Observed and simulated Indian Ocean Dipole activity since the mid-19th century and its relation to East African short rains

摘要

The coupled atmosphere-ocean phenomenon in the Indian Ocean, known as the Indian Ocean Dipole (IOD) mode, shows various influences on regional tropical climates. East Africa (EA) experiences enhanced precipitation during October-December (the annual "short rains"), and therefore it was hypothesized that a shift from wetter to drier conditions in EA after 1880 documented by proxy data (e.g., lake levels, glacier size) was favoured by a decline in positive IOD frequency. However, this proposed century-scale pattern of the IOD emerged from single data sets, while the ensemble perspective from the Coupled Model Intercomparison Project phase 5 (CMIP5) global climate model (GCM) archive is still missing. Based on rigorous test criteria that capture crucial IOD and EA climate characteristics, the present study identifies 15 runs from the CMIP5 data for the ensemble analyses. A key result of the test is that the ensemble is dominated by the GISS E2 H model (13 realizations) with a little influence from HadCM3 (2 realizations), and therefore most CMIP5 models seem unable to capture the long-term variability of EA precipitation. The selected model realizations show a significant increase in the frequency of positive IOD events in the late 20th century, in agreement with previous research and most likely due to external climate forcing. Further frequency changes are absent in the available time series, yet we find that the precipitation excess during positive IODs was stronger in the 19th century (1860-1909) than in the recent decades (1956-2005). Both thermodynamic factors on the regional scale (convection, evaporation) and dynamical processes on the large scale (strength of the zonal tropical circulation) emerge as physical basis of this changing IOD property. The higher precipitation intensity in EA during positive IOD events in the 19th century period provides a consistent mechanism that could have contributed to the moister regional climate of that time, irrespective of I