14D.1 TROPICAL CYCLONE DETECTIONS AND SEASONAL GENESIS VARIABLES IN ECMWF REANALYSIS DATA

Over the last decade climate model studies have provided an increasingly consistent message that global numbers of tropical cyclones (TC) will decrease in a warming world (e.g., Knutson et al. 2010, Walsh et al. 2015). Regional projections are less certain due to model limitations in generating realistic climatologies of TC-like circulations, and our ability to accurately detect them (e.g., Tory et al. 2013b,c). This uncertainty coupled with large natural variability within basins can make it difficult to identify a signal of change in TC formation on basin-wide and smaller spatial scales, especially in regions where TC formation is relatively infrequent in the current climate. An understanding of why TCs form where they do in the real atmosphere and in climate models, and whether the reasons are consistent, is important for understanding errors in model TC formation location and timing, and will help identify whether climate models are producing TCs for the right reason. However, the very large number of models and scenarios to be investigated makes it difficult to perform in-depth analyses to identify why these variables change in different climate scenarios. In this study we present simple diagnostics that provide an indication of TC formation favourability that can be illustrated in one or two summary images. These quantities define regions of TC favourability, consistent with the detected TCs, and provide insight into the tropical circulation responsible for the TC distribution. Our search for seasonally averaged variables that influence global TC formation distributions identified a potentially useful new ingredient for TC genesis indices: the meridional gradient of absolute vorticity ( ∗). We consider threshold values of the seasonally averaged variables to define geographic TC formation distributions, rather than the genesis index approach that uses complex mathematical functions of these variables to define TC formation densities (e.g., Emanuel and Nolan 2004, Emanuel 2010, Tippet et al. 2011, Korty et al. 2012a,b, Bruyere et al. 2012). Additional useful information is obtained by considering separately thresholds that define tropical depression (TD) formation distributions, and regions in which a large proportion of TDs become tropical storms (TSs, i.e., storms that exceed gale force intensity). Knowledge of the threshold quantities responsible for these boundaries at any particular geographic location provides useful insight into what supports and inhibits TD or TS formation there. With future applications to climate models in mind, the thresholds are constructed around detected distributions of TDs and TSs rather than observed storm distributions. This ensures greater consistency between the threshold/storm relationship developed in reanalysis data and its application to climate model data. In the next section the OWZP detection method is described, the new genesis index ingredient, and the threshold variables used are introduced. In Section 3 the TD threshold application and distribution is described and illustrated, and the significance of the results presented in Section 4. Section 5 contains a brief description of an additional set of thresholds used to define regions where a high proportion of TDs transition to TSs. The work is summarised in Section 6.