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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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D. E. Harrison and Mark Carson

Abstract

Subsurface temperature trends in the better-sampled parts of the World Ocean are reported. Where there are sufficient observations for this analysis, there is large spatial variability of 51-yr trends in the upper ocean, with some regions showing cooling in excess of 3°C, and others warming of similar magnitude. Some 95% of the ocean area analyzed has both cooled and warmed over 20-yr subsets of this period. There is much space and time variability of 20-yr running trend estimates, indicating that trends over a decade or two may not be representative of longer-term trends. Results are based on sorting individual observations in World Ocean Database 2001 into 1° × 1° and 2° × 2° bins. Only bins with at least five observations per decade for four of the five decades since 1950 are used. Much of the World Ocean cannot be examined from this perspective. The 51-yr trends significant at the 90% level are given particular attention. Results are presented for depths of 100, 300, and 500 m. The patterns of the 90% significant trends are spatially coherent on scales resolved by the bin size. The vertical structure of the trends is coherent in some regions, but changes sign between the analysis depths in a number of others. It is suggested that additional attention should be given to uncertainty estimates for basin average and World Ocean average thermal trends.

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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.

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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.

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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.

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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.

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D. E. Harrison and Robert H. Heinmiller

Abstract

We discuss the upper ocean mesoscale temperature variability field as sampled by XBT's in the joint US-USSR POLYMODE Synoptic Experiment during July 1977 to July 1978. There is a “background” persistent, relatively weak and rather large-scale pattern of variability, of alternating warm and cold areas which propagate nearly westward. For the background, very little point correlation between surface temperatures and temperatures beneath the mixed layer is found, but sub-mixed-layer temperatures above and below the “18°C water” show negative correlation. We also find a number of smaller scale, apparently discrete, features which are somewhat more intense than the background. They exhibit little consistency in direction of propagation, and pattern propagation, rather than pattern evolution, as generally observed. The time-average depth field of the 15°C isotherm exhibits spatial variations comparable in magnitude to its pointwise standard deviation values. Removing the mean by linear fits in latitude and/or longitude produces horizontal auto-correlation functions substantially different from those based on removing the time-averaged field. The statistics of the horizontal correlation are not satisfactorily determined by this data set.

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Gabriel A. Vecchi and D. E. Harrison

Abstract

The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with “active” periods of heavy rain interrupted by drier “break” periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°–2°C and local changes can exceed 3°C over 2 weeks; these changes are as large as those seen in the local climatological seasonal cycle. This raises the possibility that air–sea interaction may be a significant factor in monsoon variability; the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation. A schematic coupled air–sea oscillator mechanism is offered for further study, in which oceanic changes play a dynamical role in monsoon variability.

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D. E. Harrison and W. R. Holland

Abstract

The dynamical balances of the mean flow of a numerical model ocean general circulation experiment are examined through evaluation of regional vorticity budgets. The instantaneous flow is strongly time dependent and the effect of eddy terms in the mean budgets is of primary interest. Budgets have been computed over volumes ranging in size from less than that of a typical model eddy up to an entire wind-driven gyre, using time series of 5 and 10 year durations. The statistical reliability of terms in the budgets varies significantly with the region size; over regions the size of an eddy or smaller the reliability is often poor, but over the selected larger regions it is satisfactory. The final analysis regions are selected by requiring that each be identified clearly with some part of the mean flow and that cancellation of the locally dominant terms within each region be minimized whenever possible.

The primary mechanism for balancing the wind-stress curl vorticity input in each half basin is found to be horizontal transport of relative vorticity by the eddies across the zero wind-stress curl latitude that separates the distinct flow systems of the two half basins. However, net meridional eddy vorticity transport is generally unimportant away from the half-basin boundary latitude. Eddy horizontal transports over the analysis regions, away from the western part of the zero wind-stress curl latitude, also tend to be small. The transport flow budgets and upper layer budgets tend to be similar. The deep-layer flow is qualitatively different from these flows, a separate set of analysis regions is needed to study it, and the deep budgets are different in several respects. Away from the boundary currents and internal jets the volume integral analog of the classical geostrophic balance—vortex stretching balancing advection of planetary vorticity—holds very well. In particular, over the interior of each gyre, the net input of vorticity by the, wind balances the loss by advection of planetary vorticity to better than 10%. This result is quite different from the conclusion that would he drawn from examination of the vorticity balance at a point over much of the interior, where the divergence of the eddy relative vorticity flux is often large (but of limited statistical reliability). The eddy heat-flux divergence plays an important role in establishing the interfacial vertical velocity contribution to vortex stretching in some of the regions, and appears essential in forcing one of the deep flow currents. No simple summary of the bound current and jet region budgets can be offered, except that mean nonlinear transport often dominates eddy horizontal transport and that frictional effects can be quite small. These results are compared with classical wind-driven ocean circulation ideas and the strengths and limitations of this type of analysis for studying eddy-mean flow interaction are discussed.

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