Interannual variability of isotopic composition in water vapor over West Africa and its relation to ENSO
This study was performed to examine the relationship between isotopic composition in near-surface vapor (δ18Ov) over West Africa during the monsoon season and El Niño-Southern Oscillation (ENSO) activity using the Isotope-incorporated Global Spectral Model. The model was evaluated using a satellite and in situ observations at intraseasonal to interannual timescales. The model provided an accurate simulation of the spatial pattern and seasonal and interannual variations of isotopic composition in column and surface vapor and precipitation over West Africa. Encouraged by this result, a simulation stretching 34 years (1979-2012) was conducted to investigate the relation between atmospheric environment and isotopic signature at the interannual time scale. The simulation indicated that the depletion in the monsoon season does not appear every year at Niamey. The major difference between the composite fields with and without depletion was in the amount of precipitation in the upstream area of Niamey. As the interannual variation of the precipitation amount is influenced by the ENSO, we regressed the monsoon season averaged δ18Ov from the model and annually averaged NINO3 index, and found a statistically significant correlation (R = 0.56, P < 0.01) at Niamey. This relation suggests that there is a possibility of reconstructing past West African monsoon activity and ENSO using climate proxies.