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Effects of Super Typhoons on Cyclonic Ocean Eddies in the Western North

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Effects of Super Typhoons on Cyclonic Ocean Eddies in the Western North PUBLICATIONS Journal of Geophysical Research: Oceans AUTHOR’S PREFACE Effects of super typhoons on cyclonic ocean eddies in the TO A SPECIAL western North Pacific: A satellite data-based evaluation COLLECTION 10.1002/2013JC009575 between 2000 and 2008 Liang Sun1,2, Ying-Xin Li1, Yuan-Jian Yang1,3, Qiaoyan Wu2, Xue-Tao Chen1, Qiu-Yang Li1, Yu-Bin Li4, Special Section: and Tao Xian1 Western Pacific Ocean Circulation and Climate 1Key Laboratory of the Atmospheric Composition and Optical Radiation, CAS, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, People’s Republic of China, 2State Key Laboratory of Satellite Ocean Correspondence to: Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, Hangzhou, People’s Republic of L. Sun, 3 [email protected] China, Key Laboratory of Atmospheric Sciences and Satellite Remote Sensing of Anhui Province, Anhui Institute of Meteorological Sciences, Hefei, People’s Republic of China, 4School of Applied Meteorology, Nanjing University of Information Science and Technology, Nanjing, People’s Republic of China Citation: Sun, L., Y.-X. Li, Y.-J. Yang, Q. Wu, X.-T. Chen, Q.-Y. Li, Y.-B. Li, and T. Xian (2014), Effects of super typhoons on Abstract A composite time series of the merged satellite altimeters sea surface height anomaly (SSHA) cyclonic ocean eddies in the western data and satellite-observed sea surface temperature (SST) data were used to identify eddies in the Western North Pacific: A satellite data-based North Pacific Ocean (WNPO), where there were numbers of intense typhoons. This study systematically evaluation between 2000 and 2008, J. Geophys. Res. Oceans, 119, doi:10.1002/ investigated 15 super typhoons during the period of 2000-2008 in the WNPO to study their impacts on the 2013JC009575. pre-typhoon ocean features, e.g., the cyclonic ocean eddy (COE) feature (closed contours of SSHA < 26 cm) and neutral condition (SSHA between 26 and 6 cm). Two new COEs are generated by two super typhoons, Received 7 NOV 2013 and 18 pre-existing COEs are intensified by 13 super typhoons. 5 of the 13 super typhoons each influenced Accepted 6 AUG 2014 two pre-exisiting COEs. Although the typhoon-induced maximum cooling centers had a right bias along the Accepted article online 11 AUG 2014 tracks due to wind conditions, pre-existing COEs also play a significant role in determining the strength and location of large SST cooling. Three possible factors (maximum wind speed, typhoon translation speed and the typhoon forcing time, Tf) are employed to explain the interactions. Above all, the changes of the COE geo- metric and physical parameters (e.g., effective radius, area, SST, SSHA, and eddy kinetic energy) were mostly related to the typhoon forcing time, Tf. This is because Tf is a parameter that is a combination of the typhoon’s translation speed, intensity and size. Although the typhoons may significantly impact COEs, such samples were not commonly observed. Thus, the impact of typhoon on the strength of COEs is generally inefficient. 1. Introduction When typhoons pass over the ocean, their strong winds stir the upper ocean and generate divergent outward flows through local (entrainment, vertical mixing, and upwelling) and nonlocal (horizontal advection, horizon- tal mixing, and pressure gradients) processes [Price,1981;Lin et al., 2003; Davis and Yan 2004; Han et al., 2012]. The wind stress vector of a typhoon turns clockwise with time on the right side of the track, which is roughly resonant with the current in the mixed layer. This, together with the asymmetry of wind stress, makes the upper ocean exhibit stronger sea surface temperature (SST) cooling on the right side of typhoon track [Price, 1981; Dickey and Simpson, 1983; Stramma et al., 1986; Vincent et al., 2013]. In addition to wind stress, it has been suggested that preexisting negative sea surface height anomaly (SSHA) features or cyclonic ocean eddies (COEs) play important roles in the response of the upper ocean to typhoons [Nan et al., 2005; Walker et al., 2005; Zheng et al., 2008, 2010; Liu et al., 2009]. The COEs provide a relatively unstable thermodynamic structure that easily elevates cold and nutrient-rich water. On the other hand, COEs might be influenced by typhoons in various ways. The preexisting cyclonic ocean circulation might be intensified under appropriate conditions after the passage of a typhoon [Shang et al., 2008; Sun et al., 2009, 2012; Yang et al., 2010, 2012b]. New cyclonic eddies were generated by looping trajectory typhoons in the South China Sea [Chu et al., 2000; Hu and Kawamura, 2004]. A cyclonic ocean eddy was observed in the preexisting positive SSHA area, which was result from three sequential typhoons in September 2008 [Yang et al., 2012a]. Recently, two COEs result from long forcing time of strong wind stress curl were found in two certain locations along the trails of binary typhoons [Yang et al., 2012b]. Above studies implied that certain typhoons, such as looping trajectory SUN ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 1 Journal of Geophysical Research: Oceans 10.1002/2013JC009575 Figure 1. (a) Tracks of 15 super typhoons. (b) The forcing time for typhoon Chanchu (2006). typhoons or sequential typhoons, passing over the same region, may play a notable role in intensifying or generating COEs. However, the underlining physical processes of typhoon’s impact on COEs are still not clear. Due to the harsh conditions bought out by typhoons, little in situ data were available to study the typhoon-ocean interactions. Fortunately, microwaves can penetrate clouds with little attenuation, pro- viding an uninterrupted view of the ocean surface accompanying a typhoon [Wentz et al., 2000]. The responses of COEs to typhoons were mostly studied by using microwave satellite data. However, the features of COEs observed by satellite are very limited. COEs are mostly represented by SST and SSHA. In this paper, we explore the response of COEs to typhoon in a more complete way. COEs are gener- ally represented by geometric parameters and physical parameters. Geometric parameters include the effective radius, the area, and the distance from the COE center to the typhoon center. Physical param- eters include SST, SSHA, and eddy kinetic energy (EKE) of the COE. Then, the responses of COEs to typhoons are studied by analyzing the correlation of these COE parameters and the properties of typhoons (i.e., intensity, translation speed, and forcing time). The western North Pacific Ocean (WNPO) is a region with high frequency of typhoons and abundant COEs observed all year round. This paper primarily investigated the activity of 20 COEs influenced by 15 super typhoons during the period of 2000–2008 in the WNPO (Figure 1a). Supper typhoons are defined as storms whose maximum 1 min sustained winds speed exceeding 59 m/s, see section 2 for details. The rest of the paper is organized as follows. In section 2, data and data processing method are presented. Two typical activities of COEs induced by typhoons Dujuan (2003) , Sudal (2004), and Chanchu (2006) are described in section 3. In section 4, the influences of typhoons on COEs are fully explored. Finally, a summary and discus- sion are presented in section 6. 2. Data and Method 2.1. Satellite Data and Parameters of COE Merged altimeter data were obtained from multiple satellite sensors, including those of the systems of Jason-1, TOPEX/Poseidon, Geosat Follow-On (GFO), European Remote Sensing 2 (ERS-2), and Envi- ronmental Satellite (ENVISAT). Data were produced and distributed by the Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) organization. Near real-time merged sea surface height anomaly (SSHA) data (TOPEX/POSEIDON or Jason-11 ERS-1/2 or ENVISAT) with Mercator grids are available at www.aviso.oceanobs.com. Currently, the products are available on a daily scale with 0.25 3 0.25 resolution in the global ocean. The daily SST data were derived from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), with a spatial resolution of 0.25 3 0.25. The oceanic dynamic features are defined as positive-SSHA (anticyclonic eddy) feature (closed contours, SSHA > 6 cm), negative-SSHA (cyclonic eddy) feature (closed contours, SSHA < 26 cm), and neutral SUN ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 2 Journal of Geophysical Research: Oceans 10.1002/2013JC009575 condition (SSHA between 26 and 6 cm) [Lin et al., 2008] with a mononuclear eddy constrain [Li et al., 2014]. The eddy is usually in the form of an irregular circle with the eddy’s area of ACOE. The meridional component, ug, and the zonal component, vg, of the geostrophic velocity of the ocean cur- rent were calculated as follows: g @g g @g u 5 ; v 5 (1) g f @y g f @x where g is the acceleration of gravity, f is the Coriolis parameter, and g is the SSHA. If the eddy covers a region with n grids of 0.25 3 0.25, the EKE is computed [e.g., Xu et al., 2011]: Xn 1 2 2 EKE5 ðug1vgÞqAiHi (2) i51 2 3 where q 5 1020 kg/m is the density of seawater, Ai is the area of the ith grid, Hi is the depth of the ocean at ith grid according to the ‘‘Smith & Sandwell’’ database (the Gridded Global Relief Data (ETOPO2v2)) from National Geophysical Data Center. This database is a worldwide set of 2 min gridded ocean bathymetry derived from 1978 satellite radar altimetry of the sea surface. The changes of COE parameters induced by typhoon are defined as the differences between before COE under the influence of typhoon and COE after typhoon passage. In general, changes of COE parameters at a point might include two parts: the changes due to COE variations and the changes due to COE propagation.
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