摘要:

Observed deviations of monthly mean sea level at seven island stations along the equator in the Pacific have been superimposed on the mean dynamic topography to determine the variations of the east‐west slope of sea surface topography during the 1982/1983 El Niño. The normal east‐west slope is eliminated in January 1983 when the bulk of warm water flowing eastward has reached the coast of South America. The sea surface remains essentially flat from January to June 1983, although zonal winds are very weak until April only. The slope requires several months to be reestablished, and in October and November, sea level along the entire equator is 10 cm or more below normal, indicating a net loss of warm water from the equatorial Pac

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10419

年代:1984

数据来源: WILEY

摘要:

During the 1982–83 El Niño/Southern Oscillation event, sea level across the width of the equatorial Pacific adjusted to the reversal of the equatorial trade winds, and by the end of 1982 the normal sea level slope across the Pacific had been eliminated. The transfer of warm upper‐ocean water from the western Pacific to the eastern Pacific was accomplished by a combination of direct wind forcing as the wind anomaly crossed the basin and by mass flux induced by free equatorial waves. The importance of equatorially trapped Kelvin waves of first and second vertical mode during the onset of the 1982–83 El Niño is inferred from the cross‐correlation statistics between central and eastern Pacific sea level stations and between wind variations in the western Pacific and equatorial sea level stations to the east. The different propagation speeds of these two modes appears to be responsible for the observed change inshape of the major sea level signals during the 1982–83 event. Tentative evidence for first baroclinic mode, first meridional‐mode Rossby waves is a

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10425

年代:1984

数据来源: WILEY

摘要:

Data from 20 satellite‐tracked drifting buoys deployed in the eastern equatorial Pacific Ocean during the summer of 1979 were used to investigate the nature, effects, and energetics of currents associated with cusp‐shaped long waves observed in satellite IR imagery of the sea surface during all except El Nino years. It is shown that the long waves are associated with a vigorous pattern of mesoscale eddies lying primarily between the equator and 7°N. The spatial structure of the eddy kinetic energy closely resembles the eigenfunctions obtained from Philander's (1978) investigation of barotropic instability of that part of the South Equatorial Current lying north of the equator. Computation of the energy exchange between the eddies and the mean field suggests anefolding time of about two weeks for the eddies and a braking effect on the mean flow comparable to a change of surface wind stress of a few tenths of a dyne per square centimeter. The eddies also effect an equatorward transport of heat that amounts to about two thirds of the poleward heat transport of the divergent Ekman transport in the near‐surface

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10431

年代:1984

数据来源: WILEY

摘要:

CTD data from the EPOCS and NORPAX experiments were used to construct a mean zonal pressure gradient profile between 110°W and 150°W for the 1979 to 1981 pre‐El Niño time period. The mean zonal pressure gradient referenced to 100 dbar had a value of −4.5 × 10−7m s−2at the surface and decreased to within two standard errors of zero at 200 dbar. Below this depth the mean was not significantly different from zero. Deviations from this mean were not related to fluctuations of the zonally averaged zonal wind stress between 150°W and 110°W. Rather, the largest deviations, in the boreal springs of 1979 and 1980, appeared to be associated first with vertical‐mode Kelvin wave pulses propagating eastward along the equator. The 1979–81 mean profile was also used as a reference to examine variations in zonal pressure gradient during the 1982/83 ENSO event. The sea surface was more sharply sloped upward from the east to west in April and September 1982 before arid near the beginning of the event. The surface slope was below normal in April and October 1983. Vertical profiles of the zonal pressure gradient deviations showed large high vertical‐mode contributions during the vent than in

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10441

年代:1984

数据来源: WILEY

摘要:

The upper ocean temperature and dynamic height fields are correlated in the western North Pacific Ocean. This relationship can be used to map a relative geostrophic stream function of the Kuroshio Extension using temperature data from expendable bathythermograph (XBT) surveys. We have examined the ability of the North Pacific ship of opportunity XBT program, known as TRANSPAC, to carry out this procedure. Estimates of 600/1200 dbar geostrophic shear were generated from the XBT data utilizing objective mapping techniques. These estimates were compared with measured currents at 600 m relative to 1200 m along 152°E from 27°N to 42°N. The estimated and measured relative currents are positively correlated, with an rms error of ±6 cm/s over a measured dynamic range of 30 cm/s. However, the XBT estimated shears consistently underestimate the measured values by a factor of 2 to 3. The underestimate of the amplitude is the result of both undersampling and mapping produres that eliminate high wavenumber contributions of the dynamic height variability. These factors appear to be responsible for previous differences in amplitude between directly measured and TRANSPAC XBT estimated values of relative eddy kinetic energy and Reynolds stress. The TRANSPAC XBT survey can only monitor qualitatively the dynamic topography of the Kuroshio Extension meanders and rings. The variability outside of the extension with a temperature range of 3°C cannot be resolved because of inaccuracies in the temperature‐dynamic height relationship. To monitor the Kuroshio Extension variability in the future, it is recommended that TRANSPAC XBT resources in the western North Pacific concentrate on obtaining a denser sample of the field during the fall and winter seasons because of incomplete ship track coverage in spring and

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10451

年代:1984

数据来源: WILEY

摘要:

We derive a simple analytical model of the boundary layer induced by rotary tidal currents flowing over a horizontal bottom in the absence of strong vertical stratification, formulating explicit estimates of the shear velocity, dissipation, and friction factors relating inviscid velocity to bottom shear stress. Accordingly, these friction factors may be calibrated by an observed velocity profile through the rotary bottom boundary layer, thereby reducing the empiricism of existing depth averaged models of tidal hydraulics. We illustrate the procedure by analyzing measured M2 rotary current profiles on the outer Europeann shelf with good model accuracy; the local dissipation‐based friction factor compares favorably with a global value based upon a calibrated numerical model of the northwest European shelf as wel

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10461

年代:1984

数据来源: WILEY

摘要:

The Backus‐Gilbert method is used to evaluate the quality of estimates of sound speed anomaly and (two) current components obtained from acoustic travel time data in a horizontal ocean slice. The approach is more flexible than previous ones in that reciprocal transmissions are not required and, if present, are not rigidly coupled by addition and subtraction. Nine categories of bias error are identified. They are associated with imperfect resolution of the three unknowns and with cross contamination between them. Measures of these errors are developed that are system properties, as are stochastic error and resolution length. Bias error is calculated in numerical experiments and is combined with stochastic error to obtain mean error. The various quantities contributing to the mean error can be selectively reduced and brought into balance by trading off resolution. It is shown that it is possible in principle to estimate currents from one‐way transmission data, and results suggest that further study is merited. It is shown that array design can incorporate a mixture of reciprocal and nonreciprocal transmissions and that equal weights for reciprocal data pairs are not always opti

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10469

年代:1984

数据来源: WILEY

摘要:

Whitehead and Worthington (1982) have measured the fluxes of heat and salt due to the northward flow of Antarctic Bottom Water through a passageway 300 km wide between the Ceara Rise and the Mid‐Atlantic Ridge at about 4°N. Downstream of this “sill” the temperature and salinity of the underflowing water increase, and Whitehead and Worthington have described this change as being due to “downward” fluxes of heat and salt across isothermal surfaces. We consider the relative roles of three separate mixing processes to these “downward” fluxes of heat and salt across isotherms, and we use this information to decide between Whitehead and Worthington's two separate estimates of the volume transport of Antarctic Bottom Water: one based on current meter data, and the other based on geostrophic calculations. The slope of the θ‐Slocus of bottom water as it moves northward past the equator provides a valuable extra constraint on the relative importance of the three mixing processes. We conclude that the dominant mixing process is diapycnal (i.e., cross‐isopycnal) turbulent eddy diffusion and that the geostrophic data set of Whitehead and Worthington is consistent with the mixing ideas presented here, whereas their current me

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10479

年代:1984

数据来源: WILEY

摘要:

A scale for the thickness of layers in regular “diffusive”‐type thermohaline staircases, derived from dimensional analysis, is found to collapse oceanic data. Combining this scale with laboratory‐derived double‐diffusive flux laws, we formulate effective diffusivities for salt, heat, and density. The diffusivities depend on the Turner numberRρ, but are independent of the Brunt‐Vaisala frequency. ForRρnear 1 the diffusivities for salt and heat are approximately equal (≃10−4m2s−1). They decrease roughly asRρ−4andRρ−2, respectively, over

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10484

年代:1984

数据来源: WILEY

摘要:

The waters at Drake Passage are known to be separated horizontally into four distinct zones of relatively small geostrophic shear by three fronts in which the shear and eastward flow are large. The two northern fronts, Subantarctic Front and Polar Front, that bound the Polar Frontal Zone comprise most of the transport of the Antarctic Circumpolar Current. Vertical sections of closely spaced (∼50 km) oceanographic stations made across Drake Passage in the summers of 1975 and 1976 produced quality measurements of temperature, salinity, oxygen, silicate, nitrate, and phosphate. Based on these data, a traditional description of characteristic distributions is presented. Good sampling resolution in the vertical allows maxima in hydrostatic stability to be traced laterally (approximately along isopycnals) between individual stations. Eleven strata of maximum stability are identified. Most vertically separate distinct water masses. The most pronounced stability stratum is associated with the seasonal pycnocline; north of the Polar Front, it separates the Subantarctic Surface Waters from Subantarctic Mode Water. To the south of the Polar Front the seasonal pycnocline is found slightly above the temperature minimum characteristic of the Antarctic Surface Waters in austral summer. This stability stratum extends northward near the base of the Subantarctic Mode Water. Another stability stratum extending across the entire passage forms the upper boundary for the Circumpolar Deep Water. South of the Polar Front, this stratum is at the base of the Antarctic Surface Water; to the north it is the lower boundary of the Antarctic Intermediate Water. Although Subantarctic Mode Water and Antarctic Intermediate Water are separated by a stability stratum north of the Polar Front, there is no such separation between Antarctic Intermediate Water and the temperature minimum of the Antarctic Surface Water south of the front. This suggests that Subantarctic Mode Water and Antarctic Intermediate Water are distinct and that the latter derives from the surface waters south of the Polar Front. At greater depths a horizontally discontinuous stratum is found between the Upper and Lower Circumpolar Deep Water. Two horizontally continuous stability strata are found near the base of the Circumpolar Deep Water, marking the transition to a high‐silicate variety of denser deep water from the Southeast Pacific Basin. The densest water mass found is Weddell Sea Deep Water, which flows through a gap in the South Scotia Ridge and along the southern Scotia Sea to fill the trough in the southern Drake Passage. It is overlain by a laterally continuous stratum of relative maximum stabil

ISSN:0148-0227    

DOI:10.1029/JC089iC06p10489

年代:1984

数据来源: WILEY