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Frontiers of Earth Science

Front Earth Sci    2013, Vol. 7 Issue (4) : 447-455     DOI: 10.1007/s11707-013-0380-0
Geostrophic current estimation using altimeter data at ground track crossovers in the northwest Pacific Ocean
Yang YU1,2(), Longfei WANG1, Ziwei LI1,2, Xuan ZHOU3
1. Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China; 2. State Key Laboratory of Remote Sensing Science, Beijing 100101, China; 3. P. O. Box 5111, Beijing 100094, China
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Geostrophic current comprises a large portion of the ocean current, which plays an important role in global climate change. Based on classic oceanography, geostrophic current can be derived from pressure gradient. Assuming water density to be constant, we can estimate geostrophic current from Absolute Dynamic Topography (ADT). In this paper, we use ADT data obtained from multi-satellite altimeters to extract sea surface tilts along-track at crossover points. The calculated tilts along these two tracks can be converted into orthogonal directions and are used to estimate geostrophic current. In northwest Pacific, computed geostrophic current velocities are evaluated with Argos data. In total, 771 pairs of temporally and spatially consistent Argos measurements along with estimated geostrophic velocity datasets are used for validation. In this study, the effect of different cut-off wavelengths of the low pass filter applied to ADT is discussed. Our results show that a cut-off wavelength of 75 km is the most suitable choice for the study area. The estimated geostrophic velocity and the Argos measurement are in good agreement with each other, with a correlation coefficient of 0.867 for zonal component, and 0.734 for meridional one. Furthermore, an empirical relationship between the estimated geostrophic velocity and Argos measurement is derived, providing us a favorable and convenient approach to estimate sea surface flow velocity from the geostrophic velocity derived from altimeter data. The experimental application of the derived method on Kuroshio reveals reasonable results compared with previous studies.

Keywords geostrophic velocity      altimeter      northwest Pacific Ocean      crossover method     
Corresponding Authors: YU Yang,   
Issue Date: 05 December 2013
 Cite this article:   
Yang YU,Longfei WANG,Ziwei LI, et al. Geostrophic current estimation using altimeter data at ground track crossovers in the northwest Pacific Ocean[J]. Front Earth Sci, 2013, 7(4): 447-455.
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Yang YU
Longfei WANG
Ziwei LI
Ground track categoryRepeat period /dayTime span of data used(month/day/year)
TPJN1009/16/2002-10/08/2005 & 02/14/2009-12/31/2011
Tab.1  Time span of various ground track categories.
Fig.1  Argos drifter trace in the study area from September to November (1992-2011). The flow represented by the dense drifter trace at the west boundary of Pacific is the Kuroshio current.
Fig.2  Illustration of altimeter satellite ground tracks at crossover. and are sea surface tilt along Track 1 and Track 2 respectively, positive in northward direction. and are orthogonal component of sea surface tilt along meridional and zonal components respectively. and are angles between ground tracks and north meridian.
Fig.3  Variation of coefficient for possible track pair configuration with respect to latitude. An upper limit of of 10 is required to properly estimate the geostrophic velocity. The limitation corresponds to the latitudinal region between 15o and 60o.
Cut-off wavelength/kmR for zonal componentR for meridional component
No filter applied0.650.24
Tab.2  Correlation coefficient () between estimated geostrophic velocity and Argos measurement.
Fig.4  Scatter diagram of estimated geostrophic velocity and Argos measurement (low pass filter cut-off wavelength is 75 km). The -axis for each scatter plot is velocity from the Argos, and -axis is velocity estimated from altimeter data. The solid lines in both plots are the linearly fitted line of the comparison. Correlation coefficient is higher for the zonal component than for the meridional component.
Fig.5  Temporally averaged flow field of Kuroshio (1992-2011) derived from altimeter crossover method. The grey part on the map are regions where water depth is less than 100 m or offshore distance is less than 20 km. Surface flow velocities are not estimated in this part. Triangles denote six crossovers located on the main stream of Kuroshio.
Fig.6  Velocity variation of six crossovers at some typical locations (labeled with triangles in Fig. 5) on the main stream of Kuroshio.
CrossoverIntersected tracksLongitude/oELatitude/oNAveraged flow speed /(cm·s-1)Averaged flow direction (clockwise from north)
Tab.3  Surface velocity at six typical crossovers.
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