A statistical analysis of 51 years of NCEP-NCAR reanalysis data is conducted to isolate the separate effects of the 11-yr solar cycle (SC) and the equatorial quasi-biennial oscillation (QBO) on the Northern Hemisphere (NH) stratosphere in late winter (February-March). In a four-group [SC maximum (SC-max) versus minimum (SC-min) and east-phase versus west-phase QBO] linear discriminant analysis, the state of the westerly phase QBO (wQBO) during SC-min emerges as a distinct least-perturbed (and coldest) state of the stratospheric polar vortex, statistically well separated from the other perturbed states. Relative to this least-perturbed state, the SC-max and easterly QBO (eQBO) each independently provides perturbation and warming as does the combined perturbation of the SC-max-eQBO. All of these results (except the eQBO perturbation) are significant at the 95% confidence level as confirmed by Monte Carlo tests; the eQBO perturbation is marginally significant at the 90% level. This observational result suggests a conceptual change in understanding the interaction between solar cycle and QBO influences: while previous results imply a more substantial interaction, even to the extent that the warming due to SC-max is reversed to cooling by the eQBO, results suggest that the SC-max and eQBO separately warm the polar stratosphere from the least-perturbed state. While previous authors emphasize the importance of segregating the data according to the phase of the QBO, here the same polar warming by the solar cycle is found regardless of the phase of the QBO.The polar temperature is positively correlated with the SC, with a statistically significant zonal mean warming of approximately 4.6 ⌲ in the 10-50-hPa layer in the mean and 7.2 ⌲ from peak to peak. This magnitude of the warming in winter is too large to be explainable by UV radiation alone. The evidence seems to suggest that the polar warming in NH late winter during SC-max is due to the occurrence of sudden stratospheric warmings (SSWs), as noted previously by other authors. This hypothesis is circumstantially substantiated here by the similarity between the meridional pattern and timing of the warming and cooling observed during the SC-max and the known pattern and timing of SSWs, which has the form of large warming over the pole and small cooling over the midlatitudes during mid-and late winter. The eQBO is also known to precondition the polar vortex for the onset of SSWs, and it has been pointed out by previous authors that SSWs can occur during eQBO at all stages of the solar cycle. The additional perturbation due to SC-max does not double the frequency of occurrence of SSWs induced by the eQBO. This explains why the SC-max/eQBO years are not statistically warmer than either the SC-max/wQBO or SC minimum/eQBO years. The difference between two perturbed (warm) states (e.g., SC-max/eQBO versus SC-min/eQBO or SC-max/eQBO versus SC-max/wQBO), is small (about 0.3-0.4 ⌲) and not statistically significant. It is this small difference between perturbed states, both warmer...

By projecting surface temperature data (1959–2004) onto the spatial structure obtained objectively from the composite mean difference between solar max and solar min years, we obtain a global warming signal of almost 0.2°K attributable to the 11‐year solar cycle. The statistical significance of such a globally coherent solar response at the surface is established for the first time.

Applying Linear Discriminant Analysis on 47 years of NCEP stratospheric temperature data from 1959 to 2005, we find that the warm‐ENSO (“El Niño”) years are significantly warmer also in the stratosphere at the Northern Hemisphere polar and midlatitudes than the cold‐ENSO (“La Niña”) years, during winter. Specifically, the zonal mean, December‐February mean, 10–50 hPa mean temperature, when projected onto the coherent spatial structure that best distinguishes the two ENSO groups, classified according to the equatorial Pacific‐ocean Cold Tongue Index, is 4°K warmer in the polar stratosphere in the warm‐ENSO mean than in the cold‐ENSO mean. The difference is statistically significant at above the 95% confidence level. This is the first time statistical significance has been established for ENSO's influence on the polar stratosphere. A surprising result is that the ENSO perturbation to the polar stratosphere is comparable in magnitude to the better‐known QBO perturbation, which is 3.8°K between easterly QBO mean and the westerly QBO mean.