The Inter-annual Variation Of The Western North Pacific Tropical Cyclone Activity And Its Mechanism

Considering the warm pool (WP) in western North Pacific (WNP) is the place with not only the warmest water but also the strongest air-sea interaction in the globe, the subsurface temperature anomaly in the WP is identified as index to quantitatively define thermal state of the WNP. The interannual variation in tropical cyclones (TCs) and its mechanism is systematically explored in this dissertation using data analysis, dynamic diagnosis and numerical simulation. The follows are the main results:1. The relationship between subsurface temperature in the WP and TCs activity for the period 1959~2003 show that, during the warm state years in the WP, monsoon trough location shifts northwest which is accompanied by the shift of anomalous convection and cyclonic circulation center, hence more TCs tend to form at higher latitudes and further westward while the subsidence branch is located in the southeast quadrant to suppress TC genesis there. The consequences in cold years are reversed. Anomalous westerly and monsoon trough are likely to penetrate eastward and southward, so that more TCs form at that quadrant and tend to recurve northeast.2. To assess dynamic impacts of monsoonal circulation on TCs formation in different areas over the WNP, the WNP is divided into four domains. In the southwestern part of the WNP, the interannual variation in TCs number is attributed to the convergence of zonal wind in low troposphere; the latitudinal shear of zonal wind plays an important role in cyclogenesis at the northwestern and southeast quadrants over the WNP; the vertical shear of zonal wind is well correlated with annual TCs number in the southwest and southeast of the WNP. During the warm state in the WP, the wave train like PJ-EAP is induced to propagate from the East Asian to North America, accompanied by anomalous cyclonic circulation lying in the northwest of the WNP, which is favorable for cyclogenesis in this domain. On the contrary, during the cold state years, the circulation pattern is characterized by anticyclonic anomaly in the northwest of the WNP and cyclonic anomaly in the southeast of the WNP, which leads to more TCs formation at the southeast quadrant of the WNP.3. A large number of TCs forming in low latitude over the WNP is associated with 3~10 day filtered disturbances with synoptic scale. The main mode represents Mixed Rossby-gravity waves (MRG waves) in the middle and eastern Pacific equatorial area. MRG waves can experience the physical process with wavelength reduction and energy accumulation under the influences of convergence of zonal wind to the south of monsoon trough. Meanwhile, MRG waves gradually deviate from the equator and propagate northwestward, evolving into tropical-depression-type disturbances (TD-type disturbances) with cyclonic center tightly coupled with convection. The disturbances possess northeast-southwest tilt structure, which is conducive to obtain kinetic energy and heat energy to develop into TCs through the interaction with monsoon trough flow. In addition, it is found that the regional variation of low tropospheric wave transition is well correlated with the thermal states in the WP. When the WP is in the warm phase, monsoon trough lie in the west of the WNP, which induces the westward shift of the waves transition point along the equator. On the contrary, during the cold phase of the WP, the active convection and monsoon trough extend eastward further to the central Pacific, which produce eastward displacement of wave transition location. The interannual variation in TCs formation location is partly ascribed to regional variation of transition of tropical waves.4. The Madden-Julian Oscillation (MJO) can modulate the TCs activity over the WNP. When MJO westerly prevails in the west of WNP, TCs is likely to form in that area through the transition of barotropic energy in the high and low frequency waves, compared with MJO easterly. The probability of cyclogenesis should increase (reduce) with enhanced MJO westerly (easterly). However, the effects of modulation of MJO would be reduced in the eastern part of the WNP. Besides, the latitudinal shift of MJO could change the meridional location of monsoon trough, and modulate TCs mean location. In addition, during the warm years of the WP, MJO event is frequent and westerly is active in the west of the WNP, which benefits to TCs formation, while the situation is reversed in the cold years of the WP.5. The atmospheric model ECHAM5 is used to conduct the control and sensitivity simulations, and the results suggest that model has good skill in simulating geographic distribution of monsoon trough over the WNP responding to sea surface temperature. Despite the reduction of simulated TCs number due to coarse resolution and deficiency in parameterization process, the TCs geographic and seasonal distribution is reasonable. The sensitivity simulations show that the shift westward (eastward) of monsoon trough, which corresponds to the warm (cold) state in the WP, can cause the mean location of TCs genesis westward (eastward).