Air temperature warming along coastal California from 1951-1997
has benefited the premium wine industry, as catalogued in larger yields
and higher quality from Napa/Sonoma valleys. Climatic changes were asymmetric,
with greatest warming at night and during spring. Warming was associated
with large increases in eastern Pacific sea surface temperatures (SSTs)
and higher atmospheric water vapor. although the average temperature warming
trend was modest (1.13 degrees C/47 years), there was a 20 day reduction
in frost occurrence and a 65 day increase in frost free growing season
length. Because regional scale SSTs persist for 6-12 months, predicting
vintage quantity and quality from previous winter conditions appears possible.
This figure above shows the standardized anomalies of coastal California
average air temperatures and Pacific SSTs. SSTs (blue line and circles)
are from a 5 degree by 5 degree grid centered at 35N 125W (0.71 degrees
C over 47 years, p=0.0030); air temperatures (red line and circles) are
north and central coast divisional averages (0.94 degrees C over 47 years,
p<0.001). The increased post 1976 warming trends are part of a well
documented shift in Pacific climate (see C.C. Ebbesmeyer et al., in Proceedings
of the Seventh Annual Pacific Climate (PACLIM) Workshop, J.L. Betancourt,
V.L. Tharp, Eds. Interagency Ecological Studies Program Technical Report
26, California Department of Water Resources, Sacramento, CA, 1990, pages
115-126). The strong correlation (R=0.80) suggests that land and ocean
warming is coupled through a water vapor feedback (see for example J.A.
McGowan, D.R. Cayan and L.M. Dorman in Science volume 281 (1998)
pages 210-217). Moist Pacific air resulting from warmer SSTs, combined
with shifts in sea level pressures leading to increased southwesterly winds,
increases coastal atmospheric humidity and dewpoint temperature. Higher
dewpoint temperatures lead to higher minimum (night-time) temperatures
through the latent heat of condensation. Finally, enhanced water vapor
increases cloud cover, further increasing minimum temperatures through
re-radiation of longwave energy.
Napa/Sonoma climatic changes. (A) The average temperature increased
1.13 degrees C over 47 years (p<0.001, blue line and circles) with nearly
all warming caused by increases in minimum temperature (2.06 degrees C
over 47 years, p<0.001, red line and circles). Consequently, the diurnal
temperature range declined by 1.87 degrees C over 47 years (p<0.001).
Contribution of the minimum temperature to the average temperature increase
is higher than in global trends. (B) Napa/Sonoma frost decreased 71% over
47 years (p<0.001). If the current trend continues, Napa/Sonoma will
become a frost-free climate, indicating a fundamental shift in ecosystem
function.
This figure shows the observed relation between winter frost frequency
and wine quality ratings. Buds damaged by frosts(blue line and circles)
delay subsequent phenological events, leading to uneven maturity and poor
wine quality, (red line and circles). Decoupleing between quality and number
of frosts during the 1980s is associated with an absence of severe frosts
(minimum temperature less than -3 degrees C). In addition to improving
crop quality by reducing frost damage, a low number of frosts generally
forecasts a favorable year for Napa/Sonoma wine production, including warm
springs and low growing season diurnal temperature range.
