The atmospheric and surface response to solar cycle variability simulated with MA-ECHAM

Stergios Misios1,2 and Hauke Schmidt1

1) Max Planck Institute for Meteorology 2) International Max Planck Research School on Earth System Modelling

The nature of the problem

The 11-year solar cycle variation has a well demonstrated impact on the Northern Hemisphere winter stratosphere. During early winter of maximum solar activity years the stratospheric vortex tends to be stronger and colder in contrast to minimum phase. While there is a relative consensus on the stratospheric response, the pathways through which the solar signal is propagated into the troposphere are still under debate. Of a particular interest is the, putative dynamically driven, maximum in the lower tropical stratosphere. Moreover, the surface response in particular regions, such as the negative anomalies in peak solar activity in the eastern equatorial Pacific, further perplexes our understanding. In this work, we explore the inseparable surface-troposphere-stratosphere response of the 11-yr solar cycle with the aid of fully coupled atmosphere-ocean general circulation model.

We address these questions:

What is the origin of the secondary temperature maximum in the equatorial lower stratosphere?

How can the observed cooling in the equatorial Pacific in solar maxima be explained ?

Experimental setup•Coupled atmosphere-ocean GCM MA-ECHAM5/MPIOM•Resolution: T31 90 vertical layers, 3° degrees 40 levels•QBO: spontaneously generated [Giorgetta et al.,2006]•Observations of solar activity [Lean, 2000]•Ozone solar anomaly from HAMMONIA [Schmidt et al.,2010]•No other external forcing•Years above (below) one standard deviation of mean f10.7 flux are grouped as MAX (MIN)

The winter response in the Pacific

Coupled control experiment NO SOLAR VARIABILITY

Coupled ensemble simulations9 membersRadiation : 1952-2006

Uncoupled ensemble simulations9 membersRadiation : 1952-2006Averaged SST and SIC from control

The increased SW heating warms the upper stratosphere but the response in the winter hemisphere has dynamical origin. Focusing in low latitudes in winter (DJF), Kodera and Kuroda (2002) have proposed that during maximum solar activity reduced upwelling in the lower equatorial stratosphere should be expected due to weakening of the residual circulation by changes in planetary wave zonal mean interaction. This reduction can be translated as warming in the region.

We findWeak stratospheric SW heating and temperature response because of poor spectral resolution of ECHAM's radiation code (see fig. 1,2)The response in the troposphere is very different between coupled and uncoupled simulations with two bands of significant positive responses present only in coupled runs (see fig. 1,2)The signal in zonal winds is opposite to observations (not shown)A pronounced secondary maximum in temperature anomalies. The coupled runs show higher values suggesting that SSTs play an important role (see fig. 3).Two cells of subsequent positive and negative values for the mass streamfunction are simulated with stronger response in coupled ensemble runs (see fig. 3)

Our mechanism

Here we demonstrate that the mechanism is more complicated with a accompanied enhancement of upwelling higher up. At 20N we get the opposite picture (see fig. 3). This result raises the possibility of a solar modulated QBO which may play a role in the secondary maximum formation. This idea is supported by the downward movement apparent in fig. 4.

van Loon et al., 2007 suggested a mechanism to explain the cold tongue in MAX years. Stronger insolation in cloud free subtropical latitudes warms the surface evaporating more moisture which, when is transported to the convergence zones, strengthens the Walker and Hadley cells resulting grater ocean upwelling.

We find,Negative SST anomalies in the equatorial Pacific in peak solar activity. A stronger cooling in MIN-CLIM differences (see fig. 5)Precipitation anomalies reveal different mechanisms (see fig. 6)Reduced probability of positive Nino 3.4 index in MAX years accompanied by increased likehood of weak La Nina events (see fig.7)

Our mechanism

We extend the previously proposed mechanism suggesting that during the solar maximum years the stronger trade winds shift precipitation to the west hindering the triggering of a El Nino event (see fig.7 left). This tendency has a statistical meaning; in any one ensemble run or in observations alike, this mechanism may not be detected.For the MIN-CLIM SSTs response, the PDF comparison (see fig. 7 right) suggests that most likely, it can be attributed to the higher number of very strong La Nina events.

Fig.1 Scaled ensemble mean shortwave heating rate differences. Shading denotes 95% significance. Units in K/day

The origin of the secondary temperature maximum in the lower stratosphere

Fig.2 Scaled DJF ensemble mean zonal mean temperature MAX-MIN differences for coupled (left) and uncoupled (right) runs. Units in K

Fig.4 Annual cycle of zonal mean equatorial vertical profile of solar response in temperature. Units in K

ReferencesSchmidt et al., The solar cycle signal in a general circulation and chemistry model with internally generated QBO. JGR,2010 Giorgetta et al., Climatology and forcing of the quasi-biennial oscillation in the MAECHAM5 model, J. Climate,2006Lean, Evolution of the Sun's spectral irradiance since Maunder Minimum. GRL, 2000 Van Loon et al., Coupled air-sea response to solar forcing in the Pacific region during northern winter, JGR,2007

Kodera and Kuroda, Dynamical response to the solar cycle, JGR,2002

Fig.6 Same as figure 5 but for total precipitation. Units in mm/day

Fig.7Probability density functions of Nino 3.4 index for MAX versus climatology (left) and MIN versus climatology (right).

Fig.5 Composite differences from the 9-member ensemble mean for MAX minus climatology (left) and MIN minus climatology (right). Units in K. Shading denotes 90% significance

Fig.3 Zonal average equatorial response in DJF for coupled (left) and uncoupled (right) runs. Units in K

Fig.3 contour: mass streamfunction differences for coupled (left) nad uncoupled (right). Units in kg/sArrows: residual meridional circulation differences. Units in m/s