Intensified Impact of Northern Tropical Atlantic SST on Tropical Cyclogenesis Frequency over the Western North Pacific after the Late 1980s

Xi CAO,Shangfeng CHEN*,Guanghua CHEN,and Renguang WU，2

1Center for Monsoon System Research,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

2State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

Intensified Impact of Northern Tropical Atlantic SST on Tropical Cyclogenesis Frequency over the Western North Pacific after the Late 1980s

Xi CAO1,Shangfeng CHEN*1,Guanghua CHEN1,and Renguang WU1，2

1Center for Monsoon System Research,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

2State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029

Previous studies suggest that spring SST anomalies over the northern tropical Atlantic(NTA)affect the tropical cyclone (TC)activity over the western North Pacific(WNP)in the following summer and fall.The present study reveals that the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is not stationary.The influence of spring NTA SST on following summer–fall WNP TC genesis frequency is weak and insignificant before,but strong and significant after,the late 1980s.Before the late 1980s,the NTA SST anomaly-induced SST anomalies in the tropical central Pacific are weak,and the response of atmospheric circulation over the WNP is not strong.As a result,the connection between spring NTA SST and following summer–fall WNP TC genesis frequency is insignificant in the former period.In contrast, after the late 1980s,NTA SST anomalies induce pronounced tropical central Pacific SST anomalies through an Atlantic–Pacific teleconnection.Tropical central Pacific SST anomalies further induce favorable conditions for WNP TC genesis, including vertical motion,mid-level relative humidity,and vertical zonal wind shear.Hence,the connection between NTA SST and WNP TC genesis frequency is significant in the recent period.Further analysis shows that the interdecadal change in the connection between spring NTA SST and following summer–fall WNP TC genesis frequency may be related to the climatological SST change over the NTA region.

northern tropical Atlantic SST,tropical cyclone,western North Pacific

1.Introduction

The western North Pacific(WNP)is the most active region of tropical cyclone(TC)genesis around the world (Chan,2005).TCs can lead to disasters in many places (Emanuel,2005).A good understanding of TC activity may help to improve seasonal prediction of TC activity.Over the tropical Pacific,ENSO is a dominant mode of interannual variability,and contributes significantly to TC genesis variability over the WNP(e.g.,Chen et al.,1998;Chia and Ropelewski,2002;Chan,2005;Cao et al.,2014a;Hsu et al.,2014).For example,the frequency of TC genesis is above normal in the southeastern part but below normal in the northwestern part of the WNP during El Ni?o developing summers(Wang and Chan,2002;Li,2012;Wu et al.,2012).In addition,SST anomalies in the tropical Indian Ocean have also been found to exert a significant influenceon TC genesis throughatmosphericcirculationanomalies over the WNP(Saji and Yamagata,2003;Yoo et al.,2006;Yang et al.,2007,2010;Zhou and Cui,2008;Xie et al.,2009;Luo et al.,2012).Xie et al.(2009)suggested that the Indian Ocean SST anomalies can persist through the active TC season after an El Ni?o event has dissipated. SST anomalies in the Indian Ocean can force a Kelvin wave to the east that extends into the WNP and exerts a significant influence on TC genesis(Du et al.,2011;Zhan et al., 2011).Zhan et al.(2014)further showed that the connection between eastern Indian Ocean SST and WNP TC genesis experienced a significant interdecadal change around the late 1970s.They found that the impact of the eastern Indian Ocean SST anomalies on WNP TC genesis is significant only after the late 1970s.Zhou and Cui(2011)found that the spring SST located to the east of Australia is an effective predictor for summer TC frequency over the WNP.In addition, other factors,such as North Pacific Oscillation(Chen et al., 2014),stratospheric quasi-biennial oscillation(Chan,1995; Ho et al.,2009),NorthAtlanticOscillation/ArcticOscillation (Choi et al.,2012;Zhou and Cui,2014;Cao et al.,2015),and Antarctic Oscillation(Wang and Fan,2007),have been re-portedto contributesignificantlytothe interannualvariability of TC activity over the WNP.

?Institute of Atmospheric Physics/Chinese Academy of Sciences，and Science Press and Springer-Verlag Berlin Heidelberg 2016

Studies have found that the SST variability in the Atlantic Ocean,such as the northern tropical Atlantic(NTA) mode,Atlantic meridional mode,and Atlantic zonal mode, affects not only the weather and climate over the Atlantic (Goldenberget al.,2001;Vimont and Kossin,2007),but also the variability and predictability of ENSO(Dommenget et al.,2006;Losada et al.,2009;Ding et al.,2012;Ham et al.,2013).For example,Losada et al.(2009)investigated the tropical response to the tropical Atlantic SST anomalies using atmospheric general circulation models.They showed that SST anomalies in the tropical Atlantic produce a Gilltype response extending into the tropical eastern Pacific. Ham et al.(2013)suggested that SST anomalies over the NTA region during boreal spring may serve as a trigger for ENSO outbreaks based on observational and modeling studies.They found that spring NTA SST anomalies may exert a pronounced influence on the SST anomalies in the central–eastern Pacific in the following winter via the Atlantic to Pacific teleconnection.It should be mentioned that,in addition to ENSO,these Pacific circulation changes induced by NTA SST anomalies can also significantly modify the large-scale dynamic and thermodynamic conditions over the WNP,such as 850-hPa vorticity,700-hPa relative humidity,500-hPa vertical velocity,and barotropic energy conversion,which are closely associated with TC genesis(Huo et al.,2015;Yu et al.,2015).

Recently,Chenet al.(2015)foundthat theNTASST variability exhibits significant interdecadal changes.The standard deviation of NTA SST is relatively low from the 1900s to the early 1910s,around the 1940s,and during the 1970s to 1980s,but relatively high during the 1890s,from the 1920s to mid-1930s,around the 1960s,and after the late-1980s. Yu et al.(2015)has shown that spring NTA SST anomalies may exert pronounced influences on the SST anomalies in the central–eastern Pacific in the following summer, which could further significantly modify the large-scale conditions overthe WNP.Therefore,anychanges in the response over the central and eastern Pacific on the decadal timescale may cause changes in the relationship between WNP TC frequency and NTA SST anomalies.The main objective of the study is to document the decadal change of the interannual relationship between WNP TC frequency and NTA SST anomalies,and to examine the physical mechanisms responsible for such a change.A better understanding of change in therelationshipbetweenNTASSTandWNP TCactivitymay provide important information for the seasonal prediction of WNP TC activity.

Following this introduction,we describe the data and analysis methods in section 2.Section 3 presents evidence for the interdecadal change of the relationship between boreal spring NTA SST and following summer–fall WNP TC genesis frequency.Section 4 compares the spring NTA SST-related anomalies prior to and after the interdecadal shift to examine the plausible factors involved.A summary and discussion are given in section 5.

2.Data and method

In this study,the annual TC frequency over the WNP is obtained from the National Climate Data Center’s International Best Track Archive for Climate Stewardship(IBTrACS),Version3(Knappet al.,2010).TC datasets arecombined from differentoperationalcenters aroundthe world,including the Joint Typhoon Warning Center(JTWC),Shanghai Typhoon Institute of the China Meteorological Administration,the Regional Specialized Meteorological Center of the Japan Meteorological Agency(JMA),and others(refer to http://www.ncdc.noaa.gov/ibtracs/index.php).IBTrACS provides datasets in popular formats to facilitate analysis. Thus,the JTWC TC best-track dataset from IBTrACS is used in the study.To minimize uncertainty in identifying weak systems,TC genesis is defined as the first record of TC best-track data when the maximum wind speeds reach 20 kts (～10.3 m s-1)over the WNP.This TC definition is consistent with the study of Molinari and Vollaro(2013).We also use 25 kts(～12.9 m s-1)as a reference for TC genesis and find that the results are not sensitive to the definition of TC genesis(figure not shown).When individual years are considered,only the means in June–November(hereafter JJASON) are used because this period covers almost 80%–90%of the total climatological number of TCs over the WNP(Yeh et al.,2010).The analysis is focused upon the region extending from 120°E to the date line.The South China Sea is omitted to avoid the different interannual variation of large-scale circulations between the west and east of the Philippines(Chen et al.,1998).In addition,the TC genesis number east of the datelineismuchlowerthanthatwest ofthedateline(Lander, 1994).For these two reasons,we concentrate our analysis on the region of(0°–40°N,120°E–180°).The TC genesis frequency index is defined as the total genesis number of TCs during the JJASON season over the WNP.

The monthly mean SST data with two horizontal resolutions are extracted from the National Oceanic and Atmospheric Administration Extended Reconstructed SST version 3b(NOAA ERSSTv3b)(Smith et al.,2008).The relative humidity and atmospheric variables are taken from the National Centers for EnvironmentalPrediction–NationalCenter for Atmospheric Research(NCEP–NCAR)with a 2.5°horizontal resolution(Kalnay et al.,1996).The time period analyzed in this study is from 1958 to 2012.

Following previous studies(Ham et al.,2013;Huo et al.,2015),the NTA SST index is defined as the normalized area-averaged SST over the region of(0°–25°N,80°–15°W)during boreal spring(March–April–May,MAM).It is well known that the NTA SST anomalies are affected by the ENSO variability in the preceding winter(Alexander and Scott,2002;Chiang and Sobel,2002).To exclude the influence of ENSO on the connection between NTA SST and WNP TC activity,the ENSO signal in the preceding winter(December–January–February,DJF)is removed from the normalized NTA SST time series by means of linear regression with respect to the DJF Ni?o3.4 index.The Ni?o3.4 index is defined as the area-averaged SST anomalies over theregion of(5°S–5°N,170°E–120°W).In addition,following the studies of Ham et al.(2013)and Huo et al.(2015),longterm trends of the NTA SST index,TC genesis frequency index,and all other monthly mean variables are removed,unless otherwise specified.

3.Interdecadal change in the relationship between NTA SST and TC genesis

The normalizedtime series of the original(i.e.,long-term trend is not removed)MAM NTA SST index and the following JJASON WNP TC genesis frequency index are shown in Fig.1a.The correlation coefficient between these two time series is-0.38 for the analyzed period(1958–2012),which is statistically significant at the 95%confidence level based on the two-sided Student’s t-test.However,the relationship between these two indices appears to not be steady during 1958–2012.This is indicated by the sliding correlations between MAM NTA SST and JJASON WNP TC genesis frequency(red solid line shown in Fig.1b).Significant and negative correlations can be observed after the late 1980s, while the correlations before the late 1980s are statistically insignificant(Fig.1b).The above results may suggest an interdecadal change in the influences of MAM NTA SST on following JJASON WNP TC genesis frequency.

As the spring NTA SST is affected by the ENSO variability in the preceding winter,the observed change in the above relationship may be contributed by the change in ENSO.To examine this possibility,Fig.1b shows the 21-yr sliding correlationsbetweenthe NTA SST andTC genesisfrequencyindices after removingthe precedingwinter ENSO signal(blue dashed line shown in Fig.1b).The interdecadal change in the connection between MAM NTA SST and following JJASON WNP TC genesis frequency around the late 1980s can still be detected after removing the preceding winter ENSO signal.This confirms an intensified impact of the MAM NTA SST on following JJASON TC genesis frequency over the WNP around the late 1980s.We also examine the running correlation coefficient between the NTA SST and TC genesis frequency with windows of 19 and 23 years(figures not shown).It is found that although the correlation coefficient in a given year is related to the length of window,the interdecadal change in the connection between MAM NTA SST and JJASON TC genesis frequency around the late 1980s is still observed from both sliding correlations.Please note that in the following analysis,the precedingwinter ENSO-related anomalies have been removed from the NTA SST index.

Fig.1.(a)Normalized time series of the original MAM NTA SST index(red solid line)and the original following-JJASON TC genesis frequency index derived from the JTWC(blue dashed line)over the WNP.(b)The red solid line denotes the 21-yr sliding correlations between the original MAM NTA SST index and the original following-JJASON WNP TC genesis frequency index.The blue dashed line in(b)is the same as the red solid line but the long-term trend and preceding winter ENSO signal have been removed from the MAM NTA SST index and JJASON WNP TC genesis frequency index.The horizontal black line in(b)indicates the 95% confidence level.

To better examinethe interdecadalchangeof the relationship between the NTA SST and WNP TC genesis frequency, we select two periods to contrast the difference.From the 21-yrsliding correlations between the NTA SST and TC genesis frequency indices(blue dashed line shown in Fig.1b), the two selected periods are 1991–2011,when the negativecorrelation is the lowest(i.e.,-0.65),and 1968–88,when the correlation is the highest(i.e.,0.27).In addition,we objectively examine the interdecadal change point of the relationship by employing the Bayesian change-point method(Chu and Zhao,2004;Zhao and Chu,2010).A shift in a posterior probability in the late 1980s is detected by the Bayesian change-pointmethod(Fig.2).Hence,the selectionof the two periods of 1968–88 and 1991–2011is reasonable.

We also calculate 21-yr sliding correlations between the DJF Ni?o3.4 index and the following JJASON TC genesis frequency index over the WNP.The correlation between the two indices is insignificant(figure not shown).This indicates that the preceding DJF ENSO events cannot exert influences on the total TC genesis number over the WNP during the active TC seasons in the recent six decades,consistent with the results obtained by previous studies(e.g.,Lander,1994; Wang and Chan,2002;Chen and Huang,2008;Huo et al., 2015).

4.Mechanisms for interdecadal change in the relationship

Fig.2.(a)The posterior probability of the number of change points associated with the 21-yr sliding correlation coefficients between MAM NTA SST and the following JJASON WNP TC genesis frequency.(b)The posterior probability mass function for the first change point.

In this section,we investigate the plausible reasons related to the intensified impact of spring NTA SST on the following JJASON TC genesis frequency over the WNP around the late 1980s.It is generally accepted that seasonal TC activity is potentially controlled by the large-scale circulation patterns and thermodynamical conditions(Gray,1968;Camargoet al.,2007;Emanuel,2007).For this purpose,we first compare the MAM NTA SST-related atmospheric dynamic and thermodynamic anomalies between 1968–88 and 1991–2011.The anomalies are obtained by regression with respect to the MAM NTA SST index in the two selected periods,respectively.Figure3 shows the followingJJASON lower-level (850 hPa)and upper-level(200 hPa)circulation anomalies. Figure 4 displays anomalies of following JJASON SLP,500-hPa vertical velocity,600-hPa relative humidity,and vertical zonal wind shear between 200 hPa and 850 hPa.

During 1991–2011,obvious easterly wind anomalies are observedoverthetropicalPacificandsoutherlyanomaliesare present over subtropical East China.In addition,significant and positive SLP anomalies are seen over the WNP(Figs.3b and 4b).During 1968–88,lower-level circulation and negative SLP anomalies over the WNP are insignificant(Figs. 3a and 4a).Meanwhile,during the latter period,significant upper-level cyclonic shear anomalies are observed over the subtropical North Pacific along 20°N,which are accompanied by obvious westerly wind anomalies over the tropical Pacific along 120°–180°E and obvious easterly wind anomalies overthe subtropicalPacific between30°N and40°N(Fig. 3d).During the former period,the upper-level circulation anomalies are insignificant over the WNP(Fig.3c).

During 1991–2011,significant positive 500-hPa vertical velocity and negative 600-hPa relative humidity anomalies are mainly located in the areas of(10°–20°N,140°E–180°) and(0°–20°N,160°E–180°),respectively.Positive anomalies of vertical zonal wind shear between 200 hPa and 850 hPa are observedduringthe activeTC seasons overthe whole tropical WNP.These imply a weakenedupward motion,drier air at the middle level,and larger vertical zonal wind shear during the latter period(Figs.4d,f and h).These conditions are associated with the decrease in TC genesis frequency over the WNP in response to the positive NTA SST anomalies(Gray,1968;McBride and Zehr,1981;Emanuel,2007; Nolan,2007;Cao et al.,2014b,2014c).During 1968–88,the vertical motion anomalies,relative humidity anomalies,and vertical zonal wind shear anomalies are much weaker(Figs. 4c,e and g).Therefore,MAM NTA SST anomalies impose a muchstrongerimpactonTCgenesisfrequencyovertheWNP in the following summer and fall during the latter period than those during the former period.

To confirm that the above results are independent of the selectedreanalysisdata,atmosphericcirculationanomaliesin association with the normalized MAM NTA SST index during the former and latter periods are further calculated using the 40-yrEuropeanCenter for Medium-RangeWeather Forecasts(ECMWF)Re-Analysis(ERA-40)(Uppala et al.,2005) and ECMWF Interim Re-Analysis datasets(ERA-Interim) (Dee and Uppala,2009).The ERA-40 dataset is available from September 1957 to August 2002 and ERA-Interim is available since 1979.Thus,atmospheric circulation anomalies associated with MAM NTA SST during the former and latter epochs are calculated from ERA-40 and ERA-Interim, respectively.Results show that significant differences in at-mospheric circulation anomalies associated with MAM NTA SST during the former and latter periods can be captured by ERA-40 and ERA-Interim(Figs.3 and 5).This indicates that theresults derivedfromtheNCEP–NCAR reanalysisdataare robust.

Fig.3.Anomalies of(a,b)850 hPa and(c,d)200 hPa winds in the following JJASON obtained by regression with respect to the normalized MAM NTA SST index in(a,c)1968–88 and(b,d)1991–2011.The shading in the figures indicates that the anomalies in either direction are statistically significant at the 95%confidence level.The wind vector scale is shown in the top right(units:m s-1).

So far,we have simply demonstrated the existence of the interdecadal change in the dynamic and thermodynamicconditions associated with the JJASON WNP TC genesis frequency in response to the MAM NTA SST anomalies.Next, the possible mechanisms responsible for the interdecadal change of this remote teleconnection are further examined. We perform a linear regression analysis of the MAM NTA SST with global SST,lower-level winds,and mid-level vertical motion at various lags.The anomalies are obtained from regression on the normalized MAM NTA SST index in the two periods,respectively.Figure 6 shows the seasonal evolution of SST anomalies from the simultaneous MAM to the following SON.Figure 7 displays the associated seasonal evolutionof850-hPacirculationanomalies.Figure8displays anomalies of 500-hPa vertical velocity.

During1991–2011,negativeSST anomaliesareseenover the northeastern and southeastern Pacific in MAM,and then extend westward to the equatorial central Pacific(Figs.6b, d and f).The patterns of SST anomalies bear a close resemblance to those in the central Pacific La Ni?a developing phase(Ashok et al.,2007;Weng et al.,2007;Kao and Yu,2009;Kug et al.,2009).During boreal MAM,there are significantly positive SST anomalies in the NTA and southern Atlantic regions(Fig.6b).The warm NTA SST anomalies persist into the following JJA and SON(Figs.6d and f). Significant lower-level cyclonic circulation anomalies appear in the MAM over the North Atlantic and northeastern Pacific as a Gill-type Rossby wave response induced by the enhanced local convection(Figs.7b and 8b)(Matsuno,1966; Gill,1980).The northerly wind anomalies on the west side of the anomalous lower-level cyclonic circulation lead to the decrease in SST through enhancing total wind speed over the northeastern Pacific(Figs.6d and 7d).The surface cooling over the northeastern Pacific further results in a decrease in convective activity(Fig.8d),which in turn contributes to the formation of lower-level anticyclonic circulation anomalies over the subtropical central–eastern Pacific via a Gilltype atmospheric response(Ham et al.,2013).The northerly wind anomalies in the northeastern Pacific and easterly wind anomalies in the tropical central Pacific associated with the anticycloniccirculationanomaliesin JJA furtherintensifythe surface cooling in the central Pacific through local air–sea feedback(Figs.6d,7d and 8d).Via the above-mentioned positive air–sea feedback mechanism,the anomalous anticycloniccirculationoverthe western–centralPacific inducedbytheSSTcoolingsustains intoSON(ChenandTam,2010)and induces significant easterly wind anomalies over the tropical western Pacific(Figs.6f,7f and 8f).

Fig.4.As in Fig.3 but for anomalies of(a,b)SLP,(c,d)500-hPa vertical velocity,(e,f)600-hPa relative humidity, and(g,h)vertical zonal wind shear between 200 hPa and 850 hPa.Contour intervals are 0.1 hPa in(a,b),0.002 Pa s-1in(c,d),1%in(e,f),and 1 m s-1in(g,h).

The above-mentioned processes are consistent with the findings obtained by Ham et al.(2013)and Huo et al.(2015). Based on the study of Ham et al.(2013),NTA SST anomalies can modulate atmospheric circulation anomalies over the WNP through an eastward-propagating response from the northern Atlantic to the eastern Indian Ocean in addition to the above subtropical westward-propagating teleconnection from the northern Atlantic to the central–eastern Pacific. However,the SST signal over the eastern Indian Ocean from MAM to SON is extremely weak(figure not shown).It indicates that the Atlantic to Pacific teleconnection is fundamental in the response of TC genesis activity over the WNP to NTA SST anomalies.

Fig.5.As in Fig.3 but from(a,c)ERA-40 in 1968–88 and(b,d)ERA-Interim in 1991–2011.The shading in the figures indicates that the anomalies in either direction are statistically significant at the 95%confidence level.The wind vector scale is shown in the top right(units:m s-1).

During 1968–88,although positive SST anomalies over the NTA regions are significant during MAM,they become weaker in the following JJA and SON(Figs.6a,c and e). The MAM NTA SST-related negative SST anomalies over the subtropical northeastern Pacific from MAM to SON are much weaker than those during 1991–2011(Figs.6a,c and e).This indicates that the MAM NTA SST anomalies do not have a significant impact on the SST anomalies over the central–eastern Pacific during the former period.Accordingly,the lower-level wind and vertical motion anomalies from the NTA to the WNP are relatively weaker during the former period compared with those during the latter period (Figs.7a,c and e,and Figs.8a,c and e).As a result,the connectionbetweenthespringNTA SST andfollowingsummer–fall TC genesis frequency over the WNP is weak during the former period,as shown in Fig.1b.

It has been shown that the MAM NTA SST anomalies can influence the JJASON WNP TC genesis frequency via the SST anomalies over the central Pacific(e.g.,Chen and Tam,2010;Huo et al.,2015).Figure 9 illustrates the monthly evolution of anomalies of the 850-hPa zonal winds averaged along0°–20°N and SST averagedalong 10°S–10°N obtained fromregressionon the normalizedMAM-averagedNTA SST index from March to November during the former and latter periods.It can be seen that the most remarkable differences between the two periods are the negative SST anomalies overthecentral–easternPacific andeasterlywind anomalies over the western-central Pacific.During the latter period, significant and negativeSST anomalies are observedover the central–eastern Pacific and significant easterly wind anomalies are present over the western–central Pacific(Fig.9b). By contrast,SST and zonal wind anomalies are weak and insignificant during the former period(Fig.9a).This indicates that the Atlantic–Pacific teleconnection can be initiated by the MAM NTA SST during the latter period.Next,we further analyze the possible reasons for the difference in the Atlantic–Pacific teleconnectionbetweenthe formerand latter periods.

Hamet al.(2013)demonstratedthatnegativeSST anomalies over the subtropical northeastern Pacific are induced by the cycloniccirculationanomalies,whichare a Matsuno–Gill type response by the positive NTA SST anomalies.Northeasterly wind anomalies to the west of the cyclonic circulation anomalies over the subtropical northeastern Pacific enhance climatological wind speeds,increase upward surface latent heat flux,and result in local SST cooling.Negative SST anomalies further contribute to the formation of easterly wind anomalies over the tropical WNP,and finally result in negative SST anomalies over the tropical central–eastern Pacific in the following winter.From Figs.6a and 7a,negative SST anomalies and cyclonic circulation anomalies in MAM duringtheformerperiodoverthesubtropicalnortheasternPacific are much weaker than those during the latter period. Therefore,the MAM NTA SST anomalies cannot prolongtheir influence on the following JJASON tropical central Pacific SST.It is speculatedthatthe weakerSST anomaliesover the subtropicalnortheasternPacific in the formerperiodcompared with the latter period can be attributed to the fact that theNTASST-inducedcycloniccirculationanomaliesoverthe subtropical northeastern Pacific and Atlantic are weaker in the former period(Figs.6a and b,and Figs.7a and b).

Fig.6.As in Fig.3 but for the SST in the(a,b)simultaneous MAM and following(c,d)JJA and(e,f)SON.Contour interval for SST:0.2°C.

Fig.7.As in Fig.3 but for the 850-hPa wind(units:m s-1)anomalies in the(a,b)simultaneous MAM and following (c,d)JJA and(e,f)SON.The“C”in(b)denotes the cyclonic circulation.

In order to understand the difference in the NTA SST-related cyclonic circulation anomalies over the northeastern Pacific and Atlantic,Fig.10 displays the difference in mean

Fig.8.As in Fig.3 but for the 500-hPa vertical velocity(10-3Pa s-1)anomalies in the(a,b)simultaneous MAM and following(c,d)JJA and(e,f)SON.

Fig.9.Longitude–time cross section of anomalies of the 850-hPa zonal winds(contours)averaged along 0°–20°N and SST(shaded)averaged along 10°S–10°N obtained as regression on the normalized MAM NTA SST index from March to November in(a)1968–88 and(b)1991–2011.Only the values with an at least 90%confidence level are shown.Contoured and shaded intervals for 850-hPa zonal winds and SST are 0.4 m s-1and 0.1°C,respectively.

Fig.10.Difference in mean SST(units:°C)in MAM between 1968–88 and 1991–2011.The shading indicates that the SST difference is significant at the 95%confidence level.

SST between the latter and former periods.The mean SST during the latter epoch is significantly higher than that during the former period over the NTA region,which is consistent with the finding of Chen et al.(2015).Given the same SST anomalies in the NTA region,the atmospheric circulation wind response over the subtropical northeastern Pacific and Atlantic may be more significantly related to a larger climatological mean NTA SST pattern,attributable to the nonlinearity of dependenceof atmosphericheating on mean temperature.The stronger cyclonic circulation anomalies over the subtropical northeastern Pacific result in larger negative SST anomalies via change in surface net heat flux(Chen et al.,2015).Thus,the mean NTA SST change may be an important reason for the change in the relationship between the MAM NTA SST and following JJASON WNP TC genesis frequency.

5.Summary and discussion

This study presents evidence for an interdecadal change in the linkage between the MAM NTA SST and the following JJASON WNP TC genesis frequency.The connection is weak and insignificant before the late 1980s but strong and significant after the late 1980s.The interdecadal change is associated with the change in SST anomalies in the central Pacific induced by NTA SST anomalies

After the late 1980s,obvious positive SST anomalies in the northern Atlantic from MAM to SON induce remarkable cycloniccirculationanomaliesat the lowerlevel overthe northern Atlantic and northeastern Pacific as a Matsuno–Gill type Rossby wave response.The northerly flows on the west side of cyclonic circulation anomalies over the northeastern Pacific lead to surface cooling through the enhanced wind speed,which leads to lower-level anticyclone flow anomalies in the central–eastern Pacific.The positive feedback between the anticyclonic circulation anomalies and negative SST anomalies may induce a central Pacific type La Ni?o-like SST pattern in the following JJASON and a westward extension of the anticyclonic flows from the eastern Pacific to the WNP.In addition,upper-level cyclonic circulation anomalies,higher SLP,weakened ascending motion,enhancedverticalzonalwindshear,andnegativemid-levelrelative humidity anomalies are significant over the WNP.Those together suppress the TC genesis frequency during JJASON.

In contrast,the NTA SST-related SST over the central–eastern Pacific,lower-level and upper-level winds,vertical motion,and relative humidity anomalies in the WNP are extremely weak before the late 1980s.Thus,the influence of spring NTA SST anomalies on the WNP TC genesis frequency is weak.

Further analysis suggests that the interdecadal shift of the linkage betweenspring NTA SST and followingsummer–fall WNP TC genesis frequency can be attributed to the climatological change of the mean NTA SST.When the mean NTA SST is higher,the NTA SST anomalies can induce larger lower-level circulation anomalies over the subtropical northeastern Pacific.The lower-level circulation anomalies result in SST anomalies over the subtropical northeastern Pacific viamodifyingclimatologicalwindspeeds,whichfurthercontribute to the development of SST anomalies in the tropical central Pacific in the following summer and fall.Significant SST anomalies in the tropical central Pacific play an important role in connecting the effect of the MAM NTA SST on the following JJASON TC genesis frequency over the WNP. A numerical simulation will be conducted in the future to confirm the above hypothesis.

Previous studies have also suggested that the influence of the atmospheric circulation variability and associated SST changes on TC genesis over the WNP is unstable.For example,Cao et al.(2015)documented the weakening of the connection between the spring Arctic Oscillation and the following summer–fall TC genesis frequency over the WNP. Theyindicated that interdecadalchangein the connectionbetween spring Arctic Oscillation and the following summer–fall WNP TC can be attributed to the interdecadal change in the spatial pattern of spring Arctic Oscillation.In order to improve the seasonal prediction of the TC activity over the WNP,mucheffort has beenmade to identifya significant factor contributing to the WNP TC genesis.This study provides observational evidence for the intensified impact of the NTA SST on the WNP TC genesis frequency.Therefore,the results of the current study may potentially help improve the ability to predict seasonal TC activity over the WNP when taking the MAM NTA SST into account.

While this study provides evidence for an intensified impact of the NTA SST on TC genesis frequencyoverthe WNP, several questions need to be further examined in the future. Recent studies show that the SST anomalies over the eastern IndianOceansignificantlyaffect theinterannualvariabilityof TC genesis frequency over the WNP,and an intensified impact of the formeron the latter is apparentafter the late 1970s (Zhan et al.,2011,2014).It is found that when SST anomalies overboththe easternIndianOcean andNTA regionshave thesamesignin MAM,theWNP TC genesisfrequencyinthe followingJJASONisobviouslyenhancedorsuppressed,such as in the years of 1994,1998,2005 and 2010.Thus,the relativecontributionoftheSST anomaliesovertheeasternIndianOcean and NTA regions to the change in JJASON WNP TC genesis frequency will be examined through an atmospheric general circulation model in future work.It is noteworthy that when SST anomalies in both the eastern Indian Ocean and NTA regions are not significant,the WNP TC genesis frequency in the following JJASON is obviously enhanced, such as in the year 1996.This suggests that there may be other factors important for TC genesis variation.This will also be examined in the future.Note too that in this study we do not examine why the mean SST pattern over the Atlantic changed after the late 1980s.Observational records reveal a negative Atlantic multidecadal oscillation(AMO)phase during the late 1960s to the 1990s,followed by a positive phase afterthe1990s(Knightetal.,2005).Thepositivephaseofthe AMO in the recent period might be responsible for the intensified impact of SST over the Atlantic on the WNP TC genesis frequency.Studying this possibility is beyond the scope of the present paper.

Acknowledgements.We thank the three anonymous reviewers for their constructive suggestions and comments,which helped to improve the paper.This study was supported by the National Natural Science Foundation of China(Grant Nos.41505048,41461164005, 41275001,41475074,41505061 and 41475081)and the LASW State Key Laboratory Special Fund(Grant No.2015LASW-B04).

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22 September 2015;revised 29 November 2015;accepted 11 January 2016)

Shangfeng CHEN

Email:chenshangfeng@mail.iap.ac.cn