Bottom Pressure Data Research Articles

Observations on the horizontal and vertical structure of currents at sub-tidal frequencies in the Central North Sea

During the spring of 1986 and the summer of 1987 current, bottom pressure and CTD data were gathered in the North Sea 60 km north of the Netherlands to determine the contribution of three mechanisms to the local dynamics: wind stress, density differences and rectification due to non-linear tide-topography interaction. This was achieved by comparing the observations with simplified mathematical models each describing one of the processes. Although a ‘global’ wind-driven model disagreed rather strongly with the data if the total period of measurements is considered, reasonable agreement has been obtained for small subperiods of constants wind action. During a selected period of SW-wind stress of 0.2 N·m −2, 6±2 cm·s −1 of the dominating along-isobath current was wind-driven. Near the bottom above the steepest bottom slope additionally 5.5±1.7 cm·s −1 was due to the frontal zone. Above this slope additionally 1.8 cm·s −1 was attributed to the rectifying mechanism. It is noted, however, that the latter value was smaller than the sum of the uncertainties in the currents resulting from the other models. The observed near-bottom along-isobath current velocity was 12.3 cm·s −1. The density-induced motion was not explained by the thermal wind relation when integrated from the bottom upwards. The absence of sea-level adjustment and, to a lesser extent, internal friction had to be considered. At the same position the vertical shear of the cross-isobath current component generally was largest compared with other moorings due to the enhanced cross-isobath circulation by the density gradient and the rectifying mechanism.

Barotropic tidal dynamics of the Bay of Biscay shelf: observations, numerical modelling and physical interpretation

The first order dynamics of the Bay of Biscay shelf are mostly dominated by barotropic semidiurnal tides. The diurnal tides and currents are weak. Quarter-diurnal tides are strongly amplified on the shelf. In this paper, we investigate the two major superinertial tidal bands (semidiurnal and quarter-diurnal). A description in terms of data is given first (Section 2). Coastal sea level and original open sea data (bottom pressure and currents), harmonically analysed, are studied. A numerical model is then used to provide a coherent and dense picture of the tidal dynamics of M 2 and M 4 and their variability (Section 3). Care is taken to assess the validity of the model by comparison with bottom pressure and coastal sea level data. M 2 amplitudes are increasing crossshore by an order of 20% and M 2 phases are slowly propagating to the northwest. M 2 currents are maximum at the shelf break. M 4 amplitudes are increasing by a factor 10 between the shelf break and the coast: the shelf is actually close to resonance in its central part. M 4 phases are propagating to the north. An amphidromic point is apparent in the northern outer part of the shelf. Using the data, we then produce a shelf-scale momentum and continuity balance for M 2 (Section 4). The simple model described by Battisti and Clarke (1982a, Journal of Physical Oceanography, 12, 8–16) is applied successfully. The M 2 equilibrium may be interpreted as follows: the deepsea Kelvin wave induces on the shelf a Poincaréwave which is almost perfectly reflected. The energy fluxes are directed alongshore (southeastward) on the shelf and little dissipation is seen to occur. The alongshore variability may be explained partly by the alongshore variations of the cross-shore bathymetric profile. For the quarter-diurnal band (Section 5), the shelf is close to resonance in its central part [the Clarke and Battisti (1981, Deep-Sea Research, 28, 665–682) criterion is met: ga/(ω 2 − f 2) ∼ L, where a is an average cross-shore bottom slope, ω the frequency and L the shelf width]. A modified version of the Battisti and Clarke (1982a) theory is applied successfully to M 4. M 4 energy fluxes are directed onshore. Both onshore and alongshore phase propagation and partial reflection follow. Through resonance, M 4 is strongly amplified cross-shelf and dissipated in coastal areas. In addition to local generation, the M 4 tide is forced on the shelf from the shelf break. This is thought to result from an M 4 tide in the Bay of Biscay which is itself co-oscillating with an oceanic M 4 tide originating from the Celtic Sea and English Channel regions. The non-linear generation of M 4 at the shelf break is thought to be relatively unimportant.

Volume transport in the Alaska Coastal Current

Nine moorings were deployed in three sections in the Shelikof Strait/Semidi Islands region of the Alaskan continental shelf during the period of August 1984 to July 1985. Analysis of the resulting current and bottom pressure data, together with surface wind, provides a new understanding of transport in the Alaska Coastal Current. Using current observations, mean volume transport through the Shelikof sea valley was computed to be 0.85 × 10 6 m 3 s −1, which is in good agreement with estimates of transport obtained from hydrographic data. Approximately 75% of this flux flowed seaward through the Shelikof sea valley, with the remainder flowing along the Alaska Peninsula. Data showed the expected increase of volume transport concomitant with maximum freshwater discharge in autumn. The greatest monthly mean transport, however, occurred in winter and was related to wind forcing. On time intervals of days, fluctuations in transport were often large (up to 3.0 × 10 6 m 3 s −1), and generally geostrophic ( r = 0.79). Some of these fluctuations resulted from convergence of flow caused by the complex interaction of storms with orography. Approximately half of the fluctuations in volume transport were accounted for by the alongshore wind.

Tides on the Newfoundland grand banks

Abstract Tidal analyses of bottom pressure data are presented for 8 sites on the Grand Banks of Newfoundland. The principal diurnal and semidiurnal constituents are qualitatively similar to the charts compiled by Godin (1980) but there are quantitative differences. Five of the gauges were moored on the 400‐m isobath and results from these sites are used to specify the outer boundary conditions for a numerical model of five constituents. The model results are compared with data from the three shelf stations and coastal gauges with agreement to 0.01 m for elevation and 7 degrees for phase. Intercomparisons for tidal currents were less favourable but it is shown that the discrepancy in predicting the path of a passive drifter would be O(100 m). This is substantially less than typical errors associated with iceberg prediction models.

The subtidal behaviour of the Celtic Sea-II. Currents

Subtidal currents observed in the Celtic Sea during spring 1978 are described in terms of empirical orthogonal functions and rotary spectra. Less than half the current variance can be related to local wind forcing. It is suggested that the remaining variance is partly due to the influence of winds acting over the adjacent Irish Sea and North Sea. There is no evidence in this limited data set of a strong coupling between the Celtic Sea and adjacent North Atlantic at subtidal frequencies. Examination of the terms in the subtidal momentum equation shows that the interior of the Celtic Sea is essentially in geotrophic balance. Close to the tidally energetic St George's Channel, both bottom friction and mean tidal advection become important. A diagnostic model for estimating circulation patterns from bottom pressure and wind data is developed and successfully applied to the Celtic Sea. An attempt to use the model with both bottom pressures and coastal sea levels was less successful than with bottom pressures alone. We attribute this to the nearshore distortion of the coastal sea levels.