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Keywords:

  • CHUAN;
  • Comprehensive Historical Upper-Air Network;
  • data intercomparisno;
  • reanalysis;
  • West Africa;
  • monsoon;
  • Twentieth Century Reanalysis;
  • NCEP/NCAR Reanalysis

Abstract

The strong interannual-to-decadal variability of the West African Monsoon is subject to an active field of climate research that tries to disentangle its influencing factors and explore its predictability. Reliable observation-based data over a preferably long period are arguably the most important basis for such efforts. Here, we try to explore the quality of several data products available for the earlier period of upper-air observations (1940–1957): the Comprehensive Historical Upper-Air Network (CHUAN), the NCEP/NCAR Reanalysis (NNR), and the Twentieth Century Reanalysis (20CR). To do so, we compare wind soundings from 37 pilot balloon stations contained in CHUAN (10°S–30°N, 20°W–20°E) with the reanalyses.

The comparison with the NNR reveals seasonally and diurnally varying significant differences relative to the observations over West and Central Africa. The differences reach absolute values of several metres per second, and their spatially coherent structure strongly points to a deficiency of the reanalysis rather than observational errors. The difference fields indicate an overestimation of the strength and thickness of the low-level monsoon in all seasons and an underestimation of the Harmattan winds over the Sahel in winter. At higher levels, they point to an overestimation of the mid-tropospheric monsoon return flow and African easterly jet.

For the 20CR, the fields reveal again significant differences up to several metres per second on all levels and in all seasons. However, the direction relative to the observed monsoon flow and Harmattan/trades is opposite at the low levels. Additionally, the differences tend to be smaller and more confined to the coastal region.

Further analysis demonstrates that the observed interannual variability is only insufficiently modelled in both reanalyses. Together with the diurnal cycle of the differences, this precludes a simple correction of the reanalyses and demonstrates that, depending on the purpose of a study, one should be extremely cautious when using reanalysis products. Copyright © 2011 Royal Meteorological Society