The onset of Madden–Julian oscillation (MJO) deep convection often occurs over the western Indian Ocean and has upper-tropospheric circulation precursors that consist of eastward-circumnavigating tropical easterlies and subtropical cyclonic Rossby gyres near eastern Africa. Moreover, the evolution of the large-scale circulation and its ability to reduce subsidence may be necessary for the initial development of organized deep convection. To better understand the evolution of the circulation precursors and their interaction with convective onset, this paper analyzes the upper-tropospheric zonal momentum budget using a regional index based on the temporal progression of the meridional structure of intraseasonal outgoing longwave radiation anomalies over eastern Africa and the western Indian Ocean. The circumnavigating intraseasonal easterly acceleration produces upper-level divergence when it reaches the western extent of a region of intraseasonal westerlies and may provide a forcing for the in-phase midtropospheric upward vertical motion. For about three-quarters of the identified cases, the easterly acceleration over the western Indian Ocean is a response to the zonal pressure gradient over the region. In the composite, the negative pressure gradient force may be initially induced by the injection of negative geopotential height anomalies from the extratropics of both hemispheres to the tropics over eastern Africa, though tropically circumnavigating and local signals may also contribute to the easterly acceleration, especially in the days following convective onset.
Prior to the onset of convection over the western Indian Ocean for many Madden–Julian Oscillation (MJO) events, equatorward‐propagating upper tropospheric negative geopotential height anomalies are observed along the east coast of Africa. To identify these signals independently from the MJO, we calculate partial meridional‐temporal wavelet transforms averaged between 35 and 45°E and centred in the subtropics. During times when the amplitudes of MJO indices are strong and leading up to convective onset over the western Indian Ocean, increased wavelet power relative to the mean state is observed in the equatorward direction at intraseasonal time‐scales from both hemispheres. Regressions against groups of equatorward intraseasonal wavelet transform time series centred on negative anomalies in the subtropics show extratropical Rossby wave trains extending to the west across the Atlantic similar to signals previously observed leading up to the onset of Indian basin convection in composite MJO events. Compared to globally averaged mean‐state power spectra in the subtropics, increased equatorward propagation of upper tropospheric geopotential height anomalies occurs regionally near eastern Africa on intraseasonal time‐scales, which suggests that local mean‐state conditions, in addition to the MJO, may influence meridional wave propagation. Since these anomalies may in turn influence the onset of MJO convection over the western Indian Ocean and have higher local power, mean‐state conditions near the subtropics of eastern Africa may change the timing of the MJO.more » « less
- NSF-PAR ID:
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Page Range / eLocation ID:
- p. 380-400
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
Qualities of the meridional movements of geopotential height anomalies in the upper troposphere of the subtropics are analysed via wavelet analysis using a meridional–temporal partial Morlet wavelet. Results show that power, which represents increased presence or amplitude of waves with direct meridional movement, is increased in regions where the corresponding equatorial winds in the upper troposphere are westerly or weakly easterly. Furthermore, equatorward power is enhanced near subtropical jet exit regions whereas poleward power is enhanced in jet entrance regions. Regressions of upper‐tropospheric winds, geopotential height, and outgoing long‐wave radiation (OLR) against the wavelet transforms demonstrate that the wavelets are identifying signals with tropical–extratropical interactions that are connected to organized convection in the tropics. The relationship of power with background‐state flow characteristics, including the horizontal winds and shear, are evaluated. Instead of the zonal wind and meridional shear of the zonal wind (
du/ dy), both the meridional wind and the zonal shear of the meridional wind ( dv/ dx) appear to have a clearer relationship with the power. Power is favoured for waves whose movement is aligned in the same direction as the meridional wind, and reduced in the opposite direction. Additionally, power increases with increasing zonal shear of the meridional wind in the Northern Hemisphere and with decreasing zonal shear of the meridional wind in the Southern Hemisphere. Power in the equatorward direction is stronger than in the poleward direction and more heavily influenced by background flow characteristics. Furthermore, power for wavelets with smaller meridional and temporal scales tends to have a higher sensitivity to the background horizontal flow as compared to larger meridional and temporal scales.
We investigate how the Madden‐Julian Oscillation (MJO), the dominant mode of tropical subseasonal variability, modulates the lifecycle of cool‐season North Pacific atmospheric rivers (ARs). When the enhanced (suppressed) convection center is located over the Indian Ocean (western Pacific), more AR events originate over eastern Asia and with fewer over the subtropical northern Pacific. When the enhanced (suppressed) convection is over the western Pacific (Indian Ocean), the opposite changes occur, with more AR events originate over the subtropical northern Pacific and fewer over eastern Asia. Dynamical processes involving anomalous MJO wind and seasonal mean moisture are found to be the dominant factors impacting these variations in AR origins. The MJO‐related anomalous geopotential height patterns are also shown to modulate the propagation of the AR events. These MJO–AR lifecycle relationships are further supported by model simulations.
The effect of the Madden‐Julian Oscillation (MJO) on springtime Antarctic ozone variations is revealed for the first time from multi‐satellite reanalysis and model simulations. Twenty to 30 days after MJO Phase 8 (P8), Antarctic total column ozone (TCO) anomalies significantly decrease by up to −15 DU, associated with a wave‐1 response at around 60°S. After MJO P8, MJO‐related geopotential height anomalies in the southern hemispheric (SH) Indian Ocean emanate from subtropics to polar regions, leading to suppressed upward and poleward propagation of planetary waves (PWs) and weakened Brewer‐Dobson circulation in the SH stratosphere. This in turn results in less ozone transport from midlatitudes into the polar region and thus a negative polar TCO response. Dynamical transport plays a dominant role in modulating the Antarctic TCO after MJO P8. The magnitude of transient changes due to chemical processes is relatively weak than that caused by dynamical transport.
null (Ed.)Abstract A normalization method is applied to MJO-scale precipitation and column integrated moist static energy (MSE) anomalies to clearly illustrate the phase evolution of MJO. It is found that the MJO peak phases do not move smoothly, rather they jump from the original convective region to a new location to its east. Such a discontinuous phase evolution is related to the emerging and developing of new congestus convection to the east of the preexisting deep convection. While the characteristic length scale of the phase jump depends on a Kelvin wave response, the associated time scale represents the establishment of an unstable stratification in the front due to boundary layer moistening. The combined effect of the aforementioned characteristic length and time scales determines the observed slow eastward phase speed. Such a phase evolution characteristic seems to support the moisture mode theory of the second type that emphasizes the boundary layer moisture asymmetry, because the moisture mode theory of the first type, which emphasizes the moisture or MSE tendency asymmetry, might favor more “smooth” phase propagation. A longitudinal-location-dependent premoistening mechanism is found based on moisture budget analysis. For the MJO in the eastern Indian Ocean, the premoistening in front of the MJO convection arises from vertical advection, whereas for the MJO over the western Pacific Ocean, it is attributed to the surface evaporating process.more » « less