Impact of the El Niño type and PDO on the winter sub-seasonal North American zonal temperature dipole via the variability of positive PNA patterns

In recent years, the winter (from December to February, DJF) North American 27 surface air temperature (SAT) anomaly in midlatitudes shows a “warm west/cold east” 28 (WWCE) dipole pattern. To some extent, the winter WWCE dipole can be considered 29 as being a result of the winter mean of sub-seasonal WWCE events. In this paper, the 30 Pacific SST condition linked to the sub-seasonal WWCE SAT dipole is investigated. It 31 is found that while the sub-seasonal WWCE dipole is related to the positive Pacific 32 North American (PNA + ) pattern, the impact of the PNA + on the WWCE dipole depends 33 on the El Niño SST type and the phase of Pacific decadal Oscillation (PDO). For a 34 central-Pacific (CP) type El Niño, the positive (negative) height anomaly center of 35 PNA + is located in the west (east) part of North America to result in an intensified 36 WWCE dipole, though the positive PDO favors the WWCE dipole. In contrast, the 37 WWCE dipole is suppressed under an Eastern-Pacific (EP) type El Niño because the 38 PNA + anticyclonic anomaly dominates the whole North America. 39 Moreover, the physical cause of why the type of El Niño influences the PNA + is further examined. It is found that the type of El Niño can significantly influence the 41 location of PNA + through changing North Pacific midlatitude westerly winds 42 (NPWWs). For the CP-type El Niño, the eastward migration of PNA + is suppressed to 43 favor its anticyclonic (cyclonic) anomaly appearing in the west (east) region of North American owing to reduced NPWWs. But for the EP-type El Niño, NPWWs are 45 intensified to cause the appearance of the PNA + anticyclonic anomaly over the whole North America due to enhanced Hadley cell and Ferrell cell.

warm and drought in California with a strong cold anomaly in the east of North America, 51 whose temperature anomalies show a North American "warm west/cold east" (WWCE) 52 dipole in midlatitudes (Wang et al .2014;Lee et al. 2015;Hartmann 2015;Singh et al. 53 2016). Such a temperature dipole pattern was expected to frequently occur in the future 54 (Wang et al. 2017;Chien et al. 2019). Thus, the physical cause of the North American 55 WWCE dipole has attracted a great interest of scientists and has been an important 56 research topic in recent years (Wang et al. 2014(Wang et al. , 2015Seager et al. 2016;Lee et al. 57 2015; Yu and Zhang 2015;Hartmann, 2015;Yu et al. 2016Yu et al. , 17, 2018Singh et al. 2016;

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In this paper, we used the k-means clustering method as used in Michelangeli et 145 al., (1995145 al., ( ), Ferranti et al., (2015 and Champagne et al., (2019)  The k-means clustering algorithm was constructed by minimizing the sum of the 150 squares of distances between each sample and the corresponding cluster centroid based 151 on an iterative process, in which the Euclidean distance was used. The detail of this 152 clustering method can be found in Michelangeli et al. (1995).  (Wang et al. 2014, Lee et al. 2015Hartmann 2015;Yu and Zhang 2015;Peng 159 et al. 2018;B. Luo et al. 2020). We first analyzed the two winters to motivate our present an anticyclonic anomaly appears over the North American west coast, whereas a 170 cyclonic anomaly is located in the east part of North America. It is also found from the 171 wavelet power spectrum analysis that the timescale of the TWE variation is about 20 172 days ( Figure S1 in the supplementary file), crudely consistent with the lifetime of the It is easy to find that there are 1 PNA + and 2 PNA -(1 PNA + ) events during the 180 2013/14 (2014/15) winter according to the above definition of an individual PNA event.

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To identify the contribution of PNA to the DJF-mean WWCE SAT anomaly, Figure 2 182 shows the DJF-mean Z500 and SAT anomaly fields with and without PNA events   2018. It is seen that the Z500 anomaly mainly shows a wave train structure similar to a 238 zonally oriented PNA + pattern, thus indicating that the PNA + pattern might play a role 239 in the WWCE dipole. As noted below, the PNA + can become a zonal wave train 240 structure under certain conditions, even though it is not a typical PNA structure.

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To further understand whether the PNA + can lead to a strong WWCE dipole over   Thus, the WWCE dipole associated with C3 is related to the CP-type El Niño (Fig. 9c).

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This result leads us to infer that the different types of El Niño might be an important 315 factor influencing the WWCE dipole through the change of the PNA + pattern. S2) like C3 can produce an intense WWCE dipole over North America (Fig. 10e). But 330 such a WWCE dipole is hardly seen for the EP-type El Niño winter (Fig.10f) Fig. 11 for the CP-and EP-type El Niño winters. It is found that the 352 local Hadley cell is expanded northward for the CP-type El Niño (Fig. 11a), but 353 intensified for the EP-type El Niño (Fig.11b). The Ferrell cell is stronger during the EP-354 type El Niño (Fig. 11b) than during the CP-type El Niño (Fig.11a), as further seen from their difference (Fig.11c)  Niño can explain the difference of the associated PNA + between the CP-and EP-type 370 El Niño winters. 371 We also see that the winter zonal winds in the North Pacific midlatitudes (35 o -372 55 o N) are stronger and extend more east for an EP-type El Niño (Fig.11e) than for a 373 CP-type El Niño (Fig.11d). For this reason, the anticyclonic anomaly of the PNA + shifts 374 more east and occupies the larger region of the North America for an EP-type El Niño 375 (Fig. 10f) than for a CP-type El Niño (Fig.10e). Thus, in the EP-type El Niño winter  (Fig.12d), whose spatial shape is also consistent with the result of Kumar et al.

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(2013). The WWCE dipole is hardly seen for PDO - (Fig.12e) North American WWCE dipole (Fig. 13d) if the Pacific SST anomaly corresponds to 415 the combination of a CP-type El Niño and a PDO + (Fig. 13a). But it is relatively weak 416 under the Pacific SST anomaly condition with the combination of a CP-type El Niño 417 and a PDO - (Fig. 13b) Niño. In addition, we further examine why the CP-type or EP-type El Niño can