1. Introduction
The linkage between El Niño and the north tropical Atlantic (NTA) is one of the most robust teleconnections generated by El Niño. Significant positive sea surface temperature anomalies (SSTAs) tend to follow El Niño during its decaying spring (e.g., Enfield and Mayer 1997; Klein et al. 1999; Saravanan and Chang 2000; Alexander and Scott 2002; Lee et al. 2008; Wu and He 2019; Wu et al. 2020; Chen 2022). In addition, the variability of SSTAs over the NTA is notable for its pronounced influence on climate over the surrounding and remote regions. The NTA SSTAs can affect the precipitation over South America (Uvo et al. 1998; Giannini et al. 2000; Rodrigues et al. 2011) and the hurricane activities over the Atlantic (Wang et al. 2006; Vimont and Kossin 2007), modulate the North Atlantic Oscillation (NAO; Okumura et al. 2001; Sutton et al. 2001; Robinson et al. 2003), and induce teleconnections propagating to the tropical Pacific and East Asia (e.g., Ham et al. 2013; Hong et al. 2014; Chen et al. 2015, 2022).
The El Niño–generated NTA warming corresponds to the weakened northeasterly trade wind over the NTA via the wind–evaporation–SST feedback (e.g., Enfield and Mayer 1997; Klein et al. 1999; Giannini et al. 2000; Czaja et al. 2002; García-Serrano et al. 2017; Jiang and Li 2019). Previous studies have proposed that both the tropical teleconnection and the extratropical teleconnection are responsible for the changes in trade winds. The tropical teleconnection is the remote Gill response related to perturbations in the Walker circulation with anomalous sinking and anticyclonic circulations over the north equatorial Atlantic (e.g., García-Serrano et al. 2017), which is likely to play a secondary role in modulating the trade winds over the NTA (Jiang and Li 2019). On the other hand, the El Niño–related extratropical teleconnection is the Rossby wave train that propagates across the Pacific–North American (PNA) region with a low pressure center located over the southeastern United States and the North Atlantic (e.g., Wallace and Gutzler 1981; Enfield and Mayer 1997; Klein et al. 1999; Giannini et al. 2000; Alexander and Scott 2002). The extratropical teleconnection explains more than two-thirds of the trade wind changes over the NTA (Jiang and Li 2019), indicating the importance of the extratropical teleconnection to the El Niño–NTA relationship.
The El Niño–NTA teleconnection tends to vary with the El Niño properties (e.g., Wu and He 2019; Amaya and Foltz 2014; Taschetto et al. 2016), but similar El Niño variability does not guarantee a similar warming over the NTA (e.g., Lee et al. 2008; Wu et al. 2020; Casselman et al. 2021; Duan et al. 2021), indicating that other factors besides El Niño itself could modify the El Niño–NTA teleconnection. Previous studies have noticed that the NAO is likely to be constructive or destructive to the influence of El Niño on the NTA SSTAs through modulating the extratropical teleconnection (Jiménez-Esteve and Domeisen 2018; Casselman et al. 2021; Duan et al. 2021). Given the strong interactions between the NTA SSTAs and the NAO, the NAO can be forced or intensified by the NTA SSTAs via air–sea coupling (e.g., Okumura et al. 2001; Sutton et al. 2001; Robinson et al. 2003; Peng et al. 2006; Sung et al. 2013). Thus, the NAO that affects the El Niño–NTA connection might be the feedback from the NTA SSTAs. To avoid the disturbance from the NAO, it is better to further investigate the El Niño–NTA teleconnection without the NAO signals.
The El Niño–generated extratropical circulation usually coincides with a deepened Aleutian low (AL) over the North Pacific (e.g., Hoerling and Kumar 2002; Straus and Shukla 2002; Johnson and Feldstein 2010; Li et al. 2019; Zhi et al. 2022). The AL pressure system is a major climatic feature in the North Pacific, which not only can be affected by El Niño but also can be formed by midlatitude processes, and thus is a mixture of the El Niño–related variability and El Niño–independent variability (e.g., Trenberth and Hurrell 1994; Lau 1997; Honda et al. 2001, 2005; Larson et al. 2022). The AL is associated with a wave train propagating eastward to the North Atlantic (e.g., Honda et al. 2005; Soulard and Lin 2017; Larson et al. 2022), which results in a strong coupling between Pacific and Atlantic (e.g., Brönnimann 2007; Giamalaki et al. 2021) and affects the climate over North America and Eurasia (e.g., Honda et al. 2005; Soulard and Lin 2017; Giamalaki et al. 2021). It is probable that the AL and its related wave train could modulate the El Niño–generated extratropical teleconnection and further affect the El Niño–NTA relationship.
To confirm this and explore the physical process behind it, we would like to further investigate the connection between El Niño and the NTA SSTAs and the modulation by the AL. In this article, section 2 introduces the observational datasets. Section 3 inspects the observed characteristics of the connection between El Niño and the NTA SSTAs. Section 4 demonstrates the role of the AL in modulating this connection. Section 5 illustrates the El Niño–NTA relationship and the modulation by the AL in the model simulations that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP 6). Section 6 presents the summary and discussion.
2. Observational datasets
The SST datasets include the Hadley Center Sea Ice and Sea Surface Temperature (HadISST) dataset (Rayner et al. 2003) during 1870–2020; the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed Sea Surface Temperature version 5 (ERSSTv5; Huang et al. 2017) during 1854–2020; the Global Sea Surface Temperature Analysis (COBE-SST) dataset from the Japan Meteorological Agency (Japan Meteorological Agency 2006) during 1891–2020; and the International Comprehensive Ocean-Atmosphere Dataset (ICOADS; Freeman et al. 2017) during 1880–2020. The circulation and precipitation datasets are from the National Oceanic and Atmospheric Administration Twentieth Century Reanalysis version 2 (NOAA 20Cv2; Compo et al. 2011) during 1871–1947 and from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP–NCAR) Reanalysis version 1 (NCEP1; Kalnay et al. 1996) during 1948–2020. The linear trend and the 9-yr running mean of the datasets have been removed first to highlight the interannual variability. The same results can be obtained by using the datasets from 1948, which are said to be more reliable. The long-term records are used in this study to select more samples for more confident results.
3. Connection between the moderate El Niño and the following spring NTA SSTAs
Figure 1 is the scatterplot between the winter [December–February (DJF)] Niño-3 index (defined as the SST anomalies averaged over the central and eastern tropical Pacific: 5°S–5°N, 90°–150°W; the results are not sensitive to the ENSO index we chose) and the following spring [March–May; (MAM)] NTA index (defined as the SST anomalies averaged over the north tropical Atlantic: 0°–25°N, 15°–80°W; following Chen 2022). The positive correlations between these two indices, which range from 0.59 to 0.77 in different SST datasets, indicate that El Niño is likely to induce significant positive SSTAs over the NTA, being consistent with previous studies (e.g., Enfield and Mayer 1997; Klein et al. 1999; Saravanan and Chang 2000; Alexander and Scott 2002; Wu and He 2019; Wu et al. 2020; Chen 2022).
Scatterplot of the Niño-3 index (defined as the SSTA averaged over 5°S–5°N, 90°–150°W) in winter (DJF; x-axis units: °C) and the north tropical Atlantic index (defined as the SSTA averaged over 0°–25°N, 15°–80°W) in the following spring (MAM; y-axis units: °C) by (a) the HadISST during 1870–2020, (b) the ERSSTv5 during 1854–2020, (c) the COBE-SST during 1891–2020, and (d) the ICOADS during 1880–2020. The green (purple) circles represent the cases in which the strong El Niños are followed by the strong (weak) NTA SSTAs. The red (blue) dots represent the cases in which the moderate El Niños are followed by the strong (weak) NTA SSTAs.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
We further find that the strong El Niño events are almost always accompanied by the strong NTA SSTAs, whereas the moderate El Niño cases are followed by either the strong or weak NTA SSTAs. The strong (moderate) El Niño is defined as the winter Niño-3 index being greater than 1.6 standard deviations (greater than 0.6 standard deviations but less than 1.6 standard deviations). The strong (weak) NTA SSTA is defined as the NTA index being greater (less) than 0.6 standard deviations. As Fig. 1a shows, most of the strong El Niño events (green circles), except one case (purple circle), are followed by the strong positive NTA SSTAs. However, for the moderate El Niño cases, they are followed by either the strong (red dots) or weak (blue dots) NTA SSTAs, suggesting an uncertainty in the connection between the moderate El Niño and the following spring NTA SSTAs. The result that the strong NTA SSTAs are in response to most strong El Niño events but only to a portion of the moderate El Niño cases is true for a variety of different SST datasets (Fig. 1), suggesting that the uncertain relationship between the moderate El Niño and the NTA SSTA is robust. In the following paragraphs, the HadISST is used to further investigate the El Niño–NTA connection, but the same results can be obtained by using other SST datasets.
Based on the relationship between El Niño and the corresponding NTA SSTAs, four types of El Niño cases are categorized: the strong El Niño followed by the strong NTA SSTA (S.EN+S.NTA; green circles in Fig. 1a), the strong El Niño followed by the weak NTA SSTA (S.EN+W.NTA; purple circle in Fig. 1a), the moderate El Niño followed by the strong NTA SSTA (M.EN+S.NTA; red dots in Fig. 1a), and the moderate El Niño followed by the weak NTA SSTA (M.EN+W.NTA; blue dots in Fig. 1a). Table 1 is a list of the different types of El Niño cases. Nine of the 10 strong El Niño events are followed by the strong NTA SSTAs. Less than half of the moderate El Niño cases are followed by the strong NTA SSTAs, but others are followed by the weak NTA SSTAs. The result indicates that the connection between El Niño and the NTA SSTAs could be disturbed by other factors, and this means that the moderate El Niño cases are not guaranteed to be associated with strong positive NTA SSTAs. In the following paragraphs, the different responses to the M.EN+S.NTA type and to the M.EN+W.NTA type are compared to investigate the uncertain connection between the moderate El Niño and the corresponding NTA SSTAs.
The four types of El Niño (EN) cases in observations using the HadISST dataset from 1871 to 2020: strong El Niño followed by strong NTA SSTA; strong El Niño followed by weak NTA SSTA; moderate El Niño followed by strong NTA SSTA, and moderate El Niño followed by weak NTA SSTA. The strong (moderate) El Niño is defined as the winter Niño-3 index being greater than 1.6 standard deviations (greater than 0.6 standard deviations but less than 1.6 standard deviations). The strong (weak) NTA SSTA is defined as the NTA index being greater (less) than 0.6 standard deviations.
Figure 2 is the composite spatial patterns of SSTAs for the two types of the moderate El Niño and their differences during the decaying spring. The significant differences are in the NTA, with an NTA index of 0.29° and −0.01°C in response to the M.EN+S.NTA and the M.EN+W.NTA, respectively. Besides corresponding to the M.EN+S.NTA, the SSTAs over the North Atlantic basin display a tripole pattern with positive anomalies over the tropics and the midlatitudes but negative anomalies over the subtropics (Fig. 2a). In contrast, the SSTA over the whole Atlantic basin is relatively weak in response to the M.EN+W.NTA (Fig. 2b).
Composite spatial patterns of SSTA (°C) for (a) the moderate El Niño followed by the strong NTA SSTA, (b) the moderate El Niño followed by the weak NTA SSTA, and (c) their differences during the El Niño decaying spring. The anomalies with stippling represent the regions that are significant at the 95% confidence level using the two-tailed t test. The green dashed boxes represent the region to define the NTA index.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
Over the Pacific, the two types of El Niño have a similar pattern (Figs. 2a,b), although the SSTA over the southeastern tropical Pacific is slightly warmer for the M.EN+S.NTA (Fig. 2c). The spring Niño-3 index is 0.41°C for the M.EN+S.NTA and 0.40°C for the M.EN+W.NTA, indicating that the two types of El Niño have comparable intensity in both the mature winter (0.88° and 0.86°C, respectively) and the decaying spring. Previous studies argued that the amplitude and the decay time of El Niño are relevant to the NTA SSTAs (Lee et al. 2008; Wu and He 2019; Wu et al. 2020). We further examine the temporal evolution of Niño-3 index for the two types of moderate El Niño (not shown). The result suggests that the M.EN+S.NTA and the M.EN+W.NTA share a similar intensity and decaying phase, indicating that the different response of the NTA SSTA is not due to the El Niño amplitude and decay time. The difference between this and previous studies is likely due to the different classification of ENSO cases. We only focus on the moderate El Niño cases, but both strong and moderate ENSO cases are included in previous studies.
Significant differences are over the midlatitudes of the North Pacific (Fig. 2c), with pronounced negative SSTAs over the North Pacific coinciding with the M.EN+S.NTA (Fig. 2a), but weak anomalies over the North Pacific coinciding with the M.EN+W.NTA (Fig. 2b). The difference in the SSTAs over the North Pacific (35°–45°N, 140°W–180°) is −0.41°C (Fig. 2c; −0.53°C for the M.EN+S.NTA and −0.12°C for the M.EN+W.NTA), being comparable to the interannual variability of the SSTA in that region (with one standard deviation of 0.46°C).
The precipitation and lower-tropospheric circulation anomalies over the tropics exhibit similar patterns in response to both types of El Niño (Figs. 3a,b), with positive precipitation anomalies over the central and eastern tropical Pacific and anomalous westerlies over the central equatorial Pacific, as well as suppressed precipitation over the equatorial Atlantic and anomalous anticyclonic circulation over the tropical Atlantic. These responses represent the tropical teleconnection between El Niño and the NTA via the remote Gill response associated with perturbations in the Walker circulation (e.g., García-Serrano et al. 2017; Jiang and Li 2019). The differences in the responses to the two types of El Niño over the tropical Pacific and Atlantic are slight (Fig. 3c), suggesting a comparable strength of the tropical teleconnection generated by the M.EN+S.NTA and the M.EN+W.NTA, which is likely due to the same intensity of the two types of El Niño. One the other hand, significant differences are present over the midlatitudes of the North Pacific and the North Atlantic. The cyclonic circulation anomalies over the North Pacific and anticyclonic circulation anomalies over the subtropical North Atlantic are well organized and remarkable in response to the M.EN+S.NTA (Fig. 3a), but they are much weakened and less significant in response to the M.EN+W.NTA (Fig. 3b). The anomalies over the Indian Ocean in response to the two types of El Niño show some differences (not shown), but these are likely to be irrelevant to the El Niño–NTA teleconnection, since the model simulations show a similar M.EN–NTA connection as that in observations but do not show the differences in the Indian Ocean (Fig. 9 in section 5).
As in Fig. 2, but for (left) precipitation anomalies (mm day−1; shading) and 850-hPa wind anomalies (m s−1; vectors) and (right) 500-hPa geopotential height anomalies (m). For the 850-hPa wind anomalies, either the zonal or meridional wind anomalies significant at the 95% confidence level by the two-tailed t test are shown.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The composite of 500-hPa geopotential height anomalies for the two types of El Niño shows the extratropical teleconnection between El Niño and the NTA (Figs. 3d,e). The significant and well-organized Rossby wave train propagates across the Pacific and North American region in response to the M.EN+S.NTA (Fig. 3d). The negative center of the PNA pattern over the southeastern United States to the North Atlantic with the anomalous southwesterlies on the south edge weakens the northeasterly trade winds and, in turn, results in warming over the NTA via the wind–evaporation–SST feedback (e.g., Enfield and Mayer 1997; Klein et al. 1999; Giannini et al. 2000; Czaja et al. 2002; García-Serrano et al. 2017; Jiang and Li 2019). Thus, the strong PNA pattern (Fig. 3d), being generated by the M.EN+S.NTA, is responsible for the strong NTA SSTA (Fig. 2a), while the PNA pattern in response to the M.EN+W.NTA is relatively weak, with the negative center over the southeastern United States to the North Atlantic being absent (Fig. 3e), leading, therefore, to the weak NTA SSTA (Fig. 2b). The results suggest that the differences in the extratropical teleconnection, rather than the tropical teleconnection, result in the different strength of the El Niño–related NTA SSTA. The significant, positive geopotential height anomalies in the tropics exist for the M.EN+S.NTA but not for the M.EN+W.NTA (Figs. 3d,e). This difference is likely due to the different intensity of SSTA and precipitation anomalies over the NTA and the Indian Ocean for the two types of El Niño, rather than the influence from the tropical Pacific, since the M.EN+S.NTA and the M.EN+W.NTA share a similar strength of SSTA and precipitation anomalies over the tropical Pacific (Figs. 2 and 3a–c).
The differences in extratropical circulations in response to the two types of El Niño also can be seen in Fig. 4. Coinciding with El Niño, there are negative sea level pressure (SLP) anomalies over the North Pacific, representing the deepened AL (Figs. 4a,b). The AL accompanied by the M.EN+S.NTA is extremely strong (Fig. 4a), but it is much weakened coinciding with the M.EN+W.NTA (Fig. 4b). The SLP anomalies averaged over 50°–60°N, 140°W–180° are −2.18 hPa for the M.EN+S.NTA, comparable to one standard deviation of the SLP anomalies in situ (2.53 hPa). The enhanced AL accompanied by the M.EN+S.NTA is responsible for the intensified negative SSTA over the North Pacific (Fig. 2a), while the weakened AL coinciding with the M.EN+W.NTA is only 0.12 hPa (Fig. 4b), which is consistent with the decreased SSTA over the North Pacific associated with the M.EN+W.NTA (Fig. 2b).
As in Fig. 2, but for SLP anomalies (hPa).
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The extratropical circulations over the North Atlantic exhibit different patterns between the two types of El Niño. The M.EN+S.NTA (M.EN+W.NTA) is related to the negative (positive) SLP anomalies over the midlatitudes and positive (negative) anomalies over the high latitudes (Figs. 4a,b). It seems that the two types of El Niño occur along with the negative and positive phases of NAO. The role of the NAO in disturbing the El Niño–NTA connection has been noticed in previous studies (Lee et al. 2008; Casselman et al. 2021; Duan et al. 2021). The negative (positive) phase of NAO may work constructively (destructively) with El Niño and leads to the strong (weak) NTA SSTAs. However, the NAO can be forced or intensified by the NTA SSTAs via air–sea coupling due to the strong interactions between the NTA SSTAs and the NAO, implying that the NAO accompanying El Niño is probable due to the feedback from the NTA SSTAs (e.g., Sutton et al. 2001; Okumura et al. 2001; Robinson et al. 2003; Peng et al. 2006; Sung et al. 2013). Thus, it might be unfair to only focus on the impact of the NAO on the NTA SSTAs but neglect the modification to the NAO by the NTA SSTAs.
Besides the NAO, we find that the strength of the AL is distinct between the two types of El Niño. Does it modulate the El Niño–NTA relationship? If yes, what is the physical process behind this modulation? To answer these questions, the influences of the AL on the connection between El Niño and the NTA SSTAs are investigated after removing the NAO signals via linear regression to eliminate the interactions between the NAO and the NTA SSTAs. To remove the NAO effect, we exclude the NAO-related parts, obtained by regression of variables onto the simultaneous NAO index, from the original variables. This method is effective for removing the NAO effect, since the NTA SSTA–related circulation anomalies over the North Atlantic are much weakened after removing the NAO signals (not shown). This method focuses on the NAO-related linearity part but tends to neglect the nonlinearity part, which is beyond the scope of this study.
4. Role of the AL in modulating the El Niño–NTA connection
Figure 5 shows the composite of the two types of El Niño after removing the NAO signals. The intensity of the NTA SSTA is 0.22°C in response to the M.EN+S.NTA but only 0.06°C in response to the M.EN+W.NTA, indicating that the distinguished contrast in the NTA SSTAs induced by the two types of El Niño still exists even without the NAO signals (Figs. 5a–c). The negative North Pacific SSTA coinciding with the M.EN+S.NTA (Fig. 5a) is also stronger than that with the M.EN+W.NTA (Fig. 5b), consistent with the original composites prior to the removal of the NAO signals. Additionally, the SSTA over the midlatitudes and subtropical North Atlantic along with the M.EN+S.NTA is weakened compared to the original SSTA (Figs. 5a and 2a), but the SSTA over the NTA (0.22°C; Fig. 5a) is comparable to the original SSTA (0.29°C; Fig. 2a), implying that the NAO signals tend to be more important to the SSTA over the midlatitudes and the subtropical North Atlantic than that over the NTA.
As in Fig. 2, but for (left) SSTAs (°C), (center) sea level pressure anomalies (hPa), and (right) 500-hPa geopotential height anomalies (m). The NAO signals have been removed first via partial regression.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The intensity of the AL for the M.EN+S.NTA (with the value of −1.75 hPa; Fig. 5d) is stronger than that for the M.EN+W.NTA (with the value of −0.28 hPa; Fig. 5e), with a difference of −1.47 hPa (Fig. 5f). It means that the NAO signals barely affect the intensified AL coinciding with the M.EN+S.NTA. The extratropical teleconnection between El Niño and the NTA exhibits a strong PNA pattern induced by the M.EN+S.NTA (Fig. 5g) but a weak pattern induced by the M.EN+W.NTA (Fig. 5h), which is consistent with those in original variables (Figs. 3d–f). The results that the M.EN+S.NTA, accompanied by the strong AL, generates the strong PNA pattern and, in turn, leads to the strong NTA SSTAs can be obtained whether the NAO signals have been removed or not.
Figure 6 shows the intensity of the normalized indexes for the different types of El Niño. The moderate El Niño can be followed by either the strong or weak NTA SSTA. The El Niño event that leads to the strong (weak) NTA SSTA is accompanied by the strong (weak) AL. Moreover, whether the NAO signals have been removed or not, the same intensity of the moderate El Niño accompanied by the different strengths of the AL can lead to the different intensities of the NTA SSTA. Therefore, the uncertain El Niño–NTA SSTA connection could be due to the different intensities of the AL. The robustness of these results can be confirmed by different SST datasets.
The intensity of the normalized winter (DJF) Niño-3 index (blue bars), spring (MAM) NTA index (red bars), NAO index (orange bars), and the AL index (green bars) for different types of El Niño cases in (a) HadISST, (b) ERSSTv5, (c) COBE-SST, and (d) ICOADS. The dots show the 95% confidence intervals.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The regression of SSTA onto the AL index (defined as the SLP anomalies averaged over 50°–60°N, 140°W–180°; the AL index is multiplied by −1.0) shows that the negative AL is related to the significant positive SSTA over the central and eastern tropical Pacific and over the NTA (Fig. 7a), indicating that the negative AL coincides with both El Niño and the warming over the NTA. Furthermore, by partial regression analysis, the significant positive NTA SSTA can be related to the deepened AL even when the Niño-3 signals have been removed (Fig. 7c), suggesting the independent impacts of the AL on the SSTA over the NTA. On the other hand, the El Niño–induced NTA SSTA has somewhat weakened after removing the AL signals (Figs. 7b,d). The difference between Figs. 7b and 7d shows a similar pattern to Fig. 7c (not shown), further suggesting the isolated role of AL in affecting the NTA SSTA. The intensity of the NTA SSTA is 0.08°C induced by the normalized Niño-3 index (Fig. 7d) and 0.05°C induced by the normalized AL index (Fig. 7c), implying that the influence of the AL on the NTA SSTA cannot be ignored, when considering the El Niño–NTA connection.
Regression of spring SSTAs (°C) onto (a) the normalized AL index (defined as the sea level pressure anomalies over 50°–60°N, 140°W–180°; the AL index is multiplied by −1.0) and (b) the normalized Niño-3 index. (c), (d) As in (a) and (b), but for partial regression after removing the Niño-3 index (AL index). The anomalies with stippling represent the regions that are significant at the 95% confidence level using the two-tailed t test. The purple dashed boxes represent the region used to define the SST anomalies over the North Pacific (NP_sst index).
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The negative AL anomalies lead to the negative SSTA over the North Pacific (Fig. 7a). The deepened AL not only induces anomalous northwesterly wind that advects relatively cold dry air mass, but also affects the North Pacific oceanic conditions by altering the wind stress curl and wind speed, and thus changing the net heat flux. These atmospheric and oceanic processes lead to cooling over the North Pacific (e.g., Alexander and Scott 2008; Pickart et al. 2009; Smirnov et al. 2014; Giamalaki et al. 2021; Zhi et al. 2022). The intensity of the AL-induced negative SSTA over the North Pacific is −0.19°C (Fig. 7a), and persists to −0.12°C after removing the Niño-3 forcing (Fig. 7c), indicating the important role of the AL in maintaining the negative SSTA over the North Pacific.
The regression of 500-hPa geopotential height anomalies onto the NP_sst index (defined as the SSTA averaged over 35°–45°N, 140°W–180° (see the dashed box in Fig. 7); the NP_sst index is multiplied by −1.0) shows that the negative SSTA over the North Pacific is associated with a wave train propagating eastward with a positive center over North America and a negative center over the subtropical North Atlantic (Fig. 8a). The latter contributes to the warming over the NTA via the wind–evaporation–SST feedback. This wave train is similar to the PNA pattern generated by El Niño except for the center over the tropical Pacific, being consistent with previous studies (e.g., Honda et al. 2005; Soulard and Lin 2017; Larson et al. 2022) that suggested the AL and its related SSTA over the North Pacific are associated with a PNA-like wave train. This PNA-like wave train also can be seen without the Niño-3 signals (Fig. 8c), indicating the independent connection between the North Pacific SSTA and the wave train. On the other hand, the El Niño–related PNA pattern is weakened after the North Pacific SSTA is removed (Figs. 8b,d). The North Pacific SSTA, induced by the AL, further intensifies the AL, which modulates the El Niño–related PNA pattern and, in turn, affects the strength of the NTA SSTA. The result implies a positive feedback between the negative North Pacific SSTA and the deepened AL, being consistent with previous studies (Tatebe et al. 2017; Larson et al. 2022).
Regression of spring 500-hPa geopotential height anomalies (m) onto (a) the normalized NP_sst index [defined as the SST anomalies over 35°–45°N, 140°W–180° (the purple boxes in Fig. 7); the NP_sst index is multiplied by −1.0] and (b) the normalized Niño-3 index. (c), (d) As in (a) and (b), but for partial regression after removing the Niño-3 index (NP_sst index). The anomalies with stippling represent the regions that are significant at the 95% confidence level using the two-tailed t test.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
5. The El Niño–NTA connection and the modulation by the AL in the CMIP6 model simulations
The role of the AL in modulating the connection between the moderate El Niño and the NTA SSTA are further investigated by using 16 CMIP6 model simulations with historical runs from 1850 to 2014. The details of these models are in Table 2. Under the same criterion as that in observations, 714 El Niño cases have been selected in model simulations, including 120 S.EN+S.NTA, 39 S.EN+W.NTA, 258 M.EN+S.NTA, and 297 M.EN+W.NTA. The number of the S.EN+S.NTA is threefold that of the S.EN+W.NTA, indicating that most of the strong El Niño cases are followed by the strong NTA SSTA, being consistent with observations. However, less than half of the moderate El Niño cases are followed by the strong NTA SSTA, and the others are followed by the weak NTA SSTA. The proportion of the M.EN+W.NTA to all the moderate El Niño cases (53.5%) is close to that in observations (57.1%). The results suggest that the CMIP6 model simulations can realistically reproduce the uncertainty in the connection between the moderate El Niño and the NTA SSTA.
Description of the CMIP6 models used in this study.
Figure 9 is a composite of the SSTA and SLP anomalies based on the two types of the moderate El Niño in model simulations. The corresponding NTA SSTA is distinct between the two types of El Niño, with the intensity of 0.26°C in response to the M.EN+S.NTA (Fig. 9a) but −0.02°C in response to the M.EN+W.NTA (Fig. 9b). The model result that the moderate El Niño cases with the similar intensity could lead to the NTA SSTA with remarkable contrast is consistent with the observational evidence. Note that the different responses over the Indian Ocean are less significant than those in observations, which is likely related to the CMIP6 model’s bias in simulating intensity and spatial pattern of the El Niño–induced anomalies over the Indian Ocean, particularly for the El Niño with a relatively weak variability (e.g., Fu et al. 2021; Yang and Huang 2023).
As in Fig. 2, but for (left) SSTAs (°C) and (right) sea level pressure anomalies (hPa) in the CMIP6 model simulations.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
Over the North Pacific, the strong negative SSTA coincides with the M.EN+S.NTA (with the intensity of −0.21°C; Fig. 9a), but the weak SSTA coincides with the M.EN+W.NTA (−0.05°C; Fig. 9b). The intensified negative SSTA over the North Pacific is related to the deepened AL along with the M.EN+S.NTA (Fig. 9d). The intensity of the AL for the M.EN+S.NTA is −2.01 hPa (Fig. 9b), which is more than twofold that for the M.EN+W.NTA (with the intensity of −0.93 hPa; Fig. 9e). The deepened AL and the strengthened negative SSTA over the North Pacific coinciding with the M.EN+S.NTA, compared to those with the M.EN+W.NTA, further support the observational result that El Niño with the strong AL leads to the strong NTA SSTAs.
The modulation of the El Niño–related NTA SSTA by the AL in model simulations is further illustrated after removing the NAO signals (Fig. 10). The AL is extremely strong for the M.EN+S.NTA (with the intensity of −1.70 hPa; Fig. 10d), being twofold that for the M.EN+W.NTA (with the intensity of −0.88 hPa; Fig. 10e). The intensified AL leads to the enhanced negative SSTA over the North Pacific (Fig. 10a). The strong negative North Pacific SSTA works constructively with the El Niño forcing and, in turn, results in the strong NTA SSTA (with the intensity of 0.21°C; Fig. 10a). On the other hand, when the AL is weakened (Fig. 10e), the associated negative SSTA over the North Pacific is decreased (Fig. 10b), which reduces the strength of the El Niño–related NTA SSTA (with the intensity of −0.01°C; Fig. 10b).
As in Fig. 9, but the NAO signals have been removed first via partial regression.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
The intensity in each index for different types of El Niño in model simulations is shown in Fig. 11. The moderate El Niño with similar intensity leads to either the strong or weak NTA SSTA. This is due to the different intensity of the AL, that is, El Niño with the strong (weak) AL leads to the strong (weak) NTA SSTA. This result can be obtained whether the NAO signals have been removed or not, which is consistent with observations.
As in Fig. 6, but for those in the CMIP6 model simulations.
Citation: Journal of Climate 36, 11; 10.1175/JCLI-D-22-0656.1
Note that by using the original variables (Fig. 9), the negative phase of NAO arises for the M.EN+S.NTA (Fig. 9d), but the weak NAO signals appear for the M.EN+W.NTA (Fig. 9e), indicating that the two types of El Niño are not necessarily accompanied by the different phases of NAO. Moreover, the diversity in the strength of the NAO along with the M.EN+S.NTA is quite large (Fig. 11). The model results imply that the role of the NAO in disturbing the interannual connection between El Niño and the NTA tends to be unstable.
The present model results are based on the 16 CMIP6 model outputs combined together. We further examine the ENSO–NTA connection in individual model simulations. The results suggest that most of the 16 models realistically reproduce the positive correlation coefficient between the winter Niño-3 index and the following spring NTA SSTAs, but there are several models that overestimate (underestimate) this correlation coefficient. Further analysis suggests that the models that reproduce the strong ENSO–NTA connection tend to manifest the strong AL intensity, indicating that the intensity of AL also plays a role in the model’s ability to simulate the ENSO–NTA connection.
6. Summary and discussion
The El Niño events tend to be followed by the significant positive SSTA over the NTA. We find that the strong El Niño events are almost always accompanied by strong NTA SSTAs, whereas the moderate El Niño can be followed by either the strong or weak NTA SSTAs, suggesting the uncertainty in the connection between the moderate El Niño and the NTA SSTAs. We further investigate the possible reasons for this uncertain connection by comparing the moderate El Niño cases followed by the strong NTA SSTA (M.EN+S.NTA) with those followed by the weak NTA SSTA (M.EN+W.NTA).
The M.EN+S.NTA and the M.EN+W.NTA composites suggest that the tropical teleconnection generated by the two types of El Niño is similar, being likely due to the similar El Niño intensity. The difference in the corresponding NTA SSTAs is due to the distinct extratropical teleconnection, displaying as a well-organized and remarkable PNA pattern in response to the M.EN+S.NTA but a weakened and less significant PNA pattern in response to the M.EN+W.NTA. Further analysis suggests that the El Niño–generated extratropical teleconnection can be modulated by the strength of the AL. The extremely intensified AL coinciding with the M.EN+S.NTA leads to the enhanced negative SSTA over the North Pacific. The North Pacific SSTA is associated with a wave train propagating eastward to the North Atlantic, which strengthens the El Niño–induced PNA pattern. Therefore, El Niño accompanied by the strong AL leads to the strong NTA SSTA.
The observational results above can be further supported by the CMIP6 historical model simulations. The model simulations not only realistically reproduce the uncertainty in the connection between the moderate El Niño and the NTA SSTAs but also capture the modulation of the AL to this uncertain connection. Previous studies have noted the role of the NAO in disturbing the El Niño–NTA connection (Jiménez-Esteve and Domeisen 2018; Casselman et al. 2021; Duan et al. 2021), but the present results can be obtained even without the NAO signals, further implying the isolated role of the AL in modulating the El Niño–related NTA SSTAs.
The difference in the intensity of the AL coinciding with the M.EN+S.NTA and the M.EN+W.NTA only appears during the decaying spring (−2.18 and 0.12 hPa, respectively) but does not form during the mature winter (−2.87 and −2.58 hPa, respectively). This indicates that the different spring AL is likely due to the simultaneous midlatitude processes rather than the El Niño forcing. We further compare the synoptic-scale eddy anomalies for the two types of El Niño during spring (not shown) and find that the weakened AL for the M.EN+W.NTA during spring is due to the significant decrease in the synoptic-scale eddy activity over the North Pacific, implying that the variability of synoptic-scale eddy tends to be an internal source for the intensity of spring AL, consistent with previous studies (Lau 1988; Cai et al. 2007; Chen et al. 2020).
Previous studies indicated that both the strong and the moderate El Niños are likely to induce the strong NTA SSTA (Lee et al. 2008; Wu et al. 2020; Casselman et al. 2021). We further find a comparable strength of the NTA SSTAs in response to the S.EN+S.NTA and the M.EN+S.NTA. The spring Niño-3 index is 0.98°C for the S.EN+S.NTA, which is more than twofold that for the M.EN+S.NTA (0.41°C). However, the corresponding NTA SSTAs are 0.38° and 0.29°C, respectively, both being greater than one standard deviation (0.23°C). Moreover, the intensity of the AL accompanied by the M.EN+S.NTA is −2.18 hPa, which is close to that accompanied by the S.EN+S.NTA (−2.69 hPa). In other words, when the moderate El Niño is accompanied by an extremely strong AL, the corresponding NTA SSTA is comparable to that in response to the strong El Niño. The result can be further confirmed in the CMIP6 model simulations. The M.EN+S.NTA (0.56°C) is half as strong as the S.EN+S.NTA (1.11°C) in model simulations, but they both lead to significant NTA SSTAs (with values of 0.26° and 0.34°C, respectively). This is likely due to the comparable intensity of the AL coinciding with the M.EN+S.NTA (−2.01 hPa) and with the S.EN+S.NTA (−3.23 hPa). The result further indicates the importance of the AL in modifying the El Niño–related NTA SSTA.
Additionally, a comparison of the atmospheric circulation and NTA SSTAs in response to different combinations of the AL and El Niño suggest that the El Niño accompanied by the strong (weak) AL leads to strong (weak) warming over the NTA via generating a well-organized (weak) PNA pattern (not shown), implying that the AL is likely to be a mediator for the El Niño–NTA SSTA connection, whether the El Niño is strong or moderate. However, the strong El Niño events are almost always accompanied by the strong NTA SSTA, whereas the moderate El Niño cases are actually accompanied by either the strong or weak NTA SSTA. Thus, in this study, we mainly focused on the modulation of the AL to the uncertain connection between the moderate El Niño and the NTA SSTA.
Previous studies mentioned the teleconnection between the extratropical Pacific and the NTA SSTA (Newman et al. 2016; Larson et al. 2022). We further examine the individual role of ENSO and the AL in modulating the NTA SSTA by using partial regression analysis. Both the isolated AL index and the isolated Niño-3 index can induce significant NTA SSTAs but with different intensity (not shown). The ENSO-related NTA SSTA is stronger than the AL-related NTA SSTA, suggesting that the ENSO variability plays a dominant role in modulating the NTA SSTA. Moreover, during the research period, there are 20 years in which the strong NTA SSTAs coincide with the deepened AL. Most of these cases (15 of 20) are accompanied by El Niño events, further implying a combined influence of El Niño and the AL on the NTA SSTAs.
The present results suggest that to fully understand the moderate El Niño–related NTA SSTA variability in the decaying spring, the intensity of the simultaneous AL must be considered, implying a challenge for the prediction of the NTA SSTAs, as well as their related climate over the surrounding and remote regions.
Acknowledgments.
This study was supported by the National Key R&D Program of China (Grant 2019YFA0606703) and the National Natural Science Foundation of China (Grant 41675078; 42130504).
Data availability statement.
The HadISST dataset is available at https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html. The ERSSTv5 dataset is available at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html. The ICOADS dataset is available at https://icoads.noaa.gov/data.icoads.html. The COBE-SST dataset is available at https://psl.noaa.gov/data/gridded/data.cobe.html. The NOAA-CIRES 20th Century Reanalysis (V2) is available at https://psl.noaa.gov/data/gridded/data.20thC_ReanV2.html. The NCEP–NCAR Reanalysis version 1 is available at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. Any other data are available upon request from the corresponding author.
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