Large-scale seasonal and secular variability of the subtropical front in the western North Pacific from 1954 to 1974

Examination of the Subtropical Front in the western North Pacific along 137°E, using routine hydrographic cruise data from 1967 to 1974, reveals it to be narrow, located between 17.5° and 25°N, and associated with a sharp northward rise of the upper portion of the main thermocline which detaches itself from the deeper main thermocline near 17.5°N. The climatological front depicted in various oceanographic atlases (e.g., USSR Navy Atlas, 1976) is thus revealed as a smoothed version of the more narrow synoptic frontal feature that moves latitudinally and varies in strength on both a seasonal and a secular basis. The narrow Subtropical Front is superimposed upon the weaker north/south gradient of thermocline temperature that exists between the North Equatorial Current and the Kuroshio Countercurrent. The present study extends our knowledge on the space/time variability of the Subtropical Front by analyzing all available mechanical bathythermograph, expendable bathythermograph, and hydrographic data taken in the western North Pacific from 1954 to 1974. These data are interpolated onto a regular space grid (2.5° latitude by 5° longitude) on both a monthly climatological basis and an individual yearly basis from 1954 to 1974. Because the data distribution lacked the data density required, the method of interpolating could not reproduce the narrow character of the Subtropical Front but rather smoothed it in space and over a year's time. However, a maximum in the meridional gradient in temperature over the upper 200 m consistently appeared each year from 1954 to 1974 in the approximate vicinity of the synoptic front, although it showed much less strength. Changes in both the latitude location and the strength of this smoothed version of the Subtropical Front show interesting behavior on both the seasonal and the secular time scale. On the seasonal scale (i.e., 3 months) the Subtropical Front is twice as strong in spring as it is in fall, with some tendency for it to be located farther north when it is stronger and farther south when it is weaker. This seasonal variability has a maximum north/south displacement of ±4° of latitude in the west at 135°E, decreasing to the east, associated with a maximum range in strength of ±35% about the annual mean. On the secular scale (i.e., 1–1.5 yr) the Subtropical Front from 1954 to 1974 had a maximum range in strength of ±15% about the annual mean, and it had some tendency to be more intense when it was displaced farther north, showing a maximum north/south displacement of ±2° latitude. From 1954 to 1974 the years 1957–1958, 1963–1965, and 1970–1971 find the Subtropical Front to have been stronger than normal, in phase with the secular variability of the two major current systems both north and south of it (i.e., the Kuroshio and the North Equatorial Current). These periods of years (i.e., 1957–1958, 1963–1965, and 1970–1971) are the same as those characterizing the El Nino phenomenon in the equatorial and tropical Pacific, an indication of the more broad-scale nature of this secular variability.