The amplitudes of SPW1 during different El Nino types are presented in Fig. 1. Maximum amplitude peaks greater than 2500 meters arise in January 1964, February 1988, December 2002 (referred to as Modoki I), in the end of December 1994, December 2009, and January 2010 (Modoki II), December 1982, February 2016 (canonical El Nino). In both cases (Fig. 1c and Fig. 1o, respectively), when the observed greatest amplitudes occur in February, the maximum MEI values achieve of about 2 and appear to be in the summer. The substantial SPW1 amplitudes in December during Modoki both types can be also related to MEI values, which are of about unit and have the highest magnitude in early autumn months. Winters 1963/64 and 1987/88 have similar MEI values distribution and in both cases there is PW1 amplification, which is not so dramatic, in the beginning of cold season: in the first case – middle of November, in the second case – end of November. December 1982 (canonical El Nino) doesn’t follow this pattern. Short time intervals of increased wave activity are found during three types of El Nino in different winter months.
The observed temporal variability of the amplitudes and phases of the zonal harmonics is due to the superposition of the variability of SPWs, which can be explained by a change in the conditions of their propagation from the troposphere and/or the nonlinear interaction with the mean flow, and by the presence of traveling and standing planetary waves (PWs) in the reanalysis data. Traveling (propagating to the East and to the West of the PWs) and standing planetary waves, in turn, are also nonstationary. As a result, the situation is rather complicated and the complex Morlet transform is used to estimate the variability of their amplitudes and phases. The technique of waves’ division into traveling to the East and West, and the further recalculation of the results obtained to distinguish standing waves, are described in Pogoreltsev et al. (2009). The amplitudes of standing PW1 and traveling to the East and to the West at 2.5°N are shown in the Figs. 2–4, respectively. Despite the different origin of SST anomaly, some winters have a very similar wave activity distribution (penetration) of both the period and maximum amplitude magnitudes. In Fig. 2 there is a calm standing wave behavior during winters with Modoki I type (1979/80, 1992/93), Modoki II type (1977/78, 1994/95), and canonical type (1972/73, 1982/83): the most active waves with 3-4-day periods and amplitude peaks do not reach 25 meters. In contrast one should note the winters (1997/98 – canonical type and 2002/03 – Modoki I), when the standing PW1 with 6-day period is active in January and February. The amplitude values are of about 25 meters in February. Paying attention to traveling waves, eastward planetary waves with periods greater than 10 days and amplitudes 2–4 meters are observed every winter during Modoki I and canonical types (Fig. 3). These waves occur not so often and they are less sustainable. Traveling westward waves with different periods are active up to mid-January and in March during Modoki I excluding winter 1992/93 (Fig. 4). The same pattern holds in winter of Modoki II type in 2009/10. The 4-day and 6-day waves have the largest amplitudes, which exist in all El Nino types. The standing and traveling waves have been determined at 27.5°N (not presented) and 62.5°N (Fig. 5–7). The standing waves at 27.5°N during Modoki I winters are mainly with the periods of 10-day and greater. They have amplitude peaks in December and February. The 8-day standing waves occur during Modoki II and canonical El Nino, but in the first case the 15-20-day waves have the strongest amplitudes. They start disappearing in February and the amplitudes become roughly equal to 20 meters. It should be noted that calm March without large amplitudes of any waves features every winter. The waves traveling to the East at this latitude do not have distinctive features for Modoki I and II El Nino types. There is no any wave activity in the March and even in February (1965/66, 1972/73, and 1997/98). The activity of westward propagating waves at 27.5°N is minimal in the second half of January and in the first half of February except winter 1965/66. Evidently for the standing waves at 62.5°N it is difficult to find characteristic features to mark any El Nino type (Fig. 5). The reason is likely in different stratospheric processes and zonal flows that influence waves’ distribution and make them behave in a certain way. In canonical El Nino winters (Fig. 6f-j), there is a two-week pause while any eastward wave activity is absent, at the beginning of January. Figure 7 (k-o) demonstrates that in canonical El Nino, the westward waves with shorter periods (from 8 to 15 days) are observed at the beginning of winter. In the end of January or in February, waves with longer periods (15–20 days) occur. There is the only 10-day or 15-day waves for long time interval (of about two months) during Modoki II El Nino event.
Traveling planetary waves (the normal atmospheric modes, ultra-fast Kelvin wave, etc.), unlike atmospheric tides, do not have permanent sources, therefore they exist (observed in the results of wavelet analysis) in the form of bursts of wave activity at certain periods that are not tied to a specific time interval (Salby 1984). Therefore, to compare the wave activity of traveling planetary waves induced by different types of El Nino events, the amplitude wavelet spectra have been averaged over 5 months (from November to March). Figures 8 and 9 demonstrate the standing, eastward, and westward waves averaged over boreal winter seasons during different El Nino types at 2.5°N and 62.5°N, respectively. It is obvious that both amplitudes of nonmigrating diurnal tide (Fig. 8a, b, c) and amplitudes of Kelvin waves (Fig. 8d, e, f) are overstated all years up to 1979 regardless of El Nino type, most likely due to lack of satellite data in reanalysis data. There are two peaks of amplitudes over 8 meters after diurnal tides in Modoki I and canonical events, while there is the only one in Modoki II. The distribution of eastward propagating waves has no common features in various El Nino types considering wave amplitudes and periods. The amplitudes of diurnal migrating tide propagating to the West (Fig. 8g, h, i) are the greatest during Modoki II. It should be noted that this overstating of diurnal tides is observed in amplitudes at 27.5°N (not presented here) as well but only in the standing waves and it is the highest during Modoki II event. During this type of El Nino all westward propagating waves have their amplitudes of about 20 meters, while in Modoki I and canonical El Nino types 10-day wave dominates and the amplitude of about 30 meters is observed. The difference in the highest amplitudes is observed in standing waves at 62.5°N (Fig. 9a, b, c). The magnitudes of amplitudes do not exceed 200 meters in Modoki II event (Fig. 9b), but they reach 250 meters in Modoki I event (Fig. 9a) and the 20-day wave occur with the amplitude of 300 meters in 2015/16 (canonical type, Fig. 9c). The 16-day wave dominates among the traveling to the East with the amplitude from 20 to 50 meters and of about 100 meters in 2002/03 (Modoki I, Fig. 9d). Such strong amplitudes (92 meters) belong as well to 13-day eastward propagating waves (canonical El Nino, Fig. 10f) in 1982/83 and 2015/16.
Planetary waves provide wave fluxes of heat and / or long-lived atmospheric species. To account for these fluxes, it is usually considered the so-called residual meridional circulation (Andrews et al. 1987), which is the Brewer-Dobson circulation at the heights of the stratosphere-mesosphere. The observed variability of this circulation, as well as its climatic variability, can significantly clarify the changes in the thermal regime and ozone content in the Polar Regions during SSW events and / or depending on the ENSO, quasi-biennial oscillation (QBO), and Madden-Julian oscillation phases.
Taking into account the amplitude overstating at 2.5°N and altitude limitation of JRA-55, the residual circulation has been calculated using MERRA2 (The Modern-Era Retrospective analysis for Research and Applications, https://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/) reanalysis data for every December (Fig. 11, 12, and 13, left panels) and January (Fig. 10, 11, and 12, right panels). The distribution of residual circulation in these two months should be enough, allowing for that the wave activity begins in December and January, to assume that any dependence upon El Nino type is there. December in 1987/88 and 1992/93 shown in Fig. 10 (mid and bottom left panels) differ from one another by strength of flow and its direction especially over Pole despite the same QBO phase and previous ENSO phase as well, in both cases it was positive. In contrast, the residual circulation in December 1992 is similar to that in December 2002 (Fig. 10, top left panel) while in the latter case the opposite QBO phase is observed and neutral previous ENSO phase. However, the MEI values in fall months have identical meaning of about 0.8 in these years, and they are greater in 1987. The residual circulation in December in Modoki II event (Fig. 11 left panels) looks very similar to one in 2002 and 1992 (Modoki I), but the flow is noticeably weaker at the altitude of 50–60 km at mid-latitudes. It should be noted the strong reverse flow in this height region at 35°N directed to the South in December 1991. In January of Modoki II El Nino type (Fig. 11 right panels) the flows become very similar to those observed in 2003 and 1993 despite different QBO phases and ENSO conditions in previous summer. There is absolutely different pattern in December of canonical El Nino (Fig. 12 left panels). Every circulation differs from another be the strength of flows and their direction. The differences are smoothed out in January (Fig. 12 left panels), but still are evident.