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(a,b) Instantaneous SST (°C) and surface wind (m/s) regression patterns for the anomalous normalized (a) PMM and (b) CP ENSO indices. (c) Time series of the anomalous normalized PMM (blue) and CP ENSO (orange) indices (no units) from 1948-2016. (d) Same as (c) but for the high-frequency (HF) components of these indices. (e) Same as (c) but for the low-frequency (LF) components of these indices. Shown additionally are the LF components of the PDO (solid black) and NPGO (dashed black) indices. The maps in this figure were created using NCAR Command Language Version 6.4.0 (https://doi.org/10.5065/D6WD3XH5).
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Numerous studies demonstrated that the Pacific Meridional Mode (PMM) can excite Central Pacific (CP) El Niño-Southern Oscillation (ENSO) events and that the PMM is mostly a stochastic phenomenon associated with mid-latitude atmospheric variability and wind-evaporation-SST feedback. Here we show that CP sea surface temperature (SST) variability exhi...
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... spatial patterns of the SST and surface wind anomalies associated with the PMM (Fig. 1a) and CP ENSO (Fig. 1b) exhibit striking similarities. Note that our PMM regression pattern shows no cold SST anomalies in the eastern equatorial Pacific compared to the original PMM pattern 21 , which is likely due to our choice to use the 1948- 2016 anomalies to obtain the pattern instead of the shorter data period used in the original ...
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... spatial patterns of the SST and surface wind anomalies associated with the PMM (Fig. 1a) and CP ENSO (Fig. 1b) exhibit striking similarities. Note that our PMM regression pattern shows no cold SST anomalies in the eastern equatorial Pacific compared to the original PMM pattern 21 , which is likely due to our choice to use the 1948- 2016 anomalies to obtain the pattern instead of the shorter data period used in the original study . The time ...
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... shows no cold SST anomalies in the eastern equatorial Pacific compared to the original PMM pattern 21 , which is likely due to our choice to use the 1948- 2016 anomalies to obtain the pattern instead of the shorter data period used in the original study . The time evolutions of the PMM and CP ENSO indices exhibit a close agreement with each other (Fig. 1c), with their highest cross-correlation at zero lag (R = 0.78, significant at the 99% confidence level; Fig. 2a). Both PMM and CP ENSO indices can be well separated into high-frequency (HF; Fig. 1d) and low-frequency (LF; Fig. 1e) components via Singular Spectrum Analysis (SSA; see Methods). Again, the highest cross-correlations between ...
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... of the shorter data period used in the original study . The time evolutions of the PMM and CP ENSO indices exhibit a close agreement with each other (Fig. 1c), with their highest cross-correlation at zero lag (R = 0.78, significant at the 99% confidence level; Fig. 2a). Both PMM and CP ENSO indices can be well separated into high-frequency (HF; Fig. 1d) and low-frequency (LF; Fig. 1e) components via Singular Spectrum Analysis (SSA; see Methods). Again, the highest cross-correlations between these indices exists at zero lag for both timescales (Fig. 2a,c). Furthermore, we observe a relatively close instantaneous relationship between the LF PMM and LF CP ENSO components with the LF PDO ...
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... in the original study . The time evolutions of the PMM and CP ENSO indices exhibit a close agreement with each other (Fig. 1c), with their highest cross-correlation at zero lag (R = 0.78, significant at the 99% confidence level; Fig. 2a). Both PMM and CP ENSO indices can be well separated into high-frequency (HF; Fig. 1d) and low-frequency (LF; Fig. 1e) components via Singular Spectrum Analysis (SSA; see Methods). Again, the highest cross-correlations between these indices exists at zero lag for both timescales (Fig. 2a,c). Furthermore, we observe a relatively close instantaneous relationship between the LF PMM and LF CP ENSO components with the LF PDO and an out-of-phase ...
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... via Singular Spectrum Analysis (SSA; see Methods). Again, the highest cross-correlations between these indices exists at zero lag for both timescales (Fig. 2a,c). Furthermore, we observe a relatively close instantaneous relationship between the LF PMM and LF CP ENSO components with the LF PDO and an out-of-phase relationship with the LF NPGO (Figs. 1e and 2c). The close agreement between PMM and CP ENSO also holds on all timescales when looking at the SST and surface wind regression patterns for various lead and lag times (Figs. S1-S3), with no clear evidence that one phenomenon is leading the ...
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... mixed layer dynamics (that is the WES feed- back), thereby allowing for a fast Central Pacific-Meridional Mode (CP-MM) positive feedback process (Fig. 5). The CP-MM feedback is the missing puzzle piece that is required to explain the very close instantaneous relation- ship between these two modes of climate variability on interannual timescales (Fig. 1d). Note that our model is able to simulate meridional mode responses both in the North Pacific and in the South Pacific. Importantly, our results lead to the insight that CP ENSO and PMM cannot be considered two independent dynamical phenomena, which then has important implications for impact attribution studies, such as Murakami et al. ...
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... the CP ENSO-induced Aleutian Low variability will generate low-frequency North Pacific SST changes, which then is able to explain the close relationship between PMM, CP ENSO, PDO, and NPGO at low-frequencies (Fig. 1e). Therefore, we conclude that our proposed CP-MM feedback mechanism likely operates both on fast and slow timescales. We summarize these pathways and how they fit into our current understand- ing of the relationships between CP ENSO, the PMM, and Pacific decadal variability in Fig. 5. We expect that including a dynamical ocean in the ...
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... use the same EOF methodology (but for a single variable) on the composite (n = 20) simulated sea level pressure (SLP) anomalies and on the full (non-composite) SST anomalies separately in the North Pacific (120°E-100°W and 20°N-66°N) to investigate the CP ENSO induced teleconnections. The Multi-Taper-Method (MTM) is utilized to calculate the power spectra of the model PMM, NH PMM, and North Pacific SST PC1 42 . ...
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... However, TPDV exhibits a wider meridional extent 13,14 and shows stronger variability in the tropics than in the extra-tropics 1,3,14,15 . Recent studies have also identified that TPDV manifests the largest loadings in the equatorial central and northeastern Pacific 3,15,16 , bearing resemblance to the Central Pacific El Niño 17,18 and the Pacific Meridional Mode (PMM) patterns 19,20 . In addition, some studies suggested that TPDV exhibits a prominent periodicity of around 11 years 16,21 mainly observed after 1950 21 . ...
... The decadal sea level pressure (SLP) pattern is characterized by a band of anomalous high SLP anomalies confined to the western side of the basin while anomalous low pressure dominates in the east, especially in the extratropics. The latter structure resembles the Aleutian Low, suggesting possible atmospheric teleconnections excited by TPDV 20,46,47 . The circulation patterns in the extratropical Northern and Southern Hemispheres are similar to those associated with the North 20 and South 48 PMM, respectively. ...
... Subsurface temperature coordinate transformation and NECC pathway All data are linearly detrended before conducting analyses and anomalies are defined as deviations from the climatology of the whole period of the respective dataset. As the focus of the study is on the decadal timescale, an [8][9][10][11][12][13][14][15][16][17][18][19][20] year Lanczos bandpass filter is used 79 . For subsurface data, the temperature on pressure coordinates is converted to time-mean sigma coordinates with an increment of 0.02 kgm −3 by linear interpolation (the resulting isopycnal temperature is not necessarily salinity compensated). ...
We demonstrate the key role of off-equatorial subsurface temperature anomalies in driving the phase transition of Tropical Pacific Decadal Variability (TPDV) using observation and model experiments. During the positive phase of TPDV, anomalous atmospheric responses in the off-equatorial northwestern Pacific induce positive Ekman pumping. The resulting negative subsurface temperature anomaly generated then propagates along the North Equatorial Countercurrent pathway towards the central basin, causing a sign reversal of the equatorial sea-surface temperature anomalies around three years later. Moreover, the positive phase of TPDV possibly changes the state of the Kuroshio Extension through tropical-extratropical interaction, which subsequently projects onto the footprint of the Pacific Meridional Mode, thereby amplifying subsurface-produced disturbance 0–12 months before the cold peak phase. The cold phase is completely established after five years. Similarly, the same dynamic applies to the reversed phase, leading to a preferred decadal oscillation driven by off-equatorial subsurface temperature anomalies and extratropical-tropical ocean-atmosphere interaction.
... According to Stuecker [30] and Vimont [28], subsequent Pacific SST work should focus on both the PMM and ENSO since they are intrinsically linked at interannual timescales. While the relationships among the PMM and ENSO are somewhat well defined, SST patterns that characterize the underlying structures are limited to those patterns derived from EOFs with the associated limitations discussed above. ...
... A summary of the SSTA patterns, dominant climate drivers, and frequency trends is presented in Table 2. While previous research heavily relies on EOFs to understand tropical Pacific SST variability [11,[22][23][24][30][31][32][33], this study highlights the use of cluster analysis as advantageous over EOFs for identifying SST patterns and understanding their climatic impacts. Cluster analysis directly constructs spatial patterns and events from observed SST fields without relying on variability fractions, and it does not assume linearity, allowing for the inclusion of nonlinear relationships [35,36]. ...
El Niño–Southern Oscillation (ENSO) phases and flavors, as well as off-equatorial climate modes, strongly influence sea surface temperature (SST) patterns in the eastern tropical Pacific and downstream climate. Prior studies rely on EOFs (which characterize fractional SST variance) to diagnose climate-scale SST structures, limiting the ability to link individual ENSO flavors with downstream phenomena. Hierarchical and k-means clustering methods are used to construct Eastern Pacific patterns from the ERSST dataset spanning 1950 to 2021. Cluster analysis allows for the direct linkage of individual SST years/seasons to ENSO phase, providing insight into ENSO flavors and associated downstream impacts. In this study, four clusters are revealed, each depicting unique SST patterns influenced by ENSO and Pacific Meridional Mode (PMM) phases. A case study demonstrating the utility of the clusters was also carried out using accumulated cyclone energy (ACE) in the Atlantic and Eastern Pacific basins. Results showed that Eastern Pacific (EP) El Niño suppresses Atlantic tropical cyclone (TC) activity, while Central Pacific (CP) La Niña enhances it. Further, EP El Niño, coupled with positive PMM, amplifies ACE. Ultimately, the methods used herein offer a cleaner analysis tool for identifying dominant SSTA patterns and employing those patterns to diagnose downstream climatic effects.
... Extratropical forcings are important sources of diverse ENSO properties, especially the impacts of the North Pacific variability on the tropical sea surface temperature (SST) [7][8][9][10][11][12][13]. The North Pacific oscillation (NPO), characterized by a meridional dipole of sea level pressure (SLP) over the North Pacific [14], can induce SST anomalies that resemble the North Pacific meridional mode (NPMM) [15][16][17], significantly altering ENSO's timing, flavor, etc. [9,18,19], and therefore acting as a bridge between the mid-latitude forcings and ENSO [19,20]. Different mechanisms were previously proposed for explaining how extratropical forcings influence ENSO, including the seasonal foot-printing mechanism [10,21], the trade wind charging [22], and ocean Kelvin waves [23,24]. ...
Despite extratropical forcing has been recognized as important factors modulating the El Niño-Southern Oscillation (ENSO) properties on the interannual time scale, little is known about whether and how Arctic forcing changes the tropical sea surface temperature (SST). The current study reveals a significant link between the net surface sensible heat flux (SHF) in the Arctic and the SST anomalies in the tropical eastern Pacific (TEP). Specifically, anomalous upward SHF into the Arctic atmosphere in February leads to a warmer TEP in the subsequent spring and summer. A northeast-southwest-tilted North Pacific Oscillation-like atmospheric pattern associated with the upward Arctic SHF anomaly induces SST cooling in the subtropical North Pacific via positive Wind-Evaporation-SST feedback, which further promotes TEP SST warming via meridional heat advection, thermocline feedback, and nonlinear processes. The spring-to-summer TEP SST anomalies driven by the preceding anomalous Arctic SHF hence potentially modulate the seasonal evolution of ENSO. Our finding implies that we should take into account the Arctic-tropics linkages when comprehensively understanding the ENSO variability and improving ENSO projection skills.
... It has been well established that NPMM could affect the following ENSO events (Chang et al. 2007;Amaya et al. 2019). Stuecker et al. (2018) pointed out that the Central Pacific ENSO could in turn generate an instantaneous NPMM response by initiating atmospheric teleconnection with a strong projection on the Aleutian Low. Similarly, if the pre-existing winter NPO for ENSO years can be traced to the tropical Pacific itself, caution is warranted in determining whether it can offer additional predictive information for subsequent ENSO events beyond the commonly used tropical Pacific ocean-atmosphere conditions. ...
... consistent with previous studies suggesting that extratropical NPO signals can induce SST anomalies resembling the NPMM pattern(Chiang and Vimont 2004;Stuecker 2018).Simultaneously, there are some significant SST and wind anomalies in the subtropical South Pacific, while the spatial pattern is different from the SPMM pattern inFig. S6b. ...
El Niño-Southern Oscillation (ENSO), the dominant mode of interannual variability in the tropical Pacific, is well known to affect the extratropical climate via atmospheric teleconnections. Extratropical atmospheric variability may in turn influence the occurrence of ENSO events. The winter North Pacific Oscillation (NPO), as the secondary dominant mode of atmospheric variability over the North Pacific, has been recognized as a potential precursor for ENSO development. This study demonstrates that the pre-existing winter NPO signal is primarily excited by sea surface temperature (SST) anomalies in the equatorial western-central Pacific. During ENSO years with a preceding winter NPO signal, which accounts for approximately 60% of ENSO events observed in 1979–2021, significant SST anomalies emerge in the equatorial western-central Pacific in the preceding autumn and winter. The concurrent presence of local convection anomalies can act as a catalyst for NPO-like atmospheric circulation anomalies. In contrast, during other ENSO years, significant SST anomalies are not observed in the equatorial western-central Pacific during the preceding winter, and correspondingly, the NPO signal is absent. Ensemble simulations using an atmospheric general circulation model driven by observed SST anomalies in the tropical western-central Pacific can well reproduce the interannual variability of observed NPO. Therefore, an alternative explanation for the observed NPO-ENSO relationship is that the preceding winter NPO is a companion to ENSO development, driven by the precursory SST signal in the equatorial western-central Pacific. Our results suggest that the lagged relationship between ENSO and the NPO involves a tropical-extratropical two-way coupling rather than a purely stochastic forcing of the extratropical atmosphere on ENSO.
... 2a, b). Considering that ENSO events in the preceding winter (ND-1J0, where '-1' refers to preceding year) may impact the FMA0 NPMM (DiLorenzo et al. 2015;Stuecker 2018; Fang and Yu 2020;Fan et al. 2023), ...
The North Pacific Meridional Mode (NPMM) peaking in boreal spring influences El Niño-Southern Oscillation (ENSO) properties in the ensuring winter. Whether the precursory impact of NPMM on the spatial diversity of ENSO has decadal variation remains unknown. Using long-term reanalysis datasets, here we find that the Interdecadal Pacific Oscillation (IPO) significantly modulates the NPMM forcing on two types of ENSO. During positive IPO (+IPO) phase, a strengthened background Aleutian low and southward-shifted storm track, in comparison to negative IPO (−IPO) phase, produce stronger basin-scale negative geopotential height tendency anomalies over the North Pacific through synoptics-scale eddy–mean flow interaction. Such strong background negative tendency facilitates an Aleutian low-like pressure monopole rather than a North Pacific Oscillation (NPO)-like pressure dipole in boreal spring, leading to a weak NPMM that cannot effectively promote development of either a central Pacific (CP) or an eastern Pacific (EP) ENSO. By contrast, the NPO-like dipole enhances in boreal spring during −IPO, corresponding to stronger and more frequently occurred NPMM events that induce robust CP-ENSO-like response in boreal winter. Moreover, the −IPO-related tropical Pacific mean states and associated positive feedbacks cause a strong decrease (slight increase) in mixed layer temperature variance in the equatorial eastern (central) Pacific, further contributing to the enhanced correlation between NPMM and CP-ENSO. Thus, −IPO plays a role in the stronger impact of NPMM on CP-ENSO since the 1990s, and the modulation effects of IPO should be considered in understanding the extratropical-tropical climatic connection and ENSO spatial diversity.
... Wang, 2019). During the NPMM and CP El Niño-like phase (Figure 1e; hereafter, P2), the NPMM is characterized by weaker winds and evaporative heat loss over the central subtropical NPO (Vimont et al., 2001(Vimont et al., , 2009, where the positive footprint of SSTAs extends southwestward through the wind-evaporation-SST (WES) feedback (Stuecker, 2018;Xie & Philander, 1994), favoring anomalous cyclone (AC) formation over the western TPO (Figure 3b). During the VM and CP El Niño-like phase (Figure 1f; hereafter, P3), the anomalous warming in the central TPO and negative SSTAs in the eastern TPO and NPO generate positive and negative Walker circulation anomalies over the western and eastern TPO, respectively, resulting in the suppression of the eastward propagation of warm SSTAs and AC enhancement over the western TPO (Shi et al., 2022) (Figure 3c). ...
Plain Language Summary
This study utilizes multivariate empirical orthogonal function analysis to examine long‐term hydrographic observations and identify coherent variations in surface temperature anomalies in the Tropical and North Pacific Ocean. The leading mode displays in‐phase combinations of the East Pacific El Niño and warm Pacific Decadal Oscillation. The second mode displays a Central Pacific El Niño and North Pacific Meridional Mode‐like mode. The third mode resembles a Central Pacific El Niño and Victoria Mode‐like mode. The objective is to comprehend the synergies between the climatic variabilities in the Tropical and North Pacific Ocean, as well as the processes by which the predominant climate variabilities regulate the undocumented potential temperature variabilities in the deep South China Sea. Extrinsic forcings in the Tropical and North Pacific Ocean regulate the interannual–decadal temperature variabilities by modulating the Luzon Strait Transport. On the atmospheric bridge, these extrinsic forcings affect the upper‐layer Luzon Strait Transport by altering the local Ekman transport and Kuroshio intrusion associated with the bifurcation of the North Equatorial Current. On the oceanic pathway, climate variabilities disturb the deep‐layer Luzon Strait Transport by changing the ocean bottom pressure between the two sides of the Luzon Strait.
... As the second leading oceanatmosphere coupling mode in the North Pacific (Chiang and Vimont 2004), the PMM is considered as a key mechanism to explain the El Niño-like interdecadal pattern (Di Lorenzo et al 2015, Liguori andDi Lorenzo 2018). Both statistical results in observational data and model simulation results show that there is a close relationship between the PMM and the PDO in their low-frequency components (Di Lorenzo et al 2015, Stuecker 2018. ...
... Note that the 600 hPa relative humidity and 850 hPa relative vorticity in P1 are multiplied by 10 and are one order of magnitude smaller than in P2. changes significantly with the strengthening of PMM.During 1951During −1989, the PDO index and the PMM index are insignificantly correlated for the months of June−November (July−September). On the other hand, during the period 1990−2021 (P2), the two timeseries are positively correlated with a significant correlation coefficient of 0.76 (0.84).Stuecker (2018) found that the low-frequency component of the PMM index is correlated with the PDO index. However, the PMM-PDO relationship infigure 7is not stationary during the period 1951−2021. ...
The Pacific Decadal Oscillation (PDO) and Pacific Meridional Mode (PMM) are prominent climate modes in the North Pacific with well-established impacts on tropical cyclone (TC) genesis in the western North Pacific (WNP) basin. While previous research has primarily focused on the roles of the PDO and PMM in regulating TC genesis through the modification of large-scale environmental factors, this study investigates the evolving influence of the PDO on WNP TC genesis since the 1950s. Remarkably, our analysis reveals a shift in the PDO-TC genesis relationship, transitioning from a significant negative correlation to a significant positive correlation since the 1990s. This shift is attributed to variations in the specific large-scale factors through which the PDO affects TC genesis. Furthermore, this study suggests that these changes appear to be linked to the PMM strengthening on the interdecadal timescale in recent decades. The linkage of the PMM strengthening to the PDO-related atmospheric circulation is further confirmed by the results of a 500-year pre-industrial numerical experiment, suggesting that the PMM strengthening may result from natural internal variability. The results underscore the non-stationary relationship between PDO and WNP TC genesis, with the PMM intensity probably influencing their relationship.
... Various physical processes have been proposed to explain the pathway by which the PMM influences ENSO, including the seasonal footprinting mechanism (Chiang & Vimont, 2004;Vimont et al., 2001Vimont et al., , 2003, the trade wind charging process (Anderson et al., 2013), and the summer deep convection triggering mechanism (Amaya, 2019;Amaya et al., 2019). However, recent studies suggest that the PMM, which was a previously widely-recognized conduit for extra-tropical atmospheric variabilities to impact ENSO, actually has tropical origins (Richter et al., 2022;Stuecker, 2018;H. Zhang, Deser, et al., 2014;Y. ...
... Previous studies have noted the close agreement between the low-frequency component of the PMM and tropical central Pacific decadal variability, yet whether mid-latitude dynamics are essential in enabling such a linkage on decadal timescales remains debated (Di Lorenzo et al., 2015;Joh & Di Lorenzo, 2019;C. Liu et al., 2019;Richter et al., 2022;Stuecker, 2018;Wu et al., 2021). On shorter timescales, the PMM is frequently linked to the stochastic atmospheric forcing in the mid-latitudes (Vimont et al., 2001(Vimont et al., , 2003. ...
... For example, previous studies suggest that the circulation anomalies associated with the southern lobe of the NPO could modulate the intensity of local trade wind and produce surface latent heat flux anomalies, thereby contributing to SST variations resembling the PMM (Chang et al., 2007;Joh & Di Lorenzo, 2019;Vimont et al., 2003). In contrast, Stuecker (2018) demonstrated that the high-frequency component of PMM exhibits a strong instantaneous relationship with the central Pacific ENSO. However, it is noted that this finding was based on the PMM index which does not exclude the influence of ENSO. ...
The Pacific Meridional Mode (PMM) has long been associated with extra‐tropical air‐sea coupling processes, which are thought to influence the development of El Niño‐Southern Oscillation (ENSO). Here we show that the PMM on seasonal to interannual timescales is closely associated with a newly proposed tropical mode known as the ENSO Combination mode (C‐mode), which arises from the nonlinear interaction between ENSO and the background annual cycle in the deep tropics. The PMM exhibits a remarkable resemblance with the C‐mode in atmospheric patterns, spectral characteristics, and local impacts. Based on a simple Hasselmann‐type model, we further demonstrate that the C‐mode‐related atmospheric anomalies can effectively drive PMM‐like sea surface temperature anomalies. As the C‐mode captures seasonally modulated ENSO characteristics, the seasonal‐to‐interannual PMM variability could naturally establish a connection with ENSO, thereby offering an alternative explanation for the observed relationship between PMM and ENSO.
... This is because, as the negative PMM-associated cold SST and northeasterly anomalies move towards the equator, they strengthen the Pacific easterly trade winds and transport anomalously cold water into the tropical central Pacific, creating the conditions for another LN to occur (Fig. 3a, b, d, e). This PMM mechanism incorporates two-way interactions within the Pacific between tropical ENSO and subtropical PMM, exerting positive feedbacks that sustain or re-intensify ENSO conditions [9][10][11][12]17,44 . Various physical processes are linked to the activation of the PMM mechanism, including the WES feedback 42,44 , trade wind charging 49 , oceanic Rossby wave reflection 50 , and summer deep convection response 51 . ...
... This PMM mechanism incorporates two-way interactions within the Pacific between tropical ENSO and subtropical PMM, exerting positive feedbacks that sustain or re-intensify ENSO conditions [9][10][11][12]17,44 . Various physical processes are linked to the activation of the PMM mechanism, including the WES feedback 42,44 , trade wind charging 49 , oceanic Rossby wave reflection 50 , and summer deep convection response 51 . A similar mechanism, involving a positive PMM often induced by central Pacific El Niño events and prone to leading to multi-year El Niño events, has also been reported 12,17 ( Supplementary Text 1 for details). ...
Previous studies have emphasized the significance of a strong El Niño preceding La Niña (LN) in the formation of multi-year LN events due to the slow recharge-discharge ocean heat content process. However, observational analyses from 1900 to 2022 reveal that the majority (64%) of multi-year LN events did not necessitate a preceding strong El Niño to generate their second LN, suggesting an overemphasis on traditional views. Instead, here we show that a negative phase of the North Pacific Meridional Mode (PMM) during spring, when the first LN begins to decay, activates the mechanism responsible for triggering another LN and producing a multi-year event. The westward extension of the first LN’s cold anomalies, which interact directly with the eastern edge of the western Pacific warm pool, is highlighted as a crucial factor in the occurrence of a negative PMM. Additionally, the PMM mechanism can create a third LN, leading to triple-dip events.
... For instance, 'Atm-Slab' models (atmospheric models coupled to slab ocean models) 107,108 exhibit a frequency spectrum reddening of weather and climate variability at decadal timescales through a sequence of extratropical-to-tropical influences (ENSO precursors to ENSO development) and tropical-to-extratropical feedbacks (ENSO teleconnections) 107 , as supported by observations 109 . Indeed, model experiments 110 indicate that ENSO teleconnections from the central equatorial Pacific reinforce the NPMM and increase its persistence, resulting in the decadal NPMM variations detected in century-long coral time series from the northeastern subtropical Pacific 111 . Additionally, tropical wind anomalies associated with the Meridional Modes may induce meridional pycnocline flow (as with the Tropical Wind Charging mechanism), providing the atmospheric forcing needed to alter the strength of the STCs and produce equatorial SST anomalies. ...
... As for extratropical forcing, wind responses to tropical decadal SST anomalies might also be important in driving TPDV. Specifically, SST anomalies in the central equatorial Pacific, where decadal anomalies are more prominent, excite atmospheric Rossby waves, whose subtropical component weakens the subtropical trade winds in both hemispheres 110,120 (Fig. 5a,b). These equatorially forced subtropical wind anomalies then reinforce the equatorial anomaly through thermodynamic (for example, triggering deep convection) 100 or dynamic (for example, through changes in equatorward mass transport induced by the anomalous winds) 26 processes. ...
... lower-frequency variability 110 . Decadal timescale SST anomalies in the Atlantic and Indian Oceans also induce wind anomalies in the tropical Pacific conducive to the development of SST anomalies of the opposite sign 23,127,128,130 . ...
Naturally occurring tropical Pacific variations at timescales of 7–70 years — tropical Pacific decadal variability (TPDV) — describe basin-scale sea surface temperature (SST), sea-level pressure and heat content anomalies. Several mechanisms are proposed to explain TPDV, which can originate through oceanic processes, atmospheric processes or as an El Niño/Southern Oscillation (ENSO) residual. In this Review, we synthesize knowledge of these mechanisms, their characteristics and contribution to TPDV. Oceanic processes include off-equatorial Rossby waves, which mediate oceanic adjustment and contribute to variations in equatorial thermocline depth and SST; variations in the strength of the shallow upper-ocean overturning circulation, which exhibit a large anti-correlation with equatorial Pacific SST at interannual and decadal timescales; and the propagation of salinity-compensated temperature (spiciness) anomalies from the subtropics to the equatorial thermocline. Atmospheric processes include midlatitude internal variability leading to tropical and subtropical wind anomalies, which result in equatorial SST anomalies and feedbacks that enhance persistence; and atmospheric teleconnections from Atlantic and Indian Ocean SST variability, which induce winds conducive to decadal anomalies of the opposite sign in the Pacific. Although uncertain, the tropical adjustment through Rossby wave activity is likely a dominant mechanism. A deeper understanding of the origin and spectral characteristics of TPDV-related winds is a key priority.
