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Gunnar I. Roden

Abstract

Mesoscale sound speed fronts in the central and western Pacific are investigated on the basis of closely spaced CTD stations. In the flat-bottomed central Pacific where currents are weak, the main sound speed fronts occur neat the subarctic and subtropical water mass boundaries. In the western Pacific, where the topography is rugged and currents are strong, fronts are encountered not only near water mass boundaries, but throughout the region. Vertical oscillations of the sonocline are large and give rise to deep and at times quasi-regularly spaced (∼400 km) sound speed fronts. In the vicinity of the Emperor Seamounts a variety of sound speed fronts is observed. Frontal deflections are observed over Suiko, Nintoku, Jingu, and Kinmei seamounts. The individual patterns vary, though there is a tendency for the fronts to be deflected northward upon approaching the seamounts. A Taylor-column-like feature occurs over Suiko Seamount. Conspicuous sound speed domes occur near Jingu and Kanmu seamounts. The diameter of these domes is about 150 km and the isoson displacement relative to the surroundings reaches 300 m. The domes do not reach the sea surface but terminate at the depth of the sharp seasonal sonocline near 50 m. Significant perturbations of sound speed near seamount flanks are limited to depths above 2000 m. The largest perturbations occur within 100 km of the seamounts.

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Gunnar I. Roden

Abstract

The duration of nonseasonal sea and air temperature oscillations above and below arbitrarily fixed levels is investigated. There is close agreement between the observed mean durations and those expected from theory for a Gaussian random variable. Mean durations of sea temperature anomalies from long term monthly means above and below the 0C level vary between 3 and 5 months, those of air temperature between 2.5 and 3 months. There is almost no dependence upon latitude. The probability distributions are log-normal for durations not exceeding 2 or 3 times the mean duration. The largest observed durations above and below the 0C level are between 1 and 2 years while those above the +1.5C or below the −1.5C levels are between 1 and 2 months.

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Gunnar I. Roden

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 and 10−3 s−1 and is strongest below the base of the mixed layer. The principle of conservation of potential vorticity is applied to fronts and compared to observations.

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Gunnar I. Roden

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 chain. Meridional gradients at the sea surface are coherent with those in the upper 150 m and incoherent with those below. Meridional gradients at 300 m have a good coherence with those at greater depths. The coherence between meridional temperature and salinity gradients increases with increasing depth. The depth dependence of the spectra and coherence is attributed to different processes of gradient formation in the upper and lower layers of the sea. A comparison of the wavenumber spectra of the meridional gradients with the wavenumber spectra of zonal gradients derived from Bernstein and White's (1977) and Wilson and Dugan's (1978) data shows that in each case the dominant spectral peak occurs between about 1.5 and 3 c.p. 1000 km, indicating the prevalence of features with zonal and meridional wavelengths in the 300–600 km range.

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Gunnar I. Roden

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 essentially a salinity front, deepening from east to west, and is marked by horizontal gradients of 0.7%‰ per 36 km and weak baroclinity in the upper 50–175 m. Numerous salinity and some temperature inversions occur in the vicinity of the subtropical front.

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Gunnar I. Roden

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 similarity to the configurations of the surface isotherms and isohalines, indicating that surface thermohaline fronts are not determined by the baroclinic flow field. Instead, agreement is found between the subtropical frontal zone and the Ekman confluence zone on long time scales. A warm and saline anticyclonic eddy with large thermohaline gradients around its periphery is found near 29°30′N, 158°W. The mean baroclinic flow in the subtropical frontal zone is ∼0.04 m s−1 and does not vary with season. Perturbations from the mean flow are up to 0.4 m s−1 and vary strongly with season. Aspects of frontogenesis in the subtropical frontal zone are investigated. In the upperlayer, wind-induced differential horizontal advection of the Ekman type leads to concentration of horizontal thermohaline gradients. In the lower layer, differential vertical advection is primarily responsible for the concentration of such gradients.

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Gunnar I. Roden

Abstract

Records of sea-surface temperature, cloudiness and wind are investigated for eight ocean areas along the shipping lane between Europe and South America. There was a gradual increase in temperature over most of the tropical Atlantic between 1922 and 1938 (Bullig, 1954). The total warming over the 17-yr period varied between 0.6C and 0.9C and was strongest at latitude 7N. In the vicinity of the Cape Verde Islands, the strength of the northerly wind component decreased by about 0.8 m sec−1. Off the central Brazilian coast, the amount of cloud cover decreased by 5 to 7 per cent.

The power spectra of temperature, cloudiness and wind anomalies are investigated for the frequency range between zero and six cycles per year. It is found that most of the power of temperature anomalies is concentrated at low frequencies and that there are no periodicities. A significant and inverse relation between temperature and wind anomalies is found in the NE trade-wind region. Along the Brazilian coast there is a direct relation between temperature and north wind anomalies, and an inverse relation between temperature and cast wind anomalies. No significant relation exists between temperature and cloudiness anomalies.

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Gunnar I. Roden

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 several forcing regions by the curl of the wind stress coexist, the observed dynamic height sections show complex and asymmetric perturbations. The observed 400–600 km wavelengths are commensurate with planetary waves of diverse origin; the exact processes of their formation are poorly understood at present.

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Gunnar I. Roden

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 (STD's). 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 layer. Inflow into the gulf occurs over a broader area, mainly in the central part. North of Isla Socorro, a strong west-northwestward setting current is observed. It is about 150 km wide, 700 m deep and has maximum speeds of 33 cm sec−1 just above the high stability layer. Near the island, a narrow countercurrent with speeds up to 17 cm sec−1 is found. Off Cabo Corrientes, a strong northwestward setting current is encountered. The current is about 100 km wide, extends to a depth of 700 m, and has a maximum speed of 46 cm sec−1 near the surface.

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Gunnar I. Roden

Abstract

In the central North Pacific, temperature and salinity depth profiles show considerable fine structure, the features of which are most pronounced in the thermocline and halocline, as well as near water mass boundaries. By partitioning the observed series into mean and perturbation values, it is possible to obtain some statistical measures for the perturbations. Wavenumber spectra of temperature, salinity, sound velocity and density were computed for the wavenumber range between 0 and 167 cycles per kilometer. The perturbation spectra for the different geographical regions of the transition zone are essentially the same, for a given variable. In all cases the power density of the perturbations decreases with increasing wavenumber. The power law decay coefficient at low wavenumbers is about −8/3. It decreases gradually with increasing wavenumber. Superimposed upon the smooth decrease in power density are random fluctuations, most of them insignificant at the 95% confidence level. The perturbations are generally incoherent over distances of the order of 50 km. For a given station, temperature and salinity perturbations are coherent at low wavenumbers, but incoherent at high wavenumbers.

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