Revisiting the impact of mid-latitude cold air outbreaks on the Maritime Continent weather

The East Asian winter exhibits frequent cold air outbreaks (CAOs), or cold surges, which often cause severe cold waves in the extratropics. CAOs are also of importance for forecasters in the Maritime Continent because the associated equatorward outflows can influence tropical weather. Various CAO definitions have been documented, but a definition based on a quantitative approach was only recently proposed. Studies of the CAO impact often linked a surge index in the subtropics instead of an index in the midlatitude where CAOs basically occur. Here, we investigate the impact by using a quantitative CAO index, which is defined by integrating cold air mass flux below a threshold potential temperature over midlatitude East Asia (45°N, 90°-135°E). From a climatological analysis, tropical convections are significantly observed two-to-four days following CAO event (i.e., the peak of CAO index), indicating CAO impacts. However, case studies show that the impacts vary among independent CAO cases due to influences of associated synoptic conditions, which affect the pathways of northerlies propagation and consequently hinder or amplify the impacts. Several impact patterns and their possible causes are discussed. A better understanding of East Asian CAO variability can improve the predictability of weather over the Maritime Continent.