Study on Sea Surface Temperature (SST) front disappearance in the subtropical North Pacific

The surface subtropical front (STF) is strong in winter-spring and weak in summer-autumn. Many studies have been carried out from aspects of the STF front-genesis. In contrast, few studies examined weakening processes of the SST front in the heating season (April-August). In the present study, we try to investigate the SST front weakening processes in the STF zone using the Advanced microwave Scanning Radiometer for the Earth Observing System (AMSR-E) SSTs with temporal /spatial resolutions of one-day/12.5km. In April, the SST front is strong with high values of gradient magnitude (GM)/ Jessan-Shannon divergence (JSD); in August, SSTs become uniform (28-30^oC), and GMs (<0.8^oC/100km) and JSDs (<0.75) small(Figure 1). Since the enhanced SST front features become invisible in GM/JSD snapshots and weakly-monthly averaged images, we call this phenomenon as 'SST front disappearance (SFD)'. The statistical analyses using the 7-year AMSR-E SST indicate that the SST front weakening period is mainly from June to August; the SST PDF distribution in the STF zone has two peaks in June, negatively skewed one peak in July and the Gaussian shape with a small standard deviation in August. The SFD happens in August, however, the number of high SSTs (>30^oC) in August is smaller than that in July, which indicates that the SFD results from not only the increase of lower SST but also the decrease of higher SSTs. In June and July, the GM distribution has quite large standard deviation compared with that in May and August. It indicates that the SST front is unstable in June and July though they are stable in May with lower SSTs and in August with higher SSTs. Since it is well known that the subsurface STF is well represented by the eastward velocity referred to 400dbar, we calculate it using in situ temperature/salinity profiles. Then, it is revealed that the subsurface STF does not disappear when SFD occurs in August. Through analyses using a meteorological reanalysis data and the in situ ocean Temperature profiles, it is demonstrated that the area of high integrated heat flux (>4×10⁸J/m²) and shallow mix layer depth (<20m) corresponds to the area where GM decreases from 0.9 to 0.6^oC/100km in June - August. The SFD mechanism may be attributed to the shallow mix layer establishment by the high integrated heat flux. The details of studies using satellite and in situ data will be introduced in the presentation.