摘要:

Scalar and vector empirical orthogonal function (EOF) analyses have been performed on monthly sea surface temperature (SST) and wind stress within the tropical Atlantic region for the years 1964–1979. The long‐term average was subtracted from the 16‐year SST and wind stress monthly fields to focus on the seasonal variability and its interannual modulations. A smaller cumulative percent of variance accounted for by the first eigenvectors of wind stress reflects the greater variability of the wind than that of SST. Analysis of regional seasonal SST events requires a combination of the first three EOFs, whereas the dominant events of the seasonal cycle of the wind stress are adequately represented byE1andE2. As in the Pacific, an increase of the trade wind system is observed between the 1960s and the 1970s. Our analyses provide diagnostic insight into two types of large spacetime scale episodes. The first type is exemplified by the following sets of observations: during the spring of 1968 (spring 1976), an abnormal relaxation (intensification) of the wind stress occurred in both the northeasterlies and the southeasterlies. Subsequently, equatorially trapped anomalous warming (cooling) occurred inside the Gulf of Guinea. The second type is exemplified by the following: during 1972 to 1975, the northeasterlies (stronger) and the southeasterlies (weaker) acted out of phase, leading to a more global response of the ocean. The oceanic response was different in the northern (colder) and southern (warmer) b

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14181

年代:1986

数据来源: WILEY

摘要:

In the western tropical Atlantic, seasonal variations in the surface winds and in the ocean are dominated by an annual harmonic. A simulation with a general circulation model indicates that the response in the western side of the basin is an equilibrium one practically in phase with the local winds. It includes the following: large vertical excursions of the thermocline that have a 180° change in phase across 8°N approximately; a change in the direction of the North Brazilian Coastal Current, which flows continuously along the coast between December and May but which veers offshore near 5°N to feed the North Equatorial Countercurrent during the other months; and a seasonal reversal of the countercurrent. To the east of 30°W, seasonal changes in the model have a prominent semiannual harmonic in phase with the local winds but only partially attributable to forcing at that frequency. The transients excited by the abrupt intensification of the southeast tradewinds in May happen to have a phase essentially the same as that of the semiannual forcing. These transients decay by the end of the calendar year, so that the seasonal cycle that starts with the intensification of the winds in May can be treated as an initial value problem as far as the upper ocean, above the thermocline, is concerned. The winds along the equator determine the response of the surface equatorial layer in the Gulf of Guinea but play a minor role in the seasonal upwelling along the coast near 5°N. That upwelling is strongly influenced by changes in both components of the wind, and in the curl of the wind, over the Gulf of Gu

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14192

年代:1986

数据来源: WILEY

摘要:

In general circulation models of the seasonal cycle, westward propagating waves, with an approximate wavelength of 1000 km and period of 3 to 4 weeks, in the western equatorial Atlantic and eastern equatorial Pacific derive their energy from the kinetic and potential energy of the mean flow. There is intense downwelling in the cold crests of the wave and upwelling in the warm troughs. The local meridional heat flux associated with the waves is of the order of 100 W m−2, but their contribution to the net heat transport across the equator is small. The waves are highly nonstationary in time and inhomogeneous in spac

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14207

年代:1986

数据来源: WILEY

摘要:

In a general circulation model of the tropical Atlantic Ocean, the northwestward flowing Brazilian Coastal Current is fed by the westward South Equatorial Countercurrent and in turn loses water to the eastward Equatorial Undercurrent and the eastward North Equatorial Countercurrent. The transport of the Countercurrent decreases in a downstream direction primarily because of downwelling and then equatorward flow, in the thermocline, into the undercurrent. Some of the Countercurrent water penetrates into the Gulf of Guinea, where it flows into the southern hemisphere. The transport of the Equatorial Undercurrent decreases because upwelling, which is most intense in the western side of the basin, transfers fluid into the surface layers to sustain divergent Ekman drift which is swept westward by the South Equatorial Undercurrent. The model has a northward heat transport across all latitudes in the tropics. Seasonal variations in this transport are modest to the south of 5°S and to the north of 15°N. Across 8°N, however, the transport varies from 1.5 × 1015W in January and February to −0.1 × 1015W in August. This result implies that the zonal bands 5°S to 8°N and 8°N to 15°N act as capacitors that are out of phase. In July, August, and September the heat gained from the southern hemisphere is stored in the 5°S to 8°N band where the thermocline deepens. During this period (when the Brazilian Coastal Current turns offshore near 5°N) the thermocline between 8°N and 15°N rises as heat is lost across 15°N. When the Brazilian Coastal Current flows continuously along the coast into the Gulf of Mexico, from December into May, it transports heat from the band 5°S to 8°N to replenish the heat stored between 8°N and 15°N and to sustain the h

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14212

年代:1986

数据来源: WILEY

摘要:

Empirical search and inverse techniques were used to reanalyze historical hydrographic data obtained in the Gulf of Mexico, February 12 to March 31, 1962. From these data, eight transects, which provide basin‐wide coverage of the gulf, were constructed. On the basis of water properties the circulation of the Gulf of Mexico was characterized as a three‐layer, mass‐conservative system. Along most sections an empirical search in density coordinates was sufficient to obtain an optimum reference level that balanced the transport in the three layers. An inverse correction was necessary to obtain a mass balance for those sections that crossed the Loop Current and mesoscale eddies. The density of the optimum reference level (σ1= 32.17) obtained from this analysis corresponds approximately to the bottom of the Antarctic Intermediate Water layer that exists in the Gulf of Mexico. By using this optimum reference level, total velocity profiles at specific locations in the Gulf of Mexico were obtained. Taken together, these yielded a description of the winter circulation of this region. In general, the large‐scale circulation consists of a clockwise (anticyclonic) gyre which is most prominent in the upper 500 m. Eastward and westward transports associated with the gyre are approximately equal at about 5 × 106m3s−1. Variations to this flow are provided by the Loop Current in the eastern gulf and a cyclonic eddy in the northwestern gulf. Transports obtained for these circulation features are approximately 30 × 106and 8 × 106m3s−1, respectively, which are in agreement with values obtained from other hydrographic surveys in the

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14221

年代:1986

数据来源: WILEY

摘要:

The structure of an artificially generated sinusoidal, water wave train of fixed frequency is examined under the influence of wind. The characteristics of this wave train were obtained with the aid of capacitance‐type wave height gauges in a wind wave research facility at Stanford University. Experimental results are given for seven wind speeds in the range 140–400 cm/s and 1‐Hz, 2.54‐cm (nominal) amplitude, artificially‐generated waves. The amplitude and phase of the various wave components were deduced by a simple method utilizing their traveling wave property and their characteristic dependence upon the streamwise position in the channel. The dispersion relation and component phase speeds were also examined. It was found that (1) the amplitude of the forced and free‐traveling second harmonics compares favorably with existing theories and (2) the nonlinearities of the primary wave, the interaction between short gravity waves and the primary wave, and the advection effects of wind drift are mainly responsible for the deviation of the measured phase speeds from the linear theory. The latter results are consistent with the field measurements of Ramamonjiarisoa and Giovanangeli (1978), indicating that the apparent phase speeds at high frequencies are independent of the frequency. The measured phase speeds were also found to increase with wind speed, at a given frequency, in accord with previous laboratory measurements and theoretical c

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14237

年代:1986

数据来源: WILEY

摘要:

A previous altimeter wind speed retrieval algorithm was developed on the basis of wind speeds in the limited range from about 4 to 14 ms−1. In this paper, we use a new approach which gives a wind speed model function applicable over the range 0 to 21 ms−1. The method is based on comparing 50 km along‐track averages of the altimeter normalized radar cross section measurements with neighboring off‐nadir scatterometer wind speed measurements. The scatterometer winds are constructed from 100 km binned measurements of radar cross section and are located approximately 200 km from the satellite subtrack. The new model function agrees very well with earlier versions up to wind speeds of 14 ms−1but differs significantly at higher wind speeds. We discuss the relevance of these results to the Geosat altimeter launched in M

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14250

年代:1986

数据来源: WILEY

摘要:

Three years of current data from the offshore branch of the Labrador Current are examined. The meters were at least 400 m deep (to avoid being struck by icebergs) and positioned over the 1000‐m isobath on the eastern flank of Hamilton Bank. The currents are predominantly directed along isobath. At periods of 1–8 days the motion is bottom‐intensified; at longer periods (10–40 days) it is essentially independent of depth. One of the objectives of this paper is to determine whether or not the 10‐ to 40‐day fluctuations, which areO(5 cm s−1), can be explained by large‐scale wind forcing over the North Atlantic. Monthly variations in the transport of the North Atlantic are calculated by means of the topographic Sverdrup relationship. The Sverdrup return flow along the coast of Labrador, −〈ψw〉, is found to have a well‐defined seasonal cycle with a range of 6 Sv. The maximum southward transport generally occurs in February, which is also the month when the standard deviation of (ψw) is a maximum (9 Sv). If we assume that the monthly changes of the return flow are barotropic and confined to a boundary current of (say) 100 km width and 1 km depth (the mooring depth), then 1 Sv corresponds to a speed of 1 cm s−1. We would then expect the large‐scale wind over the North Atlantic to cause the flow along the Labrador coast to vary with a seasonal range of 6 cm s−1and exhibit monthly variations in winter of ∼9 cm s−1. However, a comparison of the (ψw) and current records shows no similarity. We therefore speculate, with some corroborative evidence from an analysis of Nain sea level, that these energetic variations in the (topographic) Sverdrup return flow along the Labrador Coast probably occur in deeper water, offshore of the current meter mooring. The cause of the barotropic current variations remains unclear. It is possible that they do not reflect genuine transport changes but are due to a meandering of the Labrador Current. Further analysis, and perhaps measurements of bottom pressure, are

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14261

年代:1986

数据来源: WILEY

摘要:

The vertical dependence of the eddy viscosity A(z) is explored using a simple analytical model and data from a closely spaced set of moorings off Long Island. Two different situations are examined, one with fairly steady flow, and the other with fairly steady wind forcing. In both cases the results differed from previous expressions for A(z), for example, in that A(z) increased instead of decreased toward the bottom boundary. This difference is a consequence of treating a baroclinic instead of a non‐stratified situation. The omission of baroclinicity also leads to larger column‐averaged values of A(z), a fact that becomes more obvious when the full velocity vector is modeled instead of just the main component. Low values (0(10) cm2/s) with an interior minimum were obtained for the steady flow, strongly baroclinic situation; and higher values (0(100) cm2/s) were required for the more strongly wind‐forced situation. Comparisons are made with the Munk and Anderson [1948] Richardson number dependency for A(z). Other aspects of the solution and its applicability to the observed flow are also disc

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14269

年代:1986

数据来源: WILEY

摘要:

The structure and salinity characteristics of saline ice slabs removed from ice sheets grown in an outdoor pool have been studied and related to the complex relative dielectric permittivity measured, utilizing free space transmission techniques, at 4.8 and 9.5 GHz. The saline ice closely simulated Arctic sea ice in its structural and salinity characteristics which were regularly monitored in a number of ice sheets grown during the winters of 1983–1984 and 1984–1985. In‐situ transmission measurements at similar frequencies were also made on the ice sheet itself using antennas located above and beneath the ice. The slab measurements were made during warming from −28°C to −2°C on slabs grown during the winter of 1983–1984 (4.75 GHz) and during a warming and cooling cycle over a slightly larger temperature range on slabs grown during the winter, 1984–1985 (4.80 and 9.50 GHz). The 1983–1984 results show both the real εr′ and imaginary εr″ parts to vary almost in direct proportion to the brine volume νBat values less than about 50‰. The 1984–1985 slabs showed only εr′ to depend strongly on νBwhile values of εr″ were considerably less than in the previous year. This difference may have been due to greater electrical conductivity in the 1983–1984 slabs, which could be related to distinct differences seen in brine pocket structure between the two winters. Values of εr′ as a function of νBwere slightly lower on the average at 9.50 GHz than at 4.80, and most values of εr′ agreed with the results of others when their temperature and salinity values were converted to values of νB. Changes in εr′ due to thermal modification (laboratory warming) seem more strongly related to a gain in air volume due to brine drainage than to the redistribution of brine inclusions. The in‐situ measurements showed extremely high attenuation for the young (<12 cm) brine‐rich ice. Good agreement was found between data for the more desalinated samples and theoretical values predicted by a previously proposed dielectric mixing model that was modified to account for the brine pocket geometry observed in thin sections, and also by including a bul

ISSN:0148-0227    

DOI:10.1029/JC091iC12p14281

年代:1986

数据来源: WILEY