摘要:

A conductivity‐temperature‐depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10°W composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar θ‐Swater within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy‐shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal “isopycnal” plane, as much as 10 × 106m3s−1of Indian Ocean water within the 25 × 106m3s−1Benguela Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of

ISSN:0148-0227    

DOI:10.1029/92JC00485

年代:1992

数据来源: WILEY

摘要:

Oceanographic measurements acquired by the USNSBartlettduring September 1989 in and north of the area of the Jan Mayen Current (JMC) show that, in terms of upper layer baroclinic flow, about half of the JMC is a wide meander in the East Greenland Current (EGC) and about half continues eastward to close the Greenland Gyre (GG) on the south. This phenomenon has been charted in the past but has elicited no comment. Surface drifters and a numerical model corroborate this behavior. At deeper depths (>100 m) the meander dissipates with the flow becoming more easterly. A drifting float at 500 m depth confirms that the flow is relatively undeviated toward the east ultimately merging with the northward flowing Norwegian Atlantic Current. Near the GG the fresh water content of the water column is low in the upper layers but high in the middepths. Where there is a substantial content of EGC water (e.g., in the JMC), the converse is usually true. The FWC of the GG computed for the period 1953–1966 was smaller by a factor of 2 in both layers than in 1989, indicating that 1989 is in another period of low salinity comparable to that of 1968, the period of the Great Salinity Anomaly described by Dickson et al. (1988

ISSN:0148-0227    

DOI:10.1029/92JC00150

年代:1992

数据来源: WILEY

摘要:

The 30 Sv transported by the Florida Current through the Straits of Florida off Miami could consist of a wind‐driven contribution of 17 Sv along with a thermohaline component of 13 Sv. The latter might flow from the South Atlantic as upper layer compensation for a net lower layer cross‐equatorial flow southward. The Sverdrup transport along 24°N in the interior North Atlantic east of 55°–60°W is about 17 Sv, potentially matching the wind‐driven component of the Florida Current. The circulation in the vicinity of 24°N and west of 55°–60°W up to the Bahama Banks contains energetic, shorter spatial scale flows, where there is a similarity between the regional pattern of the Sverdrup transport contours and the scales and structure of the observed C‐shaped pattern

ISSN:0148-0227    

DOI:10.1029/92JC00417

年代:1992

数据来源: WILEY

摘要:

A linear Wiener (least squares optimal) filter is used to predict the path of the Gulf Stream in the region between Cape Hatteras and 70°W using upstream (“Inlet”) conditions as filter inputs. Although position, angle, curvature, geostrophic velocity, and curvature vorticity (κυ) measured at 73°W were tested as multiple Inlet parameters, it was found that Inlet position alone was the best predictor. With Inlet position as the predictor, this filter is equivalent to propagating meanders downstream with a typical phase speed and growth rate. Forecasts of the downstream path position are produced, more accurate than persistence, 12 days in advance at 140 km from the Inlet, 24 days in advance at 240 km from the Inlet, and 28 days in advance at 340 km from th

ISSN:0148-0227    

DOI:10.1029/92JC00600

年代:1992

数据来源: WILEY

摘要:

Acoustic Doppler current profiler measurements made while underway in 200–240 m of water south of Key West, Florida, show wavelike signals with about 1 km horizontal wavelength and 3–5 cm s−1amplitude vertical velocity. Vertical displacements of about 5 m are inferred from coherent oscillations in surfaces of constant acoustic backscatter, in features associated with the width of the peak in the Doppler spectrum, and in fine structure of vertical current shear. While the sampling was limited, all the measurable waves occurred over irregular bathymetry in the presence of an east‐northeastward mean flowU>1 m s−1(averaged over 30‐ to 90‐m depth). The peak strain rate associated with the waves was nearly 1 × 10−3s−1at 12‐m depth, and this could have produced bands of smooth and rough water observed at the surface. We tentatively identify the waves as being topographically forced by the Florida Current flowing over a roug

ISSN:0148-0227    

DOI:10.1029/92JC00480

年代:1992

数据来源: WILEY

摘要:

Near‐inertial motion on the shelf‐slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near‐inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near‐inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near‐inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf‐slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near‐inertial energy

ISSN:0148-0227    

DOI:10.1029/92JC00588

年代:1992

数据来源: WILEY

摘要:

Temporal variations of longshore currents have recently been identified on natural beaches (Oltman‐Shay et al., 1989). Termed shear waves, these variations appear to be strongly linked to the strength of the mean longshore current. Bowen and Holnan (1989) proposed a simple model based on the shear instability of a steady bilinear longshore current profile over a constant depth to explain these observations. In this work we generalize Bowen and Holman's model. A numerical solution technique is employed to study the characteristics of shear waves generated by instabilities of smooth longshore current profiles on general bottom topographies. We find that the stability characteristics are extremely sensitive to variations in the bottom topography. The “dispersion” relationship of the shear waves is almost linear and does not seem to be very sensitive to the topography. We find that the presence of a nearshore bar greatly enhances the strength of the instability and, in certain situations, tends to trap the shear wave shoreward of the bar crest. The results also suggest that the shear on the seaward face of the longshore current profile is an important parameter in the stability problem. A Fjortoft condition is established for the instability equation. Estimates of length and time scales indicate that shear waves should be detectable in laboratory experiments. An order of magnitude analysis indicates that shear waves may be a plausible mixing mechanism in the nearshore r

ISSN:0148-0227    

DOI:10.1029/91JC02988

年代:1992

数据来源: WILEY

摘要:

Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a σtcriterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the “barrier layer”, in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt‐stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer

ISSN:0148-0227    

DOI:10.1029/92JC00407

年代:1992

数据来源: WILEY

摘要:

We examine the climatological and anomalous response of an isopycnic coordinate general circulation model, with embedded bulk surface mixed layer model, when forced by observed monthly mean wind stress variability. Our results show that model fields of sea surface temperature (SST) and currents are fairly similar to those arising in other models as well as published observations. The model generates a climatology of SST which is usually within 0.5°–1.5°C of the Comprehensive Ocean‐Atmosphere Data Set observations. Anomalous SST compares best with observations when averaged over rather large spatial regions. The mean horizontal currents tend to resemble the observed, but the amplitudes are smaller than observed (primarily because the model predicts vertical mean currents in the mixed layer rather than the surface currents) and the springtime reversal of the South Equatorial Current is absent in the model. Anomalous zonal currents are similarly reduced in amplitude though the variability is comparable to the available observations. We also describe the model's basin‐scale mean mixed‐layer structure and the mean vertical velocity field (which is fundamentally different from that of the Geophysical Fluid Dynamics Laboratory model). We discuss various aspects of the 1982–1983 El Niño in particular detail, including the heat budget of the variable‐depth tropical Pacific surface mixed layer, and the relationships among anomalies of mixed‐layer depth, thermocline depth, and SST. By these analyses, we identify the strengths and weaknesses of the model's capability to simulate observations, particularly with respect to that of other models. Throughout the paper, we attempt to identify the primary areas which require improvement in the model and suggest p

ISSN:0148-0227    

DOI:10.1029/92JC00273

年代:1992

数据来源: WILEY

摘要:

Global average temperature in a 100‐year control run of a model used for greenhouse gas response simulations showed low‐frequency natural variability comparable in magnitude to that observed over the last 100 years. The model variability was found to be barotropic in the atmosphere, and located in the tropical strip with largest values near the equator in the Pacific. The model variations were traced to complex, low‐frequency interactions between the meridional sea surface temperature gradients in the eastern equatorial Pacific, clouds at both high and low levels, and features of the tropical atmospheric circulation. The variations in these and other model parameters appear to oscillate between two limiting climate states. The physical scenario accounting for the oscillations on decadal time scales is almost certainly not found in the real world on shorter time scales due to limited resolution and the omission of key physics (e.g., equatorial ocean dynamics) in the model. The real message is that models with dynamical limitations can still produce significant long‐term variability. Only a thorough physical diagnosis of such simulations and comparisons with decadal‐length data sets will allow one to decide if faith in the model results is, or is not,

ISSN:0148-0227    

DOI:10.1029/92JC00501

年代:1992

数据来源: WILEY