Structure, genesis and scale selection of the tropical quasi‐biweekly mode

Abstract

The quasi‐biweekly mode (QBM) and the 30–60 day mode are two major intraseasonal oscillations (ISOs) in the tropics. The QBM is known to have a major influence in determining the active and break conditions of the Indian monsoon during the northern summer. A westward‐propagating equatorial Rossby wave with quasi‐biweekly period influences the Australian monsoon during the northern winter. Universality between the summer and winter QBM is established through analysis of daily circulation and convection data for 10 years. It is shown that the mean spatial structure of the QBM in circulation and convection resembles that of a gravest meridional mode equatorial Rossby wave with wavelength of about 6000 km and westward phase speed of approximately 4.5 m s−1. However, the maximum zonal wind occurs at around 5°N (5°S) during the northern summer (winter). The wave structure appears to be translated northward (southward) by about 5° during the northern summer (winter). The relationship between outgoing long‐wave radiation and circulation data indicates that the mode is driven unstable by coupling with moist convection. Similarity in temporal and spatial characteristics of the mode during the two seasons leads us to propose that the same mechanism governs the genesis and scale selection of the mode in both the seasons. An acceptable mechanism for genesis and scale selection of the QBM has been lacking. In the present study, a mechanism for genesis and scale selection of the observed QBM is proposed. A simple 2½‐layer model that includes a steady Ekman boundary layer (BL) formulation incorporating effect of entrainment mixing is constructed for the convectively coupled equatorial waves. Without influence of the background mean flow, moist feedback in the presence of frictional BL convergence drives the gravest meridional mode equatorial Rossby wave unstable with observed wavelength and period but with zonal winds symmetric about the equator. Potential temperature perturbation associated with the Rossby wave is in phase with relative vorticity perturbation at low level. The BL drives moisture convergence in phase with the relative vorticity at the top of the BL. Release of latent heat associated with the BL convergence enhances the potential temperature leading to a positive feedback. The mean flow over the Indian Ocean and western Pacific at low levels is such that the zero ambient absolute vorticity or the ‘dynamic equator’ shifts to around 5°N (5°S) during summer (winter) and results in a shift of the unstable Rossby waves towards the north (south) by about 5°. The resulting structure of the unstable Rossby mode resembles the observed structure of the biweekly mode. It is shown that neither evaporation–wind feedback nor vertical shear of the mean flow is crucial for the existence of the mode. However these processes marginally modify the growth rate and make the structure of the unstable wave more realistic. Copyright © 2004 Royal Meteorological Society

Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall over the Meghalaya–Bangladesh–western Myanmar region: covariability between the tropics and mid-latitudes

“…The horizontal structure and propagation characteristics are consistent with those found by Chen and Chen (1993), Yokoi and Satomura (2005), and Chatterjee and Goswami (2004) of n = 1 equatorial Rossby waves with submonthly period. The two vortices are not symmetric about the equator, but appear to be shifted northward by *5-7°N, consistent with the results of Chatterjee and Goswami (2004). Figure 11 shows time-longitude sections of the OLR anomaly (with only the seasonal cycle of OLR removed) and Rossbywave filtered OLR anomaly averaged from 10-20°N during June to August 1988.…”

“…Some previous studies have pointed out that the submonthly-scale/quasi-biweekly ISO in South/Southeast Asia is associated with a westward propagating n = 1 equatorial Rossby (ER) wave (Chen and Chen 1993;Annamalai and Slingo 2001;Yokoi and Satomura 2005;Chatterjee and Goswami 2004;Kikuchi and Wang 2009). The ER wave has periodicity of approximately the submonthly range (Kiladis and Wheeler 1995;Masunaga 2007;Kiladis et al 2009).…”

Intraseasonal meridional current variability in the eastern equatorial Indian Ocean

“…The present results demonstrate that the phases of the meridional currents and wind stresses synchronously propagate westward (see Figure 9), suggesting a possibility of the resonance in the eastern basin as well. There are several candidates for the atmospheric disturbances at this time-scale over the eastern Indian Ocean, which can be considered as the forcing to the oceanic ISV, such as those indicated by Chatterji and Goswami [2004], Shinoda and Han [2005], and Fukutomi and Yasunari [2005]. An air-sea interaction through modification of the sea surface temperature field by the current itself may be an important process for the ISV, although the contribution from such interaction might be small due to weak horizontal gradient in the sea surface temperature field.…”

“…Sengupta et al [2004] indicates that biweekly meridional current variability is well simulated in their moderate resolution ocean general circulation model and that the signal propagates to the west with the phase speed and zonal wavelength of 2.5$5.0 m/s and 4000$5000 km, respectively. They also suggest that the biweekly variability has characteristics of the Mixed Rossby-gravity wave and is generated by meridional wind stresses associated with the atmospheric ISVs [Chen and Chen, 1993;Chatterji and Goswami, 2004]. Another mechanism for the generation of the biweekly meridional current variability in the upperlayer of the eastern equatorial Indian Ocean is proposed by Miyama et al [2006], in which the Mixed Rossby-gravity wave is excited in the western Indian Ocean and a raypath of its energy penetrate into the deep ocean and, then, is reflected at the bottom to reach the upper eastern Indian Ocean.…”

Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

“…On the other hand, the intraseasonal variability of the zonal winds and currents is influenced by large-scale seasonal variations of the tropical atmosphere and ocean (Chatterji and Goswami 2004;Senan et al 2003).…”