Poleward propagating Tropical Intraseasonal Oscillations over the Indian Ocean

Abstract

The mechanisms of poleward propagation of tropical intraseasonal oscillations (ISOs) are not well-understood. In this thesis, we investigate this issue for two such tropical ISOs over the Indian Ocean, namely, the Boreal Summer Intraseasonal Oscillation (BSISO) during boreal summer which propagates northward from the equator, and the quasi-biweekly oscillation (QBWO) during boreal winter which propagates southwestward from the equator. We start with the BSISO and identify two types of events, one set which propagates northward over South Asia from the equatorial Indian Ocean (EIO), and the other which doesn’t. Contrasting their behaviour shows that northward propagation occurs in multiple stages after convection is initiated over the EIO. First, convection moves into the southern Arabian Sea (AS) due to moistening of the free troposphere via horizontal BSISO anomalous winds acting on the background moisture distribution, and forms a northwest-southeast (NW-SE) oriented convection band. Subsequently, in the presence of background easterly vertical shear of monsoon winds and meridional gradient of anomalous vertical velocity, a NW-SE oriented tilting term is generated that results in a tilted gyre north of the existing convective anomaly. As a result, BSISO winds over the South Asian landmass become south-easterly. In the second stage, these winds tap the ambient north-westward moisture gradient and help move convection further north over the landmass. Moreover, background winds advect anomalous moisture to initiate convection over the Bay of Bengal. For non-propagating events, though a Rossby gyre results as a response to nascent EIO convection, it is smaller, thus BSISO advection of moisture is weaker and does not initiate convection over the southern AS. In turn, the meridional gradient of anomalous vertical velocity is weak, and the background vertical shear does not generate sufficient tilting over the northern AS. Thus, the convective wind response stalls, and large-scale convection does not propagate north of 15N. In all, free-tropospheric moisture advection and vortex tilting due to the background vertical shear work together for robust northward propagation of the BSISO Next, we shift our focus to the QBWO. An analysis of outgoing longwave radiation (OLR) and winds over the South-West Indian Ocean (SWIO) yields regular, poleward propagating, largescale, convectively coupled systems of alternating cyclonic and anticyclonic circulation with a quasi-biweekly period during boreal winter. Composites from 30 years of OLR and reanalysis data show well-formed rotational gyres in the lower troposphere that can be tracked from near the equator to almost 30◦S appearing west of Sumatra and propagating towards Madagascar, i.e., with mean southwest propagation. The gyres show a marked northwest-southeast tilt, giving rise to a northeast-southwest oriented wavetrain. The scale of the gyres is about 30◦-35◦, their period is 14-18 days and they have a westward phase speed of approximately 3.5 ms−1. In early stages, the gyres are associated with weak convection, but when they move poleward and cross 10◦S, convective coupling is enhanced. Velocity fields and OLR indicate that the maxima of moist convective activity lies in the eastern sector of the gyres and a comparison between column-integrated moisture and OLR anomalies shows they are highly collocated, indicating the applicability of the moisture mode framework. A moisture budget reveals that once the gyres reach 10◦-20◦S, moistening is mainly due to northerlies in the eastern sector of the cyclonic gyre acting on the meridional gradient of background moisture, which eventually gives rise to anomalous convection in this region. This moistening process continues up to 30◦S while the gyres traverse southwestward. Subsequently, background easterlies advect anomalous moisture and along with moistening via so-called ’column-processes’, convection is observed to extend inside the gyre from the eastern side. A vorticity budget reveals that the β effect plays a leading role in the south-westward propagation, horizontal advection assists the westward movement of vorticity anomalies due to prevailing easterlies and moist coupling (via stretching) is important in reducing the speed of propagation of this mode. In fact, the relatively slow southwestward movement of the system is because moist coupling reduces the effect of β and horizontal advection terms. Moreover, as convection primarily takes place on the eastern side of the vortex, and somewhat inside the vortex too, it also follows a southwestward path along with the QBWO vortices. Overall, in this thesis, we have shown two different mechanisms of moist dynamics, where moisture and circulation actively influence each other to result in the evolution and poleward propagation of tropical ISOs.