Asymmetric climatic warming improves California vintages

Ramakrishna R. Nemani1, Michael A. White1, Daniel R. Cayan2, Gregory V. Jones3, Steven W. Running1 and Joseph C. Coughlan4

1 Numerical Terradynamic Simulation Group, University of Montana, Missoula, MT  59812 

2 Climate Research Division, Scripps Institution of Oceanography and Water Resources Division, United States Geological Survey, La Jolla, CA  92093 

3 Department of Geography, Southern Oregon University, Ashland, OR  97520 

4 Ecosystem Science and Technology Branch, NASA/Ames Research Center, Moffett Field, CA  94035

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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.
Figure 1

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.
 Figure 3

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.
 Figure 6

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.