Summer fog variability in the coast redwood region: climatic relevance and ecological implications

James A. Johnstone

Department of Environmental Science, Policy, & ManagementUniversity 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?

???

Arcata 1951-2008

Monterey 1951-2008

Primary data:

Hourly airport readings of cloud ceiling height

June-September summers

Near-continuous records (1951-2008) in the Redwood region

Fog = cloud ceiling at/below 400 m

Sonoma 2003

Hourly cloud heightSonoma County Airport

June 2003

Hourly cloud heightSonoma County Airport

Coast Redwood Forest 300m elev. Sonoma County

June 2003

Hourly cloud heightSonoma County Airport

Coast Redwood Forest 300m elev. Sonoma County

June 2003

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:

400 m

Relative Humidity Temperature

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

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)

Hours of Fog per day

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 Fog-PDO 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.

Ukiah(interior)

Berkeley(marine)

Change in TMAX contrastexemplified by Ukiah-minus-Berkeley difference.

1901-1925: 9.6°C difference

1951-2008: 6.3°C difference

50% reduction

TMAX Contrast, Northern California SST 1900-2008r = -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.)