Monthly Weather Review, submitted July 2013.
Hongyan Zhu, Matthew C. Wheeler
Centre for Australian Weather and Climate Research, Bureau of Meteorology,
Melbourne, Australia.
Adam H. Sobel
Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory
Columbia University, New York, NY.
Deborah Hudson
Centre for Australian Weather and Climate Research, Bureau of Meteorology,
Melbourne, Australia.
Abstract
The skill with which a coupled ocean-atmosphere model is able to predict precipitation over a range of time scales (days to months) is analysed. For a fair comparison across a seamless range of time scales, the verification is performed using data averaged over time windows equal in length to the forecast lead time. At a lead time of one day, skill is greatest in the extratropics around 40-60 degrees latitude, lowest around 20 degrees, and has a secondary local maximum close to the equator. The extratropical skill at this short range is highest in the winter hemisphere presumably due to the higher predictability of winter baroclinic systems. The local equatorial maximum comes mostly from the Pacific, and thus appears to be from the El Nino-Southern Oscillation. As both lead time and averaging window are simultaneously increased, the extratropical skill drops rapidly with lead time, while the equatorial maximum remains approximately constant causing the equatorial skill to exceed the extratropical at leads greater than or equal to 4 days in austral summer and greater than or equal to 7 days in boreal summer. At longer lead times, the extratropical skill eventually flattens out or increases, but does not approach the equatorial values. Comparisons with persistence confirm that the model beats persistence for most leads and latitudes, including for the equatorial Pacific where persistence is high. The results are consistent with the view that extratropical predictability is mostly derived from synoptic-scale atmospheric dynamics, while tropical predictability is primarily derived from the response of moist convection to slowly-varying forcing such as from sea surface temperature.