The responses of cyclonic and anticyclonic eddies to typhoon forcing: The vertical temperature‐salinity structure changes associated with the horizontal convergence/divergence

Abstract

The responses of the cyclonic eddies (CEs) and anticyclonic eddies (AEs) to typhoon forcing in the Western North Pacific Ocean (WNPO) are analyzed using Argo profiles. Both CEs and AEs have the primary cooling at the surface (0–10 m depth) and deep upwelling from the top of thermocline (200 m depth) down to deeper ocean shortly after typhoon forcing. Due to the deep upwelling, part of warm and fresh water at the top of AEs move out of the eddy, which leads to a colder and saltier subsurface in the AEs after the passage of the typhoon. In contrast, the inflow of warm and fresh water heats and freshens the subsurface in the CEs to compensate the cooling induced by the typhoon. This explains why the observed strong SST cooling were much less than modeled, since the AEs are more frequent than CEs in the WNPO. It indicates that there is divergence (convergence) of warm and fresh water in the surface of AEs (subsurface of CEs). The divergence‐convergence effects of the AEs and CEs lead to the secondary cooling center locate at a shallow layer of 100 m in the AEs and a much deeper layer of 350 m in the CEs. This shallow divergence‐convergence flow could lead a shallow overturning flow in upper oceans, which may potentially influences the large‐scale ocean circulations and climates.

Impact of Cyclonic Ocean Eddies on Upper Ocean Thermodynamic Response to Typhoon Soudelor

“…As the water gets deeper, the two curves become closer. As pointed out by Liu et al [29], the stronger the nonadiabatic processes, the larger the shift distance of the curve. It is worth noting that there exists a larger shift distance in our study than Liu's work, implying that the nonadiabatic process related to horizontal advection plays an important role in the upper ocean response.…”

“…As shown in Figure 3a,b, the subsurface temperature increases obviously after the typhoon. One mechanism that could result in this phenomenon is the typhoon-induced vertical mixing which brings subsurface cold water up to the surface and pushes surface warm water down to the subsurface at the same time, as discussed in previous studies [1,29]. However, it is notable that the Argo float (130.3°E, 21.6°N) inside the CE observes abnormal warming in subsurface with a maximum temperature increase of 4.37 °C, which is much larger than that (1.74 °C) outside the eddy (128.1°E, 23.4°N).…”

“…The Argo program, which collects the in situ temperature and salinity profiles in the upper ocean, provides opportunities to better understand the subsurface response to typhoons [25][26][27][28]. It appears that some important physical processes under complicated ocean backgrounds, such as the pre-existing mesoscale eddies and initial mixed layer depth, which could influence the temperature change during the passages of typhoons, might be ignored [28,29].…”

Role of the South China Sea Summer Upwelling in Tropical Cyclone Intensity

“…This is because typhoon intensity is not only related to the pre-existing atmospheric conditions but also to the upper ocean [13] [14] [15]. Evidences from numerous studies indicate that typhoon-induced strong vertical ocean mixing can bring deep cold water to the surface, and make sea surface temperature (SST) cooler than the ambient waters by up to 2˚C to 6˚C [16] [17] [18] [19]. An unusually intense SST cooling of 10.8˚C was even observed when typhoon Kai-Tak passed over the NSCS [20].…”

Introduction to Special Section on Oceanic Responses and Feedbacks to Tropical Cyclones

“…Usually, anticyclonic eddies carry warm sea surface temperature (SST) anomalies and are favorable for the rapid intensification of TCs (Mawren & Reason, ). In addition to the surface heat flux, the horizontal convergence/divergence induced by eddies are also large enough to modify the temperature and salinity anomalies induced by TCs (Liu et al, ), which indicates that a three‐dimensional ocean structure and horizontal advection are inevitable to have a comprehensive understanding of the oceanic responses to TCs. Below the upper ocean mixed layer, cold water rises due to the Ekman pumping (Zhang et al, ) and a secondary cooling center is found in both the cyclonic and anticyclonic eddies but at different depths.…”