Typhoon, tropical cyclone in the western Pacific, that passes over the ocean often leave notable traces along their track. A well-known sign is a cold wake (sea surface cooling) (D’Asaro et al., 2007; Dare & McBride, 2011; Zhang et al., 2016; Potter, 2018; Wu and Li, 2018; Yue et al., 2018; Park et al., 2019), which is frequently observed in chlorophyll blooms (Lin et al., 2003; Lin and Oey, 2016; Chacko, 2017; Pan et al., 2018; Lee et al., 2020). The typhoon-induced cold wake can be created by energetic vertical mixing or upwelling forced by cyclonic winds driving a divergence of surface water (D’Asaro et al., 2014; Liu et al., 2017). Such traces in the sea surface are restored within a week or so, while those in the sub-surface layer can persist for longer than weeks, even months (Park et al., 2011; Mei et al., 2013; Lin et al., 2017; Jeon et al., 2019; Lu et al., 2020).

The northwestern Pacific is eddy-abundant and high typhoon-activity region. When typhoons encounter warm or cold eddies, the oceanic response is significantly different; the warm (cold) eddy can usually suppress (enhance) sea surface cooling because of the thicker (thinner) upper-ocean mixed layer and more (less) warm water (Lin et al., 2005; Liu et al., 2017). In addition, typhoon-induced upwelling governed by typhoon translation speed influences cooling over the water column (Lin et al., 2017), and the trace in the subsurface layer can propagate across the basin (Jan et al., 2017).

The mesoscale eddies in the northwestern Pacific propagate westward overall and can encounter the Kuroshio, a western boundary current in the north Pacific. The eddies collided with the Kuroshio can significantly modulate the Kuroshio intensity (Tsai et al., 2015; Yan et al., 2016). The changes in the Kuroshio intensity can influence the Kuroshio intrusion into the East China Sea (ECS) through northeast Taiwan and along the ECS slope; for example, impinging cold eddies east of Taiwan increase the intrusion into the ECS (e.g., Wu et al., 2017; Ando et al., 2021; Liu et al., 2021; Shi and Wang, 2021). The intrusion into the ECS can have impacts on chlorophyll distribution and primary production in the ECS, as it occurred during the spring of 1993 in the northeast of Taiwan (Gong et al., 1996). Moreover, the intrusion causes subsurface-layer warming which have potential in ecosystem variability of the ECS (Kang and Na, 2022).

Zhang et al. (2020) reported the strengthening of the Kuroshio by intensifying Pacific-typhoon activities through enhanced positive potential vorticity anomalies by typhoon-induced changes in eddy fields. Nonetheless, the direct evidence demonstrating connectivity between Pacific-typhoons and the Kuroshio intensity is yet unclear, because it is generally difficult to track typhoon-traces directly existed in the subsurface layer due to the limited in situ observations and nonlinear eddy-evolving processes during their westward propagation revealed by the satellite altimetry (Chelton et al., 2011; Faghmous et al., 2015).

In this study, we show the connectivity between the typhoons and the Kuroshio intensity with a special focus on typhoons in October 2013 and an anomalously weak event of the Kuroshio intensity in February–May 2014. We present evidence showing the reinforcement of cyclonic cold eddies by consecutive three typhoons from in situ observations and satellite measurements including estimations of the ocean heat content (OHC) changes and sensible and latent heat fluxes at the sea surface. Then, we show that the enhanced cold eddies, the oceanic memory of typhoons, weakened the Kuroshio upstream (east of Taiwan) for approximately 3 months after the passage of typhoons over the northwestern Pacific.

An array of five pressure-recording inverted echo sounders (PIES) was deployed from June 2012 and 2013 to May 2014 in the eddy-abundant subtropical countercurrent and the North Equatorial Current regions of the western Pacific ( Figure 1 ). Four sites (F03, F05, F08, and F12) spanned from June 2012 through May 2014, and site PS1 spanned the last year from June 2013 through May 2014. The PIES records the bottom pressure (P _bot) and round-trip acoustic travel time from the seafloor to the sea surface (τ). The accuracy of τ is 0.05 ms, and that of Paroscientific quartz P _bot sensor is ±0.01% with a resolution of 0.1 mbar (Kennelly et al., 2007).

Historical hydrocasts from EN4.2.1 were used to establish a linear relationship between the geopotential height anomaly (GPHA) and the OHC anomaly with a deep reference level of 1,500 dbar ( Figure 2 ). In total, 30,175 profiles were used in regions of 10–26°N and 130–150°E. Using this relationship, the SSH anomaly was converted to an OHC anomaly.

As in previous studies (e.g., Park et al., 2012; Donohue et al., 2016), τ and P _bot were converted to steric and mass-loading sea-surface heights (SSH) using historical hydrocasts from EN4.2.1 (Good et al., 2013); the total SSH, calculated as the sum of steric and mass-loading components, was dominated by the steric component and compared with satellite-measured SSH. More details about SSH conversion and error estimates are described in Jeon et al. (2018).

The PIES-derived total SSH were in good agreement with the satellite-measured SSH (after-mentioned all-satellite product) and showed correlation coefficients in the range of 0.79–0.88 ( Figure 3 ). Two types of delayed-time mapping of absolute dynamic topography (ADT) and sea level anomaly daily products were used: two-satellite and all-satellite products. Both types were gridded with 0.25°×0.25° spatial resolution from CMEMS (https://marine.copernicus.eu/, Product user manual for sea level anomaly products, March 2020). Eddy-tracking spanning from October 2013 to May 2014 and SSH comparisons were conducted using an all-satellite product, while a two-satellite product was employed for long-term Kuroshio variability from January 1993 to December 2018 (26 years).

The Kuroshio path was defined as the maximum SSH gradient point along the Kuroshio every 10 km from the east of Taiwan to the southwest of Japan using a 26-year-mean ADT field (black and red lines, Figure 1 ). The lines perpendicular to the Kuroshio axis cover a ±100-km range from the Kuroshio core (green line, Figure 1 ). The Kuroshio intensity was estimated from the SSH gradient in the range of the perpendicular line (thick red line).

Sensible (Q_s ) and latent (Q_l ) heat fluxes between the atmosphere and the ocean (positive upward) were calculated in the typhoon region using the following bulk formulae:

where ρ_a is the air density (= 1.22 kg m ^-3), c_p is the specific heat of air at a constant pressure (=1,004 J kg^–1°C ^-1), L_vap is the latent heat of vaporization (=2.5×10⁶ J kg^–1), ∣W∣ is the wind speed, SST is the sea surface temperature, and q_s is the saturated specific humidity at the SST (here, we assumed that q_s is at 98% saturation at the SST). T_a and q_a are the air temperature and specific humidity at 10-m height above the sea surface, respectively. The exchange coefficients of sensible and latent heat fluxes, C_h and C_q , respectively, were estimated as suggested by Jaimes et al. (2015). For the SST, we used a Multi-sensor Improved SST, an optimally interpolated cloud‐free daily satellite product extracted from microwave and infrared SSTs at a spatial resolution of 10 km (Gentemann et al., 2009). The T_a and q_a were obtained from the Modern‐Era Retrospective Analysis for Research and Applications, version 2 reanalysis data (time interval of 1 hr and horizontal resolutions of 0.5° latitude × 2/3° longitude; Gelaro et al., 2017). We reconstructed the radial structure of the typhoon wind field using the method proposed by Chavas et al. (2015), referring to the maximum typhoon wind at a specific 6-hr interval location and time from the Joint Typhoon Warning Center (JTWC). SST and reconstructed wind fields were interpolated to 1-hr data to coincide with specific humidity and air temperature variables.

Isopycnal displacement or shoaling of the thermocline (η) due to storm-induced upwelling (Price et al., 1994; Babin et al., 2004; Nam et al., 2012) was calculated as η = τ / ( ρ_ofU_H ), where τ is the wind stress, ρ_o is the water density (=1022 kg m^–3), f is the Coriolis parameter (s^–1), and U_H is the typhoon translation speed. Wind stress was calculated using typhoon wind from JTWC based on the formula (Gill, 1982) and drag coefficients (Large and Pond, 1981; Trenberth et al., 1990). The η was computed as ΔSSH using the following equation (Shay et al., 2000; Walker et al., 2005): Δ S S H=−η g ^′/g , where g is gravity and g ^′ is reduced gravity (=0.015 m s^–2 as used in the similar area by Nam et al. (2012)).

In October 2013, three consecutive typhoons (Danas, Wipha, and Francisco) passed over the pre-existing cold eddies from southeast to northwest, in the region spanning from 12 to 26°N and 132 to 144°E. In situ PIES moorings and satellite altimetry measurements revealed that pre-existing cold eddies were notably reinforced by the typhoons, producing substantial sea level drops of 0.1–0.3 m. The typhoon-enhanced cold eddies propagated westward over approximately 1,000 kilometers and met the Kuroshio upstream approximately 4 months after the typhoon passages. The cold eddies significantly reduced the Kuroshio intensity in February–May 2014, recording the lowest intensity over the last 26 years and corresponding to a 24% decrease from the mean Kuroshio intensity.

Our work can provide a new mechanism linking typhoon-to-eddy-to-Kuroshio variability through the long-term oceanic memory. In a warming climate, the typhoon intensity in wind and rainfall will increase (Knutson et al., 2010; Patricola & Wehner, 2018), implying that intensified typhoons may have more chances to produce oceanic memories and their impacts on the western boundary currents such as the Kuroshio. Therefore, this new mechanism is maybe likely to play a role in Kuroshio variability in the future.

Typhoon track is at https://www.metoc.navy.mil/jtwc/jtwc.html?western-pacific. Two-satellite ADT (SEALEVEL_GLO_PHY_CLIMATE_L4_REP_OBSERVATIONS_008_057) and all-satellite ADT (SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_008_047) are available at https://resources.marine.copernicus.eu/product-detail/SEALEVEL_GLO_PHY_CLIMATE_L4_REP_OBSERVATIONS_008_057/INFORMATION and https://resources.marine.copernicus.eu/product-detail/SEALEVEL_GLO_PHY_L4_REP_OBSERVATIONS_008_047/INFORMATION, respectively. The SST is found at https://data.remss.com/SST/daily/mw_ir/. Historical hydrocasts (EN4.2.1) are at https://www.metoffice.gov.uk/hadobs/en4/. PIES-derived SSH can be downloaded https://github.com/PADOLab1/NEC_PIES.git.

CJ: primary writing, in situ observation, data processing, and calculations. DRW, HSM, SKK, and I-JM: discussion. DGK: in situ observation and discussion. J-HP: overall coordination, discussion, in situ observation. All authors modify the manuscript. All authors contributed to the article and approved the submitted version.

This work was supported by “Study on Northwestern Pacific Warming and Genesis and Rapid Intensification of Typhoon” funded by the Korean Ministry of Oceans and Fisheries(20220566)” and “The study for the cooperative use of the research vessel” funded by Korea Institute of Ocean Science and Technology”, and also supported by Pusan National University Research Grant, 2021.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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