Summer fog variability in the coast redwood region:
climatic relevance and ecological implications
James A. Johnstone
Department of Environmental Science, Policy, & Management
University of California, Berkeley
jajstone@berkeley.edu
Coast Redwood
Sequoia sempervirens (D. Don)
Modern distribution suggests a
dependence on marine
conditions / coastal fog
Research Questions:
How and why does summer fog in the redwood region vary on
annual to multi-decadal time scales?
How is its variability connected to marine conditions and regional
climate patterns (e.g. ENSO, PDO)?
How have these conditions changed over the past century?
Impacts on redwood function / ecology?
???
Primary data:
Hourly airport readings of cloud ceiling height
Arcata
1951-2008
June-September summers
Near-continuous records (1951-2008) in the Redwood region
Fog = cloud ceiling at/below 400 m
Sonoma
2003
Monterey
1951-2008
June 2003
Hourly cloud height
Sonoma County Airport
June 2003
Hourly cloud height
Sonoma County Airport
Coast Redwood Forest
300m elev.
Sonoma County
June 2003
Hourly cloud height
Sonoma County Airport
Coast Redwood Forest
300m elev.
Sonoma County
Fog raises humidity,
controls water loss by
Redwoods.
Summer Climate of the NE Pacific
Key elements:
North Pacific High
Alongshore winds
Coastal upwelling
Coastal stratus/fog
Coastal temperature inversion,
vertical humidity contrast
Mean summer profiles at Oakland:
Relative Humidity
Temperature
400 m
Summer Daily Maximum Temperatures:
Northern California
Inland-coast TMAX contrast
Data: PRISM (Oregon State U.)
Daly et al. 2004
Summer Daily Maximum Temperatures:
Northern California
Capping inversion restricts
marine layer to coastal
elevations below ~ 400m
Summer mean fog frequency
Redwood distribution ~ 30% coastal fog threshold
Diurnal cycle of ceiling height
clear sky
Interannual fog variability 1951-2008
Varies by a factor of 2.3
Hours of Fog per day
Extreme years:
1951 (14.8 hrs)
13 fog-free days (daytime hrs)
1997 (6.4 hrs)
62 fog-free days
Hours of Fog per day
Extreme years:
1951 (14.8 hrs)
13 fog-free days (daytime hrs)
1997 (6.4 hrs)
62 fog-free days
2009 near-average
(~10 hrs)
Palo Alto Airport
Fog enhanced with NE-ward
expansion of N Pacific High
Fog correlations with sea-level pressure (SLP)
r = 0.58
Fog vs. Oregon coast meridional wind speed
(r = -0.69)
Fog correlations with sea surface temperature (SST)
PDO-type SST
signature
Summer FogPDO correlation
r = -0.52
Fog correlations with summer land TMAX
Coherent pattern over the
entire U.S. west coast
Fog correlations with upper-air temperatures at Oakland
500 m
Fog vs. Oakland Inversion Strength
(r = 0.67)
T1000-2000 – T0-400
Inversion strength correlations with TMAX
Vertical and longitudinal temperature variations related:
Weak inversion permits marine air and cloud to rise and
penetrate to interior
Subsidence tends to warm the interior and
intensify the coastal inversion
Strong coupling among:
Inland-coast TMAX contrast
Inversion strength
Fog frequency
Cross-correlations: 0.65-0.85
TMAX Inland-Coast Contrast 1901-2008
Fog correlation: r = 0.84
Suggests ~33% reduction in inversion strength and fog frequency.
Change in TMAX contrast
exemplified by
Ukiah-minus-Berkeley
difference.
Ukiah
(interior)
Berkeley
(marine)
1901-1925: 9.6°C difference
1951-2008: 6.3°C difference
50% reduction
TMAX Contrast, Northern California SST 1900-2008
r = -0.73
Conclusions:
20th century weakening of TMAX contrast implies:
Greater inland penetration of marine air in summer
Weakened inversion
Reduced fog frequency
Important terrestrial climate changes linked to ocean-atmosphere
variability along the U.S. West coast and the broader N. Pacific.
Greater evaporative demands may have important implications for
coastal terrestial species and ecosystems, including redwood
(heightened drought sensitivity?)
Further work:
Stable isotope tree-ring calibration:
4 Redwood sites:
~50-yr analysis of
δ18O, ∆13C (discrimination)
∆13C: Expected positive relationship with humidity / fog
Positively correlated with:
Summer fog frequency
∆13C: Expected positive relationship with humidity / fog
Negatively correlated with:
Coastal SST
PDO
δ18O correlated with sprintime pressure
over the California coast
and much of the globe
(Multiple local factors appear to be involved)
Research funded in part by:
The Save-the-Redwoods League
The Berkeley Atmospheric Sciences Center
The National Science Foundation
Thanks to collaborators:
Todd Dawson (Berkeley)
John Roden (Southern Oregon Univ.)