Precipitation fluctuations over global land areas since the late 1800's

Abstract

An analysis of southern hemisphere land precipitation records for the last 100 years indicates an increase in mean annual precipitation since the 1940's, with positive anomalies, compared to the 1921–1960 reference period, occurring during approximately the last 15 years in all seasons except southern summer (December–February). There is little or no temporal correlation with corresponding precipitation indices for the northern hemisphere (Bradley et al., 1987a). Furthermore, while trends in the northern hemisphere temperate regions were opposite those in the northern tropical areas, in the southern hemisphere both zones exhibit similar trends. The change toward higher precipitation in middle latitudes begins about 10 years earlier in the northern than in the southern hemisphere (in the 1940's versus the 1950's, respectively). For the northern hemisphere, the overall linear trend from 1890 to 1986 is not significantly different from zero. However, a decline is evident since the 1950's, primarily due to lower rainfall amounts south of about 30°N. Globally, the trend is toward higher values in annual and seasonal precipitation, except for the boreal summer season (June–August). Most of the observed increases, however, took place from about 1940 to the mid‐1950's, after which time the record has displayed little overall trend, instead exhibiting decade‐long fluctuations. The observations are only broadly consistent with zonally averaged profiles of precipitation changes derived from general circulation model (GCM) simulations of climate using doubled atmospheric CO2 concentrations, although we note that there is considerable variability in precipitation response from one model to another. One possible discrepancy occurs in the northern hemisphere tropics, where most GCMs indicate relatively little change in precipitation in response to CO2 doubling (Gutowski et al., 1988), whereas in the past couple of decades there has been a major decline in rainfall in this region. The overall increase in southern hemisphere precipitation is consistent with marine observations for the tropics and extratropical southern latitudes, which show an increase in sea surface temperature of about 0.3°C since the mid‐1970's and an increase in surface wind speed of between 0.4 and 0.8 m/s since 1950. The potential increase in evaporation could have contributed to the observed rainfall trends. We note, however, that precipitation in tropical land areas is strongly influenced by the phases of the El Niño/Southern Oscillation (ENSO), being generally lower during warm events and higher during the opposite cold phase. The effects of the strong ENSO warm events of 1982–1983 and around 1940 are clearly evident in the global record, as are precipitation peaks during cold years, such as 1974–1975 and 1954–1956.