Intra-seasonal variabilities of oceanic low-level cloud in summertime North Pacific and its interaction with sea surface temperature

Oceanic low-level cloud plays a key role to determine radiation budget at the surface or top-of-atmosphere and modulate regional and global climate variabilities. In particular, an interaction process between oceanic low-level cloud and sea surface temperature (SST) is important to understand co-variability of atmosphere, sea surface, and oceanic subsurface layer. Despite the importance of role of the low-level cloud, the detailed process of evolution of the cloud and the related air-sea interaction process is not well investigated on shorter time-scale than seasonal variation.

In the present study, we investigated the intra-seasonal variability (ISV) of oceanic low-level cloud cover (LCC) in the summertime western north Pacific by using satellite observational and reanalysis datasets from 2003 to 2016. Typical time-scales of LCC variability are synoptic scale (3-7-day) and intra-seasonal scale (20-100-day), which is shorter than seasonal variation. Some of important LCC controlling factors are SST and horizontal temperature advection (Tadv), whose dominant time-scales are similar with that of LCC. We focused on intra-seasonal timescale in this study because coherence among each variable on intra-seasonal timescale is higher than synoptic one. From band-pass filtering method and phase composite analysis to investigate the evolution process of LCC, it is found that ISV of LCC is almost stationary. It implies that the local LCC controlling factors are important to evolution of low-level cloud. The results also showed that dry-cold Tadv from poleward is a trigger to increasing in LCC, but SST is a follower to LCC due to cloud radiative effect for shortwave (SW) radiation. Evolution of LCC is also corresponded to positive tendency of RH. The positive tendency of RH is induced by change in not specific humidity but temperature, i.e. decreasing saturation water vapor pressure due to decreasing temperature. We further quantitatively investigated the impact of LCC to SST by oceanic mixed-layer temperature budget analysis, showing that an importance of anomalous dry-cold Tadv for cooling of SST by not only enhanced latent heat release but also decreasing in downward SW radiation at the sea surface caused by more LCC.