[AAS06-P12] Unusual growth in intense typhoon occurrences over the Philippine Sea in September after the mid-2000s
キーワード：tropical cyclone, decadal change
During the global warming hiatus period (1998–
present), a pronounced increase in the number of intense
typhoon occurrences was identified over the Philippine
Sea (PS: 5°–25°N, 125°–140°E) in September after the
mid-2000s. Comparing two periods before and after the
mid-2000s indicates that intense typhoons rarely occurred
over the PS in September before the mid-2000s, with a frequency
of fewer than 0.4 per year, but reached up to nearly
1.5 per year after the mid-2000s. The abrupt increase in
intense typhoon occurrences over the PS was primarily
attributed to increased tropical cyclone (TC) genesis and
favorable large-scale conditions for TC intensification. The
increase in TC genesis number over the PS was caused
by contributory dynamical conditions, including positive
low-level relative vorticity anomalies and anomalous
ascents, which corresponded to a southwestward shift and
strengthening of the monsoon trough. In addition, among
the favorable large-scale conditions, the increased relative
humidity that resulted from intensified moisture flux
convergence exerted essential effect on the TC intensification.
These changes in atmospheric environmental conditions
favoring intense typhoon occurrences over the PS
were primarily associated with the change in the tropical
Indo-Pacific sea surface temperature (SST) around the mid-
2000s. Besides that, the positive feedback TCs exerted on
the circulation was also conducive to the unusual growth in
intense typhoon occurrences over the PS. And note that the
role of SST anomalies in the air–sea interaction is the key
to interpret why the unique phenomenon only occurred in
September
present), a pronounced increase in the number of intense
typhoon occurrences was identified over the Philippine
Sea (PS: 5°–25°N, 125°–140°E) in September after the
mid-2000s. Comparing two periods before and after the
mid-2000s indicates that intense typhoons rarely occurred
over the PS in September before the mid-2000s, with a frequency
of fewer than 0.4 per year, but reached up to nearly
1.5 per year after the mid-2000s. The abrupt increase in
intense typhoon occurrences over the PS was primarily
attributed to increased tropical cyclone (TC) genesis and
favorable large-scale conditions for TC intensification. The
increase in TC genesis number over the PS was caused
by contributory dynamical conditions, including positive
low-level relative vorticity anomalies and anomalous
ascents, which corresponded to a southwestward shift and
strengthening of the monsoon trough. In addition, among
the favorable large-scale conditions, the increased relative
humidity that resulted from intensified moisture flux
convergence exerted essential effect on the TC intensification.
These changes in atmospheric environmental conditions
favoring intense typhoon occurrences over the PS
were primarily associated with the change in the tropical
Indo-Pacific sea surface temperature (SST) around the mid-
2000s. Besides that, the positive feedback TCs exerted on
the circulation was also conducive to the unusual growth in
intense typhoon occurrences over the PS. And note that the
role of SST anomalies in the air–sea interaction is the key
to interpret why the unique phenomenon only occurred in
September