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 SummaryThe 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.

In the past 40 years, the global annual mean surface temperature has experienced a non-uniform warming, differing from the spatially uniform warming simulated by the forced responses of large multi-model ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, intermittent and spatially varying warming trends along the Northern Hemisphere (NH) mid-latitudes and a slight cooling in the tropical eastern Pacific. In particular, this “wavy” pattern of temperature changes over the NH mid-latitudes features strong cooling over Eurasia in boreal winter. Here, we show that these non-uniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically-driven teleconnection is associated with a Pacific SST pattern resembling the Interdecadal Pacific Oscillation (IPO), and hereafter referred to as the IPO-related Bipolar Teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial non-uniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades.

The South Asia high (SAH) is a prominent circulation system of the Asian summer monsoon, exerting profound influences on the weather and climate in China and surrounding regions. Its formation and maintenance is closely associated with strong summertime continental heating in the form of surface sensible heat flux and the latent heat release in connection with the Asian monsoon. In this study, the possible response of the South Asian high intensity to the thermal condition change in the Tibetan Plateau is examined with four modern reanalysis datasets, including the Modern-Era Retrospective Analysis for Research and Applications (MERRA), MERRA version 2 (MERRA-2), the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim), and the Japanese 55-year Reanalysis (JRA-55). Despite the surface air warming in the four modern reanalysis datasets, reduced surface wind speed in three of the reanalysis datasets, and decreased surface sensible heat flux in the MERRA-2 dataset, there is no statistically significant trend in the SAH intensity over the period 1979–2015. One of the possible reasons is that the response of the upper-level circulation to the thermal condition change of the Tibetan Plateau occurs mainly in the 200-hPa subtropical westerly jet stream, which is located far away from the center of the South Asian high. Thus the South Asian high intensity is not particularly sensitive to the thermal condition change of the Tibetan Plateau, while the center of the South Asian high intensity over the plateau exhibits a northward trend over the period.

It has been projected that the influence of anthropogenic climate change on tropical cyclone (TC) intensity could be detected by the end of the century although significant increasing trends in TC intensity metrics have been found based on the currently available historic records. The human influences on TC intensity have been debated for about two decades because of a lack of quantitative assessment of the contributions of large-scale environmental factors and track shifting. As an extension of a previous study, we show that the observed rise in the percentage of intense TCs in the western North Pacific basin over the past 56 years resulted from the combined influence of the track shifting and temporary changes in environmental factors. The influence of environmental factors was primarily owing to the decrease of environmental vertical wind shear and the warming of sea surface temperature. While a small part of the observed rise in the percentage of intense TCs resulted from SST warming, the track shifting increased the TC intensification time by 18.2% (11.3 hours) over the past 56 years, accounting for more than one-third of the observed percentage increase in intense TCs. Since track shifting is also projected in the global warming experiments, this study suggests that anthropogenic climate change may intensify TCs by shifting TC prevailing tracks.