Quasi-biweekly Oscillations During the Boreal Summer

We study the large-scale westward-propagating quasi-biweekly oscillation (QBWO) in the
global tropics. During the boreal summer, these waves exhibit significant activity over
Southeast Asia and the Western Pacific Ocean. Although comparatively weaker, this os-
cillation is also found across much of the northern tropics throughout this season. The
structure of the QBWO shows a strong resemblance to equatorial Rossby waves, but a
few fundamental features vary across different regions and result in different growth and
propagation mechanisms. Due to their cyclonic-anticyclonic gyres, once formed, these
waves can trigger or suppress extreme events, such as tropical cyclones/depressions. In
fact, these large-scale systems can also influence heatwaves/regional temperature changes,
intense/suppressed rainfall events, and changes in humidity.
Composites from multiple decades of data reveal significant differences between circula-
tion and convection structures in various tropical regions. Convective coupling modifies
the theoretically predicted structure of the equatorial Rossby waves [1] in relatively moist
regions, such as the Western Pacific, Bay of Bengal, and the Arabian Sea. Specifically,
in the very moist regions over the Bay of Bengal and the Arabian Sea, convection is
collocated with circulation, instead of the expected quadrature lag in these variables [2].
A vorticity budget indicates that while meridional advection of planetary vorticity is the
primary controller of the tendency in both moist and dry regions, other terms are essential
in approximating the evolution of the vorticity anomaly. Planetary stretching hinders the
propagation, while horizontal advection by the zonal wind supports it in the dry regions.
In moist regions, while stretching appears to aid growth, it is required in combination
with horizontal vorticity advection to match the vorticity tendency. The moisture budget
illustrates that in relatively dry regions, the zonal mean advection of perturbed moisture
in regions with strong easterlies contributes to the evolution of moisture. On the other
hand, in moist regions, horizontal advection of the background moisture by the anoma-
lous winds and a combination of vertical advection, evaporation, and precipitation are
crucial for approximating the moisture tendency. These results help us develop a better
understanding of the QBWO and lead the way for simplified theoretical models of this
intraseasonal tropical mode of variability.
[1] T. Matsuno, Journal of the Meteorological Society of Japan. Ser. II, 44(1):25–43, (1966).
[2] Y. Nakamura and Y.N. Takayabu, Journal of the Atmospheric Sciences, 79(1):247–262,
(2022).