Spatial and temporal variation of north-west pacific tropical cyclone under the background of upper ocean warming

Under the background of global warming, the activities of north-west pacific (NWP) tropical cyclones (TCs) are undergoing significant changes. The TC frequencies have been characterized by an initial slow increase followed by a rapid increase and then a decrease, the past 33 years. During the 21st century, the TC frequency of the NWP has clearly decreased. However, the three TC origin types in the NWP have experienced different types of changes. The TC frequencies of origin 1 (10 degrees similar to 22 degrees N,110 degrees similar to 120 degrees E) and origin 2 (8 degrees similar to 20 degrees N,125 degrees similar to 145 degrees E) are both increasing, but the TC frequency of origin 3 (5 degrees similar to 20 degrees N,145 degrees similar to 155 degrees E) is decreasing. Under the background of upper ocean warming, the average TC duration has shown a decreasing trend (-0.27d/10a), while the TC mean and maximum intensity has increased (0.93 m/s/10a and 1.57 m/s/10a, respectively). Therefore, the potential threats of TC activities to NWP coastal countries are likely to intensify. The changes in the thermal state of the upper ocean have many effects on TC activities. Sea surface temperature is not the main factor affecting the frequency of TCs. However, the response of TCs to the upper ocean heat content is obvious.