A novel scheme for identifying principal modes in geophysical variability with application to global precipitation

摘要

Any assessment of future climate changes requires knowledge of the full range of variability in the El Ni?o?Southern Oscillation (ENSO) phenomenon. The recent availability of 1–2 decades of satellite-based high-resolution measurements allows, in theory, the principal modes of many geophysical quantities to be fully resolved and well recovered at seasonal to decadal timescales. In this paper, a three-dimensional (x, y, T, where T is period) harmonic extraction scheme aimed at revealing spatially/temporally independent variability modes is proposed. This scheme is applied to a 308-month Global Precipitation Climatology Project (GPCP) data set spanning January 1979 through August 2004, allowing 11 principal precipitation modes to be clearly identified. In addition to the well-known annual, semiannual, and seasonal cycles, seven interannual modes (six in the equatorial Pacific and one in the equatorial Indian Ocean) with intrinsic periods ranging from 1.5 to 7.7 years are identified, and the geophysical background of their generation is discussed in the context of a joint influence by the western Pacific warm pool and the eastern Pacific cold tongue. Also, a well-defined decadal mode of 13.9 years is found in the central equatorial Pacific. The spatial/temporal structures of these principal modes are provided in detail. An important characteristic of the identified precipitation modes is that they are separated in space and incoherent in time. Identification of interannual precipitation variability as a single ENSO mode in some previous studies appears to be oversimplified and potentially misleading. Our results serve as a significant contribution to the understanding and prediction of the ENSO-induced precipitation anomaly as well as its related global and regional climate change.