The NOAA CPO Modeling, Analysis, Predictions, and Projections (MAPP) program hosted a webinar on the topic of Persistent Flow Regimes and Extremes on Friday, February 19, 2016. The announcement is provided below. This Webinar was co-sponsored by the Office of Weather and Air Quality and the National Earth System Prediction Capability.
University of Oxford, Oxford, UK
Blocking Impacts and Climate Change
Blocking events can lead to high-impact heat waves in summer and cold snaps in winter. Blocking is often cited as a problem area for climate models, raising questions over the confidence we should place in projected changes in extremes. Here we show that blocking is now reasonably well simulated by some current climate models. The models tend to agree on a reduction of blocking occurrence in response to climate change, and in addition there are robust changes in the impact of blocking events which can be linked back to well-understood changes in thermal advection.
NASA Global Modeling Assimilation Office, Greenbelt, MD, USA
Warm Season Drought Development over North America: The Role of Stationary Rossby Waves
Summertime quasi-stationary Rossby waves are known to play a key role in Northern Hemisphere atmospheric circulation and surface meteorology variability on subseasonal time scales. In particular, such waves have been crucial to the development of a number of recent short-term warm season heat waves and droughts over North America (e.g. the 1988, 1998 and 2012 summer droughts). Here we present a case study of a stationary Rossby wave event that developed during 20 May-15 June 1988 and led to the severe 1988 North American drought, based on simulations with the NASA Goddard Earth Observing System (GEOS-5) atmospheric general circulation model. Specifically, we investigate the roles of stationary Rossby wave sources, north Pacific mean jet stream and soil moisture feedback over North America in contributing to the development of stationary Rossby waves over North America and the subsequent rapid development of drought conditions there. Our results highlight the crucial importance of the north Pacific jet stream in guiding and constraining the path and speed of wave energy propagation. They also suggest that stationary Rossby waves can serve as a potential source of predictability for subseasonal development of droughts over North America.
NOAA Earth System Research Laboratory/Global Systems Division, Boulder, CO, USA
Identification of Predictability Processes Related to Atmospheric Blocking Toward the Goal of Improving Forecast Lead-Time of These Events
Predictability of blocked and broader quasi-stationary hemispheric global circulation patterns is an important question even for multi-week and even medium-range forecasting. Three earth-system processes related to blocking onset, duration, and cessation with some longer time-scale are atmospheric MJO events, stratospheric wave breaking, and land-surface memory. We will describe some initial studies of predictability and frequency for blocking and these related processes from a 16-year retrospective period of 32-day runs using the coupled atmospheric-ocean FIM-HYCOM model compared to a similar record from the NOAA CFSv2 coupled model. Accurate frequency can be considered broadly as a necessary but not sufficient condition for actual prediction of these events. For instance, frequency of blocking (500 hPa Tibaldi-Molteni metric) is shown to be relatively constant over forecast duration from this study. This effort will be continued within the US coupled modeling community goal to improve week 3-4 forecast skill.