Seasonality and Co-variability of Sea Surface Temperature Front and South boundary of Low-Cloud Cover in the mid-latitude North Pacific

Oceanic low-level clouds, including fog, stratus and stratocumulus, interact strongly with sea surface temperature (SST) in mid-latitude SST frontal regions in summer. To understand the processes of the interaction, it is important to clarify the relationship between the SST front and low-level cloud cover (LCC) and its meridional gradient. In this study, we investigated seasonality and co-variability of the SST front (SSTF) and the south boundary of LCC (strength and position) in the North Pacific (NP). We used MODIS 8-day mean cloud product (MYD08_E3) and OISST daily mean product (AVHRR-Only). To find the SSTF and LCC boundary (LCCB), we defined them at the location of their minimum/maximum meridional gradient for SST/LCC in each longitude bin. Our target region is NP (30N-50N, 150E-150W), and target period is from 2003 to 2016. First, we examined climatological seasonal changes in SSTF and LCCB and found that the changes in strength of SSTF are opposite between the western and eastern NP. Correlation coefficient between the positions of SSTF and LCCB is positive except for wintertime western NP and is the largest in summertime eastern NP (r=0.61). In addition, we investigated the changes in vertical structure of ocean mixed layer with shift of SSTF and LCCB by using the gridded monthly data based on Argo (MOAA-GPV). When LCCB shifts poleward, only the near-surface temperature in the southern flank of SSTF increases. In eastern NP, the pattern of temperature change with SSTF shift is similar to that of LCCB, implying the change in SSTF position in the eastern NP may be affected by the LCCB shift. In contrast, an increase in temperature not only near the surface but also in deeper part ( 500 m) can be seen in the western NP. Thus, position of SSTF change in the western NP are associated with oceanic processes. Our results imply that the low-level cloud radiative forcing to variability of the SSTF depends on season and region, especially to the strength of SSTF.