摘要:

The interpretation of transient tracer observations depends on difficult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions. Recent studies have attempted to circumvent this problem by making use of a derived quantity, age, based on the simultaneous distribution of two complementary tracers, such as tritium and its daughter, helium 3. The age is defined with reference to the surface such that the boundary condition takes on a constant value of zero. We use a two‐dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms. An interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age. We define an idealized “ventilation age tracer” that is conservative with respect to mixing, and we explore how its behavior compares with that of the tracer‐based ages over a range of advective and diffusive par

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09377

年代:1990

数据来源: WILEY

摘要:

A Lagrangian drifter was deployed in a cold filament off northern California as part of the Coastal Transition Zone program. The drifter was equipped with an optical package (consisting of a spectroradiometer, a fluorometer, and a beam transmissometer) suspended at 8.5‐m depth and a water sampler suspended at 16.3‐m depth. The drifter was recovered after 8 days. Optical, chemical, and biological properties changed considerably as the drifter moved offshore in the cold filament. Concentrations of phytoplankton chlorophyll increased rapidly in the first 2 days, in parallel with the disappearance of nitrate and nitrite. After this initial period, chlorophyll decreased gradually over the next 6 days with prominent diurnal fluctuations present in the last 3 days. Water transparency also showed similar long‐term as well as diurnal fluctuations. The phytoplankton community became increasingly dominated by large centric diatoms throughout the deployment. Although total cell volume was higher towards the middle of the deployment, this increase occurred without a parallel increase in chlorophyll. In addition, total particulate concentrations were highest nearshore. Although the drifter slippage was approximately 1 cm/s, the biological, chemical, and physical characteristics of the water were affected by both in situ changes and vertical motions of the water. These results are generally consistent with results from other up welling st

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09393

年代:1990

数据来源: WILEY

摘要:

Hydrographic and velocity profiles were made through a small baroclinic cyclonic eddy during the Arctic Internal Wave Experiment in the Canada Basin in April 1985. The maximum measured azimuthal velocity was 0.38 m s−1at a depth of 115 m, with velocities decaying to near zero at 30 and 270 m. Maximum isopycnal displacements at the closest approach to the eddy's axis were about 40 m. The deduced radius of maximum velocity isr0= 7±2 km, and the total radius is about 13 km. Kinetic energy dissipation rate, ε, was enhanced within 70 m of the surface throughout the transect, and for radiusr

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09411

年代:1990

数据来源: WILEY

摘要:

The primary aim of the present study is to compare two sea surface temperature (SST) climatologies produced from merchant ship observations over the tropical Atlantic Ocean. The first climatology uses measurements taken between 1911 and 1972. The second uses 1964–1984 data. The second climatology is generally warmer by a few tenths of degrees. Part of the difference is due to changes in the temperature measurement techniques with time. The difference also shows, especially along shipping routes, some spatial discrepancies which are not due to instrument changes. It is demonstrated the difference is mostly caused by inhomogeneity in the spatial and temporal density of the available observations used to construct the first climatology. Using a simple technique to eliminate spurious bias, the analysis allows isolation of the long‐term trend of SST anomalies in the tropical Atlantic Ocean during this century. South of the thermal equator, a warming trend occurs principally during boreal summers and extends to the present with an increasing range when calculated over the past 26‐year period. In the north, such a warming is more questionable between the 1940s and 1970s. It is more evident during the past 26‐year period but with a temperature range less than half of that for the southern basin. Thus large abnormal patterns have a tendency to straddle the thermal equator, including a north‐south climate dipole. Three spatially limited areas with a cooling trend are suggested. The reliability of SST anomalies along the tropical section of the Europe‐South America shipping track in recent years is als

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09421

年代:1990

数据来源: WILEY

摘要:

Microstructure observations near upwelled fronts indicate considerable variation in the structure of vertical mixing across the frontal region. Observations of cool filaments off northern California indicate that within the cool (dense) core of filaments the raised pycnocline inhibits the penetration to middepths of surface‐generated mixing. The microstructure profiles are used to estimate the available wind energy for mixing as a function of pycnocline, or mixed layer depth. A greater portion of energy input at the surface is available for entrainment of dense fluid through the pycnocline and into the surface mixed layer where the pycnocline is shallow. Hence surface‐forced mixing may cause a more rapid increase in mixed layer density within the cool filament than outside the filament, resulting in an enhanced horizontal density gradient in the mixed layer. Assuming the flow adjusts towards geostrophy, the enhanced horizontal density gradient at the front could result in an accelerated mixed layer in the direction of the preexisting geostrophic flow. Proportions relating the gain in potential energy to the wind energy vary with pycnocline depth and differ by as much as an order of magnitude from the findings of Denman and Miyake (1973) and Davis et al. (1981). Horizontal variability of pycnocline erosion may not be properly taken into account in some models and should more realistically be parameterized by including dependence on pycnocline de

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09433

年代:1990

数据来源: WILEY

摘要:

Using a two‐layer β plane model with bottom friction, we investigated the shelf/slope responses induced by localized offshore forcing sources which (1) propagate in the along‐slope direction and (2) have oscillating amplitudes. The two cases represent typical forcing patterns of cutoff eddy propagations and unstable meanders in western boundary currents. When the localized forcing source propagates in the along‐slope direction, it tends to induce components of bottom‐trapped topographic Rossby waves (TRWs) with a similar cross‐slope structure, and the shelf/slope response depends strongly upon the magnitude of the propagating speedc. In the in viscid limit, maximum response occurs whencis such that the induced bottom‐trapped TRW components may freely cross the slope region. When bottom friction is considered, however, the shelf/slope response due to the bottom‐trapped TRWs is important only in largecvalue cases. In smallcvalue cases, we found the shelf/slope response is determined by surface‐intensified wave components, whose existence is due to the planetary β effect. For observed warm eddies withc= 3 ∼ 5 cm s−1, the result of the present study suggests that both bottom‐trapped TRWs and surface‐intensified baroclinic waves are significant in determining the shelf/slope response. In cases when the localized forcing source oscillates in amplitude, we found that a localized response peak exists in the shelf/slope region. Along a fixed cross‐slope section this response peak in the frictional case tends to shift offshore as the oscillating period of the forcing source increases. This result is opposed to the inviscid result, which shows that the response peak is independent of the forcing's oscillating period. The result for the frictional case is qualitatively consistent with that observed across the continental rise south of Cape Cod, thus suggesting the importance of bottom friction in determini

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09447

年代:1990

数据来源: WILEY

摘要:

Polynyas and leads are openings in pack ice due to divergences in ice drift and to local melting. They are the vents and windows to the polar oceans. In winter they are a major source of brine during freezing and a locus for gas exchange. Large sensible heat fluxes, together with evaporation and longwave radiation from a very small percentage of open water and thin ice, dominate regional heat budgets. In summer, solar radiation is absorbed by open water but is reflected from snow‐covered pack ice. Experiments and models describing these processes are reviewe

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09461

年代:1990

数据来源: WILEY

摘要:

Large areas of elevated phytoplankton pigment concentrations were observed using CZCS satellite data in the Weddell Sea marginal ice zone and adjacent regions during the austral summer‐autumn transition. The study was made in conjunction with in situ observations of pigment levels and ice data derived from satellite passive microwave observations. The area of satellite investigation includes the eastern and western parts of the Weddell Sea and the western part of the Antarctic Peninsula. Phytoplankton blooms, about 200 km wide and extending several hundred kilometers along the ice edge, were observed. Higher pigment values in the south than in the north indicate that irradiance is not the only factor controlling spatial distributions of phytoplankton pigment concentrations at this time. A time series in Marguerite Bay also shows the persistence of a bloom for at least 12 days and significant effects on spatial distribution of the phytoplankton due to ocean and sea ice movement. An unusual pattern of sea ice formation in the Weddell Sea was also observed in a series of CZCS images near the Greenwich meridian. Turbulent oceanic and atmospheric forcings are suggested during the early stages of development of sea ice in the region and may account for the low pigment concentrations observed adjacent to the area. This study shows that ice edge phytoplankton blooms are not simply a spring‐summer feature but extend into the austral autumn and that they may contribute significantly to regional productiv

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09481

年代:1990

数据来源: WILEY

摘要:

The relationship between sea ice and weather, one of the least known components of the climatic system, could be an important factor for the climate of high latitudes. The annual cycle of the sea ice extent is characterized by a asymmetric development, with the sea ice area slowly advancing toward the equator in the winter and rapidly retreating in summer. In this study the seasonal asymmetric behavior of ice extent and the changes in sea ice concentration are shown to be linked to the atmospheric convergence line (ACL) around Antarctica. It is found that the relative positions of the ACL characterized by the half‐year cycle exert a strong influence upon the mean movement of the sea ice. It is also observed from the investigations of the areal concentration of the sea ice that a decrease in ice concentration prior to the sea ice retreat is needed for a rapid retrea

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09497

年代:1990

数据来源: WILEY

摘要:

A dynamic‐thermodynamic sea ice model is coupled to a one‐dimensional model of the oceanic mixed layer and pycnocline and is applied to the Weddell Sea. This model prognostically determines the vertical oceanic heat flux from the mixed layer dynamics in contrast to earlier sea ice modeling where the oceanic heat flux was prescribed. In addition to the standard simulation, polynya and paleoclimate experiments were performed to investigate the effects of sea ice dynamics. Furthermore, the mixed layer‐pycnocline model is compared to the original Kraus‐Turner a

ISSN:0148-0227    

DOI:10.1029/JC095iC06p09513

年代:1990

数据来源: WILEY