A team of researchers looked at the solar influence on Southern Hemisphere Westerly Winds (SWW). These winds influence rainfall patterns and ocean currents in the Southern Hemisphere. Varma et al infer rainfall patterns by looking at iron deposits in marine sediments near Chile, which are apparently higher during drier conditions and lower during wetter times. They compared these to both Beryllium (10Be) and Carbon-14 (14C) which they use to estimate solar activity.
The end result is they find that the westerly winds shift northwards towards the equator during lower solar activity, and conversely move southwards towards the poles during higher solar activity. The shifting wind patterns move the rainfall. An effect is apparent in records for the last 3,000 years.
In graph a below, 10Be (solar activity) and Fe (rainfall) have a decent correlation coefficient (r) of 0.45, while the 14C (solar activity) and Fe (rainfall) correlation in b has a lower correlation (r) of 0.21. Varma et al say:
“the large correlation coefficient for 10Be would suggest that ca. 20% (i.e., r2) of late Holocene rainfall and hence SWW variability could be attributable to solar forcing.”
They conclude that the current models don’t give the sun a large enough role.
“…we propose that the role of the sun in modifying Southern Hemisphere tropospheric circulation patterns has probably been underestimated in model simulations of past climate change.
Note that all this has the caveat that correlation is not causation. We don’t understand the mechanisms involved. Then there is that slightly awkward point that correlation does not hold up for records older than 3000 years (it is “close to zero”), and Varma et al wonder whether the dates are inaccurate for the older records which could explain the lack of correlation. Hmm. Three thousand years is a long time.
Speculation about possible mechanisms
Varma et al talk of mechanisms that amplify the solar effect through both “top down” and bottom up” processes, and think that both types of mechanisms are needed to generate these significant shifts in response to very small changes in solar TSI.
The top down mechanism:
“…a “top-down” mechanism, which influences the troposphere via stratospheric ozone responses to variations in ultraviolet radiation, has been proposed by Haigh (1996). In her model, increase in ultraviolet radiation and resulting rising ozone concentration, induced heating in the lower stratosphere during the Southern Hemisphere summer. As a consequence, strengthened stratosphere easterly winds caused the tropospheric subtropical westerly jets to move poleward, the tropical Hadley cell to broaden, and the SWW to move southward.
Valma point out that 70% of the shortwave incoming solar energy (that is not reflected back to space) ends up at the surface, and they looked at models of the “bottom up” possibilities:
As expected, a general surface cooling is induced by the TSI reduction (Fig. 7a). In addition to this general cooling, a more pronounced reduction in surface temperature is observed in the mid latitudes of, e.g., the central South Pacific sector (Fig. 7a, c, and d). By means of general atmospheric circulation modelling and scaling arguments, it has recently been shown that a reduction of the mean global surface temperature decreases the width of the Hadley cell (Frierson et al., 2007) and shifts the eddy-driven surface westerlies that result from the balance between vertically integrated eddy momentum convergence and surface drag towards the equator (Lu et al., 2010). Since the reduction in TSI is only 0.15%, the global cooling effect is small and additional feedbacks are required to induce a significant change in the westerlies.
One important feedback is associated with enhanced Ekman divergence and resulting upwelling (vertical velocity) around 40 S in austral summer (Fig. 7b). The upwelled cold water is transported northward in the Ekman surface layer, leading to enhanced SST cooling just north of 40 S (Fig. 7c). This cooling results in a meridional SST gradient anomaly that lies very close to (or just equatorward of) the subtropical jet. It has been shown that such an anomalous subtropical surface temperature gradient causes a strengthening of the jet along with an equatorward shift of the eddy-driven surface westerlies (Brayshaw et al., 2008; Lu et al., 2010), thus providing a positive feedback on the initial SWW shift in our simulations.
In austral winter, the coldest surface temperature anomalies are found around Antarctica (Fig. 7d), mainly due to increased sea-ice concentrations (up to 7%) reducing ocean-atmosphere heat fluxes and increasing the surface albedo. The resulting stronger meridional surface temperature gradient at high latitudes, however, is accompanied by a poleward shift of the surface westerlies (Chen et al., 2010; Lu et al., 2010), due to enhanced high-latitude baroclinic wave generation. This results in a winter SWW shift that is opposite to the other seasons.
The Southern Hemisphere Westerly Winds (SWW) constitute an important zonal circulation that influences large-scale precipitation patterns and ocean circulation. Variations in their intensity and latitudinal position have been suggested to exert a strong influence on the CO2 budget in the Southern Ocean, thus making them a potential factor affecting the global climate. In the present study, the possible influence of solar forcing on SWW variability during the Holocene is addressed. It is shown that a high-resolution iron record from the Chilean continental slope (41° S), which is interpreted to reflect changes in the position of the SWW, is significantly correlated with reconstructed solar activity during the past 3000 years. In addition, solar sensitivity experiments with a comprehensive global climate model (CCSM3) were carried out to study the response of SWW to solar variability. Taken together, the proxy and model results suggest that centennial-scale periods of lower (higher) solar activity caused equatorward (southward) shifts of the annual mean SWW.
Varma, V., Prange, M., Lamy, F., Merkel, U., and Schulz, M.: Solar-forced shifts of the Southern Hemisphere Westerlies during the Holocene, Clim. Past, 7, 339-347, doi:10.5194/cp-7-339-2011, 2011. [abstract] [PDF]