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MJO, Rossby and Kelvin Waves

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  • Location: Coventry,Warwickshire
  • Location: Coventry,Warwickshire

    The MJO is a major contributor to the global weather patterns, so for those who want to understand a little bit more about it here is a brief overview of my current understanding of the MJO and Rossby and Kelvin Waves.

    First lets talk a little bit about waves or more specifically Rossby and Kelvin waves. These can occur both in the Atmosphere and in the Ocean and it is important to be clear about the difference between the two.

    Oceanic Rossby waves take the form of slight height changes in the sea and more apparent changes in the depth of the thermocline. These can take months or years to cross an oceanic basin and have there orrigin in anomalous atmospheric pressure patterns.In the North Pacific, for instance, a Rossby wave, after the 10 years or so that it takes to cross the basin, can push the Kuroshio Current northwards and affect weather on the North America continent. This might have happened already in 1993, the culprit Rossby wave being an effect of the 1982-83 El Niño.The important thing about oceanic rossby waves are that they are slow and westward moving. Kelvin waves move faster and eastwards taking about 70 days to cross the Pacific. See in the link below how an easterly wind anomaly at the equator can produce these waves and in the subsequent link how they are reflected to ultimately produce a pattern which has similarities to the el nino,la nina pattern.

    Oceanic Rossby and Kelvin wave Thory

    The Evolution of Oceanic Kelvin and Rossby waves

    The point here for me is that strong MJO events have large impacts of weather patterns and probably contribute to la nina and el nino events. We should note however that el nino and la nina tend to closely follow the volumes of warm water (20C+) at the equator so the MJO does not have it all its own way.

    Warm Water Volume and ENSO

    We recognise Rossby waves in the atmosphere as the long waves in the jetstream but there are also Kelvin waves in the atmosphere which travel eastwards around the world typically taking 40-50 days which show up as a pressure anomally. These lesser known waves may actaully play a most important role is triggering the MJO cycle.

    Perhaps we should just note that gravity waves are a different phenomenon and although Atmospheric Rossby waves are thought of as planetary waves I prefer to use this term for those waves in the Stratosphere, Mesosphere and Ionosphere.

    So onto the MJO events which have there orrigins in enhanced convection over the tropical western Pacific which create a low pressure which radiates rapidly eastward as a dry equatorial Kelvin wave over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the sea level pressure anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle. So we have a trigger which circulates the world over a set period (typically 50days) with one event triggering the next.Here we should note that the MJO Phases do not coincide with this circulation but reflect the eastward migration of convection once convection has been triggered.

    Once convection fires at the start of the cycle you will get a Rossby wave response with pressure troughs to the north and south of the area of convection. The low pressure will bring colder air in to the west of the convection killing of convection while eastward moving warm air spreads the convection eastwards. Eastwards of the low pressure systems will be strong anticyclones (high pressure) which will give strongly easterly winds at the equator. These pressure systems affect the mid latitude jetstream and hence the pattern across the north Pacific, the US and to some extent the North Atlantic and the UK.

    MJO Phase 2 or 3 weather pattern response

    MJO Phase 6 or 7 weather pattern response

    At the moment cool waters in the central pacific due to la nina are tending to damp down the eastward movement of convection while anomalous highs ahead of the convection will be acting to enhance la nina and slowly move it Westwards. Phase 5 through 8 of the MJO can result in a high pressure anomaly towards Alaska and a deep trough down into the central US. There are some suggestions that this high pressure towards Alaska ridges into the arctic region causing a displacement of the stratospheric vortex forcing the arctic oscillation to trend negatively. Perhaps I will revisit this when I know a bit more.

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