Keith R. Thompson

Interannual variability in Mediterranean heat and buoyancy fluxes

Abstract The flux of heat through the Strait of Gibraltar is known well enough that the Mediterranean Sea may be used as a climate test basin. After adjusting reported winds for changes in observing practice, the COADS for 1946 to 1988 was used together with standard heat flux formulas to estimate the long-term mean heat flux into the sea, giving 36 W mm2 more than is compatible with the Gibraltar exchange. As the estimated latent heat flux is consistent with the freshwater budget, it is suggested that standard formulas overestimate insulation in the Mediterranean. If a constant adjustment factor is used for the insulation, or for the latent heat loss, interannual variability of ±15 W m−2 is found in the total heat flux. Changes in the latent heat flux dominate, with contributions from both the humidity of the air and the saturation humidity at the temperature of the sea surface. The buoyancy flux from the sea is also examined and shows that the contributions from precipitation and runoff are import...

Maximum likelihood spectral fitting: The Batchelor spectrum

Abstract A simple technique for fitting spectra that is applicable to any problem of adjusting a theoretical spectral form to fit observations is described. All one needs is a functional form for the theoretical spectrum and an estimate for the instrumental noise spectrum. The method, based on direct application of the maximum likelihood approach, has several advantages over other fitting techniques. 1) It is unbiased in comparison with other least squares or cost function–based approaches. 2) It is insensitive to dips and wiggles in the spectrum, due to the small number of fitted parameters. It is also robust because the range of wavenumbers used in the fit is held fixed, and the built-in noise model forces the routine to ignore the spectrum as it gets down toward the noise level. 3) The method provides a theoretical estimate for error bars on the fitted Batchelor wavenumber, based on how broad or narrow the likelihood function is in the vicinity of its peak. 4) Statistical quantities that indicate how w...

Madden-Julian Oscillation and sea level: Local and remote forcing

The Madden-Julian Oscillation (MJO) is the dominant mode of atmospheric variability in the tropical atmosphere on intraseasonal time scales (i.e., weeks to seasons). This study examines the connection between the MJO and global sea level measured by altimeters over the last 17 years. We first identify regions exhibiting a significant (both statistical and practical) relationship between sea level and the MJO. The first region consists of the equatorial Pacific and western coastal zones of North and South America. Consistent with previous studies, we identify wind-driven equatorially trapped Kelvin waves that propagate eastward along the equatorial Pacific and then transform into coastal trapped waves that propagate poleward along the western coasts of North and South America. The second region includes the shallow waters of the Gulf of Carpentaria (off Australia’s north coast) and the adjacent Arafura and Timor seas. Setup by onshore winds is shown to be the dominant physical process. Finally, the northeastern Indian Ocean is shown to be a complex region involving a combination of equatorially trapped Kelvin waves, coastal trapped waves, and westward-propagating Rossby waves exhibiting characteristics of both local and remote forcing. The implications of the results for deep and coastal ocean forecasting are discussed.

Time-Averaged Forms of the Nonlinear Stress Law

Abstract On the assumption that the mean velocity and the probability distribution of the higher frequency fluctuating motions are known, an expression for the mean surface stress is given. For the case of isotropic background variations, the mean stress is shown to be a simple nonlinear function of the mean velocity and the standard deviation of the fluctuations. Results should be useful in studies concerning the stress at the bottom of either the ocean or the atmosphere. For use in the oceanic case, a constant drag coefficient is considered. For the atmospheric case, the drag coefficient is a function of wind speed. Results are compared for several previously proposed forms of this functional dependence.

Estimation of Low-Frequency Wind Stress Fluctuations over the Open Ocean

Abstract A simple, approximate formula for mean wind stress is given in terms of the mean and variance of the wind fluctuations over the averaging period. The formula is nonlinear with respect to the mean wind speed. The formula is tested using 3 h wind observations from eight North Atlantic Ocean Weather Ships. Mean wind stress is calculated 1) by vector averaging the 3 h wind stresses and 2) by applying the approximate formula. For an averaging period of 4 months the two methods agree to within ±0.025 Pa, 95% of the time. For an averaging period of 1 month the approximate formula slightly overestimates the stress. This is due to skewness in the probability density function of the observed 3 h wind fluctuations. An expression for the modification of the mean stress due to skewness is given. A straightforward method is described for the estimation of vector mean wind and variance fields, and thus mean stress fields, over the open ocean. To cheek the method, the long-term stress field of the North Atlantic...

The adjoint method of data assimilation used operationally for shelf circulation

A real-time shelf circulation model with data assimilation has been successfully used, possibly for the first time, on the outer Nova Scotian Shelf. The adjoint method was used to infer the time histories of flows across the four open boundaries of a 60 km × 60 km shallow-water equation model of Western Bank. The aim was to hindcast and nowcast currents over the bank so that a patch of water (initially 15 km in diameter) could be resampled over a 3-week period as part of a study of the early life history of Atlantic cod. Observations available in near real time for assimilation were from 14 drifting buoys, 2 telemetering moored current meters, the ships acoustic Doppler current profiler and the local wind. For the postcruise hindcasts presented here, data from two bottom pressure gauges and two more current meters are also available. The experiment was successful, and the patch was sampled over a 19-day period that included two intense storms. In this paper we (1) document the model and how the data are assimilated, (2) present and discuss the observations, (3) demonstrate that the interpolative skill of the model exceeds that of simpler schemes that use just the current velocity data, and (4) provide examples of how particle tracking with the model enables asynoptically acquired data to be displayed as synoptic maps, greatly facilitating both underway cruise planning and postcruise data analysis. An interesting feature of the circulation on the bank was a nearly stationary eddy atop the bank crest. Larvae within the eddy were retained on the bank in a favorable environment until the onset of the storms. The variable integrity of the eddy may contribute to fluctuations of year-class success.

Perspectives on Field Studies and Related Biological Models of Bivalve Growth and Carrying Capacity

Marine bivalves are cultured throughout the world, and in many places such as Atlantic Canada and New Zealand, culture of mussels and other groups has grown exponentially in the last decade (Hickman 1989; Mallet 1989). Acceptable culture sites are limited due to habitat suitability, road access, and competing recreational or commercial use such as wild fisheries. As available culture space becomes filled up with stock, there may be a depression of individual bivalve growth rate and an increase in mortality caused by several factors associated with overcrowding. Suspension feeders have a remarkable capacity to filter the water column such that they are food limited at high culture density (Navarro et al. 1991). There are several indications that production maxima have been reached or exceeded as culture is continually expanded to the detriment of food supply (Mallet 1989; Hickman et al. 1991; Perez Comacho et al. 1991). This phenomenon is also documented for natural populations of both infaunal and epifaunal bivalves including mussel beds (Peterson and Black 1987; Frechette and Grant 1991; Smaal 1991; Bayne and Hawkins 1992). Moreover, culture research has demonstrated major site differences in growth rate (Mallet et al. 1986) confirming that environmental conditions can regulate shellfish production. Overcrowded culture conditions also lead to increased incidence of shellfish diseases (Dijkema and van Stralen 1989). Finally, high culture biomass may produce a negative feedback to the local environment through organic loading and anaerobic conditions beneath culture leases (Dahlback and Gunnarsson 1981), potentially leading to degradation of culture environments.

Is the Exchange through the Strait of Gibraltar Maximal or Submaximal

The Mediterranean Sea and Strait of Gibraltar can take either of two possible states. In the first the Mediterranean is overmixed, with minimum salinity difference between it and the Atlantic and maximal exchange through the Strait. In the second the Mediterranean is not overmixed, the salinity difference is greater than for the overmixed case and the exchange is submaximal. Interannual and longer term variability in air—sea interaction over the whole Mediterranean will be much more readily detectable at the Strait of Gibraltar if the exchange there is submaximal than if it is maximal, so it is important to establish the present state of the exchange. We review indirect evidence from a variety of observations and theories and find support for both interpretations. We conclude that the system is close to maximal exchange and may flip from one state to the other on various time scales up to many years, but with a tendency for maximal exchange early in the year, and submaximal later in the year.

Skewness of sea level variability of the world's oceans

Skewness of sea level variability for the worlds oceans is calculated using gridded altimeter data for the period 1993-2001. Many well-known ocean features can be identified in the skewness map, including the Gulf Stream, Kuroshio Extension, Brazil-Malvinas Confluence, and the Agulhas Retroflection. It is shown, through an idealized example and results from a quasi-geostrophic model, that sea level skewness can be used to identify the mean path of unstable ocean jets and also regions dominated by eddies with a preferred sense of rotation. These ideas are confirmed with a more detailed analysis of the skewness fields for the northwest Atlantic and Agulhas Retroflection region. Finally, it is argued that sea level skewness, like variance, is a potentially powerful diagnostic for testing the realism of high-resolution ocean circulation models.

Subtidal circulation on the Scotian Shelf: Assessing the hindcast skill of a linear, barotropic model

Currents measured during the winter of 1985–1986 by four moorings on the inner Scotian Shelf are used to assess the hindcast skill of a three-dimensional circulation model forced by local wind stress and coastal sea level. The model is linear and barotropic. The integration scheme is based on the modification of the Galerkin spectral method proposed recently by Sheng and Thompson [1993]. The skill of the model is measured by the variance of the hindcast errors divided by the variance of the observations (henceforth γ2). The model is most effective within 30 km of shore (0.51≤γ2≤0.66). At the mooring in the Nova Scotia Current, a surface intensified southwestward jet with its center approximately 50 km from shore, the model fails to capture the bulk of the variance at current meters within 70 m of the surface (0.81≤γ2≤0.87). The skill of the model is lowest at the mooring 65 km from shore (0.92≤γ2≤0.94). To put these measures of skill into perspective, the currents are hindcast using a linear statistical model with the same inputs as the circulation model. The statistical model is optimal in the sense no other linear model with these inputs can achieve a lower hindcast error variance. For half of the current meters within 30 km of shore and two in the Nova Scotia Current the skill of the circulation model is not significantly lower than that of the statistical model. The largest discrepancies in the skill of the two models are found at the offshore mooring. We show that the suboptimal performance of the circulation model is due in part to the assumption of a spatially uniform wind field. We speculate that another contributing factor is the assumed form of the sea level profile along the open boundary that is upstream in the sense of coastal trapped wave propagation. Finally, the statistical model is used to estimate the increase in skill of the circulation model that may result from the assimilation of hydrographic data and additional coastal sea levels.