摘要:

A four‐box model of the ocean is examined in which a surface and deep layer of equal thickness and large volume are connected to two polar reservoirs of smaller volume. While the four reservoirs are thoroughly mixed, there is no significant mixing between them; exchange is due to flow between reservoirs with the flow out of or into the polar reservoirs driven by pressure gradients. Newtonian boundary conditions are imposed on the temperature and salinity of the surface layer and the two polar reservoirs; the deep water is forced only by flows into it from other boxes. The temperature and salinity forcing is applied with a different time scale for each variable. When the time scales are nearly equal, the system responds as a simple component fluid with a single solution for each forcing. When the salinity time scale is long compared with temperature, up to four multiple equilibria can be present for a given forcing. These equilibria are unstable to finite amplitude perturbations, and numerical solutions exhibit catastrophic transitions from one mode to another. When the deep water formation is totally inhibited as a result of a transition, the system can go into a long‐term climatic oscillation, quasi‐periodic in nature. The system possesses four “attractors” corresponding to deepwater formation in either, both, or neither of the two polar r

ISSN:0148-0227    

DOI:10.1029/91JC02147

年代:1991

数据来源: WILEY

摘要:

Independent temperature microstructure samples of Gregg (1977) in the main thermocline of the mid‐Pacific in different years and seasons are used to infer space‐time average vertical heat fluxes and space‐time average turbulence and temperature dissipation rates. Lognormality plots of these data suggest that mixing rates in thermocline layers are approximately lognormal and very intermittent, with intermittency factors σ2lnC̄150mthat increase with depth from 3 to 6 asKzincreases from ≈ 0.2 to 1.6 cm2s−11, and ϵ ≈ (0.6–1.6)×10−4cm2s−3. Decorrelation length scale ranges for turbulence with such large intermittency factors should be large by the Kolmogorov intermittency (third universal similarity) hypothesis. Available microstructure data suggest that mixing concentrates near fronts and is dominated by powerful bursts of turbulence that fossilize and radiate breaking internal waves. Equations for the estimation of vertical heat flux from temperature dissipation rate measurements are rederived by a control volume rather than Reynolds averaging method that emphasizes the necessity for large averaging length and time scales and does not exclude frontal mixing and nonturbulent mixing processes that are neglected by most applications of the Osborn and Cox (1972) model. The indicated heat fluxesqzare about 6 and 3 W m−2at 0.1 and 1 km depths, consistent with deeper bulk flow estimates ofqz≈ 2 W m−2from Munk (1966) but much larger than those inferred from the Gregg (1989) thermocline ε correlation that decrease fromqz≈ 1 to 0.1 W m−2in this depth range. It is suggested that the Gregg (1989) values ofqz, ̄2lnϵ≈ 0.04–0.9,Kz≈ 0.06 cm2s−1, and ϵ ≈ 7×10−6cm2s−3for the ocean main thermocline are underesti

ISSN:0148-0227    

DOI:10.1029/91JC02122

年代:1991

数据来源: WILEY

摘要:

Hydrographic data collected in September 1980 over the Blake Plateau were analyzed using a combination of empirical search and inverse techniques. Five sections, extending from the continental shelf break eastward across the Blake Plateau, were positioned at approximately 1° intervals between 28° and 32°N. The empirical search procedure was applied to four closed regions (boxes), constructed from adjacent sections, where each region was assumed to consist of two conservative layers. Five geostrophic velocity sections were obtained using the average optimum reference level for the four boxes. The inverse technique provided barotropic correction velocities that caused all layers to conserve mass. The sections and the resulting transport streamline fields revealed the presence of a cyclonic feature whose meridional and zonal extent was about 200 km in both directions. This feature was shown to be a coalescing Gulf Stream ring in a late stage of decay. Water mass analyses, current meter data, and earlier studies were used to support this hypothesis. The cyclonic advection of Gulf Stream water around the ring formed a meander in the eastern Gulf Stream wall. Topography appeared to be affecting this flow, suggesting that other rings entering the region would be similarly influenced. Evidence of recurring rings at or near this location and hence, meanders of the eastern Gulf Stream wall, was found in hydrographic data collected by NOAA over a 12‐month period from 1965 to 1966. These data suggested that two such events, lasting about 4 weeks each, occurred during this period. The frequency of these events was in agreement with earlier findings, while the duration of these events was supported by current meter observat

ISSN:0148-0227    

DOI:10.1029/91JC01931

年代:1991

数据来源: WILEY

摘要:

Since 1985 various investigators have proposed that Norwegian Sea deep water (NSDW) is formed by mixing of warm and saline deep water from the Arctic Ocean with the much colder and fresher deep water formed by convection in the Greenland Sea (GSDW). We here report on new observations which suggest significant modification and expansion of this conceptual model. We find that saline outflows from the Arctic Ocean result in several distinct intermediate and deep salinity maxima within the Greenland Sea; the southward transport of the two most saline modes is probably near 2 Sv. Mixing of GSDW and the main outflow core found over the Greenland slope, derived from about 1700 m in the Arctic Ocean, cannot by itself account for the properties of NSDW. Instead, the formation of NSDW must at least in part involve a source which in the Arctic Ocean is found below 2000 m. The mixing of various saline outflows is diapycnal. While significant NSDW production appears to occur in northern Fram Strait, large amounts of saline Arctic Ocean outflow also traverse the western Greenland Sea without mixing and enter the Iceland Sea. During the past decade, deep convection in the Greenland Sea has been greatly reduced, while deep outflow from the Arctic Ocean appears to have continued, resulting in a markedly warmer, slightly more saline, and less dense deep regime in the Greenland Sea.

ISSN:0148-0227    

DOI:10.1029/91JC02013

年代:1991

数据来源: WILEY

摘要:

Water whose ultimate source is the overflow at the Denmark Strait can be traced using maps of salinity, dissolved oxygen, and silica concentration on deep density surfaces south along the American continent to at least 5°S. Several property extrema whose resolution is at the limit of accuracy of the measurements occur on these density surfaces, which nevertheless may indicate areas of stronger mixing along the crest of the Mid‐Atlantic Ridge. Transport estimates for that part of the southward flow within 600 km of the western boundary near 10°N suggest that 10±3×106m3s−1of Lower North Atlantic Deep Water moves south there. The total southward deep water transport near 10°N is estimated to be closed to 25

ISSN:0148-0227    

DOI:10.1029/91JC01878

年代:1991

数据来源: WILEY

摘要:

We present the hypothesis that the remote forcing is one of the mechanisms that determine the semiannual reversal of the upper ocean circulation in the Bay of Bengal. A simple reduced‐gravity oceanic model is used to explain the hypothesis. Three numerical experiments have been performed to compare the oceanic responses to a remote forcing, a local forcing and a combined forcing. It is found that the long Rossby waves excited by the remotely forced Kelvin waves contribute substantially to the variability of the local circulation. This research indicates that equatorial Kelvin waves are one of the major factors determining the circulation in the eastern equatorial Indian Ocea

ISSN:0148-0227    

DOI:10.1029/91JC02424

年代:1991

数据来源: WILEY

摘要:

Measurements of CO2concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin‐wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO2from this region. The sensitivity of this flux to the uncertainty in the CO2gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO2to the atmosphere during austral autumn (February to May) ranges from −0.03 (ocean influx) to +0.09 (ocean efflux) GT of carbon depending on the combination of wind field and wind‐dependent exchange coefficient

ISSN:0148-0227    

DOI:10.1029/91JC02064

年代:1991

数据来源: WILEY

摘要:

A reconstruction of the prebomb Δ14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ14C content are found distinctly south of the equator. Prebomb, low‐Δ14C surface water appears to owe its origin to the upwelling of ∼15°C water off the coast of Peru. The low‐Δ14C water upwelling off Peru is shown to be derived from the “13° Water” thermostad (11°–14°C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same Δ14C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ14C levels were observed in 9°–10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low‐Δ14C water upwelling off Peru and the thermostad water in the undercurrent both originate as ∼8°C water in the subantarctic region of the southwest Pacific. This prescription points to the “lighter variety” of Subantarctic Mode Water (7°–10°C) as a possible source. Because prebomb Δ14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels Of Δ14C should be particularly unaffected by diapycnal mixing with warmer overlying water types. We argue that successively less dense features of the South Pacific thermocline, like the Subantarctic Mode Water, the equatorial 13°C Water, and the Peru upwelling, may be part of a single process of thermocline ventilation. Each evolves from the other by diapycnal alteration, while prebomb Δ14C is nearly conserved. Detailed comparisons are made between the coral Δ14C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ14C data suggest a southern hemisphere thermocline origin for the equatorial Δ14C minimum, the model produces its Δ14C minimum by upwelling abyssal water to the surface via the equatorial divergence. In an appendix to the paper we present a new set of coral Δ14C measurements produced over the last 10 years at Lamont‐Doherty Geological Observatory and compile a post‐1950 set of published coral

ISSN:0148-0227    

DOI:10.1029/91JC02063

年代:1991

数据来源: WILEY

摘要:

Detailed observations obtained from airborne meteorological measurements in an area along the Baltic coast in the southeast of Sweden are presented. The measurements were taken during late spring/early summer when the water in the Baltic is generally cool compared to typical daytime land surface temperatures which generated significant surface temperature contrasts between land and water. The measurements show the coastal marine boundary layer to be very far from the homogeneity often assumed in numerical models. The cases presented focus mainly on mesoscale circulations driven or modified by this temperature contrast and related turbulence structure such as thermal jets and sea breeze circulations and show a high degree of temporal as well as small‐scale spatial variability. Even though the synoptic scale wind direction in this area is highly variable, due to traveling mid‐latitude cyclones, the surface temperature patterns show great consistency from day to day, with the coldest water always being along the coast with a pronounced minimum localized at the southeastern part of the coastline. It is not clear to what extent the sea surface temperature inhomogeneities are responsible for some of the observed small‐scale variability, nor is it clear if they are results of ocean processes alone or are interacting in any significant way with the marine boundary layer close to the

ISSN:0148-0227    

DOI:10.1029/91JC02100

年代:1991

数据来源: WILEY

摘要:

This paper deals with the removal of ambiguities existing on the direction of the wind measured by satellite scatterometers. It is shown that neural networks are well adapted to solving this problem. Results using ERS. 1 simulated scatterrometric wind fields are presented. It is found that 99% of the ambiguities can be removed by imposing a rate of 25% ambiguity at 180° and 90

ISSN:0148-0227    

DOI:10.1029/91JC02216

年代:1991

数据来源: WILEY