On the spatial scale dependence of long-term persistence in global annual precipitation data and the Hurst Phenomenon

Precipitation deficits are the main physical drivers of droughts across the globe, and their level of persistence can be characterised by the Hurst coefficient H (0.5<H<1), with high H indicating strong long-term persistence (LTP). Previous analyses of point and gridded annual global precipitation datasets have concluded that LTP in precipitation is weak (H∼0.6) which is inconsistent with higher values of H for large river basins e.g. the Nile. Based on an analysis of gridded annual precipitation data for eight selected regions distributed across the globe, an important new finding is that H increases with the spatial scale of averaging, with mean H values at the grid and regional scale of 0.66 and 0.83, respectively. The discovery of enhanced LTP at the regional scale of averaging of precipitation has important implications for characterising the severity of regional droughts, as well as LTP in the annual flows of large rivers and recharge to major aquifers. Teleconnections with known modes of low frequency variability in the global climate system are demonstrated using correlation analysis and stepwise regression. Despite having several constituent regions exhibiting LTP, the Northern Hemisphere surprisingly has no LTP; this is shown to result from different modes of low frequency climatic variability cancelling each other out. LTP for the Southern Hemisphere is moderate, and weak for Global average precipitation. LTP in Blue Nile basin scale precipitation is shown to explain the Hurst Phenomenon in naturalised annual flows for the River Nile, more than seventy years after its discovery by Hurst.