If the solar-irradiance/terrestrial-temperature aspects of sun-climate relationships can be characterized as a mechanism looking for an effect, other aspects of current activity can be thought of as effects looking for a mechanism. The literature contains a long history of correlations between weather and climate anomalies of one kind or another and some aspect of solar variability, often simply the sunspot number. The field acquired a rather poor reputation, largely because of a few cases in which there was a suspicion of bias in data selection or in which the statistical significance of the observed correlations was questionable. In some cases, however, neither of these problems arose, and the evidence seemed to point to a real relationship of some kind, although a plausible mechanism to explain the correlations was lacking.
In recent years, the field has been revived through the work of K. Labitzke in Germany and H. van Loon in the United States on a remarkable periodic oscillation in a number of meteorological parameters that appears to be well synchronized with the 11-year solar cycle. The oscillation was originally discovered in stratospheric temperatures over the northern polar regions in winter, and its phase with respect to the solar cycle depended on the phase of the equatorial quasi-biennial oscillation (QBO) in zonal stratospheric winds [Labitzke, 1987]. More recently, however, the oscillation, now known as the ten-to-twelve year oscillation (TTO), has been discovered in a wide range of atmospheric parameters, including some in the troposphere, where it is strongest in northern summer over the western ocean-dominated hemisphere [Labitzke and van Loon, 1992], and is independent of the phase of the QBO. Over most of the northern hemisphere the July-August tropospheric temperatures are higher during solar maxima and lower during solar minima. The relationships appear to meet statistical significance criteria, although the applicability of standard significance tests to data with a strong periodic component is not clear. The oscillation in the lower stratosphere has recently been reviewed by van Loon and Labitzke [1993].
The reality of the associations has been questioned on statistical grounds by several authors, including Salby and Shea [1991] and Dewan and Shapiro [1991]. The chief objection centered around the question of aliasing effects related to undersampled low-frequency variance, which according to the authors could account for the high correlation found in the stratosphere when the data were divided according to the phase of the QBO. Tinsley and Heelis [1993] have argued, however, that the aliasing objection is invalid in the case of a quasi-periodic signal such as the QBO. While the point does not yet appear to be completely settled, the aliasing argument would in any case not affect the tropospheric correlations that were found to be independent of the QBO. The same authors have also criticized these, however, on the basis of statistical significance.
While the work of Labitzke and van Loon has clearly shown the existence of an oscillation in many atmospheric parameters with a period in the vicinity of 11 years and a phase that is related to that of solar activity, there is reluctance to accept a relationship to the 11-year solar cycle. This skepticism is likely to remain unless a credible mechanism linking the sun and the atmosphere is found. As mentioned above, the 11-year cycle in solar total irradiance that has been seen so far is too small to account for the correlation, and in any case an 11-year cycle in insolation would be severely damped by thermal inertia in the oceans (but could conceivably survive in land temperatures). Decadal-scale variations have also been detected in long-term sea-surface temperature and associated meteorological data (e.g., Deser and Blackmon, 1993), and changes in ocean circulation patterns have been suggested as a possible cause. The question is far from being settled yet, and has important implications for our understanding of global climate change.