Extratropical–Tropical Interaction during Onset of the Australian Monsoon: Reanalysis Diagnostics and Idealized Dry Simulations

Abstract

The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics.For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical-tropical interaction is northeastward Rossby wave propagation from the cyclogenesis region toward the Tropics at upper levels. At these levels, westerly winds extend to nearly 10°S and provide a favorable background flow for such propagation. The process eventually results in the amplification of an equatorward-extending midlatitude upper trough and tropical ridge, which appears to trigger the development of the underlying monsoon trough. To test the hypothesis, the influence of high-latitude cyclogenesis on the tropical circulation is investigated with the aid of an idealized, dry, three-dimensional, baroclinic wave channel model. The initial state consists of (i) a zonally constant baroclinic region centered on 40°S, from which the high-latitude cyclogenesis develops, (ii) a weak monsoon trough at 15°S, and (iii) a subtropical jet at 25°S.The major findings from the simulations are as follows: 1) There is evidence of northeastward Rossby wave propagation from the cyclogenesis region toward low latitudes. 2) Consistent with theoretical studies, the subtropical jet plays a key role by providing a favorable westerly background flow for group propagation into the Tropics. 3) High-latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity, and divergence in the monsoon trough. 4) Vorticity and divergence changes are consistent with enhancement of the monsoon trough (increases in low-level cyclonic vorticity) and the potential for triggering a large-scale convective outbreak (changes in upper-level divergence).

Climate perspective on the large‐scale circulation associated with the transition of the first South Atlantic hurricane

“…Our argument is that a high latitude blocking can dynamically influence the formation of an area of weak shear over the subtropics. This is similar, for instance, to the process described by Davidson et al (2007), when the high latitude flow over the Indian Ocean was found to potentially influence the outcome of the monsoon trough in the Australian region. In the case of Catarina, a likely mechanism is a slow propagation of weak shear from the south as the blocking builds up.…”

“…At present, there is no climatology of hybrid and transitioning systems in the SA, and their behaviour is still poorly understood in the SH, although some insights have recently been gained in the Australia region (Davidson et al, 2007), where both tropical and extra-tropical transitions seem to be very important mechanisms. Future studies addressing these patterns in more detail are much needed, in particular, on the basis of a more general largescale approach in order to further explore and test the interactions proposed here.…”

On the intensification and recurvature of tropical cyclone Tracy (1974)

“…This is seen as a series of troughs and ridges (marked with symbols 'T' and 'R') emanating from the cyclogenesis region and extending northeastwards to low latitudes. This structure is very similar to that described in Davidson et al (2007) during onset events. The proposed mechanism, which still requires further consolidation, is that amplifying Rossby Waves are refracted towards the tropics by the subtropical jet and as part of the wave propagation, the strengthening upper level tropical ridge, with accelerating upper level easterly winds on its northern side, influence large-scale tropical cloudiness and the monsoon.…”

“…Cross-checking against more recently-available ERA40 data was conducted to confirm consistency of diagnostic results. Similar to the findings in Davidson et al (2007), apart from some differences in the moisture analysis and in the intensity of the diagnosed ascent field, the data sets are very similar. Note that for simulations, because of the diabatic initialization used here (see description below), there is less sensitivity to the vertical motion and moisture fields contained in the raw re-analysis data.…”

“…The method used to monitor the LSE is similar to that described in Davidson et al (2007). We trace the evolution of extratropical-tropical interactions in the form of upperlevel propagating Planetary Rossby Waves, which appear to emanate from high-latitude cyclogenesis events over the South-West Indian Ocean, and monitor local circulation changes over the Australian monsoon.…”