Solar and volcanic forcing of North Atlantic climate inferred from a process-based reconstruction
Authors: Sjolte Jesper , Sturm Christophe , Adolphi Florian , Vinther Bo M. , Werner Martin , Lohmann Gerrit , Muscheler Raimund 
Published: August 2018

Abstract. The effect of external forcings on atmospheric circulation is debated. Due to
the short observational period, the analysis of the role of external forcings
is hampered, making it difficult to assess the sensitivity of atmospheric
circulation to external forcings, as well as persistence of the effects. In
observations, the average response to tropical volcanic eruptions is a
positive North Atlantic Oscillation (NAO) during the following winter.
However, past major tropical eruptions exceeding the magnitude of eruptions
during the instrumental era could have had more lasting effects. Decadal NAO
variability has been suggested to follow the 11-year solar cycle, and
linkages have been made between grand solar minima and negative NAO. However,
the solar link to NAO found by modeling studies is not unequivocally
supported by reconstructions, and is not consistently present in observations
for the 20th century. Here we present a reconstruction of atmospheric winter
circulation for the North Atlantic region covering the period 1241–1970 CE.
Based on seasonally resolved Greenland ice core records and a 1200-year-long
simulation with an isotope-enabled climate model, we reconstruct sea level
pressure and temperature by matching the spatiotemporal variability in the
modeled isotopic composition to that of the ice cores. This method allows us
to capture the primary (NAO) and secondary mode (Eastern Atlantic Pattern) of
atmospheric circulation in the North Atlantic region, while, contrary to
previous reconstructions, preserving the amplitude of observed year-to-year
atmospheric variability. Our results show five winters of positive NAO on
average following major tropical volcanic eruptions, which is more persistent
than previously suggested. In response to decadal minima of solar activity we
find a high-pressure anomaly over northern Europe, while a reinforced
opposite response in pressure emerges with a 5-year time lag. On centennial
timescales we observe a similar response of circulation as for the 5-year
time-lagged response, with a high-pressure anomaly across North America and
south of Greenland. This response to solar forcing is correlated to the
second mode of atmospheric circulation, the Eastern Atlantic Pattern. The
response could be due to an increase in blocking frequency, possibly linked
to a weakening of the subpolar gyre. The long-term anomalies of temperature
during solar minima shows cooling across Greenland, Iceland and western
Europe, resembling the cooling pattern during the Little Ice Age
(1450–1850 CE). While our results show significant correlation between
solar forcing and the secondary circulation pattern on decadal (r=0.29,
p<0.01) and centennial timescales (r=0.6, p<0.01), we find no
consistent relationship between solar forcing and NAO. We conclude that solar
and volcanic forcing impacts different modes of our reconstructed atmospheric
circulation, which can aid in separating the regional effects of forcings and
understanding the underlying mechanisms.