Representation of Spatial Variability of the Water Fluxes over the Congo Basin Region

The Congo Basin, being one of the major basins in the tropics, is important to the global climate, yet its hydrology is perhaps the least understood. Although various reanalysis/analysis datasets have been used to improve our understanding of the basin’s hydroclimate, they have been historically difficult to validate due to sparse in situ measurements. This study analyzes the impact of model resolution on the spatial variability of the Basin’s hydroclimate using the Decorrelation Length Scale (DLCS) technique, as it is not subject to uniform model bias. The spatial variability within the precipitation (P), evaporation/evapotranspiration (E), and precipitation-minus-evaporation (P-E) fields were investigated across four spatial resolutions using reanalysis/analysis datasets from the ECMWF ranging from 9–75 km. Results show that the representation of P and P-E fields over the Basin and the equatorial Atlantic Ocean are sensitive to model resolution, as the spatial patterns of their DCLS results are resolution-dependent. However, the resolution-independent features are predominantly found in the E field. Furthermore, the P field is the dominant source of spatial variability of P-E, occurring over the land and the equatorial Atlantic Ocean, while over the Southern Atlantic, P-E is mainly governed by the E field, with both showing weak spatial variability.

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On the Structure of the Regional-Scale Circulation over Central Africa: Seasonal Evolution, Variability, and Mechanisms

Journal of Climate ◽

10.1175/jcli-d-19-0176.1 ◽

2020 ◽

Vol 33 (1) ◽

pp. 145-162 ◽

Cited By ~ 3

Author(s):

Georges-Noel T. Longandjo ◽

Mathieu Rouault

Keyword(s):

Atlantic Ocean ◽

Pressure Gradient ◽

Surface Pressure ◽

Central Africa ◽

Lower Troposphere ◽

Congo Basin ◽

Equatorial Atlantic ◽

Seasonal Evolution ◽

Westerly Jet ◽

Equatorial Atlantic Ocean

AbstractAtmospheric circulation over central Africa is dominated by the tropical easterly jet, the African easterly jet, and the low-level westerly jet. In the lower troposphere, a zonal overturning cell occurs over central Africa, but the mechanisms driving its formation, seasonal evolution, and variability are still unclear. Here, using reanalyses (ERA-Interim, NCEP-2, and JRA-55) and the ECHAM5.3 atmospheric model forced by observed sea surface temperature, we highlight the existence, in the lower troposphere, of a separated single, closed, counterclockwise, and shallow zonal overturning cell, namely, the Congo basin cell. This Congo basin cell persists year round, with maximum intensity and width in August/September and minimum intensity and width in May. This shallow cell extracts heat from the warm central Africa landmass through latent and internal energies and transports it to the cold eastern equatorial Atlantic Ocean, reminiscent of the mixed Carnot–steam cycle. Indeed, the monsoon-like circulation triggered by the zonal surface pressure gradient between the warm central Africa landmass and surrounding cold oceans produces mass convergence at the Congo Air Boundary, providing necessary upward motion to air parcels to destabilize the atmosphere over central Africa. As result, convective updrafts depend on underlying moist static energy and the induced low-level westerly jet, controlled by the near-surface land–ocean thermal contrast through the zonal surface pressure gradient between the warm central African landmass and cold eastern equatorial Atlantic Ocean, rather than the midlevel easterly jet. This midlevel easterly jet is formed by the mechanical work that balances the convection associated with the saturation and rainfall. Furthermore, the efficiency of the Congo basin cell determines seasonality over central Africa.

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