Abstract

We present the first record covering several consecutive years of the stable isotopic composition of oxygen and hydrogen in rainfall (δ18 O and δD) from the year-round rainfall zone on the south coast of South Africa. The dataset includes 140 daily rainfall samples collected between January 2009 and December 2012 (excluding May/June 2012), and a previously published record from 2006 to 2007. The δ18O and δD of daily samples range from -12.1 to 4.3‰ and from -84.1 to 26.3‰, respectively; monthly amount-weighted averages of δ18O and δD range from -7.6 to 1.0‰ and from -44.2 to 12.8‰, respectively. Examination of the relationship of δ18O and δD with temperature (T) and rainfall amount (P) shows that correlation with temperature is higher for both event-based (δ18O vs. T: Spearman’s Rank correlation coefficient (rs) = 0.52, δ18D vs. P: rs =-0.30; δD vs. T: rs = 0.47, δD vs. P: rs = -0.24) and weighted monthly average data (δ18D vs. T: rs = 0.62, δ18D vs. P: rs =-0.49; δD vs. T: rs = 0.45, δD vs. P: rs = -0.41). The seasonal amplitude of δ18O and δD is larger in Mossel Bay than at two sampling stations in Cape Town and at other mid-latitude coastal sites, and the slope of the relationship between δ18D and temperature is steeper at Mossel Bay. This suggests that rainfall at Mossel Bay, where synoptic conditions leading to rainfall include complex interactions of tropical-subtropical and temperate pressure systems, is affected by other parameters than rainfall at Cape Town, where temperate frontal systems predominate.

The local Meteoric Water Lines (LMWL) for daily rainfall data (δD = 7.35 [6.88,7.84]* δ18O + 11.21 [9.97, 12.52], 95% CI in brackets) and weighted monthly averages (δD = 7.70 [6.77, 8.76]* δ18D + 12.10 [9.47, 15.08] ) are similar to the Global Meteoric Water Line (δD = 8* δ18O + 10) and also correspond well with the LMWLs for Cape Town and Pretoria. Deuterium excess (d-excess) values of single rain events vary widely; monthly averages at Mossel Bay over the whole sampling period are mostly between 10 and 20‰, with lower values during November and December (5 and 6‰) that are related to evaporation from falling raindrops. A comparison of seasonal changes in δ18O and δD to main atmospheric circulation types that cause rainfall shows that the high δ18O and δD values at Mossel Bay between January and April are associated with more frequent occurrence of convective rainfall, whereas lower values are related to stratiformfrontal or orographic rainfall.

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