The influence of equatorial rossby waves on the flow, convection and tropical cyclones over the Australian Tropics and Southeastern Indonesia

This thesis examines Equatorial Rossby Waves (ERWs) and their modulation of the low to middle level flow, convection and tropical cyclogenesis associated with the passage of over northern Australia, southeastern Indonesia and adjoining waters during the Austral summer wet season. ERWs are equatorially trapped, convectively coupled, westward propagating waves, which are symmetric about the equator. In the Southern Hemisphere the enhanced (suppressed) convective phase of these waves is associated with northwesterly (southeasterly) low to middle level wind anomalies. In the western Pacific, in the absence of dominating low to middle level easterly flow, ERWs manifest themselves as twin cyclonic circulations on either side of the equator. In this thesis ERWs are identified using 9.7 to 48 day band pass filtered outgoing long-wave radiation (OLR) data. Nine strong ERW events, and also thirteen tropical cyclone (TC) genesis events where the initial circulation formed in the Banda Sea, are investigated using a case study and compositing approach. Principal component analysis was employed to investigate all Austral wet season ERW events from January 1979 to 30 April 2011. These data indicated that on average there were 11 ERW events moving through Australian longitudes each wet season. Conclusions reached in this thesis are based on the two sets of events/case studies and the principle component analysis. Analysis of the data over Australian longitudes indicates that there are two classes of ERW events, in which both the propagation of the ERWs and the modulation of the flow, convection and TC genesis differ. These classes are associated with the pre-existing synoptic situation over the region. The first class is characterised by a pre-existing monsoon trough. Four of the nine case studies fall into this class, the remaining five case studies fall into the second class. This class is characterised by a pre-existing, near equatorial trough and easterly low to middle level mean flow extending southward over northern Australia. For the first class of ERW event the focus of enhanced convection and precipitation is over northern Australia and the Coral Sea with the monsoon trough moving south and the flow to its north strengthening and becoming more northerly. Pre-existing circulations within the trough can intensify or new circulations can form. These circulations sometimes reach tropical cyclone intensity. Over the northern Australian, southeastern Indonesian region during the mid wet season months of January through March the monsoon trough is the persistent synoptic feature. Hence over this region during these months most ERW events fall into the first class of event. During these months ERWs propagate to the southwest and tend to weaken. They have an average wave length of 6090km, with an average westward ground based phase speed of 4.4ms-1. For the second class of ERW event a typical scenario occurs when the Southern Hemisphere member of the ERW twin moves westward from the Pacific basin and loses its structure as it traverses the Papua New Guinea (PNG) highlands then rapidly reforms to the west of PNG, thus initiating the formation of the monsoon trough to the north of Australia. In individual events the subsequent development and movement of this system and the trough affect the deep layer flow and precipitation over northern Australia and southeastern Indonesia. In a number of scenarios investigated in this thesis the circulation reached tropical cyclone intensity. In these scenarios the system initially moved to the west and then generally recurved towards the southeast, making landfall on the northwest Australian coast. During the early and late wet season months of November, December and April the near equatorial trough is the typical synoptic situation, so most ERW events fall into the second class of event. During these months ERWs have an average wave length of 6570km, with an average westward phase speed of 4.8ms-1. On average during these months ERWs propagate parallel to the equator and maintain their strength as they move through Australian/Indonesian longitudes. ERWs were found to modulate tropical cyclogenesis (TC genesis) over the Australian/eastern Indonesian region. During the mid wet season months ERWs modulate TC genesis in the east and central parts of the region and in far west. During the early and late wet season ERWs modulate TC genesis in the central parts of the region. Thirteen TCs were found to have initially formed as circulations in the Banda Sea. Eleven of these systems formed in the early or late wet season. The initial formation and movement of each of these eleven systems were intimately associated with the passage of active ERW phases. Subsequent to their initial formation as cyclonic circulations most of the thirteen systems rapidly intensified to tropical cyclone strength and beyond in a favourable environment provided by an active phase of a Madden Julian Oscillation (MJO). These systems can bring severe weather to the eastern Indonesian Archipelago with heavy rainfall and landslides due to steep topography and very deep soil profiles.