ARS | Publication request: INTRA TO MULTIDECADAL TERRESTRIAL RAINFALL REGIMES AT THE END OF THE 20TH CENTURY

Submitted to: International Journal of Climatology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2003
Publication Date: November 20, 2003
Citation: Mauget, S.A. 2003. Intra to multidecadal terrestrial rainfall regimes at the end of the 20th century. International Journal of Climatology.

Interpretive Summary: Atmospheric greenhouse gases (GHG) such as CO2 prevent the Earth from cooling by absorbing part of the Earth's outgoing long-wave radiation and directing it back to the Earth's surface. There are 3 reasons why this additional radiative energy might be most directly expressed in increased rainfall rates: 1) oceans cover ~ 71% of the Earth's area, 2) oceans are better absorbers of long-wave radiation compared to land surface types, and, 3) that energy is returned to the atmosphere mainly in the form of increased water vapor. To seek evidence of GHG-related climate change, a new approach to time series analysis was used here to evaluate annual rainfall rates over a number of continental regions during 1901-1998. When this method was used to test time series of annual rainfall averaged over North America during that time, it found that 8 of the 10 wettest years occurred during 1972-1998. Similarly, over areas of northern Europe and the former Soviet Union it found that 7 of the 10 wettest years occurred during 1978-1998. Wet and dry periods were also found over other continental areas in the final decades of the 20th century, some of which are interestingly consistent with computer-model derived predictions of the effects of greenhouse gases on global rainfall patterns. However, the late century North American and northern European wet periods stood out in terms of being very inconsistent with 'business-as-usual' climate variability. The origins of these wet regimes, and the possible implications regarding GHG-related climate change, are discussed.

Technical Abstract: Intra- to multi-decadal (IMD) variation in annual precipitation over terrestrial regions during 1901-1998 was evaluated here through the calculation of Mann-Whitney U statistics over running time windows of 6-30 years duration. This time series analysis approach was used as the foundation of a survey method intended to detect IMD precipitation regimes over continental areas, and also to evaluate IMD variation in time series of annual precipitation spatially averaged over specified regions. As in previous work that focused on the continental United States, an unprecedented incidence of high ranked annual rainfall conditions over North America was also found here during 1972-1998. Over northern Europe a comparably significant incidence of wet years was indicated during 1978-1998. Wet and dry regimes were also found over other continental areas in the final decades of the 20th century, some of which are interestingly consistent with the GCM-derived projections of the effects of greenhouse gases (GHG) on global rainfall patterns. However, the late century North American and northern European wet periods stood out in terms of being highly inconsistent with stationary climate variability. The origins of these wet regimes, and the possible implications regarding GHG-related climate change, are discussed.