Relationship between Typhoon Activities and ENSO Evolution in 2009/2010

When a tropical disturbance in the western North Pacific is developed to a tropical depression and further a tropical storm, many atmospheric and oceanic factors including vertical wind shear, upper level outflow, monsoon trough and the phase of the Madden-Julian Oscillation (MJO), sea surface temperature (SST), and ocean heat content (OHC) are important. On the other hand, El Nino and Southern Oscillation (ENSO) plays a crucial role to determine the large-scale circulation pattern as well as ascending and descending air motion in the tropical Pacific. Intra-seasonal and inter-annual frequency and spatial distribution of tropical cyclone formation in the region can be greatly affected by ENSO cyclone and its evolution. But the studies focused on when and how ENSO transition is occurred, how tropical cyclone formation can be affected, are somewhat limited, though many studies of the relationship between tropical cyclone activities and ENSO (especially at the phase of El Nino or La Nina) have been carried out. The purpose of this study is to investigate the relationship between the typhoon activity and ENSO evolution, with special emphasis on the change in the large-scale atmospheric circulation pattern during the ENSO transition period. Climatological occurrence data of tropical cyclones in the western North Pacific since 1951 was used. The data was produced by the RSMC Tokyo. In order to analyze the temporal evolution of ENSO with time series, SST anomaly data within Nino 3.4 region from the U.S. NOAA was also employed. The SST anomaly data at a given month was filtered and obtained trough the three-month moving average. Large-scale atmospheric circulation patterns and their anomalies at the levels of 850, 500, and 200 hPa were analyzed by using the NCEP reanalysis data. Only fourteen typhoons with the maximum sustained wind greater than 34 knots were formed in the western North Pacific in 2010 whereas climatological annual numbers of typhoon (in period from 1981 to 2010) are between 25 and 26. The typhoon numbers in 2010 correspond to the record-breaking number in the annual typhoon numbers since 1951. The result of this study suggests that the ENSO evolution (especially in transitional case from El Nino to La Nina) speed (i.e., how fast), magnitude (i.e., how much decrease), and time (i.e., in which season the transition starts and ends) can be some important factors for suppressing tropical cyclone formation in the western North Pacific. This kind of rapid transitional change in the ENSO cycle (very similar to the “seesaw”) seems to be closely linked with vertical air motions in the tropical and subtropical pacific region directly or sometimes indirectly, affecting tropical cyclone genesis and formation processes. The DJF(December-January-February) SST anomaly (unit is degree C) in Nino 3.4 in 2010 was plus 1.6 with the peak intensity at the El Nino phase, but the SON(September-October-November) anomaly in the same year was minus 1.5 with the peak intensity at the subsequent La Nina phase. Neutral condition was lasted only for two months, AMJ (April-May-June) and MJJ (May-June-July). The SST anomaly was rapidly decreased by 3.1 degree C during only nine months. The similar case can be found in 1997/1998, exhibiting the SST anomaly decrease by 3.9 degree C during thirteen months. In 1998, annual typhoon numbers were sixteen, recording the second minimum since 1951. Widespread subtropical anticyclone circulation pattern induced by rapid ENSO transition seems to be the main reason to explain why so small numbers of typhoons were formed in 2010 and 1998.