Gunnar I. Roden

Thermohaline Structure and Baroclinic Flow Across the Gulf of California Entrance and in the Revilla Gigedo Islands Region

Abstract The thermohaline structure, vertical stability and baroclinic flow between the Gulf of California entrance and the Revilla Gigedo islands are investigated by means of closely spaced salinity and temperature depth profiles (STDs). The outstanding feature of the upper ocean is the 10–30 m thick layer of high stability centered at about 50 m. This layer separates the tropical surface water from the thin shallow salinity minimum below, which is most pronounced between 80 and 130 m. The outstanding feature of the deep water is the marked increase in abyssal temperatures east of Isla Socorro, apparently due to heat flow. Baroclinic flow in the Revilla Gigedo islands region is characterized by high-speed flow near capes and islands. Outflow from the Gulf of California takes place in a narrow high velocity core near its western side. The width of the high-speed core is of the order of 30 km and speeds >30 cm sec−1 occur down to 700 m. The high-speed flow is accomplished by a break in the high stability ...

On the Subtropical Frontal Zone North of Hawaii During Winter

Abstract Oceanic fronts in the subtropical frontal zone north of Hawaii are investigated and related to atmospheric forcing. Particular attention is paid to the winter of 1974 when a detailed study was made of the thermohaline structure aboard the R.V. Thomas G. Thompson. In that winter, well-defined fronts occurred at 34, 31 and 28°N. In the upper 100 m, these fronts are nearly vertical and are characterized by temperature, salinity and sound velocity gradients of up to 2°C (27 km)−1, 0.3‰ (27 km)−1 and 12 m s−1 (27 km)−1, respectively. Horizontal density gradients across the northern two fronts are small because of compensating horizontal temperature and salinity gradients. A thin layer of increased stability is encountered between 100 and 125 m. Below this layer, there are prominent lateral intrusions of cool and low-salinity subsurface water under warmer and higher salinity surface water, at latitudes north of 31°N and longitudes east of 155°W. The 0/1500 db dynamic height topography bears no similari...

Oceanic Subarctic Fronts of the Central Pacific: Structure of and Response to Atmospheric Forcing

Abstract The oceanic fronts in the subarctic region of the central North Pacific are investigated and related to atmospheric forcing. The thermohaline structure indicates a near balance between horizontal temperature and salinity gradients, resulting in weak density gradients and the absence of baroclinic jets. Large temperature inversions are found inside the permanent subarctic halocline and pycnocline, which are attributed to overrunning of cool, low-salinity water from the north over warmer, more saline water from the west. During summer, a shallow SOFAR channel is found between the bottom of the seasonal pycnocline and the top of the permanent pycnocline. The frontal structure in the latitude belt between 39° and 47°N is complicated and is characterized by multiple thermohaline and sound velocity fronts. Frontogenesis in the mid-Pacific depends strongly upon differential advection of the Ekman type. During fan, characteristic magnitudes of temperature frontogenesis due to this cause are 1°C (100 km)−...

Large-Scale Upwelling off Northwestern Mexico

Abstract The seasonal variation of large-scale upwelling off northwestern Mexico is investigated by means of the divergence of the Ekman transport, computed for a 1° latitude-longitude grid. It is found, by using such a close grid, that the vertical velocity is resolved in sufficient detail to bring out many important features of upwelling that agree favorably with the temperature distribution. The limitations of the method are discussed.

Electronic Digitization and Sensor Response Effects on Salinity Computation from CTD Field Measurements

Abstract Spikes are often observed in salinity profiles computed from measurements of conductivity, temperature and pressure. Many of these spikes are not real and are the result of a mismatch in the response functions of the sensors. Some of the spikes are also due to the sequential sampling technique used by most digitizers whereby the sensors are not sampled at the same time or position. We derive expressions to linearly correct for these two causes of spikes. When the corrections are applied to measurements in the North Pacific, a significant reduction in the number and size of the spikes is observed in high gradient regions such as the thermocline.

On Long-Wave Disturbances of Dynamic Height in the North Pacific

Abstract Long-wave disturbances of dynamic height in the North pacific are investigated by means of closely spaced STD (salinity, temperature, depth) stations and are related to atmospheric forcing. Wavelike disturbances with length scales between 400 and 600 km are common in the latitude range between 20° and 50°N. The wave amplitudes are larger in the western than in the central and eastern Pacific, and they depend upon season. A large eastward decrease in wave amplitude is observed near the Emperor seamount chain. The wavelike disturbances extend to several hundred meters’ depth, with an exponential-type decrease in wave amplitude. The baroclinic currents associated with the dynamic height perturbations are an order of magnitude larger than the mean currents. The perturbations are related to atmospheric forcing, in particular the curl of the wind stress field. Where the distribution of the curl is zonal, meridional waves are excited, which are reflected in north–south dynamic height sections. Where sev...

Mesoscale Thermohaline, Sound Velocity and Baroclinic Flow Structure of the Pacific Subtropical Front During the Winter of 1980

Abstract The three-dimensional thermohaline. sound velocity and baroclinic flow structure of the. pacific sub-tropical front during January and February 1990 are discussed. The front is meander-like, with a wavelength of 180 km, a wave amplitude of 55 km, and is centered around latitude 30°N. On 37 km spatial and 10-day time scales the front barely moves, but its intensity increases due to an intrusion of cold and low-salinity water from the north. The front is nearly vertical in the upper 130 m and slopes southward below. The strongest cross-frontal differences are 1.7°C for temperature, 0.6‰ for salinity. 0.4 kg m−3 for density and 13 m s−1 for sound velocity and occur beneath the base of the mixed Layer. The baroclinic flow field indicates a meandering current with maximum speeds of 0.3 m s−1. Cyclonic eddies occur north and anti-cyclonic eddies occur south of the front. Satellite-tracked drifters confirm these patterns. Vertical shear of the baroclinic flow in the frontal region varies between 10−4 an...

Thermohaline Structure, Fronts, and Sea-Air Energy Exchange of the Trade Wind Region East of Hawaii

Abstract The thermohaline structure, fronts, and sea-air energy exchange of the trade wind region east of Hawaii are investigated. The wind stress, heat, salt and buoyancy flux fields at the sea surface are evaluated on a 1° latitude-longitude grid and are related to the main thermohaline features of the upper ocean sampled meridionally at 36-km intervals. It is shown that there exists a close relation between the configuration of the above-mentioned fields and the meridional temperature and salinity structure. Two oceanic fronts occur at the borders of the trade wind region. The southern, or doldrum, front occurs in the zone of strongest net evaporation gradients, between latitudes 11 and 12N, and is characterized by salinity gradients of 1%o per 36 km and strong baroclinity in the upper 50 m. The northern, or subtropical, front is encountered between latitudes 31 and 33N in the region of confluence of the California current with the trade wind drift. During summer and fall, the subtropical front is esse...

Bispectra and Cross-Bispectra of Temperature, Salinity, Sound Velocity and Density Fluctuations with Depth off Northeastern Japan

Abstract Off northeastern Japan, depth profiles of temperature, salinity, sound velocity and density show considerable structure between 30 and 240 m. The structure is asymmetric with respect to the mean and the amplitudes are 10 times larger here than in similar latitudes in the mid-Pacific. By partitioning the observed series into mean and perturbation series, it is possible to compute the bispectrum and the bicoherence for each variable, as well as the cross-bispectrum and the cross-bicoherence for different variables. The computations were carried out for the wavenumber plane bounded by |k1|≤167 cpkm, cycles per kilometer (cpkm), |k2|≤167 cpkm, |k1 + k2|≤167 cpkm, with an areal resolution of 48.2 (cpkm)2. The “third-power density” of the bispectra decreases toward the Nyquist wavenumber by about three orders of magnitude. The rate of decrease is not constant, but depends upon both wavenumber and direction. There is no statistically significant structure superimposed upon the slopes. The absence of pro...

The Depth Variability of Meridional Gradients of Temperature, Salinity and Sound Velocity in the Western North Pacific

Abstract In the western North Pacific, meridional gradients of temperature, salinity and sound velocity show considerable variation with depth. Gradients of frontal intensity (more than three times the rms value) occur in the upper 600 m of the ocean. Fronts in the surface layer are spaced at irregular intervals. Many deep fronts have no surface manifestation and are spaced at intervals between 300 and 600 km. A spectral analysis of the meridional gradients as functions of depth and longitude was carried out for the wavenumber range between 0 and 13.4 cycles per 1000 km (c.p. 1000 km). The shape of the power density spectra strongly depends on depth. In the upper 150 m the shape is irregular. Between 300 and 600 m, the spectra show a well-defined peak between 1.5 and 3.3 c.p. 1000 km and a sharp decrease in power beyond 10 c.p. 1000 km. While the shape of the power density spectra shows little variation with longitude, there is a substantial decrease in the total power when crossing the Emperor seamount c...