Sang-Ki Lee -- Is there an optimal ENSO pattern that enhances large-scale atmospheric processes conducive to tornado outbreaks in the U.S? -- The record-breaking U.S. tornado outbreaks in the spring of 2011 prompt the need to identify long-term climate signals that could potentially provide seasonal predictability for U.S. tornado outbreaks. Here we use both observations and model experiments to show that a positive phase Trans-Niño may be one such climate signal. Among the top ten extreme outbreak years during 1950-2010, seven years including the top three are identified with a strongly positive phase Trans-Niño. The number of intense tornadoes in April - May is nearly doubled during the top ten positive Trans-Niño years from that during ten neutral years. Trans-Niño represents the evolution of tropical Pacific sea surface temperatures (SSTs) during the onset or decay phase of the El Niño-Southern Oscillation. A positive phase Trans-Niño is characterized by colder-than-normal SSTs in the central tropical Pacific and warmer-than-normal SSTs in the eastern tropical Pacific. Modeling experiments suggest that warmer-than-normal SSTs in the eastern tropical Pacific work constructively with colder-than-normal SSTs in the central tropical Pacific to force a strong and persistent teleconnection pattern that increases both the upper-level westerly and lower-level southeasterly over the central and eastern U.S. These anomalous winds advect more cold and dry upper-level air from the high-latitudes and more warm and moist lower-level air from the Gulf of Mexico converging into the east of the Rockies, and also increase both the lower-tropospheric (0 ~ 6 km) and lower-level (0 ~ 1 km) vertical wind shear values therein, thus providing large-scale atmospheric conditions conducive to intense tornado outbreaks over the U.S.
Rong Fu -- Assessing Future Changes of Drought and extreme temperature and over the South-Central United States Projected by the CMIP5 Models -- Nine climate models that participated in the Inter-governmental Panel for Climate Change (IPCC) Fifth Assessment Report (CMIP5) realistically capture the general patterns of seasonal cycle, the probability distributions of rainrate (P) and surface daily maximum and minimum temperature (Tmax, Tmin), and evapotranspiration (ET) over the south-central United States (SC US). However, most of them have wet and cold biases in precipitation and Tmax, and underestimate non-rainy days and heavy to violent rainfall events and overestimate moderate rain. These biases are consistent with their underestimates of the latitudinal gradient of 500 hPa geopotential height in winter and spring, the strength and extent of the mid-tropospheric geopotential ridge, and the lower tropospheric westerly winds in summer. The former allows more frequent passages of synoptic disturbances during winter and spring, whereas the latter weakens the circulation pattern in favor of summer droughts. Although a few models can partially capture the Pacific-North American wave-train patterns, the models cannot fully capture the tele-connection patterns associated with El Niño-Southern Oscillation (ENSO) and its influence on rainfall anomalies over the SC US. Most of the models cannot reproduce the observed global SST warming mode and its relationship with an increase of summer rainfall over the SC US. The poorer performing models tend to be the outliners in climate projections. Exclude these models from the ensemble projections lead to a weaker projected increase of extreme warm Tmax, Tmin, and stronger decrease of surface net water deficit (P-ET) over the SC US by 2973-2099 relative to 1979-2005 under both the RCP4.5 and RCP8.5 scenarios.
Grant Branstator -- Excitation of Transient Rossby Wavetrains and their Influence on Midlatitude Weather -- Persistent Rossby wavetrains are a familiar phenomenon because of their role in interannual variability, including the global response to ENSO events. That Rossby wavetrains are also important on shorter time scales is evident from the example in the first part of our presentation, which considers US heat waves in a 12,000 year climatological SST experiment with the CAM3 atmospheric GCM. The most common location for these heat waves is the Great Plains, and our analysis indicates that these events are associated with, and probably initiated by, transient Rossby waves that originate far upstream.
One source of such transient Rossby waves is intra-seasonal tropical rainfall, which comprises a large fraction of tropical rainfall variability. Linear theory suggests that the structure of the midlatitude circulation anomalies induced by such, potentially moving, sources of heat will be different from the familiar great circle wavetrains produced by steady sources. But systematically investigating these anomalies in a realistic setting, like a GCM, is computationally prohibitive. To overcome this difficulty we have employed the Fluctuation Dissipation Theorem from statistical physics. Using it we find that the scale, amplitude and reach of circulation anomalies produced by moving heat sources are strongly dependent on the time dependent behavior of tropical heating. For example, sources moving eastward at a rate of about 5m/s have a noticeably weaker midlatitude influence than steady sources. Nonetheless, we find that subseasonal circulations produced by moving sources interact with and change the position and structure of midlatitude synoptic events and associated extremes, provided the sources move no faster than a typical MJO disturbance.
Brian Colle -- Future Prediction of Eastern North American and Western Atlantic Extratropical Cyclones in the CMIP5 Models During the Cool Season -- Extratropical cyclone track density, genesis frequency, deepening rate, and maximum intensity distributions over eastern North America and the western North Atlantic were analyzed for 15 Coupled Model Intercomparison Project phase 5 (CMIP5) models for the historical period (1979-2004) and three future periods (2009-2038, 2039-2068, and 2069-2098). Some of the results were compared with the North American Regional Climate Change Assessment Program (NARCCAP) runs for the historical and 2039-2068 periods. The cyclones were identified using an automated tracking algorithm applied to sea-level pressure every 6 hours. The model results for the historical period were evaluated using the Climate Forecast System Reanalysis (CFSR). The CMIP5 models were ranked given their track density, intensity, and overall performance for the historical period. It was found that 6 of the top 7 CMIP5 models with the highest spatial resolution were ranked the best overall. These models had less underprediction of cyclone track density, more realistic distribution of intense cyclones along the U.S. East coast, and more realistic cyclogenesis and deepening rates. The best 7 models were used to determine projected future changes in cyclones, which included a 10-30% decrease in cyclone track density and weakening of cyclones over the western Atlantic storm track, while in contrast there is a 10-20% increase in cyclone track density over the eastern U.S., including 10-40% more intense (< 980 hPa) cyclones and 20-40% more rapid deepening rates just inland of the U.S. East coast. The 850 hPa winds were also analyzed for a few of the models, which also suggested stronger future (mid-21st century) storms along the U.S. East coast. Some of the reasons for these CMIP5 model differences were explored for selected models based on model generated Eady growth rate, upper-level jet, surface baroclinicity, and surface precipitation.
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