Study on SST front disappearance in the subtropical North Pacific using microwave SSTs

We investigated the processes relating to the weakening of the SST front in the subtropical front (STF) zone using the Advanced Microwave Scanning Radiometer for the Earth Observing System SSTs for 7?years with temporal/spatial resolutions of 1?day/12.5?km. In April, the SST front is strong with a high gradient magnitude (GM) and Jensen–Shannon divergence (JSD) band; in August, SSTs become uniform (28–30?°C), together with small GMs (<0.8?°C/100?km) and JSDs (<0.75). Since the SST front features become invisible in GM/JSD snapshots and weekly–monthly averaged images, we call this phenomenon ‘SST front disappearance (SFD)’. The SFD occurs in August, but the number of high SSTs (>30?°C) in August is smaller than that in July, which indicates that the SFD results from not only the increase in lower SSTs but also the decrease in higher SSTs. In June and July, the GM distributions have quite large standard deviations compared to those in May and August. We also investigated the vertical profile of STF using in situ temperature/salinity profiles. It was revealed that the SFD influence extends to 50?m depth. The area of high integrated heat flux and shallow mixed layer depth were found to correspond to the area where the GM decreases from 0.9 to 0.6?°C/100?km during June–August. Quantitative analyses confirmed that the SFD mechanism may be attributable to the establishment of the shallow mixed layer by the high integrated heat flux from May to July. From July to August, the SST heating/cooling in the north/south of the SST front may accelerate the SFD.