Stratocumulus Response to a SST Front in Kuroshio Extension: In Situ Observations and Mechanism

The effects of sea surface temperature (SST) fronts on the marine atmospheric boundary layer (MABL) structures and stratiform clouds are less understood on synoptic time scales due to the lack of in-situ observations. A research vessel (R/V) dongfanghong 2 of the Ocean University of China captured a rapid development of stratocumulus over a SST front in Kuroshio Extension (KE) on 12 April 2014, when the R/V cruised northward along 145 °E (Fig. 1). During the track, the SST decreased rapidly from 20 °C to less than 15 °C in a distance of ~50 km. The cloud top and cloud base increased and the cloud coverage expanded significantly with the deepening of the marine atmospheric boundary layer up to 3000 m over the warm flank of the SST front.

By analyzing the observations and model results, the present study reveals the mechanisms in the processes of the cloud development. A positive heat flux center formed on the warm flank (downwind) of the KE front due to the slow response of air to the forcing of the KE front. Induced by the heat flux center, a weak low pressure trough developed, leading to vertical motion in the MABL. The turbulent mixing in the MABL was favorable to the downward transport of westerly momentum from aloft to sea surface which, in turn, further intensified the surface heat flux. This SST front effect allowed the MABL to deepen and the height of the cloud base/top to increase downwind the KE front.

On the other hand, a warm air advection from Honshu, Japan arrived at the heat flux center. The SST front effect combined the warm advection to produce a pronounced warm air center near surface at 09-12 UTC 12 April, which further led to the deepening of the mixing layer, the rapid development of the clouds.

Model (WRF) results indicate that the warm sea surface south of the SST front enhanced the surface latent heat fluxes by 200 W/m², leading to local ascending motion of 0.05 m/s in the MABL. The warm-air advection from Honshu warmed the lower level of the atmosphere about 2K and reduced the sea level pressure by about 0.5 hPa. The SST front provide a sea surface background condition and the warm-air advection generate an effect of trigger. This study is helpful to understand the physical mechanisms in atmospheric responses to oceanic fronts at synoptic time scales.