Temporal and spatial variabilities in ISCCP clouds over East Asia and their implication of climate change

It is generally accepted that convective storms are deepened over the tropics, causing expanded dry subsidence zones and then mid-latitude storm tracks shifted poleward under the global warming condition. Although debate still remains about the response of cloudiness and mid-latitude storm intensity to the warming, recent study indicated that the total cloud fraction has decreased in mid-latitude storm-track regions over the past 25 years. This may suggest accelerated global warming because the decreased cloud fraction can induce positive cloud feedback. In contrast to the decreasing trend of the total cloud fraction, it was reported that deep convective cloud amount has been increased in North Pacific Ocean off East Asia (Bender al., 2011; Zhang et al., 2007). Because the cloud feedback is controlled not only by the total cloud amount but also by the cloud types, it may be natural to examine the trend of individual cloud type. First, this study objectively classified ISCCP clouds using a K-means clustering algorithm unlike traditional classification using three criteria of cloud optical thickness and cloud top pressure. After that, we conducted Empirical Orthogonal Function (EOF) analysis to monthly frequencies of cloud types over East Asia for the past 25 years. The results suggest that thick high and middle-level clouds have increased while thin high and middle-level clouds have decreased, and the trends of low-level clouds are opposite to the trends of high and middle-level clouds. The trends are considered to be due to changes in the water vapor transport from tropics to mid-latitude.