Abstracts Prepared by Workshop Speakers - The Effects of Solar Variability on Earth's Climate - page 43

variation of the tropical upwelling rate, which would in turn advectively modulate ozone concentrations
in the lower stratosphere, consistent with observations. Second, it is possible that there is a significant
troposphere-ocean response to solar variability that is driven mainly by direct changes in total solar
irradiance (TSI). This “bottom-up” mechanism would then reduce planetary wave amplitudes in the
troposphere near solar maxima, which would also modulate the BDC, the tropical upwelling rate, and
ozone concentrations in the lower stratosphere, as is observed. These mechanisms are not mutually
exclusive and both may be important. For example, top-down forcing from the upper stratospheric
response may, in principle, produce significant indirect effects on surface climate, which would then have
dynamical feedbacks on the stratosphere. However, if it is found that top-down UV forcing mainly
produces the lower stratospheric response, then support would be obtained for the view that top-down
solar UV forcing is the primary driver of solar-induced tropospheric climate change. If, on the other
hand, the observed lower stratospheric response is primarily a consequence of bottom-up dynamical
feedbacks from a troposphere-ocean response that is driven mainly by changes in TSI, then it would
follow that direct TSI forcing of near-surface climate is the main driver of solar-induced climate change.
Current work focuses mainly on investigation of the bottom-up mechanism for producing the
lower stratospheric response to 11-year solar forcing at low latitudes. Specifically, we are investigating
whether a statistically significant solar cycle response of the troposphere-ocean system exists that has
characteristics consistent with producing the observed lower stratospheric response through a
modification of planetary wave amplitudes. To characterize the troposphere-ocean response, a multiple
linear regression statistical model is applied to Hadley Centre sea level pressure (SLP) and sea surface
temperature (SST) data, which are available back to ~ 1870. In agreement with previous authors, the most
statistically significant positive response is obtained for SLP in the North Pacific during northern winter,
consisting of a weakening and westward shift of the Aleutian low near solar maxima relative to solar
minima. This response is similar to that which occurs during the cold (La Niña) phase of the El Niño-
Southern Oscillation. To test whether the response is indeed solar (rather than a consequence of aliasing
from a few strong ENSO events), the analysis is repeated for two separate time periods (1880-1945 and
1946-2009). It is found that the North Pacific SLP response to 11-year solar forcing is approximately
repeatable during the two time periods, supporting the reality of the solar response. An associated
response of North Pacific wintertime SST is also obtained but is less repeatable for separate time periods.
In addition, a marginally significant SLP decrease over eastern Europe is obtained near solar maxima
relative to solar minima.
The “La Niña-like” character of the North Pacific SLP response is in agreement with previous
analyses using compositing methods 13 and with some climate model studies. 14 It also agrees with some
paleoclimate studies, which have found evidence for La Niña-like conditions in the Pacific region during
periods of prolonged solar activity increases, such as the “medieval climate anomaly.” 15 Both the positive
North Pacific SLP response and the negative eastern European SLP response under solar maximum
conditions correspond to regions of known tropospheric precursors of anomalous stratospheric circulation
changes. 16 Increases in North Pacific SLP tend to weaken and shift westward the Aleutian low, while
decreases in eastern European SLP tend to weaken and shift eastward the Siberian high. To first order,
this weakens the wave one quasi-stationary Rossby wave forcing at northern middle to high latitudes,
which allows a strengthening of the polar vortex and a deceleration of the BDC. The observed SLP
13 See, for example, H. van Loon, G. Meehl, and D. Shea, Coupled air-sea response to solar forcing in the
Pacific region during northern winter, Journal of Geophysical Research 112:D02108, 2007.
14 See, for example, G. Meehl, J. Arblaster, K Matthes, F. Sassi, and H. van Loon, Amplifying the Pacific
Climate System response to a small 11-year solar cycle forcing, Science 325(5944):1114-1118, 2009.
15 See, for example, M. Mann, Z. Zhang, S. Rutherford, R. Bradley, M. Hughes, D. Shindell, C. Ammann, G.
Faluvegi, and F. Ni, Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly,
Science 326(5957):1256-1260, 2009.
16 See, for example, C. Garfinkel, D. Hartmann, and F. Sassi, Tropospheric precursors of anomalous Northern
Hemisphere stratospheric polar vortices, Journal of Climate 23:3282-3299, 2010.