The Impact Of The Large-scale Circulation Decadal Adjustment On The Generation Of Tropical Cyclones In The Northwest Pacific

Tropical Cyclone(TC)genesis over the Western North Pacific(WNP)is the focus of tropical atmospheric research.Understanding the change of TC genesis at various time scales is of great significance for TC genesis forecasting.Previous studies have extensively discussed the inter-annual or inter-decadal variability of TC genesis.These studies have reported significant inter-decadal changes of WNPTC genesis.The TC genesis large-scale are generally controlled by large-scale dynamic and thermodynamic conditions,and these environmental factors affecting TC genesis are generally associated with the large-scale circulation.For example,the monsoon trough is considered to be an important large-scale system that affects the generation of TC over the WNP.In this study,in order to better distinguish the influence of these large-scale circulations on the changes of WNPTC genesis,the large-scale flow patterns favorable for TC genesis have been classified into five basic flow patterns based on the objective classification method: monsoon shear line(SL),monsoon confluence region(CR),monsoon gyre(GY),easterly wave(EW),and Rossby wave energy dispersion from the preexisting TC(PTC).Based on these classifications,this paper further studies the interdecadal changes of TC genesis frequency in May-October over the WNP during the two periods of1979-1997 and 1998-2012,and the contribution of TCs associated with these large-scale flow patterns to the interdecadal changes of WNPTCs is also investigated.The main conclusions are as follows:(1)The apparent decrease in TC genesis from 1998-2012 was due to the significant decrease in TC genesis associated with the PTC pattern and to a somewhat lesser degree,TC genesis associated with the GY pattern.In contrast,TC genesis associated with the SL pattern show a small increase,which seems to contradict the weakened monsoon circulation since 1998 but corresponds well to cyclonic anomalies over the Philippines region.In addition,it did not find that the TC genesis associated with the SL and CR patterns experienced significant interdecadal changes form observations.(2)Due to the phase transition of the Pacific Interdecadal Oscillation(PDO)since 1998,the eastern Pacific has cooled and exhibited an anomalous distribution of SST similar to Mega La Ni(?)a pattern.Decreased TC genesis associated with the GY pattern and increased TC genesis associated with the EW pattern are closely related to the strengthening of Pacific easterly waves in response to the Mega La Ni(?)a-like pattern that predominated during 1998-2012.Weakened easterly shear over the eastern WNP is not conducive to the development and propagation into the southeastern WNP of Rossby wave trains induced by preexisting TC energy dispersion.Consequently,there is a significant reduction of TC genesis associated with the PTC pattern and a weakening in the contribution of TC genesis associated with the PTC pattern to TC genesis associated with the EW pattern.(3)An increased correlation between TCs associated with the SL/GY/EW patterns and central Pacific(CP)-type ENSO during 1998-2012 is observed.A stable and robust association between TC genesis associated with the CR pattern and tropical North Atlantic sea surface temperature is observed regardless of decadal climate regime shifts.However,there is no significant link between TC genesis associated with the PTC pattern and more CP ENSO events during 1998-2012,but there is a strong association between the Pacific meridional mode and TC genesis associated with the PTC pattern during 1979-1997.More observational analyses and numerical simulations are needed to further investigate the underlying physical mechanism.