NE Pacific coastal warming due to changes in atmospheric circulation
A. Jan 1900 - Mar 2015
Atmospheric sea-level pressure (SLP) anomalies predict coastal SST variations and changes from Jan 1900 to the present. Most warming occurred from 1920 to 1940 due to sustained regional SLP reductions. Similar forcing produces interannual and decadal SST warming events. Update: The March 2015 SST anomaly is the highest in the entire record (+1.51 ºC). Historically anomalous low pressure and cyclonic winds have prevailed over NE Pacific during the past 24 months.
B. Jan 1980 - Mar 2015. NE Pacific monthly coastal SST anomalies (red) and SST modeled from regional SLP. Recent warming from January 2013 to March 2015 occurred in response to low SLP over the NE Pacific, consistent with long-term forcing. Gray shading marks data beginning in January 2013 that were not included in the study. Negative SLP anomalies generate anomalous cyclonic winds, reducing the mean anticyclonic flow and thus winds speeds over the Arc. The drop in wind speeds reduces evaporation rates, producing positive surface latent heat fluxes and SST increases.
In a study co-authored with Nate Mantua, I identified a strong long-term connection between NE Pacific atmospheric sea-level pressure (SLP) and coastal sea surface temperatures (SSTs) within a coastal 'Arc' region stretching around North America. From this strong and consistent relationship, we concluded that changes in SLP and ocean winds can account for virtually all sea surface warming in the Arc from 1900 to 2012 and nearly all warming in the coastal western United States as well.
This conclusion presents a new perspective on causes of NE Pacific and western US warming, and indicates that well-known dynamical mechanisms driving 'multidecadal variability' can also generate century-long regional warming as well. The findings raised some controversy, leading to front-page coverage in the Seattle Times and features by the New York Times, environmentalresearchweb, and numerous other media outlets.
In order to address some initial scientific questions, we released a summary and FAQ via the the NOAA SW Fisheries Science Center
NE Pacific SLP and SST
NE Pacific SLP (area within box) SSTARC (area enclosed within bold contour)
Johnstone, J. A. and Mantua, N. J. 2014. Atmospheric controls on northeast Pacific temperature variability and change, 1900-2012. Proceedings of the National Academy of Sciences.111, 14360-14365.
Each summer the northern California coastline is blanketed by low stratus cloud and fog. In a 2010 study co-authored with Todd Dawson at Berkeley, I used hourly airport records to illustrate the recent fog history and mechanisms of fog variability. Updated data from Arcata and Monterey shows the recent 2014 fog season in historical context back to 1951 (below). The 2014 June-September summer fog frequency for Northern California (Arcata and Monterey combined) was 7% above normal. Fog frequency was slightly above average despite the warm surface conditions throughout much of the coastal NE Pacific.
Stable isotopes in coast redwood tree rings reveal legible spring-summer climate signals
Johnstone, J. A., Roden, J. S. and Dawson, T. E. 2013. Oxygen and carbon stable isotopes in coast redwood tree rings respond to spring and summer climate signals. Journal of Geophysical Research: Biogeosciences 118, 1438-1450. pdf
Northern California fog
Johnstone, J. A. and Dawson, T. E. 2010. Climatic context and ecological implications of summer fog decline in the coast redwood region. Proceedings of the National Academy of Sciences 107, 4533-4538.
Western US precipitation
Johnstone, J. A. 2010. A quasi-biennial signal in western US hydroclimate and its global teleconnections. Climate Dynamics 36, 663-680. pdf