Variations in Observed Global and Regional Precipitation during the Past Three Decades: Global Warming, PDV and More

Global data sets of surface temperature, atmospheric water vapor and precipitation for the last three decades are utilized to examine variations in precipitation related to inter-annual forcing (ENSO, volcanoes), inter-decadal forcing (Pacific Decadal Variation [PDV]) and the global warming trend. Relations among globally-integrated surface temperature, water vapor and precipitation for these various time scales are calculated with similarities and differences noted. While water vapor increases as expected with Clausius-Clapeyron (~7%/C) with changes at all scales, precipitation varies 2-4%/C with ENSO and volcanoes and near 0%/C for the 30-year trend using GPCP precipitation analyses.

While the observations indicate a small or very small change in global mean precipitation, there are distinct regional changes or trends in precipitation during the period. It also is noted that a “climate shift” has occurred about 1998 that has been related to changes in the Pacific Ocean on an inter-decadal scale, the PDV. A very large question with both scientific and practical impacts is “How does the pattern of precipitation change noted in the observations relate to global warming and/or PDV-related forcings over the relatively short period of global precipitation record.” This study tries to answer this question using the global data sets and techniques to separate out the global warming and PDV signals. EOF analyses of longer-record (1949-2010) SST anomalies within the Pacific basin (60oN-60oS) and over the global oceans confirm the existence of a strong, PDV-related climate regime shift around 1998/1999 discovered in past studies. Therefore, the observed linear changes/trends in both precipitation and tropospheric water vapor during 1988-2010 result from a combined impact of global mean surface warming and the PDV shift. In particular, in the tropical central-eastern Pacific, a band of increases along the equator in both precipitation and water vapor sandwiched by strong decreases south and north of it are likely caused by the opposite effects from global-mean surface warming and PDV-related, La Niña-like cooling in the tropical central-eastern Pacific. This narrow band of precipitation increase could also be evidence for the effect of global mean surface warming. The core of these results has been reported by Gu and Adler (2012) in Climate Dynamics.

Although further analysis and testing are required, one result of the study is an observation-based estimate of the pattern of precipitation trends that might be expected due to solely global warming.