摘要:

Time changes in global mesoscale sea level variances were observed with satellite altimetry between November 1986 and March 1988, showing significant, geographically coherent seasonal patterns. The NE Pacific and NE Atlantic variances show the most reliable patterns, higher than their yearly averages in both the fall and winter. The response to wind forcing appears as the major contributor to the NE Pacific and Atlantic signals; errors in the estimated inverse barometer response due to errors in atmospheric pressure, residual orbit errors, and errors in sea state bias are evaluated and found to be negligible contributors to this particular signal. The equatorial regions also show significant seasonal patterns, but the uncertainties in the wet tropospheric correction prevent definitive conclusions. The western boundary current changes are very large but not statistically significant. Estimates of the regression coefficient between sea level and significant wave height, an estimate of the sea state bias correction, range between 2.3% and 2.9% and vary with the type of orbit correction applied.

ISSN:0148-0227    

DOI:10.1029/JC094iC12p17959

年代:1989

数据来源: WILEY

摘要:

We have developed a new technique for extracting global mesoscale variability from satellite altimeter profiles having large radial orbit error (∼3 m). Long‐wavelength radial orbit error, as well as other long‐wavelength errors (e.g., tides, ionospheric‐atmospheric delay, and electromagnetic bias), are suppressed by taking the derivative (slope) of each altimeter profile. A low‐pass filter is used to suppress the short‐wavelength altimeter noise (λ3 km) adjacent to continental shelves, spreading ridges, and oceanic plateaus. Variability is low in shallower areas (<3 km). Along the ACC, the meso‐scale variability appears to be organized by the many shallow areas in its path. We do not see convincing evidence that variability is higher downstream from topographic protrusions. Instead, the areas of highest variabil

ISSN:0148-0227    

DOI:10.1029/JC094iC12p17971

年代:1989

数据来源: WILEY

摘要:

A numerical model of the Indian Ocean, driven by climatological monthly mean winds, realistically simulates the major features of the large scale upper ocean circulation observed in the southern hemisphere and equatorial regions. The principal feature in the tropical Indian Ocean is a basin‐wide clockwise southern hemisphere (cyclonic) gyre comprised of the South Equatorial Current to the south, the South Equatorial Countercurrent to the north, and the East African Coastal Current in the west. Rossby waves propagate westward in the shear zone between the South Equatorial Current and the South Equatorial Countercurrent, and are obstructed and partially reflected by the banks along the Seychelles‐Mauritius Ridge (60°E). A region of high eddy activity northwest of Madagascar is an extension of the tropical gyre and is a tropical analog to the Gulf Stream recirculation region. Oscillations in meridional transport at the equator have westward phase speed and eastward group velocity and are the result of mixed Rossby‐gravity (Yanai) waves forced by oscillations in the highly nonlinear western boundary current region. Oscillations with 40‐ to 50‐day periods are seen in most currents. These oscillations cannot be atmospherically forced, as the shortest period in the mean monthly wind forcing is 60 days. Mean transports in the western basin agree with observations. Small (2 Sv) mean throughflow from the Pacific to the Indian Ocean at the eastern open boundary is due to wind‐forced Indian Ocean dynamics alone and is within the range of observations of throughflow from

ISSN:0148-0227    

DOI:10.1029/JC094iC12p17985

年代:1989

数据来源: WILEY

摘要:

Four meridional transects of the turbulent dissipation rate ε and of shear were taken across the equator along 140°W in late 1984. Averaging the upper 110 m of the transects, we found that the vertically averaged ε as well as shear showed maximum values within ±1° latitude. Because a strong diurnal cycle of ε aliased the measurements, we cannot extract spatial variations with good accuracy. However, when ε is separated into the contributions from the thermocline and from the surface mixed layer, consistent patterns emerge. In the thermocline, ε was strongest where the Richardson number was lowest, i.e., near the equator—consistent with the production of turbulence by the shear of the undercurrent. In the surface mixed layer near the equator, ε was usually larger than predicted by similarity scaling, but the discrepancy varied with the diurnal cycle of shear. When the mean shear was low, after prolonged mixing by nighttime convection, ε was close to the similarity scaling. We therefore attribute the excess dissipation to shear

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18003

年代:1989

数据来源: WILEY

摘要:

Experiments with a low resolution, multilayered model of the oceanic general circulation with wind and thermal forcing were carried out in order to identify processes controlling important aspects of the general circulation. Emphasis was placed on the sensitivity of the model's solution to the sea surface thermal boundary condition. For long restoring time scale, wind‐driven circulation dominated the solution, resembling the circulation in the North Pacific. For short restoring time scale, strong thermohaline circulation driven by deep water formed at high latitude dominated the solution, resembling the circulation in the North Atlantic. Accordingly, other aspects of the circulation, such as the baroclinic structure of the currents and the meridional mass flux partition, changed with the restoring time scal

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18011

年代:1989

数据来源: WILEY

摘要:

A new class of nearshore waves based on the shear instability of a steady longshore current is discussed. The dynamics depend on the conservation of potential vorticity but with the background vorticity field, traditionally the role of Coriolis in larger scale flows, supplied by the shear structure of the longshore current. The resulting vorticity waves are longshore‐progressive with celerities roughly equal toV0/3, whereV0is the peak longshore current velocity. A natural frequency scaling for the problem isfs, the shear of the seaward face of the longshore current. While the instability can span a range of frequencies and wavenumbers, a representative frequency is given by 0.07fs, typically in the range of 10−3–10−2Hz (called the Far Infragravity, or FIG, band because frequencies are just below those of the infragravity band). Wavelengths are of the order of 2x0, wherex0is the width of the longshore current. Growth is exponential with ane‐folding time that is typically half of a wave period. Field data, presented in the companion paper [Oltman‐Shay et al., this issue], demonstrate the presence of energetic motions from a natural beach whose behavior matches the theory in many aspects. Results from the model suggest that shear instability will be more important on barred, rather than monotonic beach profiles, a result of the stronger shears expected over the bar crest. Since vorticity waves will probably have a profound effect on cross‐shore mixing as well as longshore current dissipation, we expect the dynamics of barred and monotonic beaches to show fundamenta

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18023

年代:1989

数据来源: WILEY

摘要:

A new type of alongshore progressive wave with periods and alongshore wavelengths of the order of 102seconds and meters, respectively, has been observed in the surf zone. These periods fall into the lower end of the much studied infragravity frequency band previously shown to contain surface gravity edge and leaky waves. However, their short wavelengths (more than an order of magnitude smaller than the mode 0 edge wave) distinguish these new waves from surface gravity waves. In addition, they are only observed in the presence of mean longshore current and they change celerity (O(1 m/s)) and direction with the mean current. Alongshore wavenumber‐frequency spectra clearly identify these waves, distinct from edge and leaky waves, by their approximately linear dispersion line at wavenumbers greater than the mode 0 edge wave dispersion curve. Their rms horizontal velocities can exceed 30 cm/s. These waves are shown to be consistent with a model [Bowen and Holman, this issue] of vorticity waves generated by the shear instability of the mean longshore curren

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18031

年代:1989

数据来源: WILEY

摘要:

The Mackenzie shelf is a broad, estuarine region bordering the southeastern Beaufort Sea in the Arctic Ocean. Its fields of temperature and salinity result from the modification of offshore water masses by river inflow, ice melting and freezing, solar insolation, and air‐sea exchange. We here relate water masses resident on the Mackenzie shelf to the large‐scale oceanography of the Arctic mediterranean. The summertime exchange between the shelf and open ocean is largely confined to waters lying above the main halocline (S34.2 psu). Cross‐shelf property distributions show that individual water masses maintain their structural identity (i.e., core properties and buoyancy frequency) as they move across the shelf and participate in the estuarine circulation. Shelf waters are strongly influenced by river inflow; however, the concept of a single “plume” issuing from the incoming river and forming a strictly two‐layered structure over uniform shelf water is misleading, since a variety of temperature, salinity, and turbidity fronts co‐exist on the shelf at

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18043

年代:1989

数据来源: WILEY

摘要:

The distributions of δ18O, salinity, temperature, and nutrients have been used to quantify water sources to the Mackenzie shelf in the Beaufort Sea. Comparison of water mass analyses with satellite imagery confirms that the meteoric (runoff) water is associated with the Mackenzie plume. The seasonally variable surface layer for the shelf is viewed as cycling between a “reverse estuary” in winter, when the polar mixed layer (PML) is formed, and a positive estuary in summer when the shelf waters respond to freshwater inputs (runoff and ice melt). We infer a standing stock of 3.7 m fresh water at the end of summer 1986, of which 30% owes its origin to the melting of sea ice; our data coupled with river flow imply a freshwater flushing time for the Mackenzie shelf at about 150 days. To re‐form the PML during winter requires the removal of this seasonal fresh water through the combined processes of flushing and ice formation: once this fresh water has been removed, continued ice growth can produce “new” brine which would be observed as a deeper and saltier PML from the previous year. A simple geochemical model shows that autumn conditions (freshwater accumulation) and the rate of flushing are important controls on the potential of the shelf to produce “new” brine and that winter runoff, were it to distribute evenly across the shelf, is sufficient to inhibit br

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18057

年代:1989

数据来源: WILEY

摘要:

An array of current meters was deployed along the continental shelf and upper slope from April 15 to July 1, 1982, as part of the Warm Core Rings Experiment. The array consisted of four mooring sites along the 325‐m isobath making an “X” with another line of moorings extending from the 60‐m to the 900‐m isobath. The purpose of the array was to examine the current and temperature variability induced at the shelf break and upper slope by the formation and passing of ring 82B. The velocity records consisted of 2‐ to 10‐day fluctuations embedded on a long‐period signal with a period ranging from 15 to 26 days, increasing offshore. The temperature records contained three time scales: The 2‐ to 10‐day scale, longer (about 1–2 months) scales due to movements of the shelf‐slope front induced by the ring, and an increasing trend due to seasonal warming. The 2‐ to 10‐day fluctuations of velocity and temperature were not statistically coherent with each other or with the weak surface wind stress but appear to be visually correlated with small (sub‐ring) scale undulations along the shelf‐slope front as seen in the advanced very high resolution radiometer (AVHRR) sea surface temperature imagery on an event by event basis. The observed periods and across‐shore scales of the long‐period motions were used with barotropic topographic Rossby wave (TRW) theory to estimate alongshore wavelengths of the order of 1000 km for these motions, westward and slightly offshore phase velocities, and onshore group velocity. Friction and wave refraction will not allow barotropic TRWs of the observed period to penetrate the upper slope unless the source is located nearby on the continental rise, which strongly suggests Gulf Stream warm core ring 82B as the energy source for the waves. The observed wave energy was used with the radiation model of Louis and Smith (1982) to estimate the rate at which energy was being lost from the source of the waves. The decay rate calculated from the moorings and the model was 0.24×10−13J d−1, averaged over 6–14 weeks after ring formation, compared with 0.81×10−13J d−1for the decay rate of ring 82B (Olson et al., 1985) based on observed changes in the ring's av

ISSN:0148-0227    

DOI:10.1029/JC094iC12p18071

年代:1989

数据来源: WILEY