Climate dynamics

Abstract:

Mid-latitude weather and climate are dominated by the jet streams and associated eastward-moving storm systems. Occasionally, however, these are blocked by persistent anticyclonic regimes known as blocking. Climate models generally predict a small decline in blocking frequency under anthropogenic climate change. However, confidence in these predictions is undermined by, among other things, a lack of understanding of the physical mechanisms underlying the change. Here we analyze blocking (mostly in the EuroAtlantic sector) in a set of sensitivity experiments to determine the effect of different parts of the surface global warming pattern. We also analyze projected changes in the impacts of blocking such as temperature extremes. The results show that enhanced warming both in the tropics and over the Arctic act to strengthen the projected decline in blocking. The tropical changes are more important for the uncertainty in projected blocking changes, though the Arctic also affects the temperature anomalies during blocking.

Abstract:

© 2016 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.The 11-year solar cycle signal in December–January–February (DJF) averaged mean-sea-level pressure (SLP) and Atlantic/European blocking frequency is examined using multilinear regression with indices to represent variability associated with the solar cycle, volcanic eruptions, the El Niño–Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). Results from a previous 11-year solar cycle signal study of the period 1870–2010 (140 years; ∼13 solar cycles) that suggested a 3–4 year lagged signal in SLP over the Atlantic are confirmed by analysis of a much longer reconstructed dataset for the period 1660–2010 (350 years; ∼32 solar cycles). Apparent discrepancies between earlier studies are resolved and stem primarily from the lagged nature of the response and differences between early- and late-winter responses. Analysis of the separate winter months provide supporting evidence for two mechanisms of influence, one operating via the atmosphere that maximises in late winter at 0–2 year lags and one via the mixed-layer ocean that maximises in early winter at 3–4 year lags. Corresponding analysis of DJF-averaged Atlantic/European blocking frequency shows a highly statistically significant signal at ∼1-year lag that originates primarily from the late winter response. The 11-year solar signal in DJF blocking frequency is compared with other known influences from ENSO and the AMO and found to be as large in amplitude and have a larger region of statistical significance.

Abstract:

The 11-year period solar cycle in the sun’s output impacts the winter surface climate of Northern Europe and the Atlantic. This occurs through a chain of dynamical processes, illustrated below, that we are only only just starting to understand. Using the HadGEM model to conduct a series of sensitivity experiments, I aim to improve this understanding, and perhaps the predictability of N. Europe winters.

Abstract:

The 11-year solar cycle signal in December-January-February (DJF) averaged mean sea level pressure (SLP) and Atlantic / European blocking frequency is examined using multi-linear regression with indices to represent variability associated with the solar cycle, volcanic eruptions, the El Nino Southern Oscillation (ENSO) and the Atlantic Multi-decadal Oscillation (AMO). Results from a previous 11-year solar cycle signal study of the period 1870–2010 (140 years; ~13 solar cycles) that suggested a 3–4 year lagged signal in SLP over the Atlantic are confirmed by analysis of a much longer reconstructed dataset for the period 1660–2010 (350 years; ~32 solar cycles). Apparent discrepancies between earlier studies are resolved and stem primarily from the lagged nature of the response and differences between early and late winter responses. Analysis of the separate winter months provide supporting evidence for two mechanisms of influence, one operating via the atmosphere that maximises in late winter at 0–2 years lags and one via the mixed-layer ocean that maximises in early winter at 3–4 year lags. Corresponding analysis of DJF-averaged Atlantic / European blocking frequency shows a highly statistically significant signal at ~1-year lag that originates primarily from the late winter response. The 11-year solar signal in DJF blocking frequency is compared with other known influences from ENSO and the AMO and found to be as large in amplitude and have a larger region of statistical significance.