Unfavorable environmental conditions for tropical cyclone genesis over the western North Pacific during the Last Interglacial based on PMIP4 simulations

Dubin Huan,Qing Yan,Ting Wei

a Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, Chinab Key Laboratory of Meteorological Disaster/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science and Technology, Nanjing, Chinac State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, Chinad University of Chinese Academy of Sciences, Beijing, China

Keywords: Tropical cyclone LAST Interglacial PMIP4

ABSTRACT Investigating the variation in tropical cyclone (TC) activity over the western North Pacific (WNP) in past warm periods helps to better understand TC behaviors in a warming future.In this study,the authors analyze the changes in large-scale TC genesis factors and the associated mechanisms over the WNP during the Last Interglacial (LIG),based on multimodel outputs from the Paleoclimate Modelling Intercomparison Project Phase 4.The results show that potential intensity exhibits a general decrease over the WNP during the LIG in the storm season,dominated by a weakened thermodynamic disequilibrium.The moist entropy deficit shows an overall increase over the WNP,arising from the decreased mid-tropospheric moisture and weakened vertical temperature contrast.Vertical wind shear enhances over the central WNP but weakens over the southwestern WNP,which is induced by the changes in the meridional tropospheric temperature gradient and hence high-level zonal winds.The absolute vorticity shows a general decrease over the WNP,partially linked with the decreased SST over the western tropical Pacific.Based on a genesis potential index,the authors suggest a decrease in genesis potential over the WNP during the LIG,indicating unfavorable conditions for TC genesis.The results highlight the important role of Earth’s orbit in regulating TC activity,which may shed light on TC behaviors in a warmer climate.

1.Introduction

The western North Pacific (WNP) is the most active ocean basin for tropical cyclone(TC)genesis,accounting for approximately one-third of the global total (Chan,2005).These TCs exert tremendous socioeconomic impacts over East and Southeast Asia (Mei and Xie,2016) and regulate the extreme precipitation and heat waves that take place there(e.g.,Kitoh and Endo,2019;Wang et al.,2023).Given the important influence of TCs over the WNP,extensive efforts have been made to decipher their variations and underlying mechanisms.Recent studies have suggested that TC frequency exhibits a significant decreasing trend since 1850 over the WNP(Chand et al.,2022),with a decadal decrease around the late 1990s(e.g.,Zhao et al.,2018).Meanwhile,the location of TC genesis has migrated poleward and landward (Daloz and Camargo,2018;Wang and Toumi,2021) and the TC translation speed has slowed during the past several decades (Kossin,2018;Lai et al.,2020),leading to an increase in flood risks in China’s coastal regions(Lai et al.,2020).However,there is still no strongly detectable evidence revealing the anthropogenic impact on contemporary TCs (Knutson et al.,2019),and no consensus on the behaviors of TC activity (e.g.,frequency,tracks,and translation speed) under a warming future(Knutson et al.,2020).To advance our understanding of TC activity over the WNP under global warming,we can draw support from paleoclimatology to investigate how TCs behaved in past warm periods.

The Last Interglacial (～130–115 ka;LIG) is one of the warmest periods of the past 800 ka (Past Interglacials Working Group of PAGES,2016),during which the climatic background was highly similar to the present interglacial,excluding anthropogenic impacts(Tzedakis,2003).Sea surface temperature(SST) experienced warming of～0.5?C during the LIG relative to the preindustrial period (Hoffman et al.,2017),together with partially melted ice sheets and 6–9 m higher sea level(Dutton et al.,2015).Although these changes were induced by changes in Earth’s orbit and thereby insolation,the LIG exhibited comparable warming to that under the low emission scenario arising from increasing greenhouse gas concentrations (Gulev et al.,2021).Therefore,the LIG provides an opportunity to place TCs in a warmer context and explore how they may vary with global warming.However,the paucity of geological evidence for storms during the LIG hampers our comprehension of TC activity at that time.

Numerical simulations have been considered as an effective tool in examining spatial patterns of paleoclimatic changes and associated dynamic mechanisms.Extensive efforts have been made to decipher the climatic changes during the LIG (e.g.,Chen et al.,2022;Jiang et al.,2022,2023).Regarding TCs over the WNP,existing modeling studies,however,have mainly focused on the mid-Piacenzian,Last Glacial Maximum,mid-Holocene,and last millennium (e.g.,Koh and Brierley,2015;Korty et al.,2012a,b).Recently,Yan et al.(2021) suggested a westward migration of TC genesis potential and potential intensity over the North Atlantic during the LIG.However,the variation in TC activity over the WNP during the LIG still remains elusive.

Hence,here,we use multimodel outputs from the Paleoclimate Modelling Intercomparison Project Phase 4 (PMIP4) to investigate the variations in TC genesis factors over the WNP during the LIG and further analyze the change in genesis potential.Such an investigation will help improve our knowledge on the response of TCs to changes in orbital parameters,and has implications for the behaviors of TCs in a warmer climate.

2.Methodology

2.1.Data

We use the outputs from 11 climate models(Table S1)participating in PMIP4 with the same experimental protocol (i.e.,piControl and lig127k experiments) (Otto-Bliesner et al.,2017).The main changes in external forcings during the LIG relative to the preindustrial period are Earth’s orbital parameters.Larger obliquity,larger eccentricity,and a perihelion close to the boreal summer solstice during the LIG gave rise to the change in solar radiation distribution and magnitude at the top of the atmosphere,with increased insolation in boreal summer relative to the preindustrial period (Berger and Loutre,1991;Otto-Bliesner et al.,2017).Greenhouse gas concentrations were slightly lower during the LIG compared with the preindustrial period,with CO2set to 275 ppm(284.3 ppm)in the lig127k(piControl)experiment.Note that the effect of changes in the length of months(i.e.,the paleo-calendar effect)in the model outputs for the LIG has been adjusted,using the method of Bartlein and Shafer (2019).

2.2.Genesis factors and genesis potential index

It has been considered that potential intensity,mid-tropospheric moisture,vertical wind shear,and absolute vorticity are the main factors for TC genesis.Potential intensity represents the theoretical maximum wind velocity of a storm and is defined as (Bister and Emanuel,2002)

whereCkrefers to the surface enthalpy exchange coefficient,Cdrepresents the drag coefficient,Tois the mean outflow temperature,kis the enthalpy of an ambient boundary layer parcel,andis the air saturation enthalpy at the sea surface.Higher potential intensity means that a TC may achieve greater intensity and thus favors genesis.-krepresent the impacts of thermodynamic efficiency and thermodynamic disequilibrium,respectively.To quantify the contributions of the two terms to potential intensity,both sides of Eq.(1)are taken as natural logarithms:

Moist entropy deficit (χ)is used to measure the moisture content of the mid-troposphere.It is defined as(Emanuel et al.,2008)

wheres＊andrepresent the moist entropies saturated in the free troposphere(600 hPa)and at the sea surface,respectively,andsmrefers to the moist entropy in the mid-troposphere (600 hPa).A larger moist entropy deficit means the mid-troposphere is farther from saturation,which is unfavorable for TC genesis.

Vertical wind shear is vital to TC genesis,because larger vertical wind shear could damage the structure of convection and the warm core in the high level of a storm(Korty et al.,2012a).We define vertical wind shear as the magnitude of the horizontal wind difference between 200 and 850 hPa.In addition,low-level(850 hPa)absolute vorticity creates the convergence necessary as the basis of convection development(Nolan et al.,2007).

Next,we use a genesis potential index (GPI) to combine the four genesis factors and examine the potential variation in TC genesis.The GPI used here is defined as (Emanuel,2010;Korty et al.,2012a)

whereais a normalizing coefficient,ηis low-level (850-hPa) absolute vorticity,PI is the potential intensity,χis the moist entropy deficit,and VS is the vertical wind shear between 200 and 850 hPa.More information on the genesis factors and GPI is provided in Yan et al.(2019a).

3.Results

Based on the multimodel ensemble mean(MEM),potential intensity generally exhibits negative anomalies over the WNP during the LIG relative to the preindustrial period in the storm season (from July to October),with local positive anomalies over the southwestern corner(Fig.1(a)).The most intense change in potential intensity locates over the north of the WNP(20?S–30?N).This pattern shows high consistency among the PMIP4 models and suggests a decrease in the theoretical maximum intensity of TCs during the LIG,which is unfavorable for TC genesis.Although SST sees an overall warming over the WNP(Fig.1(d)),the relative SST (defined as the difference between local SST and the tropical mean (20?S–20?N)),an effective approximation for potential intensity (Korty et al.,2012a;Yan et al.,2017),decreases over the central WNP and increases over the southwestern side,resembling the pattern of the potential intensity anomaly(Fig.1(b)).Furthermore,the variation in potential intensity is associated with changes in thermodynamic efficiency and thermodynamic disequilibrium.Thermodynamic efficiency generally decreases over the WNP during the LIG(Fig.1(c)),which is in turn tied to the increased outflow temperature,despite the increase in SST (Fig.1(d)).Compared with the thermodynamic efficiency,the thermodynamic disequilibrium shows a similar pattern to the potential intensity and dominates the variation in potential intensity (Fig.1(e)),which largely arises from the enhanced surface wind speed,based on the equation in Emanuel(2007)(Fig.1(f)).

Fig.1.Differences in genesis factors and large-scale environmental conditions between the LIG and preindustrial period during the storm season based on the MEM:(a)two times the natural logarithm of potential intensity(units:m s-1);(b)relative SST(units: ?C);(c)natural logarithm of thermodynamic efficiency(units:m s-1);(d) SST (units: ?C);(e) natural logarithm of the thermodynamic disequilibrium (units: m s-1);(f) wind speed at 1000 hPa (units: m s-1).Black dots represent the regions where ≥8 models share the same sign of change as the MEM.

The moist entropy deficit shows a general increase over the WNP during the LIG in the storm season,according to the MEM,especially over the northeastern side (Fig.2(a)).The PMIP4 models show close agreement in the sign of change.A larger moist entropy deficit hampers the formation of TCs over the WNP during the LIG.Moreover,the increase in the moist entropy deficit is mainly induced by the decreased moisture in the middle troposphere,which is in turn linked with descending motion(Fig.2(b)).On the other hand,the decreased vertical temperature contrast between the sea surface and mid-troposphere weakens the moisture transport from the sea surface and also contributes to the increased moist entropy deficit (Fig.2(c)).

Vertical wind shear broadly shows a quasi-meridional dipole pattern over the WNP during the LIG based on the MEM,with positive anomalies over the central WNP and negative anomalies over the southwestern WNP(Fig.2(d)).This pattern is found in the majority of PMIP4 models.As strong vertical wind shear obstructs TC genesis,this illustrates a southwestward migration of favorable conditions for TC genesis over the WNP during the LIG.Moreover,the variation in vertical wind shear is largely attributed to the change in upper-level zonal winds.Our results show that westerlies in the upper troposphere are generally strengthened over the WNP (Fig.2(e)).The anomalous westerlies (～5?–25?N)strengthen (weaken) the climatological westerlies (easterlies) over the central(southwestern)WNP,which is in turn linked with the changes in meridional temperature gradient in the troposphere(Fig.2(f)).

The absolute vorticity at the lower level largely decreases over the WNP during the LIG relative to the preindustrial period(Fig.3(a)).This decreased absolute vorticity is observed in the majority of PMIP4 models and suggests that conditions are unfavorable for TC formation.Moreover,change in absolute vorticity may be associated with the circulation anomaly at 850 hPa.Our results illustrate an anomalous anticyclonic circulation in the lower troposphere over the southwestern WNP(Fig.3(b)).This anomalous anticyclone may be further tied to the decreased SST over the western tropical Pacific,which results in suppressed precipitation and diabatic cooling,together with anomalous convergence in the upper level(Fig.3(c)).The anomalous diabatic cooling then triggers the low-level anticyclonic circulation anomaly over the northwest of the heating source,based on the Gill response(Gill,1980).

The MEM of GPI shows a general decrease across the majority of the WNP during the LIG,with a slight increase along the east of the Philippines,which is found in the majority of models (Fig.4(a)).This indicates unfavorable conditions for TC genesis over the WNP and hence a southwestward migration of TC location.Moreover,change in GPI is closely tied to variations in genesis factors.The generally decreased GPI over the WNP is associated with changes in the four genesis factors above,whereas the increased GPI along the east of the Philippines is tied to the reinforced potential intensity and weakened vertical wind shear(Figs.1(a),2(a),2(d),and 3(a)).Furthermore,changes in these genesis factors are linked with variations in environmental conditions discussed above.Additionally,to further analyze the relative contribution of each genesis factor to the GPI change during the LIG relative to the preindustrial period,the GPI is recalculated with three genesis factors in the preindustrial period and the remaining one in the LIG.Thus,the contribution of the remaining factor can be quantified by the difference between the two GPIs(Camargo et al.,2007;Xu and Huang,2015).The results indicate that changes in potential intensity,moist entropy deficit,and vertical wind shear result in the dipole pattern of GPI over the WNP,especially the wind shear,whereas decreased absolute vorticity leads to the decreased genesis potential over the southern WNP(Fig.S1).When averaged over the whole WNP,changes in the four genesis factors all contribute positively to the decreased GPI over the WNP,with more profound effects of vertical wind shear and absolute vorticity,though the spread among individual models for the role of moist entropy deficit is relatively larger(Fig.4(b)).

Fig.4.(a) Differences in GPI between the LIG and preindustrial period during the storm season based on the MEM.Black dots in (a) represent the regions where ≥8 models share the same sign of change with the MEM.(b)Differences in GPI between the LIG and preindustrial period during the storm season(black box)resulting from changes in potential intensity(PI),moist entropy deficit(χ),vertical wind shear(VS),absolute vorticity(AV),and nonlinear effect,spatially averaged over the WNP (5?–30?N,120?E–180?).The units of GPI are events m-2 10-13/ season.(c) Differences in zonal steering flows (shading;units: m s-1) between the LIG and preindustrial period and the climatological steering flows(vectors;units:m s-1)during the preindustrial period in the storm season based on the MEM.

As the GPI provides little information on TC tracks,we further discuss the changes in large-scale steering flows over the WNP during the LIG.The steering flows here are defined as pressure-weighted tropospheric mean flow from 850 to 300 hPa and are widely used to represent the TC motion during the storm season (e.g.,Wu and Wang,2004).The MEM results show that anomalous easterlies locate at the southern (～5?–15?N) and northern (～30?–40?N) parts of the WNP(Fig.4(c)).This suggests more favorable conditions for westward TC tracks.Taking the changes in GPI and steering flows into consideration,it can be summarized that conditions are unfavorable for TC genesis over the WNP during the LIG relative to the preindustrial period,but there may be an increased possibility for TC motion shifting westward.

4.Conclusions

In this study,we use the outputs from 11 PMIP4 models to investigate the changes in large-scale genesis factors over the WNP during the LIG.Our results indicate that potential intensity exhibits a general decrease over the majority of the WNP during the LIG,dominated by a weakened thermodynamic disequilibrium.The moist entropy deficit is generally increased over the WNP during the LIG,especially over the northeastern side,which arises from decreased mid-tropospheric moisture and a weakened vertical temperature contrast between the sea surface and mid-troposphere.Vertical wind shear shows a dipole pattern over the WNP,with enhanced wind shear over the central WNP and a weakened one over the southwestern side,which is induced by the changes in meridional tropospheric temperature gradient and hence high-level zonal winds.The absolute vorticity largely reduces over the WNP,which is explained by the decreased SST over the western tropical Pacific and thereby the low-level anticyclone anomaly over the southwestern WNP induced by diabatic cooling.

Regarding the GPI,the MEM shows a general decrease in genesis potential over the WNP during the LIG,except for east of the Philippines where the GPI becomes higher,indicating a southwestward migration of TC location.Changes in all four genesis factors contribute positively to the decreased GPI over the WNP,with dominant roles of vertical wind shear and absolute vorticity.Additionally,the anomalous westward steering flows imply that there is increased favorability for a westward TC track during the LIG relative to the preindustrial period.

Our results suggest a decrease in genesis potential and unfavorable conditions for TC formation over the WNP during the LIG.It is meaningful to compare our findings with those in other past warm periods and future projections.Korty et al.(2012b) indicated a southwestward shift of genesis potential over the WNP during the mid-Holocene based on PMIP2 models,later confirmed by models from PMIP3 (Koh and Brierley,2015).Additionally,decreased genesis potential was found over the WNP during the mid-Piacenzian warm period as well,based on PMIP3 models (Koh and Brierley,2015) and Community Earth System Model (Yan et al.,2019b).These results suggest a similar GPI change between the LIG and other past warm intervals,probably reflecting the relatively consistent response of TC genesis potential to rising temperature in the past.Besides,TC frequency is projected to decrease over the WNP for a 2?C anthropogenic warming,but with low confidence(Knutson et al.,2020).These projections share similarities with the decreased genesis potential over the WNP during the LIG.

There are,however,limitations that should be noted.As TC records are only traced back to several millennia,it is hard to constrain our results directly.Besides,as the GPI used here provides little information on TC tracks and landfall,high-resolution regional climate simulations are needed to achieve more detailed information on TC activity over the WNP during the LIG,although we have analyzed the changes in steering flows.Nevertheless,our investigation places the behaviors of TCs in the context of a past warm period that shares similarities with the present and near future.Our results may advance our understanding of the response of TCs to rising temperatures induced by Earth’s orbit and shed light on TC activity in a warming future.

Funding

This study was funded by the National Natural Science Foundation of China [grant number 41888101].

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.aosl.2023.100395.