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Kern E. Kenyon

Abstract

Extensive STD data at a 100 km station spacing from a recent winter cruise (March–April 1976) along 35°N between California and Japan are used to describe the large-scale (5000 km) shallow salinity minimum within the upper 500 m in the eastern half of the transect in relation to the salinity structure above and below it, and to the mesoscale (500 km) variations in salinity in the western Pacific. The shallow salinity minimum, which has a vertical scale of order 100 m, was found to be a continuous feature less than 200 m from the sea surface on more than 50 consecutive stations from about 125°W to about 175°W. The depth of the shallow salinity minimum has a large-scale maximum of about 160 m near 135°W and it decreases to about 60 m east and west of the maximum. The salinity, temperature and density at the shallow salinity minimum increase westward monotonically, and the salinity deficits between the salinity at the minimum and at the salinity maxima above and below the minimum decrease westward. The vertical scale of the shallow salinity minimum increases westward. The shallow salinity minimum lies in the main pycnocline where the vertical stability is large; the maximum vertical stability occurs between the bottom of the surface layer and the depth of the shallow salinity minimum. Also within the main pycnocline and about 20 m above the shallow salinity minimum is a maximum in dissolved oxygen concentration, which was found over the same longitude range as that of the shallow salinity minimum. The salinity and oxygen data along 35°N are consistent with a source of low-salinity and high-oxygen water at the sea surface in high latitudes.

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Kern E. Kenyon

Abstract

Two physical ideas are combined to derive an expression for the depth of wave influence of a surface gravity wave without assuming the fluid motion is irrotational. One idea is that in the reference frame which makes the wave steady, the static pressure difference between crest and trough balances the dynamic pressure difference between crest and trough. The other idea is that at the depth of wave influence, the pressure under the crest equals the pressure under the trough due to the vertical acceleration of the fluid being downward at the crest and upward at the trough. The depth of wave influence is directly proportional to the wave length and independent of the wave height if the wave height is small compared to the wave length.

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Kern E. Kenyon

Abstract

A large-scale longitudinal variation in surface temperature in the North Pacific is described by an analysis of some existing data and some newer data. When monthly mean surface temperatures are plotted against longitude at mid-latitudes there is nearly always a temperature maximum in the eastern Pacific and a minimum in the central Pacific. Long-term averages of monthly mean temperature data, as well as independent surface temperature data from individual cruises, show a similar maximum and minimum. At 35°N and 40°N the mean positions of the maximum and minimum are further west in summer than in winter, and for a given month are further west at 35°N than at 40°N. The east-west scale of the temperature feature is defined to be the longitudinal distance between the positions of the adjacent temperature extremes and is shown to have an average value of about 2000 km nearly independent of season and latitude. The temperature scale is defined to be the difference between the maximum and minimum temperatures and is shown to have an average value of about 2°C at 40°N and 35°N and 1°C at 30°N. Based on these scales and the mean north-south temperature gradient, two separate north-south scales of the surface temperature feature are defined which are of order 100 and 1000 km. The data suggest a direct relationship between variations in the temperature scale and the east-west scales, with large (small) temperature scales often corresponding to large (small) east-west scale. The effect of the longitudinal temperature variation on the, seasonal range of surface temperature, as well as its connection with surface temperature anomalies, is discussed. It is suggested that the longitudinal temperature variation may be an indicator of a large-scale mechanism of poleward heat transport in the ocean.

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Kern E. Kenyon

Abstract

The influence of longitudinal variations in wind stress curt on the steady circulation in a rectangular ocean basin is investigated analytically in a linear barotropic model. It is shown that longitudinal variations in the interior circulation are produced by longitudinal variations in the magnitude of a wind stress curl of constant (negative) sign. If friction is small in the interior, the east-west variation in southward flow is directly proportional to that of the applied wind stress curl in accordance with the Sverdrup vorticity balance. Examples show that when the wind stress curl has a maximum in the center of the basin, the southward flow is also concentrated in the center of the basin. When the wind stress curl has a minimum in the center of the basin, the southward flow is concentrated in two regions, one on either side of the minimum curl. This variation in southward flow causes a concentration in cast-west flow along the northern and southern boundaries of the basin, which is not obvious from the Sverdrup balance. If the minimum value of the wind stress curl term is smaller than the friction term in the vorticity equation, there is northward flow in the center of the basin which is part of a closed anticyclonic gyre in the eastern half of the basin. Longitudinal variations in wind stress curl can also produce differences in the northward flow in the western boundary current compared to the case of a wind stress curl which is independent of longitude.

Observations of monthly mean sea surface temperatures are presented which show a quasi-steady longitudinal variation with an east-west scale of order 1000 km, an inferred north-south scale of order 100 km, and an amplitude of order 1°C at mid-latitudes in the central and eastern North Pacific. When plotted as a function of longitude, the monthly mean temperatures at 35 and 40N show a relative minimum near the central longitude of the basin. This longitudinal temperature variation exists in a long-term annual average and it also exhibits apparent seasonal changes. It is suggested that the quasi-steady longitudinal temperature variation is a real feature of the North Pacific and that it is associated with the large-scale wind-driven ocean circulation. The longitudinal distribution in mean sea surface temperature is consistent with the circulation in the dynamical model which is produced by a qualitatively realistic longitudinal variation in mean wind stress curl.

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Kern E. Kenyon and David Sheres

Abstract

Linear momentum of surface gravity waves changes with time during refraction by a horizontally variable current, as is predicted by ray theory; the momentum change per unit time requires a force by the current on the waves. According to Newton’s third law, the waves apply an equal but opposite force back on the current. The wave force of linear waves on the current is calculated for a steady horizontal shear current and it is found to be directly proportional to the wave momentum times the shear in the current. For a current like the Gulf Stream it is theoretically possible for the wave force on the current to be as large as the Coriolis force on the current to the depth of wave influence; the effect on equatorial surface currents is likely to be even more significant. Considering the reasonable conjecture that the orbital angular momentum of the waves cannot be exchanged with the current, the growth or decay of the wave amplitude in the shear current is computed as well. An exponential growth or decay of the amplitude is obtained with the e-folding scale being proportional to the current shear. A comparison between the calculated wave force and the Coriolis force for reported data describing the reflection of waves by the Gulf Stream is presented. The potential effects of the wave force on the surface extent of such currents and their observations by remote sensing, including possible bias in estimation of their transport capacity, are discussed. Instances of potential positive and negative feedback acting during the interaction between the waves and the current are outlined.

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Wilton Sturges and Kern E. Kenyon

Abstract

Several independent data sources suggest that there is a net upper-layer mass flux O(3 Sv) (Sv ≡ 106 m3 s−1) to the west in the central Gulf of Mexico, even though the western gulf is a closed basin. A plausible explanation is that this net flux is pumped downward by the convergent wind-driven Ekman pumping, as is typical of all midlatitude anticlyclonic gyres. The downward flux can follow isopycnals to depths O(500–600 m) and deeper by eddy mixing; a mechanism for forcing deep water to the south through the Yucatan Channel is provided by the intrusion and ring-shedding cycle of the Loop Current. Potential vorticity maps show that a deep flow from the western gulf back to the Yucatan Channel is likely.

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