Extratropical Forcing Triggered the 2015 Madden–Julian Oscillation–El Niño Event

In this paper, we report the triggering effect of extratropical perturbation on the onset of an atypical Madden–Julian Oscillation (MJO) and onset of the 2015–16 El Niño in March 2015. The MJO exhibited several unique characteristics: the effect of extratropical forcing, atypical genesis location and timing in the equatorial western Pacific, and the extremity of amplitudes in many aspects. The southward-penetrating northerly associated with the extratropical disturbances in the extratropical western North Pacific contributed to triggering the deep convection and westerly wind burst (WWB) and onset of the MJO over the anomalously warm tropical western Pacific in early March. The persisting strong WWB forced downwelling Kelvin wave-like oceanic perturbation that propagated eastward and led to the onset of the 2015–16 El Niño. The proposed novel extratropical forcing mechanism explaining the unique extratropics–MJO–El Niño association, based on both data diagnostics and numerical experiments, warrants further attention for a more detailed understanding of the onset of the MJO and its potential effect on El Niño.


Results
Role of the extratropical-tropical interaction on the Madden-Julian Oscillation onset. Figure 1 presents the evolution of near-surface anomalous atmospheric fields during late February to early March in 2015. Before the MJO onset in early March, an atypically strong high-pressure system persisted over the extratropical western North Pacific in late February 2015. The accompanying dry and cold northerly to the west of the dateline penetrated southward to the tropical North Pacific, followed by the flaring of a deep convection in the equatorial western Pacific. The development of vertical circulation (averaged over 150°E-180°E; Supplementary Figure 1) further demonstrated the lead-lag relationship between the subsiding northerly in the extratropical western North Pacific and development of the deep convection in the tropical western Pacific. The Hovmüller diagrams shown in Fig. 2a further demonstrate the close temporospatial relationship between sea level pressure (SLP) and northerly anomalies in the subtropical North Pacific between 135°N and the dateline. The physical process of extratropical disturbance to trigger the MJO initiation was similar to the effect of the cold surge on the initiation of the intraseasonal oscillation 14,18 . The extratropical disturbance associated northeasterly anomaly advected cool and dry air southward to the warm and moist tropics. The cool and dry air above the warm moist ocean surface produced an unstable atmospheric environment favorable for the development of deep convection.
Following the arrival of extratropically originated perturbations, a tropical convection and strong WWB occurred to the west of the dateline in early March (Fig. 2b). As shown in Fig. 1e and f, the westerly anomaly in the tropical western Pacific, the easterly anomaly in the tropical eastern Pacific, the negative SLP anomaly over the tropical Pacific east of 150°E, and the deep convection west of the dateline formed a pattern resembling the theoretical Gill-Matsuno perturbation 22,23 . The emergence of this convection-circulation pattern signaled the onset of an MJO that later strengthened to an unprecedented amplitude (MJO real-time multivariate index 24 of 4.62 on March 16, 2015 compared with the previous record of 4.02 on February 14, 1985; http://www.bom.gov.au/ climate/mjo/) during its eastward propagation over the tropical Pacific in March (Fig. 2c) 13 .
In contrast to the canonical MJOs, which typically originate in the tropical IO, the MJO in spring 2015 initiated in the western Pacific. Our analysis revealed that only 3 out of the 75 large-amplitude MJOs (occurring in 1975, 2013  the extratropical SLP, northerly, WWB, tropical SLP anomaly crossing the central and eastern equatorial Pacific (Fig. 1c), and the MJO strength. This observation revealed the distinct characteristics associated with this MJO: the effect of extratropical forcing, atypical genesis location and timing, and the extremity of amplitudes in many aspects.
The warm ocean surface in the equatorial western Pacific might be another favorable condition conducive to MJO occurrence. A vertical cross section of the monthly ocean temperature along the equator (Supplementary Figure 3) revealed that the upper ocean in the central Pacific was approximately 1.5 K warmer in February-April 2015 than the long-term mean. This warming in the central Pacific was markedly higher than that during the onset of the 1997-98 El Niño. A previous study 13 reported the effect of this warm water on MJO development in early spring 2015. An analysis of the SST evolution since early 2014 indicated that the warm SST in the central Pacific was primarily the remaining positive SST anomaly from the aborted 2014 El Niño (data not shown). The extratropical forcing in late February likely triggered the anomalous convection over this warm water and initiated the rigorous atmosphere-ocean interaction in the tropical western and central Pacific and onset of a strong MJO event.  the 2015 event, initiated near the dateline (Fig. 3b). The WWB location was the critical factor, which was evident by comparing the 2015-16 and 1997-98 El Niño events ( Fig. 3a and b and Supplementary Figure 4). Both El Niño events demonstrated a similar temporospatial evolution of the atmospheric and oceanic fields. The first oceanic Kevin wave-like perturbation in 1997 initiated in the western Pacific (150°E), whereas the 2015 event occurred near the dateline. This distinction resulted from the difference in the genesis location of the 1997 and 2015 MJO events: for example, the IO versus the west of the dateline (Supplementary Figure 5).

Effect of Madden-Julian Oscillation on triggering the 2015-16 El Niño onset. Supplementary
The main physical processes for an MJO triggering the El Niño onset are described as follows. The MJO-associated deep convection and strong low-level westerlies in the west cool the underlying SST through the cloud-radiation-SST and wind-evaporation-SST feedback. The subsidence branch of the anomalous west-east overturning circulation in the east enhances the downward shortwave radiation and warms the SST in the eastern Pacific. Therefore, the MJO-associated atmospheric circulation anomalies create a cool-warm SST contrast between the equatorial western and eastern Pacific, which provides favorable conditions for El Niño development. Numerical experiments. In this section, we provide evidence from two sets of numerical experiments (see Experimental Setting) to support our hypothesis. The first experiment is to address the effect of the MJO-associated WWB on forcing the oceanic Kelvin wave [6][7][8] . Figure 3d shows the Hovmüller diagrams (averaged over 2°S-2°N) of the simulated depth of 20 °C (D20) obtained from the oceanic simulations that were forced by three prescribed pulses of westerly wind stress. Three pulses of eastward warm water starting in March, May, and July were simulated (Fig. 3b). The simulated eastward propagation speed of warm water was approximately 1.7 ms −1 , slightly slower but comparable with the observed propagation speed of approximately 2-3 ms −1 . The numerical experiment confirmed that the observed oceanic Kelvin wave-like perturbations were triggered by the MJO-associated WWB. Moreover, the experiment revealed that the eastward-moving oceanic perturbation resulted in substantial SST warming in the central and eastern Pacific (data not shown).
The second set of expriments examines the effect of extratropical forcing on initiating the MJO. The Hovmüller diagram of 200-hPa velocity potential of the atmospheric experiment shown in Fig. 4b and c is a compilation of the Day 13 simulated results from 59 global-and tropical-nudging simulations, respectively. The global-nudging simulations successfully reproduced the eastward-propagating signals (Fig. 4a), whereas the tropical-nudging simulations mainly produced a stationary convection to the west of the dateline. Plots for 10-m winds presented in Supplementary Figure 6  The extratropical forcing in triggering the MJO onset was primarily from the North Pacific. By contrast, no similar extratropical disturbances in the South Hemisphere were observed ( Fig. 1a and Supplementary Figure 1). To further confirm this argument, a new experiment, the NP-nudging run, in which the nudging by the observed extratropical daily fluctuations was applied only to the North Pacific (10°N-90°N, 110°E-130°W) to isolate the origin of major extratropical forcing. The simulated 10-m zonal winds of the NP-nudging run resemble that of the global-nudging run, indicating that the extratropical forcing that triggered the westerly wind burst associated with the MJO was primarily from the North Pacific.

Conclusion and Discussion
An atypical MJO initiated to the west of the dateline in early March 2015 and rapidly amplified to an unprecedented magnitude over the warm SST in the central and eastern Pacific on March 16. Following the MJO, the SST in the equatorial central-eastern Pacific encountered rapid growth and ultimately evolved to a strong El Niño comparable with the 1982-83 and 1997-98 events. Before the MJO onset, we observed a persisting high-pressure 1. Observational analysis indicated that the strong cold northerly, which was associated with a persisting high-pressure system in the extratropical western North Pacific, penetrated southward to the tropical western Pacific and triggered the tropical convective instability that led to the onset of the MJO at an atypical location, namely west of the dateline. The critical effect of the extratropical disturbances on the MJO onset was confirmed by numerical experiments by using an atmospheric general circulation model coupled with an ocean mixed layer model. Extratropical forcing was the unique characteristic of the reported MJO-El Niño event. The onset of El Niño by an MJO has been observed often. However, according to our review of relevant literature, the present study is the first to report the onset of an El-Niño-inducing MJO in the western Pacific triggered by extratropical perturbations. Extremity was another unique characteristic. Several aspects of perturbations, such as extratropical and tropical SLP, northerly, and the MJO reached unprecedented amplitudes. The reasons for these unique characteristics remain unknown. However, our study revealed the possible effect of extratropical forcing, which has not been considered previously, on the onset of MJO and El Niño. Such a mechanism, although it might not occur frequently, warrants further attention and may elucidate the onset of an MJO and its potential effect on El Niño.

Methods
Data. The following observational data were used in this study: (1)  Description of the atmospheric simulations. The atmosphere-ocean (mixed layer) coupled model ECHAM5-SIT, which realistically simulates MJO 28,29 , was used to investigate the triggering effect of extratropical forcing on the onset of an MJO. To identify the effect of extratropical perturbations, we conducted a series of hindcast experiments initialized at 00UTC daily from February 1 to March 31. The simulations were nudged toward the observed U, V, T and Q twice daily in the first 5 days and once daily on Days 6-10; thereafter, the nudging was stopped. In the global-nudging experiment, the simulations were nudged toward the observed daily global perturbations. In the tropical-nudging experiment, nudging by observed daily fluctuations was applied to the 10°S-10°N tropical band and by climatological means elsewhere. Because extratropical nudging was the main difference between two experiments, a more realistic simulation in the global-nudging experiment can be attributed to the forcing effect of the extratropical perturbations. A North Pacific (NP) nudging experiment was also conducted, by applying nudging to the North Pacific (10°N-90°N, 110°E-130°W)  Experimental Setting. In the present study, we conducted two simulations following the Coordinated Ocean-Ice Reference Experiment II protocol 32,33 , which uses interannual varying atmospheric forcing data sets for the 1948-2007 period. The initial conditions for two simulations were identical, starting from the March of the year 371, corresponding to the 60-year repeated forcing in March 1958. Without any changes, a control simulation (control case) was continuously integrated for 8 months until October of the same year. To evaluate the tropical ocean response under strong winds, a simulation (test case) was conducted by uniformly adding 10 m/s to the zonal winds within 140°E-180°E and 3°S-3°N in March, May, and July. We selected 10 m/s on the basis of the 8-m/s westerly wind anomaly at 850 hPa observed in 2015. The 10-m/s of the westerly anomaly is approximately 0.14 N/ m 2 of the zonal wind stress forcing to the ocean model 34 . For simplification, the uniform 10 m/s westerly anomaly was applied instead of using the observed wind fields, to simulate three pulses of westerly winds occurring from March to July in 2015 (Fig. 3a) which were assumed to trigger three oceanic Kelvin Waves at the Equator (Fig. 3b). Another experiment using the observed wind stress anomaly as forcing, which was more persistent than the three idealized westerly events, was also conducted. In this experiment, the separation of three oceanic events was not as clear as shown here but the first eastward-propagating Kelvin wave initiated in March was clearly simulated.

Figure Source.
All the figures were created by authors using NCAR Command Language (NCL) 35 [version 6.3.0], a product of the Computational & Information Systems Laboratory at the National Center for Atmospheric Research (NCAR) and sponsored by the National Science Foundation, is a free interpreted language designed specifically for scientific data processing and visualization.