Asymmetry of seasonal sea surface temperature variation in the sea around Korean Peninsula

Abstract

Sea Surface Temperature (SST) is an important component of climate and weather systems at various time scales. In this study, asymmetric seasonal variation of SST in the seas around Korean Peninsula is investigated using reanalysis and observation data. Specifically, the asymmetry of the SST seasonal variation is estimated quantitatively, and is compared with heat advection and surface net heat flux using SST data and atmospheric variables from ECMWF.

Two definitions were used to quantify the asymmetry of the seasonal variation

. day ratio defined as a ratio of number of days in increasing temperature trend to total days. Amplitude ratio means ratio of semi-annual amplitude to annual amplitude of temperature variation implies that the seasonal variation of temperature is asymmetric if day ratio is far from 0.5 or amplitude ratio is large.

Day ratio of SST shows an opposite feature to air temperature. Air temperature increases slowly, whereas SST decreases slowly in the study area. Spatial distribution of SST amplitude ratio is more complex than that of air temperature. There are substantial disagreement between the day ratio and the amplitude ratio over the highly asymmetric regions, such as northern East/Japan Sea and Kuroshio path. Phase difference between annual and semi-annual components causes this disagreement.

Heat advection and surface heat flux were estimated using model SST and current data. Incoming heat advection shows negative correlation with the asymmetry index of the SST seasonal variation, whereas surface net heat flux has positive correlation with the SST asymmetry index. Model sensitivity test, which performs no current along the boundary, shows prominent cause of asymmetry in seasonal variation different feature. Asymmetry decreases significantly when there is no current.