Clive E. Dorman

Evaporation Minus Precipitation and Density Fluxes for the North Atlantic

Abstract Estimates of evaporation (E) over the North Atlantic Ocean by Bunker have been combined with estimates of precipitation (P) by Dorman and Bourke to produce new annual and seasonal maps of E–P and surface density flux. Although uncertainties about precipitation algorithms and exchange coefficients still presist, it is felt that the high spatial resolution of these data set permits an estimate of E–P that is a significant improvement over previous work. The maps of E–P show considerably more detail than earlier maps, including a previously uncharted minimum with a northeast to southwest trend across the subtropical gyre. The two regions of maximal E–P display a close connection with continental air flows off Africal and North America, suggesting that the relative narrowness of the North Atlantic contributes to its status as a net evaporation basin. The zonally integrated E–P values are combined with river runoff estimates to obtain the meridional flux of freshwater, which can be compared with fluxe...

The Marine Layer off Northern California: An Example of Supercritical Channel Flow

Abstract During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern California continental shelf, only interrupted for periods of a few days, when weak or southerly winds occur. In the course of the Coastal Ocean Dynamics Experiment (CODE), fixed station and observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters in a region extending up to 40 km offshore and 100 km along the coast. These observations suggest that atmospheric conditions during the spring and summer usually fall into one of three categories: the surface wind can be everywhere weak (Pattern 1), it can blow at large speeds in a uniform pattern (Pattern 2), or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales (Pattern 3), The latter pattern, which o...

Buoy observations of the atmosphere along the west coast of the United States, 1981–1990

The distribution of statistical properties of the meteorological and sea surface temperature fields along the west coast of the United States is described based on 10-yearlong observations from buoys deployed by the National Data Buoy Center. The observations suggest that properties vary differently in each of three different regions along the coast: the Southern California Bight which remains sheltered from strong wind forcing throughout the year; the central and northern California coast up to Cape Mendocino, the site of persistent equatorward winds; and the Oregon-Washington coast, where traveling cyclones and anticyclones produce vigorous and variable forcing. Over most of the region the variance in the wind speed is roughly equally divided between the annual cycle and the synoptic forcing, corresponding to periods between 5 and 50 cycles per year. Two seasons, summer and winter, are sufficient to describe the annual cycle. During the summer, two distinct wind speed maxima occur along the coast, one near Point Conception and the other off northern California, between Point Reyes and Point Arena. In the winter a single maximum occurs, located near Point Conception. The atmospheric pressure generally increases with latitude along the coast, but the annual cycle of atmospheric pressure has a different phase, depending on location; off the coast of California, highest pressures are found during the winter, while off Oregon and Washington, the highest pressures occur during the summer. Fluctuations in air and sea temperature are highly correlated, and the sea temperature is usually higher than the air temperature, in the winter. Examination of vertical soundings of the atmosphere at Oakland, Vandenberg Air Force Base and San Diego during the same period of time reveals that a well-defined inversion separates the marine boundary layer (MBL) from the free atmosphere above nearly 90% of the time during the summer and half the time during the winter. Station soundings consistently overestimate the MBL thickness, but the results do suggest that the MBL is supercritical part of the time in the vicinity of the three sites. An attempt is made to examine the interannual variability and compare it to the Southern Oscillation index, although the results are limited because the record length is short compared with interannual timescales. Spatially averaged temperature anomalies increase during winter 1982–1983, coincident with the large El Nino event.

The lower atmosphere over the Gulf of California

Automated meteorological observations at coastal and island stations, instrumented aircraft flights, and coastal soundings were made between July 1983 and June 1985 to define the variations of the lower atmosphere over the northern half of the Gulf of California during the two important climatic seasons, the mid-latitude winter and the subtropical summer. A marine layer is well defined over water during both seasons but dissipates within a few kilometers inland. The winter large-scale pressure field is dominated by the Great Basin high over the southwestern United States. Modulated by upper level synoptic activity, it causes 3 to 6 days events of northwesterly winds (8–12 m s−1) directed along the gulfs axis, which are coherent over basin scales, and bring cool, dry desert air over the gulf. The vertical scale of these winds is 700–1000 m over land on the western side of the gulf, close to the height of the Baja California mountains, but only 100–400 m over water, defined by the wintertime inversion. Cross-gulf winds, related to topographic effects during upper level trough passages over Baja California, are particularly intense in the northwestern portion of the gulf. The winter marine layer is defined by a 1° to 4.5°C temperature inversion and a dew point temperature of 6°–11°C (a moisture content of 6–8 g kg−1); weak subsidence reduces dew point values to −4°C above the inversion. Winter winds above the marine layer are coherent (0.8) across the width of the gulf. The summer large-scale pressure field is dominated by a thermal low over the southwestern United States, and drives weak (2–5 m s−1) southeasterly winds, also directed primarily along the gulf, which are less spatially uniform than winter winds. Air temperature and moisture content are rather constant, and topographic effects are absent. The summer marine layer is on the order of 200–300 m thick, with dew point temperatures of 26°–28°C (21–24 g kg−1), and capped by a weak temperature inversion (1°–2°C) over water. Aloft, the dew point temperature is 17°–21°C, and winds are weak and uncorrelated across the width of the Gulf. During June and early July 1984, four week-long pulses of southeasterly winds (10 m s−1) support the existence, reported previously, of moisture surges from the gulf into the Sonoran desert. A dynamical analysis of subdiurnal motions shows that the cross-gulf momentum balance is geostrophic, the along-gulf balance is ageostrophic, with the along-gulf pressure gradient opposed by friction at the surface. These results are consistent with a scale analysis of the equations of motion in the marine layer over the Gulf of California.

Seasonal patterns of surface wind stress and heat flux over the Southern California Bight

Patterns of wind stress and heat flux between the atmosphere and the ocean over the Southern California Bight are described based on observations from buoys and ships. During the winter, the wind stress is spatially homogeneous and temporally variable, with strong events corresponding to low-pressure systems sweeping through the area. During the summer, spatial patterns are more persistent, with large gradients. Inshore of a line running approximately between Point Conception and Ensenada, Mexico, winds are weak. Offshore wind speeds are comparable in magnitude to those found over the continental shelf north of Point Conception. The boundary is the location of maximum wind stress curl, and the spatial resolution afforded by California Cooperative Fisheries Investigation (CalCOFI) observations suggests maximum wind stress curls over 3 times larger than the values proposed by Nelson [1977]. Net heat flux estimates derived from the CalCOFI measurements are somewhat larger than the values proposed by Nelson and Husby [1983], due to differences in latent heat flux estimates. Possible mechanisms responsible for the spring-summer spatial structure in the wind and the relationship between these gradients and the properties of the underlying ocean are discussed.

Coastal Perturbations of Marine-Layer Winds, Wind Stress, and Wind Stress Curl along California and Baja California in June 1999

Abstract Month-long simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) with a horizontal resolution of 9 km have been used to investigate perturbations of topographically forced wind stress and wind stress curl during upwelling-favorable winds along the California and Baja California coasts during June 1999. The dominant spatial inhomogeneity of the wind stress and wind stress curl is near the coast. Wind and wind stress maxima are found in the lees of major capes near the coastline. Positive wind stress curl occurs in a narrow band near the coast, while the region farther offshore is characterized by a broad band of weak negative curl. Curvature of the coastline, such as along the Southern California Bight, forces the northerly flow toward the east and generates positive wind stress curl even if the magnitude of the stress is constant. The largest wind stress curl is simulated in the lees of Point Conception and the Santa Ba...

Evidence of Kelvin Waves in California's Marine Layer and Related Eddy Generation

Abstract Evidence suggests that an internal solitary Kelvin wave exists in the marine layer along California. The marine layer is lifted over the central coast by a weak cyclonic circulation. This “bump,” initially 850 m high, moves to the north along the coast at 6 m s−1. The undisturbed layer depth is 100–200 m thick. The crest height of the wave decreases to 500 m farther north. Winds under the raised marine layer are southerly. The leading edge of the wave is easily followed by satellite as the thickened marine layer is marked by overcast stratus. A greatly curved offshore leading edge indicates that nonlinear effects are important. Offshore scale in the overcast is about 300 km in the south and 50 km in the north. Surface pressure gradient alongshore is closely related to the marine layer depth. The surface wind shifts when the leading and trailing edge of the wave passes. Northerly wave progression ceases at the sharp bend formed by Cape Mendocino. At this time, a vortex is formed in the marine laye...

The Structure and Variability of the Marine Atmosphere around the Santa Barbara Channel

The Santa Barbara Channel is a region characterized by coupled interaction between the lower-level atmosphere, the underlying ocean, and the elevated topography of the coastline. The nature of these interactions and the resulting weather patterns vary between summer and winter. During summer, synoptic winds are largely controlled by the combined effect of the North Pacific anticyclone and the thermal low located over the southwestern United States, resulting in persistent northwesterly winds. A well-defined marine atmospheric boundary layer (MABL) with properties distinct from the free atmosphere above is a conspicuous feature during the summer. The wind has different characteristics in each of three zones. Maximum winds occur in the area extending south and east from Pt. Conception (zone 1), where they initially increase as they turn to follow the coast, then decrease farther east. Winds are usually weak in zone 2, located in the easternmost part of the channel, offshore from the Oxnard plain. Winds are also weak in zone 3, sometimes reversing to easterly at night, in a narrow band located along the mainland coast. Summer air temperature at the surface follows the SST closely and varies significantly with location. Summer sea level pressure gradients are large, with the lowest pressure occurring on the northeast end of the Santa Barbara Channel. Diurnal variations are strongest in summer, although the modulation is weakest in zone 1. The diurnal variation is parallel to the coast in all of zone 3 but the Oxnard plain, where it is perpendicular to the coast. The height of the marine layer varies between 300 m in late afternoon and 350 m in late morning. In winter, synoptic conditions are driven by traveling cyclones and sometimes accompanied by fronts. These are usually preceded by strong southeast winds and followed by strong northwest winds. Atmospheric parameters are distributed more uniformly than in summer, and diurnal variations are greatly reduced. Sea level air temperature and pressure are more spatially uniform than in the summer. Spatial variations in the observed fields in the summer are consistent with a hydraulic model of the MABL as a transcritical expansion fan. The summertime situation is governed by a coupled interaction between the atmosphere and the underlying water. The ocean influences the density of the MABL to the extent that it behaves distinctly from the free atmosphere above, resulting in strong winds polarized in the direction parallel to the coast. In turn, these winds provoke an upwelling response in the coastal ocean, which in part determines the surface properties of the water.

Highlights of Coastal Waves 1996

Some of the highlights of an experiment designed to study coastal atmospheric phenomena along the California coast (Coastal Waves 1996 experiment) are described. This study was designed to address ...

Bora event variability and the role of air‐sea feedback

1) was conducted to improve forecast momentum and heat flux fields, and to evaluate surface flux field differences for two consecutive bora events during February 2003. (COAMPS 1 is a registered trademark of the Naval Research Laboratory.) The strength, mean positions and extensions of the bora jets, and the atmospheric conditions driving them varied considerably between the two events. Bora 1 had 62% stronger heat flux and 51% larger momentum flux than bora 2. The latter displayed much greater diurnal variability characterized by inertial oscillations and the early morning strengthening of a west Adriatic barrier jet, beneath which a stronger west Adriatic ocean current developed. Elsewhere, surface ocean current differences between the two events were directly related to differences in wind stress curl generated by the position and strength of the individual bora jets. The mean heat flux bias was reduced by 72%, and heat flux RMSE reduced by 30% on average at four instrumented over-water sites in the two-way coupled simulation relative to the uncoupled control. Largest reductions in wind stress were found in the bora jets, while the biggest reductions in heat flux were found along the north and west coasts of the Adriatic. In bora 2, SST gradients impacted the wind stress curl along the north and west coasts, and in bora 1 wind stress curl was sensitive to the Istrian front position and strength. The two-way coupled simulation produced diminished surface current speeds of 12% over the northern Adriatic during both bora compared with a one-way coupled simulation.