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Strengthening of the Kuroshio Current by Intensifying Tropical Cyclones

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Strengthening of the Kuroshio Current by Intensifying Tropical Cyclones RESEARCH OCEAN CIRCULATION geostrophic adjustment, was first considered by Rossby in 1938 (19) and has since been in- Strengthening of the Kuroshio current by intensifying vestigated in a variety of contexts both linear and nonlinear (20–23). Common to all those tropical cyclones studies is that the flows in question hold less energy at their end states than they do initially, Yu Zhang1*, Zhengguang Zhang1, Dake Chen2,3, Bo Qiu4, Wei Wang1 and energy is dispersed in the form of inertial gravity waves ( 19, 24). It is therefore antici- A positive feedback mechanism between tropical cyclones (TCs) and climate warming can be pated that eddies under the influence of strong seen by examining TC-induced energy and potential vorticity (PV) changes of oceanic geostrophic storms may be perturbed and subsequently eddies. We found that substantial dissipation of eddies, with a strong bias toward dissipation of undertake adjustment processes that atten- anticyclonic eddies, is directly linked to TC activity. East of Taiwan, where TCs show a remarkable uate them (25). intensifying trend in recent decades, the ocean exhibits a corresponding upward trend of positive Evidence continues to mount that both PV anomalies. Carried westward by eddies, increasing numbers of positive PV anomalies impinge on mechanisms described above exist, working the Kuroshio current, causing the mean current to accelerate downstream. This acts in opposition together to reduce the strength of anticyclonic to decreasing basin-scale wind stress and has a potentially important warming impact on the eddies but acting against each other in chang- extratropical ocean and climate. ing cyclonic eddies. Which process dominates, and what overall influence the TCs exert on an underlying eddy field, remains unclear. To this n a warming climate, very intense tropical ways. For example, in central areas of TCs, end, a survey of eddies’ lifetime evolution—in cyclones (TCs; categories 4 and 5 on the strong and positive wind stress curl domi- particular, immediately after their meeting Downloaded from Saffir-Simpson scale) are becoming more nates, producing divergent surface currents, with TCs—is necessary. The advent of satellite I intense and frequent, as proposed by theo- forcing cool water upward from below, and altimetry has enabled global and synoptic retical and modeling studies (1–4)and elevating isopycnals as much as 100 m, in the mapping of eddies with prominent surface supported by observations (5, 6). This behavior same sense as the circulation of cyclonic eddies signatures (26). The later combination of data raises the question of what feedbacks can be and the associated upward displacements of from two altimeters has provided a time-series induced by TC changes on climate. Much prior isopycnals. Hence, oceanic cyclonic eddies can record of global maps of ocean eddies at high http://science.sciencemag.org/ work has been focused on TCs’ interaction be enhanced or generated (16–18)whiletheir spatial and temporal resolutions [DT-2014 daily with the quiescent upper ocean. For example, mirror images, anticyclonic eddies, can be “two-sat merged” gridded product provided the decay of cold ocean wakes produced by weakened. In outer regions of TCs, the wind by archiving, validation, and interpretation storm-driven mixing causes net heating of stress curl is negative and very weak. Although of satellite oceanographic data (AVISO) (27)]. the upper ocean and has been suggested to the effects are to strengthen anticyclonic ed- On the basis of that record, an eddy-tracking have important effects on global climate (7, 8). dies yet weaken cyclonic eddies, opposite to data archive was constructed, providing at The ocean is anything but quiescent or uni- those inside TCs’ cores, the amplitudes are daily frequency the center, amplitude, polar- form, though, and is full of dynamical struc- much reduced. In the meantime, at the small- ity, radius, and rotational velocity for those tures of different scales.Amongthem,rotating est scale satisfying geostrophy, mesoscale eddies eddies identified and tracked by altimetry (28). structures with spatial scales of order O (10 to are always close to geostrophic balance. If this In addition, Argo floats have provided depth 100 km), temporal scales of order O (100 days), balance is disturbed through processes such as profiles of temperature and salinity, from which on May 28, 2020 and vertical extents of 1000 m or more, usually the shoaling of isopycnals by strong storms, eddies’ vertical structures can be extracted referred to as mesoscale eddies, are the most the flow will adjust itself back to a state of (29). Taking advantage of both AVISO and ubiquitous and energetic; they play an essen- geostrophic balance. This process, known as Argo data, a law for the universal structure of tial role in transporting material and energy, and they modulate large-scale ocean circula- 9–11 tion ( ). Fig. 1. Changing rate of eddy energy Because mesoscale eddies are ubiquitous, versus distance from TC center. their encounters with TCs are not rare. By Mean changing rate of eddies’ total modulating subsurface density as well as ther- energy is plotted against radial distance, mal conditions, these synoptic-scale structures averaged over 15 days after passing of have been found to greatly affect the evo- TCs, for anticyclonic eddies (red curves) ’ lution of a storm s strength, posing a big and cyclonic eddies (blue curves). challenge for operational prediction of TC in- Curves with circles are for all TCs in the 12–15 tensity ( ). Meanwhile, TCs can also have western North Pacific Ocean between a large impact on eddies through more than 1993 and 2014; those with squares are one mechanism and in different or opposite for more intense winds with speed greater than 40 m s–1. 1Physical Oceanography Laboratory/CIMST, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, P. R. China. 2State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, P. R. China. 3Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, P. R. China. 4Department of Oceanography, University of Hawai‘iat Manoa, Honolulu, HI, USA. *Corresponding author. Email: [email protected] Zhang et al., Science 368, 988–993 (2020) 29 May 2020 1of6 RESEARCH | RESEARCH ARTICLE eddies was uncovered (30), making it possible storm center generally experience substantial different origins. In the Kuroshio Extension to accurately infer an eddy’s three-dimensional weakening afterward. On average, the energy region, a high eddy kinetic energy level arises structure from its surface signal. Using 20 years decay rate declines from the storm center mono- from the meandering of the current itself (32), of these datasets in combination with TC ob- tonically outward, asymptotically approaching while along latitudes around 25°N, eddies are servations in the western North Pacific, we the background dissipation level that is weaker generated in the shearing flows between the carried out an analysis of the evolution of than the central maximum value by a factor of eastward Subtropical Counter Current located three-dimensional eddy energy and potential 2 to 3 (red curves in Fig. 1). Cyclonic eddies’ in the 18° to 25°N band and the underlying vorticity fields with a special focus on storm responses, although close to those of anticy- westward North Equatorial Current (33). After effects. [Potential vorticity (PV) is a dynam- clonic eddies’ responses farther away from the generation, these eddies propagate mainly west- ically conserved quantity depicting the ten- storm, exhibit a tendency to strengthen pro- ward at speeds of a few centimeters per second, dency of a rotating fluid to spin.] Our results gressively as they approach the storms’ centers evolve as they move, enter and pass through the indicate that the interaction with TCs can in- (blue curves in Fig. 1). Thus, TCs not only cause region of strong TC activity, and finally collide troduce an important, newly recognized fea- the entire eddy field to decay substantially with the Kuroshio before they disappear (32). ture of the eddy field, which, together with more quickly than otherwise might have oc- Second, seasonal variations of the eddies’ decay the strengthening trend of TCs in a warming curred but also make cyclonic eddies relatively rates in the two regions have different origins. climate, has potentially strong influence on stronger than anticyclonic eddies. In the TC-active region, the seasonal variation large-scale ocean circulation and climate. As seasonal phenomena, TCs develop mostly of the decay rate is in phase with that of TC during the months of June through November, activity (Fig. 2B), and so is the difference of Eddy response to TCs with peak activity occurring usually in late sum- decay rates between anticyclonic and cyclonic Figure 1 shows the results of a survey of the mer (31). If TCs’ effect is dominant in driving eddies (see supplementary materials). Both rates of energy change of eddies during the eddy dissipation, the year-round energy evolu- pieces of evidence indicate TCs’ vital role in Downloaded from 15-day period after their encounters with TCs tion of eddies should exhibit similar seasonality. causing eddy dissipation. In the Kuroshio Ex- in the western North Pacific Ocean (see sup- In the North Pacific Ocean, only two areas tension region, the seasonal variation of the plementary materials). The rate of change de- demonstrate strong seasonal variation of the decay rate is almost 180° out of phase with pends on factors including storm wind speed, decay rates of eddies (Fig. 2A), and both are that in the south, being strong in winter and eddy polarity, and the radial distance, r,be- near the western boundary: One located at spring but weak in summer and autumn (Fig. tween an eddy and the storm’scenteratthe 20° to 30°N and 120° to 130°E, east of Taiwan, 2C).
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