摘要:

Monthly adjusted sea level (ASL) was analyzed to examine the characteristics of sea level variation at Ulleung Island in the East (Japan) Sea from 1979 to 1992. It is remarkable that the nonseasonal variation is comparable with the mean seasonal variation in amplitude at Ulleung Island, whereas the mean seasonal variation is dominant along the eastern coast of the Korean peninsula. At Ulleung Island the variation in the mean seasonal, interannual, and short‐term component variability is 54, 13, and 30% of the total ASL variance, respectively. ASL variability is highly correlated with the upper layer heat content around Ulleung Island. There are two factors that contribute to large nonseasonal variation of the ASL around the island. One is the location of the polar front that divides the northern cold water region and the southern warm water region. The other is the presence of a warm eddy that frequently forms near Ulleung Island. When the northern cold water region extends southward to Ulleung Island, the heat content of the upper layer decreases, and ASL around the island falls. In contrast, when the southern warm water region extends northward to the island, the heat content increases, and ASL rises greatly. The movement of the front with a period of 1–2 years causes interannual variation of ASL. When a warm eddy approaches Ulleung Island, the heat content increases, and ASL rises. The short‐term variation in ASL is caused primarily by the migration of the warm eddy around Ulleung I

ISSN:0148-0227    

DOI:10.1029/2001JC000895

年代:2002

数据来源: WILEY

摘要:

Using average sections along the Bosphorus taken in September 1994 with a loosely tethered profiler and an acoustic Doppler current profiler,Gregg et al.[1999] found the exchange flow between the Sea of Marmara and the Black Sea to be quasi‐steady but far from satisfying the hydraulic control conditions for two‐layered flows. Here we examine synoptic sections and use images from an acoustic backscatter system to provide the first detailed look at the flow and water mass changes in the Bosphorus and to assess how well the flow satisfies the hydraulic assumptions. Thirty kilometers long, 28–100 m deep, and 0.75–3 km wide, the Bosphorus has bathymetry for more complex than that used in analytic or numerical models of exchange flows. The particulars affect dynamics in important ways. For instance, owing to changes in channel shape, the narrowest section, known as the contraction, is not also the minimum in cross‐sectional area. Rather, it is a transition between the wider northern half and the narrower southern half of the strait, and some places south of the contraction have slightly smaller areas. Sharp bends occur throughout the strait and often produce flow separations as well as directing upper and lower flows to opposite sides of the channel. Never<28% of the water column, the interface thickens to 75% in the southern half of the strait as a result of intense mixing downstream of the contraction. As a consequence of the strong mixing and numerous flow separations, we conclude that the exchange flow may be at least partly controlled by friction instead of being a simple hydra

ISSN:0148-0227    

DOI:10.1029/2000JC000485

年代:2002

数据来源: WILEY

摘要:

The evolution of oceanographic conditions in the upwelling region off northern Chile (18°–24°S) between 1996 and 1998 (including the 1997–1998 El Niño) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5°S), Iquique (20.5°S), and Antofagasta (23.5°S) and a time series of vertical temperature profiles off Iquique. Spatial patterns of sea surface temperature and salinity from May 1996 to March 1997 followed a normal seasonal progression, though conditions were anomalously cool and fresh. Starting in March 1997, positive anomalies in sea level and sea surface temperature propagated along the South American coast to 37°S. Maximum sea level anomalies occurred in two peaks in May–July 1997 and October 1997 to February 1998, separated by a relaxation period. Maximum anomalies (2°C and 0.1 practical salinity units (psu)) extended to 400 m in December 1997 within 50 km of the coast. March 1998 presented the largest surface anomalies (>4°C and 0.6 psu). Strong poleward flow (20–35 cm s−1) occurred to 400 m or deeper during both sea level maxima and weaker (10 cm s−1) equatorward flow followed each peak. By May 1998, SST had returned to the climatological mean, and flow was equatorward next to the coast. However, offshore salinity remained anomalously high owing to a tongue of subtropical water extending southeast along the Peruvian coast. Conditions off northern Chile returned to normal between August and December 1998. The timing of the anomalies suggests a connection to equatorial waves. The progression of the 1997–1998 El Niño was very similar to that of 1982–1983, though with different timin

ISSN:0148-0227    

DOI:10.1029/2001JC001002

年代:2002

数据来源: WILEY

摘要:

An assessment of four open ocean boundary stream function conditions based on different data sources for a regional model of the Southeast Pacific Ocean (SEP; 103°W to the Chilean coast and 16.5°S to 54°S) is presented. The model run, with the boundary conditions which most successfully describes the SEP, is forced with a stream function extracted from a global ocean circulation model and is suitable for forming the basis of climate process studies in the SEP. Investigations of other model boundary conditions based on observational data, two using a Sverdrup balance argument to determine the stream function from two commonly used wind stress climatologies and a third using a geostrophic balance argument to determine the stream function from WOCE hydrographic data, were less successful. The main difficulty experienced in all of the model runs was the lack of formation of a sufficiently fresh salinity minimum at intermediate depths. This is linked to the inadequate production of Antarctic Intermediate Water (AAIW) within the model domain or insufficient AAIW propagating across the open boundaries. Comparisons between the model runs highlight the level of uncertainty in aspects of the oceanography of the SEP that govern water mass formation in the regi

ISSN:0148-0227    

DOI:10.1029/2000JC000539

年代:2002

数据来源: WILEY

摘要:

A nonhydrostatic three‐dimensional numerical model is used to investigate convection along slanted isopycnal surfaces and subsurface anticyclone generation in a baroclinic jet system appropriate for the Beaufort Sea environment. The current is initially bounded to the left by a coastal wall and subsequently subject to surface friction exerted by sea ice and cooling at the sea surface. Despite spatially uniform cooling rates used in the model, convection favors limited areas with high cyclonic vorticity. In a meandering current system, rigorous convection occurs primarily in meander troughs where the current turns cyclonically. The cyclonic vorticity enhances convection, which in turn enhances cyclonic vorticity. The positive feedback mechanism, previously known in atmospheric literature as convective instability of the second kind, often strengthens convection and the consequent subsurface anticyclones by as much as 1 order of magnitude. Subsequent seaward detrainment in subsurface depths of interest moves anticyclones from the cyclonic side of the jet to the anticyclonic side. The subsurface anticyclones may be strengthened for the second time because of favorable changes of ambient vorticity. Thus downward convection along slanted isopycnal surfaces of a left‐bounded baroclinic jet (similar to segments of the Beaufort Sea anticyclonic gyre) is an efficient way to produce subsurface anticylcones and propel them seaward afterw

ISSN:0148-0227    

DOI:10.1029/2001JC000786

年代:2002

数据来源: WILEY

摘要:

The baroclinic response of tide and tidal currents in the Yellow and East China Seas is investigated using a two‐layer numerical model. Seasonal variability in the M2tide, especially the smaller summer amplitudes prevailing along the Korea/Tsushima Strait [Kang et al., 1995], is investigated by a series of numerical experiments with varying degrees of stratification specific to winter and summer. Model results show that the summer amplitudes of the M2tide around the southwestern tip of the Korean peninsula and Korea/Tsusima Strait decrease, with a peak decrease of about 14 cm off the southwestern tip of the Korean peninsula, while the summer amplitudes in other coastal regions tend to increase. This seasonal variability generally coincides with the observations. These models results indicate that seasonal stratification has several noticeable effects on the tides, including varying degrees of current shear, varying frictional dissipation, and varying barotropic energy flux. In particular, it drives complicated seasonal variability in the M2tide, with a peak amplitude modulation of nearly 5% off the southwestern tip of the Korean peninsula. The seasonal variation of barotropic M2energy flux through the eastern entrance of the Yellow Sea is thought to induce the corresponding variability in the M2amplitude in the Korea/Tsusima Strait, with smaller amplitudes found in the summe

ISSN:0148-0227    

DOI:10.1029/2001JC000838

年代:2002

数据来源: WILEY

摘要:

A series of experiments with an ocean general circulation model of the Mediterranean forced by artificial (but realistic) surface fluxes of heat and fresh water is performed over 100 year periods. The model has a stable thermohaline circulation under the baseline fluxes. Small increases/decreases in excess evaporation (±8%) produce a linear strengthening/weakening of this thermohaline circulation with more/less water formation and strait transports. The water properties do not change much, and the responce is consistent with submaximal exchange at Gibraltar, where changes in volume and freshwater transport can be achieved mainly by changing the interface level between inflowing and outflowing waters. Larger changes in excess evaporation lead to nonlinear responses for increasing and decreasing values that are quite different. Reducing evaporation (−20%) causes both western and eastern deep water formation to cease, leaving a shallower intermediate circulation only. Increasing evaporation (+25%) initially strengthens the overturning but also produces a hydraulic jump‐like feature at Gibraltar, which mixes inflowing and outflowing waters, decreasing the salt flux out of the basin. As a result, the basin salinity increases greatly over 140 years, until the overturning collapses and the Gibraltar mixing ceases, leaving a shallow intermediate circulation above highly saline and stagnant deep waters. The possible relevance of these experiments to recent changes in Mediterranean fluxes and water properties is discu

ISSN:0148-0227    

DOI:10.1029/2000JC000550

年代:2002

数据来源: WILEY

摘要:

Gas mining leads to saucer‐like surface depressions. In the North Sea, gas is currently mined at several offshore locations. The associated bed depression has a similar spatial extent as offshore tidal sandbanks, which are large‐scale bed patterns covering a significant part of the North Sea bottom. The morphological time scales of bed depressions and tidal sandbanks are similar, so that significant interaction between these features is expected. In this paper we allow the bed depression to become morphologically active. A simple depression model based on a homogeneous soil is tuned with data of a bed depression near the Dutch barrier island of Ameland. Next, this subsidence model is included in a morphodynamic model. We show that this model is able to explain tidal sandbanks, which represent natural bed behavior. Here we approximate the solution by an expansion up to first order. The zeroth‐order solution of the model is a flat bed with a spatially uniform, time‐independent current. The first‐order solution is investigated using a Fourier transformation. In general, we observe significant interaction between the bed depression and the natural sandbank formation process. The process of induced bed depression triggers and intensifies the natural morphological behavior of the offshore seabed. The model also shows essential differences between modeling a morphodynamically active marine bottom depression and a bottom depression below the threshold for sediment motion. The maximum bed level depression in the active case is significantly larger, and the circular shape of depression contours is affected by stretching toward the preferred orientation of the tidal sandbank formatio

ISSN:0148-0227    

DOI:10.1029/2001JC000851

年代:2002

数据来源: WILEY

摘要:

In the tropical Atlantic the vertical distance between Antarctic Intermediate Water (AAIW) and upper North Atlantic Deep Water (uNADW) cores (defined by their salinity extrema) exceeds 1000 m. In this thick layer, two additional water mass signals, characterized mainly by temperature extrema, are identified in the western boundary region. They are the intrusive flows of upper Circumpolar Deep Water (uCDW) and shallow uNADW (suNADW). The latter is considered a water type of uNADW. Since uCDW is a northward flow as a lower phase of AAIW and suNADW is a southward flow as an upper phase of NADW, a reference of level of no motion (LNM) is defined between uCDW and suNADW at the neutral density surface σN= 27.70 (at about 1150 m). This surface is coincident with a minimum of dianeutral velocity and measured velocity by lowered acoustic Doppler current profiler. Dianeutral motion processes are examined across the five selected neutral surfaces σN= 27.45, 27.60, 27.70, 27.80, and 27.90 that span the depth range between AAIW and uNADW and follow the major water mass signals. Dianeutral transport shows a decreasing tendency toward the middle σN= 27.70 neutral surface and reaches a minimum of 0.6 Sv (1 Sv = 106m3s−1), integrated in the tropical region between 10°N and 10°S. Thus the weakening of dianeutral motion and required minimum of epineutral advection at LNM seem to be re

ISSN:0148-0227    

DOI:10.1029/2000JC000520

年代:2002

数据来源: WILEY

摘要:

In the framework of the Bremerhaven Regional Ice Ocean Simulations a coupled sea ice‐ocean model, based on a dynamic‐thermodynamic sea ice model and the S‐coordinate Primitive Equation Model, has been developed. The model is run on a circumpolar grid that is focused on the Weddell Sea. It includes the major Antarctic ice shelves; ice shelf‐ocean interaction is described by the sea ice model's thermodynamic component. Numerical simulations are forced with 6 hourly data of the European Centre for Medium‐Range Weather Forecasts reanalysis of the period 1985–1993 and validated against observations of hydrography as well as sea ice extent, thickness, and drift. Qualitatively and quantitatively, a good agreement with observations is achieved. The basin‐wide circulation features a double‐cell structure of the Weddell Gyre with maximum transports close to recent observations. Summer sea ice coverage is underestimated, but winter ice extent, as well as sea ice thickness and drift, is

ISSN:0148-0227    

DOI:10.1029/2000JC000741

年代:2002

数据来源: WILEY