Analysis of Surface and Subsurface Geostrophic Currents Derived from Satellite Altimetry and In-situ Hydrographical Data

摘要

Studies have shown that ocean circulations are highly important for the climate stability and human life Their variations are also highly connected to potential natural hazards; therefore continuous monitoring of ocean circulations has been a highly respected issue over the past centuries The research uses multiple satellite altimetry data satellite-only geoid model (GOCE or GRACE) in-situ hydrographical data to determine mesoscale geostrophic current velocities globally To reduce the errors remain in ADT the research adopts conventional pointwise approach spectral approach and profile approach to process Sea Surface Height (SSH) and geoid models In-situ current meter observations at 23 stations fixed at 10m depth from TAO/TRITON and PIRATA were taken as ground truth Results show that when adopting the spectral approach around 70%~90% of stations gives Root Mean Square (RMS) smaller than or at same accuracy level (within 1 cm/s) compared with the pointwise approach and the averaged RMS is about 10~15 cm/s while there are over 90% of stations giving RMS smaller than or at same accuracy level with pointwise approach and averaged RMS is around 8~10 cm/s when adopting profile approach which better improves the conventional pointwise approach GOCE geoid model was also proved to perform better than GRACE geoid in determining geostrophic currents from time-variant perspective with 60%~80% of stations giving smaller RMS On the other hand the average correlation coefficients are all around 0 6~0 7 and 0 3~0 4 in zonal and meridional direction respectively with no significant discrepancy when adopting different approach or geoid model (average difference within 0 1) The correlation coefficients between geostrophic current velocities volume transports through the Gulf Stream (GS) Labrador Current (LC) and wintertime North Atlantic Oscillation (NAO) were estimated while the correlations of Kuroshio Current (KC) and El Ni?o/Southern Oscillation (ENSO) were evaluated by using Multivariate ENSO Index (MEI) Results show the correlation coefficient of 0 7 with 1-year lag between GS and wintertime NAO in zonal direction which may relate to the north-southward shift of GS pathway while LC velocities show the correlation coefficient of 0 5 in meridional direction with zero-lag The meridional volume transport through the transect also shows the same maximum correlation coefficient and lag time Such fast response may due to the barotropic nature of LC variability In the North Pacific Ocean transects through the Bashi Channel the northeast of Taiwan and Kuroshio Extension were chosen Comparatively higher correlations with MEI are in the meridional currents through the Kuroshio Extension and near-surface zonal currents through the Bashi Channel with maximum negative correlation coefficient of -0 4 and -0 3~ -0 4 for volume transports; Zonal currents through Kuroshio Extension shows maximum positive correlation coefficient of 0 4 and 0 3 for volume transports Results indicate that correlations in the transect through Bashi Channel and Kuroshio Extension are all higher than those in the transect of the northeast of Taiwan where only gives correlation coefficient of 0 1~0 2

獎項日期	2015 2月 6
原文	English
監督員	Chung-Yen Kuo (Supervisor)