Abstract
Abstract. This study was performed to examine the relationship between isotopic composition in near-surface vapor (δ18Ov) over western 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 daily 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 western Africa. Encouraged by this result, we conducted a simulation stretching 34 years (1979–2012) to investigate the relationship between atmospheric environment and isotopic signature on an interannual timescale. 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 relationship suggests that there is a possibility of reconstructing past western African monsoon activity and ENSO using climate proxies.
Highlights
The El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual variability in the tropics (Dai et al, 1997) and plays an important role in the variability of precipitation, temperature, and circulation patterns on this timescale
The average is negatively biased (Yoshimura et al, 2011) and the latitudinal gradient is weaker in IsoGSM, bias and overestimated gradient are found in SCIAMACHY when compared with ground-based Fourier-transform spectrometers (Scheepmaker et al, 2014)
We presented the interannual variability of δ18Ov in western Africa and its relationship to ENSO using the nudged IsoGSM model (Yoshimura et al, 2008)
Summary
The El Niño–Southern Oscillation (ENSO) is the strongest mode of interannual variability in the tropics (Dai et al, 1997) and plays an important role in the variability of precipitation, temperature, and circulation patterns on this timescale. A. Okazaki et al.: Interannual variability of isotopic composition drought that hit western Africa during the 1970s and 1980s prompted researchers to study the factors controlling western African rainfall variability on interannual to multidecadal timescales (e.g., Folland et al, 1986; Palmer, 1986; Janicot et al, 1996; Giannini et al, 2003; Shanahan et al, 2009; Mohino et al, 2011a, b). We explore the factors governing the interannual variability of monsoon season δ18Ov, which is the source of precipitation and controls δ18Op variability (Risi et al, 2008a), over western Africa and how the ENSO signal is imprinted.
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