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Han-Ching Chen, Chung-Hsiung Sui, Yu-Heng Tseng, and Bohua Huang

Abstract

This study investigates the sudden reversal of anomalous zonal equatorial transport above thermocline at the peak phase of ENSO. The oceanic processes associated with zonal transport are separated into low-frequency ENSO cycle and high-frequency oceanic wave processes. Both processes can generate a reversal of equatorial zonal current at the ENSO peak phase, which is a trigger for the rapid termination of ENSO events. For the low-frequency process, zonal transport exhibits slower and basinwide evolution. During the developing phase of El Niño (La Niña), eastward (westward) transport prevails in the central-eastern Pacific, which enhances ENSO. At the peak of ENSO, a basinwide reversal of the zonal transport resulting from the recharge–discharge process occurs and weakens the existing SST anomalies. High-frequency zonal transport presents clear eastward propagation related to Kelvin wave propagation at the equator, reflection at the eastern boundary, and the westward propagating Rossby waves. The major westerly wind bursts (easterly wind surges) occur in late boreal summer and fall with coincident downwelling (upwelling) Kelvin waves for El Niño (La Niña) events. After the peak of El Niño (La Niña), Kelvin waves reach the eastern boundary in boreal winter and reflect as off-equatorial Rossby waves; then, the zonal transport switches from eastward (westward) to westward (eastward). The high-frequency zonal transport can be represented by equatorial wave dynamics captured by the first three EOFs based on the high-pass-filtered equatorial thermocline. The transport anomaly during the decaying phase is dominated by the low-frequency process in El Niño. However, the transport anomaly is caused by both low- and high-frequency processes during La Niña.

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Chang-Hoi Ho, Jong-Jin Baik, Joo-Hong Kim, Dao-Yi Gong, and Chung-Hsiung Sui

Abstract

The present work examines interdecadal variations of typhoon tracks in the western North Pacific (WNP) during the boreal summer (June–September) for the period 1951–2001. Typhoon tracks are expressed as percentage values of the total number of typhoon passages into a 5° × 5° latitude–longitude grid box with respect to the total number of typhoons formed in the WNP. The analysis period is divided into two interdecadal periods: ID1 (1951–79) and ID2 (1980–2001). From ID1 to ID2, typhoon passage frequency decreased significantly in the East China Sea and Philippine Sea, but increased slightly in the South China Sea. The time series of typhoon passage frequency over the East China Sea and South China Sea further reveal a regime shift in the late 1970s, while those over the Philippine Sea indicate a continuous downward trend of −9% decade−1.

The interdecadal changes in typhoon tracks are associated with the westward expansion of the subtropical northwestern Pacific high (SNPH) in the late 1970s. The expansion of the SNPH to the southeast coast of Asia may result in a larger elliptic pathway of typhoon migration. This is consistent with the westward shift of the typhoon tracks from ID1 to ID2, resulting in an increase of typhoon passage frequency in the South China Sea and a decrease in the East China Sea. The change of typhoon tracks is partly due to the westward shift of major typhoon formation regions associated with a warmer sea surface temperature in the South China Sea. The decreasing typhoon passage frequency over the Philippine Sea is due to less typhoon formation in recent decades. This is consistent with the decreasing cyclonic relative vorticity in the lower troposphere.

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Bin Wang, Michela Biasutti, Michael P. Byrne, Christopher Castro, Chih-Pei Chang, Kerry Cook, Rong Fu, Alice M. Grimm, Kyung-Ja Ha, Harry Hendon, Akio Kitoh, R. Krishnan, June-Yi Lee, Jianping Li, Jian Liu, Aurel Moise, Salvatore Pascale, M. K. Roxy, Anji Seth, Chung-Hsiung Sui, Andrew Turner, Song Yang, Kyung-Sook Yun, Lixia Zhang, and Tianjun Zhou

Abstract

Monsoon rainfall has profound economic and societal impacts for more than two-thirds of the global population. Here we provide a review on past monsoon changes and their primary drivers, the projected future changes, and key physical processes, and discuss challenges of the present and future modeling and outlooks. Continued global warming and urbanization over the past century has already caused a significant rise in the intensity and frequency of extreme rainfall events in all monsoon regions (high confidence). Observed changes in the mean monsoon rainfall vary by region with significant decadal variations. Northern Hemisphere land monsoon rainfall as a whole declined from 1950 to 1980 and rebounded after the 1980s, due to the competing influences of internal climate variability and radiative forcing from greenhouse gases and aerosol forcing (high confidence); however, it remains a challenge to quantify their relative contributions. The CMIP6 models simulate better global monsoon intensity and precipitation over CMIP5 models, but common biases and large intermodal spreads persist. Nevertheless, there is high confidence that the frequency and intensity of monsoon extreme rainfall events will increase, alongside an increasing risk of drought over some regions. Also, land monsoon rainfall will increase in South Asia and East Asia (high confidence) and northern Africa (medium confidence), decrease in North America, and be unchanged in the Southern Hemisphere. Over the Asian–Australian monsoon region, the rainfall variability is projected to increase on daily to decadal scales. The rainy season will likely be lengthened in the Northern Hemisphere due to late retreat (especially over East Asia), but shortened in the Southern Hemisphere due to delayed onset.

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