ON THE ANALYSIS OF 500-MILLIBAR ZONAL WIND PROFILES AND INDEX CYCLES
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Mean geostrophic west-wind profiles are presented from seven years of 500-mb data. A method is out-lined for analyzing these profiles objectively. The appearance and significance of index cycles shown on this objective analysis are discussed. Comparisons are made between short-period continuity and inter-annual variability.Abstract The length and velocity scales appropriate to geostrophic adjustment are discussed for both neutral and lapse conditions. Similarity arguments based on these scales lead to (i) an improved derivation of the relation between geostrophic and surface winds for the neutral case and (ii) the result that, in lapse conditions, the Coriolis force does not affect the relation between geostrophic and surface winds to the first order. Nevertheless a region of weak geostrophic adjustment may be defined. The case of inversion conditions is also discussed.
Geostrophic current
Similarity (geometry)
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Experiments with insertion of height data in a free surface spectral model were conducted to explain the effectiveness of local geostrophic correction of the wind field in accelerating the assimilation of the height data by the model. It is shown that the local geostrophic correction projects most of the inserted data on to the slow manifold of the model.
Geostrophic current
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On the basis of CTD data obtained on 4 repeat-observed stations located at 121°E,19.5°N ~21°N in the Luzon Strait of the cruise in the summer of 2008,the role of disturbances caused by internal tides in temperature and salinity profiles in calculating of the geostrophic currents is discussed.The result shows that the role of disturbances caused by internal tides in temperature and salinity profiles in calculating of the geostrophic currents is significant.When we use hydrographic data to calculate the geostrophic currents we have to take measures to remove the disturbances caused by internal tides in temperature and salinity profiles,unless there would be great errors.By using the time-averaged CTD data,the geostrophic currents in the Luzon Strait is calculated.The result reveals that the significant part of the geostrophic currents appear above 350 m.Kuroshio intrusion mainly happens between 19.5°N and 21°N,especially at the upper layer.Between 19.5°N and 21°N,the water volume transport shows a in at upper and flowing out at deeper layer vertical structure,above 350 m the transport is westward(2.6 Sv) and below 350 m it is eastward(3.1 Sv).The distribution of the geostrophic currents we get is confirmed by the salinity distribution along 121°E Section.
Geostrophic current
Temperature salinity diagrams
Dynamic height
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Abstract : Dynamical features of polar oceans, captured by traditional treatment of hydrographic data sets, are only two fields: dynamical height of sea surface relative to certain depth, and geostrophic currents by assuming a certain level of no motion. Much information about the polar water is lost by such a treatment. In fact, the ocean flow is not purely geostrophically balanced. It contains two parts: geostrophic currents and ageostrophic circulations. The geostrophic currents are obtained from the hydrographic data sets (traditional physical oceanographic treatment), and the ageostrophic circulation is forced the geostrophic flow (called the geostrophic forcing) and surface wind field. Therefore, the three-dimensional circulation (both geostrophic and ageostrophic) can also be calculated by the hydrographic and surface wind data sets. There are potentially significant errors with the traditional treatment of the hydrographic data (e.g., only computing geostrophic currents), particularly in regions having strong temperature and salinity gradient, such as in the west Spitsbergen current. Neither geostrophic current nor dynamical height can provide detail information about the three-dimensional flow field near the west Spitsbergen current. In order to diagnose the three dimensional flow field, a new theory should be adapted.
Geostrophic current
Hydrographic survey
Forcing (mathematics)
Sea-surface height
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Geostrophic velocity and transport of water in the Drake Passage relative to a newly defined zero reference layer indicate that the circumpolar current is basically north of 59 degrees S, with its axis north 57 degrees S, and that the total volume transport exceeds 200 x 10(6) cubic meters per second. The calculated geostrophic velocities are consistent with results of descriptive water-structure studies.
Geostrophic current
Circumpolar star
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Thermal wind relation is applied to compute the upper 1000-m layer meridional geostrophic velocity across the 18°N section in the South China Sea(SCS),based on the hydrographic data collected during the open cruises of northern SCS from 2005 to 2008 and merged altimetry data.The geostrophic velocity is compared with the Acoustic Doppler Current Profilers(ADCP) observations,and the upper 1000-m layer geostrophic volume,heat and salt transports across the 18°N section in the SCS are estimated.The results indicate that the meridional geostrophic velocities display belt distribution along the section during the cruises.The estimated geostrophic velocities are in good agreement with the ADCP data except at few stations.Sea surface height distribution derived from altimeter data shows that the distribution of meridional geostrophic velocities is closely related with mesoscale eddies.The total geostrophic volume,heat and salt transports of 2005 2007 in the upper 1000 m are all southward across 18°N,and the three-year-averaged values are 11.8 Sv,0.38 PW,and 418.8 Gg.s 1,respectively.However,the transport varies greatly from year to year.The volume,heat and salt transports of 2005 were the largest,while those of 2007 were the smallest.The geostrophic volume,heat and salt transports of 2008 from 110°E to 117°E are 7.3 Sv,0.22 PW,and 259.4 Gg.s 1,respectively.
Geostrophic current
Thermal wind
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This paper examines apparent ageostrophic deviations at 300 and 200 mb over the United States, their relationship to upper-air synoptic patterns, and the extent to which ageostrophic deviations affect wind forecasts made from geostrophic maps. For spot winds, ageostrophic RMS vector deviations are 19 kn at 300 mb and 20 kn at 200 mb. For the same set of data, geostrophic wind forecasts (36-hr) result in a 34-kn RMS vector error if verified by the geostrophic winds, and 38-kn RMS error if verified by the rawins. Prognostic errors of route winds decrease as the route length increases: for a route length 1900 n mi, the 36-hr geostrophic wind forecast gives RMS vector error 17 kn if verified by geostrophic wind and 19 kn if verified by the rawins. Instrumental uncertainties inherent in the present upper-wind reports prevent the use of the rawins as an absolute standard for verification. In all, it appears that a medium-period forecast of spot and route winds can be made with tolerable approximation from the geostrophic prognostic charts.
Thermal wind
Geostrophic current
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The ratio between observed surface and geostrophic wind speed has been investigated from observations at the German Bight, taking geostrophic wind and the air-sea temperature difference as parameters. The ratio decreases with increasing geostrophic wind and increasing stability. While stability is an important parameter for light to moderate winds, variation of the ratio with geostrophic wind speed cannot be neglected, taking the full range of geostrophic wind speeds into consideration. From the Navier-Stokes equations, such a variation is to be expected. For light winds, the (local) surface wind may exceed the (mesoscale) geostrophic wind. Both effects together can be described approximately by a linear relation between the surface wind and geostrophic wind, with a slope of 0.56 and a constant term b>0 varying with stability. The residual error was 2 m/s. Variation with latitude is inferred from the Navier-Stokes equations.
Thermal wind
Geostrophic current
Wind Stress
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