Donald V. Hansen

Turbidity maxima in partially mixed estuaries: A two-dimensional numerical model

Abstract A steady-state numerical model has been developed to help explain the presence of the turbidity maximum in partially mixed estuaries. The model supports the hypothesis that estuarine dynamics are primarly responsible for its occurrence. The magnitude and location of the turbidity maximum is shown to depend upon the settling velocity (particle size) of the sediment, the amount of sediment introduced at both the ocean and river source, and the strength of the estuarine circulation. This phenomenon is not simply related to the river input of sediments, since results show the existence of a turbidity maximum for a sediment source which is predominately oceanic. Sediment flux stream functions show the presence of a closed cell when a turbidity maximum exists.

On the nature of decadal anomalies in North Atlantic sea surface temperature

North Atlantic sea surface temperature data from the Comprehensive Ocean-Atmosphere Data Set were used to investigate the behavior of temperature anomalies on multiple-year timescales during the period 1948-1992. Monthly anomaly time series for each 2° square from the equator to 70°N were low-pass filtered at 4 years and normalized by the local standard deviation. Attention is focused on the extreme events, the upper and lower deciles, of the anomaly time series. A 45 -year sequence of January maps shows the already familiar phenomena of generally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The years 1978 through 1982 were largely devoid of persistent strong anomalies, but moderate cold conditions returned during 1983-1986. Warm conditions dominated the North Atlantic from 1987 onward. Within these thermal epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20° of latitude or longitude, but they range from barely detectable to spanning the width of the basin, the latter especially in lower latitudes. Most of the anomalies move long distances along certain preferred paths. These paths generally follow the routes of the subarctic and subtropical gyres. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approximate route of the North Atlantic Current. Midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast. The speed of these movements (1-3 km d -1 ) is generally less than the expected speed of the near-surface ocean circulation. Simple ideas about the effects of beta dynamics and air-sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies. The long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and propagation of features.

Tropical Pacific Ocean Mixed Layer Heat Budget: The Pacific Cold Tongue

Abstract Data from satellite-tracked drifting buoys and VOS/XBT profiles for the years 1979–95 were used to evaluate the seasonal cycle of how major oceanic processes redistribute heat in the cold tongue region of the tropical Pacific. The most active processes for the annual cycle are local heat storage and heat export by entrainment of upwelling and by mean meridional advection. Heat export by zonal advection, however, is not negligible, and meridional eddy heat fluxes associated with tropical instability waves effect a negative feedback that offsets a considerable fraction of that produced by the mean meridional advection. All of these processes mimic the essentially one cycle per year of the surface wind stress, as do those of the depths of both the bottom of the surface mixed layer and the thermocline. Because it is associated with poleward Ekman transports, upwelling, and baroclinic adjustment near the equator, the zonal wind stress component appears to be the more important. The meridional wind str...

Anticyclonic current rings in the eastern tropical Pacific Ocean

Observations from satellite-tracked drifting buoys, expendable bathythermograph and conductivity-temperature-depth data, and Geosat altimeter data are used to describe anticyclonic eddies that occur in small numbers off the Pacific coast of Central America. These eddies are similar in many respects to the well-known warm-core rings that are observed north of the Gulf Stream off the Atlantic coast of North America, except that they occur in an environment that also is warm, and they contain considerably greater kinetic energy. It is hypothesized that they are formed as a result of conservation of potential vorticity when the North Equatorial Countercurrent (NECC) turns northward upon approaching the eastern boundary during its autumnal maximum. The rings so formed have a strongly nonlinear character which causes them to propagate westward between 9°N and 14°N with a speed in excess of that of long Rossby waves. Due to a relatively small available potential energy content, these rings have a dissipation time scale of about 6 months and perhaps end by collision with and reabsorbtion into the NECC. The rings account for the observed enhancement of surface kinetic energy, and probably for the seaward transport of waters enriched in copper.

A Two-dimensional Numerical Model of Estuarine Circulation: The Effects of Altering Depth and River Discharge

Steady-state numerical solutions are obtained for a two-dimensional, vertically stratified model of a partially mixed estuary. The boundary at the seaward end of the estuary is considered to be open, with the profiles of salinity, vorticity and streamfunction obtained by extrapolating interior dynamics out to the boundary. A salinity source is maintained at the bottom at the mouth. Zero salt flux is required at a free-slip top and no-slip bottom boundary. Zero salinity and a parabolic velocity profile are maintained at the head of the estuary. A number of cases are run for various estuarine parameters; the river transport and Rayleigh number being the two parameters that have the most pronounced effect. The river transport is varied by adjusting the mean freshwater velocity, Ur. Decreasing Uf allows salt as well as the stagnation or null point to penetrate upstream. The estuarine circulation weakens, but expands over a larger portion of the estuary. The position of the stagnation point, with respect to the seaward boundary, varies as Ur-6’8 for U, > I cm/s and as Ufms” for Ur < I cm/s. Increasing the Rayleigh number, by deepening the estuarine channel, H, results in an increased circulation as well as strong intrusion of salinity and inward migration of the stagnation point. The horizontal location of the stagnation point is found to be proportional to Ra and therefore, varies as HS.

Oceanographic Observations of the 1982 Warming of the Tropical Eastern Pacific

Moored current meter, sea level, hydrographic, and surface drifter measurements show the large changes that took place in the eastern tropical Pacific during the onset of the warm episode of 1982. In August the near-surface flow at 0�, 110�W reversed direction to eastward. By October the sea surface temperature in the equatorial zone increased by 5 degrees Celsius above the long-term monthly mean value, sea level rose by 22 centimeters at the Gal�pagos Islands, and the thermocline was displaced downward by 50 to 70 meters along the equator and the South American coast.

Evaluation and diagnosis of surface currents in the National Centers for Environmental Prediction's ocean analyses

Ensemble average currents from the 15 m depth level of the National Centers for Environmental Predictions (NCEP) analyses of the tropical Pacific Ocean are evaluated against surface mixed layer current observations obtained from an extensive set of satellite-tracked drifting buoys. These averages display many climatological characteristics of the region but are not intended to serve as a climatology because the data from the analyses are trimmed to match the time-space distribution of the observations. Substantial discrepancies between the analyses and the observations are revealed. First, the near-equatorial meridional currents and divergence have approximately twice the magnitude in the analyses as in the observations. This discrepancy is largely independent of whether temperature profile data are assimilated or not and is attributed to the parameterization of vertical viscosity. Second, the zonal flow in both the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC) is much stronger in the analyses than in the observations, especially in the western Pacific. This discrepancy is associated with assimilation of temperature profile data. It arises because salinity is an active variable in the underlying analysis model but is not controlled by boundary fluxes or other observations. Under the uncontrolled influence of advection and strong horizontal diffusion the salinity distribution becomes nearly homogeneous. Consequently, the analyses do not account for observed temperature-salinity correlations when density is computed following assimilation of temperature profile data. This leads to erroneous pressure gradients that drive excessively strong geostrophic currents and force large accelerations near the western boundary. Our results indicate that it is important to consider the consequences on the density structure of neglecting salinity during the assimilation of temperature data. We recommend that surface salinity observations from drifting buoys and volunteer observing ships be initiated to improve the ocean analyses.

Temporal Sampling Requirements for Surface Drifting Buoys in the Tropical Pacific

Abstract Drifting buoy data from the eastern tropical Pacific Ocean are used to evaluate the degradation of sea surface temperature and current information incurred by reducing the number of transmissions from drifting buoys using the ARGOS system for position finding. Buoy locations are interpolated at uniform time intervals using an optimum interpolation method known as Kriging, which provides also an estimate of the rms position error. It is found that the published standard for surface current measurement for the TOGA Program (5 cm s−1) can be met with transmissions on one day of three in the Southern Hemisphere. Due to stronger mesoscale variability in the Northern Hemisphere the standard would be jeopardized by reducing transmissions even to one day of two. The standard for observation of sea surface temperature (0.1°C) can be met in either hemisphere with transmissions on one day of four. The Lagrangian decorrelation times for the Northern Hemisphere region of the eastern tropical Pacific are estim...

Estimation of salinity profiles in the upper ocean

A new algorithm is presented for estimating salinity profiles in the upper ocean from measurements of temperature profiles and surface salinity. In application to the eastern tropical Pacific the method replicates a large fraction of the variability of salinity in the upper few tens of meters and provides modest to substantial improvement at nearly all levels. Estimated salinity profiles are able to characterize barrier layers, regions formed by a halocline within the thermal mixed layer. The rms error of geopotential height calculations based on estimated salinity profiles is reduced more than 50% by this method relative to methods not using surface salinity. Even without the surface salinity measurement some reduction of error in geopotential heights can be obtained relative to previous methods.

Evaluation of Sea Surface Temperature Measurements from Drifting Buoys

Abstract Three drift-buoy designs have been deployed since 1988 in substantial numbers in the tropical Pacific Ocean by United States participants as part of the Tropical Ocean Global Atmosphere (TOGA) Pan Pacific Surface Current Study. These include the Low Cost Tropical Drifter designed and built at the Atlantic Oceanography and Meteorological Laboratory, the Low Cost Drifter (LCD) designed and built by the Massachusetts Institute of Technology Draper Laboratories, and the Minister Drifter designed and built at the Scripps Institution of Oceanography and built by Tecnocean Inc., San Diego, California, which has subsequently become known as the World Ocean Climate Experiment standard drifter. This report contains an evaluation of the performance of the sea surface temperature measurement system carried by these buoy designs. Based on comparisons of the monthly mean SST derived from the available XBT and CTD casts and on intercomparisons of among each of the buoy types, all three designs appear to include...