Impacts of two types of El-Niño on the winter North Pacific storm track

In the present study, the impacts of eastern Pacific (EP) and central Pacific El-Niño on the winter North Pacific storm track (WNPST) are investigated, and the possible reasons for the different responses of the WNPST to the two types of El-Niño are revealed. It is found that only EP El-Niño episodes have a distinct influence on the strength and movement of the WNPST. During EP El-Niño episodes, the WNPST is significantly enhanced and extended equatorward. The patterns of atmospheric baroclinicity anomalies are consistent during the two types of El-Niño. The enhancement and equatorward extension of the WNPST during EP El-Niño episodes can be attributed to anomalous baroclinic energy conversion. In addition, EP El-Niño episodes can also intensify the strength of the WNPST by warming the lower-tropospheric air upstream of the WNPST, which generates more synoptic-scale disturbances entering the WNPST.


Introduction
The North Pacific storm track (NPST) is one of the most active regions of bandpass filtered synopticscale disturbances in the Northern Hemisphere (Blackmon et al 1977) and is consistent with the tracks of surface cyclones and anticyclones in the middle latitudes (Wallace et al 1988, Penny et al 2013. As an important link to the tropical influence in mid-high latitudes (Held et al 1989, Chang et al 2002, synoptic-scale disturbances in the NPST occur frequently and propagate eastward (Charney 1947, Blackmon 1976, Zhao and Liang 2019, transporting abundant moisture, kinetic energy and heat poleward. As a result, NPST exerts large influence, especially in winter, on atmospheric circulation and climate variability (Jin 2010, Luo et al 2016.
Previous studies have examined significant intraseasonal (Lau 1988, Nakamura and, seasonal (Nakamura 1992, Lee et al 2011, interannual (Nakamura et al 2002, Harnik andChang 2004 in the NPST. The upper-tropospheric jet over the North Pacific and El Niño-Southern Oscillation (ENSO) prominently modulate the interannual variation in the NPST Shukla 1997, Harnik andChang 2004). During El-Niño episodes, the winter NPST (WNPST) and upper-tropospheric jet are intensified and moves southeastward due to the strengthened Hadley circulation over the East Pacific and vice versa during La-Niña episodes (Held et al 1989, Trenberth and Hurrell 1994, Chang et al 2002, Wang et al 2017. In recent years, it has been widely accepted that El-Niño episodes can be divided into two types, canonical eastern Pacific (EP) El-Niño episodes and central Pacific (CP) El-Niño episodes (Kao and Yu 2009), according to the locations of the maximum sea surface temperature anomalies (Yu and Kao 2007, Ashok et al 2007. Previous studies noted significant differences between the two types of El-Niño on the global climate (Ashok et al, 2009, Kug et al 2009, Mo 2010, Wang et al 2014, Infanti and Kirtman 2016, Tan et al 2016. In terms of the storm track, Ashok et al (2009) reported that during CP El-Niño episodes, the strength of the austral winter storm track (AWST) over Australia is weakened prominently due to anomalous blocking over Australia, while the AWST over South America is intensified. The researchers believed that the influences of CP El-Niño on the AWST in the Southern Hemisphere are distinctly different from and stronger than those of EP El-Niño (Ashok et al 2007).
Thus, two questions are put forward: do the impacts of the two types of El-Niño on the WNPST have significant and robust differences like those in the Southern Hemisphere? If so, what is the reason for the differences between the two types of El-Niño? As a useful and practical tool, the energy budget diagnosis has widely been applied to investigate the storm track variations (Mak and Cai 1989, Lee et al 2012, Wang et al 2017, Ma and Zhang 2018, Zhao and Liang 2019. In addition, the variations of storm track intensity may also be attributed to the upstream seeding effect by the amount of baroclinic wave activity entering the storm track (Orlanski 2005, Lee et al 2010, Penny et al 2013. To answer these two questions, we investigate the impact of El-Niño on the WNPST again by distinguishing the EP El-Niño and the CP El-Niño to verify whether there exists a difference, and we attempt to find the possible reason by analysing the role of atmospheric baroclinicity, the energy conversion and the upstream seeding effect.

Data and methods
The daily atmospheric data used in this research, including horizontal winds and air temperature, are provided by the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) global atmospheric reanalysis datasets (Kalnay et al 1996) with a horizontal resolution of 2.5 • × 2.5 • . Monthly sea surface temperature (SST) data from the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) dataset (Rayner et al 2003) with a horizontal resolution of 1 • × 1 • are utilized. The time span of the data used in this study is 1948 to 2018. The winter in the present study refers to the time period of December to February.
The Lanzcos bandpass filter is used to isolate synoptic-scale (2.5-6-day) disturbances from the NCEP/NCAR daily data. The WNPST in the free atmosphere is characterized by the meridional eddy heat flux at 850 hPa (Nakamura et al 2002, Gan andWu 2015), the eddy kinetic energy at 500 hPa (Takahashi andShirooka 2014, Afargan andKaspi 2017) and the variance in meridional wind velocity at 300 hPa (Lee et al 2010, Hoskins andHodges 2019).
To distinguish the EP and CP El-Niño episodes, the method from Ham and Kug (2012) is adopted for simple calculation. Specifically, if the normalized winter mean Niño 3 (4) index is larger than its one

Event
Years selected in observations EP El-Niño 1958, 1966, 1973, 1983, 1987, 1992, 1998CP El-Niño 1969, 1988, 1995, 2005and 2015 standard deviation and the magnitude of the Niño 3 (4) index is larger than the Niño 4 (3) index, an episode is considered to be an (a) EP (CP) El-Niño. Based on this method, table 1 presents the El-Niño classification and the years of two types of El-Niño episodes from the HadISST dataset. In addition, we also use the Niño index in Ren and Jin (2011) to classify El-Niño episode to verify our results (not shown). It is found that the results are consistent. Figure 1 shows the composite WNSPT anomalies related to the two types of El-Niño. From figure 1, we can find that the EP and CP El-Niño episodes have totally different impacts on the strength and movement of the WNPST. In the lower troposphere, a negative meridional eddy heat flux anomaly appears over southern Alaska in EP El-Niño years (figure 1(a)), while positive anomalies occur in the equatorward flank of the climatological mean. It is obvious that the peak area and southeastern part of the climatological eddy kinetic energy are prominently strengthened in the middle troposphere in EP El-Niño years (figure 1(b)), whereas the northern part over the Bering Sea is remarkably attenuated. In addition, the response of the meridional wind variance to the EP El-Niño case has a well-defined meridional asymmetric structure in the upper troposphere (figure 1(c)), with a robust positive anomaly emerging in the region south of 45 • N, especially the southern climatology of the meridional wind variance, and some slightly negative anomalies occur in the poleward flank. The WNPST in the troposphere is uniformly significantly intensified and extends equatorward. However, except for the appearance of some significant positive anomalies in the two flanks of the climatological mean of the meridional wind variance in the upper troposphere (figure 1(f)), the responses of the WNPST to the CP El-Niño episodes fail the significance test, which indicates that the CP El-Niño has little influence on the strength and movement of the WNPST and is obviously different from the impact of EP El-Niño on the WNPST. To make the results more convincing, a more modern reanalysis dataset, ERA-Interim (Dee et al 2011), is used to corroborate the findings. The different impacts of two types of El-Niño on the WNPST can also be seen in the satellite era (Figures not shown). Considering the consistent variation in the tropospheric WNPST (figure 1), the WNPST is characterized by the meridional eddy heat flux (Nakamura et al 2002, Gan andWu 2015) for convenience in the following chapters.

The role of atmospheric baroclinicity
Previous studies have documented that the WNPST is augmented and extended equatorward during El-Niño episodes without distinguishing different types (Held et al 1989, Straus andShukla 1997), and the anomalies in the WNPST and upper-tropospheric jet are consistent during El Niño events (Liu et al 2014). To identify the role of atmospheric baroclinicity associated with the upper-tropospheric jet over the North Pacific, figure 2 shows the composite zonal wind anomaly at 300 hPa and the vertically integrated (from 925 to 700 hPa) maximum Eady growth rate (EGR) anomaly. According to the linear theory of baroclinic instability, EGR can be calculated by σ = 0.31N -1 f∂u/∂z (Lindzen and Farrell 1980), where σ denotes EGR, N denotes the Brunt-Väisälä frequency, f denotes the Coriolis parameter, and ∂u/∂z is the vertical shear of zonal wind. The EGR acts as a suitable indicator to measure the atmospheric baroclinicity, which is expected to feed synopticscale transient eddies to form a well-organized storm track (Hoskins and Valdes 1990, Kuwano-Yoshida and Minobe 2017. During EP El-Niño episodes, a robust positive upper-tropospheric zonal wind anomaly is slantingly elongated from the Kuril Islands to the Hawaiian Islands and a remarkable negative anomaly arises over the northwest coast of North America ( figure 2(a)). However, the sizable dipole anomalies of upper-tropospheric zonal wind are zonally oblong east of the date line during CP El-Niño episodes ( figure 2(b)). In fact, we can find that there are no fundamental differences between the patterns of upper-tropospheric zonal wind responses to the two types of El-Niño. For the atmospheric baroclinicity represented by the EGR, a significant and robust negative EGR anomaly is located over southern Alaska during EP El-Niño episodes (figure 2(c)), which corresponds well to the negative WNPST anomaly ( figure 1(a)). In addition, the prominent positive EGR anomalies near the Hawaiian Islands contribute to the enhancement and equatorward extension of the WNPST. The pattern of EGR anomalies during CP El-Niño episodes (figure 2(d)) is basically consistent with that during EP El-Niño episodes without fundamental differences due to the inherent connection between the EGR and upper-tropospheric zonal wind. However, the responses of the WNPST to the EP and CP El-Niño are totally different, indicating that the atmospheric baroclinicity anomaly is not the main reason for the different behaviours of the WNPST in response to the two types of El-Niño, and CP El-Niño cannot influence the WNPST by affecting the atmospheric baroclinicity just as EP El-Niño.

Barotropic and baroclinic energy conversion
To reveal the possible reasons for the different behaviours of the WNPST in response to EP and CP El-Niño, the eddy kinetic energy budget which is fundamental for the development of the WNPST (Lee et al 2011) is investigated in the present study. According to Mak and Cai (1989), Cai and Mak (1990), Cai et al (2007) and Ma and Zhang (2018), the barotropic energy conversion (B TEC ) and baroclinic energy conversion (B CEC ) can be expressed as follows: where u, v and ω denote three-dimensional wind, p 0 represents 1000 hPa, g refers to the gravity acceleration, T denotes air temperature, is the gas constant for dry air, C v and C p are the specific heat of dry air at a constant volume and constant pressure, respectively, and θ is the potential temperature. B TEC denotes barotropic energy conversion from the kinetic energy of time-mean flow to the eddy kinetic energy. B CEC1 denotes baroclinic energy conversion from the mean available potential energy to the eddy available potential energy and B CEC2 denotes baroclinic energy conversion from the eddy available potential energy to eddy kinetic energy. The overbar represents the climatological mean. The prime in the superscript represents the 2.5-6-day filtered synoptic component. Figure 3 shows the composite B TEC , B CEC1 and B CEC2 anomalies related to EP and CP El-Niño episodes. Noteworthy negative B TEC anomalies occur near the Hawaiian Islands ( figure 3(a)), resulting in an anomalous B TEC from eddy kinetic energy to the mean kinetic energy of basic flow. Thus, the anomalous B TEC during EP El-Niño episodes attenuates the strength of the southeastern WNPST and is not beneficial to the equatorward extension. Significantly and robustly positive B CEC1 anomalies zonally elongated over the North Pacific ( figure 3(b)) provide a favourable condition for B CEC2 by converting the mean available potential energy to the eddy available potential energy. Figure 3(c) shows that prominently positive B CEC2 anomalies appearing at the southern flank of the WNPST directly enhance the strength of the WNPST; in addition, prominently negative B CEC2 anomalies arising over the west coast of North America weaken the strength of the WNPST, which is conducive to the equatorward and eastward movement of the WNPST. Although the impacts of B TEC and B CEC2 on the strength of the WNPST near the Hawaiian Islands are opposite, the magnitude of the B CEC2 anomaly is larger than that of B TEC , resulting in the southward extension of the WNPST during EP El-Niño episodes. However, the B TEC and B CEC anomalies are not significant in the CP El-Niño episodes (figures 3(d)-(f)), indicating that the barotropic and baroclinic energy conversion may be an important way that the two types of El-Niño modulate the WNPST.

The lower-level temperature upstream of the WNPST
It is widely accepted that EP and CP El-Niño episodes can excite significantly different extratropical teleconnection patterns (Ashok et al, 2009, Kug et al 2009, Mo 2010, Wang et al 2014, Infanti and Kirtman 2016, Tan et al 2016. Considering that the extratropical atmospheric circulations associated with El-Niño have impacts on the strength and movement of the WNPST on an interannual time scale, the EP and CP El-Niño may exert influences on the WNPST by affecting certain extratropical climate systems via atmospheric teleconnections. Lee  Based on the strength index of the WNPST (Yang et al 2020), which sets a threshold that is the median of the WNPST amplitudes of all of the grids within a domain of (25 • -65 • N, 130 • E-120 • W) and then the mean of the values greater than the threshold in all the grids is defined as the strength index of the WNPST, the temperature anomalies at 850 hPa associated with the strength of the WNPST are found to be distinctly positive upstream of the WNPST (figure 4), which is accompanied by an enhanced seeding effect producing more intense storm track eddies (Orlanski 2005).
The lower-level anomalous anticyclone related to the El-Niño over the Northwest Pacific, especially the southerly on the western side, links the East Asian climate and El-Niño episodes (Zhang et al 1996. Figure 5(a) shows that the anomalous anticyclone related to EP El-Niño guides remarkably warm air to move northward over the Kuroshio, which tends to heat the lower-level atmosphere upstream of the WNPST. However, the anomalous southerly during CP El-Niño episodes (figure 5(b)) is not as robust and significant as that during the EP El-Niño episodes due to the far westward anomalous anticyclone related to the westward sinking side of the Walker circulation excited by the CP El-Niño.
The lower-tropospheric prominently positive temperature anomalies over the Northwest Pacific are associated with the EP El-Niño ( figure 5(c)). However, there are no significant temperature anomalies over the Northwest Pacific during the CP El-Niño episodes (figure 5(d)), indicating that the CP El-Niño cannot affect the WNPST by influencing the lowerlevel temperature upstream of the WNPST through the seeding effect.

Summary and discussion
In the present study, based on the NCEP/NCAR reanalysis and HadISST dataset, we investigate the impacts of the EP and CP El-Niño on the WNPST and explore the role of baroclinicity and the possible reasons for the different responses of the WNPST to the two types of El-Niño.
EP and CP El-Niño episodes have totally different impacts on the strength and movement of the WNPST. During the EP El-Niño episodes, the WNPST in the troposphere tends to become significantly enhanced and extend equatorward. However, there is no prominent change in the strength and movement of the WNPST during the CP El-Niño episodes. The anomalous atmospheric baroclinicity related to the upper-tropospheric jet over the North Pacific is found to provide favourable conditions for the enhancement and equatorward extension of the WNPST during both types of El-Niño, which means that the atmospheric baroclinicity cannot explain the behaviour of the WNPST during the CP El-Niño episodes.
The obvious differences of B TEC and B CEC are found between the EP and CP El-Niño episodes. The patterns of prominently anomalous B CEC , including the B CEC1 and B CEC2 , are basically consistent with that of the WNPST during the EP El-Niño episodes. However, the B TEC and B CEC anomalies are not significant in the CP El-Niño episodes. In addition, the significantly anomalous southerlies related to the anomalous anticyclone over the Northwest Pacific induced by the EP El-Niño tend to warm the lower troposphere upstream of the WNPST, resulting in the intensified strength of the WNPST.
It should be noted that the links between baroclinicity, baroclinic conversion and storm track amplitudes are quite complex (Chang et al 2002). In addition, apart from the dry mechanism, such as the aforementioned EGR and energy conversion, the WNPST is also impacted by the moisture effects (Chang et al 2002, Lee et al 2011, including the latent heat release associated with meridional moisture flux and large-scale condensation and the moist baroclinic instability associated with diabatic conversion, which is sometimes more important than the dry mechanism (Ma et al 2017;Kuwano-Yoshida and Minobe 2017). The mismatched patterns between the EGR anomalies and WNPST anomalies may be attributed to the moisture effect.
The contrast of the influences of the two types of El-Niño on the wintertime storm tracks in the Northern Hemisphere and Southern Hemisphere is notable. In addition, corresponding to the two types of El-Niño, the impacts of the two types of La-Niña on the WNPST are investigated (not shown), revealing the similar but opposite results with a significant damping effect of EP La-Niña, instead of CP La-Niña, on the WNPST. In the present study, the asymmetry in the impacts of the two types of El-Niño on the WNPST is only documented through reanalysis data; however, the sample sizes of two types of El-Niño events are limited. Therefore, climate model data with longer time series, such as the Coupled Model Intercomparison Project 6 (CMIP6), is needed to further investigate and verify this issue. In addition, whether two types of El-Niño can affect the WNPST through ocean processes remains to be further studied. Furthermore, a recent study shown that CP El-Niño can be divided into different types (Wang et al 2018). The influence of different types of CP El-Niño on the WNPST is an interesting and leading-edge issue we will focus on next. ance project of the National University of Defense Technology (ZXBJGB02). The authors would like to thank three anonymous reviewers, Professor Sir Brian Hoskins and Akira Kuwano-Yoshida for their helpful and crucial comments. The first author thanks Professor Lifeng Zhang for the useful discussion and constructive comments in the course 'Atmospheric Dynamics and Numerical Simulation' . The NCEP/NCAR reanalysis dataset was obtained online (https://www.esrl.noaa.gov/psd/data/gridded/ reanalysis/). The HadISST SST was obtained online (https://www.metoffice.gov.uk/hadobs/index.html).

Data availability statement
The data that support the findings of this study are openly available at the following URL/DOI: https://www.esrl.noaa.gov/psd/data/gridded/ reanalysis/.