Dominant Role of the Ocean Mixed Layer Depth in the Increased Proportion of Intense Typhoons during 1980-2015

Over the past decade extensive studies have been undertaken to understand the increasing trend in the proportion of intense tropical cyclones (categories 4 and 5 on the Saffir-Simpson scale). The trend has been found globally and in some individual basins since the late 1970s. This study quantifies the contributions of various factors that control the proportion of intense typhoons. It is demonstrated that the increase of the proportion of intense typhoons during 1980-2015 is consistent with the corresponding changes in the ocean/atmosphere environment. The proportion change resulted from the temporal variations of the environmental parameters (sea surface temperature, ocean mixed layer depth, outflow temperature, and vertical wind shear), as well as the shifts of tropical cyclone prevailing tracks. The nonuniform spatial distribution of environmental parameters makes the shifts of tropical cyclone prevailing tracks contribute at least half the increase of the proportion of intense typhoons. The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons. Although the maximum potential intensity theory and numerical modeling project an increase of tropical cyclone intensity in a warming climate, the effects of the temporal change of the ocean mixed layer depth and the prevailing track change were not taken into account in the projection. This study suggests that the increase of the proportion of intense typhoons in the western North Pacific basin could be larger than the projection in previous studies. Plain Language Summary The increasing trend in the proportion of intense tropical cyclones (categories 4 and 5 on the Saffir-Simpson scale) has been found globally and in some individual basins since the late 1970s. Although extensive studies have been undertaken over the past decade, its attribution still is a subject of controversy. This study quantifies the contributions of various factors that control the proportion of intense typhoons in the western North Pacific and found that the increase of the proportion of intense typhoons during 1980-2015 is consistent with the corresponding changes in the ocean/atmosphere environment. The proportion change resulted from the temporal variations of the environmental parameters (sea surface temperature, ocean mixed layer depth, outflow temperature, and vertical wind shear), as well as the shifts of tropical cyclone prevailing tracks. The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons. This study suggests that the tropical cyclone intensification could be larger than the projection since the change of the ocean mixed layer was not taken into account in previous studies.