Provisionally Corrected Surface Wind Data, Worldwide Ocean-Atmosphere Surface Fields, and Sahellan Rainfall Variability

M. Neil Ward Hadley Centre for Climate prediction and Research, Meteorological Office, Bracknell, Berks, England

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Abstract

Worldwide ship datasets of sea surface temperature (SST), sea level pressure (SLP), and surface vector wind are analyzed for a July-September composite of five Sabelian wet years (1950, 1952, 1953, 1954, 1958) minus five Sahelian dry years (1972, 1973, 1982, 1983, 1984) (WD). The results are compared with fields for a number of individual years and for 1988 minus 1987 (8887); Sahelian rainfall in 1988 was near the 1951–80 normal, whereas 1987 was very dry. Before performing the analyses, an extensive study of the geostrophic consistency of trends in pressure gradients and observed wind was undertaken, motivated by the suggestion that changes in observational practice on board ships have introduced an upward trend in reported wind speed during the last 40 years.

The results suggest that, during the period 1949–88, there is a mean increase in reported wind speed of about 16% that cannot be explained by trends in geostrophic winds derived from seasonal mean SLP. Estimates of the wind bias are averaged for 18 ocean regions. The trend in every 2° lat × 2° long wind time series is adjusted according to the mean bias of the region in which the box resides.

A map of correlations between Sahelian rainfall and SLP in all available ocean regions is shown to be field significant. Remote atmospheric associations with Sahelian rainfall are consistent with recent suggestions that SST forcing from the tropical Atlantic and the other ocean basins may contribute to variability in seasonal Sahelian rainfall. It is suggested that wetter years in the Sahel are often accompanied by a stronger surface monsoonal flow over the western Indian Ocean and low SLP in the tropical western Pacific near New Guinea, and there is also a suggestion of increased cyclonicity over the extratropical eastern North Atlantic and northwest Europe. All of these features were present in WD and 8887 fields. In the tropical Atlantic, WD shows many of the features identified by previous authors. However, the 8887 fields do not reflect these large-wale tropical Atlantic changes. Instead there is only local strengthening of the pressure gradient and wind flow from Brazil to Senegal. Further individual years are presented (1958, 1972, 1975) to provide specific examples of some of the patterns identified in WD, while July-September 1983 is shown to have an SLP anomaly pattern over the tropical oceans that is quite unique in the period 1949–88.

Abstract

Worldwide ship datasets of sea surface temperature (SST), sea level pressure (SLP), and surface vector wind are analyzed for a July-September composite of five Sabelian wet years (1950, 1952, 1953, 1954, 1958) minus five Sahelian dry years (1972, 1973, 1982, 1983, 1984) (WD). The results are compared with fields for a number of individual years and for 1988 minus 1987 (8887); Sahelian rainfall in 1988 was near the 1951–80 normal, whereas 1987 was very dry. Before performing the analyses, an extensive study of the geostrophic consistency of trends in pressure gradients and observed wind was undertaken, motivated by the suggestion that changes in observational practice on board ships have introduced an upward trend in reported wind speed during the last 40 years.

The results suggest that, during the period 1949–88, there is a mean increase in reported wind speed of about 16% that cannot be explained by trends in geostrophic winds derived from seasonal mean SLP. Estimates of the wind bias are averaged for 18 ocean regions. The trend in every 2° lat × 2° long wind time series is adjusted according to the mean bias of the region in which the box resides.

A map of correlations between Sahelian rainfall and SLP in all available ocean regions is shown to be field significant. Remote atmospheric associations with Sahelian rainfall are consistent with recent suggestions that SST forcing from the tropical Atlantic and the other ocean basins may contribute to variability in seasonal Sahelian rainfall. It is suggested that wetter years in the Sahel are often accompanied by a stronger surface monsoonal flow over the western Indian Ocean and low SLP in the tropical western Pacific near New Guinea, and there is also a suggestion of increased cyclonicity over the extratropical eastern North Atlantic and northwest Europe. All of these features were present in WD and 8887 fields. In the tropical Atlantic, WD shows many of the features identified by previous authors. However, the 8887 fields do not reflect these large-wale tropical Atlantic changes. Instead there is only local strengthening of the pressure gradient and wind flow from Brazil to Senegal. Further individual years are presented (1958, 1972, 1975) to provide specific examples of some of the patterns identified in WD, while July-September 1983 is shown to have an SLP anomaly pattern over the tropical oceans that is quite unique in the period 1949–88.

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