A Study of Low-Frequency, Wind-Driven, Coastal-Trapped Waves along the Southeast Coast of Australia

Coastal-trapped waves (CTWs) along the southeast coast of Australia were investigated based on a frictional, wind-driven long-wave theory. It was found that low-frequency sea level anomalies (SLAs) were continuously propagating from the south coast up along the east coast as CTWs, mainly forced by the alongshore wind stress. Three main subinertial peaks existed in the spectral characteristics of the SLAs, with periods of 14.2, 10.2, and 7.8 days, respectively. Power spectral density distributions of the peaks showed that the CTW amplitudes varied significantly along the southeast coast. For idealized linear and exponential shelves, a theoretical analysis indicated that the fundamental factor influencing the eigenvector of mode 1, and therefore the CTW amplitude, was the offshore water depth. This theoretical work was well supported by eight sensitivity cases. Four additional cases were conducted, and time-averaged energy fluxes were calculated to identify the energy source of the CTWs in the Australian Coastal Experiment (ACE) region. It was shown that both the local wind stress and the wind stress in Bass Strait contributed to the CTWs in the ACE region, with the latter playing a more important role. The remaining CTW energy came from remote forcing farther west of Bass Strait. The energy flux calculation also showed that the CTW energy flux was almost constant along the investigated coast because of the balance between frictional dissipation and the energy gain from the alongshore wind stress; the significant variations in the power spectral density (PSD) of the subinertial peaks were mainly due to the variations in the modal eigenvectors caused by the shelf geometry.

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼ 30° S) based on a high-resolution atmospheric regional model (WRF)

Two physical mechanisms can contribute to coastal upwelling in eastern boundary current systems: offshore Ekman transport due to the predominant alongshore wind stress and Ekman pumping due to the cyclonic wind stress curl, mainly caused by the abrupt decrease in wind stress (drop-off) in a cross-shore band of 100 km. This wind drop-off is thought to be an ubiquitous feature in coastal upwelling systems and to regulate the relative contribution of both mechanisms. It has been poorly studied along the central-northern Chile region because of the lack in wind measurements along the shoreline and of the relatively low resolution of the available atmospheric reanalysis. Here, the seasonal variability in Ekman transport, Ekman pumping and their relative contribution to total upwelling along the central-northern Chile region (∼ 30° S) is evaluated from a high-resolution atmospheric model simulation. As a first step, the simulation is validated from satellite observations, which indicates a realistic representation of the spatial and temporal variability of the wind along the coast by the model. The model outputs are then used to document the fine-scale structures in the wind stress and wind curl in relation to the topographic features along the coast (headlands and embayments). Both wind stress and wind curl had a clear seasonal variability with annual and semiannual components. Alongshore wind stress maximum peak occurred in spring, second increase was in fall and minimum in winter. When a threshold of −3 × 10−5 s−1 for the across-shore gradient of alongshore wind was considered to define the region from which the winds decrease toward the coast, the wind drop-off length scale varied between 8 and 45 km. The relative contribution of the coastal divergence and Ekman pumping to the vertical transport along the coast, considering the estimated wind drop-off length, indicated meridional alternation between both mechanisms, modulated by orography and the intricate coastline. Roughly, coastal divergence predominated in areas with low orography and headlands. Ekman pumping was higher in regions with high orography and the presence of embayments along the coast. In the study region, the vertical transport induced by coastal divergence and Ekman pumping represented 60 and 40 % of the total upwelling transport, respectively. The potential role of Ekman pumping on the spatial structure of sea surface temperature is also discussed.

Continental Shelf Baroclinic Instability. Part I: Relaxation from Upwelling or Downwelling

There exists a good deal of indirect evidence, from several locations around the world, that there is a substantial eddy field over continental shelves. These eddies appear to have typical swirl velocities of a few centimeters per second and have horizontal scales of perhaps 5–10 km. These eddies are weak compared to typical, wind-driven, alongshore flows but often seem to dominate middepth cross-shelf flows. The idea that motivates the present contribution is that the alongshore wind stress ultimately energizes these eddies by means of baroclinic instabilities, even in cases where obvious intense fronts do not exist. The proposed sequence is that alongshore winds over a stratified ocean cause upwelling or downwelling, and the resulting horizontal density gradients are strong enough to fuel baroclinic instabilities of the requisite energy levels. This idea is explored here by means of a sequence of idealized primitive equation numerical model studies, each driven by a modest, nearly steady, alongshore wind stress applied for about 5–10 days. Different runs vary wind forcing, stratification, bottom slope, bottom friction, and Coriolis parameter. All runs, both upwelling and downwelling, are found to be baroclinically unstable and to have scales compatible with the underlying hypothesis. The model results, combined with physically based scalings, show that eddy kinetic energy generally increases with bottom slope, stratification, wind impulse (time integral of the wind stress), and inverse Coriolis parameter. The dominant length scale of the eddies is found to increase with increasing eddy kinetic energy and to decrease with Coriolis parameter.

Seasonal Evolution of the Upwelling Process South of Cape Blanco*

Bursts of upwelling-favorable winds lasting 4–20 days occur year-round south of Cape Blanco, a major headland on the Oregon coast. The ocean’s response to these events was studied using moored current, temperature, and salinity data; satellite SST data; and a few across-shelf sections near the mooring site. The mooring was located at 42°26.49′N, 124°34.47′W, 6 n mi off Gold Beach, Oregon, from May 2000 to October 2003. After the spring transition but before upwelling jet separation, equatorward wind stress produced a steady upwelling response much the same as a long, straight coast. Currents and winds had similar spectral characteristics with a peak near 15 days. After jet separation, upwelling-favorable winds forced a much more variable current consisting of a series of thin equatorward jets that evolved and moved offshore across the mooring, with shorter time scales than the wind stress forcing. During autumn, the equatorward wind stress weakened slightly and a transition period occurred, with the flow often poleward along the bottom. During winter, the water column was unstratified during poleward winds and currents with little variation in SST across the shelf. Winter upwelling restratified the water column from the bottom up by drawing cold, salty water onshore along the bottom, with little or no change in SST. This scenario was modulated by strong intraseasonal and interannual variability in the ocean and atmosphere. A wavelet transform analysis of alongshore wind stress and the first empirical orthogonal mode of the alongshore currents revealed strong energy peaks in the 30–70-day band. These signals were particularly clear in the ocean and were not coherent with the local wind stress, suggesting they were due to Kelvin waves of equatorial origin. The shift toward longer (40–45–60 days) periods from 2000 to 2003 was consistent with decreasing (warming) northern oscillation index, suggesting that the period as well as the energy of the intraseasonal waves may be important in transmitting heat poleward during warm years.

Reconstructing the stock-recruit relationship for Northeast Arctic cod using a bioenergetic index of reproductive potential

Correlation and simulation analyses suggest that, for the Northeast Arctic (NA) cod stock, the total lipid energy (TLE (kJ)) contained in the livers of mature females is proportional to total egg production, making TLE a potential predictor of recruitment. Accordingly, the TLE of NA cod was estimated for a 51-year time period (1946-1996), using estimates of numbers at length derived from virtual population analysis, modelled values of proportion mature and weight at length, and observed values of the liver-condition index. A significant linear relationship between TLE and recruitment to age 3 was observed. The temporal trend in TLE suggests that the reproductive potential of the NA cod stock has been in decline since the mid-1970s. A multiple-regression model that included TLE, mean temperature, and mean alongshore wind stress as independent variables explained approximately 43% of the variation in recruitment. Reconstructing stock-recruit relationships using more sensitive measures of reproductive potential is the first step in resolving environmental effects on recruitment and in developing biological reference points that are more effective in stock conservation.

Wind-induced upwelling along the west coast of North America, 1899–1988

Alongshore geostrophic wind stresses (AWS) were used as an "index" of wind-induced coastal upwelling/downwelling for eight coastal stations from Baja California to Alaska for 1899–1988. For winters since around 1940, downwelling has intensified along Alaska and northern British Columbia, while upwelling has increased along Baja California. El Niño events induced strong winter coastal downwelling poleward of 40°N. During summer, upwelling has increased since around 1940 along southern British Columbia to Baja California, while from 1899 to 1940, upwelling declined along southern California to Baja California. Empirical orthogonal function analysis of the AWS showed that the first mode consisted of the AWS at the eight stations all varying in-phase, while the second mode had the northern four stations out-of-phase with the southern four stations. Off southern British Columbia, correlations between coastal sea level and AWS and between sea surface temperature and AWS were both strong during winter but insignificant during summer. In contrast, correlation between salinity and alongshore wind stress was insignificant during fall and winter, but strong during spring and moderate during summer. Summer AWS was positively correlated with both the fatness of sardine and the condition factor of herring off British Columbia.