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Article Impact of Major in 2016 on Sea Surface Features in the Northwestern Pacific

Dan Song 1 , Linghui Guo 1,*, Zhigang Duan 2 and Lulu Xiang 1

1 Institute of Physical Oceanography and Remote Sensing, Ocean College, University, No. 1 Zheda Road, Zhoushan 316021, ; [email protected] (D.S.); [email protected] (L.X.) 2 32020 Unit, Chinese People’s Liberation Army, No. 15 Donghudong Road, Wuhan 430074, China; [email protected] * Correspondence: [email protected]

 Received: 22 August 2018; Accepted: 24 September 2018; Published: 25 September 2018 

Abstract: Studying the interaction between the upper ocean and the typhoons is crucial to improve our understanding of heat and momentum exchange between the ocean and the atmosphere. In recent years, the upper ocean responses to typhoons have received considerable attention. The sea surface cooling (SSC) process has been repeatedly discussed. In the present work, case studies were examined on five strong and super typhoons that occurred in 2016—LionRock, Meranti, Malakas, Megi, and Chaba—to search for more evidence of SSC and new features of typhoons’ impact on sea surface features. Monitoring data from the Central Meteorological Observatory, China, (SST) data from satellite microwave and infrared remote sensing, and sea level anomaly (SLA) data from satellite altimeters were used to analyze the impact of typhoons on SST, the relationship between SSC and pre-existing eddies, the distribution of cold and warm eddies before and after typhoons, as well as the relationship between eddies and the intensity of typhoons. Results showed that: (1) SSC generally occurred during a passage and the degree of SSC was determined by the strength and the translation speed of the typhoon, as well as the pre-existing sea surface conditions. Relatively lower sea level (or cold core eddy) favors causing intense SSC; (2) After a typhoon passed, the SLA obviously decreased along with the SSC. The pre-existing positive SLAs or warm eddies decreased or disappeared during the typhoon’s passage, whereas negative SLAs or cold eddies were enhanced. It is suggested that the presence of warm eddies on the path has intensified the typhoons; (3) A criterion based on the ratio of local inertial period to application time of the typhoon wind-forcing was raised to dynamically distinguish slow- and fast-moving typhoons. And subcritical (slow-moving) situations were found in the LionRock case at its turning points where a cold core eddy was generated by long-time forcing. Moreover, the LionRock developed into a super typhoon due to reduced negative feedback when it was stalling over a comparably warmer sea surface. Therefore, the distinctive LionRock case is worthy of further discussion.

Keywords: typhoon; sea surface temperature; sea level anomaly; sea surface cooling; warm eddy; cold eddy

1. Introduction The Northwestern Pacific Ocean is the largest ocean area with typhoons, hurricanes, or tropical cyclones, accounting for more than 30% of the total number worldwide [1]. As severe weather phenomena, typhoons not only cause serious damage to coastal areas, but also constantly change the thermodynamic characteristics of the ocean during their passage. It is generally thought that typhoons play an important role in regulating the momentum, heat, and material exchange between the upper

Water 2018, 10, 1326; doi:10.3390/w10101326 www.mdpi.com/journal/water Water 2018, 10, 1326 2 of 18 ocean and the atmosphere. Improving our understanding of typhoon–ocean interactions is required to formulate more accurate typhoon predictions [2–6]. During a typhoon, one of the most remarkable responses of the ocean is a significant reduction in sea surface temperature (SST) [7–14], known as sea surface cooling (SSC). The observed drop in SST could vary tremendously from about 2 ◦C[11] to even 11 ◦C[10]. Typhoon-generated SSC and current velocity increases are often strongly rightward-biased [7,15–17], and shifts toward the typhoon track at depths exceeding roughly 100 m [18–20]. However, the left side of the typhoon track may also experience significant SSC due to intense [21,22]. The degree of SSC is generally thought to be determined by the intensity and translation speed of the typhoon and the pre-existing ocean conditions, such as SST, sea surface height (SSH), mixed layer depth (MLD), thermocline structure, etc. It has been suggested that the typhoon’s translation speed has vital importance to typhoon–ocean interaction. Slow-moving intense typhoons generate strong upwelling, which enhances the entrainment underneath and reduces the rightward bias [7], while fast-moving ones cause a transient surface intensified response and expose themselves for a shorter period to the cooling sea surface, both of which tend to reduce the negative SST feedback [2]. Lin et al. [23] indicated that a lower translation speed demands a much higher enthalpy flux to maintain the intensity of a , which is consistent with the argument that there exists a minimum translation speed for intensification whose value grows with the intensity of tropical cyclones [2]. A criterion to determine whether the typhoon is slow was raised upon the so-called Froude number (the ratio of translation speed to first baroclinic mode speed) [24]. A subcritical (slow-moving) situation was found to have facilitated SSC and mixed layer deepening due to the absence of inertial-gravity waves in the wake of the typhoon. Moreover, the interactions between typhoons and ocean mesoscale eddies were also emphasized. The SST inside pre-existing cyclonic (cold) eddies drops remarkably after the passage of a typhoon because cold deep water is easily raised due to uplifted thermocline and large current shear under the base of the mixed layer [10,25–33]. A tremendous SSC of 10.8 ◦C was recorded when Typhoon Kai-Tak passed over the cyclonic West Eddy in 2000 [28]. However, in contrast to the significant SSC, only 10% of the cyclonic eddies passed by super typhoons were intensified during the period 2000–2008 [32]. This may suggest that only under slow-moving typhoons can strong upwelling due to long-time forcing efficiently enhance and enlarge the cyclonic eddies. In this case, new cyclonic eddies were also found to be generated [32,34–36]. Compared with cyclonic eddies, anticyclonic (warm) eddies are not significantly impacted by typhoons. The comparably warmer surface and higher heat content in anticyclonic eddies, however, tends to reduce the negative feedback due to SSC induced by upwelling and vertical mixing [37,38]. Lin et al. [39] reported that super caused a weak SSC of only 0.5 ◦C inside a warm eddy, quite smaller than the value (2 ◦C) outside, while the typhoon was rapidly intensified due to the presence of the warm eddy. On the contrary, the rapid collapse of category 4 hurricane Kenneth when it stalled over a cold eddy was reported [40]. It was said that the slow translation speed of Kenneth caused significant SSC of 8–9 ◦C which turned the enthalpy fluxes to negative and cut off direct ocean energy flux to the hurricane. In the present work, we examined the impact of five major typhoons passing the Northwestern Pacific Ocean in 2016. We analyzed meteorological records and remote-sensing data and focused on the following: (1) the impact of the typhoon on SST and the relationship between SSC and pre-existing eddies; and (2) distribution of cold and warm eddies before and after a typhoon and the relationship between the eddy and intensity of typhoon. More evidence for typhoon-induced SSC was found and the pre-existence of a warm eddy was suggested to intensify the typhoon during its passage. Moreover, a subcritical (slow-moving) situation was found in the LionRock case at its turning points which is worth further discussion in the following research. WaterWater 20182018,, 1010,, 1326x FOR PEER REVIEW 3 of 18

2. Data and Methods 2. Data and Methods Five strong and super typhoons (Table 1) that occurred in the Northwestern Pacific Ocean from Five strong and super typhoons (Table1) that occurred in the Northwestern Pacific Ocean from 20 August 2016 to 5 October 2016 were selected for case studies in the present work, including 20 August 2016 to 5 October 2016 were selected for case studies in the present work, including LionRock, Meranti, Malakas, Megi, and Chaba. These five typhoons occurred consecutively with LionRock, Meranti, Malakas, Megi, and Chaba. These five typhoons occurred consecutively with considerable impact, and followed three different types of path (Figure 1). The typhoon monitoring considerable impact, and followed three different types of path (Figure1). The typhoon monitoring data were released by the Central Meteorological Observatory on the internet in real time data were released by the Central Meteorological Observatory on the internet in real time (http: (http://typhoon.nmc.cn/web.html). The data included typhoon center positions, minimum air //typhoon.nmc.cn/web.html). The data included typhoon center positions, minimum air pressures, pressures, maximum wind speeds, and average translation speeds. maximum wind speeds, and average translation speeds.

Table 1. Information about the five typhoons in 2016. Table 1. Information about the five typhoons in 2016. Name Code No. Duration (h) Path Type Max Wind Speed (m/s) Name Code No. Duration (h) Path Type Max Wind Speed (m/s) LionRock 1610 276 SW to NE 50 LionRock 1610 276 SW to NE 50 Meranti 1614 147 NW (sea path) 70 Meranti 1614 147 NW (sea path) 70 MalakasMalakas 1616 1616 177 177 NW NW to to NE NE 52 52 Megi Megi1617 1617 135 135 NW NW 52 52 Chaba Chaba1618 1618 180 180 NW NW to to NE NE 60 60

Figure 1.1. Paths map of fivefive majormajor typhoonstyphoons inin 2016.2016. The dotteddotted colored lines show the trajectories of thethe typhoons,typhoons, and thethe colorcolor throughthrough eacheach dotdot representsrepresents thethe intensityintensity of thethe typhoontyphoon inin levelslevels ofof thethe Beaufort wind scale.scale. The time interval betweenbetween everyevery twotwo dotsdots onon thethe pathpath isis 1212 h.h.

The SST datadata werewere obtainedobtained usingusing microwavemicrowave (MW) remote-sensing (TMI, AMSR-E, AMSR2, and WindSat)WindSat) and and infrared infrared (IR) (IR) remote remote sensing sensing (Terra (Terra MODIS MODIS and Aqua and MODIS). Aqua MODIS). The time resolutionThe time isresolution 1 day, and is the1 day, spatial and resolution the spatial is resolution 9 km (ftp://ftp.remss.com/sst/daily_v04.0/mw_ir/ is 9 km (ftp://ftp.remss.com/sst/daily_v04.0/mw_ir/).). To avoid To avoid statistically insignificant effects, we considered the cooling of the typhoon track area

Water 2018, 10, 1326 4 of 18 statistically insignificant effects, we considered the cooling of the typhoon track area during the typhoon’s birth and disappearance as being caused by the typhoon crossing. Therefore, we selected the lowest temperature of the grid during the statistical period as the temperature after typhoon transit, and the temperature field of the previous day was subtracted from the temperature field generated by the typhoon to obtain the cooling distribution of the study area. Satellite altimeters provided us with a useful method of obtaining long-term data on global sea level anomaly (SLA), which has been proven to be in good agreement with in situ observations, such as the tide gauge [41]. The SLA data are released by the Satellite Oceanographic Data Center (AVISO) of the Centre National d’Etudes Spatiales through its official website. This data are mainly composed of TOPEX/POSEIDON, Jason-1, and ERS/Envisat altimeter satellites, with a time resolution of 1 day and a spatial resolution of 0.25◦. The data included SLA and geostrophic flow velocity (u/v) data. In the study area, the determination of the center and extent of cold and warm eddies was performed using the geostrophic velocity field.

3. Results

3.1. LionRock developed from a disturbance in the Pacific open ocean. It moved slowly northwestward under the guidance of the subtropical high, and the typhoon intensity gradually strengthened. On 17–18 August 2016, it encountered a high-level eddy in the troposphere, resulting in a significant reduction in and intensity. However, on August 19–20, it experienced the Fujiwhara effect (two typhoon inter-rotational effects) with the Kompasu in the development stage. It turned south and entered the relatively warm ocean surface. Convection occurred, and its intensity increased. LionRock developed into a tropical storm. Subsequently, on August 21–22, LionRock displayed the Fujiwhara effect with and continued to move southward. The Fujiwhara effect occurs when cyclones are in proximity to one another; their centers will begin orbiting cyclonically about a point between the two systems due to their cyclonic wind circulations [42]. It became a strong tropical storm and developed into a typhoon at 8:00 a.m. on August 24, with a maximum wind speed of 35 m/s. At 2:00 p.m., it developed into a strong typhoon with a maximum wind speed of 42 m/s and was oriented in the southwest direction. At 2:00 a.m. on August 26, LionRock was affected by the Western North Pacific subtropical high (WNPSH) and changed its direction from southwest to northeast, with its intensity gradually strengthening. At 2:00 p.m. on August 28, it developed into a super typhoon and maintained its strength until August 29 at 2:00 p.m. From 8:00 p.m. on August 29 to 2:00 a.m. on August 30, it gradually turned to the northwest. It made in at 5:00 p.m. on August 30 and reached at 5:00 a.m. on August 31, 2016. LionRock caused a certain degree of cooling (>2 ◦C) in a large area where it passed over the sea (Figure2). The cooling of the right side of the path was more obvious than that of the left side, and the cooling was greatest at the turning point of the typhoon path and when the typhoon wind speed was relatively large. The cooling area at the turning point of the typhoon path was concentrated in a circular area. The cooling center of the typhoon straight-through area was about 100 km on the right side of the typhoon path, and the cooling area was also approximately circular with a cooling radius of about 200 km. The SLA distribution (Figure3a) before the passage of Typhoon LionRock was obviously related to the cooling distribution. From Figure3a, before the typhoon crossing (19 August 2016), where the SLA value was positive (i.e., it was in a warm eddy area), the cooling was low (<3 ◦C). When the SLA before a typhoon crossing is negative (i.e., it is in a cold eddy area), the cooling caused by LionRock was relatively high and the cooling eddy center was close to the center of the cold eddy. The SLA distribution (Figure3a) before the passage of Typhoon LionRock was obviously related to the cooling distribution. Before the typhoon crossing, where the SLA value was positive (i.e., it was in a warm eddy area), the cooling was low (<3 ◦C). When the SLA before a typhoon crossing was negative (i.e., it is in a cold eddy area), the cooling was relatively high and the cooling eddy center was close to the center of the cold eddy. Water 2018, 10, x FOR PEER REVIEW 5 of 18 Water 2018, 10, 1326 5 of 18 Water 2018, 10, x FOR PEER REVIEW 5 of 18

Figure 2. Cooling of the sea surface after Typhoon LionRock passed through. The color bar FigurerepresentsFigure 2. Cooling2. theCooling degree of the of seaofthe surfacecooling sea surface afterin °C. Typhoon after The Typhoon mean LionRocking LionRockof passed the color through. passed of Thethethrough. colortyphoon barThe represents trajectorycolor bar the is degreeconsistentrepresents of cooling with the Figure indegree◦C. The1. of meaningcooling in of the°C. colorThe ofmean theing typhoon of the trajectory color of is the consistent typhoon with trajectory Figure1 .is consistent with Figure 1.

FigureFigureFigure 3.3. 3.Distribution Distribution Distribution ofof of coldcold cold and andand warm warmwarm eddieseddieseddies alongsidealongside the the path path of of Typhoon Typhoon LionRock LionRock (a )( abefore) before thethethe typhoon typhoon typhoon was was was generatedgenerated generated (19(19 (19 AugustAugust August 2016), 2016),2016), ((bb)) duringduringduring the thethe typhoon typhoontyphoon (25 (25 August August 2016), 2016), (c )( cduring) during the the the typhoontyphoontyphoon (27(27 (27 AugustAugust August 2016),2016), 2016), andand and ( (d(dd))) after afterafter Typhoon TyphoonTyphoon LionRockLionRockLionRock passed passedpassed (1 (1(1 September SeptemberSeptember 2016). 2016). The The color color on on thethethe typhoon typhoon typhoon pathpath path isis is consistentconsistent consistent with withwith FigureFigureFigure1 1.. TheThe boldboldbold blackblackblack solid solid line lineline outside outsideoutside the thethe blue blueblue * *(cold* (cold(cold eddy) eddy)eddy) andandand the the the red red red * (warm* *(warm (warm eddy) eddy)eddy) represents representsrepresents the boundarythe boundary of that ofof eddy. thatthat eddy. Rededdy.solid Red Red contourssolid solid contours contours represent represent represent positive seapositivepositive level anomalysea sea level level anomaly (SLA), anomaly whereas (SLA), (SLA), whereasbluewhereas dashed blue contours dasheded contourscontours represent represent represent negative ne ne SLAgativegative or SLA zero.SLA or or Thezero. zero. colored The The starscoloredcolored on thestars stars path on on the represent the path path represent represent the location thethe oflolocation the typhoon of thethe typhoontyphoon during theduring during day. the the day. day.

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AtAt the the same same time, time, it it can can bebe seenseen from Figure 33dd that after after LionRock LionRock passed passed (1 (1 September September 2016), 2016), thethe position position of coldof cold and and warm warm eddies eddies and theirand radiitheir changedradii changed significantly. significantly. The cold The eddies cold increasededdies andincreased moved towardand moved the area toward close the to thearea typhoon close to path, the typhoon and the warmpath, eddiesand the weakened warm eddies and movedweakened away fromand the moved typhoon away path. from In particular,the typhoon on 25path. August, In particular, the typhoon on reached25 August, the turningthe typhoon point andreached a new the cold eddyturning was generatedpoint and a at new the cold turning eddy point was (boxgenerated A in Figure at the3 turningb). At 2:00 point a.m. (box on A 26 in August,Figure 3b). the At typhoon 2:00 turneda.m. fromon 26 southwestAugust, the to typhoon northeast, turned and the from cold southwes eddy generatedt to northeast, in the boxand Athe disappeared. cold eddy generated On 27 August, in thethe typhoon box A traveleddisappeared. to the On box 27 B August, region, the forming typhoon a new traveled strong to cold the eddybox B on region, the left forming side of a the new path (Figurestrong3c). cold LionRock eddy on also the strengthenedleft side of the the path cold (Fig eddiesure 3c). in theLionRock southwest also andstrengthened northeast the to some cold extent.eddies in Figurethe southwest4 shows and the northeast maximum to some wind extent. speed and the typhoon center pressure at various points along theFigure typhoon 4 shows path, the asmaximum well as thewind day’s speed SLA. and Two the differenttyphoon center degrees pressure of typhoon at various enhancement points (windalong speed the typhoon increase path, and pressureas well as decrease) the day’s occurredSLA. Two after different LionRock degrees passed of typhoon the high enhancement SLA area and (wind speed increase and pressure decrease) occurred after LionRock passed the high SLA area and continued for some time. On 24 August, the typhoon passed the high value SLA zone (15–30 cm), continued for some time. On 24 August, the typhoon passed the high value SLA zone (15–30 cm), the the maximum wind speed of the typhoon increased from 30 to 50 m/s, and the central pressure rapidly maximum wind speed of the typhoon increased from 30 to 50 m/s, and the central pressure rapidly dropped from 980 hpa to 940 hpa. From 27 August to 29 August, LionRock passed through a relatively dropped from 980 hpa to 940 hpa. From 27 August to 29 August, LionRock passed through a high-value SLA area twice (>15 cm). The center wind speed increased twice, from 42 s to 52 m/s, relatively high-value SLA area twice (>15 cm). The center wind speed increased twice, from 42 s to 52 andm/s, the and center the pressurecenter pressure decreased decreased by 20 hpa.by 20 However, hpa. However, when thewhen typhoon the typhoon passed passed through through regions withregions low SLAwith values,low SLA it values, maintained it maintained its strength its strength or wasweakened or was weakened to a certain to a certain extent. extent.

FigureFigure 4. 4.Variations Variations in in maximum maximum wind wind speed speed (blue (blue solid solid line) line) and and central central pressure pressure (green (green dotted dotted line) ofline) the Typhoon of the Typhoon LionRock LionRock (upper ),(upper and change), and change in SLA in (black SLA solid(black line, solidbottom line, bottom) on the) pathon the of path Typhoon of LionRock.Typhoon Black LionRock. dotted Black boxes dotted indicate boxes where indicate the where typhoon the changedtyphoon ch dramatically.anged dramatically.

3.2.3.2. Meranti Meranti and and Malakas Malakas TyphoonTyphoon Meranti Meranti formed formed on on the the afternoonafternoon ofof 10 September 2016. 2016. It It strengthened strengthened into into a typhoon a typhoon ◦ ◦ (at(at 17.6 17.6°N, N, 131.7 131.7°E) E) on on the the early early morningmorning of 12 September, strengthened strengthened into into a astrong strong typhoon typhoon at at ◦ ◦ ◦ 8:008:00 a.m. a.m. on on 12 12 September September (at(at 18.018.0° N,N, 130.4130.4° E),E), and and strengthened strengthened to to a asuper super typhoon typhoon at at18.1° 18.1 N, N, 129.9129.9°◦ E) E) at 11:00at 11:00 a.m. a.m. on on 12 12 September. September. In In 24 24 h, h, the the wind wind speed speed of of Meranti Meranti increased increased from from 25 25 to to 62 62 m/s. Thism/s. meant This Merantimeant Meranti was a rapidlywas a rapidly increasing increasing typhoon. typhoon. At 2:00 At p.m. 2:00 on p.m. 13 September,on 13 September, Meranti Meranti reached itsreached peak intensity its peak and intensity the maximum and the windmaximum speed wind near itsspeed center near reached its center 70 m/s reached (over 70 Beaufort m/s (over scale 17),Beaufort stronger scale than 17), the stronger peak strength than the of peak this strength year’s no. of 1this typhoon, year’s no. Nibbert 1 typhoon, (65 m/s, Nibbert 17 or (65 above). m/s, 17 It or was theabove). strongest It was typhoon the strongest in the global typhoon sea in area the during global 2016.sea area Meranti during traveled 2016. Me northwestranti traveled all the northwest way to the southall the of Taiwanway to andthe south on 15 of September and it landed on 15 September on the coast it oflanded Xiangan on the District, coast ,of Xiangan , District, China (atXiamen, 24.57◦ N, Fujian, 118.23 China◦ E). (at 24.57° N, 118.23° E).

Water 2018, 10, x FOR PEER REVIEW 7 of 18 Water 2018, 10, 1326 7 of 18 As was developing, another tropical depression was occurring near the typhoon formation at 08:00 a.m. on 13 September 2016, and it gradually developed into Typhoon no. 15, MalakasAs Typhoon (at 15.8° Meranti N, 132.1° was developing, E). Malakas another made tropical its way depression to the northwest was occurring at the nearbeginning the typhoon of its formationgeneration, at 08:00and a.m.it gradually on 13 September developed 2016, into and ita graduallystrong typhoon. developed It intoturned Typhoon northward no. 15, Malakason the ◦ ◦ (atsoutheastern 15.8 N, 132.1 side E).of MalakasTaiwan madeon 17 itsSeptember way to the an northwestd then turned at the beginningnortheast on of its18 generation, September. and The it graduallyintensity of developed the system into continued a strong to typhoon. be strong. It turned After landing northward on onthethe southern southeastern side of sideKyushu of Taiwan in Japan on 17on September19 September, and it then gradually turned decreased. northeast on 18 September. The intensity of the system continued to be strong.Although After landing Meranti on was the southern very intense, side of it Kyushudid not in have Japan a onstrong 19 September, cooling effect. it gradually It only decreased.caused a smallAlthough degree of Meranticooling in was the very southwestern intense, it part did of not the have a strong and cooling the east effect. side Itof only Taiwan, caused with a smalla maximum degree temperature of cooling in drop the southwestern of about 2 °C part(Figure of the 5a). Philippines The largest and area the of east cooling side was of Taiwan, close to with the ◦ astrong maximum cold eddy temperature shown in drop zone of aboutC in Figure 2 C (Figure 6a. Malakas5a). The caused largest a arealarge of range cooling of wascooling, close and to thethe strongcooling cold was eddyobviously shown rightward in zone biased C in Figure (Figure6a. 5b). Malakas caused a large range of cooling, and the coolingThe was formation obviously dates rightward of Typhoons biased Meranti (Figure and5b). Malakas were quite close to each other and their affectedThe areas formation were datessimilar. of The Typhoons degree Meranti of SST coo andling Malakas caused were by them, quite however, close to each was othervery different. and their affectedThe main areas reason were that similar. the two The typhoons degree of caused SST cooling a great caused difference by them, in cooling however, may was be that very Meranti’s different. Thepath main had reasona high thatSLA thelevel two (Figure typhoons 6a). causedAt the asame great time, difference after Typhoon in cooling Meranti may be thatcrossed, Meranti’s the area’s path hadSLA awas high reduced SLA level to (Figuresome extent6a). At (Figure the same 6b), time, and after the TyphoonSLA distribution Meranti crossed,also changed. the area’s This SLA created was reducedfavorable to conditions some extent for (FigureTyphoon6b), Malakas and the to SLA cool distribution down. At the also same changed. time, Meranti This created moved favorable quickly conditionson the ocean, for and Typhoon the time Malakas of interaction to cool down. between At the Meranti same time,and its Meranti passage moved through quickly the sea on thearea ocean, was andshort, the and time this of may interaction also be betweenthe cause Meranti of its limited and its impact. passage through the sea area was short, and this mayAfter also be the the two cause typhoons of its limited passed impact. the sea area, the SLA in the sea area showed different degrees of reductionAfter (Figure the two 6). typhoons In particular, passed after the seathe area,two typhoons the SLA in strengthened, the sea area showedthe existing different warm degrees eddy (at of reduction20° N, 124.75° (Figure E in6 ).Figure In particular, 6a, box C) after disappeared. the two typhoons Meranti’s strengthened, transit caused the a existing greater warmreduction eddy than (at ◦ ◦ 20MalakasN, 124.75 in theE SLA in Figure level.6 Thea, box SLA C) level disappeared. had already Meranti’s fallen transitsomewhat caused during a greater the pass reduction of Malakas, than Malakaswhich caused in the a SLAcertain level. degree The of SLA cooling level after had it already passed. fallen somewhat during the pass of Malakas, whichAs caused shown a in certain Figure degree 7, the of SLA cooling level after of the it passed. area through which Meranti passed was relatively high As(20–40 shown cm), in which Figure 7may, the be SLA one level of the of thereason areas throughfor Meranti’s which strength Meranti passed(maximum was relativelywind speed high of (20–4070 m/s), cm), as typhoon which may enhancement be one of the occurs reasons in high-value for Meranti’s SLA strength regions. (maximum At 12:00 windp.m. on speed 12 September, of 70 m/s), aswhen typhoon the typhoon enhancement passed occurs a SLA in area high-value with a relati SLAvely regions. high At value 12:00 of p.m. 20–40 on cm, 12September, the maximum when wind the typhoonspeed of passedthe typhoon a SLA areaincreased with a from relatively 62 to high 70 valuem/s, and of 20–40 the system cm, the grew maximum into a wind super speed typhoon. of the typhoonHowever, increased when Meranti from 62passed to 70 an m/s, area and with the a rela systemtively grew low intoSLA a value, super the typhoon. typhoon However, maintained when its Merantioriginal passedstrength an or area changed with a relativelyonly little. low In SLAaddition, value, the the apparent typhoon maintainedweakening its of original Meranti strength occurred or changedwhen it arrived only little. on land. In addition, the apparent weakening of Meranti occurred when it arrived on land.

Figure 5.5.( a()a Map) Map of coolingof cooling distribution distribution after Typhoonafter Typhoon Meranti Meranti passed. (bpassed.) Map of(b cooling) Map distributionof cooling ◦ afterdistribution Typhoon after Malakas Typhoon passed. Malakas The passed. color bar The represents color bar the represents degree of the cooling degree in ofC. cooling The color in °C. of The the typhooncolor of the path typhoon is consistent path is with consistent Figure 1with. Figure 1.

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FigureFigure 6. MapMap of ofthe the cold cold and andwarm warm vortexes vortexes distribution distribution caused caused by Typhoons by Typhoons Meranti Merantiand Malakas. and Malakas.(a) Before (a )Typhoon Before Typhoon Meranti Meranti passed passed(9 September (9 September 2016). 2016).(b) After (b) After Typhoon Typhoon Meranti Meranti passed passed (17 (17September September 2016). 2016). (c) (Beforec) Before Typhoon Typhoon Malakas Malakas passed passed (12 (12 September September 2016). 2016). ( (dd)) After TyphoonTyphoon MalakasMalakas passed (21 September September 2016). 2016). The The color color on on the the typhoon typhoon path path is consistent is consistent with with Figure Figure 1. The1. Thebold bold black black solid solid line line outside outside the the blue blue * (cold * (cold eddy) eddy) and and the the red red * * (warm eddy) representsrepresents thethe boundaryboundary ofof thatthat eddy.eddy. RedRed solidsolid contourscontours representrepresent positivepositive SLA,SLA, whereaswhereas blueblue dasheddashed contourscontours representrepresent negativenegative SLASLA oror zero.zero.

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FigureFigure 7.7.7. VariationsVariationsVariations in inin maximum maximummaximum wind windwind speed speedspeed (blue (blue(blue solid solidsolid line) line)line) and andand central centralcentral pressure pressurepressure (green (green(green dotted dotteddotted line) ofline)line) Typhoon ofof TyphoonTyphoon Meranti MerantiMeranti (upper ((),upperupper and), change), andand changechange in SLA inin (blue SLASLA solid (blue(blue line, solidsolidbottom line,line,) bottom onbottom the) path) onon ofthethe Typhoon pathpath ofof Meranti.TyphoonTyphoon Black Meranti.Meranti. dotted BlackBlack boxes dotteddotted indicate boxesboxes where indicateindicate the typhoon whwhereere the changedthe typhoontyphoon dramatically. changedchanged Thedramatically.dramatically. part where TheThe the partpart line suddenlywherewhere thethe disappears lineline suddenlysuddenly in the disappearsdisappears SLA is when inin thethe the SLSL typhoonAA isis whenwhen passed thethe typhoontyphoon onto land. passedpassed ontoonto land.land.

Malakas’Malakas’ historyhistoryhistory includes includesincludes three threethree distinct distinctdistinct growths growthsgrowths and andand two twotwo reductions, reductions,reductions, shown shownshown in Figure inin FigureFigure8. The 8.8. three TheThe growthsthreethree growthsgrowths occurred occurredoccurred after Malakas afterafter MalakasMalakas passed an passedpassed area with anan arar a relativelyeaea withwith aa high relativelyrelatively SLA value. highhigh MalakasSLASLA value.value. continued MalakasMalakas to growcontinuedcontinued after to beingto growgrow generated, afterafter beingbeing on gene 14gene Septemberrated,rated, onon 20161414 SeptemberSeptember from 9:00 20162016 a.m. from tofrom 9:00 9:009:00 p.m. a.m.a.m. Malakas toto 9:009:00 passed p.m.p.m. MalakasMalakas an area withpassedpassed a relatively anan areaarea withwith low a SLAa relativelyrelatively value ( −lowlow12–2 SLASLA cm), valuevalue and (( the−−12–212–2 typhoon’s cm),cm), andand growth thethe typhoon’styphoon’s stagnated. growthgrowth The maximum stagnated.stagnated. wind TheThe speedmaximummaximum and centerwindwind speedspeed pressure andand of centercenter typhoon pressurepressure were of maintained.of typhoontyphoon werewere From maintained.maintained. 15 September FromFrom at 9:001515 SeptemberSeptember a.m. to 8:00 atat a.m.9:009:00 ona.m.a.m. 16 toto September, 8:008:00 a.m.a.m. onon Malakas 1616 September,September, passed MalakasMalakas through passedpassed an area throughthrough witha anan relatively areaarea withwith high aa relativelyrelatively SLA value highhigh (20–40 SLASLA valuevalue cm). The(20–40(20–40 maximum cm).cm). TheThewind maximummaximum speed windwind of the speedspeed typhoon ofof thethe increased typhootyphoonn from increasedincreased 42 to fromfrom 50 m/s, 4242 toto and 5050 them/s,m/s, central andand thethe pressure centralcentral droppedpressurepressure dropped bydropped 10 hpa. byby From 1010 hpa.hpa. 15 From SeptemberFrom 1515 SeptemberSeptember at 3:00 p.m. atat 3:003:00 to 9:00 p.m.p.m. p.m. toto 9:009:00 on p.m.p.m. 17 September, onon 1717 September,September, Malakas MalakasMalakas passed throughpassedpassed throughthrough an area withanan areaarea a relatively withwith aa relativelyrelatively low SLA lowlow value SLASLA (− value7–3value cm). ((−−7–37–3 After cm).cm). the AfterAfter typhoon thethe typhoontyphoon maintained maintainedmaintained its strength itsits forstrengthstrength a period, forfor aa it period,period, weakened itit weakenedweakened and then andand passed thenthen through passedpassed an ththroughrough area with anan areaarea a relatively withwith aa relativelyrelatively high SLA highhigh value. SLASLA Then value.value. the typhoonThenThen thethe strengthened typhoontyphoon strengthenedstrengthened again at 3:00 againagain p.m. atat 3:00 on3:00 18 p.m.p.m. September onon 1818 SeptemberSeptember 2016. 2016.2016.

FigureFigure 8. 8. SimilarSimilar to to Figure Figure 7 77 but butbut for forfor Typhoon TyphoonTyphoon Malakas. Malakas.Malakas.

3.3.3.3. MegiMegi TyphoonTyphoon MegiMegi waswas generatedgenerated onon thethe NorthwestNorthwest PaPacificcific OceanOcean 21402140 kmkm southsouth ofof TaitungTaitung City,City, TaiwanTaiwan ProvinceProvince atat 8:008:00 a.m.a.m. onon 2323 SeptemberSeptember 202016.16. AfterAfter thethe typhoontyphoon waswas generated,generated, itit headedheaded northwestnorthwest towardtoward TaiwanTaiwan Island.Island. OnOn 2525 September,September, itit becamebecame aa strongstrong tropicaltropical stormstorm atat 19.4°19.4° N,N,

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3.3. Megi was generated on the Northwest Pacific Ocean 2140 km south of Taitung City, Water 2018, 10, x FOR PEER REVIEW 10 of 18 Taiwan Province at 8:00 a.m. on 23 September 2016. After the typhoon was generated, it headed ◦ ◦ northwest139.8° E. It toward became Taiwan a super Island. typhoon On 25 on September, 27 September, it became with a strong a center tropical pressure storm of at 935 19.4 hpaN, 139.8and a E.maximum It became wind a super speed typhoon of 52 onm/s. 27 On September, the same with day, a centerit landed pressure in Hualian of 935 hpaCounty, and aTaiwan maximum and windweakened speed after of 52 landing. m/s. On Then, the it same landed day, in it Hui’an landed County, in Hualian Fujian, County, China Taiwan through and the weakened Taiwan Strait. after landing.As shown Then, itin landed Figure in 9, Hui’an the cooling County, of Fujian,the Typh Chinaoon throughMegi path the area Taiwan is obvious. Strait. The maximum coolingAs temperature shown in Figure was 9greater, the cooling than 5 of°C, the and Typhoon the resulting Megi cooling path area distribution is obvious. did The not maximum stop at the ◦ coolingright side temperature of the path. was In the greater early than period 5 C, of and typhoon the resulting development, cooling it distribution caused a temperature did not stop drop at the of right1–2 °C. side The of thecooling path. area In the was early mainly period distributed of typhoon on development, the left side it of caused the typhoon’s a temperature path. drop On 24 of ◦ 1–2September,C. The coolingMegi developed area was mainly from a distributed tropical depressi on the lefton sideto a oftropical the typhoon’s storm, causing path. On a 24temperature September, ◦ Megidrop of developed 1–2 °C on from the right a tropical side of depression the typhoon to a path tropical and storm, gradually causing strengthening a temperature into dropa typhoon. of 1–2 OnC onboth the sides right of side the of typhoon the typhoon path, path we found and gradually a significant strengthening temperature into drop a typhoon. of more On than both 3 sides °C. On of the 27 ◦ typhoonSeptember, path, Megi we developed found a significant into a super temperature typhoon an dropd caused of more strong than cooling 3 C. On (>5 27 °C) September, centered on Megi the ◦ developedtyphoon track into area. a super Comparing typhoon andFigures caused 9 and strong 10b, cooling the positions (>5 C) of centered cooling oncenters the typhoon match well track those area. Comparingof the cold Figureseddies,9 and and the10b, more the positions cold eddies of cooling there centersare, the match more wellobvious those the of thecooling. cold eddies,On the andcontrary, the more in areas cold eddieswhere the there warm are, theeddies more were obvious strong, the the cooling. cooling On was the contrary,significantly in areas lower where than the in warmthe surrounding eddies were area. strong, the cooling was significantly lower than in the surrounding area.

Figure 9.9. Map of coolingcooling distribution after Typhoon MegiMegi transit.transit. The color barbar value represents the degree ofof coolingcooling inin ◦°C.C. The color of the typhoontyphoon path isis consistentconsistent withwith FigureFigure1 1..

After Megi passed through the research area, thethe SLA in the area demonstrated a certain degree of reduction (Figure 1010).). Before the typhoon passage,passage, there were two small warm eddies and a small cold eddy on on the the southeast southeast side side of of Taiwan Taiwan Island Island (F (Figureigure 10a, 10 a,box box E). E).After After the thetyphoon typhoon passage, passage, the thethree three small small eddies eddies merged merged into into one one big big cold cold eddy eddy and and moved moved toward toward the the typhoon typhoon path path (Figure (Figure 10b).10b). TheThe SLASLA ofof thethe coldcold eddyeddy centercenter waswas reducedreduced fromfrom− −15 toto −−3030 cm cm and and the radius of the cold eddy increasedincreased fromfrom 3030 toto 200200 km.km. As shownshown inin FigureFigure 1111,, atat thethe beginningbeginning ofof Megi’sMegi’s formation,formation, thethe areaarea MegiMegi passedpassed was a high-value SLA area (0–20 cm), and the maximum wind speed and pressure in the typhoontyphoon center continued toto growgrow until until it it became became a stronga strong typhoon typhoon with with a maximum a maximum center center wind wind speed speed if 52 if m/s 52 andm/s centerand center pressure pressure of 935 of hpa.935 hpa. From From 3:00 3:00 p.m. p.m. on 26 on September 26 September to 1:00 to 1:00 a.m. a.m. on27 on September, 27 September, after after the typhoonthe typhoon passed passed an area an area with with a SLA a lowSLA value low value (−13–0 (−13–0 cm), thecm), typhoon the typhoon intensity intensity weakened weakened after being after maintainedbeing maintained for some for time. some The time. maximum The maximum wind speed wind ofspeed the typhoon of the typhoon center droppedcenter dropped to 40 m/s to 40 before m/s thebefore typhoon the typhoon landed landed in Taiwan, in Taiwan, and the and central the pressurecentral pressure increased increased to 960 hpa. to 960 After hpa. landing After landing in Taiwan, in MegiTaiwan, continued Megi continued to weaken. to weaken.

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Figure 10. Map of the cold and warm eddy distribution caused by Typhoon Megi (a) before passage Figure 10. Map of the cold and warm eddy distribution caused by Typhoon Megi (a) beforebefore passagepassage (22 September 2016) and (b) after passage (September 30). The color on the typhoon path is consistent (22 September 2016)2016) andand ((b)) after passage (September(September 30).30). The color on the typhoon path is consistent with Figure 1. The bold black solid line outside the blue * (cold eddy) and the red * (warm eddy) with Figure1 1.. TheThe boldbold blackblack solidsolid lineline outsideoutside thethe blueblue ** (cold(cold eddy)eddy) andand thethe redred ** (warm(warm eddy)eddy) represents the boundary of that eddy. Red solid contours represent positive SLA, whereas blue represents thethe boundary boundary of of that that eddy. eddy. Red Red solid solid contours contours represent represent positive positive SLA, whereasSLA, whereas blue dashed blue dashed contours represent negative SLA or zero. contoursdashed contours represent represent negative negative SLA or zero.SLA or zero.

Figure 11. Variations inin maximum maximum wind wind speed speed (blue (blue solid solid line) line) and and central central pressure pressure (green (green dotted dotted line) Figure 11. Variations in maximum wind speed (blue solid line) and central pressure (green dotted ofline) Typhoon of Typhoon Megi Megi (upper (upper), and), change and change in SLA in (blueSLA (blue solid line,solidbottom line, bottom) on the) on path the ofpath Typhoon of Typhoon Megi. line) of Typhoon Megi (upper), and change in SLA (blue solid line, bottom) on the path of Typhoon BlackMegi. dottedBlack dotted box indicates box indicates where thewhere typhoon the typhoon changed changed dramatically. dramatically. The part whereThe part the where line suddenly the line Megi. Black dotted box indicates where the typhoon changed dramatically. The part where the line disappearssuddenly disappears in the SLA in is the when SLA the is typhoonwhen the passed typhoon onto passed land. onto land. suddenly disappears in the SLA is when the typhoon passed onto land. 3.4. Chaba 3.4. Chaba was formed in the Northwestern Pacific on 28 September 2016. Its center was Typhoon Chaba was formed in the Northwestern Pacific on 28 September 2016. Its center was locatedTyphoon on the Chaba ocean 580was km formed northeast in the of Northwestern in the UnitedPacific Stateson 28 (14.6September◦ N, 150.0 2016.◦ E). Its Aftercenter being was located on the ocean 580 km northeast of Guam in the United States (14.6° N, 150.0° E). After being generated,located on the it traveled ocean 580 westward km northeast at a speedof Guam of 20–25in the kmUnited per States hour to(14.6° the N, vicinity 150.0° ofE). the After Malakas being generated, it traveled westward at a speed of 20–25 km per hour to the vicinity of the Malakas generationgenerated, site.it traveled Then it westward turned tothe at northwesta speed of and 20–25 became km aper super hour typhoon to the atvicinity 8:00 a.m. of onthe2 Malakas October. generation site. Then it turned to the northwest and became a super typhoon at 8:00 a.m. on 2 Thegeneration minimum site. pressure Then it inturned the center to the reached northwest 920 hpa.and Thebecame maximum a super wind typhoon speed at reached 8:00 a.m. 60 on m/s. 2 October. The minimum pressure in the center reached 920 hpa. The maximum wind speed reached TheOctober. typhoon The wentminimum north topressure east of in the the East center China reached Sea, and 920 the hpa. strong The typhoon maximum continued wind speed until 7:00reached p.m. 60 m/s. The typhoon went north to east of the Sea, and the strong typhoon continued on60 4m/s. October The typhoon 2016. Then went it weakened north to east and of turned the Ea tost the China northeast Sea, and and the finally strong hit thetyphoon coast nearcontinued South until 7:00 p.m. on 4 October 2016. Then it weakened and turned to the northeast and finally hit the ’suntil 7:00 Pusan. p.m. on Later, 4 October it moved 2016. into Then the Sea it weaken of Japaned and and weakened turned to into the annortheast extratropical and finally cyclone. hit the coast near ’s Pusan. Later, it moved into the Sea of Japan and weakened into an .

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WhenWhen ChabaChaba waswas stillstill inin thethe develdevelopmentopment stage,stage, itit traveledtraveled westward,westward, causingcausing thethe SSTSST toto dropdrop byby 1–21–2When °C°C (Figure(Figure Chaba was12).12). Most stillMost in ofof the thethe development coolingcooling areaarea stage, waswas locatedlocated it traveled onon the westward,the leftleft sideside causing ofof thethe path, thepath, SST andand to thenthen drop thethe by ◦ 1–2typhoontyphoonC (Figure turnedturned 12 ). toto Most thethe of northwestnorthwest the cooling andand area graduallygradually was located strengthenedstrengthened on the left side intointo of theaa supersuper path, andtyphoon,typhoon, then the causingcausing typhoon aa turnedsignificantsignificant to the reductionreduction northwest inin and thethe gradually SSTSST (>2(>2 strengthened°C).°C). TheThe seasea into surfacesurface a super coolingcooling typhoon, causedcaused causing byby Typhoon aTyphoon significant ChabaChaba reduction waswas inmainlymainly the SST distributeddistributed (>2 ◦C). The inin seaareasareas surface withwith cooling lowlow SLASLA caused values.values. by TyphoonComparedCompared Chaba toto thosethose was mainly ofof thethe previous distributedprevious typhoons,typhoons, in areas with thethe lowcoolingcooling SLA effecteffect values. causedcaused Compared byby ChabaChaba to those waswas ofrelativelyrelatively the previous small.small. typhoons, TheThe mainmain the reasonreason cooling maymay effect bebe that causedthat thethe by SSTSST Chaba droppeddropped was relativelysignificantly,significantly, small. andand The thethe main seawaterseawater reason maymixedmixed be thatlayerlayer the deepdeep SSTened.ened. dropped TheThe significantly, waterwater temperaturetemperature and the seawater ofof thethe shallowshallow mixed layer seasea deepened.waterwater waswas Theslightlyslightly water differentdifferent temperature toto thethe of SST.SST. the shallow sea water was slightly different to the SST. Similarly,Similarly, ChabaChabaChaba causedcaused aa certaincertain degreedegree ofof declinedeclindeclinee inin thethe SLASLA valuevalue ofof thethe areaarea throughthrough whichwhich itit passed.passed. After After the the typhoon typhoon passed, passed, the the area’s area’s lowloloww SLA SLA valuevalue increased increased andand aa strongstrong coldcold eddyeddy waswas generatedgenerated in in thethe typhoon’styphoon’s track track (Figure (Figure 13b,1313b,b, boxbox F).F). AfterAfter thethe typhoontyphoon transited,transited, the the warmwarm eddyeddy aroundaround thethethe typhoon typhoontyphoon path pathpath gradually graduallygradually weakened weakenedweakened or even oror even disappeared.even disappeared.disappeared. For example, ForFor example,example, before the beforebefore typhoon thethe crossedtyphoontyphoon this crossedcrossed sea area, thisthis there seasea area,area, was there athere warm waswas eddy aa warmwarm in the eddyeddy eastern inin thethe part easterneastern of Taiwan partpartIsland ofof TaiwanTaiwan (Figure IslandIsland 13a, (Figure(Figure box G). After13a,13a, boxbox the G).G). typhoon AfterAfter transited,thethe typhoontyphoon the transited,transited, main body thethe of mainmain the warm bodybody eddy ofof thethe turned warmwarm away eddyeddy from turnedturned the awayaway typhoon fromfrom path, thethe andtyphoontyphoon its intensity path,path, andand weakened. itsits intensityintensity weakened.weakened.

FigureFigure 12.12. MapMap of ofof cooling coolingcoolingdistribution distributiondistribution after afterafter Typhoon TyphoonTyphoon Chaba ChabaChaba passed. passed.passed. The TheThe color colorcolor bar barbar value valuevalue represents representsrepresents the degreethethe degreedegree of cooling ofof coolingcooling in ◦ C. inin The°C.°C. TheThe color colorcolor of the ofof typhoonthethe typhoontyphoon path pathpath is consistent isis consistentconsistent with withwith Figure FigureFigure1. 1.1.

FigureFigure 13.13.13. Map MapMap of ofof the thethe cold coldcold and andand warm warmwarm eddy eddyeddy distribution distributiondistribution caused causedcaused by Typhoon byby TyphoonTyphoon Chaba: (Chaba:aChaba:) before ((aa Typhoon)) beforebefore ChabaTyphoonTyphoon passed ChabaChaba (27 passedpassed September (27(27 SeptemberSeptember 2016) and ( 2016)b2016)) after andand Typhoon ((bb)) afterafter Chaba TyphoonTyphoon transit ChabaChaba (6 October transittransit 2016). (6(6 OctoberOctober The color 2016).2016). on theTheThe typhoon colorcolor onon paththethe typhoontyphoon is consistent pathpath is withis consistentconsistent Figure1 with.with The FiFi boldguregure black 1.1. TheThe solid boldbold line blackblack outside solidsolid thelineline blue outsideoutside * (cold thethe eddy) blueblue ** and(cold(cold the eddy)eddy) red * andand (warm thethe eddy) redred ** represents(warm(warm eddy)eddy) the boundaryrepresentsrepresents of thethe that boundaryboundary eddy. Red ofof solid thatthat contours eddy.eddy. RedRed represent solidsolid contourscontours positive SLA,representrepresent whereas positivepositive blue SLA,SLA, dashed whereaswhereas contours blueblue represent dasheddashed contourscontours negative represent SLArepresent or zero. negativenegative SLASLA oror zero.zero.

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As shown in FigureFigure 1414,, atat thethe beginningbeginning ofof thethe generationgeneration ofof Chaba,Chaba, the SLA value of the sea area where the typhoon passed waswas smallsmall ((−−1717 to to 6 6 cm), cm), and and the the maximum maximum wind wind speed speed and pressure of the typhoon center did notnot changechange much.much. From From 30 September to 2 October 2016, the typhoon passed through a region with a high SLA value (8–22(8–22 cm) and continued to strengthen and maintain its strength for for a a long long time. time. When When the the typhoon typhoon was was strongest, strongest, the the wind wind speed speed reached reached 60 60m/s m/s and and the centerthe center pressure pressure was was 920 920 hpa. hpa. After After passing passing thro throughugh the the shallow shallow sea sea area, area, the the typhoon typhoon gradually weakened and disappeared.

Figure 14. Variations ininmaximum maximum wind wind speed speed (blue (blue solid solid line) line) and and central central pressure pressure (green (green dotted dotted line) line)of Typhoon of Typhoon Chaba (Chabaupper ),( andupper change), and in change SLA (blue in solidSLA line,(bluebottom solid )line, on the bottom path of) Typhoonon the path Chaba. of TyphoonBlack dotted Chaba. box Black indicates dotted where box the indicates typhoon where changed the typhoon dramatically. changed dramatically.

4. Discussion 4. Discussion 4.1. Impact of Tropical Cyclones on Sea Surface Temperature (SST) 4.1. Impact of Tropical Cyclones on Sea Surface Temperature (SST) To our knowledge, there have been many observations and simulations of sea temperature To our knowledge, there have been many observations and simulations of sea temperature changes caused by typhoons, but most have been limited to the study of certain cases. We studied changes caused by typhoons, but most have been limited to the study of certain cases. We studied five consecutive typhoons that occurred in the Western Pacific in 2016, including LionRock, Meranti, five consecutive typhoons that occurred in the Western Pacific in 2016, including LionRock, Meranti, Malakas, Megi, and Chaba. Through the analysis of SST data and SLA data from ocean remote sensing, Malakas, Megi, and Chaba. Through the analysis of SST data and SLA data from ocean remote we found that the typhoon-induced cooling of the sea surface is roughly distributed on the left and sensing, we found that the typhoon-induced cooling of the sea surface is roughly distributed on the right sides of the typhoon track, with a range of about 100 km. We found that in areas where the left and right sides of the typhoon track, with a range of about 100 km. We found that in areas where SLA was positive before the typhoon, the degree of SSC after the typhoon passage was less than the SLA was positive before the typhoon, the degree of SSC after the typhoon passage was less than 3 ◦C. In areas where the SLA was negative before the typhoon, the degree of SSC after the typhoon 3 °C. In areas where the SLA was negative before the typhoon, the degree of SSC after the typhoon passage was usually greater than 3 ◦C. The cooling center was closer to the center of the cold eddies. passage was usually greater than 3 °C. The cooling center was closer to the center of the cold eddies. Among them, Typhoon LionRock caused the largest cooling area, as shown in Figure2. In the warm Among them, Typhoon LionRock caused the largest cooling area, as shown in Figure 2. In the warm eddies area where the SLA was positive, the degree of cooling was small, and was concentrated in eddies area where the SLA was positive, the degree of cooling was small, and was concentrated in the range of 2 to 3 ◦C. However, in the cold eddies area near the typhoon path with negative SLA, the range of 2 to 3 °C. However, in the cold eddies area near the typhoon path with negative SLA, the the cooling rate was relatively large, both above 3 ◦C and even reaching 5 ◦C. Moreover, at the turning cooling rate was relatively large, both above 3 °C and even reaching 5 °C. Moreover, at the turning point of Typhoon LionRock, a new cold eddy was formed, and the cooling rate was significantly point of Typhoon LionRock, a new cold eddy was formed, and the cooling rate was significantly higher than other regions (>5 ◦C).Typhoon Megi had an obvious cooling in the path area, as shown in higher than other regions (>5 °C).Typhoon Megi had an obvious cooling in the path area, as shown Figure9. The degree of cooling caused by the early typhoon was relatively small. However, after Megi in Figure 9. The degree of cooling caused by the early typhoon was relatively small. However, after developed into a super typhoon, it caused significant cooling in the path area. In addition, the location Megi developed into a super typhoon, it caused significant cooling in the path area. In addition, the location of the cooling area caused by Megi was basically the same as the distribution of cold eddies,

Water 2018, 10, x FOR PEER REVIEW 14 of 18 Water 2018, 10, 1326 14 of 18 and the cooling was more obvious in the area where the cold eddies were strong, while in the warm eddy areas, the cooling was lower than the surrounding area. of theIn cooling particular, area Malakas caused by passed Megi to was the basically left (Figur thee same6c) and as Megi the distribution passed over of the cold center eddies, (Figure and the10) coolingof a cold was eddy more located obvious to the in east the areaof Taiwan where Island. the cold The eddies intense were SSC strong, over whilethe cold in eddy the warm (shown eddy in areas,Figures the 5b cooling and 9) was suggested lower than that the both surrounding the shallower area. mixed layer and the uplifted thermocline structureIn particular, favored Malakasthe entrainment passed to to the bring left (Figuredeep 6coldc) and water Megi to passed the surface over the during center the (Figure typhoon 10) ofpassage. a cold As eddy a comparison, located to the Meranti east of passed Taiwan to Island. the southwest The intense of the SSC same over cold the eddy cold eddyat a distance (shown inof Figuresaround5 250b and km9 )away suggested from thatits track both (Figure the shallower 9a), and mixed it caused layer a and relatively the uplifted weaker thermocline SSC over structure the cold favorededdy center the entrainment(Figure 5a). to bring deep cold water to the surface during the typhoon passage. As a comparison, Meranti passed to the southwest of the same cold eddy at a distance of around 250 km away from4.2. Impact its track of Translation (Figure9a), andSpeed it causedof Tropical a relatively Cyclones weaker SSC over the cold eddy center (Figure5a).

4.2. ImpactThe time of Translation interval between Speed of typhoon Tropical Cyclonespositions indicated by colored dots on the tracks in Figure 1 is 12 h. Therefore, all the five typhoons are generally fast-moving except the LionRock near its The time interval between typhoon positions indicated by colored dots on the tracks in Figure1 is turning points (note that the local inertial period at 30° N is 24 h). Since neither subsurface historical 12 h. Therefore, all the five typhoons are generally fast-moving except the LionRock near its turning data nor any numerical model was applied here, accurate calculations of the first baroclinic mode points (note that the local inertial period at 30◦ N is 24 h). Since neither subsurface historical data nor speeds at each point are not available. However, an estimation of no more than 3 m/s in the any numerical model was applied here, accurate calculations of the first baroclinic mode speeds at sub-tropical and mid-latitude ocean is in principle reasonable, which means also only the LionRock each point are not available. However, an estimation of no more than 3 m/s in the sub-tropical and near its turning points reflected a subcritical situation of slow-moving typhoon, following the mid-latitude ocean is in principle reasonable, which means also only the LionRock near its turning criterion raised by Pan and Sun [24] upon the Froude number (the ratio of translation speed to first points reflected a subcritical situation of slow-moving typhoon, following the criterion raised by Pan mode baroclinic speed). and Sun [24] upon the Froude number (the ratio of translation speed to first mode baroclinic speed). Here, another criterion upon the ratio of local inertial period to application time of the typhoon Here, another criterion upon the ratio of local inertial period to application time of the typhoon wind-forcing is suggested, which reads: wind-forcing is suggested, which reads: D𝐷7 T𝑇I // U𝑈h wherewhere T𝑇I representsrepresents thethe locallocal inertialinertial period,period, D𝐷7 meansmeans thethe levellevel 77 wind-forcingwind-forcing diameterdiameter ofof thethe typhoon,typhoon, andand U𝑈his is the the translation translation speed. speed. The The LionRock LionRock case case is is shown shown in in Figure Figure 15 15..

FigureFigure 15.15. RatioRatio ofof locallocal inertialinertial periodperiod toto applicationapplication timetime ofof thethe typhoontyphoon wind-forcing.wind-forcing. MissMiss valuesvalues fromfrom 2525 AugustAugust toto earlyearly 2626 indicateindicate zerozero translationtranslation speed.speed.

AsAs shownshown inin FigureFigure 1515,, subcriticalsubcritical (slow-moving)(slow-moving) situationssituations were foundfound nearnear thethe twotwo turningturning pointspoints of LionRock around 22 22 August August and and from from 24 24 to to 26 26 August. August. In Inparticular, particular, from from 25 25to toearly early 26 26August, August, LionRock LionRock stalled stalled at atits its southwestern southwestern tip tip for for almost almost one one day. day. These subcritical situationssituations ◦ generatedgenerated strongstrong upwelling, upwelling, caused caused a significant a significant surface surface temperature temperature drop ofdrop nearly of 5nearlyC, and 5 reduced°C, and thereduced rightward the rightward bias of SSC, bias which of SSC, is consistentwhich is cons to theistent discussions to the discussions in literature in literature [7]. [7]. AnotherAnother noticeablenoticeable situationsituation is thethe comparablycomparably higherhigher initialinitial seasea surfacesurface nearnear thethe southwesternsouthwestern mostmost turningturning points (Figure3 3a).a). TwoTwo warm warm eddies eddies were were found found to to the the right right of of its its track. track.It It means means that that

Water 2018, 10, 1326 15 of 18 the LionRock was stalling over a warmer surface for quite a long time, absorbing vast energy, growing into a super typhoon and speeding up to move northeastward. It left behind a cold eddy generated by strong upwelling, which disappeared in two days possibly due to horizontal advection. This process can also be clearly seen in Figure4.

4.3. Relationship between Tropical Cyclones and Sea Level Anomaly (SLA) We found that when a typhoon crosses over a sea area, it causes SLA to respond strongly. The main action was that the SLA obviously decreased. After Typhoon LionRock passed over the sea, the cold eddies strengthened, while the warm eddies weakened, and the center of the cold eddy moved toward the area close to the typhoon path, while the warm eddy moved away from the typhoon path. After the passage of Typhoons Meranti and Malakas, SLA showed a different degree of reduction. After Typhoon Megi passed over, the small cold eddy that existed before the typhoon merged and intensified, and moved closer to the typhoon path. Similarly, the passage of Typhoon Chaba also created a low value SLA area, which was located in the southwest of the northwestern Pacific greater, and generated a stronger cold eddy at the typhoon path. We also found that the mesoscale eddies, which existed in the upper ocean before the typhoon, not only played an important role in the extent and location of the cooling of the sea surface, but also had an important impact on the typhoon intensity during the typhoon passage. During Typhoon LionRock, two different degrees of typhoon intensification occurred after the typhoon passed the higher (or warm eddies) areas of SLA. However, when the typhoon passed through regions with low SLA values, it maintained a constant intensity or weakened to a certain extent. The apparent weakening of Typhoon Meranti also occurred when it passed SLA-reduced areas or arrived on land, and the three growths in the life of Malakas occurred after passing through the relatively high value area of SLA. At the beginning of Typhoon Megi, when it passed through the area with high SLA, the wind speed increased, the pressure decreased, and the typhoon continued to grow. However, at the beginning of Typhoon Chaba, the SLA of the passing sea area was small, so the maximum wind speed and center pressure of the typhoon did not change much. The above results support the view that a typhoon passing through a warm eddy region (i.e., relatively high SLA area) will increase in intensity. After passing through a cold eddy region (i.e., relatively low SLA area), the typhoon intensity will remain unchanged or gradually weaken.

5. Conclusions In the present work, five strong or super typhoons in the Northwestern Pacific Ocean that occurred in 2016 were selected for case studies. Satellite altimeter data and SST data were used to analyze typhoon-induced sea surface response. After a typhoon passed, it caused a significant cooling of the sea surface. The degree of SSC was determined by the intensity, the translation speed of the typhoon, as well as pre-existing sea surface conditions. In general, the SST was intensively reduced after a typhoon passed when the initial SLA was negative, and the cooling centers were consistent with the centers of the cold eddies. On the contrary, if the initial SLA was positive, the degree of SSC was not obvious. Due to wind-induced upwelling, after a typhoon passed, the SLA obviously decreased. Therefore, typhoons have a significant impact on the distribution of cold and warm eddies. Pre-existing cold eddies are enhanced and their centers move toward the typhoon track, while warm eddies are weakened (or even disappeared) and moved away from the typhoon track. Considering the feedback from the ocean to the typhoon, when a typhoon passed over a high SLA region, the maximum wind speed significantly increased and the central air pressure decreased, which means that the typhoon has been intensified. When it passed over a low SLA region, the maximum wind speed remained unchanged or decreased. Upon a criterion based on the ratio of local inertial period to application time of the typhoon wind-forcing, subcritical (slow-moving) situations were found in the LionRock case. In particular, Water 2018, 10, 1326 16 of 18

LionRock was stalling at its southwest-most turning points over a comparably warmer sea surface for more than one day. It absorbed vast energy, developed into a super typhoon and speeded up to move northeastward, leaving behind a cold core eddy generated by strong upwelling. This distinctive case is worthy of further discussion in subsequent research.

Author Contributions: Conceptualization, D.S.; Methodology, D.S. and L.G.; Software, L.G. and Z.D.; Validation, D.S., L.G. and L.X.; Formal Analysis, L.G.; Investigation, Z.D.; Resources, L.G. and Z.D.; Data Curation, L.G. and Z.D.; Writing-Original Draft Preparation, L.G.; Writing-Review & Editing, D.S. and L.X.; Visualization, L.G., Z.D. and L.X.; Supervision, D.S.; Project Administration, D.S.; Funding Acquisition, D.S. Funding: This research was funded by the Ministry of Science and Technology of the People’s Republic of China under grant number 2016YFC1401404, the NSFC-Zhejiang Joint Fund for the Integration of Industrialization and Informatization under grant number U1709204, the Zhejiang Provincial Natural Science Foundation of China under grant number LY17D060003, and the Key Laboratory of Ocean Circulation and Waves of the Chinese Academy of Sciences under grant number KLOCW1704. Acknowledgments: We would like to thank Shuangyan He, Jiawang Chen and some other colleagues for their inspiring advices on the present work. We also deeply thank Jin Liu for his help in programming, and Jianlei Fan for proofreading. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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