Analysis of interdecadal variation of tropical Pacific thermocline based on assimilated data

The interdecadal variation of Pacific thermocline represented by depth anomalies of 25 sigma(theta) isopycnal surface calculated from SODA data set is analyzed. The climatological depth of 25 sigma(theta) gisopycnal surface is quite close to the depth of 20degreesC isotherm in the tropical Pacific. The EOF1 mode of the 25 sigma(theta) isopycnal surface accounts for 26.4% of the total variance and its associated pattern is of east-west direction. The centers of positive and negative extremes are located near 10degreesS over the southern Pacific and the correlation coefficient with zero-lag between the corresponding EOF1 time coefficient and PDO index is -0.67. This shows that there is very close relation between the southern tropical Pacific and PDO. The wavelet analysis of detrended EOF1 time coefficient reveals that there are two dominant time scales of about 3similar to7 and 30 a respectively. An apparent abruptness of mean value occurred in the late 1970s. EOF2 mode accounts for 12.4% of the total variance and its pattern is an ENSO-related one. The correlation coefficient between the EOF2 time coefficient and NINO3 index is -0.68. The wavelet analysis of EOF2 time coefficient reveals that there are two leading time scales of about 2similar to7 and 10similar to15 a respectively. On an interdecadal scale, the zonal change is consistent along the equator and is seesaw along 10degreesS; there is consistent polarity in the tropics along 165degreesE, but reverse polarity between around equator and other tropical region along 120degreesW. In all the four profiles mentioned above, the regime shift occurred in the late 1970s. The evolving characteristics of anomalies can be explained mostly by the anomalies of ocean currents during a complete cycle on an interdecadal scale.