- Reinterpretation of the Tropical Cyclone and Ocean Interaction Jia-Shiuan Tsai 2016-03-27 Taiwan is located at the northwestern Pacific Ocean, suffering the destructive tropical cyclones (TC, which is also called typhoon in Asia) every year. While the typhoon brings damages, it also becomes one of the popular research topics in Taiwan. I.-I. Lin, a professor of the Department of Atmospheric Sciences, National Taiwan University (NTU), has published some significant papers that described new discoveries of the interaction between tropical cyclone and ocean. Those works were a success of national and international cooperation, participated by National Taiwan Normal University (NTNU) and Academic Sinica in Taiwan, together with groups from the USA, China, and Hong Kong. The ocean condition has been considered as a key factor for TC research, and it may affect TC development and intensification; however, there’s still no complete theory that can well explain the TC-Ocean interaction. Traditionally, the warm ocean with high sea surface temperature (SST) is supposed to provide huge sensible heat and latent heat, and then transfer to the energy for TC. Thus, a warmer ocean condition is believed to nourish more destructive TCs. On the other hand, while the TC passes through an ocean, the high wind stress it brings will accelerate the upper ocean and mix colder water from lower layer, and the SST will drop. Recently, the damage caused by typhoon is more severe, which is believed relative to the global warming. However, I.-I. Lin from NTU and Johnny C.L. Chan from City University of Hong Kong published an article on Nature Communications in 2015 [1], stating that the typhoon destructive potential has decreased in recent decades. It was found that the high SST provides a favorable environment for typhoon development, and the typhoon intensity is strengthened dramatically, but the annual TC numbers in typhoon season and the average TC duration decreased at the same time. (Fig. 1) Under global warming, the atmospheric circulation, such as easterly trade wind and Walker cell, is strengthening; thus, both the lower-tropospheric trade wind and the upper-tropospheric westerlies enhanced, and the vertical wind shear increased. Accompany with the low-level relative vorticity in typhoon genesis region decreasing, the atmospheric condition is considered becoming worse for typhoon genesis. Hence, the genesis position of typhoons was found shifted northwestward, which was shown in Fig. 2. When the TC formed westward, the path that TC traveled is shorter than normal, and the duration of a typhoon would be less. Thus, the Power Dissipation Index (PDI), which is a quantified index for the typhoon destructive potential and identified to the combination among the typhoon frequency, duration and intensity, illustrated to be decreased by 35% in recent decades, shown as Fig. 1. Under global warming scenario, PDI could also decrease 15%. Another research, also published on Nature Communications by a Taiwan-China cooperation [2], found that the ocean condition will even suppress TC intensification under global warming. They investigated outputs from 22 climate models under global warming scenario and focused on the western North Pacific (WNP) and the North Atlantic (NA) regions. When the TC passes over the ocean, there is a TC-induced ocean cooling effect (OCE), strong TC wind mix the colder subsurface (deeper) water with the surface water to reduce the SST, so that the available air-sea sensible and latent heat fluxes will decrease. However, the subsurface ocean temperature warms slower than the surface ocean, and the vertical temperature gradient is then sharpened. As a result, the OCE, which is a function of the initial ocean vertical profile and TC condition, is stronger under global warming, and it will Another research, also published on Nature Communications by a Taiwan-China cooperation [2], found that the ocean condition will even suppress TC intensification under global warming. They investigated outputs from 22 climate models under global warming scenario and focused on the western North Pacific (WNP) and the North Atlantic (NA) regions. When the TC passes over the ocean, there is a TC-induced ocean cooling effect (OCE), strong TC wind mix the colder subsurface (deeper) water with the surface water to reduce the SST, so that the available air-sea sensible and latent heat fluxes will decrease. However, the subsurface ocean temperature warms slower than the surface ocean, and the vertical temperature gradient is then sharpened. As a result, the OCE, which is a function of the initial ocean vertical profile and TC condition, is stronger under global warming, and it will Another research, also published on Nature Communications by a Taiwan-China cooperation [2], found that the ocean condition will even suppress TC intensification under global warming. They investigated outputs from 22 climate models under global warming scenario and focused on the western North Pacific (WNP) and the North Atlantic (NA) regions. When the TC passes over the ocean, there is a TC-induced ocean cooling effect (OCE), strong TC wind mix the colder subsurface (deeper) water with the surface water to reduce the SST, so that the available air-sea sensible and latent heat fluxes will decrease. However, the subsurface ocean temperature warms slower than the surface ocean, and the vertical temperature gradient is then sharpened. As a result, the OCE, which is a function of the initial ocean vertical profile and TC condition, is stronger under global warming, and it will decrease the SST and lead to suppress the TC intensification. There is another interesting research about the typhoon-El Niño relationship on Scientific Report by Taiwan-USA cooperation. [3] A Gaia -like process was found in the El Niño-TC relationship, which is shown at Fig. 3. During El Niño, the eastern Pacific is warmer than normal, providing a favorable place for TC genesis and causing a positive feedback for TC. The TC genesis region shifts southeastward and TC would travel longer distance before it dissipates. As consequence, the TC could achieve higher intensity during El Niño. However, this research discovered a negative feedback to TC intensification that works as a damper to restrain typhoons from over-intensified. During El Niño, there is a strong pre-existing subsurface shoaling over the western North Pacific Ocean, and the subsurface water was much colder than the normal years. The OCE during El Niño reduced the heat flux supply chain for TC, providing a damper of TC intensification. The formation of TC is related to both atmospheric and ocean conditions, and the mechanisms are complex. These investigations analyzed the huge climate data, and give a reinterpretation of the TC-ocean interaction. Reference: [1] I-I Lin and Johnny C.L. Chan, Recent decrease in typhoon destructive potential and global warming implications, Nat. Commun., 6:7182, 2015. [2] Ping Huang, I-I Lin, Chia Chou and Rong-Hui Huang, Change in ocean subsurface environment to suppress tropical cyclone intensification under global warming, Nat. Commun., 6:7188, 2015. [3] Zhe-Wen Zheng, I.-I. Lin, Bin Wang, Hsiao-Ching Huang & Chi-Hong Chen, A Long Neglected Damper in the El Niño—Typhoon Relationship: a ‘Gaia-Like’ Process, Sci. Rep. 5:11103, 2015. Gaia: a hypothesis states there are self-regulation mechanisms among the organisms and inorganic surroundings on Earth, keeping our planet suitable for life survival. Reference Matthew H. Alford, Thomas Peacock, Jennifer A. MacKinnon, Jonathan D. Nash, Maarten C. Buijsman, Luca R. Centuroni, Shenn- Yu Chao, Ming-Huei Chang, David M. Farmer, Oliver B. Fringer, Ke- Hsien Fu, Patrick C. Gallacher, Hans C. Graber, Karl R. Helfrich, Steven M. Jachec, Christopher R. Jackson, Jody M. Klymak, Dong S. Ko, Sen Jan, T. M. Shaun Johnston, Sonya Legg, I-Huan Lee, Ren-Chieh Lien, Matthieu J. Mercier, James N. Moum, Ruth Musgrave, Jae-Hun Park, Andrew I. Pickering, Robert Pinkel, Luc Rainville, Steven R. Ramp, Daniel L. Rudnick, Sutanu Sarkar, Alberto Scotti, Harper L. Simmons, Louis C. St Laurent, Subhas K. Venayagamoorthy, Yu-Huai Wang, Joe Wang, Yiing J. Yang, Theresa Paluszkiewicz & Tswen-Yung (David) Tang. The formation and fate of internal waves in the South China Sea, Nature, vol. 521, pp.65, 2015. DOI: 10.1038/nature14399. Assistant Professor Ming-Huei Chang Institute of Oceanography minghueichang@ntu.edu.tw Professor Sen Jan Institute of Oceanography senjan@ntu.edu.tw Professor Joe Wang Institute of Oceanography wang@oc.ntu.edu.tw Professor Yiing Jang Yang Institute of Oceanography yjyang67@ntu.edu.tw Professor Tswen-Yung (David) Tang Institute of Oceanography (Deceased) Life cycle of the internal waves in the South China Sea