How do Midlatitude Blocks and Wave Amplitude Respond to Changes in the Midlatitude-to-Pole Near-Surface Temperature Gradient? A Study with an Idealized Dry GCM

Wednesday, 17 December 2014
Pedram Hassanzadeh, Zhiming Kuang and Brian Farrell, Harvard University, Cambridge, MA, United States
Atmospheric blocks and amplified midlatitude planetary waves can lead to weather extremes such as heat waves, cold spells, droughts and heavy precipitation events. While assessing the influence of climate change on extreme weather events is of significant scientific and societal importance, how blocks and wave amplitude respond to changes in the large-scale atmospheric circulation is not well-understood. For example, whether the weakened midlatitude westerlies and Z500 meridional gradient, caused by a reduced midlatitude-to-pole near-surface temperature difference in the Northern Hemisphere (associated with Arctic Amplification), lead to an increase in atmospheric blocking events and wavier jet streams remains unclear, despite a number of recent studies addressing this issue using observation and comprehensive GCMs.

Here we address the question of how blocks and wave amplitude respond to changes in the midlatitude-to-pole near-surface temperature difference using an idealized dry GCM with a simple setup as described in Held and Suarez (1994). Quasi-stationary long-lived blocks that split or substantially displace the jet are common in simulations made with this model, which is free of topography and zonally asymmetric forcing. Reducing the pole-to-equator surface temperature difference in the forcing of this model leads to a mean-state with smaller midlatitude-to-pole near-surface temperature difference and Z500 meridional gradient, smaller variances of temperature and Z500, and slower jet streams, which are consistent with changes expected from Arctic Amplification. Using long high-resolution simulations, we find a robust decline in blocked area and wave amplitude as the pole-to-equator surface temperature difference in the forcing is reduced. These findings are insensitive to numerical resolution, strength of hyperdiffusion, and parameters used in the blocking and wave amplitude indices. Statistical and dynamical diagnoses are used to investigate the underlying physics of these responses.

With the limitations of an idealized approach borne in mind, our findings provide a complement to the ongoing studies with a hierarchy of GCMs and observations examining the response of blocks and wave amplitude to changes in the large-scale atmospheric forcing arising from climate change.