Is the November 2018 Madden Julian Oscillation (MJO) as a Possible Trigger for an El Nino?

1. The Madden Julian Oscillation (MJO)

The Madden Julian Oscillation (MJO) is the dominant form of intra-seasonal (30 to 90 days) atmospheric variability in the Earth’s equatorial regions (Zang 2005). It is characterized by the eastward progression of a large region of enhanced convection and rainfall that is centered upon the Equator. This region of enhanced precipitation is followed by an equally large region of suppressed convection and rainfall. The precipitation pattern takes about 30 – 60 days to complete one cycle, when seen from a given point along the equator (Madden and Julian 1971, 1972).

At the start of the enhanced convection phase of an MJO, a large region of greater than normal rainfall forms in the far western Indian Ocean and then propagates in an easterly direction along the equator. This region of enhanced rainfall travels at a speed of ~ 5 m/sec across the Indian Ocean, the Indonesian Archipelago (i.e. the Maritime Continent) and on into the western Pacific Ocean. However, once it reaches the central Pacific Ocean, it speeds up to ~ 15 m/sec and weakens as it moves out over the cooler ocean waters of the eastern Pacific.

Wheeler and Hendon (2004) have developed a Real-time Multivariate MJO (RMM) Index that tracks the strength and movement of the enhanced-convection phase of MJOs. This index is based upon the first two Empirical Orthogonal Functions (EOFs) of the combined fields of the equatorially averaged (15°S to 15°N) outgoing longwave radiation (OLR), 850 hPa zonal wind and 200 hPa zonal wind data for the period from 1979 to 2001.

In order to track the strength and movement of the enhanced-convection phase of MJOs, a two-dimension phase-space diagram is created by plotting the daily values of RMM2 against RMM1 (Wheeler and Hendon 2004) [Please see the diagram on the left-hand side of the following figure for the phase-space diagram for October/November 2018- BOM 2018]. In this phase diagram, the distance of a given daily data point from the origin [i.e. SQRT((RMM1^2)+(RMM2^2))] is an indicator of the strength of the enhanced-convection in the MJO. Distances greater than 1.00 indicate significant MJO activity. Additionally, the curve formed by joining the sequential daily data point, moves in an anticlockwise direction about the origin, with the anti-clockwise movement indicating the systematic eastward propagation of the MJO. The 360 degrees of the phase diagram is divided into eight octants which roughly correspond to actual physical locations along the Earth’s equator (Wheeler and Hendon 2004).

The bottom right-hand side of the following figure shows the approximate locations of the first seven octants in the phase diagram. The eighth octant covers the sector that extends along the equator from East Africa to the International Date Line.

One class of MJO that is of particular interest for this study are those that propagate into the western Pacific Ocean (i.e. octants 6 and 7 for the MJO Phase). The reason why this class of Pacific Penetrating Madden Julian Oscillations (PPMJO) is special is that they are often associated with westerly wind bursts (WWB’s) that are thought to play an important role in triggering the onset of El Nino events.

The diagram in the top right-hand corner of the figure above shows the MJO phase (i.e. MJO octant number) versus time diagram for 2018 [up to November 17th]. The current MJO event starting on the 31st of October is highlighted on the far right of this diagram.

2. A Connection Between Lunar Declination & Lunar-Induced Accelerations of the Earth.

If you remove the annual and bi-annual seasonal component of the changes in the Earth’s LOD, you are left with the abrupt slowdowns in the Earth's rotation rate (roughly) once every 13.7 days cause by the effects of the lunisolar tides. A more detailed investigation of these semi-monthly changes in Earth’s rotation rate shows that the maxima in LOD (where the Earth is rotating most slowly) occur within a day or so of the time that the Moon crosses the Earth's Equator. This tells you that the slow down in the rotation rate (i.e. the maximum in LOD) is a direct result of the lunar tidal bulge in the Earth's oceans (and atmosphere) being dragged across the Earth's Equator by the Moon. The slow down occurs for much the same reason that a twirling ice-skater slows down their rate of spin by extending their arms i.e. by the conservation of angular momentum.

Similarly, when the lunar tidal bulges in the Earth’s oceans (and atmosphere) reach their furthest point from the Earth’s equator (i.e. highest latitudes where lunar standstills take place), this brings the tidal bulges to their closest point to the Earth’s rotation axis. When this happens, the Earth’s rotation rate increases for the same reason a twirling ice-skater speeds up their rate of spin by pulling in their arms (again by the conservation of angular momentum).



Hence, the bi-monthly variations in LOD are a:

a) maximum (i.e. the slowest rotation rate) near times when the lunar tidal bulges cross the Equator

b) minimum (i.e. the fastest rotation rate) near times when the lunar tidal bulges reach lunar standstill

The following figure shows the connection between the rotation rate of the Earth and the Cardinal locations of the Moon's declination. This figure shows that the maximum rate of change in the Earth's rotation rate take place at (or near) times of lunar standstills (i.e. when the Moon's declination is furthest from the Earth's Equator) and lunar crossing of the Equator [Note the red crosses which mark the times of maximum change in the Earth's rotation rate]. 

The next figure shows the deviations in the Earth's angular velocity between late-October and late-November 2018 [Sidorenkov 2009 - Note that the angular velocity can be converted to LOD in seconds by multiplying by -86400]. Highlighted in this figure are the rough times of the lunar standstills and the lunar crossings of the Earth's Equator. Remember that these correspond to the times of maximum change in the lunar-induced rotation rates of the Earth. Three points of maximum change in rotation rates, A, B, and C have been highlighted as they are used in the next discussion point.

3. Are Westerly Wind Bursts About to be Produced by the NOV 2018 MJO & Could They Trigger an El Nino Event?

During ENSO neutral or La Nina conditions, the trade winds in the equatorial Pacific Ocean blow steadily from the east. However, just prior to the onset of an El Nino event, the easterly trade winds dramatically weaken.

Lian et al. (2014) and Chen et al. (2015) have shown that for every major El Nino event since 1964, the drop off in easterly trade wind strength has been preceded by a marked increase in westerly wind bursts (WWB) in the western equatorial Pacific Ocean. These authors contend that the WWB generate easterly moving equatorial surface currents which transport warm water from the warm pool region into the central Pacific. In addition, the WWB create downwelling Kelvin waves in the western Pacific that propagate towards the eastern Pacific where they produce intense localized warming [McPhaden 1999]. It is this warming that plays a crucial in the onset of El Nino events through its weakening of the westerly trade winds associated with the Walker circulation.

A meteorological phenomenon that produces WWB along the Earth's Equator is close  Typhoon/Cyclone pairs that straddle the Earth's equator. The equatorial winds between the two intense low-pressure systems of a Typhoon/Cyclone pair blow steadily from west to east, weakening the prevailing equatorial trade winds.

Consider the following hypothesis: Typhoon/Cyclone pairs are triggered by the abrupt changes in the Earth's rotation rate that are induced by the Moon over its monthly cycle.

The left-hand side of the following figure shows a surface wind map of the Earth for 12:00 UT on the 19/11/2018. This wind map shows that there are three (or possibly four) Typhoon//Cyclone pairs staddled along the Equator between East Africa and the Western Pacific Ocean [These are marked with red "L" symbols connected by orange lines]. Also shown in this diagram is the path of the November MJO event between the 31st of October and the 19th of November (long red horizontal curve). Along the path, there are three verticle dark lines showing where the MJO event was located when the lunar-induced rotation rate of the Earth was changing at its fastest rates. These took place on the 4th of November (just south of the Indian sub-continent), the 12th of November (off the East coast of Kalimantan), and the 20th of November (just to the east of New Britain and New Ireland).

Amazingly, the location of the November MJO event when the lunar-induced changes in the Earth's rotation rate where are maximum appear to be close to the location of the Typhoon/Cyclone pairs.

Could this be the link between the appearance of WWBs in the western equatorial Pacific Ocean (that are thought to be responsible for triggering El Nino events) and the Perigean New/Full Tidal cycle?

Perhaps when the strongest Perigean New/Full moon tidal events occur close to the times of lunar crossing of the Equator and lunar standstill, the powerful lunar-induced changes in the Earth's rotation rate, are conducive to forming strong WWBs in the western equatorial Pacific ocean, and it these WWB's that trigger Ell Nino events.

References:

Zhang, C., 2005: Madden-Julian Oscillation. Reviews of Geophysics, 43, 1-36.

Madden R. and P. Julian, 1971: Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific, J. Atmos. Sci., 28, 702-708.

Madden R. and P. Julian, 1972: Description of global-scale circulation cells in the tropics with a 40-50 day period. J. Atmos. Sci., 29, 1109-1123.

Wheeler, M. C., and H. H. Hendon, 2004: An all-season real-time multivariate MJO index:

Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917-1932.

Australian Bureau of Meteorology (BOM), 2018: http://www.bom.gov.au/climate/mjo/  last accessed 20/11/2018.


Sidorenkov, N.S., 2009: The Interaction Between Earth’s Rotation and Geophysical Processes, Weinheim: Wiley.

Lian, T., D. Chen, Y. Tang, and Q. Wu (2014), Effects of westerly wind bursts on El Niño:

A new perspective, Geophys. Res. Lett., 41, 3522–3527.


Chen, D., Lian T., Fu C., Cane, M.A., Tang, T., Murtugudde, R., Song X., Wu, Q. and Zhou, L., 2015, Strong influence of westerly wind bursts on El Niño diversity, Nature Geoscience, Vol. 8 No 5, pp. 339 – 345.

McPhaden, M. J. (2004), Evolution of the 2002/03 El Niño, Bull. Am. Meteorol. Soc., 85, 677–695. http://www.pmel.noaa.gov/tao/proj_over/pubs.html