Surface observed global land precipitation variations
during 1900-1988

Dai, A., I.Y. Fung, and A.D. Del Genio

J.  Climate, 10,  2943-2962.  1997.


The authors have analyzed global station data and created a gridded dataset of
monthly precipitation for the 1900-1988 period. Statistical analyses suggest that discontinuities associated with instrumental errors are large for many high-latitude
station records although they are unlikely to be significant for the majority of the
stations. The first leading EOF in global precipitation fields is an ENSO-related
pattern concentrating mostly in the low latitudes. The second leading EOF depicts
a linear increasing trend (~2.4 mm/decade) in global precipitation fields during the
1900-1988 period. Consistent with the zonal precipitation trends identified in
previous analyses, the EOF trend is seen as a long-term increase mostly in North
America, mid- to high-latitude Eurasia, Argentina and Australia. The spatial patterns
of the trend EOF and the rate of increase are generally consistent with those of the precipitation changes in increasing CO2 GCM experiments.

The North Atlantic Oscillation accounts for ~10% of December-February precipitation variance over North Atlantic surrounding regions. The mode suggests that during high-NAO-index winters, precipitation is above normal in northern (>50°N) Europe,
the eastern United States, northern Africa and the Mediterranean; while below-normal
precipitation occurs in southern Europe, eastern Canada and western Greenland.

Wet and dry months of one standard deviation occur at probabilities close to those
of a normal distribution in midlatitudes. In the subtropics, the mean interval between
two extreme events is longer. The monthly wet and dry events seldom (probability <5%)
last longer than 2 months. ENSO is the single largest cause for global extreme
precipitation events. Consistent with the upward trend in global precipitation, globally,
the averaged mean interval between two dry months increased by ~28% from
1900-1944 to 1945-1988. The percentage of wet areas over the U.S. has more than
doubled (from ~12% to >24%) since the 1970s while the percentage of dry areas has
decreased by a similar amount since the 1940s. Severe droughts and floods comparable
to the midwest U.S. 1988 drought and 1993 flood have occurred 2-9 times in each of
several other regions of the world during this century.