摘要:

The Coastal Ocean Dynamics Experiment (CODE) was designed to identify and study those dynamical processes which govern the wind‐driven motion of water over the continental shelf. The initial effort in this multiyear, multi‐institutional research program was to obtain high‐quality data sets of all the relevant physical variables needed to construct accurate kinematic and dynamic descriptions of the response of continental shelf waters to strong wind forcing in the synoptic band covering 2‐ to 20‐day variability. Two small‐scale, densely instrumented field experiments, each approximately 4 months long, were conducted in spring and summer 1981 (CODE 1) and 1982 (CODE 2). A more sparsely instrumented, long‐term, large‐scale component was also conducted in conjunction with a separate but related Large‐Scale West Coast Shelf Experiment (informally called “SuperCODE”) to help separate the local wind‐driven response in the region of the small‐scale experiments from motions generated in some distant region along the coast and to investigate the seasonal cycles of atmospheric forcing, water structure, and coastal curr

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01455

年代:1987

数据来源: WILEY

摘要:

During the preparation of this volume, William D. “Bill” Grant, one of our dear friends and fellow Coastal Ocean Dynamics Experiment (CODE) scientists, died of cancer on October 7, 1986. Our shock and disbelief have given way to deep sadness as we come to realize his absence. His intelligence, wit, and friendship will be greatly missed.Bill was a gifted scientist whose wisdom, dedication, and boundless energy had led him to a preeminent position in his principal field of research on bottom boundary layers beneath waves and currents. In nine short years, Bill had emerged from the completion of his doctoral research at the Massachusetts Institute of Technology to the rank of Senior Scientist in the Department of Ocean Engineering at the Woods Hole Oceanographic Institution. At the time of his death, Bill was recognized as one of the world's leading authorities on the hydrodynamics of bottom boundary layers on continental shelves. Bill enjoyed the application of sound theoretical models to the explanation of geological and oceanographic phenomena observed in the marine environment. His scientific interests spanned a broad range of topics including the nature and structure of turbulent bottom boundary layers in both shallow continental shelf regions like CODE and deep oceanic domains like the High Energy Benthic Boundary Layer Experiment (HEBBLE), and the role that these bottom layers have in affecting both oceanic circulation and sediment transport patterns. Bill had the unique ability to generate physically realistic models which he then tested through intensive field investigations. He was at once a theoretician, laboratory experimentalist, and seagoing ocean scientist, and he excelled in all areas of his resea

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01465

年代:1987

数据来源: WILEY

摘要:

As part of the Coastal Ocean Dynamics Experiment (CODE), meteorological instruments were deployed on buoys and at coastal stations and instrumented aircraft flights and coastal soundings were made to study the three‐dimensional structure of the marine boundary layer over the continental shelf off northern California during the 1981 and 1982 upwelling seasons. These measurements show that after the atmospheric spring transition the airflow in the marine layer is dominated by the North Pacific high, and the surface wind field over the shelf is characterized by periods of strong (7–15 m/s), upwelling‐favorable alongshelf winds lasting for up to 30 days, interrupted by shorter periods of much weaker winds directed either equatorward or poleward. These periods of weak or reversed winds typically last several days and are called wind relaxations, even though they are primarily associated with coastally trapped perturbations of the marine layer along the central and northern California coast and not with a large‐scale weakening of the North Pacific high. The atmospheric boundary layer measurements made in CODE suggest a simple conceptual model which can explain much of the physiology or structure of the marine layer and associated surface wind field during periods of persistent upwelling‐favorable winds. During these periods, which represent the quasi steady state regime during the upwelling season, the inversion base of the marine layer drops eastward toward the coast until it intersects the coastal mountain range at a height of several hundred meters, and the associated thermal wind produces an alongshelf wind jet which has a maximum speed just below the inversion base. Turbulent mixing tends to homogenize any stratification in the marine layer beneath the jet and couple the jet to the ocean surface, producing strong upwelling‐favorable winds over the shelf. Day/night heating/cooling over the narrow coastal strip beneath the marine layer generates a weak cross‐coast secondary circulation which causes the core of the alongshelf jet to drop in elevation and shift onshore. This diurnal change in the marine layer structure explains both the daytime acceleration of the surface winds observed over and near the coast and its offshore decay and the associated offshore increase in the subdiurnal alongshelf wind. Thus the quasi‐steady component of the wind stress has a significant curl over the inner shelf during periods of active upwelling. This mean summer atmospheric boundary layer regime is occasionally interrupted by synoptic and/or mesoscale events or anomalous conditions. Analysis of the CODE observations suggests five types of events, two primarily synoptic‐scale conditions which lead to stronger‐than‐normal upwelling‐favorable winds over the shelf and three primarily mesoscale events which lead to wind relaxation. About half of the wind relaxation events observed in 1981 and 1982 are believed to be associated with either coastal‐trapped gravity currents or internal Kelvin waves which propagate north‐ward in the marine layer along the central and northern Califo

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01467

年代:1987

数据来源: WILEY

摘要:

The vertical and horizontal structure of the atmospheric boundary layer near the northern California coast was investigated during spring upwelling conditions as part of the 1981 Coastal Ocean Dynamics Experiment. Two daytime aircraft flights were devised to measure mean and turbulent quantities for 25‐km tracks along and across the continental shelf from 30‐ to 1200‐m altitude. The Pacific high offshore and an inland thermal low characterized the synoptic situations, and equatorward winds resulted, which produced intense ocean upwelling. The wind profile was found to be jet shaped: maximum wind speeds were 25 m s−1and 16 m s−1at 400 and 100 m, respectively, in the two flights. Above and below the heights of the jet speed maxima, the wind speed decreased rapidly. Sharp density inversions existed at the levels of the peak wind speeds; in one flight the inversion also sloped down toward the coast. Turbulence was effectively confined below the inversions. Strong, zonal temperature gradients above the inversion between the hot land and cool marine air caused a thermal wind for which the wind speed decreased with height. Below the inversion, turbulent stress divergence was large and effective in transferring the horizontal momentum of the wind to the sea, slowing the wind. There was also a small thermal wind below the inversion but opposite to the one above owing to the well‐mixed air temperature following the sea surface temperature, which was cold at the coast and relatively warmer offshore owing to the upwelling. These effects explain the jet‐shaped wind profile. The mean momentum equations show that an acceleration toward the coast can balance the pressure gradient, Coriolis, and turbulent stress divergence forces, and there were some observations to support this. Complete balance was not obtained for the turbulent kinetic energy budget; inferred dissipation rates w

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01489

年代:1987

数据来源: WILEY

摘要:

North winds along the northern California coast in the summer of 1982 were interrupted by six events with southerly winds. Four of these events occurred under similar circumstances. One of these four events is presented in detail. At the start of this event the marine layer is thickened in the southern California bight. A couple of days later the marine layer thickens from Point Conception to Monterey. Then the marine layer thickness increase surges to the north along the coast to Point Arena, where progression stopped and an eddy formed. In this surging stage, winds switched to southerlies as the leading edge of the event passed. A day later, the leading edge surged further to the north. Inshore winds were southerly, and the lifted marine layer extended to Cape Blanco in Oregon. This event is interpreted as a gravity current surging up the coast. The basic reservoir is the thickened marine layer in the southern California bight. Northerly progression is interrupted by a blocking wave near Point Arena where an eddy forms. It is hypothesized that most of the major wind reversals along the northern California coast during the summer are gravity currents or Kelvin waves in the marine layer that are formed to the south.

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01497

年代:1987

数据来源: WILEY

摘要:

Seasonal cycles of coastal wind stress, adjusted sea level (ASL), shelf currents, and water temperatures off the west coast of North America (35°N to 48°N) are estimated by fitting annual and semiannual harmonics to data from 1981–1983. Longer records (9–34 years) of monthly ASL indicate that these two harmonics adequately represent the long‐term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles. We characterize the differences between fall/winter and spring/summer as follows: For fall/winter, monthly mean winds north of 35°N are northward for 3–6 months (longer in the north than in the south); south of 35°N, the mean winds are near zero or weakly southward; monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures. For spring/summer, monthly mean wind stresses are southward at all latitudes for 3–6 months (longer in the south than in the north), sea levels are low, and water temperatures are relatively cool; monthly mean currents at 35 m depth over the shelf are southward for 1–6 months (longer at the shelf break than over midshelf and longer in the north than in the south), while the deeper currents are less southward or northward. The magnitudes of the seasonal cycles of all variables are maximum between approximately 38°N and 43°N, generally decreasing slightly to the north and greatly to the south. At each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1–2 months. The seasonal cycles of all variables show a south‐to‐north progression (south leads north by 1–2 months). At 48°N, annual mean currents at 50 m depth over the shelf break oppose the annual mean wind (northward wind and southward current). Similarly, at 35°N, annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind (southward wind and northward current). From 35°N to 43°N, both summer and winter regimes are dominated by

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01507

年代:1987

数据来源: WILEY

摘要:

Past measurements off the coast of central Oregon and Washington have shown that the rapid change from northward monthly mean winter winds to southward summer winds forces a “spring transition” of the coastal ocean: sea levels and temperatures drop, and mean surface currents shift from northward to southward. Current and water temperature data from 35°N to 48°N from 1981 and 1982, and sea level and wind stress data from 1971–1975 and 1980–1983, show the transition to have a large alongshore scale, typically 500 to 2000 km; the large‐scale wind stress appears to be the forcing mechanism at latitudes north of approximately 37°N. South of 37°N, sea level usually falls more gradually before the northern transition event. Both wind and sea level events generally progress from south to north over a 3‐ to 10‐day period, but this is not always true. Several aspects of the spring transition reflect coastal trapped wave dynamics. Previous studies at 45°N found persistent vertical shear of the southward summer current, associated with a cross‐shelf density gradient. During 1982 the shear and the density front develop over the shelf break immediately after the transition at 43°N and to the south, but they are much less persistent than at 48°N. The stronger winds between 38°N and 42°N and the narrower shelf result in an offshore displacement of the density front and vertical shear past the shelf break, leaving the water over the shelf less stratified and more subject to barotropic reversals of the current than that farther north, where the front s

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01527

年代:1987

数据来源: WILEY

摘要:

The spring transition between winter and summer conditions on the northern California continental shelf occurred suddenly, over a period of days, in late March in 1981 and in mid‐April in 1982. The spring transition marks the onset of the upwelling season over the continental shelf and is a response to an atmospheric spring transition associated with a sudden seasonal change in the large‐scale atmospheric pressure field over the North Pacific. The 1981 and 1982 spring transitions over the northern California shelf are characterized by a drop in the water temperature over the shelf of 2°–4°C and a reduction of the temperature difference across the water column at midshelf from 2°C to 0.5°C. Isotherms and isopycnals, which were level prior to the spring transition, slope upward toward the coast after the spring transition. The mass balance for the 1982 spring transition event is roughly two dimensional, with offshore flow near the surface balanced by weaker subsurface onshore flow. The alongshore momentum balance during the 1982 spring transition is not two dimensional and includes the establishment of an alongshore pressure gradient which opposes the wind and apparently drives a poleward flow observed during a period of weak wind following the spring transition. The cross‐shelf momentum balance is geostrophic, with density fluctuations contributing to the cross‐shelf pressure gradient. The heat budget for the spring transition event is roughly two dimensional, with the drop in temperature due primarily to a cross‐shelf exchange of heat associated with the cross‐

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01545

年代:1987

数据来源: WILEY

摘要:

Moored observations of winds, currents, and temperature made over the northern California shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the Coastal Ocean Dynamics Experiment (CODE) are described. In both field experiments (CODE 1 and CODE 2), the onset of the upwelling season is triggered by the strengthening of the North Pacific high off northern California and signalled by the presence of water colder than 9°C near the bottom over the shelf. Vertical temperature stratification over the shelf is reduced during active upwelling. Local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction, and the mean wind stress is directed equatorward. Response of alongshelf currents to wind forcing is similar at all depths over the shelf. The response of the cross‐shelf current is more complex. In the surface layer, offshelf flow is significantly and positively correlated with the upwelling‐favorable alongshelf wind stress. Beneath this layer a weak return flow exists. The thickness of the upper mixed layer varies greatly, reaching 60 m during upwelling events. Thus direct wind forcing affects a substantial fraction of the water over the shelf. Low‐frequency wind stress, current, and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure. The most energetic mode typically accounts for 75% or more of the variance of wind stress and temperature. Over the middle and inner shelf, the most energetic current mode accounts for 90% or more of the variance, is dominated by he alongshelf component, and veers to the left with depth. The low‐frequency alongshelf flow fluctuations are quasi‐barotropic, with some vertical shear but no reversal with depth; vertical shear in alongshelf current and the cross‐shelf density gradient obey the thermal wind relation. The simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross‐shelf transects in CODE 2. Volume transport over the shelf is often nearly two‐dimensional, but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope. Correlation of low‐frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf, where a 15 cm−1alongshelf flow lags a 1 dyn cm−2alongshelf wind stress by the order of 10 hours. Currents over the upper slope are not significantly correlated with local wind. Alongshelf transport over the shelf is highly correlated with local wind stress, explaining 80% of the variance in the low‐frequency transport during CODE 2. Volume transport is best correlated with wind stress approximately 30 km to the south. Winds in the CODE area during spring and summer are characterized by strong and persistent upwelling‐favorable winds interrupted by shorter periods of weak winds. Regression analysis indicates that in the absence of wind, a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the CODE area. An upwelling‐favorable wind counters this jet over the shelf, so that the alongshore flow is everywhere equator‐ward during strong wind events. Since the maximum response to wind forcing is found at midshelf and near surface, the mean current over the upwelling season is strongly sheared in both the vertical and cross‐shelf directions. Mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward, while the deeper flow at midshelf and through most of the water column over the

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01569

年代:1987

数据来源: WILEY

摘要:

During the Coastal Ocean Dynamics Experiment (CODE) along the northern California continental shelf in the spring and summer of 1981 and 1982, the first comprehensive effort to obtain all of the observations necessary to infer pressure fluctuations throughout the water column was made. This is a description of the subtidal fluctuations in measured bottom pressure, internal pressures inferred from moored conductivity and temperature observations, and the water column total pressure anomalies computed from a combination of the two components. The pressure field during CODE 2 (April‐July 1982) is found to be more energetic than that in CODE 1 (April‐July 1981), with more than 70% total pressure variance concentrated at frequencies of less than (15 days)−1. The spatial structure of the pressure fluctuation field is characterized by coastal intensification and a tendency for poleward phase propagation. Like the pressures, most of the CODE across‐shelf pressure difference variance is found at the lower frequencies, with more than 60% of the total across‐shelf pressure difference variance at frequencies of less than (15 days)−1The CODE across‐shelf pressure differences are found to be in geostrophic balance with both alongshelf currents and the total alongshelf transport. The alongshelf pressure difference variance is generally found to be concentrated at the lower frequencies much like that of the across‐shelf pressure differences. Most of the pressure and pressure difference variance is found to be highly coherent with wind stress. The 20‐day band pressure variability is apparently related to the response of the pressure field during so‐called “relaxation events.” The barotropic component of the alongshelf geostrophic transport (i.e., across‐shelf pressure difference) has been found to be almost perfectly coherent with regional wind stress τyand to propagate poleward within the CODE region at the same rate as τyand the alongshelf pressure gradient is very important in accelerating the alongshelf flow. The 5‐day band pressure variability in the CODE region appears to result from local wind stress fluctuations. The alongshelf pressure gradient was found to be of secondary importance relative to the wind in acc

ISSN:0148-0227    

DOI:10.1029/JC092iC02p01605

年代:1987

数据来源: WILEY