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Irina Rudeva and Ian Simmonds

Abstract

Presented here is a global analysis of frontal activity variability derived from ERA-Interim data over the 34-yr period of January 1979–March 2013 using a state-of-the-art frontal tracking scheme. In December–February over that epoch, there is a northward shift of frontal activity in the Pacific in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the largest trends are identified in the austral summer and are manifested by a southward shift of frontal activity over the Southern Ocean.

Variability of frontal behavior is found to be closely related to the main modes of atmospheric circulation, such as the North Atlantic Oscillation (NAO) for the Atlantic–European sector in the NH and the southern annular mode (SAM) in the middle and high latitudes of the SH. A signal associated with El Niño and hence emanating from the tropics is also apparent in the behavior of frontal systems over the Pacific by a reduction in the number of fronts in the middle South Pacific and intensification of frontal activity in high and low latitudes throughout the year. It is shown in general that the associations of the large-scale modes with frontal variability are much stronger than with cyclones. This indicates that the quantification of the behavior of fronts is an important component of understanding the climate system. At the very high latitudes, it is also shown here that, in the recent years of rapid sea ice reduction in the Arctic, there have been fewer summer fronts observed over the Canadian Arctic.

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Irina Rudeva and Sergey K. Gulev

Abstract

Climatology of the atmospheric cyclone sizes and their change over the cyclone life cycle is analyzed on the basis of tracking 57 yr of NCEP–NCAR reanalysis sea level pressure data over the Northern Hemisphere. To quantify the atmospheric cyclone sizes a coordinate transform was used, which allows for the collocation of the cyclone center with the virtual pole and for the establishment of a unique coordinate system for the further determination of cyclone geometry. This procedure was incorporated into a numerical cyclone tracking scheme and provided quantitative estimation of cyclone geometry at every stage of the cyclone development. Climatological features of the distribution of the cyclone size characteristics (effective radius, asymmetry) are considered for the cyclones with different central pressure, deepening rate, and lifetime. Mean effective cyclone radius may experience significant changes, ranging from 300–400 km over the continents to more than 900 km over the oceans. There is found to be a strong dependence of the cyclone effective radius on the cyclone lifetime and intensity, implying the largest cyclone sizes for the most intense and long-living transients. Analysis of size changes during the cyclone life cycle implies that the cyclone radius increases during the development stage from 50% to 150%. Size evolution during the cyclone life cycle implies a universal dependence of the normalized cyclone effective radius and the normalized cyclone age. The actual maximum cyclone radius can be determined from these two nondimensional parameters and cyclone central pressure. Further application of the analysis of cyclone size and shape are discussed.

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Irina Rudeva and Sergey K. Gulev

Abstract

Composite analysis of North Atlantic midlatitudinal winter cyclones is performed using NCEP–NCAR reanalysis data for the 60-yr period from 1948 to 2007. The composites were developed using an advanced methodology involving the coordinate transform of cyclones into a nondimensional azimuthal coordinate system and the further collocation of fields. Composite analysis is performed for air–sea turbulent fluxes, heat content, precipitable water, and precipitation for 576 oceanic cyclones generated in the Gulf Stream area in winter (January–March) from 1948 to 2007. For the region of cyclone generation over the Gulf Stream, composites were analyzed for different cyclone intensities. Over the whole North Atlantic, composites were developed throughout the life cycle and for different cyclone types classified by the regions of their migration. These classifications allow the case-to-case variability to be minimized and the robustness of the composite to be boosted. In the region of cyclone generation over the Gulf Stream, characteristics of the composites strongly depend on the cyclone intensity quantified through the radial sea level pressure difference between the cyclone’s edge and its center. Stronger cyclone intensity implies larger turbulent fluxes in the rear of a cyclone and stronger precipitation in the forward part. Cyclones gradually dry with the water content and precipitation rate decreasing by about 40% and 50%–70%, respectively, during the lifetime. Although composites of air–sea turbulent fluxes show locally very strong positive fluxes in the rear part of the cyclone, the total air–sea turbulent fluxes provided by cyclones are not significantly different from the averaged background fluxes. This shows that the formation of extreme air–sea fluxes by cyclones is connected to the larger-scale circulation conditions, particularly to the cyclone–anticyclone transition zones.

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Natalia Tilinina, Sergey K. Gulev, Irina Rudeva, and Peter Koltermann

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.

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Maria Hatzaki, Helena A. Flocas, Ian Simmonds, John Kouroutzoglou, Kevin Keay, and Irina Rudeva

Abstract

An objective climatology of anticyclones over the greater Mediterranean region is presented based on the Interim ECMWF Re-Analysis (ERA-Interim) for a 34-yr period (1979–2012) and the Melbourne University automatic identification and tracking algorithm. The scheme’s robustness and reliability for the transient extratropical propagation of anticyclones, with the appropriate choices of parameter settings, has been established and the results obtained here present new research perspectives on anticyclonic activity affecting the Mediterranean. Properties of Mediterranean anticyclones, such as frequency, generation and dissipation, movement, scale, and depth are investigated. The highest frequency of anticyclones is found over continental areas, while the highest maritime frequency occurs over closed basins exhibiting also maxima of anticyclogenesis. There is a significant seasonality in system density and anticyclogenesis maxima, this being associated with the seasonal variations of the larger-scale atmospheric circulation that affect the greater Mediterranean region.

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Lukas Papritz, Stephan Pfahl, Irina Rudeva, Ian Simmonds, Harald Sodemann, and Heini Wernli

Abstract

In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (>75th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.

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Jennifer L. Catto, Erica Madonna, Hanna Joos, Irina Rudeva, and Ian Simmonds

Abstract

Extratropical cyclones are responsible for many extreme precipitation events in the midlatitudes. Warm conveyor belts (WCBs) and fronts are known to be related to the uplift and hence the precipitation within cyclones. The authors have investigated the link between WCBs and fronts and how such a link impacts the occurrence of extreme precipitation events. WCB trajectories have been calculated from the ERA-Interim dataset, and low-level (below 790 hPa) and midlevel (790–600 hPa) WCBs have been considered. These have been matched with objectively identified fronts (i.e., characterized by an overlap of WCB and front somewhere along the front). About 10% of cold fronts, 8% of warm fronts (identified using a thermal criterion), and 15% of wind fronts (identified using a wind shift method) are matched with WCBs, while up to 70% of WCBs are matched with fronts. Some WCBs, especially in the Southern Hemisphere, are not matched with either type of front (up to 70% east of Australia). The relationship between WCBs and fronts does not change much between the low levels and midlevels, indicating that the WCBs are already strongly associated with fronts during the lowest part of their ascent, although in the Southern Hemisphere the WCBs are more often related to warm fronts during their midtropospheric ascent. In parts of the midlatitudes, more than 60% of extreme precipitation events match either cold or warm fronts, and up to 90% of these have matched WCBs. Fronts associated with WCBs are found to be between 2 and 10 times more likely to produce extreme precipitation events than fronts without associated WCBs.

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Irina Rudeva, Ian Simmonds, David Crock, and Ghyslaine Boschat

Abstract

This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.

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Urs Neu, Mirseid G. Akperov, Nina Bellenbaum, Rasmus Benestad, Richard Blender, Rodrigo Caballero, Angela Cocozza, Helen F. Dacre, Yang Feng, Klaus Fraedrich, Jens Grieger, Sergey Gulev, John Hanley, Tim Hewson, Masaru Inatsu, Kevin Keay, Sarah F. Kew, Ina Kindem, Gregor C. Leckebusch, Margarida L. R. Liberato, Piero Lionello, Igor I. Mokhov, Joaquim G. Pinto, Christoph C. Raible, Marco Reale, Irina Rudeva, Mareike Schuster, Ian Simmonds, Mark Sinclair, Michael Sprenger, Natalia D. Tilinina, Isabel F. Trigo, Sven Ulbrich, Uwe Ulbrich, Xiaolan L. Wang, and Heini Wernli

The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of weak cyclones, and distribution in some densely populated regions. Consistency between methods is better for strong cyclones than for shallow ones. Two case studies of relatively large, intense cyclones reveal that the identification of the most intense part of the life cycle of these events is robust between methods, but considerable differences exist during the development and the dissolution phases.

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