Impact of Background Conditions on the Structure and Propagation of the Boreal Summer Quasi-Biweekly Oscillation

TL;DR

This study analyzes the westward-propagating boreal summer QBWO across a moisture–wind gradient in the northern tropics, linking background moisture and mean flows to its structure and propagation. Using 21 years of NOAA OLR and ERA5 reanalysis data, a 10–25 day QBWO filter, and Day 0 composites over nine tropical regions, it identifies robust features such as collocated OLR and moisture anomalies and upright vertical structures indicative of a first baroclinic mode. A vorticity budget shows regime-dependent balances, with mean-flow advection and planetary stretching governing dry regions, and eddy advection of background vorticity becoming essential in very moist regions with westerly flow; a parallel moisture budget reveals a similar shift toward eddy-driven background-moisture advection in moist zones. The results provide constraints for model evaluation and motivate the development of moisture–vorticity coupled theories and simplified QBWO models tailored to regional background moisture and wind conditions.

Abstract

We examine the westward-propagating quasi-biweekly oscillation (QBWO) during boreal summer, with a focus on how background moisture and winds shape its structure and propagation. In dry regions, convection lags the circulation by nearly a quarter cycle, whereas in very moist regions it becomes nearly in-phase and extends across the QBWO gyre. As the background moistens, moisture anomalies increasingly align with the QBWO circulation. Despite differences in environmental moisture and wind conditions, several structural features remain robust: outgoing longwave radiation and moisture anomalies stay collocated, moisture and pressure-velocity anomalies remain vertically upright, and the filtered winds retain a first-baroclinic mode structure. A vorticity budget shows that, although the planetary vorticity-gradient term is important, both planetary stretching and horizontal advection are needed to explain the vorticity tendency- and their relative importance shifts with the moisture regime. In dry and moderately moist regions with easterly mean flow, mean winds primarily advect vorticity anomalies. In contrast, in very moist regions with westerly flow, anomalous winds instead advect the background vorticity. An analogous transition occurs in the moisture budget: in dry and moderately moist environments, zonal mean flow advection dominates, but in very moist regions, strong background moisture gradients allow eddy advection of the mean moisture field to become the leading term. In the moist regime, vertical advection, precipitation, and evaporation also contribute substantially to the moisture tendency. Overall, the QBWO behaves like a mean-flow-driven linear mode in dry and moderately moist regions with easterly background winds, but shifts toward a regime dominated by eddy advection of background vorticity and moisture in very moist regions characterized by westerly flow.

Figures (12)

• Figure 1: Mean column integrated (1000 hPa to 250 hPa) moisture and mean winds at 850 hPa during the boreal summer (JJAS).
• Figure 2: Composite OLR (shading) and 850 hPa horizontal wind (quivers) anomalies on Day 0 in the northern hemisphere tropics. The mentioned regions are chosen by the methodology described before.
• Figure 3: Composite OLR (shading) and moisture (solid and dashed contours denote positive and negative values respectively) anomalies on Day 0. Interval of contours is $0.6\kg/\m^2$. The regions are the same as those in Figure \ref{['F1']}.
• Figure 4: Hovmöller diagram for column moisture (color shading) and vorticity (contours, positive in solid and negative in dashed) anomalies, the difference between adjacent contours is $5\times10^{-7}\text{s}^{-1}$. Averaged values across latitudes $20^\circ\text{N}-10^\circ\text{N}$ are plotted in each case, the Y-axis shows lead/lag from Day 0. The left panel (a)) shows a composite for the dry Central African region and the right one (b)) represents a composite for the moist Bay of Bengal.
• Figure 5: Vertical structure of pressure velocity (shading, upper row) and moisture (contours, upper row) with latitude (a)) and longitude (b)) on Day 0 of the QBWO composite over the Bay of Bengal. Solid (dashed) contours denote positive (negative) moisture anomalies, and the contour interval is 8.0d-5kg/kg. The lower panels show the vertical structures of zonal velocity with latitude (c)) and meridional velocity with longitude (d)) in the first and second column, respectively.