Larry Horowitz - The chemistry and dynamics of the stratosphere and the coupling between the stratosphere and troposphere are investigated using a hierarchy of GFDL atmospheric models. The AM3 chemistry-climate model, the atmospheric component of the GFDL CM3 coupled model used for CMIP5, is shown to successfully simulate the major observed features of the distribution, variability, and depletion trends of stratospheric ozone. A version of AM3, nudged to reanalysis winds, is shown to reproduce the principal features of deep stratospheric intrusions. Transport of stratospheric ozone to the surface is found to drive a substantial portion of observed synoptic variability in surface ozone over the western U.S. during spring, contributing to exceedances of the NAAQS threshold. Variations in the stratospheric Brewer-Dobson circulation (BDC) are found to be correlated, in CM3 and reanalysis data, with changes in tropical mean surface temperature. This correlation, which is robust across a range of timescales and climate forcing scenarios, results from changes in upper tropospheric temperatures and zonal winds, modifying wave dissipation in the lower stratosphere. A new non-hydrostatic version of the cubed-sphere finite volume dynamical core (FV3) improves the representation of Kelvin waves and is shown to improve dramatically the simulation of the quasi-biennial oscillation (QBO).Judith Perlwitz - It is widely recognized that tropospheric and stratospheric circulation are closely coupled two-ways and that a degraded representation of stratospheric processes in climate models affects simulated tropospheric climate. This talk evaluates main aspects of the dynamical coupling in models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) with the focus on the downward connection. We introduce a new dynamical metric of troposphere-stratosphere coupling based on extreme stratospheric planetary-scale wave heat flux events. In reanalysis the stratospheric heat flux extremes are linked instantaneously to high-latitude planetary-scale wave patterns in the troposphere and zonal wind, temperature, and mean sea level pressure anomalies in the Atlantic basin. The metric when applied to CMIP5 models illustrates that models with a degraded representation of stratospheric heat flux extremes exhibit robust biases in the troposphere relative to ERA-Interim reanalysis. In particular, models with biased stratospheric extremes exhibit a biased climatological stationary wave pattern and Atlantic jet stream position. The stratospheric biases are connected to model lid height, but it is not sufficient for assessing the tropospheric impacts. Overall, the results suggest that a metric based on stratospheric heat flux extremes should be used in conjunction with metrics based on extreme polar vortex events in multimodel assessments of troposphere-stratosphere coupling.Ming Cai - Extreme weather events such as cold air outbreaks pose great threats to human life and socioeconomic well-being of the modern society. In the past, our capability to predict their occurrences is constrained by the 2-week predictability limit for weather. We demonstrate here for the first time that the day-to-day variations of air mass transported into the polar stratosphere, referred to as “the pulse of the stratosphere”, can be predicted with a useful skill 4–6 weeks in advance by operational forecast models. We further show that anomalous strengthening of this mass transport is closely tied to massive cold air outbreaks in mid-latitudes. In particular, we reveal that the three massive cold air outbreaks over North America in January and February of 2014 were preceded by three episodes of extreme mass transport into the polar stratosphere with peak intensities reaching a trillion tons per day, twice of that on an average winter day. Therefore, our capability to predict the pulse of the stratosphere with operational forecast models opens up a new opportunity for 30−day forecasts of massive continental-scale cold air outbreaks, such as those occurring over North America in the 2013–14 winter. An ongoing real time forecast experiment inaugurated in the winter of 2014–15 has confirmed the feasibility of forecasting cold air outbreaks one month in advance. As recorded in the website (www.amccao.com), we have been very successful in forecasting cold air outbreaks and winter storms at a lead time of 3−6 weeks in advance.
Americans’ health, security and economic wellbeing are tied to climate and weather. Every day, we see communities grappling with environmental challenges due to unusual or extreme events related to climate and weather. In 2011, the United States experienced a record high number (14) of climate- and weather-related disasters where overall costs reached or exceeded $1 billion. Combined, these events claimed 670 lives, caused more than 6,000 injuries, and cost $55 billion in damages. Businesses, policy leaders, resource managers and citizens are increasingly asking for information to help them address such challenges.
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