Wen-Zhou Zhang

Typhoons typically enhance phytoplankton biomass in the upper ocean, as evidenced by elevated sea surface chlorophyll ‐a concentrations (Chl‐ a ). However, subsurface Chl ‐a responses remain debated, leading to uncertainty about whether vertically integrated Chl ‐a over upper depth increases or decreases. Using a coupled physical‐biogeochemical model, this study reveals that the depth‐averaged Chl ‐a response to a typhoon exhibits distinct spatial patterns relative to the storm's track. Near the typhoon's path, intense vertical mixing penetrates beyond the nutricline, transporting deep nutrients into the upper ocean. The supplied nutrients fuel phytoplankton growth in both surface and subsurface layers, thereby increasing depth‐averaged Chl‐ a over upper 150 m. However, the increase in subsurface Chl ‐a driven by biogeochemical processes is smaller than the decrease caused by physical processes, resulting in a net reduction of Chl ‐a in the subsurface layer. This ultimately leads to a significant increase in the surface layer, a minor decrease in the subsurface layer, and an overall rise in total Chl‐ a concentration. In contrast, areas farther from the track experience mixing that exceeds the mixed layer depth but remains shallower than the nutricline. Here, mixing redistributes subsurface Chl ‐a to the surface without uplifting new nutrients. Consequently, surface Chl ‐a rises, while subsurface levels decline, leaving the depth‐averaged Chl ‐a largely unchanged. These spatially divergent responses explain why observed vertical Chl ‐a profiles vary across different cross‐track locations. Clarifying the spatial variability of upper ocean Chl ‐a dynamics under typhoon forcing is critical for accurately assessing typhoons' ecological impacts, including their role in marine carbon cycling and productivity.

We innovatively investigated three‐dimensional biogeochemical responses to tropical cyclones (TCs) in the Northwest Pacific using a composite analysis of simulations from a coupled physical‐biogeochemical model. TC‐induced upwelling and vertical mixing cause a vertical redistribution of chlorophyll a concentration (Chl ‐a ), increasing surface Chl‐ a while decreasing subsurface Chl ‐a , within a 100‐km radius of the TC center. Simultaneously, these processes bring nutrient‐rich deep waters into the upper layer. Following the TC passage, the significant increases in both Chl ‐a and nutrients lead to a high Chl ‐a anomaly in the subsurface layer that persists for over 30 days. Westward geostrophic advection, driven by the meridional density gradient, tends to shift the subsurface nutrients and Chl ‐a anomalies toward the left side of the generally northwestward TC track. The total biomass in the upper ocean, as indicated by integrated Chl ‐a , decreases shortly after the TC passage and subsequently starts to rise, ultimately resulting in a net increase.

The Gulf Stream (GS) transports a massive amount of heat northward to high latitudes and releases sensible and latent heat to the atmosphere, playing an important role in the North Atlantic and European climate change. The change trends of the GS transport and pathway are still uncertain to date. Our analyses of altimeter observations from 1993 to 2016 indicate that the linear trends in surface maximum speed, transport and latitudinal location of the GS are significant east of 61ºW at the 95% level while they are small and not significant between 72ºW and 61ºW. The weakening trend of the GS during the period from 1993 to 2016 is accompanied with a southward-shifting path, which is associated with the decline of the North Atlantic Oscillation (NAO) and possibly reduction in the Atlantic meridional overturning circulation (AMOC).

With global warming, the global ocean is experiencing more and stronger marine heatwaves (MHWs) and less and weaker marine cold spells (MCSs). On the regional scale, the complex circulation structure means that the changes in sea surface temperature (SST) and extreme temperature anomaly events in the Taiwan Strait (TWS) exhibit unique regional characteristics. In summer (autumn), the SST in most regions of the TWS has a significant increasing trend with a regionally averaged rate of 0.22 °C (0.19 °C) per decade during the period 1982–2021. In winter and spring, the SST in the western strait shows a significant decreasing trend with a maximum decreasing rate of −0.48 °C per decade, while it shows an increasing trend in the eastern strait. The annual mean results show that the TWS is experiencing more MHWs and MCSs with time. The frequency of the MHWs in the eastern strait is increasing faster than that in the western strait. In the western region controlled by the Zhe-Min Coastal Current, the MCSs have an increasing trend while in the other areas they have a decreasing trend. The MHWs occur in most areas of the TWS in summer and autumn, but the MCSs are mainly concentrated in the west of the TWS in spring and winter. The cooling effect of summer upwelling tends to inhibit the occurrence of MHWs and enhance MCSs. The rising background SST is a dominant driver for the increasing trend of summer MHWs. By contrast, both the SST decreasing trend and internal variability contribute to the winter MCSs increasing trend in the strait.

Wind power input to oceanic near‐inertial oscillations (NIOs) plays a crucial role in sustaining the global ocean conveyor belt. However, the impact of tropical cyclones (TCs) on wind power input to NIOs, despite being the most vigorous atmospheric dynamics capable of exciting NIOs, is often overlooked in global estimations due to their transient nature and a lack of observations. Utilizing hourly wind and ocean current records, we quantified the wind power on NIOs induced by TCs from 1990 to 2019. Our findings reveal that the wind power on NIOs due to TCs is estimated to be between 0.028 and 0.065 TW, which accounts for a significant proportion, that is, 8%–17%, of that over the globe. This study highlights the importance of incorporating the wind power induced by TCs when estimating the global wind power on NIOs, as its impact is non‐negligible. Our findings contribute to a better understanding of the global energy balance by improving the estimation of wind power on NIOs.

We used Biogeochemical-Argo (BGC-Argo) float observation profiles and satellite data to investigate the physical-biogeochemical processes of a phytoplankton biomass rise in the Kuroshio region south of Japan during a period of two sequential tropical cyclones (TCs)—Hagibis and Neoguri—in October 2019. During TC Hagibis, prominent sea surface cooling and surface chlorophyll-a (Chl-a) increase occurred within a pre-existing cyclonic eddy (CE) south of Japan. Because of TC-induced mixing and upwelling, the maximum cooling happened at the depth of 57 m where water temperature dropped by 6°C. The dramatic mixing and upwelling redistributed chlorophyll-a vertically (reducing subsurface Chl-a and increasing surface Chl-a) with little augment of depth-integrated Chl-a in the upper ocean above 160 m depth. Meanwhile, the mixing and upwelling transported nutrients from the subsurface to the surface layer. In the week after Hagibis, the depth-integrated Chl-a greatly increased. Subsequently, TC Neoguri obviously enhanced the augment of phytoplankton biomass although it was weaker than Hagibis. The upwelling induced by Hagibis increased nutrients in the water below 80 m, providing a very favorable condition for the subsequent TC Neoguri to further promote the growth of phytoplankton. The intense precipitation accompanying with Neoguri brought the coastal water with rich terrestrial material to offshore ocean, increasing nutrients and decreasing salinity in the subsurface layer. These both contributed to the marked increase in Chl-a during Neoguri. Our results demonstrated that the two sequential TCs worked together with a cyclonic eddy to cause a drastic and complex Chl-a enhancement event in the Kuroshio region south of Japan.

Eddies are ubiquitous mesoscale dynamics in oceans worldwide that prominently influence the upper ocean. Specifically, the presence of preexisting eddies can modulate oceanic responses to passing typhoons, for which the underlying mechanism requires further investigation. In 2013, Typhoon Soulik traveled over 2,000 km in the western North Pacific, encountering multiple eddies, offering a suitable opportunity for investigating the impact of eddies on typhoon-induced upper ocean responses. A coupled ocean-atmosphere model was developed for this event, and the simulations were validated with multiple observations. The surface cold wake after Typhoon Soulik was successfully reproduced by the coupled model, showing a multicore structure, which was due to the preexisting mesoscale cyclonic eddies along the typhoon path. An experimental case excluding eddies was also tested in the numerical model. The comparison between the control and experimental cases revealed that sea surface cooling was enhanced by eddies via the vertical mixing and horizontal advection of eddy-related currents. In particular, vertical mixing enhanced sea surface cooling beneath the typhoon center due to the stronger vertical stratification within the preexisting cyclonic oceanic eddies. The eddy-related current with a horizontal temperature gradient contributed to advection, enhancing sea surface cooling at the cooling center and leading to a rightward-shifted pattern of enhanced sea surface cooling. This study enriches the understanding of the complex upper ocean dynamics under the combined effects of typhoons and eddies.