Simulated impacts of two types of ENSO events on tropical cyclone activity in the western North Pacific: large-scale atmospheric response

Abstract

The present paper uses an atmospheric general circulation model to explore large-scale atmospheric response to various El Niño-Southern Oscillation events associated with tropical cyclone (TC) activity in the western North Pacific. The simulated response is basically consistent with and confirms the observed results. For eastern Pacific warm (EPW) event, anomalously wet ascent occurs over the tropical central/eastern Pacific and dry descent is over the western Pacific. This Walker circulation is associated with anomalous low-level convergence, reduced vertical wind shear (VWS), and enhanced genesis potential index (GPI) in the southeast sub-region. These are consistent with the observed increase of the TC formation in the southeast sub-region but decrease in the northwest sub-region during July–September (JAS) and the increase in the southwest and northwest sub-regions during October–December (OND). In addition, the strong westerly anomalies of the TC steering flow prevail in the East Asian coast, suppressing the TC northwestward or westward tracks. For eastern Pacific cold (EPC) event, all of the simulated variables show almost a mirror image of EPW. For central Pacific warm event, the anomalous Walker circulation shifts westward because of the westward shift of the maximum SST anomaly forcing. The anomalous subsidence associated with the western branch of the Walker circulation during OND shifts northward to the South China Seas, resulting in a decrease of the TC genesis there. The TC steering flow patterns during JAS are favorable for TCs to make landfall over Japan and Korea. Compared with EPC, the descending motion in the central/eastern Pacific is much stronger for central Pacific cold (CPC) event, accompanied by more enhanced VWS and reduced GPI in the southeast sub-region. Therefore, CPC provides a more adverse environment to the TC formation there during JAS and OND, consistent with the observed decrease of TC formation there. Moreover, the easterly anomalies of the TC steering flow dominate the tropics during JAS, enhancing TC activity in the east coast of China. Additionally, the convection over the western Pacific moves to the South China Sea during OND, favoring the TC genesis there.

Appendices

Appendix 1: Location of TC formation in observations

The composited TC number anomalies with the two types of ENSO events during the peak season of JAS and late season of OND over the different sub-regions are presented in Fig. S1. Dot (cross) filled bars indicate statistically significant above (below) climatology at the 90 % confidence level. During JAS, EPW corresponds to a significant increase (decrease) of TC genesis number in the SE (NW) sub-region. The TC genesis number in EPC year is reduced in the SW sub-region, whereas it is enhanced in the NW and NE sub-regions. CPW shows no statistically distinct difference of the TC genesis from climatology in all sub-regions, while CPC suppresses the formation in the SE sub-region. For the late season of OND, there is a significant decrease of the TC genesis number in the SW and NW sub-regions during EPW years, but an increase in the SE sub-region during CPW years. ENSO influence on TCs formed in the SCS is pronounced only during the late season of OND. CPW (CPC) is associated with a decreased (increased) TC formation in the SCS during OND.

Appendix 2: TC track density and steering flow in observations

We calculate the TC track density in the WNP by counting the number of TCs forming within and passing through each 5°′5° grid box for a given season. The TC steering flow is calculated as the vertically-integrated wind from 850 to 300 hPa, normalized by (850–300-hPa). Figures S2 and S3 show the composites of TC track density and steering flow anomalies for EPW, EPC, CPW and CPC during JAS and OND, respectively. During JAS, associated with the positive and negative anomalies of TC track density in the SE and NW sub-regions for EPW events are the easterly and westerly steering flow anomalies, respectively (Fig. S2a). For EPC events, the positive anomalies of TC track density are found in the northern portion of the WNP (Fig. S2b), due to the EPC-induced increase of TC genesis there (Fig. S1a). For CPW events, an anomalous anticyclonic steering flow is located over the sea east of Japan, which favors TCs to move northwestward. Meanwhile, the westerly steering flow anomalies occupy the tropics, suppressing the westward movement (Fig. S2c). For CPC events, an anomalous cyclonic steering flow occupies the sea east of China, and easterly anomalies prevail in the tropics. Associated with these TC steering flow patterns, there is a zonal dipole of TC track density anomalies with positive values in the east coast of China and negative values in the eastern part of the WNP (Fig. S2d).

During OND, an anomalous anticyclonic steering flow is found over the Philippines in EPW events, which suppresses TC activity there (Fig. S3a). An EPC year shows almost a mirror image of the EPW distribution (Fig. S3b). For CPW events, the westerly anomalies of the TC steering flow prevail in the tropical WNP, corresponding to a decrease of TC track density over Indo-China, the SCS and Philippines (Fig. S3c). For CPC events, due to the increased number of TCs formed in the SCS, the positive anomalies of TC track density are located over the southeast coast of China. The negative anomalies of TC track density are in the eastern part of the WNP due to the decrease of TCs formed there during CPC years (Fig. S3d).