A Study On The Impacts Of Global Warming And Interdecadal Pacific Oscillation On The Long-term Changes Of Tropical Cyclone Activity

Tropical cyclone(TC)genesis activity(including genesis frequency,tracks and intensity)is easily affected by multi-time scale factors.For a long time,previous studies mainly foucused on investigating mechanisms controlling TC activity from synoptic,intraseasonal to interannual time scales.However,the causes of interdecadal and long-term changes of TC activities is still not well understood.So,in this study,we focus on the long-term changes of tropical cyclone activities,investigating the impacts of external forcing and atmospheric internal climate variability.We systematically analyzed and verfied the mechanisms responsible for the interdecadal variability and long-term change of TC activity using the statistical methods and atmospheric circulation models.Five main conclusions are summerized as follows:(1)Find that the intense tropical cyclone(Category 3-5)tracks significantly increased over the coastal region of East Asia since 1980 while decreased over the southeastern part of western North Pacific(WNP)and attribute it to impact of global warming hiatus(GWH).Previous studies have proposed that GWH is mainly induced by global warming(GW)and negative phase of interdecadal Pacific oscillation(IPO).GWH was characterized by a La Nina-like cooling in the tropical Eastern Pacific accompanied with the Indian Ocean and the tropical Atlantic Ocean warming.Here we show that the recent GWH SST pattern contributed significantly to the increased occurrence frequency of intense tropical cyclones in the coastal regions along East Asia since 1998.The GWH associated with sea surface temperature anomalies triggered a pair of anomalous cyclonic and anticyclonic circulations and equatorial easterly anomalies over the WNP,which favored TC genesis and intensification over the western WNP but suppressed TC genesis and intensification over the southeastern WNP due to increased vertical wind shear and anticyclonic circulation anomalies.Results from atmospheric general circulation model experiments demonstrate that the SST anomalies related to GWH affect intense TC tracks over the WNP.(2)Discuss the different responses of Northern Hemisphere TC changes to El Nino-like and La Nina-like warming,respectively.GW could be two faces:an El Nino-like or a La Nina-like warming pattern.Using the geophysical fluid dynamical laboratory(GFDL)high resolution atmospheric model(HIRAM),we designed two sensitive experiments based on two warming patterns.We found that(a)both the El Nino-like and La Nina-like warming lead to a decrease of TC genesis frequency in the WNP and an increase in the eastern North Pacific(ENP).The El Nino-like SST warming suppressed north Atlantic(NA)TC genesis while the La Nina-like SST warming promoted NA TC genesis;(b)although both the El Nino-like and La Nina-like warming suppressed WNP TC genesis,the mechanism is distinct:the El Nino-like SST warming,combined with the Indian Ocean warming,induced a dipole cyclonic and anticyclonic circulation anomalies over the WNP and suppressed the TC genesis over the western WNP while the La Nina-like warming mainly intensified the equatorial easterly and suppressed the WNP TCs.(3)Reveal that an abrupt decrease of WNP TCGF since 1998 is mainly contributed by the phase shift of the Interdecadal Pacific Oscillation(IPO)from positive to negative since 1998.Some studies have already documented the phenomenon that the WNP TCGF significantlt decreased since 1998 based on the data from 1980-2015.Here,we used an objective clustering method and catergorized the WNP TC genesis frequency(TCGF)into three distinct clusters and found that(a)this abrupt decrease is primarily due to the decrease in a cluster of TCs(C1)that mostly formed over the southeastern WNP,south of 15°N and east of the Philippines,and no significant change of TCGF exists over other regions.(b)IPO in recent negative phase(1998-2013)shows a La Nina-like SST anomalies,which leads to the intensified equatorial easterly.So,the monsoon trough weakened and shift westward.Combined with intenfied vertical wind shear over the southeastern WNP,the WNP genesis over southeastern WNP significantly decreased.These results were further confirmed by similar analyses based on longer datasets(1945-2015)and also the outputs from a 500-year pre-industrial general circulation model experiment using the Geophysical Fluid Dynamical Laboratory(GFDL)coupled model-3.(4)Unraveling the reason for the interdecadal change around mid-1970s of the interannual relationship between WNP TCGF and SSTG.Previous study addressed the significant interannual relationship between spring(averaged from March to May)SSTG(defined as the SST gradient between South Pacific(40°-20°S,160°E-170°W)and western Pacific warm pool(EQ-16°N,125°-165°E))and June-October averaged TCGF over the WNP from 1980-2011.However,we found that this interannual relationship between SSTG and WNP TCGF is not always stable.The interannual relationship is insignificant from 1945-1974(the prior period)although it is significant from 1979-2015(the post period).Two reasons caused this interdecadal change:(a)In prior period,the eastern Pacific(EP)ENSO suppressed the impact of the SSTG on WNP TCGF.The EP El Nino induced the anomalous cyclonic circulation over the eastern WNP while anomalous anticyclonic circulation over the western WNP,a significant dipole pattern,which suppressed the impact of SSTG.However,the central Pacific(CP)El Nino increased since 1979,which can induce the cyclonic circulation anomalies over the WNP and promote the impact of the SSTG on WNP TCs.(b)In prior period,the persistence of the SSTG is very poor through summer while,in recent period,the SSTG can persist from spring to summer.Further,we used the general circulation model(ECHAM model)to verify the results and discussed the relationship between IPO and the shift between EP and CP ENSO.(5)Objectively separate different impacts of GW and IPO on north hemisphere(NH),including western North Pacific(WNP),eastern North Pacific(ENP)and North Atlantic(NA),TC tracks and further evaluate the relative contribution of GW and IPO to the change of TC tracks over each basin.The external forcing induced GW and internal climate change of IPO tangled,which makes long-term change of TC activity complicated.Here in this study,based on singular value decomposition(SVD)method and a suite of model experiments,we separate the distinct impacts of GW and IPO on TC tracks.We found that during 1960-2014,(a)GW leads to an upward trend of TC track(TCT)frequencies over the eastern North Pacific(ENP)and North Atlantic(NA)and a downward trend over the western North Pacific(WNP).On one hand,combined with the Indian and Atlantic SST warm,the GW suppressed TC genesis over the WNP but promoted over thw ENP.On ther other hand,the GW shows a La Nina-like SST warming pattern,suppressing the WNP TCGF but promoting ENP TCGF.(b)the positive(negative)phase of IPO significantly promotes(suppresses)TC genesis over Pacific region but suppresses(promotes)NA TC genesis.The relative importance analysis shows that the IPO dominated the WNP TCTs while the GW impact is secondary;the NA TC activity is mainly influenced by the GW;and both the GW and IPO significantly modulated the ENP TC activity.