Northern Hemisphere Extratropical Cyclone Activity Research Articles

Northern Hemisphere Extratropical Cyclone Activity in the Twentieth Century Reanalysis Version 3 (20CRv3) and Its Relationship with Continental Extreme Temperatures

In this study, we detect and track extratropical cyclones using 6-hourly mean sea level pressure data taken from the Twentieth Century Reanalysis version 3 (20CRv3) over the period 1951–2015 and compare them with those in the Interim and fifth generation of ECMWF reanalyses over the period 1979–2018. Three indices were employed to characterize cyclone activity, including cyclone count, cyclone intensity, and a cyclone activity index (CAI) that combines the count and intensity. The results show that the cyclone indices in the three datasets have comparable annual climatologies and seasonal evolution over the northern extratropical land and ocean in recent decades. Based on the cyclone indices over the period 1951–2010 in 80 ensemble members of 20CRv3, cyclone count and intensity are negatively correlated in winter and tend to be positively and weakly correlated in summer. The interannual CAI variability is dominated by the cyclone count variability. Regional mean cyclone activity can be well represented using the ensemble average cyclone index. We then examined the linkage of the cyclone activity in 20CRv3 and observed cold and warm extremes over Eurasia and North America over the period 1951–2010. In winter, the principal components of interannual cold and warm extreme anomalies are more correlated with the regional mean cyclone count index over Eurasia, while they are more correlated with the cyclone intensity index over North America. The temperature anomalies associated with the regional and ensemble mean cyclone count index explain about 10% (20%) of interannual cold (warm) extreme variances averaged over Eurasia. The temperature anomalies associated with the mean cyclone intensity explain about 10% of interannual cold and warm extreme variances over North America. Large-scale atmospheric circulation anomalies in association with cyclone activity and the induced temperature advection drive temperature anomalies over Eurasia and North America. In summer, circulation and thermal advection anomalies associated with cyclone activity are weak over the two continents. Hence, that season’s relationship between cyclone activity and extreme temperature variability is weak.

Subseasonal Prediction of Wintertime Northern Hemisphere Extratropical Cyclone Activity by SubX and S2S Models

AbstractThe prediction of wintertime extratropical cyclone activity (ECA) on subseasonal time scales by models participating in the Subseasonal Experiment (SubX) and the Seasonal to Subseasonal Prediction (S2S) is assessed. Consistent with a previous study that investigated the S2S models, the SubX models have skillful predictions of ECA over regions from central North Pacific across North America to western North Atlantic, as well as East Asia and northern and southern part of eastern North Atlantic at 3–4 weeks lead time. SubX provides daily mean data, while S2S provides instantaneous data at 0000 UTC each day. This leads to different variance of ECA. Different S2S and SubX models have different reforecast initialization times and reforecast time periods. These factors can all lead to differences in prediction skill. To fairly compare the prediction skill between different models, we develop a novel way to evaluate the prediction of individual model across the two ensembles by comparing every model to the Climate Forecast System, version 2 (CFSv2), as CFSv2 has 6-hourly output and forecasts initialized every day. Among the S2S and SubX models, the European Centre for Medium-Range Weather Forecasts model exhibits the best prediction skill, followed by CFSv2. Our results also suggest that while the prediction skill is sensitive to forecast lead time, including forecasts up to 4 days old into the ensemble may still be useful for weeks 3–4 predictions of ECA.

Subseasonal to Seasonal Prediction of Wintertime Northern Hemisphere Extratropical Cyclone Activity by S2S and NMME Models

AbstractIn this study, the prediction of wintertime extratropical cyclone activity from subseasonal to seasonal timescale in current dynamical models' reforecasts is investigated. On seasonal time scales, the North American Multi‐Model Ensemble (NMME) models show skillful predictions over the eastern North Pacific, North America and the western North Atlantic with at least 5 months lead. The prediction skill is highly related to El Nino‐Southern Oscillation (ENSO), as using the ENSO‐related SST pattern gives rise to prediction skill with very similar spatial pattern and amplitude. On subseasonal time scales, models in the Seasonal to Sub‐seasonal Prediction (S2S) dataset have skillful predictions up to 4 weeks lead over regions from the eastern North Pacific to the western Atlantic, as well as northern Europe, the eastern Atlantic and East Asia. Generally, forecast skill improves with a larger ensemble size. The subseasonal prediction skill from the Pacific to the western Atlantic is related to ENSO, and that over eastern Atlantic, Europe and East Asia are associated with stratospheric polar vortex anomalies. Current models do not show much skill from the Madden‐Julian Oscillation (MJO), as the MJO impact on extratropical cyclone activity is not well captured by the models. European Centre for Medium‐Range Weather Forecasts (ECMWF) model has the highest single model subseasonal prediction skill. The prediction skill in the ECMWF model is higher than its estimated potential predictability, likely because the signal‐to‐noise ratio is too low in the model hindcasts.

A Modeling Investigation of Northern Hemisphere Extratropical Cyclone Activity in Spring: The Linkage between Extreme Weather and Arctic Sea Ice Forcing

Arctic sea ice exhibits rapid reductions and large fluctuations during recent decades in conjunction with a warming climate. Arctic sea ice shows a prominent seasonal cycle with the maximum extent in spring and the minimum extent in fall. In this study a suite of modeling experiments, which were solely forced with observed time-varying sea-ice concentration for spring 1979 to 2008 was conducted using NCAR’s Community Atmosphere Model to identify sea ice impacts on extratropical storms and associated surface climate parameters when they at the peak of their seasonal variability. Storms are the linkage between the large-scale circulation changes, forced by Arctic sea ice and local extreme weather events through strong winds, high precipitation, and high/low air temperature. In this study, a storm identification and tracking algorithm indicates that reduced sea-ice cover enhances Arctic storm activity. As a consequence, surface climate parameters such as surface air temperature (SAT) and precipitation increases in spring over the Arctic. The changes in the Arctic have a direct and indirect effect on the extratropical storm activity over the mid-latitudes. In contrast to the Arctic, storm activity weakens over Eurasia in the years with less sea ice. Further analysis of the surface climate indicates a warmer and dryer Eurasia for years with reduced sea ice.

Observed and projected decrease in Northern Hemisphere extratropical cyclone activity in summer and its impacts on maximum temperature

AbstractExtratropical cyclones cause much of the high‐impact weather over the midlatitudes. With increasing greenhouse gases, enhanced high‐latitude warming will lead to weaker cyclone activity. Here we show that between 1979 and 2014, the number of strong cyclones in Northern Hemisphere in summer has decreased at a rate of 4% per decade, with even larger decrease found near northeastern North America. Climate models project a decrease in summer cyclone activity, but the observed decreasing rate is near the fastest projected. Decrease in summer cyclone activity will lead to decrease in cloud cover, giving rise to higher maximum temperature, potentially enhancing the increase in maximum temperature by 0.5 K or more over some regions. We also show that climate models may have biases in simulating the positive relationship between cyclone activity and cloud cover, potentially underestimating the impacts of cyclone decrease on accentuating the future increase in maximum temperature.

Comparing Cyclone Life Cycle Characteristics and Their Interannual Variability in Different Reanalyses

Abstract Characteristics of Northern Hemisphere extratropical cyclone activity were compared for five concurrent reanalyses: the NCEP–U.S. Department of Energy (DOE) reanalysis (herein NCEP–DOE), the Japanese 25-year Reanalysis Project (JRA-25), the ECMWF Interim Re-Analysis (ERA-Interim), the National Aeronautics and Space Administration's Modern-Era Retrospective Analysis for Research and Applications (NASA-MERRA), and the NCEP Climate Forecast System Reanalysis (NCEP-CFSR), for the period 1979–2010 using a single cyclone tracking algorithm. The total number of cyclones, ranging from 1400 to more than 1800 yr−1, was found to depend strongly on the spatial resolution of the respective reanalysis. The largest cyclone population was identified using NASA-MERRA data, which also showed the highest occurrence of very deep cyclones. Of the reanalyses, two (NCEP–DOE and ERA-Interim) are associated with statistically significant positive trends in the total number of cyclones from 1% to 2% decade−1. These trends result from moderate and shallow cyclones contributing to approximately 90% of the total cyclone count on average. The number of very deep cyclones (<960 hPa) in the North Atlantic increased in most reanalyses until 1990 and then declined during the last decade. In the North Pacific, the number of these events reached a peak in 2000 and then decreased during the last decade. The winter pattern is characterized by robust trends in cyclone numbers, with an enhancement of the North Atlantic storm track and a weakening of the North Pacific subtropical storm track. In the summer, there is a robust intensification of the Mediterranean storm track and a decrease in counts over the North Atlantic. Interannual variability and decadal-scale variations of the cyclone counts are highly correlated among the reanalyses, with the greatest agreement in moderate and deep cyclones.

Are Greenhouse Gas Signals of Northern Hemisphere winter extra-tropical cyclone activity dependent on the identification and tracking algorithm?

For Northern Hemisphere extra-tropical cyclone activity, the dependency of a potential anthropogenic climate change signal on the identification method applied is analysed. This study investigates the impact of the used algorithm on the changing signal, not the robustness of the climate change signal itself. Using one single transient AOGCM simulation as standard input for eleven state-of-the-art identification methods, the patterns of model simulated present day climatologies are found to be close to those computed from re-analysis, independent of the method applied. Although differences in the total number of cyclones identified exist, the climate change signals (IPCC SRES A1B) in the model run considered are largely similar between methods for all cyclones. Taking into account all tracks, decreasing numbers are found in the Mediterranean, the Arctic in the Barents and Greenland Seas, the mid-latitude Pacific and North America. Changing patterns are even more similar, if only the most severe systems are considered: the methods reveal a coherent statistically significant increase in frequency over the eastern North Atlantic and North Pacific. We found that the differences between the methods considered are largely due to the different role of weaker systems in the specific methods.

Response of Northern Hemisphere extratropical cyclone activity and associated precipitation to climate change, as represented by the Community Climate System Model

The projected effects of rising CO2 levels on Northern Hemisphere extratropical cyclone activity and cyclone‐associated precipitation are examined for September–May, using output from version 3 of the Community Climate System Model (CCSM3). A cyclone identification algorithm was applied to a five member ensemble of CCSM3 20th and 21st century output, along with a method of isolating precipitation produced by each cyclone. Mean seasonal statistics describing cyclone activity and the character of associated precipitation were calculated over several study regions for 20 a periods. The dominant change in cyclone activity is a marked midlatitude decrease in frequency during autumn, winter, and spring. Few significant shifts in storm tracks or cyclone intensity were identified. Total daily precipitation from these events is found to increase into the 21st century, largely because of increases in available atmospheric moisture with rising temperatures. This thermodynamic increase in precipitation leads to large rises in total seasonal cyclone‐associated precipitation over high latitudes, while over midlatitudes the thermodynamic increase is offset by the dynamic effect associated with decreased cyclone frequency.

Northern Hemisphere extratropical cyclone activity for four mid‐season months

AbstractCyclone frequency, cyclogenesis and the primary and secondary tracks followed by extratropical cyclones within the Northern Hemisphere are examined for the four mid‐season months: January, April, July and October. The data for these four months were compiled from several sources for the period 1958–1977. Cyclogenesis was defined as the point of origin of a cyclone provided it maintained a closed isobar for 24 hours. Cyclone frequency was determined from the number of occurrences of an extratropical cyclone passing through a 5° latitude‐longitude grid box. Analyses of the grid values of cyclone frequency reveal relative maxima which define the cyclone tracks. The tracks are partitioned into primary and secondary tracks depending on the size of the relative maxima of cyclone frequency.Cyclogenesis is concentrated in the lee of major mountain ranges and along the east coasts of Asia and North America, with cyclogenesis in coastal North America found nearer the coastline than in the Asian area. A major cyclogenetic region also exists in the Gulf of Genoa‐Northern Italy region of the Mediterranean Sea, but here cyclogenesis is confined to the cooler seasons. As expected the areas of most frequent cyclogenesis shift poleward during the warmer months. Similarly, cyclone frequency exhibits a poleward displacement during the warmer months. It is also noted that whereas winter (January) is characterized by a significantly greater cyclone frequency over the ocean than over land, summer (July) finds the frequency over the continent about as large as over the oceans. Similarly there are significant poleward displacements of cyclone tracks in summer, but the displacement is in no way uniform over the hemisphere. These results can be compared with earlier hemispheric studies of cyclone activity.