Summer Russian heat waves and their links to Greenland's ice melt and sea surface temperature anomalies over the North Atlantic and the Barents–Kara Seas

In this paper, because our purpose is to understand what leads to the internal variability of the summer heat wave over west Russia and its regional change, we need to examine the spatial structure of the HWF over west Russia. For this reason, all the fields we present below are linearly detrended and the EOF analysis of the HWF over west Russia is performed. We show the EOF1 and EOF2 modes of the linearly detrended summer HWF anomalies with a latitude weighting of grid data over west Russia (40°–70°N, 25°–75°E) for 1979–2017 and the time series of their corresponding normalized principal components (PC1 and PC2) in figures 1(c)–(f). It is found that the heat wave occurs over the west Russian midlatitudes mainly to the west of 50°E and in the mid-latitudes (40°–60°N), with a central latitude of about 50°N for the EOF1 mode (figure 1(c)), but mainly to the east of 50°E and at high latitudes (50°–75°N) with a central latitude of 65°N for the EOF2 mode (figure 1(d)). Thus, the Russian heat wave pattern associated with the EOF1 (EOF2) mode may be referred to as the mid-latitude (high-latitude) Russian heat wave to represent the Russian heat waves in different regions. The two heat wave modes are also not influenced by the latitude weighting of the grid data (not shown). Moreover, we see that the 2010 Russian heat wave, investigated by previous studies (e.g. Dole et al 2011, Schneidereit et al 2012, Trenberth and Fasullo 2012), belongs to the group of mid-latitude Russian heat waves because the PC1 time series has a very strong peak in 2010 (figure 1(e)). However, the high-latitude HWF increased significantly during 2010–2017 (figure 1(f)), thus implying that the Russian heat waves have tended to take place at higher latitudes after 2010.

To examine the types of atmospheric circulation patterns that are linked to the mid-latitude and high-latitude Russian heat waves, we first show the JJA-mean Z500 and SAT anomalies regressed onto the PC1 and PC2 time series of the HWF in figures 2(a) and (b). It is found that the mid-latitude Russian heat waves correspond to summer midlatitude Ural anticyclones confined to the midlatitudes (40°–60°N) and at the longitudes of 30°–70°E with anticyclonic centers at 47°E and 55°N (figure 2(a)), whereas the high-latitude Russian heat waves are associated with summer high-latitude Ural anticyclones confined to the high latitudes (50°–75°N) and at the longitudes of 50°–100°E with anticyclonic centers at 67°E and 64°N (figure 2(b)). We also note that the mid-latitude Ural anticyclones seem to result from the propagation of wave trains from the North Atlantic to Eurasia along the south of Greenland (figure 2(a)). In contrast, the high-latitude Ural anticyclones originate from the propagation of high-latitude wave trains from Greenland to Eurasia (figure 2(b)). As revealed by the daily composites (figure 6) presented below, the positive JJA-mean Z500 anomalies over Greenland (the south of Greenland) correspond to Greenland blocking (North Atlantic blocking). Thus, the generation of the summer mid-latitude (high-latitude) Ural anticyclones is likely to be associated with the appearance of summer North Atlantic blocking (Greenland blocking).

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Because North Atlantic blocking (Greenland blocking) is often coupled with North Atlantic SST (GrIS) anomalies (Ballinger et al 2019, Kwon et al 2020), it is useful to examine the types of summer SST (GrIS) anomaly that correspond to North Atlantic blocking (Greenland blocking). We show the JJA-mean SST anomalies from the HadISST dataset and meltwater production over Greenland (referred to as the Greenland Ice Melts, hereafter) regressed onto the PC1 and PC2 time series of the summer HWF in figures 2(c)–(f). It can be seen that the mid-latitude Russian heat waves (HWF EOF1) correspond to strong positive SST anomalies over the North Atlantic midlatitudes (40°–60°N) mainly in the south of Greenland and a negative SST anomalies over the Barents–Kara Seas (BKS, 35°–70°E, 65°–80°N) (figure 2(c)). Compared to other regions, relatively strong positive SST anomalies mainly appear at high latitudes in the North Atlantic, especially over Baffin Bay, Davis Strait, the Labrador Sea and the east of the southern tip of Greenland (BDL, hereafter) and over BKS during the high-latitude Russian heat waves (HWF EOF2) (figure 2(d)). We also find that the Greenland Ice Melts, especially on the northeast side of Greenland's landmass, are stronger during the high-latitude heat waves (figure 2(f)) than the mid-latitude heat waves (figure 2(e)). These results lead us to conclude that the midlatitude Russian heat waves are associated with positive North Atlantic midlatitude SST anomalies and negative SST anomalies over BKS, whereas the high-latitude Russian heat waves are most likely linked to intense Greenland Ice Melts and positive SST anomalies over BDL and BKS. These results are not significantly changed by changes in the domain size of the EOF analysis. Our further calculation indicates that the results for a large domain (40°–70°N, 20°–90°E) (Figs. S2-S3 in the supplementary material) are consistent with figures 1–2 for a small domain (40°–70°N, 25°–75°E). It is also noted that the EOF1 and EOF2 modes of the JJA-mean HWF are approximately separated during 1979–2017, even though they slightly depend on the size of the EOF domain. The two heat wave modes can still be found even when SVD is used (figures S4-S5 in the supplementary material).

Furthermore, we can see that the positive North Atlantic SST pattern in figure 2(c) resembles the extratropical SST anomalies of the positive AMO⁺ or the positive phase of the Atlantic multidecadal variability (AMV) (Ghosh et al 2019, their figure 1(b)). Although Ghosh et al (2019) investigated the roles of the positive and negative AMV SST anomalies in the central to eastern European summer climate, they did not examine the role of the extratropical AMO⁺ SST anomalies in Russian heat waves. As opposed to their study, in this paper we mainly investigate the conditions under which mid-latitude (high-latitude) Russian heat waves can occur. It is shown that the extratropical North Atlantic SST anomalies of the AMO⁺ mainly influence the mid-latitude Russian heat wave, whereas the high-latitude Russian heat waves are mainly linked to intense Greenland ice melts.

To demonstrate whether the UBs that result from the propagation of upstream wave trains are related to changes in SST anomalies and Greenland Ice Melts, it is useful to examine the daily evolution of the composite UBs associated with mid-latitude (high-latitude) Russian heat waves. To investigate this problem, any normalized PC1 (PC2) time series with at least a 0.5 standard deviation (STD) in the summer HWF EOF1 (EOF2) mode is defined as a strong mid-latitude (high-latitude) heat wave summer, which is the case for 1981, 1988, 1995, 1998, 1999, 2006 and 2010 (1981, 1982, 1988, 1989, 1990, 1991, 1998, 2003, 2004, 2007, 2012 and 2016). Using the TM index, we find that there are 24 and 35 UB events in the Ural regions 30°–90°E and 40°–100°E for strong mid-latitude (7 cases) and high-latitude (12 cases) heat wave summers, which correspond to 3.42 and 2.92 UB events per summer, respectively. Thus, UB events are relatively more frequent in strong mid-latitude heat wave summers than in strong high-latitude heat wave summers.

We show the time-mean composite daily Z500 and SAT anomalies averaged from lag −10 to 10 d (lag 0 denotes the peak day of blocking) in figure 3 for the strong mid-latitude and high-latitude heat wave summers. It is found that the positive center of the time-mean composite Z500 anomalies of the UB events is located near 45°E and 55°N for the mid-latitude heat waves (figure 3(a)), but near 70°E and 65°N for the high-latitude heat waves (figure 3(b)). Thus, it is reasonable that the UB events associated with the mid-latitude (high-latitude) heat waves are referred to as mid-latitude (high-latitude) UB events hereafter. Because the time-mean position of the composite UB anticyclones is consistent with the positive center of the regressed JJA-mean Z500 anomalies against the HWF PC1 (PC2) time series, individual mid-latitude (high-latitude) UB events are the main contributors to the summer-mean mid-latitude (high-latitude) Ural anticyclones, as seen from figures 2(a) and (b). This indicates that the maintenance of mid-latitude (high-latitude) UB events is responsible for the occurrence of strong summer mid-latitude (high-latitude) Russian heat waves. The calculation using the time-mean WAF vector of Takaya and Takaya (2001) shows that mid-latitude (high-latitude) UBs seem to originate from the propagation of upstream wave trains (blue arrows in figures 3(a) and (b)). Below, we will present a daily composite WAF vector associated with UB events, to demonstrate that the occurrences of the mid-latitude (high-latitude) UB events result from decays of North Atlantic blocking (Greenland blocking).

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As noted above, the mid-latitude and high-latitude Russian heat waves can correspond to different summer North Atlantic SST, Arctic SST and GrIS anomaly patterns. In this subsection, we further evaluate which of the SST and GrIS anomalies is important for the mid-latitude (high-latitude) UBs. We show the time series of normalized JJA-mean SST anomalies averaged over the North Atlantic ($4{{\text{0}}^{\circ} }{\text{ - 6}}{{\text{0}}^{\circ} }N$,${\text{2}}{{\text{5}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }W$), BKS (${\text{6}}{{\text{5}}^{\circ} }{\text{ - 8}}{{\text{0}}^{\circ} }N$, ${\text{3}}{{\text{5}}^{\circ} }{\text{ - 7}}{{\text{0}}^{\circ} }E$), BDL (${\text{6}}{{\text{0}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }N$, ${\text{2}}{{\text{0}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }{\text{W}}$) and JJA-mean Greenland Ice Melts in figures 4(a)-d. Their correlation coefficients with the HWF PC1 and PC2 time series and the correlation coefficients of the summer AMO indexes (Fig. S6 in the supplementary material) with the HWF PC1 and PC2 timeseries are presented in table 1. The regressed JJA-mean Z500 and SAT anomalies onto the normalized time series of domain-averaged SST anomalies and Greenland Ice Melts are shown in figures 4(e)-h. It can be observed that the mid-latitude (high-latitude) Ural anticyclones are mainly related to the positive extratropical North Atlantic SST anomalies (Greenland ice melting) through the generation of North Atlantic blocking (Greenland blocking) (figures 4(e), h).

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We also see that the center of the mid-latitude (high-latitude) Ural anticyclones of the JJA-mean Z500 regressed into the North Atlantic SST anomaly (Greenland Ice Melt) time series is located near 58°E and 56°N (62°E and 58°N) under the condition that the role of the BKS SST anomalies is excluded. The Z500 anticyclonic (cyclonic) anomalies in response to the positive (negative) BKS SST anomalies are centered near 64°E and 69°N. Clearly, when the BKS SST anomalies are not considered, the positions of the mid-latitude (high-latitude) Ural anticyclones obtained are very different, compared to the results in figures 2(a)–(b). Based on this, we conclude that the BKS SST anomalies are also important for the regional change of UBs, because positive (negative) BKS SST anomalies can correspond to positive (negative) Z500 anomalies over the BKS (figure 4(f)). As noted below, the positions of the positive JJA-mean Z500 anomalies obtained for mid-latitude (high-latitude) Russian heat waves become more consistent with those in figures 2(a)–(b), when both the extratropical North Atlantic SST anomalies (Greenland Ice Melts) and the BKS SST anomalies are considered. When a positive BDL SST anomaly corresponds to a positive Z500 anomaly over Greenland or a Greenland anticyclone (figure 4(g)), the Ural anticyclone resulting from this Greenland anticyclone appears to be rather weak in this case. This suggests that the role of a BDL SST anomaly is secondary to the occurrence of a high-latitude UB, compared to the role of a Greenland Ice Melt.

The correlation calculation (table 1) shows that the mid-latitude Russian heat waves, as denoted by the HWF EOF1 mode, have significant positive correlations of 0.40, 0.41 and 0.38 (p < 0.05) with the AMO indexes, the midlatitude North Atlantic SSTs and the BDL SSTs, although the HWF PC1 exhibits a modest significant negative (positive) correlation of −0.28 (0.3) (p < 0.1) with the BKS SST (Greenland Ice Melts). This hints that the mid-latitude Russian heat waves are more closely related to the positive extratropical North Atlantic SST anomalies, BDL SSTs and AMO⁺. However, we find that while the HWF PC1 has a significant positive correlation with the BDL SST anomalies, the role of the BDL SST anomalies in the mid-latitude heat waves (figure 4(g)) is relatively weak compared to that of the extratropical North Atlantic SST anomalies (figure 4(e)). Although the positive extratropical North Atlantic SST anomalies resemble the AMO⁺ SST patterns, our correlation calculation further reveals that positive extratropical North Atlantic SST anomalies (for an example, see figure 2(c)) are likely to be due to the AMO⁺, because their positive correlation coefficients with the summer AMO indexes (Fig. S6) were able to reach 0.50 (p < 0.01) during 1979–2017. The positive BDL SST anomalies are also likely to be part of the AMO⁺, because they have a positive correlation of 0.35 (p < 0.05), although they are relatively weak compared to the extratropical anomalies of the AMO⁺ SST. The North Atlantic midlatitude SST anomalies will be referred to as the extratropical AMO⁺ SST anomalies hereafter.

We can see from table 1 that the high-latitude Russian heat waves, as denoted by the HWF EOF2 mode, have a large positive correlation of 0.45 (p < 0.01) with the Greenland Ice Melts and a modest significant correlation of 0.30 (p < 0.1) with the BKS SST anomalies, but no significant correlations with the AMO indexes, North Atlantic SST anomalies and BDL SST anomalies. Although the BKS SST anomalies show a significant negative correlation of −0.37 (p < 0.05) with the BDL SST anomalies, they have an insignificant negative correlation of −0.13 (−0.21) with the extratropical AMO⁺ SST (Greenland Ice Melt) anomalies. We further find that the Greenland Ice Melts have a large positive correlation of 0.68 (p < 0.01) with the BDL SST anomalies, though the BDL SSTs show a modest significant positive correlation of 0.29 (p < 0.1) with the extratropical AMO⁺ SST anomalies. Thus, the Greenland Ice Melts are more likely due to the positive BDL SST anomalies. It is possible that the BDL SST anomalies play an indirect role in the variability of Russian heat waves via the GrIS or Greenland Ice Melt changes. This leads us to infer that high-latitude Russian heat waves are mainly related to Greenland Ice Melts and BKS SST anomalies. The new results are different from the previous findings of Trenberth and Fasullo (2012), who noted that tropical North Atlantic SST anomalies can have a large contribution to Russian heat waves in 2010. However, here we find that Greenland Ice Melts play a large role in high-latitude Russian heat waves. The numerical experiments of Ghosh et al (2019) also indicated that mid-latitude Ural anticyclones mainly arise from the forcing of positive extratropical North Atlantic SST anomalies rather than from the forcing of tropical North Atlantic SST anomalies as found in Trenberth and Fasullo (2012) for the 2010 summer Russian heat wave. As opposed to the previous studies, we further find that the Greenland Ice Melts and the BKS SST anomalies are also important for the positions and strengths of summer Russian heat waves.

Because the Greenland ice melt (GIM) or extratropical North Atlantic SST anomaly (${\text{SS}}{{\text{T}}_{{\text{NA}}}}$) has an insignificant correlation with the BKS SST anomaly (${\text{SS}}{{\text{T}}_{{\text{BKS}}}}$), one can use a binary linear regression model to estimate the combined contribution of ${\text{SS}}{{\text{T}}_{{\text{NA}}}}$ (GIM) with ${\text{SS}}{{\text{T}}_{{\text{BKS}}}}$ to the mid-latitude (high-latitude) Russian heat waves by reconstructing the HWF PC1 (PC2) time series. Using ${\text{PC}}1 = {\alpha _1}{\text{SS}}{{\text{T}}_{{\text{NA}}}} + {\alpha _2}{\text{SS}}{{\text{T}}_{{\text{BKS}}}} \times \left( { - 1.0} \right)$, we show the reconstructed HWF PC1 time series in figure 5(a), where ${\alpha _{\text{1}}}{\text{ = 0}}{\text{.38}}$ (${\alpha _{\text{2}}}{\text{ = 0}}{\text{.23}}$) is the regression coefficient of (SST_BKS) onto the PC1 time series of the HWF EOF1 mode. The reconstructed HWF PC2 time series from ${\text{PC}}2 = {\alpha _1}{\text{GIM}} + {\alpha _2}{\text{SS}}{{\text{T}}_{{\text{BKS}}}}$ is also shown in figure 5(b), where ${\alpha _{\text{1}}}{\text{ = 0}}{\text{.53}}$ (${\alpha _{\text{2}}}{\text{ = 0}}{\text{.42}}$) is the regression coefficient of GIM (SST_BKS) onto the PC2 time series of the HWF EOF2 mode. As shown in table 1, the reconstructed HWF PC1 (PC2) time series has a significant positive correlation of 0.47 (0.60) (p < 0.01) with the HWF PC1 (PC2) time series in figures 1(d)–(f). Clearly, the inclusion of ${\text{SS}}{{\text{T}}_{{\text{BKS}}}}$ can increase the positive correlation of the extratropical AMO⁺ SST anomalies (Greenland Ice Melts) with mid-latitude (high-latitude) Russian heat waves. The JJA-mean Z500 and SAT anomalies regressed onto the reconstructed HWF PC1 and PC2 time series (figures 5(c) and (d)) further reveal that the center of the Ural anticyclone in the regressed JJA-mean Z500 anomaly field against the reconstructed HWF PC1 (PC2) time series in figures 5(a) and (b) is located near 53°E and 55°N (64°E and 65°N), closer to that in figure 2(a) and (b) when the negative (positive) BKS SST anomalies are considered, though there is a small difference with those in figures 2(a) and (b). Thus, the presence of mid-latitude (high-latitude) UBs is more likely due to the combined effects of the extratropical AMO⁺ SST anomalies and the negative BKS SST anomalies (the Greenland Ice Melts and the positive BKS SST anomalies), though they stem from the decay of North Atlantic blocking (Greenland blocking) and the propagation of associated wave trains. While the extratropical AMO⁺ SST anomalies (Greenland Ice Melts) play a major role, the negative (positive) anomalies of the BKS SST seem to influence the location and strength of UBs. A comparison between figures 4 and 5 indicates that negative (positive) BKS SST anomalies tend to cause southwestward- (northeastward-) displaced UBs with weak (strong) intensities through the presence of negative (positive) Z500 anomalies over the BKS. The combination of an extratropical AMO⁺ SST anomaly and a negative BKS SST anomaly (a Greenland Ice Melt and a positive BKS SST anomaly) can make an UB anticyclone have an anticyclonic center closer to that of a mid-latitude (high-latitude) UB associated with the HWF EOF1 (EOF2) mode (figures 2(a) and (b)) than the role of an extratropical AMO⁺ SST anomaly (Greenland Ice Melt) without the effect of a BKS SST anomaly. Of course, other factors play a role in the locations where summer UB anticyclones occur, which deserve further investigation.

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We conclude from the above results that extratropical AMO⁺ SST anomalies over the North Atlantic and negative BKS SST anomalies can combine to cause mid-latitude Russian heat waves over west Russia through the generation of mid-latitude UBs, even though the North Atlantic SST anomalies play a bigger role than the negative BKS SST anomalies. In contrast, the Greenland Ice Melts and positive BKS SST anomalies combine to result in high-latitude Russian heat waves via the generation of high-latitude UBs. Below, we create a daily composite to reveal the physical mechanism of UB generation and to demonstrate that the propagation of Rossby wave trains associated with the evolution of North Atlantic blocking (Greenland blocking) leads to mid-latitude (high-latitude) UBs.

To further explore the causes of the generation of mid-latitude (high-latitude) UBs, we create daily composites of instantaneous Z500 and SAT anomalies as well as WAF vectors for UB events during the blocking evolution process. Two-day-interval sequences of composite daily Z500 and SAT anomalies during the period from lag −20 to 2 d are shown in figure 6 for mid-latitude and high-latitude UB events, where lag 0 denotes the peak day of the UB and the blue arrow represents the WAF vector at 500 hPa. Although mid-latitude (high-latitude) UBs mainly take place during the period from lag −10 to 10 d, North Atlantic blocking (Greenland blocking) can appear prior to the UB, from lag −20 to −8 d (figures 6(a) and (b)). It is found that North Atlantic blocking mainly occurs on the south side of Greenland, which peaks at lag −18 d and is still strong before lag −20 d (figure 6(a)). When this North Atlantic blocking decays from lag −18 to −12 d, the WAF vector points to the Ural region (figure 6(a)). Along with the disappearance of the North Atlantic blocking from lag −8 to 0 d, an intensified WAF vector toward the Ural region is clearly seen, which subsequently leads to a mid-latitude UB via the propagation of wave trains from the North Atlantic midlatitudes to the Ural region. On the other hand, we can see that although the Greenland blocking shows a temporal evolution behavior similar to that of North Atlantic blocking, it peaks at lag −12 d (figure 6(b)). Along with the decay and disappearance of Greenland blocking from lag −12 to 0 d, intensified WAF toward the Ural region is also seen, which leads to the formation of a high-latitude UB. These results suggest that the decay and energy dispersion of North Atlantic blocking (Greenland blocking) is the major process in the establishment of mid-latitude (high-latitude) UBs via wave train propagation from the North Atlantic (Greenland) to the Ural region.

It is useful to further examine what the precursors of the mid-latitude (high-latitude) UB events are. We first show the temporal variations of daily SST anomalies averaged over the North Atlantic ($4{{\text{0}}^{\circ} }{\text{ - 6}}{{\text{0}}^{\circ} }N$,${\text{2}}{{\text{5}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }W$), BKS (${\text{6}}{{\text{5}}^{\circ} }{\text{ - 8}}{{\text{0}}^{\circ} }N$, ${\text{3}}{{\text{5}}^{\circ} }{\text{ - 7}}{{\text{0}}^{\circ} }E$), BDL (${\text{6}}{{\text{0}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }N$, ${\text{2}}{{\text{0}}^{\circ} }{\text{ - 7}}{{\text{5}}^{\circ} }{\text{W}}$), the daily Greenland Ice Melts and the daily NAO indexes during the whole lifecycle of UBs in figure 7. This figure shows that although the extratropical AMO⁺ SST and BKS SST anomalies vary slightly with the evolution of the UBs, the extratropical AMO⁺ SST anomalies (figure 7(a)) are always stronger during the evolution process (from lag −20 to 20 d) of UBs for the mid-latitude UBs than for the high-latitude UBs. In contrast, the Greenland Ice Melts (figure 7(d)) are always stronger for the high-latitude UBs than for the mid-latitude UBs. Thus, the strong extratropical AMO⁺ SST (Greenland Ice Melt) anomalies are a favorable precursor of the North Atlantic blocking (Greenland blocking). We also noted that the SST anomalies over the BKS do not strongly depend on the evolution of UBs, because their temporal change is small regardless of whether UBs are present or absent (from lag −20 to −10 d) (figure 7(b)). In other words, the BKS SST anomalies are not a response to UBs. Instead they are more likely to be a driver of relatively weak stationary anticyclones over the BKS (figure 4(f)) as some part of the high-latitude UBs through increased surface heat fluxes (Zhang et al 2018). In addition, it is noted that the variations of the BDL SST anomalies are unstable during the period prior to UBs and their difference between the mid-latitude and high-latitude UBs (figure 7(c)) is weaker than that of the extratropical North Atlantic or BKS SST anomalies, thus indicating that the role of the BDL SST anomalies seems to be weak in the establishment of UBs. We also see that the negative NAO^- index prior to the UB onset is larger for high-latitude UBs than for mid-latitude UBs (figure 7(e)). Thus, the presence of a strong NAO^- pattern resembling Greenland blocking can also be considered to be a precursor of high-latitude UBs.

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We further show the time-mean composite daily Z500 and SAT, Greenland Ice Melt (meltwater production over Greenland) and SST anomalies averaged from lag −20 to −10 d during the period prior to UB in figure 8 to understand whether the mid-latitude (high-latitude) UBs are related to the presence of a strong precursor North Atlantic blocking (Greenland blocking). It can be seen from figure 8 that strong blocking anticyclones appear over the North Atlantic (Greenland) prior to the mid-latitude (high-latitude) UBs (figures 8(e)–(f), which corresponds to stronger (weaker) extratropical AMO⁺ SST anomalies over the North Atlantic (figures 8(c)–(d). For these cases, a prior negative (positive) SST anomaly is also seen over the BKS for the mid-latitude (high-latitude) UB, in agreement with the daily composite results in figure 7. In contrast, the Greenland Ice Melts prior to the onsets of UBs are stronger especially on the northeast side of Greenland's landmass for the high-latitude UBs (figure 8(b)) than for the mid-latitude UBs (figure 8(a)). Thus, it is likely that the presence of prior North Atlantic blocking (Greenland blocking) is more closely related to strong prior extratropical AMO⁺ SST anomalies (Greenland Ice Melts). The numerical results of Ghosh et al (2017, 2019) indicated that a strong extratropical AMO⁺ SST anomaly can reinforce a large-scale anticyclone resembling a North Atlantic blocking in the warm SST region through the forcing of the positive heat flux and Rossby wave source, which subsequently forms an east-west wave train from the North Atlantic to the Ural region. A strong Greenland Ice Melt has a similar effect, which can also reinforce a Greenland blocking, though Greenland blocking can be initiated by synoptic-scale eddies (Zhang and Luo 2020) or diabatic heating (Steinfeld and Pfahl 2019). Thus, a mid-latitude UB is, to some extent, mainly due to the forcing of a strong prior extratropical AMO⁺ SST anomaly, whereas a high-latitude UB is mainly due to the forcing of a strong prior Greenland Ice Melt.

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