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Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.
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Hydrodynamics of regional and seasonal variations in Congo Basin precipitation
The processes that determine the seasonality of precipitation in the Congo Basin are examined using the atmospheric column moisture budget. Studying the fundamental determinants of Congo Basin precipitation seasonality supports process-based studies of variations on all time scales, including those associated with greenhouse gas-induced global warming. Precipitation distributions produced by the ERA5 reanalysis provide sufficient accuracy for this analysis, which requires a consistent dataset to relate the atmospheric dynamics and moisture distribution to the precipitation field. The Northern and Southern Hemisphere regions of the Congo Basin are examined separately to avoid the misconception that Congo Basin rainfall is primarily bimodal. While evapotranspiration is indispensable for providing moisture to the atmospheric column to support precipitation in the Congo Basin, its seasonal variations are small and it does not drive precipitation seasonality. During the equinoctial seasons, precipitation is primarily supported by meridional wind convergence in the moist environment in the 800–500 hPa layer where moist air flows into the equatorial trough. Boreal fall rains are stronger than boreal spring rains in both hemispheres because low-level moisture divergence develops in boreal spring in association with the developing Saharan thermal low. The moisture convergence term also dominates the moisture budget during the summer season in both hemispheres, with meridional convergence in the 850–500 hPa layer as cross-equatorial flow interacts with the cyclonic flow about the Angola and Sahara thermal lows. Winter precipitation is low because of dry air advection from the winter hemisphere subtropical highs over the continent.
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- Award ID(s):
- 1939880
- PAR ID:
- 10331053
- Date Published:
- Journal Name:
- Climate Dynamics
- ISSN:
- 0930-7575
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00382-021-06066-3
