Abstract
In this study, we reexamine the effect of two types of El Niño Southern Oscillation (ENSO) modes on Madden Julian Oscillation (MJO) activity in terms of the frequency of MJO phases. Evaluating all-season data, we identify two dominant zonal patterns of MJO frequency exhibiting prominent interannual variability. These patterns are structurally similar to the Wheeler and Hendon (Mon. Weather Rev. 132:1917–1932, 2004) RMM1 and RMM2 spatial patterns. The first pattern explains a higher frequency of MJO activity over the Maritime Continent and a lower frequency over the central Pacific Ocean and the western Indian Ocean, or vice versa. The second pattern is associated with a higher frequency of MJO active days over the eastern Indian Ocean and a lower frequency over the western Pacific, or vice versa. We find that these two types of MJO frequency patterns are related to the central Pacific and eastern Pacific ENSO modes. From the positive to the negative ENSO (central Pacific or eastern Pacific) phases, the respective MJO frequency patterns change their sign. The MJO frequency patterns are the lag response of the underlying ocean state. The coupling between ocean and atmosphere is exceedingly complex. The first MJO frequency pattern is most prominent during the negative central-Pacific (CP-type) ENSO phases (specifically during September–November and December-February seasons). The second MJO frequency pattern is most evident during the positive eastern-Pacific (EP-type) ENSO phases (specifically during March–May, June–August and September–November). Different zonal circulation patterns during CP-type and EP-type ENSO phases alter the mean moisture distribution throughout the tropics. The horizontal convergence of mean background moisture through intraseasonal winds are responsible for the MJO frequency anomalies during the two types of ENSO phases. The results here show how the MJO activity gets modulated on a regional scale in the presence of two types of ENSO events and can be useful in anticipating the seasonal MJO conditions from a predicted ENSO state.
Highlights
The Madden Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO) are the two leading modes in intraseasonal and interannual time scales in the tropics, and of immense importance to the global climate variability[1,2,3,4]
We identify the spatial modes of MJO frequency variation by employing Empirical Orthogonal Function analysis (EOF) analysis on MJO frequency anomaly (Fig. 1a,b)
We find that MJO frequency pattern associated with the first two EOFs do not change appreciably over time (Supplementary Fig. S2)
Summary
The Madden Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO) are the two leading modes in intraseasonal and interannual time scales in the tropics, and of immense importance to the global climate variability[1,2,3,4]. In boreal summer, the RMM index sometimes fails to isolate the MJO signal from the northward propagating intraseasonal oscillation (ISO) signal[7] These limitations must be taken into account while using the RMM index in climate studies. The Indo-Pacific warm pool conditions modulate the MJO characteristics both on interannual to long-term climate time s cales[18] Besides this simultaneous connection between the MJO and ENSO, Hendon et al.[19] observed a lag relationship between the MJO activity in spring (March–April–May) and the state of ENSO in subsequent winter (December–January–February). Hsu and Xiao[24] discussed the reason behind the strengthened MJO propagation over the Indian Ocean during central Pacific ENSO events compared to eastern Pacific ENSO events Overall, these studies are mainly focused on the interannual variation of the strength of MJO convection or the amplitude. The frequency of MJO phase 2,3 and 4,5 increases during EP El Niño winters and CP El Niño winters r espectively[22]
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