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It is common practice to use a 30-year period to derive climatological values, as recommended by the World Meteorological Organization. However this convention relies on important assumptions, of which the validity can be examined by deriving the uncertainty inherent to using a limited time-period for deriving climatological values. In this study a new method, aiming at deriving this uncertainty, has been developed with an application to precipitation for a station in Europe (Westdorpe) and one in Africa (Gulu). The weather generator framework is used to produce synthetic daily precipitation time-series that can also be regarded as alternative climate realizations. The framework consists of an improved Markov model, which shows good performance in reproducing the 5-day precipitation variability. The sub-seasonal, seasonal and the inter-annual signals are introduced in the weather generator framework by including covariates. These covariates are derived from an empirical mode decomposition analysis with an improved stability and significance assessment. Introducing covariates was found to substantially improve the monthly precipitation variability for Gulu. From the weather generator, 1,000 synthetic time-series were produced. The divergence between these time-series demonstrates an uncertainty, inherent to using a 30-year period for mean precipitation, of 11 % for Westdorpe and 15 % for Gulu. The uncertainty for precipitation 10-year return levels was found to be 37 % for both sites.
Until now, the NW Indian Ocean was sparsely covered with coral proxy records, and records from the Maldives Archipelago do not exist. The first such coral proxy record from the central Maldives is presented in this study. It originates from a massive Porites lutea (Quoy and Gaimard, 1833) colony that was sampled March 2007 in the lagoon of Rasdhoo Atoll (4°N/ 73°W), which is located in the central Maldives. The record spans a period of 90 yrs and reaches back to 1917 AD with monthly to bimonthly resolution. This study investigates temporal variations of the skeletal stable oxygen (delta18O) and carbon (delta13C) isotopes, the strontium-to-calcium (Sr/Ca), and the annual extension-rates, and their relationship to historical climate variations 1917-2007. Annual extension-rates show an increase over the 20th century, and are correlated with instrumental sea surface temperatures (SST). The interannual variation of the extension-rates within 2.5-4 years is driven by the El Niño-Southern Oscillation (ENSO). The amount of skeletal extension during the summer months is triggered by variations in the strength of the SW monsoon. Interannual and decadal variability in monsoon current activity (18-19 yrs) and rainfall over India are an expression of the summer monsoon strength. This is the reason why a statistical link between coral extension-rates and precipitation over India can be established. This implies that annual extension-rates in corals can be used as a new proxy for Indian monsoon variability on decadal resolution. The delta18O record exhibits the 20th century warming trend that is influenced by the effect of monsoon-induced cooling. delta18O also reveals interannual ENSO triggered variability, which is due to ENSO-forced variations in SST and sea surface salinity (SSS). A decadal variation at 12-14 yrs cannot be linked to SST variations in the NW Indian Ocean, but with decadal variations of SSS. They could be caused by ENSO- forced variations of the monsoon currents during the mature phase of ENSO teleconnections in the Indian Ocean in boreal winter. The Sr/Ca record does not indicate a significant warming, in spite of the observed SST rise at the sampling site. Changes in seawater Sr/Ca cannot be excluded. Nevertheless, interannual ENSO forcing is still evident. Evidence for the Pacific Decadal Oscillation (PDO) is found during 1917-1955. Afterwards, the Sr/Ca data indicate the disappearance of PDO forcing. By the combination of Sr/Ca and delta18O it is possible to detect ~80% of historical El Niño and La Niña events at the sample site. This study confirms the notion that interannual to multi-decadal climate fluctuations in the Pacific play a crucial role for climate variability in the Indian Ocean.