At the time of writing, areas around the Northern Territory (TC Lam) and Queensland (TC Marcia) in Australia are in the path of severe cyclones (category 4 at the moment, with the Queensland one - TC Marcia predicted to reach category 5 - Category scales used for Australia and Fiji). These cyclones seem to be connected by a monsoonal trough, as indicated by the dashed line in the weather map below from Australia's Bureau of Meteorology:

enter image description here

Note: it is summer here at the moment.

Also, these cyclones are a very large distance apart (about 1900km in a direct line), and these cyclones are not merging at all. According to the Meteorological Bureau, TC Lam is moving south through the Arnhem Land area of the Northern Territory; and TC Marcia is going to 'hook' when it goes inland and head towards the Brisbane area to the south coast of Queensland (but it is expected to weaken to a tropical low well before then). The projected paths (based on latest and consistent information from the Bureau) is shown as red arrows in the image below:

enter image description here

What is the meteorological mechanism that has resulted in the linked "double" severe cyclone event?

Edit - update: TC Lam has made landfall at cat 4, TC Marcia is now making landfall at cat 5

  • $\begingroup$ Just a side note: At the time of writing, I am currently in one of the towns in the path of Severe TC Marcia. $\endgroup$
    – user889
    Feb 19 '15 at 10:03
  • $\begingroup$ I am sure that this phenomena, or something very similar, has occurred somewhere before. $\endgroup$
    – user889
    Feb 19 '15 at 14:12
  • $\begingroup$ No, it isn't, however what is unusual is 2 severe cyclones (category 4 and 5) hitting at the same time, 'attached' by a trough... also 24 hours ago TC Marcia was only a category 1... it is now expected to be a category 5 at landfall. A smaller intensification also occurred with TC Lam. $\endgroup$
    – user889
    Feb 19 '15 at 14:35
  • 1
    $\begingroup$ Could the monsoonal trough somehow locally increase the baroclinicity? This would then favor regions along it to become unstable and form vortices. $\endgroup$ Feb 19 '15 at 19:46
  • $\begingroup$ Is this a chicken and egg question? Did the cyclone development cause the linkage or did the trough cause or intensify the cyclones. I would guess that the best way to answer the question would be to run the models with systematic changes to starting conditions. ... and good luck! $\endgroup$
    – haresfur
    Feb 19 '15 at 22:20

Let us state the obvious reasons first:

  1. Warm SSTs along the Timor Sea as well along the Queensland coast favour the formation of Tropical Cyclones -

    The following two URLs show the SSTs along the Northern as well as Eastern Coast of Australia for the referenced periods. a) http://earth.nullschool.net/#2015/02/15/2100Z/ocean/primary/waves/overlay=sea_surface_temp/orthographic=136.27,-24.99,512

    b) http://earth.nullschool.net/#2015/02/20/2100Z/ocean/primary/waves/overlay=sea_surface_temp/orthographic=136.27,-24.99,512

    The threshold SST required for tropical cyclogenesis is 26.5 C as shown in The Threshold Sea Surface Temperature Condition for Tropical Cyclogenesis

  2. The role of equatorial waves in tropical cyclongenesis and in this case interaction of equatorial waves with the monsoon trough. Results of filtered OLR as shown in the image below shows the presence of an Equatorial Rossby Wave over those regions and the presence of blue shades indicates negative OLR (enhanced convection).

    enter image description here

    Source - Tropical Wave Monitoring

    This reference The Role of Equatorial Rossby Waves in Tropical Cyclogenesis. Part II: IdealizedSimulations in a Monsoon Trough Environment clearly states that

" When just the MT is simulated without the ER wave anomaly fields, the MT remains quasi-steady and TC genesis does not occur. It is only when the ER wave is imposed on the MT that TC genesis is initiated. The results imply that the ER wave–MT interactions produce more TCs than would otherwise occur if no such interactions took place."

  1. A question that can be raised is that we do know that presence of MJO also promotes tropical cyclongenesis as shown in this reference Diagnosis of the MJO Modulation of Tropical CyclogenesisUsing an Empirical Index. So how can does one state unequivocally that it is a Equatorial Rossby Wave and not the MJO's interaction with the MT that favours tropical cyclogenesis ?

    If we look at the strength of the MJO signal during and after the formation of TC Lam & Maria

    as shown in these set of images

    enter image description here

    The amplitude of MJO signal is currently weak (less than 1 being present inside the circle) and is unlikely to be a significant contributor to the formation of tropical cyclones.

    This conclusion can be enhanced by looking at the filtered OLR contributions from MJO -

    enter image description here

    The presence of positive OLR(in yellow over the Northern/Eastern part of Australia indicating suppressed convection.

  2. Both the MJO as well as tropical waves favour the formation of twin cyclones as shown by references Dynamical Aspects Of Twin Tropical cyclones Associated with the Madden Julien Oscillation and A Case Study of an Outbreak of Twin Tropical Cyclones

Update In an earlier question that I had asked Relation between direction of shear and cyclone formation one can clearly see a working example of easterly shear favoring the formation of tropical cyclone. So easterly shear as shown in the map below is defined as the difference in the zonal winds at two different pressure levels (850 hPa and 200 hPa). Since by convention easterly winds are noted to be negative increasing positive values of shear is easterly shear(as one travels vertically) and the opposite is westerly shear. The following shear plot is a mean of several days and one can see the easterly shear over the Australian regions where the cyclone formation took place.

$zonal\_shear = U_{850} - U_{200} $

The data is from NCAR NCEP reanalysis.

enter image description here

  • $\begingroup$ There’s a reason powerful twin cyclones smashed Australia last week. It’s called the equatorial Rossby wave. “(The cyclones didn’t form because of) monsoonal bursts, as they often do in our region, but due to another atmospheric phenomenon which passed through the Australian region,” says cyclone expert Professor Jon Nott. That phenomenon is this equatorial Rossby wave thing, and according to the Bureau of Meteorology, it produces westerly winds, tropical cloud and rainfall as its passes over a region. End result: cyclones. $\endgroup$
    – gansub
    Apr 20 '15 at 10:38

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