Inter-annual and decadal variabilities of upper ocean in the East Sea (Sea of Japan)

Abstract

To examine inter-annual and decadal variabilities of upper ocean in the southwestern region of the East Sea we analyze historical hydrographic data provided by National Fisheries Research and Development Agency. Statistical analysis of water temperature measured at ten selected levels from surface down to 300 m depth is made. Seasonal variation of temperature is dominant in the upper 150 m depth, while not obvious below 150 m depth. Low frequencies longer than the annual period are studied in terms of normalized temperature anomaly, that is defined as spatial average over 50 stations where water depth is deeper than 300 m in the study region. Grand wavelet spectrum of each level shows 2 peaks around periods of 3-4 years and 10-20 years, which might be related to oceanographic signals such as the El Nino and the North Pacific climate regime shift. We found that inter-annual signal diminishes and the decadal signal becomes stronger as the water depth gets deeper. Wavelet spectrum of each level shows clear vertical thermal structure in the context of inter-annual and decadal variability. Decadal variability arises consistently throughout all levels although data using in this analysis are not quite long enough to discuss the variability in detail.