The Role Of Tropical Atlantic SST Anomalies In Modulating Western North Pacific Tropical Cyclone Activity

Based on tropical cyclone (TC) best-track datasets, NCEP/NCAR reanalysis, SST data from the Met Office Hadley Centre and NOAA OLR data, the connections between north tropical Atlantic (NTA) sea surface temperature (SST) anomalies and TC activity over the western North Pacific (WNP) and associated physical mechanisms are investigated, and a multiple linear regression model for the seasonal prediction of the June-October (typhoon season) WNP TC genesis frequency is developed in this study. In addition, the impact of NTA SST anomalies on South China Sea summer monsoon (SCSSM) and the influence of SCSSM on WNP TC activity on intraseasonal time scale are explored. The main results obtained are as follows:1. We demonstrate a remarkable negative correlation of WNP TC genesis frequency with the (preceding) boreal spring NTA SST anomalies. Our analysis suggests that major factors for TC genesis including distributions of large-scale vorticity and mid-tropospheric humidity are rendered unfavourable by remote teleconnections while barotropic energy conversion from the large-scale flow is suppressed. As shown in recent studies, the remote teleconnection from the Atlantic is sustained and enhanced throughout the typhoon season through local air-sea interactions.2. The simultaneous correlationship between NTA SST anomalies and WNP TC frequency is also significant. Regression analysis suggests that the warm NTA SST during typhoon season produces a zonally overturning circulation anomaly, with ascending over the tropical Atlantic/eastern Pacific and descending over the equatorial central Pacific. The anomalous descending induces a low-level anomalous anticyclonic circulation over the WNP and therefore inhabits TC formation. And vice versa.3. A multiple linear regression model for the seasonal forecast of the WNP TC genesis frequency is developed using four predictors. Predictors are derived from large-scale atmospheric and oceanic variability during previous winter and spring, including NTA SST anomaly index, western Pacific subtropical high index, India-Burma trough index and Nino3.4 index. The TC frequency of 1979-2012 hindcasted by this prediction model indicates that the model can reasonably forecast the variability of WNP TC genesis frequency. And the temporal stability of this statistical model is verified through the cross validation method.4. The NTA SST anomalies are significantly correlated with the SCSSM in spring and summer for 1979-2012 after excluding the impact of ENSO during previous winter. The remote effect of NTA SST warming (cooling) during spring and summer produces a low-level anomalous anticyclonic circulation over the WNP, therefore strengthens (weakens) the WNP subtropical high and suppresses (enhances) the SCSSM activity.5. The SCSSM activity shows a intraseasonal oscillation feature, with active and inactive phases occurring alternatively. The amount of WNP TC genesis in active phase is far more than that in inactive phase. A majority of TCs form in the west of 135°E during the active phase, whereas in the east of 135°E in the inactive phase. Due to the different large-scale steering flow, the WNP TCs entering the region of west of 135°E and south of 25°N prefer straight-moving tracks in active phase, and recurving tracks in inactive phase.