The influence of solar variability on North Atlantic climate David Jackson*, Jeff Knight*, Adam Scaife*, Sarah Ineson*, Nick Dunstone*, Lesley Gray †, Mike Lockwood #, and Amanda Maycock ‡ *Met Office Hadley Centre, † University of Oxford, # University of Reading, ‡ University of Cambridge 11 th European Space Weather Week Liege, Belgium, November 2014

Contents The North Atlantic Oscillation and European winter climate variability Observed and simulated solar links Upper stratospheric mechanism Solar UV perturbation experiments Lags between the solar cycle and the NAO response Impacts of a possible “grand solar minimum”

Winter North Atlantic Oscillation

Northern Europe in Winter depends largely on which way the wind blows: Weakened Pressure Gradient Cold advection into Europe Cold, calm and dry Strengthened Pressure Gradient Warm Europe Mild, stormy and wet Winter 1999/00 Winter 2009/10 Winter 1962/63

North Atlantic Oscillation NAO+: more active storm track, more westerly advection of air NAO-: less active, southward shifted, less advection from the Atlantic Most important mode of year-to-year variability in North Atlantic-European winter climate Linked to storminess:

Regional climate effects of the solar cycle in observations and models

Observed solar variability and NAO related climate 2m temperature 11 year solar cycle –ve NAO and more blocking at solar minimum Woollings et al., 2010, GRL

Observed solar variability Kuroda and Kodera, 2002, JMSJ Solar maximum minus solar minimum from the 11 year cycle Descending wind anomalies, Winter only, strongest in NH N. Hemisphere winter S. Hemisphere winter

Lesley Gray

Models: some experiments have shown encouraging signs: (eg Matthes et al, JGR, 2006) But generally, models produce mixed results Results with more comprehensive upper atmosphere physics No sign of westerly anomaly at solar maximum (Tsutsui et al., JGR, 1999)

UV perturbation experiments

Experiments Hadley Centre ocean-atmosphere climate model. 85 levels – well resolved middle atmosphere - upper boundary at 85km Solar minimum (80 yrs): control run Solar maximum (20 x 4 yrs): perturbation of +1.2Wm -2 to nm UV band. Only the UV is altered. Based on apparent UV change in SIM satellite data (Harder et al. 2009) Climatological ozone mesosphere stratosphere troposphere Generally previous model results are mixed Ineson et al., Nat. Geosci., SIM measured a decline in ultraviolet from that is a factor of 4 to 6 times larger than typical previous estimates

Cooling of the equatorial stratopause at solar minimum Annual zonal mean temperature Weaker meridional temperature gradient Weakened westerly flow Solar min – Solar max

Solar Variability Effects – mainly winter Similar to wave-mean flow interactions seen in other contexts Ineson et al., Nat. Geosci., Oct Nov Dec Jan Feb Mar Solar min – Solar max

Mechanism: descent through the stratosphere increase in planetary wave driving F  deceleration just below easterly wind anomaly  descent of the anomaly zonal mean zonal wind (contours) and EP flux divergence (cols) After Andrews and McIntyre 1978 Solar min – Solar max

Winter surface climate response (solar min – solar max) Sea level pressure Surface temperature

Lags in the solar response

An associated signature in the ocean Perhaps this is the source of the memory… Grey et al., JGR, Observed relationship in NAO mslp, SST LAGS solar cycle by ~2-3y

We performed transient experiments using the Hadley Centre atmosphere- ocean climate model (HadGEM3) Two 12-member transient ensembles: Constant solar forcing UV-only ( nm) solar variability with amplitude based on SIM satellite observations (1.2 Wm -2 peak to trough) Each uses other known forcings: GHGs, aerosols, ozone, volcanoes By looking at the difference in responses in the two ensembles, the solar UV influence can be isolated. MSLP (UV min-max - Const min-max )

Sub-Surface North Atlantic response Evidence of re-emergence of signals below the summer mixed layer Oceanic memory for NAO lag

Climate response to a ‘Grand Solar Minimum’

Experiments Control: Hadley Centre IPCC RCP8.5 simulation (HadGEM2-CC) to 2100 Scenarios based on most rapidly declining Lockwood (2010) total solar irradiance (TSI) scenario. Decline to approximately 2050 then level EXPT-A: relative proportion of changes in each spectral band same as in CMIP5 control simulation EXPT-B: all the change is put into the UV band ( nm) Overall, A and B have a very similar TSI decline of about 1.75Wm -2 Effect on global mean warming small. As is effect on winter European surface T

Climate effect of a long-term decline in solar activity Ineson et al., 2015, submitted The climatic effect of a long-term decline in solar forcing appears to be very similar to that for the minimum of the solar cycle Solar scenario minus RCP8.5, average MSLPSurface Temperature

Solar influence compared to climate change in IPCC scenarios Ineson et al., 2015, submitted In specific regions such as Southern Europe, future solar forcing could be nearly as important as which emissions scenario is followed (here for winter rainfall). } Grand solar min scenarios average

Using a larger estimate of UV variability than has previously been used, it is possible to reproduce the amplitude of the observed NAO response to the solar cycle in the Hadley Centre climate model (HadGEM3) The model shows a ‘top down’ mechanism linking anomalies in the tropical upper stratosphere and the NAO via interaction between planetary waves and zonal wind anomalies Other effects, such as ozone changes and heating in the lower stratosphere remain as candidates to modify or enhance these signals Using ensembles of simulations of the period it is possible to reproduce the observed lag of 3-4 years in the NAO response The solar cycle response is also seen following an early 21 st century decline in solar activity towards a ‘grand solar minimum’ Regionally, the scale of the solar impact is a large fraction of the difference between emissions scenarios. This argues for scenarios of natural forcing alongside anthropogenic forcing Summary

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