Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: Kevin Keay x
  • All content x
Clear All Modify Search
Ian Simmonds and Kevin Keay

Abstract

This paper presents a new climatology of Southern Hemisphere (SH) extratropical cyclones. This has been compiled by applying a state-of-the-art cyclone tracking scheme to the 6-hourly National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) global reanalyses spanning the period 1958–97. The results show there to be, on average, between 35 and 38 cyclonic systems per analysis (depending on season), with the greatest density [exceeding 6 × 10−3 cyclones (deg lat)−2] found south of 60°S in all seasons and in the Indian and west Pacific Oceans in autumn and winter. For the most part, there is a net creation of cyclones (i.e., cyclogenesis exceeds cyclolysis) north of about 50°S, and a net destruction to the south of this latitude. Having said this, the most active cyclogenesis takes place south of 45°S. The NCEP–NCAR reanalyses indicate that most SH cyclogenesis occurs at very high latitudes, and the axis of the maximum lies on, or to the south of, 60°S. This is in agreement with the deductions of many modern studies of SH cyclone behavior. The region is also host to even greater levels of cyclolytic activity.

The authors consider measures of the importance and influence (e.g., for eddy fluxes) of cyclonic systems. It is suggested that the “depth” of a system (the pressure difference between the center and the “edge” of a cyclone) is a relatively bias-free and useful measure of a cyclone’s status and effect on the circulation. The greatest climatological depths are seen to lie at about 60°S, well to the north of the circumpolar trough and of the region of greatest cyclone density. The mean lifetime of cyclones that last at least 1 day is just over 3 days. Those that are located between 50° and 70°S (at their half-lifetime) endure, on average, almost one day longer than all other systems. The mean track length of winter systems is 2315 km, which reduces to 1946 km in summer.

The significance of the work presented here lies in a number of factors. First, the climatology has been derived from 40 yr of analysis, a period longer than any considered heretofore. Further, the (re)analyses used can be regarded as one of the best representations of the global atmosphere. The availability of these analyses at 6-hourly intervals means that the uncertainties with tracking of cyclones are greatly diminished. Finally, it has been compiled using one of the most sophisticated and reliable automatic cyclone finding and tracking schemes. This climatology of SH extratropical cyclones is arguably the most accurate and representative set yet assembled.

Full access
Ian Simmonds and Kevin Keay

Abstract

An analysis of the variability and trends exhibited by many aspects of Southern Hemisphere (SH) mean sea level extratropical cyclones during the period 1958–97 is presented. The investigation is undertaken by applying a state-of-the-art cyclone finding and tracking scheme to the 6-hourly reanalyses produced by the National Centers for Environmental Prediction. The outcome of this is arguably the most reliable analysis of SH cyclone variability undertaken to date.

Across the 40-yr period the annual and seasonal mean cyclone densities have undergone reductions at most locations south of about 40°S (with the greatest reductions near 60°S), and increases to the north. This pattern of change resembles the “high-latitude mode” identified in many studies of SH circulation features. It is shown that the mean radius of SH extratropical cyclones displays almost everywhere a significant positive trend, and there are also increases in annual mean cyclone “depth” (i.e., the pressure difference between the center and the “edge” of a cyclone).

The annual average number of cyclones per SH analysis rose from the start of the period to a maximum of about 39 in 1972. Since then, the numbers have shown an overall decline, the counts in the 1990s being particularly low. Similar behavior was evident when the count was confined to the 30°–50°S and 50°–70°S latitude bands. Least squares best fit to the three time series exhibit significant slopes of −0.58, −0.26, and −0.58 cyclones per analysis per decade, respectively. Between 30° and 70°S the annual mean number of cyclones found per analysis assumed a maximum about 1970, but that number has dramatically decreased by about 10% since then. (This analysis suggests that the downward trends in cyclone numbers are associated with a warming Southern Hemisphere.) The overall structure of the time series of annual cyclone per analysis over 30°–50°S and 50°–70°S are similar, but their year-to-year changes are shown to be negatively correlated; hence, there tends to be an interannual compensation of cyclone density between the middle and higher latitudes.

The extent to which changes in the semiannual oscillation over the last few decades could be said to have influenced how cyclones are distributed across seasons is briefly examined. The results show, in particular, that the interannual relationship between spring and winter cyclone density cannot be explained in terms of a response to a change in the amplitude of the semiannual oscillation.

Full access
Luke Osburn, Kevin Keay, and Jennifer L. Catto

Abstract

Wintertime extratropical cyclones in the east Pacific region are the source of much of the precipitation over California. There is a lot of uncertainty in future projections of Californian precipitation associated with predicted changes in the jet stream and the midlatitude storm tracks. The question this work seeks to answer is how the changes in the frequency and the intensity of extratropical cyclones in the Pacific storm track influence future changes in Californian precipitation. The authors used an objective cyclone identification method applied to 25 CMIP5 models for the historical and RCP8.5 simulations and investigated the changing relationships between storm frequency, intensity and precipitation. Cyclone data from the historical simulations and differences between the historical and RCP8.5 simulations were used to “predict” the modeled precipitation in the RCP8.5 simulations. In all models, the precipitation predicted using historical relationships gives a lower future precipitation change than the direct model output. In the future, the relationship between track density and precipitation indicates that for the same number of tracks, more precipitation is received. The relationship between track intensity and precipitation (which is quite weak in the historical simulations) does not change in the future. This suggests that other sources, likely enhanced moisture availability, are more important than changes in the intensity of cyclones for the rainfall associated with the storm tracks.

Full access
Damien Irving, Ian Simmonds, and Kevin Keay

Abstract

Mesoscale cyclones play an important role in the weather and climate of the Southern Hemisphere (SH) mid-to-high latitudes. However, the relatively small size and short lifetime of these systems, combined with the lack of available conventional data in this region, means that there is a poor understanding of their climatological characteristics. In this study, the University of Melbourne cyclone-finding algorithm was applied to relatively high-resolution scatterometer-derived surface pressure fields, obtained from the Department of Atmospheric Sciences at the University of Washington, to produce a decade-long (1999–2008) seasonal climatology of mesoscale cyclone activity over the ice-free regions of the Southern Ocean.

The frequency of mesoscale cyclone activity was found to be highest just to the north of the sea ice zone, reaching a maximum over the Amundsen and Bellingshausen Seas (ABS), while the southern Indian Ocean was associated with systems of the largest depth, intensity, and momentum flux at the ocean surface. These spatial patterns in mean mesoscale cyclone characteristics showed a broad resemblance to those reported in existing synoptic-scale cyclone climatologies. Maximum wind speed data indicated that SH polar lows may be more frequent than the current literature suggests, while strong positive trends identified in mesoscale cyclone frequency over the ABS may represent a contributing factor to the rapid warming observed in that region over recent years. Partial correlation analyses indicated a link between mesoscale cyclone frequency and the southern annular mode.

Full access
Ian Simmonds, Craig Burke, and Kevin Keay

Abstract

The Arctic region has exhibited dramatic changes in recent times. Many of these are intimately tied up with synoptic activity, but little research has been undertaken on how the characteristics of Arctic cyclones have changed. This paper presents a comprehensive analysis of Arctic (here defined as the domain north of 70°N) cyclones diagnosed with the Melbourne University cyclone tracking scheme applied to the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR (NCEP1) and NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP)-II (NCEP2) reanalysis sets (the last two extending to the end of 2006). A wide variety of cyclone characteristics is presented as befits these complex features.

In winter the highest density of cyclones is found between Norway and Svalbard and to the east to the Barents and Kara Seas, and significant numbers are found in the central Arctic. In summer the greatest frequencies are found in the central Arctic. The total number of cyclones identified in the ERA-40 record exceeds those in the two NCEP compilations. The mean size of cyclones shows similar maxima in the central Arctic in both winter and summer. By contrast, the greatest mean system depth in winter (in excess of 8 hPa) is found to the southeast of Greenland, although average depths exceed 6 hPa over a considerable portion of the basin. In summer the deepest cyclones are found in the central portion of the Arctic.

The analysis shows that the total number of cyclones in winter exceeds that in summer, a result in contrast to earlier studies. This difference comes about primarily due to the greater numbers of “open strong” systems in winter in all reanalyses. Cyclones in this category are associated with very active synoptic situations; it is of importance that they be included in cyclone counts but would not be considered in many cyclone identification schemes. Since 1979 neither the ERA-40 nor the NCEP2 sets show significant trends in any of the cyclone variables considered. However, over the entire record starting in 1958 the NCEP1 reanalysis exhibits a significant increase in summer cyclone frequency (due mainly to the increase in closed strong systems). Both NCEP1 and ERA-40 also reveal significant increases in the number of summer closed strong cyclones, as well as in their mean depth and intensity in that season.

Interannual variations in Arctic cyclone numbers are closely related to the Arctic Oscillation (AO) index in the full reanalyses records. An even stronger relationship is found between the AO and the number of deep cyclones. These relationships have still held in the last decade when the AO has returned to more normal values but the summer and fall sea ice extent has continued to decrease.

Full access
Ian Simmonds, Kevin Keay, and Eun-Pa Lim

Abstract

The recent NCEP–Department of Energy (DOE) Reanalysis-2 update of the original NCEP–NCAR dataset provides what is arguably the highest quality analyses spanning two decades available for the high southern latitudes. It therefore offers an excellent starting point from which to assemble a modern, comprehensive, and reliable picture of synoptic activity in the subantarctic region. This set, covering the “modern satellite” era from January 1979 to February 2000, is used herein. In addition, the exploration in this study has been conducted with sophisticated feature-tracking and trajectory analysis software.

It is shown that the high southern latitude cyclone system density is greatest in the Indian Ocean and to the south of Australia near, or to the south of, 60°S. The numbers in winter exceed those in summer, except over a few, but important, regions such as the Bellingshausen Sea. The Antarctic coastal region is confirmed as one of high cyclonicity, as is that in the northern part of the Antarctic Peninsula and over and to the north of Drake Passage. Cyclolysis is much more confined to the near-coastal region. The mean intensity, radius, and depth of subantarctic cyclones assume their largest values near 60°S.

It is shown that the rate of change of cyclone central pressure is not a particularly useful gauge of intensification in the Southern Hemisphere, where large spatial variations of climatological pressure are found. When appropriate adjustments are made, it is found that the “corrected” central pressure of cyclones is seen to increase along the track for most systems found south of 45°S. The paper also documents the range of starting points of 4-day 500-hPa trajectories that reach points on the Antarctic coast. The broad frequency distribution reflects the very energetic nature of synoptic activity in the region. The counts of cyclones in the 21 yr of NCEP–DOE analyses show negative trends over most of the subantarctic region. At the same time, however, the annual mean cyclone intensity, radius, and depth all exhibit increases.

Finally, the frequency of occurrence of rapidly developing cyclones (or “bombs”) in the subantarctic environment is determined, and it is found that they are not uncommon features. Their number shows a maximum in winter but, unlike the Northern Hemisphere situation, many are also found in summer.

Full access
Ian Simmonds, Kevin Keay, and John Arthur Tristram Bye

Abstract

Presented here is an objective approach to identify, characterize, and track Southern Hemisphere mobile fronts in hemispheric analyses of relatively modest resolution, such as reanalyses. Among the principles in its design were that it should be based on broadscale synoptic considerations and be as simple and easily understood as possible. The resulting Eulerian scheme has been applied to the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA)–Interim and a climatology of frontal characteristics, at both the 10-m and 850-hPa levels, derived for the period 1 January 1989–28 February 2009. The knowledge of the character of these features is central to understanding weather and climate over the hemisphere.

In both summer and winter the latitude belt 40°–60°S hosts the highest frequency of frontal points, but there are significant zonal asymmetries within this band. The climatology reveals that the longest fronts are in the Indian Ocean where mean lengths exceed 2000 km. The mean frontal intensity over the hemisphere tends to be greater at 850 hPa than at 10 m, and greater in winter than in summer. The frontal intensity also shows its maximum in the Indian Ocean. In the mean, the meridional tilt of these fronts is northwest–southeast over much of the midlatitudes and subtropics, and increases with latitude toward the equator. The tilts are of overwhelmingly opposite sign in the coastal Antarctic and subantarctic regions.

Broadly speaking, the number of fronts and their mean length and mean intensity exhibit maxima in winter in the midlatitudes (30°–50°S), but show a sizeable semiannual variation (maxima in fall and spring) during the year at higher latitudes.

Full access
Helena A. Flocas, Ian Simmonds, John Kouroutzoglou, Kevin Keay, Maria Hatzaki, Vicky Bricolas, and Demosthenes Asimakopoulos

Abstract

In this study, an updated and extended climatology of cyclonic tracks affecting the eastern Mediterranean region is presented, in order to better understand the Mediterranean climate and its changes. This climatology includes intermonthly variations, classification of tracks according to their origin domain, dynamic and kinematic characteristics, and trend analysis. The dataset used is the 1962–2001, 2.5° × 2.5°, 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The identification and tracking of the cyclones was performed with the aid of the Melbourne University algorithm. It was verified that considerable intermonthly variations of track density occur in the eastern Mediterranean, consistent with previous studies for the entire Mediterranean, while further interesting new features have been revealed. The classification of the tracks according to their origin domain reveals that the vast majority originate within the examined area itself, mainly in the Cyprus area and the southeastern Aegean Sea, while the tracks that originate elsewhere most frequently enter from the west. Deeper cyclones follow the southwest track originating from the area between Algeria and the Atlas Mountains. A greater size characterizes the westerly tracks (southwest, northwest, and west), while the northwest tracks propagate faster over the study area. A negative trend of the track frequency was found on an annual basis that can be mostly attributed to the winter months, being associated with variations in the baroclinicity. This negative trend is more prominent for the westerly and northeasterly tracks, as well as for those originating in the northern part of the examined area.

Full access
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.

Full access
Pandora Hope, Kevin Keay, Michael Pook, Jennifer Catto, Ian Simmonds, Graham Mills, Peter McIntosh, James Risbey, and Gareth Berry

Abstract

The identification of extratropical fronts in reanalyses and climate models is an important climate diagnostic that aids dynamical understanding and model verification. This study compares six frontal identification methods that are applied to June and July reanalysis data over the Central Wheatbelt of southwest Western Australia for 1979–2006. Much of the winter rainfall over this region originates from frontal systems. Five of the methods use automated algorithms. These make use of different approaches, based on shifts in 850-hPa winds (WND), gradients of temperature (TGR) and wet-bulb potential temperature (WPT), pattern matching (PMM), and a self-organizing map (SOM). The sixth method was a manual synoptic technique (MAN). On average, about 50% of rain days were associated with fronts in most schemes (although methods PMM and SOM exhibited a lower percentage). On a daily basis, most methods identify the same systems more than 50% of the time, and over the 28-yr period the seasonal time series correlate strongly. The association with rainfall is less clear. The WND time series of seasonal frontal counts correlate significantly with Central Wheatbelt rainfall. All automated methods identify fronts on some days that are classified as cutoff lows in the manual analysis, which will impact rainfall correlations. The front numbers identified on all days by the automated methods decline from 1979 to 2006 (but only the TGR and WPT trends were significant at the 10% level). The results here highlight that automated techniques have value in understanding frontal behavior and can be used to identify the changes in the frequency of frontal systems through time.

Full access