Changes in Tropical Cyclone Activity for the Western North Pacific during the Last Decades Derived from a Regional Climate Model Simulation

The enhanced global tropical cyclone (TC) activity in the last years stimulates the research community to verify TC climatology and define changes in TC statistics over the last decades. The observations (‘best track data') of TC activity in the western North Pacific (WNP) basin are leading to the ambiguous trend results. This fact reduces the feasibility to diagnose recorded TC activity and attribute to the large-scale environmental factors. Therefore there is a strong demand for an alternative data sets, which will be sufficiently long and homogenous to derive TC climate statistics for the WNP in the last decades. We employ an atmospheric regional model (CCLM) to derive TC activity changes, by dynamically downscaling NCEP/NCAR re-analyses for the last decades. The comparison of simulated TC with more recent observations (JMA) has shown, that CCLM can successfully simulate variability of TC activity on time scales from inter-annual to inter-decadal. Changes in simulated intense tropical cyclones are associated with the large-scale patterns of Maximum Potential Intensity (MPI) and sea surface temperature (SST). Additionally, such relationship is verified and confirmed by observed TC data set (BTD provided by Japan Meteorological Agency). For the period 1948-2011, annual numbers of TC days in the region of WNP show an increasing trend, with a short decrease in the last decade. Increasing impact of the intense TCs is found in the subtropical latitudes, and weaker storms - over the South China Sea region. TC activity in the south-eastern part of the WNP shows negative tendencies. Overall, the results indicate an increase and north-westward shift of TC tracks for the period 1948-2011. The patterns of TC activity are related to MPI fields. Increasing TC trends, especially along the SE Asia coast and in the subtropical latitudes, correlate significantly with the atmospheric fields approaching towards more favourable for TC genesis conditions.