**3. Upper ocean current response to TCs**

#### **3.1. In the open ocean**

Ekman pumping. The warm water is extracted into the subsurface from the surface by the

The SCS is the largest semi‐enclosed marginal sea of the WNP. Many TCs pass through the SCS and some are born in this sea every year [3]. Compared with the open ocean of the WNP, the SCS displays a similar but some different temperature response to typhoons due to its

Using the Princeton Ocean Model, Chu et al. [19] showed that Typhoon Ernie (1996) induced a significant SST cooling with a rightward bias in the SCS, similar to that in the open ocean. But it also caused some unique responses such as the SST warming in the region from southwest of Taiwan Island to northwest of Luzon Island because of the convergence between the northward coastal current west of Luzon and the Kuroshio intrusion current through the

Owing to the shallow pre‐typhoon mixed layer and thermocline, Typhoons Kai‐Tak (2000), Lingling (2001) and Megi (2010) generated a very large SST drop of 10.8°C, 11°C and 8°C, respectively, in the SCS [8, 20, 21]. Chiang et al. [8] suggested that upwelling (account for 62%) dominated vertical mixing (31%) in producing the SST cooling under the influence of Kai‐Tak, a weak and slowly moving typhoon. Megi's translation speed was 5.5–6.9 m/s over the ocean east of the Philippines, faster than 1.4–2.8 m/s over the SCS, and the pre‐typhoon mixed layer depth in these two regions was about 40 m and 20 m, respectively [22]. As a result, the SST cooling in the former was only 1–2°C, quite smaller than that in the SCS. Based on the mooring observations in the northern SCS during Megi, Guan et al. [23] showed that the temperature cooling occurred in the entire observed water column (60–360 m), which was mainly caused

After comparing the temperature responses to TCs in the SCS and in the tropical ocean of the NWP, Mei et al. [24] found that under the influence of TCs with an identical intensity, the SST cooling in the SCS is more than 1.5 times that in the tropical ocean, which could be attributed to the shallower mixed layer and stronger subsurface thermal stratification in the former. Numerical simulations showed that Typhoon Nuri (2008) induced a stronger SST cooling in the SCS than in the open ocean of the WNP when it travelled northwestward from the open ocean to the SCS [25]. Sun et al. [25] indicated that three processes are responsible for the different regional responses. Firstly, the SCS has a thinner mixed layer, which makes it easier to entrain cooler subsurface water into the surface layer. Secondly, the cyclonic background vorticity in SCS allows stronger current shears and turbulent eddy diffusivity to be generated, however, the background vorticity in the open ocean is anticyclonic. Finally, as the typhoon moved to higher latitude in SCS, the larger Coriolis frequency in the SCS is more favorable for producing stronger wind‐current resonance and then stronger inertial amplitudes and

downwelling and subsequently moves downstream with Kuroshio current.

unique hydrological environment and complex topography.

6 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

**2.3. In the SCS**

Luzon Strait.

turbulence.

by typhoon‐induced upwelling.

Geisler [26] used a two‐layer ocean model to investigate the linear response of ocean to a moving hurricane. He concluded that inertial‐gravity waves are the dominant feature of the upper ocean response if TC's translation speed exceeds the phase speed of the first baroclinic mode, and inversely the oceanic response is a barotropic, geostrophical, and cyclonic gyre with upwelling in the storm's center. In the upper ocean, a TC typically generates near‐inertial currents on the right side of the TC track (looking in the moving direction) in the Northern Hemisphere and causes the resonance between wind stress vectors and currents [4]. The wind stress rotates clockwise with time on the right of the track, which is homodromous with the near‐inertial currents. In contrast, the wind stress rotates anticlockwise, against the near‐ inertial currents, on the left. The shear of near‐inertial current across the mixed layer base can deepen the mixed layer. The near‐inertial currents decay rapidly within a few inertial cycles, propagating downward into the thermocline and even deep ocean as near‐inertial internal waves [27, 28].

Based on the observations from drifters during 1985–2009, Chang et al. [29] illustrated the composited near‐surface ageostrophic currents under all recorded TCs with various intensity levels in the WNP. Strongest current is shifted to the right of the TC track. On average, the maximum velocity increases with the intensity of the TC and that for category 4 and 5 TCs may exceed 2.0 m/s. Moreover, they found that the near‐surface current responses depend on the TC translation speed as presented by Geisler [26]. Hormann et al. [30] also observed a rightward shift of maximum near‐inertial currents associated with Typhoon Fanapi (2010). Observed peak current magnitudes are up to 0.6 m/s and the e‐folding decay time of the strong near‐inertial currents within the cold wake is about 4 days.

Analyzing the observations of the upper ocean currents induced by four category‐5 typhoons [Chaba (2004), Maon (2004), Saomai (2006) and Jangmi (2008)] in the WNP, Chang et al. [31] found that besides the rightward shift, the maximum mixed layer current velocity increased with the decreasing translation speed of the four typhoons. The maximum current velocities varied from 1.2 m/s to 2.6 m/s when the translation speed of typhoons changed from 8.1 m/s to 2.9 m/s. Additionally, the maximum current velocity shows a proportional relationship with the Saffir–Simpson hurricane scale of typhoons.

#### **3.2. Influence on the Kuroshio current**

The Kuroshio is a strong western boundary current in the WNP, similar to the Gulf Stream in the North Atlantic. Under the influence of Typhoon Hai‐Tang (2005), the Kuroshio axis northeast of Taiwan Island moved onto the shelf [16, 18]. The average speed of the Kuroshio surface current increased by 18 cm/s after the typhoon's passage. The northward wind in the eastern part of the typhoon caused a coastal upwelling along the east coast of Taiwan Island and an east‐west sea level slope was set up. This sea level slope generated a northward geostrophic current which enhanced the Kuroshio and pushed it onto the shelf northeast of Taiwan Island [18].

Zheng et al. [15] simulated and described the response of the Kuroshio to Typhoon Morakot (2009) in the region east of Taiwan Island. When Morakot came to the Kurosho, the south‐ ward wind before the typhoon center forced the surface flow of the Kuroshio to slow down to zero. Subsequently, the surface flow strengthened as Morakot got closer to the Kuroshio. There was a sudden speedup in the surface flow due to the disappearance of strong south‐ ward wind and the release of accumulated potential energy. Before Morakot approached the Kuroshio, the Kuroshio main stream was shifted eastward for more than 1.5°. The shift‐ ed main stream returned to its original location when the typhoon center went through the Kuroshio. As the surface flow slowed down, the Kuroshio main core shifted from the sur‐ face to the depth of 50–100 m and its maximum speed decreased from more than 1.3 m/s to less than 1.1 m/s.

Using three ADCPs deployed in the area east of Taiwan Island, Yang et al. [32] revealed that during the period of 2014–2015, the volume transport of the Kuroshio was reduced by six typhoons which moved almost from southeast to northwest in the region east of the island, but intensified by two typhoons which travelled northward.

#### **3.3. In the SCS**

As frequently appearing strong weather systems, typhoons have potential influence on lo‐ cal currents and large‐scale circulations in the semi‐enclosed SCS. The large‐scale circula‐ tions in the SCS are controlled mainly by strong northeast monsoon wind in winter and by weak southwest monsoon wind in summer. As a result, a basin‐wide cyclonic gyre appears in winter and it is replaced by a large diploe structure (a cyclonic gyre in the north and an anticyclonic gyre in the south) in summer. Under the influence of typhoons, both the cy‐ clonic and anticyclonic gyres are intensified in summer while the northern and southern parts of the cyclonic gyre are intensified and weakened, respectively, in winter except Octo‐ ber and November when both are intensified [33]. Additionally, accumulative effect of ty‐ phoons can affect mean mesoscale structures: weakening the cyclonic eddy northeast of Luzon Island and enhancing the cyclonic and anticyclonic eddies off Vietnam central coast [33].

Typhoons often trigger near‐inertial waves or near‐inertial oscillations (NIOs) in the SCS (e.g., [23, 34, 35]). Based on ADCP observations in the northern continental shelf of the SCS, Sun et al. [35] showed that Strong NIOs were generated by Typhoon Fengshen (2008) and lasted for about 15 days after the passage of the typhoon. A similar phenomenon was induced by Typhoon Chanchu (2004) in the west of the SCS [36]. Using the observations from an ADCP mooring deployed in the northern SCS, Chen et al. [34] found that Typhoon Nangka (2009) triggered an intensive NIOs while Typhoon Linfa (2009) did not, although they both passed by the mooring in the same month of June. This is because the mooring was located in the right of Nangka's moving track but in the left of Linfa's track. As a result, the wind stress affecting the mooring location rotated clockwise during Nangka but counterclockwise during Linfa. Yang et al. [37] demonstrated that the second baroclinic mode dominated in the NIOs appear‐ ing after the passage of Typhoon Nesat (2011) in the northern SCS.

In the SCS, the signals of internal solitary waves (ISWs) often appear in current observations during the influence periods of typhoons. Xu et al. [38] observed a series of ISWs excited by a tropical storm Washi (2005) in the Northwestern SCS. The response of the ISWs was related to direct wind forcing and remote forcing from the inertial internal waves generated by Washi. Such ISWs were also observed in the northern SCS after the passage of Typhoon Neast (2011) [39] and in other marginal seas of the WNP after a typhoon passed over [40].

#### **3.4. In the Taiwan Strait**

geostrophic current which enhanced the Kuroshio and pushed it onto the shelf northeast of

Zheng et al. [15] simulated and described the response of the Kuroshio to Typhoon Morakot (2009) in the region east of Taiwan Island. When Morakot came to the Kurosho, the south‐ ward wind before the typhoon center forced the surface flow of the Kuroshio to slow down to zero. Subsequently, the surface flow strengthened as Morakot got closer to the Kuroshio. There was a sudden speedup in the surface flow due to the disappearance of strong south‐ ward wind and the release of accumulated potential energy. Before Morakot approached the Kuroshio, the Kuroshio main stream was shifted eastward for more than 1.5°. The shift‐ ed main stream returned to its original location when the typhoon center went through the Kuroshio. As the surface flow slowed down, the Kuroshio main core shifted from the sur‐ face to the depth of 50–100 m and its maximum speed decreased from more than 1.3 m/s to

Using three ADCPs deployed in the area east of Taiwan Island, Yang et al. [32] revealed that during the period of 2014–2015, the volume transport of the Kuroshio was reduced by six typhoons which moved almost from southeast to northwest in the region east of the island,

As frequently appearing strong weather systems, typhoons have potential influence on lo‐ cal currents and large‐scale circulations in the semi‐enclosed SCS. The large‐scale circula‐ tions in the SCS are controlled mainly by strong northeast monsoon wind in winter and by weak southwest monsoon wind in summer. As a result, a basin‐wide cyclonic gyre appears in winter and it is replaced by a large diploe structure (a cyclonic gyre in the north and an anticyclonic gyre in the south) in summer. Under the influence of typhoons, both the cy‐ clonic and anticyclonic gyres are intensified in summer while the northern and southern parts of the cyclonic gyre are intensified and weakened, respectively, in winter except Octo‐ ber and November when both are intensified [33]. Additionally, accumulative effect of ty‐ phoons can affect mean mesoscale structures: weakening the cyclonic eddy northeast of Luzon Island and enhancing the cyclonic and anticyclonic eddies off Vietnam central coast

Typhoons often trigger near‐inertial waves or near‐inertial oscillations (NIOs) in the SCS (e.g., [23, 34, 35]). Based on ADCP observations in the northern continental shelf of the SCS, Sun et al. [35] showed that Strong NIOs were generated by Typhoon Fengshen (2008) and lasted for about 15 days after the passage of the typhoon. A similar phenomenon was induced by Typhoon Chanchu (2004) in the west of the SCS [36]. Using the observations from an ADCP mooring deployed in the northern SCS, Chen et al. [34] found that Typhoon Nangka (2009) triggered an intensive NIOs while Typhoon Linfa (2009) did not, although they both passed by the mooring in the same month of June. This is because the mooring was located in the right of Nangka's moving track but in the left of Linfa's track. As a result, the wind stress affecting the mooring location rotated clockwise during Nangka but counterclockwise during Linfa.

but intensified by two typhoons which travelled northward.

8 Recent Developments in Tropical Cyclone Dynamics, Prediction, and Detection

Taiwan Island [18].

less than 1.1 m/s.

**3.3. In the SCS**

[33].

A strait is a special sea area that connects two different sea waters. Thus, it is of great importance for material exchange and energy transfer between the two sea waters. Here we take the Taiwan Strait joining the ECS and the SCS as an example of strait in the WNP. The Taiwan Strait is a wide and long channel with an average depth of about 60 m, bounded by the Chinese Mainland to the west and Taiwan Island to the east.

Several typhoons pass over or pass by the Taiwan Strait every year. A typhoon can induce strong southward currents and reduce or reverse northward transport through the Taiwan Strait temporarily [41–44]. Chen et al. [41] observed that the northward currents in the northern end of the strait were reversed after the passage of Typhoons Rusa and Sinlaku in 2002. Based on buoy observations and numerical model simulations, Zhang et al. [42] found that five typhoons reversed northeastward current in the middle of the Taiwan Strait and induced five southward transport events through the strait during the period of 27 August to 5 October 2005. These southward transport events were directly forced by wind stress and/or along‐strait water level gradient associated with the typhoons [42]. A similar southward transport event during Typhoon Krosa (2007) was simulated by Lin et al. [44]. The observations of the drifters deployed in the Taiwan Strait revealed current reversal when Typhoon Hai‐Tang (2005) traversed the strait [43].

However, the southward transport event does not always occur under the effect of typhoons. Zhang et al. [45] identified four typhoons in 2005–2009 that enhanced northward transport through the Taiwan Strait. These typhoons travelled westward in the area south of the strait or moved northward from the south to the north along special tracks, resulting in a weak southward atmospheric forcing in the early stage and a strong northward atmospheric forcing in the later stage. Meanwhile, the effect of ageostrophic process generated by the atmospheric forcing also contributed to the enhanced northward transport.

The accumulative effect of all typhoons can modify monthly mean transports during the typhoon season and even annual mean transport through the Taiwan Strait. Based on numer‐ ically simulated results, Zhang et al. [46] demonstrated that if the effects of typhoons are considered, the monthly mean transport and annual mean transport are reduced by up to 0.45 Sv and 0.09 Sv (more than 10%), respectively, compared with those without typhoons.
