Single-year and double-year El Niños

Compared with well documented and frequent occurrence of multi-year La Niña, double-year El Niño is less frequent and has not been well investigated. Both of them are a discrepancy from the cyclic behavior of the El Niño-Southern Oscillation and deserve investigation. Here, we demonstrate the diversity of single- and double-year El Niño events in their strengths, flavors, as well as associations with the recharge/discharge processes. The possible different climate impacts are also discussed. During 1950–2021, 75% of El Niño events persist for one year, and 25% of them last for two years. Both central and eastern Pacific type El Niños occur in the single-year and double-year El Niños with various strengths. On average, there is no relationship between the initial time and duration of an El Niño event. Compared with the single-year El Niños, the averaged warm water volume (WWV) is larger in the peak and declines slower for the double-year El Niños, suggesting that a persistently recharged heat condition of the equatorial Pacific is a precondition for the emergence of a second-year El Niño. The faster decline of WWV in the single-year El Niños is associated with the in-phase decrease of its intraseasonal-interseasonal and interannual components, while the slower decline of WWV in the double-year El Niños is determined by the interannual component. In addition, the single-year and double-year El Niño may have different impacts on regional climate.


Introduction
Accurate short-term climate prediction is a need for society. For the short-term climate prediction, El Niño-Southern Oscillation (ENSO) plays a crucial role and is the major source of the predictability of the global climate at seasonal-interannual time scales (National Research Council 2010;Hu et al. 2020a). ENSO is a quasi-periodic phenomenon with a reoccurrence every 2-7 years (McPhaden et al. 2021). Its growth and persistency are controlled by positive ocean-atmosphere feedbacks among the wind stress, sea surface temperature (SST), and thermocline depth fluctuations in the tropical Pacific (Bjerknes 1969), while its phase transition and decay are determined by a variety of negative feedbacks (Jin 1997;Wang 2018).
Although there have significant progress in understanding ENSO since Bjerknes (1969), unfortunately, successfully predicting ENSO is still a challenge sometimes (e.g., Barnston et al. 2012;Zhu et al. 2014;Hu et al. 2020b). Such challenge is mainly due to the noncyclic feature of ENSO, which is linked to the asymmetry or diversity of ENSO (Choi et al. 2013;Timmermann et al. 2018). For example, on average, the amplitudes of SST anomalies in the eastern equatorial Pacific are larger in El Niño than in La Niña during their peaks. Spatially, major SST anomalies are in the eastern tropical Pacific in La Niña, while sometimes in the central (CP) and sometime in the eastern (EP) tropical Pacific in El Niño.
Another asymmetry is that the discharge associated with El Niño is stronger than the recharge linked to La Niña (Kessler 2002;Hu et al. 2017;Li et al. 2020;2022a). Recently, Li et al. (2022a) found that on average, the amplitude of the asymmetric part of thermocline fluctuation associated with ENSO is about 1/4 of that of the total thermocline fluctuation associated with ENSO. Such asymmetry leads to the asymmetric evolution of ENSO: both El Niño and La Niña are mostly followed by La Niña. As a result, double-dip La Niñas are observed very often, but it is seldom for two-year consecutive El Niño (Okumura and Deser 2010;Okumura et al. 2011;Choi et al. 2013;Hu et al. 2014;An and Kim 2018;Kim and Yu 2021;Li et al. 2022a; Fig. 1). Based on the nonlinearity of the atmospheric processes (Hoerling et al. 1997;An and Kim 2018) and asymmetry associated with the recharge and discharge process (Kessler 2002;Hu et al. 2017), different mechanisms have been proposed for the asymmetric duration between El Niño and La Niña. Okumura and Deser (2010) and Okumura et al. (2011) emphasized the asymmetric role of the Indian Ocean in the ENSO cycle transition. During El Niño, delayed basin warming of the Indian Ocean induces anomalous surface easterly winds in the western equatorial Pacific, leading to the phase transition. During La Niña, the anomalous easterly wind forced by the reduced convection in the equatorial Pacific is stronger than the surface westerly wind induced by the Indian Ocean cooling because of the westward shift of the Pacific convection anomalies, prolonging the La Niña condition. Hu et al. (2014) proposed an ocean mechanism based on the asymmetric recharge and discharge processes. They argued that the reflected cold water of the previous La Niña from the southern American coast propagates westward as Rossby waves and interrupts the oceanic heat recharge process, leading to the emergence of a second-year La Niña. As a result, a strong La Niña is mostly followed by a relatively weaker La Niña, except for the presence of a strong downwelling Kelvin along the equatorial Pacific. Wu et al. (2019) noted that the duration of La Niña event preceded by a strong El Niño favor to persist into the second year due to large heat discharge of the equatorial Pacific and delayed adjustments of the tropical Atlantic and Indian Oceans to the tropical Pacific cooling.
Compared with well documented and frequent occurrence of multi-year La Niña, double-year El Niño is less frequent and has not been well investigated. Wu et al. (2019) argued that an early developed El Niño normally decays quickly after the mature phase because of the early arrival of delayed negative oceanic feedback and fast adjustments of the tropical Atlantic and Indian Oceans to the tropical Pacific Ocean warming. By contrasting multiyear El Niño events with single-year ones, Kim and Yu (2020) suggested that multiyear El Niño events may be caused by a negative North Pacific Oscillation (NPO) in the first-year winter via subtropical Pacific coupling processes and by a negative Indian Ocean Dipole (IOD) in the first-year fall via tropical inter-basin coupling processes. The phase information of the preceding winter NPO and fall IOD together can be used to project the evolution characteristics of El Niño events, particularly the re-intensified multiyear and transitional single-year events. Kim and Yu (2021) mentioned a possible connection between the onset location and the evolution of El Niños. When the onset is located east (west) of 155°E, the event has a strong tendency to reverse (maintain) its phase, leading to cyclic (multiyear) evolution. Recently, Ding et al. (2022) proposed a two-way interaction between NPO and El Niño: the NPO during boreal winter can trigger a CP El Niño during the subsequent winter, which excites atmospheric teleconnections to the extratropics that reenergize the NPO variability, then re-triggers another El Niño event in the following winter, resulting in persistent El Niño-like states.
Nevertheless, the multi-year duration of both El Niño and La Niña is a discrepancy from the cyclic behavior of the ENSO theory, such as the recharge and discharge oscillator (Jin 1997), thus deserves investigation. For example, (a) are there any differences in the SST and warm water volume (WWV) anomalous evolution between the single-and double-year El Niños? (b) What are the differences in different time sale variations between the single-and doubleyear El Niños? These are the focus of this work. The paper is organized as follows. After the introduction, the data and methods used in the analyses are described in Sect. 2; Sect. 3 shows the diversity of single-year and double-year El Niños; a summary with discussion is given in Sect. 4.

Data and methods
SSTs are from HadISST1 with a 1 o x 1 o resolution (Rayner et al. 2003). To exclude the influence of the interdecadal variations and long-term trends and to focus on the interseasonal-interannual time scale feature of ENSO, the relative Niño3.4 index is adopted to represent ENSO, which is defined as the difference between the SST anomaly averaged in (5°S − 5°N, 170°W − 120°W; the Niño3.4 region) and the SST anomaly averaged in the whole tropics (0°-360°, ENSO year classification follows the NOAA Climate Prediction Center (CPC) (https://origin.cpc.ncep.noaa.gov/ products/analysis_monitoring/ensostuff/ONI_v5.php). An El Niño (a La Niña) is referred to that 3-month running mean Niño 3.4 index is above 0.5 °C (blow − 0.5 °C) with continuation for a minimum of 5 consecutive overlapping seasons. In the CPC definition, the conventional Niño 3.4 index is calculated with SST from ERSST.v5 ) and climatology based on centered 30-year base periods updated every 5 years. All the El Niño (a La Niña) years during January 1950-December 2021 are listed in Table 1 according to NOAA/CPC's classification. In the following analyses, year "0" refers to the development year, year "1" is the decay year, and year "2" represents the year after the decay year.

Diversity of single-year and double-year El Niños
From Table 1; Fig. 1, we can see that most of the El Niño events persist for one year, a few of them last for two years, and no one for three years. In contrast, most La Niña events last for two years, and some of them persist for one year or three years. Statistically ( Fig. 2 For single-year El Niños and based on the relative Niño3.4 index, the strengths vary from weak to extremely strong (Fig. 3a), and their average declines to below average in the spring of the decay year. While for double-year El Niños (Fig. 3b), the strengths of the two peaks in their averages are comparable, and all the first-year peaks of the between GODAS and ORAs4 during the common period (Jan1979-Dec2016) are larger than 0.9 along the equatorial Pacific Ocean (not shown), indicating the consistency of D20 anomalies along the equatorial Pacific Ocean between GODAS and ORAs4.
The atmospheric circulation variables (surface pressure and zonal wind stress) from the National Centers for Environmental Prediction and the National Center for Atmospheric Research (NCEP/NCAR) reanalysis for the period from January 1949 to the present (Kalnay et al. 1996). The precipitation over land is analyzed using the data from Chen et al. (2002) which are derived from gauge observations from over 17,000 stations collected in the Global Historical Climatology Network and the Climate Anomaly Monitoring System datasets. The data span the period since January 1948 with a 1 o x1 o resolution.
To identify the contributions of different time scales to the intensity and evolution of ENSO recharge and discharge processes, Ensemble Empirical Mode Decomposition (EEMD) is adopted for decomposing the WWV index. EEMD is adaptive and derives optimal frequencies from the data itself, instead of using a priori "global" basis functions of rigid periods in Fourier transform-based time series analysis, which naturally separate components of different time scales (Wu and Huang 2009).

Fig. 2 Percentages (%) of single-, double-, and triple-year El Niños (red bars) and La Niñas (blue bars) during 1950-2021
As a key indicator of the ENSO phase turnaround (Jin 1997;Hu et al. 2017), the WWV index shows apparent differences between the averages of the single-year and double-year El Niños. The WWV index decreases quickly since the peak of single-year El Niño and becomes negative since January of the decay year (January(1); Fig. 5a), consisting with the recharge and discharge oscillator paradigm (Jin 1997). While, compared with the single-year El Niños (Fig. 5a), for the double-year El Niños (Fig. 5d), their averaged WWV is larger in the peak, declines slower, and reaches negative until July(1), suggesting that a persistently recharged heat condition of the equatorial Pacific is a precondition for the emergence of a second-year El Niño.
In addition to the impact of low-frequency variation and long-term trend on the ENSO (Yeo et al. 2016;Li et al. 2019), Li et al. (2022b) recently noted that La Niña decay in the boreal spring and early summer is mainly controlled by the intraseasonal-interseasonal variation (2-12 months). From Fig. 5b, c, e, and f, we can also see that both the intraseasonal-interseasonal and interannual components of the WWV index have appreciable differences in the averages relative Niño3.4 index in each El Niño don't exceed 2 o C (extreme El Niño). That is in contrast to La Niña.  argued that a strong La Niña is favorable to be followed by a second-year La Niña, which, on average, is weaker than the first-year La Niña. Also, from Fig. 3, we can see that on the average (the black thick curves), the times of the relative Niño3.4 index turning to positive are close in the single-year and double-year El Niños. Thus, statistically, there is no relationship between the initial time and duration of an El Niño event, differing from Wu et al. (2019).
To identify the flavor of El Niño, i.e., CP EP events, the difference between the Niño3 and Niño4 indices (Niño3 minus Niño4) is displayed (Kao and Yu 2009). When the difference is positive (negative) in the peak phase of an El Niño, it is called EP (CP) El Niño. From Fig. 4, we can see both positive and negative differences present in the singleyear and double-year El Niños, meaning that both CP and EP type El Niños occur in the single-year and double-year El Niños. we examine the connection between the evolutions of the WWV index and surface zonal wind stress anomalies along the equatorial Pacific, which are represented by the average in the Niño3 and Niño4 regions (5 o S-5 o N, 160 o E-90 o W; Fig. 6). From Fig. 6, we can see the appreciable differences in the wind stress anomalies along the equatorial Pacific Ocean in both the single-year and double-year El Niño. That is consistent with the diversity shown in the relative Niño3.4, Niño3-Niño4, and WWV indices (Figs. 3, 4 and  5). For the single-year El Niño composites, the decline of WWV index from positive to negative from year(0) to year(1) (discharging; Fig. 5a) is associated with westerly wind stress anomaly (Fig. 6a), while easterly wind stress anomaly since the middle of year(1) is linked to the increase of the WWV index. That fits the recharge/discharge paradigm (Jin 1997) and the cyclic feature of ENSO. However, for the double-year El Niño composites, the persistently positive WWV index (Fig. 5a) is accompanied by an overall small zonal wind stress anomaly along the equatorial Pacific (Fig. 6b). That might imply that the recharge/discharge process is less affectional, resulting in the non-cyclic evolution of ENSO, e.g. double-year El Niño.
The single-year and double-year El Niños may have different impacts on regional climate. As an example, Fig. 7 shows the composites of SST (shading over oceans), land precipitation (shading over land), and surface pressure (contours) anomalies in the winter of the mature phase of single-year El Niños (represented by S_D(0)JF(1)), and in the winter of the mature phase of the 1st and 2nd years of the double-year El Niños (represented by D_ D(0)JF(1) and D_D(1)JF(2), respectively). Although the broad-scale patterns are similar in the composites (Fig. 7a-c), there are a lot of detailed differences in their amplitudes and regional distributions. For example, the SST anomalies in the eastern tropical Pacific are more significant in S_D(0)JF(1) (Fig. 7a) than in D_ D(0)JF(1) (Fig. 7b) and D_D(1)JF(2) (Fig. 7c), and it is the opposite in the tropical North Atlantic Ocean. In the Indian Ocean, the SST anomalies are more significant in S_D(0)JF(1) (Fig. 7a) and D_D(1)JF(2) (Fig. 7c) than in D_ D(0)JF(1) (Fig. 7b).
The surface pressure anomalies also display appreciable differences among the three composites in the regions including the high-latitudes of Eurasia, the North Pacific Ocean, and the North Atlantic Ocean. These SST and surface pressure anomaly differences lead to regional precipitation differences. For example, the land precipitation anomaly composites in southeastern China are above normal in S_D(0)JF(1) and D_D(1)JF(2), consisting with the surface pressure anomaly contrast between smaller positive anomalies in southeastern China and larger positive anomalies in the Northwestern Pacific (Fig. 7a, c). Such surface pressure gradient favors transporting moisture to southeastern China between the single-year and double-year El Niños. For the single-year El Niño average (Fig. 5b, c), the intraseasonalinterseasonal and interannual components of the WWV index have an in-phase decline (discharging) since the decay of the El Niños, leading to the enhanced discharging of the equatorial Pacific (Fig. 5a). While, for the double-year El Niño average, there is a slightly recharging tendency after the decay of the first-year El Niños for the intraseasonalinterseasonal component (Fig. 5e). For the interannual component, compared with the single-year El Niños (Fig. 5c), the decrease of the WWV index in the double-year El Niños (Fig. 5f) is slower and the WWV index reaches negative in the late spring and early summer of year(1). As a result, the total WWV index decays slower and the equatorial Pacific is still in the recharged phase until July of year(1) (Fig. 5d), unfavorable for the phase transition.
In the recharge/discharge paradigm (Jin 1997), the heat exchange between the equator and off-equator is linked to the non-equilibrium between the zonal mean equatorial thermocline depth and the surface zonal wind stress. Here, The possible different climate impacts of single-and double-year El Niño events are also discussed.
During 1950-2021, 75% of El Niño events persist for one year, and 25% of them last for two years. On average, there is no relationship between the initial time and duration of an El Niño event. Historically, both central and eastern Pacific type El Niños occur in the single-year and doubleyear El Niños with various strengths. Compared with the single-year El Niños, the averaged WWV index is larger in the peak and declines slower for the double-year El Niños, suggesting that a persistently recharged heat condition of the equatorial Pacific is a precondition for the emergence of a second-year El Niño. The faster decline of the WWV index in the single-year El Niños is associated with the in-phase decrease of its intraseasonal-interseasonal and interannual components, while the slower decline of the WWV index in the double-year El Niños is determined by the interannual component. and convergences in the region (Wu et al. 2003). While the below normal precipitations in D_ D(0)JF(1) in southeastern China are associated with an opposite surface pressure anomaly contrast between southeastern China and the Northwestern Pacific (Fig. 7b). In central North America, the precipitation is significantly wet in S_D(0)JF(1) and D_ D(0)JF(1), and near average in D_D(1)JF(2). That is linked to negative and positive surface pressure anomalies in the region, respectively.

Summary and discussion
Compared with well documented and frequent occurrence of multi-year La Niña, double-year El Niño is less frequent and has not been well investigated. Nevertheless, the multiyear duration of both El Niño and La Niña is a discrepancy from the cyclic behavior of the ENSO and deserves investigation. In this work, we demonstrate the diversity of singleand double-year El Niño events in their strengths, flavors, as well as associations with the recharge/discharge processes.  Each El Niño or La Niña is unique, implying that in addition to the interference among different time scales, the interactions between the tropics and extratropics are an important contributor to the ENSO diversity and complication. For instance, Kim and Yu (2021) argued that El Niño events initiated by subtropical Pacific atmosphere-ocean coupling show larger uncertainty in their decay evolution patterns than those initiated by tropical Pacific atmosphereocean coupling. The onset location of subtropical Pacificonset El Niño, which interacts with the eastern edge of the western Pacific warm pool, is a key factor controlling its decay evolution. Recently, Ding et al. (2022) hypothesized a connection between the North Pacific Oscillation (NPO) and multi-year El Niños. Thus, extra-tropical influence may also be an important contributor to the different durations of El Niños.
Last, we must make clear that in addition to potential physics differences, the differences in the composites and averages might be partially due to the sampling, since the sample sizes are small, especially for the double-year El Niños. Nevertheless, it is necessary to further investigate the physics in the single-year and double-year El Niños, their different impacts on regional climate and the associated mechanisms, as well as the possible prediction skill differences between single-year and double-year El Niños, which benefits the seasonal-interannual climate prediction.