally a local variance maximum in the range of 30- to 60-day periods. Within this near-equatorial region, the meridional wind variance is roughly half the zonal wind variance and is found primarily between about 3-day and 6-day periods and at the annual period. Poleward of about 5 degrees of latitude, the interannual variability in zonal wind diminishes sharply, and the zonal and meridional wind variances become increasingly comparable. The zonal wind energy level in the 3- to 60-day band decreases as one moves farther from the equator, until the more energetic winds typical of subtropical latitudes arise. Coherence calculations typically show zonal wind coherence significant at the 95% level at all energetic periods when islands axe within 200–300 km of each other meridionally, and within 1000–1500 km zonally. The meridional wind tends to be less coherent. A minimal sampling array for tropical surface wind variability in this region should have meridional sampling about every 2° and zonal sampling about every 15° for the zonal wind, and perhaps half these distances for the meridional wind.

Full access
D. S. Luther and D. E. Harrison

Abstract

The utility of studying low-frequency surface weather phenomena with long time series of meteorological data from tropical Pacific islands is demonstrated. The wind stress changes associated with El Niño events in the period 1950–78 are examined at seven locations. Zonal wind stress anomalies at the equator near the date line often exhibit strengthening and subsequent weakening of the trade winds prior to each El Niño, as originally suggested by Wyrtki. An exception is the weak 1963 El Niño, which is preceded by meridional wind stress anomalies at the equator. The strongest zonal and meridional wind stress anomalies, however, occur well after the first occurrence of anomalously warm water off the coast of Peru for each El Niño, in agreement with prior analyses of merchant marine data. Away from the equator, variability of the wind stress anomalies from one El Niño to the next is strong, leading to numerous discrepancies with published profiles of the “mean” El Niño wind changes.

Power spectra of wind stress from three island stations are compared with concurrent wind stress spectra computed from merchant marine data. Many disparities are found and can be attributed to (sometimes severe) aliasing in the ship data. Possible aliasing errors in the ship data time series are estimated by randomly subsampling the island data in order to mimic the ship data sampling. Sampling criteria, which depend upon the scientific application, are suggested in order to limit the alias noise in the ship data to acceptable amounts.

Full access
D. E. Harrison and D. S. Gutzler

Abstract

We examine the variability of monthly mean winds at 850 mb and the surface from five island stations in the tropical western Pacific Ocean. Climatological winds and (850 mb-surface) wind shear are evaluated and used to construct time series of monthly mean wind and shear anomalies. Wind variance at 850 mb tends to be substantially greater than at the surface, and large temporal variations in shear are found. Prominent anomalies are associated with El Niño–Southern Oscillation periods. Composite El Niño event anomalies are examined; it is found that the westerly wind anomalies associated with warm central Pacific sea surface temperatures are stronger at 850 mb than at the surface, and that the anomalous (850 mb-surface) shears are as large as the surface wind anomalies themselves.

Several simple techniques are described to investigate the feasibility of estimating surface wind anomalies from 850 mb wind anomalies. Because strong correlations exist between the zonal winds at these levels, zonal estimate errors can be reduced to ≈0.5 m s−1 if known shear statistics are included in the estimate algorithm. Estimates which extrapolate cloud level wind anomalies to the surface using only climatological shear are shown to produce much greater surface wind errors. If these results are representative and if accurate monthly mean winds at 850 mb can be obtained from cloud motion vectors, then very useful low-frequency surface wind fields can be derived from cloud motion data.

Full access
A. M. Chiodi and D. E. Harrison

Abstract

Globally, the seasonal cycle is the largest single component of observed sea surface temperature (SST) variability, yet it is still not fully understood. Herein, the degree to which the structure of the seasonal cycle of Southern Hemisphere SST can be explained by the present understanding of surface fluxes and upper-ocean physics is examined. It has long been known that the annual range of Southern Hemisphere SST is largest in the midlatitudes, despite the fact that the annual range of net surface heat flux peaks well poleward of the SST peak. The reasons for this discrepancy (“falloff of the annual range of SST”) are determined here through analysis of net surface heat flux estimates, observed SST, and mixed layer depth data, and results from experiments using two different one-dimensional ocean models. Results show that (i) the classical explanations for the structure of the annual range of SST in the Southern Hemisphere are incomplete, (ii) current estimates of surface heat flux and mixed layer depth can be used to accurately reproduce the observed annual range of SST, and (iii) the prognostic mixed layer models used here often fail to adequately reproduce the seasonal cycle at higher latitudes, despite performing remarkably well in other regions. This suggests that more work is necessary to understand the changes of upper-ocean dynamics that occur with latitude.

Full access