摘要:

Evidence is presented for local and remote wind‐forcing of deep ocean currents in three frequency bands (corresponding to 3.7–8 day, 8–22 day, and 23–82 day periods) at a two‐year mooring in the eastern North Atlantic. Coherence between current meter velocities and estimated wind stress curl is largest for the deepest instrument (3000 m) and decreases toward the surface. The strongest evidence is found at 3000 m depth in the high frequency (3.7–8 day) band, where both components of velocity are significantly coherent with wind stress curl. Meridional velocity at 3000 m is also significantly coherent with wind stress curl in the low frequency (23–82 day) band, while zonal velocity in the low frequency band and both velocity components in the intermediate frequency (8–22 day) band are only marginally coherent. The results are compared with a stochastically‐forced linear quasi‐geostrophic model that includes a simple representation of the mid‐Atlantic Ridge. Good agreement between predicted and observed coherences is found for the 3000 m currents in the high frequency band, but only partial agreement is found between predicted and observed coherences in the low frequency band, and little agreement in the intermediate band. Kinetic energy levels at 3000 m are well predicted in all three bands. For the shallower instruments (1100 m and 500 m), observed coherences are at most marginally significant and energy levels are considerably higher than the theory predicts. Including an upper‐layer mean flow in the model improves the prediction of energy levels at th

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11359

年代:1990

数据来源: WILEY

摘要:

The role of the ocean in transferring angular momentum between the atmosphere and the solid Earth is examined for periods longer than a day. Changes in the total angular momentum of the ocean can occur through the action of external torques such as surface wind stresses and continental torques (i.e., body forces acting on lateral boundaries), which represent momentum exchange with the atmosphere and the solid earth respectively. Analysis of these external torques suggests that in general, the related angular momentum exchanges are mostly associated with barotropic variability in the ocean. Solutions for a mid‐latitude basin, obtained with a shallow water numerical model forced by simple periodic zonal winds, illustrate how the momentum exchanged with the atmosphere is quickly transmitted to the solid Earth through the continental torque, with the bottom friction torque being negligible. For wind forcing confined to the middle of the ocean basin, both free waves and forced variability with large horizontal decay scales can participate in the interaction with the boundaries. The fast time scales involved in oceanic barotropic adjustment permit a rapid equilibration between the forcing regions and the coastal boundaries. The angular momentum transfer mechanism between the ocean and the solid Earth is consistent with the observed coherence at no discernible time lag between atmospheric and solid Earth angular momentum variation

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11369

年代:1990

数据来源: WILEY

摘要:

To initialize the barotropic and baroclinic modes, numerical ocean prediction models need information both above and below the main thermocline. Forecasts of upper ocean mesoscale variability using real and simulated data show high sensitivity to the Subthermocline pressure (STP) field. Results using simulated data indicate that the accuracy of this field may be the limiting factor on the time scale for mesoscale oceanic predictive skill. Satellite altimetry provides a potentially abundant source of sea surface height (SSH) data, but there is no comparable source of Subthermocline information on the horizon. We investigate statistical techniques to infer Subthermocline pressure anomalies from SSH data, a problem complicated by the weak correlation between the fields. This problem is addressed by using the degrees of freedom available in the data and by describing them in an efficient manner to suppress noise, eliminate unskilled or redundant estimators and to prevent ill‐conditioned matrix inversions. Multilinear regression, empirical orthogonal function (EOF) regression and principal estimator patterns are compared using data simulated by a numerical ocean model and error models. Numerous questions that need to be addressed for proper application of the statistical techniques are investigated. Topics include noise suppression and the impact of the noise on accuracy. These topics are studied as a function of decorrelation distance in the noise and the presence or absence of noise in dependent and independent data sets. In this context we also investigate dependent data set size requirements, the criteria for choosing estimators, the number, areal coverage, and spacing of sampling locations used in the estimators, the effect of the ocean dynamical regime on the results, the effects of ocean model imperfections or changes in population statistics on the results, SSH versus ΔSSH in an orbital repeat period as estimators, and the effect of orbital repeat period on Subthermocline estimates from ΔSSH data. In the presence of 40% rms noise the usual significance tests are much too conservative for this application. (However, we also found that EOFs calculated from spatially correlated, temporally uncorrelated noise can pass a popular EOF significance test based on uncorrelated noise.) Although more difficult to suppress than uncorrelated noise, correlated noise did not markedly increase the error in these tests. Spatial coverage of the estimators was found to be an important parameter, and four to five ascending‐or descending tracks per wavelength were sufficient for uniformly accurate estimates whether beneath or between altimeter tracks. In most of these tests, ΔSSH proved a better estimator than SSH, but for ΔSSH resolving the shortest major time scale is a necessity. High accuracy is not required for Subthermocline pressure anomalies to substantially enhance upper ocean forecast skill in a numerical ocean prediction model. In results to be reported elsewhere, a statistically inferred STP field substantially enhanced the skill of a Gulf Stream forecast model where the SSH was initialized from oceanic observations. The inferred STP field allowed the model to show forecast skill in comparison to persistence of the initia

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11375

年代:1990

数据来源: WILEY

摘要:

Global ocean tide loading charts of the radial displacement, the potential divided byg(gravity acceleration), and the gravity effect have been computed using the 11 constituents M2, S2, N2, K2, K1, O1, P1, Q1, Mf, Mm, Ssaof Schwiderski's tidal model. These new charts have a resolution of 1° × 1° on the continents as well as on the oceanic area. A description of Farrell's convolution method to compute the loading effects is given, and an estimate of the numerical errors leads to the conclusion that these global charts have a precision better than 2.5% independent of the accuracy of Schwiderski's maps. The current approximation of the loading effects by a proportionality relation with the local oceanic tides is also compared with Farrell's convolution method. Departures of several centimeters systematically appear, in particular over the continental shelves. We then show that the maps of the oceanic tides deduced from satellite altimetry could be corrected for the loading effect by an iterative computational procedure based on our algorithm of Farrell's convoluti

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11411

年代:1990

数据来源: WILEY

摘要:

Satellite Advanced Very High Resolution Radiometer (AVHRR) imagery and Argos tracked drifters have revealed the existence of a narrow, warm surface poleward current along the Iberian coast during autumn and winter. Six near‐surface drifter tracks indicated a net northward flow between September 1986 and March 1987, although individual drifter trajectories also provided evidence of eddy activity and episodes of equatorward flow. A SeaSoar survey during September 1986 in the region 37°–43°N, 9°–11°W showed that poleward flow at that time was associated with the northward advance of a warm, salty tongue of water throughout the upper 400 m layer. Infra‐red imagery confirmed the northward penetration of the warm current and its progression around Cape Finisterre along the north coast of Spain during the course of the winter. A similar situation is indicated by satellite imagery during other years. Current meter data show that poleward flow occurred along the continental slope down to depths greater than 600 m. Conductivity, temperature, and depth (CTD) casts show the presence of Mediterranean Intermediate Water (MIW) at depths greater than 600 m. The MIW does not flow as a simple northward flowing jet along the continental slope off Iberia, but instead shows evidence of eddies within the flow. The poleward surface current is in the opposite sense to the general summer circulation of the region, when cool upwelled water is advected equatorward. It is postulated that the poleward flow is generated by thermohaline forcing when the southward trade winds, characteristic of the summer months, weaken or reverse. Intermittent poleward flow appears to be a common characteristic of the Eastern Boundary throughout the North Atlantic from West Africa to the No

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11425

年代:1990

数据来源: WILEY

摘要:

Different orbital scenarios for satellite missions are evaluated with respect to their ability to provide a satisfactory altimeter data set as input into an assimilation procedure. The assimilation is realized, using the technique of “nudging” (Anthes, 1974), into a numerical simulation of the general oceanic circulation at mid‐latitudes involving strong variability fields of the Gulf‐Stream‐like jet stream. The numerical model is multilayered and quasi‐geostrophic, and it considers basin‐scale circulation with high resolution. The basic orbital scenarios under consideration are four in number: the two ERS 1 and Topex/Poseidon nominal mission parameters (with 3‐ and 10‐day orbital periods, respectively) and two extra possibilities for Topex/Poseidon: a 17‐day orbital period scenario (close to the Geosat parameters) and a 29‐day orbital period scenario. All four cases are compared for assimilation experiments realized under identical conditions. The criterion of efficiency is to ensure that model flows converge toward observations, i.e., the fact that surface information is sufficient to recreate via the model the realistic three‐dimensional ocean circulation. In the conditions of experiments, results show that the 10‐ to 17‐day range is able to satisfy the convergence condition to some extent. This is not the case for the two scenarios outside this range. The effects of subsequent undersampling (subcycles) are examined in the 10‐day case and found to be of little importance as long as sampling is sufficiently regular. Preliminary tests to take into account data and

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11443

年代:1990

数据来源: WILEY

摘要:

A series of observing system simulation experiments (OSSE) is performed on a simulated data set which was designed to mimic the wind‐forced response of the tropical Pacific ocean. This data set was constructed by adding random perturbations to the FSU monthly mean pseudostress anomaly data. These perturbed pseudostress anomaly fields were used to drive a simple linear model whose output was sampled to form synthetic observations. The statistics of the perturbations are given by estimates of the errors in the pseudostress data calculated in an earlier study. OSSE are performed using simulated sea level height data from island tide gauge stations and from selected TOGA‐TAO (Tropical Ocean and Global Atmosphere Thermal Array for the Ocean) mooring sites. Data from the TOGA‐TAO moorings are assumed, in one experiment, to consist of sea level height data, identical to that from tide gauges. In a further experiment, mooring data consist of amplitudes of the first two vertical modes. Errors in the OSSE are seen to be comparable to errors obtained in comparison to real data where such comparisons are available. Assimilation of data at six island stations results in noticeable, but not dramatic improvement in the analysis, as was noted in an earlier study with actual observations. OSSE using simulated mooring data showed accuracy in the sea level height field comparable to that of the instruments across much of the basin. Inclusion of the separate baroclinic modes resulted in negligible improvement. Simulated fields of 20° isotherm depth anomaly were also produced. Results were similar to the sea level height results. As in the simulation of sea level height, inclusion of the separate baroclinic modes resulted in negligible impro

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11461

年代:1990

数据来源: WILEY

摘要:

We examined 21 Coastal Zone Color Scanner (CZCS) images of the southeastern Bering Sea to map the near‐surface distribution of phytoplankton during 1979 and 1980 and compared this information with the mesoscale (100–1000 km) distribution of phytoplankton inferred from ship sampling. Pigment patches coherent over length scales of about 1000 km along isobaths were detected in the satellite data. The patches straddled boundaries of hydrographic domains that were previously defined from the ship observations. Smaller patches (order of 100 km diameter and less) were common. The imagery indicated that open‐water phytoplankton blooms occur first in late April in coastal waters, peak in early May over the middle shelf, and decay rapidly afterwards, reaching concentration minima in June in both regions. These patters show that earlier ship observations are valid for most of the eastern Bering shelf. Images from August and September 1979 revealed the occurrence of fall blooms, previously unrecorded in this area perhaps because of scarce ship observations after summer. Interannual variation in the distribution of biomass was evident, with April–May 1979 showing less pigment near the surface than April–May 1980. These changes were in part related to aliased sampling by the CZCS. Also, the wind was downwelling favorable in late April 1979 but upwelling favorable during late April 1980. CZCS values underestimated the ship data by a factor of at least 2 during spring 1979. During spring of 1980 the CZCS and ship‐based data were similar. The apparent errors remain unexplained, but it appears that in particular time aliasing of the evolution of the blooms by the CZCS leads to serious problems in comparisons with ship time series. It also seems that region‐specific pigment algorithms are required for proper quantitative interpretation of remote sensing data from t

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11483

年代:1990

数据来源: WILEY

摘要:

Satellite‐derived pigment concentrations from the west coast time series (WCTS) are averaged into monthly mean fields over the California Current system (CCS) for the period July 1979 to June 1986. Errors caused by the scattering algorithm used in the WCTS are reduced by an empirical correction function, although winter values (November–February) remain unreliable. For the March–October period we look at both the mean seasonal development and the nonseasonal monthly anomalies of pigment concentration. These are compared with fields of alongshore wind stress, mixing power of the wind (u*3) and wind stress curl. Outside of the Southern California Bight there is a strong seasonal cycle with a spring‐summer maximum, a northward progression of high pigment concentrations from California to Oregon and a double maximum off Washington (spring and summer, with a lull in between). Within the Southern California Bight, seasonality is low, with a relative minimum in late summer. Off Baja California the pattern is similar to that off northern California. In regions where previous work has been done, there is general agreement with the seasonal cycles found here. Nonseasonal variability in pigment concentration over the large‐scale CCS (400 km wide) is most closely related statistically to synoptic fields of wind stress curl. Within 100 km of the coast, the strongest relations are between pigment concentration and bothu*3and alongshore wind stress. Correlations with these wind variables account for only 25% of the monthly variance in anomalous satellite‐derived pigment concentrations. This is partly due to the noise in both wind and pigment data sets but also demonstrates the fact that much of the anomalous pigment variability is not a response to anomalous wind forcing on these time scales. Correlations are also low between anomalous pigment concentrations and anomalous sea level heights, which serve as a crude proxy for the strength of the alongshore current over the shelf. The largest nonseasonal anomaly in the data occurred during the 1982–1983 El Niño, which caused a large‐scale decrease in pigment concentration, stronger and longer lasting in the south t

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11501

年代:1990

数据来源: WILEY

摘要:

Two‐dimensional wave slope spectra have been measured in the large Delft wind‐wave facility using an imaging optical technique and digital image processing. The data cover wavelengths from 0.4 to 24cm and wind speeds (U10) from 2.7 to 17.2 ms−1. The spectral densities of small gravity waves at higher wind speeds are proportional tok−3.5andu*. Capillary‐gravity and capillary waves show features which clearly manifest that the energy balance for these waves is much different from that for gravity waves. The degree of saturation is approximately constant at a given wind speed, but strongly increases with friction velocity (∝u*2.5). A sharp cutoff, which is almost independent of the wind speed, occurs at a wavelength of

ISSN:0148-0227    

DOI:10.1029/JC095iC07p11531

年代:1990

数据来源: WILEY