摘要:

Four years of the TOPEX/POSEIDON altimeter data are used to produce estimates of statistical space‐time scales of ocean variabilities for the sea surface height. A three‐dimensional (space‐time) correlation function with an anisotropic directional dependence is assumed. The function has Gaussian distributions in radial directions in the space‐time coordinates and ellipsoidal contour surfaces. The function can reveal the space‐time scales and propagations of the variabilities and statistical errors in the altimeter data. We evaluated the space‐time scales using the best fit correlation function to the altimeter data. The scales show geographical differences. This means that dominant variabilities depend on regions. We show an optimum interpolation (OI) method as an example of the application of the fitted correlation function. The OI is applied to the altimeter data in the space‐time domain to make grid point data with higher accuracy than those obtained from a usual spatial (two‐dimensional) OI (2‐D OI). The values of the correlation function are used for the correlation coefficients of the first guess error in the OI. A propagation diagram obtained from the space‐time OI (3‐D OI) shows clear propagations of planetary disturbances with larger amplitude. The grid point values from the 3‐D OI show better correspondences with sea levels from tide gauges than those from the 2‐D OI. The advantage is due to the data of the different cycles being additionally adopted and the correlation function reflecting propagation o

ISSN:0148-0227    

DOI:10.1029/1999JC900247

年代:2000

数据来源: WILEY

摘要:

Sea level variations relative to a 4‐year mean in the Indian Ocean north of 10°S are examined during 1993–1996 using both a numerical reduced gravity model with realistic coastline geometry and wind stress and sea level measurements from the TOPEX/Poseidon altimeters. The annual signal is found to be composed of propagating as well as nonpropagating features. The propagation speeds in the model and altimetry generally agree to within 25% or less. Complex empirical orthogonal function (CEOF) decomposition yields a separation between the annual and semiannual cycles (46 and 30% of the respective variance for the model, and 40 and 26% for the altimetric measurements, respectively). The propagation of these signals across the ocean basin is indicated by the spatial phase functions. Both temporal phase functions are steady for the annual cycle, though the amplitudes are modulated in time. The results for the semi‐annual cycles are similar, but the temporal phase functions are disrupted for ∼12 months starting in 1994. This may be due to an unusually strong monsoon during that time. The correlation between model sea level variation and those measured by altimetry is highly variable in both space and time. Low‐frequency filtering of the sea level anomalies, obtained by summing the two largest CEOF modes (the annual and semiannual cycles), improves the correlation. The filtered anomalies correlate in time as high as 0.9 in the western Arabian Sea and as high as 0.7 south of the equator and in the eastern Bay of Bengal. There are pockets of poor correlation (as low as −0.4) in the eastern Arabian Sea, central Bay of Bengal, and central equatorial region. These areas tend to contain recurring Rossby wave interactions as represented by the 1.5‐layer model. Each area is associated with a “phase nexus” (analogous to an amphidromic point in tidal theory) or a strong gradient of the model spatial phase functions. The spatial correlation between the filtered anomalies is typically 0.6 over much of the observation period but contains unexplained declines as low as 0.2 during a few months in bot

ISSN:0148-0227    

DOI:10.1029/1999JC900231

年代:2000

数据来源: WILEY

摘要:

A subset of the global ocean temperature climatology is used to characterize the observed seasonal variability in the near‐surface thermal structure of the tropical Indian Ocean. In the near‐surface isothermal layer the seasonal variability is least in the warm pool region and increases poleward. Over much of the tropical Indian Ocean, local surface heat fluxes overwhelm horizontal advection in the seasonal evolution of the mixed layer temperature. Entrainment cooling is weaker by an order of magnitude than the other two processes. Several dynamical processes produce the most prominent signals in the thermocline in different geographic regions, such as coastal upwelling/downwelling with associated reversals in the flow off Arabia, off southwest India, and off the east coast of India, Ekman‐driven thermocline deepening in the central Arabian Sea, convergence of waters caused by Wyrtki Equatorial Jets in the eastern equatorial Indian Ocean, Ekman divergence of a dome of cold waters just south of the equator in the west, propagating Kelvin waves in the coastal regions of the Bay of Bengal, and propagating Rossby waves from off southwest India to Somalia and in the Indo‐Pacific throughflow region. For the spatiotemporal resolutions considered in this study the penetration of the seasonal cycle of temperature appears to be limited to 150 m depth or so. Numerical model solutions have revealed the causative mechanisms for some of the prominent signals seen in the observations, in particular, those driven by long‐period propagating waves in regions of large stratification, whose structure was not revealed by earlier climatologies to date, as revealed by this new cl

ISSN:0148-0227    

DOI:10.1029/1999JC900220

年代:2000

数据来源: WILEY

摘要:

Western boundary disturbances detected in the first multicentury integration of the National Center for Atmospheric Research Climate System Model are analyzed, and the role they play in the model interdecadal variability is investigated. The boundary signals propagate southwestward along the coast of North America. At depth they reach the equator and propagate along the equator but with decreasing amplitude away from the western boundary. Their southwestward propagation is associated with the evolution of the western boundary current system (the Gulf Stream system in the upper part of the water column and the Deep Western Boundary Current at depth) from weaker than average to stronger than average conditions. Thus the boundary disturbances seem to play an important role in the model thermohaline circulation variability at interdecadal timescales. They show similarities with the boundary “waves” found in ocean‐only models exhibiting interdecadal variability, and their role in the model's response to changes in the rate of sinking appears to be in agreement with the theory of the spin‐up of the deep ocean circulation. The main result of this study is that the similarities with ocean‐only results and theories support the idea that interdecadal timescales of variability originate from the ocean

ISSN:0148-0227    

DOI:10.1029/1999JC900229

年代:2000

数据来源: WILEY

摘要:

The recent El Niño–La Niña cycle exhibited striking patterns of current and salinity variability in the upper equatorial Pacific Ocean. This evolution is described from mid‐1996 through 1998 using a remarkable data set of 35 meridional conductivity‐temperature‐depth (CTD)/acoustic Doppler current profiler (ADCP) sections along with buoy data. The sections, nominally from 8°S to 8°N between 165°E and 95°W, were occupied over the course of 27 months. A wide range of current variability was sampled with currents that appeared or disappeared some time during the cycle, including an equatorially trapped eastward surface current, the Equatorial Undercurrent, the northern branch of the South Equatorial Current, and the North Equatorial Countercurrent. Basin‐wide, interannual changes in upper ocean and pycnocline zonal transports were as large as 64±32×106m3s−1. Changes in the salinity structure included a deep and fresh mixed layer in the central equatorial Pacific that built up during the El Niño and was then disrupted by upwelled salty water with the onset of La Niña, a very fresh mixed layer observed in the eastern equatorial Pacific late in the El Niño, and a reduction in the strength of the meridional equatorial salinity gradient within the pycnocline to one third of the usual value during the El Niño. Finally, the zonal transports above the thermocline from 5°S to 5°N were well correlated with the rate of change of warm‐water volume west of the i

ISSN:0148-0227    

DOI:10.1029/1999JC900280

年代:2000

数据来源: WILEY

摘要:

In order to avoid the “false North Pacific Intermediate Water (NPIW)” problem a general circulation model is driven by the winter‐like North Pacific wind that can separate the modeled Kuroshio from the coast around 36°N. In the model a salinity minimum appears around 26.8σθin the North Pacific subtropical region, which does not outcrop in the North Pacific. Formation processes of the salinity minimum are examined. Comparing them with observations and discussing them sufficiently, this study clarified that NPIW salinity minimum is formed just above the less saline water influenced largely by the Oyashio water. The formation of NPIW is basically controlled by the rapid increase of the Oyashio layer thickness around 26.5–26.8σθ, which layer structure is determined by Okhotsk Sea Mode Water pycnostad at 26.8–26.9σθand the winter sea surface density of ∼26.5σθ. For NPIW salinity minimum formation it is most important that the gradient of the Oyashio mixing ratio exceeds the “critical gradient” around 26.4–26.7σθ. NPIW salinity minimum density is almost determined by the heaviest density of this range (26.7σθ), which is ∼0.1σθless dense than the bottom of the rapid increase of the Oyashio layer thickness. Then cabbeling leads to an increase in its density to 26.8σθ. The Sea of Okhotsk is very important for the formation of NPIW through the formation of the Oyashio thick layers, and it practically dete

ISSN:0148-0227    

DOI:10.1029/1999JC900261

年代:2000

数据来源: WILEY

摘要:

Ocean heat transport in the South Atlantic of a coupled climate model has been diagnosed and compared with observational estimates. The coupled model overestimates the northward heat transport in the South Atlantic. This corresponds to an underestimate of the northward flux of bottom water across World Ocean Circulation Experiment (WOCE) section A11 (the model transports less than 1 Sv compared with climatological estimates of 6 Sv). The magnitude of the southward outflow of North Atlantic deep water agrees well with observational estimates while the northward flux of surface and intermediate waters is larger than observational estimates. We show that with the correct water mass properties a realistic northward flux of bottom water across 30°S is only possible, in the climate model, if the depths of the channels linking the Argentine and Brazil basins are properly represented. We find that an increase in the northward flow of bottom water corresponds to an increase in the southward flow of deep water, rather than a reduction in the northward flux of surface and intermediate waters. This indicates that the upper and lower limb of the overturning are decoupled in the model. As a consequence, increasing the northward transport of bottom water leads to a small reduction of the northward heat transport across 30°S. The magnitude of the heat transport from the Indian to the Atlantic ocean in the model (the warm water path) is surprisingly large for a noneddy resolving model. We find that the magnitude of the Indonesian Throughflow is important in climate resolution models for determining the strength of the warm water path. However, the relative strengths of the warm and cold water paths do not significantly change the heat transported across 30

ISSN:0148-0227    

DOI:10.1029/1999JC900259

年代:2000

数据来源: WILEY

摘要:

In March–April 1995, as part of the World Ocean Circulation Experiment section A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shelf (72.5°S, 16.5°W) to South Georgia (55°S, 34.5°W). Chlorofluorocarbon (CFC‐11, CFC‐12, and CFC‐113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Water (defined as waters with potential temperatures less than 0°C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potential temperatures less than −0.7°C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing northern Weddell Gyre than in the westward flowing southern limb. Our CFC measurements suggest that distinct sources of Weddell Sea Bottom Water exist in the western Weddell Sea, in agreement with previous descriptions based on potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0°C and −0.7°C, confirm the long‐recognized sources for this water mass in the western and southwestern Weddell Sea. In the southern Weddell Gyre at about 20°W and along the Antarctic continental slope, Weddell Sea Deep Water with potential temperatures around −0.45°C shows a deep CFC maximum about 1000 m above the seafloor. CFC concentrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30°W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea. Instead, these tracers suggest that an important portion of the Weddell Sea Deep Water in the southern Weddell Gyre originates outside the western Weddell Sea, probably near the Amery Basin and environs, around 75°E. These features of the circulation and renewal of the deep Weddell Gyre should be carefully considered in simulations dealing with fluxes, pathways, and formation rate

ISSN:0148-0227    

DOI:10.1029/1999JC900263

年代:2000

数据来源: WILEY

摘要:

During the summer of 1996 the nuclear submarine USSPogyoccupied a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin). CTD/Niskin bottle casts extending to 1600 m were carried out at eight stations, providing the first high‐quality temperature, salinity, CFC, tritium, and3He data obtained from this region, although XCTD data had previously been collected in this region. These data, along with data from stations at the basin boundary to the south and west, reveal the presence of well‐ventilated intermediate water beneath the halocline in the center of the Canada Basin, indicating renewal times of the order of 1–2 decades. The least ventilated intermediate water was observed at the northern end of the Canada Basin along the southern flank of the Alpha Ridge. Intermediate water is derived from the Atlantic Ocean and enters the Arctic Ocean through Fram Strait and the Barents Sea. It flows around the Arctic basins in boundary currents and splits in the eastern Amundsen Basin with one branch crossing the Lomonosov Ridge and flowing along the East Siberian continental slope and the other flowing along the Eurasian flank of the Lomonosov Ridge. From the 1996 Scientific Ice Expedition (SCICEX 96) observations we conclude that the branch that flows along the East Siberian continental slope transports this water to the Chukchi Rise, where it apparently enters the central Canada Basin with some flow continuing along the boundary to the southern Canada Basin. The Fram Strait Branch Water mixes extensively with waters from the Canadian Basin during its transit along the East Siberian continental slope, being diluted by a factor of about 5 by the time it reaches the central Canada Basin. The Barents Sea Branch Water does not undergo such extensive mixing and is diluted by a factor of only about 2 when it reaches the central Can

ISSN:0148-0227    

DOI:10.1029/1999JC900233

年代:2000

数据来源: WILEY

摘要:

Thin sea ice plays a central role in the surface heat and mass balance of the Arctic Ocean. In order to develop understanding of these balances we describe and analyze highly resolved temperature data taken through the air/sea/ice interface during the transition from an ice‐free to an ice‐covered Arctic Ocean surface. The data were taken to observe the thermodynamic evolution of a lead, a process that has previously only been accessible to measurement techniques confined to the lead edge. Our detailed analysis of the field data is guided by recent theoretical and experimental advances in understanding the phase dynamics of directionally solidified alloys. Because of the dearth of direct observations we also present time series of the relevant heat fluxes inferred from our data and demonstrate the controlling influence that the internal phase evolution has on these quantities. We have previously examined the stability of the brine trapped in a growing sea ice matrix both theoretically and experimentally and now find that haline convection, driven from within the growing layer, is consistent with this previous work and with the nature of direct turbulence measurements. The importance of this process is that although ice growth is continuous, the local brine flux commences abruptly, only after some time, in contrast to what had previously been supposed. Hence the ice growth process itself is a source of intermittency in oceanic boundary layer turbulence. Furthermore, we find that in this particular situation the sea ice growth is not simply a square root function of time, in contrast to the model typically used in numerical simulations. By far the most practical methods of studying lead convection are numerical simulations and laboratory models, and a strong conclusion of this study is the importance of the proper treatment of the boundary conditions describing the buoyancy f

ISSN:0148-0227    

DOI:10.1029/1999JC900269

年代:2000

数据来源: WILEY