Study On The Low-frequency Variability Of The Kuroshio And Kuroshio Extension Based On The Satellite Altimeter

The sea level anomalies(SLA) and mesoscale eddies in the Kuroshio south ofJapan and the Kuroshio Extension region east of Japan are studied by EOF analysisusing altimetry data from CNES merged by Topex/Poseidon、Jason-1/2andERS/Envisat. Next，the mechanism of the Low-Frequency changes in the KuroshioExtension is studied.And then the formation and decay process of the2004/05Kuroshiolarge meander south of Japan is examined. The main conclusions are summarized asfollows:Firstly, the temporal and spatial variations of the sea level anomalies andmesoscale eddies in the region of the Kuroshio and its extension is examined by EOFanalysis and wavelet spectrums based on the satellite altimeter data from the period of1993-2010. The results of the EOF analysis as follows: The EOF1is a seasonal mode,which indicates that the sea level of the total region tends to increase. Compared withthe EOF1of the wind stress curl, it is found that their spatial structure and temporalvariability are close, and the correlation coefficient is0.65. It can be then concluded thatthe seasonal variations of the wind stress over the North Pacific play a key role in theseasonal variations of the sea level in the Kuroshio. The EOF2of the SLA shows thevariations of the southern recirculation gyre, whose significant cycle is8-10years.It issuggested that the decadal variations of the southern recirculation gyre is associatedwith the PDO. The EOF3of the SLA reveals the meandering state of the Kuroshio. Thecyclonic and anti-cyclonic eddies in the south of Japan can characterize the formation ofthe meandering state of the Kuroshio, and the amplitude of the meandering is related tothe duration time, intensity and location of these eddies. Then, the process of the Low-Frequency in the Kuroshio Extension is analyzed. Itis suggested that the changes of wind stress in the eastern North Pacific may be causedby PDO variations.The wind stress will cause the changes of SSH through Ekmanpumping. And the decadal variations of the strength, location and mesoscale eddy fieldof the KE and recirculation gyre may due to the SSHA signals which are produced inthe eastern North Pacific and then propagate westward.Secondly, the generation and development processes of a small meander southeastof Kyushu, which triggered the Kuroshio large meander in2004, is studied using amerged satellite altimeter data. It is found that a disturbance from the East China Sea(ECS) that accompanies a high potential vorticity (PV) anomaly and a cyclonic eddypropagating westward from the Kuroshio recirculation (KR) play important roles for thegeneration and development of the small meander. The cyclonic eddy triggers an initialcyclonic anomaly southeast of Kyushu. When the high PV anomaly passes through theTokara Strait, a large positive vorticity is generated due to the stretching of the watercolumn is intensified. The positive vorticity is transported to the southeast of Kyushu bythe vorticity advection and accumulates there. As a result, the small meander isgenerated. The decay and amplification processes of the small meander illuminate thatthe disturbance from the ECS contributes to the horizontal scale of the small meander.Tracing back the disturbance, it is revealed that the disturbance is a frontal wavegenerated by collision between a strong anticyclonic eddy and the Kuroshio east ofTaiwan. The frontal wave propagates along the Kuroshio accompanying high PVanomaly generated around the continental shelf edge by viscosity, and influence thegeneration and development of the trigger meander by transporting a large amount ofhigh PV anomaly to the Tokara Strait.Finally, the decay of the2004/05Kuroshio large meander is examined by using amerged satellite altimeter data. The Kuroshio takes the typical large meander(LM) inSeptember2004to January2005. Then two path transition occurs in late January andJune2005on the Izu Ridge. And after the second path transition,the meander amplitude decrease rapidly, until the LM disappears. The results of the vorticity diagnostic suggestthat these path transitions are caused by disturbances propagating along the Kuroshio. Inaddition, the decrease of the LM amplitude almost coincides with these path transitions.It is suggested that two possible mechanisms is responsible for the weakening of the LM.(1) Extraction of high potential vorticity(PV) water from the LM by disturbancescoming from the upstream of the Kuroshio.(2) Outflow of high-PV water of the LM tothe downstream.