The interplay between the Point Año Nuevo upwelling center, an offshore anticyclonic mesoscale eddy, and the waters of the Monterey Bay was studied during a series of up‐ and downwelling favorable wind events during August 2000. The upwelling events were characterized by the appearance of cold, salty water at Point Año Nuevo at the north end of the bay that subsequently spread southward across the mouth of the bay as the winds continued. During the downwelling/relaxation events, the surface current and temperature response was dominated by the onshore translation of the offshore eddy and by local surface heating in the bay itself. The circulation within the bay was cyclonic during both wind regimes but slightly more barotropic under poleward forcing. The ICON model, a nested, data assimilating, sigma coordinate model, was used to simulate the upwelling and relaxation events and calculate the subsurface current and density fields. The model reproduced the dominant current and temperature patterns outside the bay, including the southward flowing upwelling filament, the movement of the offshore eddy, the poleward flow off Point Sur, and the circulation within the bay. The model salinity fields at the surface and 100 m levels show that during upwelling, the bay was filled with higher‐salinity water stemming from the Point Año Nuevo upwelling center to the north. During downwelling, the source water for both the surface and 100 m levels was the colder, fresher California Current water offshore, which had advected southward well past Point Piños during the previous upwelling event.
Abstract. Four oceanographic moorings were deployed across the South China Sea continental slope near 21.85ââN, 117.71ââE, from 30 May to 18 July 2014âââââââ for the purpose of observing high-frequency nonlinear internal waves (NLIWs) as they shoaled across a rough, gently sloping bottom. Individual waves required just 2âh to traverse the array and could thus easily be tracked from mooring to mooring. In general, the amplitude of the incoming NLIWs tracked the fortnightly tidal envelope in the Luzon Strait; they lagged by 48.5âh and were smaller than the waves previously observed to the southwest near the Dongsha Plateau. Two types of waves, a waves and b waves,âââââââ were observed, with the b waves always leading the a waves by 6â8âh. Most of the NLIWs were remotely generated, but a few of the b waves formed locally via convergence and breaking at the leading edge of the upslope-propagating internal tide. Waves incident upon the moored array with amplitude less than 50âm and energy less than 100âMJâmâ1 propagated adiabatically upslope with little change of form. Larger waves formed packets via wave dispersion. For the larger waves, the kinetic energy flux decreased sharply upslope between 342 and 266âm, while the potential energy flux increased slightly, causing an increasing ratio of potential-to-kinetic energy as the waves shoaled. None of the waves met the criteria for convective breaking. The results are in rough agreement with recent theory and numerical simulations of shoaling waves.
Abstract Four current-meter moorings and 12 pressure sensor–equipped inverted echo sounders (PIES) were deployed during summer 2011 in the South China Sea. The goal of the experiment was to obtain synoptic observations of the large-amplitude nonlinear internal waves from the near field to the far field as they propagated west-northwest across the sea. The program was unique because it was the first to observe the latitudinal variability of the wave crests in addition to the transformations along a single east–west transect. The waves were strongest down the center of the PIES array along roughly 20°45′N and were weaker off axis in both directions. Both a-waves and b-waves arrived earlier in the south than the north, but with different lag times indicating different propagation directions and therefore different sources. The waves were classified by their arrival patterns and source locations and not by their amplitude or packet structure. The Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) model, calibrated against the array, showed that the a-waves developed out of the internal tide spawned in the southern portion of the Luzon Strait and the b-waves originated in the north. The northern tides were refracted and suffered large dissipative losses over the shallow portion of the western ridge, whereas the southern tides propagated west-northwest unimpeded, which resulted in a-waves that were larger and appeared sooner than the b-waves. The results were consistent with previous observations that can now be understood in light of the full three-dimensional structure of the internal waves and tides in the northeastern South China Sea.
Abstract. Four oceanographic moorings were deployed across the South China Sea continental slope near 21.85∘ N, 117.71∘ E, from 30 May to 18 July 2014 for the purpose of observing high-frequency nonlinear internal waves (NLIWs) as they shoaled across a rough, gently sloping bottom. Individual waves required just 2 h to traverse the array and could thus easily be tracked from mooring to mooring. In general, the amplitude of the incoming NLIWs tracked the fortnightly tidal envelope in the Luzon Strait; they lagged by 48.5 h and were smaller than the waves previously observed to the southwest near the Dongsha Plateau. Two types of waves, a waves and b waves, were observed, with the b waves always leading the a waves by 6–8 h. Most of the NLIWs were remotely generated, but a few of the b waves formed locally via convergence and breaking at the leading edge of the upslope-propagating internal tide. Waves incident upon the moored array with amplitude less than 50 m and energy less than 100 MJ m−1 propagated adiabatically upslope with little change of form. Larger waves formed packets via wave dispersion. For the larger waves, the kinetic energy flux decreased sharply upslope between 342 and 266 m, while the potential energy flux increased slightly, causing an increasing ratio of potential-to-kinetic energy as the waves shoaled. None of the waves met the criteria for convective breaking. The results are in rough agreement with recent theory and numerical simulations of shoaling waves.
The Sand Dunes 2014 field experiment in the South China Sea was located in a region frequented by large amplitude (40-80 m) nonlinear internal waves. Four moorings spanning 386-266 m across the slope near 21° 52’N, 117° 36.5’E observed wave arrival patterns during June 3-19, 2014, that were similar to those previously observed nearby consisting of large (a-waves) arriving diurnally with smaller (b-waves) in between, with all amplitudes modulated by the fortnightly tidal beat. Small waves continued to gain amplitude as they shoaled, whereas large waves maxed out near 342 m then became smaller between 342 and 266 m. The number of waves/packet increased upslope due to wave dispersion. With just one exception, wave breaking and trapped cores were not observed. This contrasts with previous observations to the southwest near Dongsha Island where the wave energies (550 vs. 300 MJ) and bottom slopes were larger. Most of the incident waves can be attributed to remote forcing in the Luzon Strait, although some b-waves were formed locally due to convergence of the internal tide. A new result was the arrival of double a-waves two hours apart near spring tide on June 16-19. Some possible forcing scenarios for these double a-waves will be presented.
Abstract 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.
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