The identification of distinct patterns in California temperature trends

Regional changes in California surface temperatures over the last 80 years
are analyzed using station data from the US Historical Climate Network and the
National Weather Service Cooperative Network. Statistical analyses using annual
and seasonal temperature data over the last 80 years show distinctly different spatial
and temporal patterns in trends of maximum temperature (Tmax) compared to
trends of minimum temperature (Tmin). For trends computed between 1918 and
2006, the rate of warming in Tmin is greater than that of Tmax. Trends computed
since 1970 show an amplified warming rate compared to trends computed from 1918,
and the rate of warming is comparable between Tmin and Tmax. This is especially
true in the southern deserts, where warming trends during spring (March–May) are
exceptionally large. While observations show coherent statewide positive trends in
Tmin, trends in Tmax vary on finer spatial and temporal scales. Accompanying the
observed statewide warming from 1970 to 2006, regional cooling trends in Tmax
are observed during winter and summer. These signatures of regional temperature
change suggest that a collection of different forcing mechanisms or feedback
processes must be present to produce these responses.