A Preliminary Climatology of Extratropical Transitions in the Southwest Indian Ocean Kyle S. Griffin Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York ABSTRACT Tropical cyclones (TCs) in the Southwest Indian Ocean (SWIO) historically have received less attention than TCs in other ocean basins. One possible reason for the comparatively few TC-related studies in the SWIO is that significant in situ data limitations have only been partially addressed since the beginning of the modern satellite era. The absence of significant landmasses poleward of 25 S that can be impacted by TCs experiencing extratropical transition (ET) is a possible second factor in the fewer available TC-related studies in the SWIO. The purpose of this presentation will be to present the results of a SWIO TC/ET climatology. A SWIO climatological analysis is constructed for the TC season (defined to be July through June) for 1999-2009 using the RSMC La Reunion and JTWC best-track datasets. Gridded data from the ECMWF interim reanalysis are used to provide a synoptic overview of the individual events. An analysis of the synoptic patterns and the geographical distribution of SWIO ET events will be presented utilizing composite plots from ESRL/PSD. The statistical results will then be briefly compared to ET climatologies from other ocean basins. 1. Introduction Of all the tropical cyclone basins around the world, the Southwest Indian Ocean continues to be the least studied in peer-reviewed literature. This seeming lack of scholarly interest in the region is perhaps not without cause. To begin, data records of both observations and historical tropical cyclones are poor and unreliable at times, especially in the pre-satellite era. Also, tropical cyclones (TC) that form in the region do not frequently make landfall, and even when they do, the countries affected by such tropical cyclones are certainly not home to a large meteorological community. However, despite these difficulties, the Southwest Indian basin has the potential to yield a set of results unachievable in any other basin. One of the significant aspects of examining the Southwest Indian Ocean is the lack of significant localized effects in the primary regions of extratropical transition (ET). The poleward extent of land within the basin is only about 25ºS, at the southern tip of Madagascar. Even upstream, the most southern tip of South Africa only extends to approximately 35ºS, thus leaving the vast majority of the mid-latitudes free of any possibly orographic effects. As such, the purity of this basin makes it an excellent place to examine ET, from both a climatological and a storm-centric perspective. This paper will examine the basin from that larger scale and provide a preliminary climatology spanning the previous eleven tropical cyclone seasons. 2. Data and Methodology While Regional Specialized Meteorological Centers (RSMC) around the world maintain historical tropical cyclone records (including RSMC La Reunion for the Southwest Indian Ocean basin) not all contain as much detail as the HURDAT databases maintained for tropical cyclones within the RSMC Miami area of responsibility. As such, there is no official record of extratropical transitions, either in the sense of their timing or even their occurrence. The Joint Typhoon Warning Center (JTWC) notes if a system becomes extratropical at the last advisory, but these notations are not always carried over into the post-season archives. Also, as there is no objective definition of ET (Hart and Evans 2001, 2003), a general, subjective process was used to determine which TCs to include in this climatology. Maps of sea level pressure, surface wind, and 1000-500hPa thickness (obtained from ECMWF Interim reanalyses, 1.5º resolution) were examined every 12 hours for systems in which a recurvature was
observed or which traveled noticeably poleward out of the tropics. Such a method does potentially allow some systems to be missed, but these systems would likely be very small in number and not portray a representative ET. Using these maps, approximate beginning and end times were established to the nearest 12 hourly interval for each ET event, with estimations favoring a longer ET period to ensure the full scope of transitions was captured. This strategy, based off both RSMC La Reunion and JTWC best track data (advisory data only for the 2009 season), found 42 TCs that experienced ET during the eleven season period encompassing the tropical cyclone seasons from 1999 through 2009. An additional system, termed a subtropical disturbance by RSMC La Reunion, also underwent ET, but was not included in this climatology due to its unprecedented classification and lack of an official name. In the process of recording ET events, a difference was noticed between events occurring in the western portion of the basin versus those observed in the eastern portion of the basin. The western region - the Madagascar region - is defined to include systems which recurved west of 55ºE, or if no recurvature occurred, finished ET west of that longitude. An eastern region was also defined from 55ºE to 90ºE (the eastern boundary of RSMC La Reunion's area of responsibility). ET events were also classified by the post-et evolution of the cyclone, specifically whether its central pressure continued rising or if the system restrengthened in its extratropical phase. Each ET event was classified under this general definition of strengthening or weakening, as well as by the region that the transition occurred in. Minding these groupings, mean and anomaly composites of several synoptic parameters were created using Earth System Research Laboratory/Physical Science Division's plotting tools in order to distinguish the different synoptic patterns that dominate each style of event. 3. Event Distribution Each tropical cyclone season in the Southern Hemisphere lasts from July 1 st through June 30 th, corresponding with the seasonal equivalents of the northern hemisphere season. Historical data show a focus in tropical cyclone activity in the Southwest Indian Ocean during the period from late November through April, though systems can occur year-round. During the 1999-2009 period observed in this study, ET events occurred solely during the months from December through April, while TC activity spanned every month of the year. Fig. 1 shows this distribution of TC occurrence, which broadly peaks during the months of January, February, and March. This peak coincides with the peak numbers of ET events, also shown in Fig. 1 during the months of January and February. To compare this with the Atlantic basin and other better-studied basins of the Northern Hemisphere, a simple six month shift is needed in order to make such comparisons. Doing so shows the Southwest Indian Ocean TC activity peak occurs during the boreal equivalents of July, August, and September. More interestingly, however, is that the ET peak corresponds to boreal July and August, a full one to two months ahead of the Atlantic basin's numerical peak in mid-september (Hart and Evans, 2001). The difference between climatological maxima in ET probability is even greater, as the Southwest Indian basin peaks with figures just over 50% in January and February, as shown in Table 1, while the Atlantic probabilistic peak occurs during October at a slightly lower probability. This seasonally-adjusted difference could be the result of several differences between the two basins, with perhaps the most significant and most likely difference being the presence of mid-latitude land mass in the Atlantic basin, often forcing many early season tropical cyclones that would otherwise develop and recurve (as they do in the Southwest Indian Ocean) to landfall and weaken, thus forcing the ET peak later into the season. Another potential cause of that type of discrepancy may lie in the presence of stronger oceanic mid-latitude cyclones during the heart of the Southwest Indian Ocean season. These mid-latitude troughs and their associated cold fronts could create more opportunities for ET events or allow the conditions needed for ET to occur to reach farther equatorward than the mid-latitude pattern in the Atlantic typically does. A more detailed examination of these differences and a larger data set would be needed before stating any of these conjectures with certainty. When considering all tropical cyclones in the basin, ET occurred in
approximately 36.5% (42/115) cases, a figure nearly identical to the long-term value for the Western Pacific basin (Klein 1997). Values of TC and ET events per year are presented in Fig. 2, and when averaged yield a mean annual value of approximately 3.8 of the 10.5 TC events per year resulting in ET. 4. Synoptic patterns associated with ET events As previously stated, ET events were classified using two criteria: post-et evolution and region of recurvature. A plot of the distribution of events within these calculations can be noted in Fig. 3. When an ET event occurs which results in a strengthening system, it commonly coincides with the approach of a mid-latitude trough from the west (Sinclair 2002) and is accompanied by a frontal zone. In viewing the MSLP anomalies of all ET events in Fig. 4a, there is no strongly evident representation of an anomalous mid-latitude trough, though both a common location of recurving TCs and the subtropical ridge are presented quite well. The surface features that distinguish between the cyclones that strengthen after ET and those that do not become evident in Figs. 4b and 4c. A composite of strengthening storms shows an anomalous pressure trough around 40ºE, a common place for midlatitude cyclones or their trailing frontal boundaries to queue before sweeping in to capture and enhance a TC undergoing ET. Similarly, for events when the TC does not strengthen after ET (Fig. 4c), composites show a longitudinally expanded anomaly associated with the subtropical ridge, indicative of a lack of significant mid-latitude troughs. Most ET events in this situation are likely associated with decaying frontal zones that have little remaining upper level support to aid in restrengthening the cyclone. The broad subtropical high aids in blocking transitioning TCs from progressing poleward and interacting with other baroclinic zones, instead leaving the entire system to decay beneath the subtropical ridge. This setup can be seen in Fig. 5, as weaknesses in subtropical ridging can be observed near or just downstream of the specific region of transition, with the offset most easily evident in Figs. 5a and 5d. A setup presenting similar results does not appear to be common in Northern Hemisphere basins, as large landmasses poleward of the subtropical ridge likely moderate the intensity of mid-latitude cyclones, thus allowing the subtropical ridges to remain more intact. In general, this pattern seems to be typical of ET events which result in weakening extratropical systems, but a storm-relative analysis would need to be performed before making such conclusions with certainty. 5. Summary During the period from the 1999 Southern Hemisphere tropical cyclone season through the end of the 2009 season, an average of between 10 and 11 tropical cyclones each year form within or pass through the waters of the Southwest Indian Ocean. Of these storms, just under 4 experience some form of ET, which represents 36.5% of the total number of TCs per year. These values, while measured over a relatively short range of years, seem to be towards the lower end of the global spectrum, falling more in line with Northwest Pacific values presented in Klein 1997 than the 10-15% higher values from other basins such as the Atlantic, as referenced in Hart and Evans 2001. The investigation into common synoptic patterns as presented here is only in its most preliminary stages; much further analysis remains possible and is almost certain to provide a valuable basis that could potentially be used to establish the Southwest Indian Ocean as a standard basin, against which other TC basins could be compared. Regardless of its use as a baseline, the unique continent-free mid-latitude regions of the basin provide the opportunity to study numerous events in greater (computer-analysis based) detail as well as create composites charts free of any possible contamination or bias due to the presence of land masses in a subset of the cases. The opportunities of the Southwest Indian Ocean could be quite large, and it will be interesting to see future research based within this basin in coming years.
REFERENCES Evans, J. L. and R. E. Hart, 2003: Objective Indicators of the Life Cycle Evolution of Extratropical Transition for Atlantic Tropical Cyclones. Mon. Wea. Rev., 131, 909-925. Hart, R. E. and J. L. Evans, 2001: A Climatology of the Extratropical Transition of Atlantic Cyclones. J. Climate, 14, 546-564. Jones, S. C. et al, 2003: The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions. Wea. Forecasting, 18, 1052-1092. Klein, P., 1997: Extratropical Transition of western North Pacific tropical cyclones. M.S. Thesis, Dept of Meteorology, Naval Postgraduate School, 101 pp. [NTIS ADA-341-420] Sinclair, M. R., 2002: Extratropical Transition of Southwest Pacific Tropical Cyclones. Part I: Climatology and Mean Structure Changes. Mon. Wea. Rev., 130, 590-609.
TC and ET events by month, 1999-2009 30 25 20 Number of events 15 10 5 0 July August October December February September November January March April May June Month Fig 1. Distribution of TC occurrences (light green shading) and ET events (dark green shading) per month during the 1999-2009 Southwest Indian Ocean tropical cyclone seasons. No ET events during these years were noted outside of the December-April time frame.
TC and ET events by year, 1999-2009 14 12 10 Number of TCs 8 6 4 2 0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Year Fig. 2. Tropical cyclone occurrence by year (light green shading) and the number of ET events per year (dark green shading) over the period 1999-2009. ET Events by region and end result, 1999-2009 25 20 Number of Events 15 10 5 0 Madagascar Open Ocean Weak Strong Fig 3. Classification of ET events by region and post-et evolution. Sample of 42 ET events, 1999-2009. Weak refers to systems experiencing post-et weakening, and likewise for Strong. Region of Recurvature
(a) (b) Fig. 4. MSLP composite anomalies, in hpa, for (a) all 42 ET events (b) the 25 ET events resulting in post-et strengthening, and (c) the 17 ET events resulting in post-et weakening. (c)
a b c d Fig. 5. Composite anomaly maps for ET events resulting in weakeing in the SW Indian Ocean. (a) shows 500hPa height anomalies in meters for the defined Madagascar region, while (b) is the same as (a) for the defined open ocean region. (c) and (d) are MSLP anomaly maps in units of hpa for the same data sets as (a) and (b), respectively.