The Impact of Consecutive Tropical Cyclones on Changes in Environmental Factors and Phytoplankton Distributions in Overlapping Areas

Abstract

Tropical cyclones are known to have significant ecological impacts, particularly on marine productivity. This study investigates the effects of two tropical cyclones (TC “MARIA” and TC “AMPIL”) on changes in environmental factors and phytoplankton in overlapping marine areas during August 2024. Our findings indicated that TC “MARIA”, despite its lower wind speeds, resulted in significant increases in surface chlorophyll-a (Chl-a) due to its prolonged duration, while depth-integrated Chl-a showed a declining trend, suggesting limitations on phytoplankton growth due to water column instability and reduced light availability. In contrast, TC “AMPIL”, with its higher wind speeds and faster translation speed, caused more immediate disturbances, leading to increases in surface Chl-a. However, the depth-integrated Chl-a did not significantly increase, as phytoplankton growth was hindered by the succession of the two typhoons. Additionally, we observed a pronounced cooling in sea surface temperature after both typhoons, likely linked to ongoing mixing processes and atmospheric influences. This study can provide us with more insights into the interaction between tropical cyclone dynamics and marine ecology.

1. Introduction

Since the 1990s, the rate of ocean warming has surged significantly, becoming a prominent global phenomenon [1]. This rise in temperature is contributing to an increase in the frequency of extreme weather events [2,3,4]. In recent decades, a clear seasonal shift towards earlier occurrences of global super typhoons has been observed [3]. Additionally, there is a noticeable trend of strengthening among weaker typhoons worldwide [4]. In this context of increasing tropical cyclone activity, the occurrence of dual and consecutive typhoons is becoming more frequent [5,6]. Therefore, it is essential to intensify research on the impacts of such events, particularly in relation to their combined effects on marine ecosystems and climatic patterns.

Tropical cyclones typically cause a reduction in sea surface temperature in the ocean [7,8,9,10,11]. The most pronounced cooling usually takes place on the right side of the typhoon’s trajectory [12,13,14]. In the northwestern Pacific, the peak cooling is usually detected two to three days following the typhoon’s passage. Recovery from this cooling may take several days to weeks, depending on variables such as the intensity of the storm and its translation speed [11,15,16]. Moreover, tropical cyclones can trigger phytoplankton blooms along their trajectories through various physical mechanisms such as mixing, entrainment, and upwelling [9,17,18,19]. Typhoons significantly enhance chlorophyll-a (Chl-a) concentrations and promote phytoplankton growth, accounting for about 20–30% of the new primary production in the ocean [9].

With the launch of satellite sensors designed to study earth science, an increasing number of studies have shown that tropical cyclones significantly increase the concentration of surface Chl-a. Babin et al. found that after a tropical cyclone passes, the Chl-a concentration in cooling areas significantly rises, which is related to the injection of nutrients into oligotrophic waters [20]. The phytoplankton blooms triggered by tropical cyclone “Damrey” in the South China Sea were analyzed, revealing that vertical mixing and upwelling led to two distinct phytoplankton blooms in the South China Sea [19]. Sun et al. also discovered that prolonged influence from a tropical cyclone can lead to significant responses of phytoplankton biomass in the upper ocean [21]. The extent and nature of these phytoplankton blooms are further influenced by specific characteristics of the typhoons themselves, including their translation speed and intensity [5,20]. Generally, strong typhoons can exert significant forcing over the ocean surface, leading to intense vertical mixing and entrainment, which brings cold, nutrient-rich water to the surface. Additionally, slower-moving typhoons can intensify the cooling of the surface and subsurface layer and stimulate higher phytoplankton biomass production because of their prolonged duration of influence.

Under the backdrop of frequent extreme weather events, successive typhoons can have cumulative effects on the ocean. Huang et al. [6] analyzed the impact of two typhoons on phytoplankton blooms in the Northwest Pacific, highlighting that the intensity and translation speed of the typhoons, as well as pre-existing oceanic conditions (such as mixed layer depth and eddies), play significant roles in influencing phytoplankton proliferation. Similar observations in the Arabian Sea revealed that [22], following the passage of two consecutive typhoons, there were two brief peaks in surface Chl-a concentrations. This phenomenon is primarily associated with the redistribution of Chl-a and the influx of nutrients from the deeper layers. The impact of tropical cyclones of varying intensities on surface Chl-a and depth-integrated Chl-a concentrations differs significantly. Research indicates [23] that strong tropical storms do not have a noticeable effect on either surface Chl-a or depth-integrated Chl-a concentrations; typhoons increase surface Chl-a concentrations, while depth-integrated Chl-a remains relatively unchanged; conversely, super typhoons lead to significant increases in both surface Chl-a and depth-integrated Chl-a concentrations. These findings underscore the complex dynamics of marine ecosystems in response to extreme weather events, necessitating further investigation into the underlying mechanisms. This study aims to examine the impacts of two distinct typhoons, tropical cyclone “MARIA” (TC “MARIA”) and tropical cyclone “AMPIL” (TC “AMPIL”), which differ significantly in their intensity and translation speed, on phytoplankton in the overlapping marine areas during August 2024. Understanding these differences is crucial for elucidating how varying cyclone characteristics influence marine ecosystems, particularly phytoplankton dynamics. Specifically, this study focuses on the following three aspects: (1) changes in environmental factors caused by the two typhoons with different intensities and translation speeds; (2) variations in surface Chl-a concentration and integrated chlorophyll levels within the euphotic layer before and after the typhoons; and (3) a preliminary analysis of the cooling phenomenon observed after consecutive typhoons.

2. Data and Methods

2.1. Data

2.1.1. Typhoon Tracks

The trajectory and intensity changes in TC “MARIA” and TC “AMPIL” in the northwest Pacific (Figure 1) were sourced from the International Best Track Archive for Climate Stewardship (IBTrACS), which is managed by the National Centers for Environmental Information (NCEI) [24]. The dataset utilized in this study included 3-hourly movement speed and maximum sustained wind speed (MSW), as well as the longitude and latitude of the centers of these two typhoons (https://doi.org/10.25921/82ty-9e16, accessed on 25 September 2024). TC “MARIA” formed on 7 August 2024 in the northwest Pacific and passed through our study area during 10 to 11 August. After TC “MARIA” disappeared on 12 August, TC “AMPIL” appeared at the same time in the northwest Pacific and passed through our study area again on 17 August.

2.1.2. Satellite Data

Wind speed data at 6 h intervals were obtained from the Cross-Calibrated Multi-Platform (CCMP) dataset, which offers a spatial resolution of 0.25° × 0.25°. The CCMP gridded surface vector winds were generated through a combination of satellite observations, moored buoy data, and model wind information and finally classified as a 3-level ocean vector wind analysis product (https://www.remss.com/measurements/ccmp/, accessed on 25 September 2024).

2.1.3. Reanalysis and Model Simulation Data

The hourly total precipitation data and the temperature of air at 2 m (T_(2m)) above the sea surface were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF), which offers a spatial resolution of 0.25° × 0.25° (https://www.ecmwf.int/, accessed on 25 September 2024).

The daily Chl-a data were sourced from the Copernicus Marine Environment Monitoring Service (CMEMS), which offers a spatial resolution of 0.25° × 0.25°. This dataset comprised daily and monthly average files of biogeochemical parameters across the global ocean, covering 50 vertical levels from the surface to a depth of 5700 m. The surface and vertical distributions of CMEMS Chl-a data closely aligned with the BGC-Argo measurements (R² = 0.8427, p < 0.01), indicating the utility of the CMEMS Chl-a data within the study area (Figure 2). The daily mixed layer depth (MLD) data, with a resolution of 0.083° × 0.083°, were also sourced from the CMEMS (https://data.marine.copernicus.eu/, accessed on 25 September 2024).

2.1.4. In Situ Data

During the TC “MARIA” and TC “AMPIL” passage, the Argo float 5906393 happened to pass through our research area. We selected the water temperature and salinity data of Argo from 1 August to 25 August 2024 to investigate the ocean response to the passage of two consecutive typhoons (https://fleetmonitoring.euro-argo.eu/dashboard?Network=BGC, accessed on 25 September 2024). The BGC-Argo 2902878 we used to validate CMEMS Chl-a data in Figure 2 is obtained from https://biogeochemical-argo.org/ (accessed on 25 September 2024).

2.2. Methods

2.2.1. Calculation of Ekman Pumping Velocity

In this study, the Ekman pumping velocity (EPV) is computed to indicate the intensity of upwelling using the following method [25]:

$$\tau =C_D{\rho }_{air}\left|U\right|U$$

(1)

$$EPV=curl({\displaystyle \frac{\tau }{\rho f}})$$

(2)

In Formulas (1) and (2), EPV refers to the Ekman pumping velocity (m·s⁻¹) caused by wind stress ($\tau$) curling; $C_D$ represents the drag coefficient; $U$ represents wind speed at 10 m; ${\rho }_{air}$ represents the air (1.29 kg·m⁻³) density; $\rho$ represents the seawater (1.025 × 10³ kg·m⁻³) density; $f$ is the Coriolis parameter ($f=2\omega sin\varphi$).

2.2.2. Calculation of Depth-Integrated Chl-a

The computation of depth-integrated Chl-a was carried out using the following method [23,26]:

$${Chl-a}_{\left(int\right)}={\sum }_{i=1}^{n-1}{\displaystyle \frac{{Chl-a}_{\left(i\right)}+{Chl-a}_{(i+1)}}{2}}\times (D_{i+1}-D_i)$$

(3)

In Formula (3), Chl-a_(int) refers to the depth-integrated Chl-a in water column (mg·m⁻²) (here, Chl-a_(int) is determined by integrating from the surface down to a depth of 200 m), Chl-a_(i) represents the Chl-a concentration at the layer (i) (mg·m⁻³), D_i represents the thickness of layer i (m), and n represents the number of layers.

3. Results

3.1. Temporal Variations in Temperature and Salinity

The variations in temperature and salinity reflect the significant impact of TC “MARIA” and TC “AMPIL” on the marine environment of the study area. Both the ocean temperature and salinity underwent considerable changes during the typhoon periods. During the period from 1 August to 11 August, as the Argo float drifted and TC “MARIA” arrived, the temperature abruptly decreased from around 30 °C to 28.5 °C (Figure 3). The salinity also exhibited a corresponding decline, dropping from 34.45 psu to 34.15 psu (Figure 3). After TC “MARIA” passed, the temperature began to rise again, reaching nearly 30 °C, indicating that the marine environment experienced a significant disturbance. The fluctuations in salinity were particularly pronounced; on the 15 August, there was a notable peak in both temperature (30.16 °C) and salinity (34.36 psu), suggesting a possible influence from the Kuroshio Current. During the passage of TC “AMPIL”, the decline in temperature and salinity was not as pronounced as that caused by TC “MARIA”. From 19 August to 25 August, the impact of the typhoon gradually weakened, and salinity saw a rebound; however, the temperature continued to decline, even dropping to levels lower than those during the typhoon period.

3.2. Variations in Wind, EPV, MLD, and Total Precipitation

The parameters such as wind, EPV, and MLD underwent drastic changes, reflecting the significant impact of the tropical cyclones on the marine environment during the passage of the TC “MARIA” and TC “AMPIL” (Figure 4). During the passage of TC “MARIA” from 10 to 11 August, the wind speed sharply increased, peaking at around 15 m·s⁻¹. Subsequently, when TC “AMPIL” passed through on 17 August, the wind speed further escalated significantly, reaching a high of approximately 18 m·s⁻¹. Outside of these tropical cyclone periods, the wind speed generally remained at a relatively lower level, fluctuating between 5 and 10 m·s⁻¹. The mean translation speed of TC “MARIA” is 2.79 m·s⁻¹, the mean MSW is 24.73 m·s⁻¹, and the duration is 42 h (

Table 1. The mean translation speed, mean maximum sustained wind (MSW), and duration of TC “MARIA” and TC “AMPIL” passing through our study area.

	Mean Translation Speed (m·s−1)	Mean MSW (m·s−1)	Duration (h)
TC “MARIA”	2.79	24.73	42
TC “AMPIL”	5.05	47.59	18

). In comparison, the mean translation speed of TC “AMPIL” is 5.05 m·s⁻¹, the mean MSW is 47.59 m·s⁻¹, and the duration is 18 h (Table 1). During the passage of TC “MARIA”, the EPV exhibited strong positive and negative fluctuations. Specifically, from 10 to 11 August, the EPV experienced significant variations, reaching a peak of approximately 2.3 × 10⁻⁵ m·s⁻¹. When TC “AMPIL” passed through on 17 August, the EPV increased again, reaching a high value of around 1.5 × 10⁻⁵ m·s⁻¹. In addition, minor changes in domain size (black box in Figure 1) do not affect the results. During both tropical cyclones, the MLD showed a significant increase, with depths reaching between 25 and 35 m. This deepening of the mixed layer highlights the extent to which the storm-induced mixing altered the stratification of the water column. After the passage of the typhoons, from 19 August to 25 August, the mixing in the study area remained strong, with depths measured at approximately 15 to 20 m. This sustained mixing indicates ongoing disturbances in the water column, even after the cyclones had moved on. Furthermore, there was a noticeable increase in total precipitation during the passage of both tropical cyclones. From 8 to 11 August, the total precipitation associated with TC “MARIA” reached approximately 120 mm. In comparison, during TC “AMPIL”, the total precipitation was around 70 mm. Heavy precipitation (apparently caused by AMPIL’s leading edge) started early, on 16 August (Figure 4), which is why the time when AMPIL passed on 17 August was significantly different from the time of the salinity decline on 16 August (Figure 3). This difference in rainfall amounts reflects the varying intensities and characteristics of the two storms, with TC “MARIA” exhibiting a more prolonged precipitation event.

Based on the time-series data of the EPV presented in Figure 4, the influence of TC “MARIA” was most pronounced during the period from 10 to 11 August. During this time, significant changes in EPV values were observed. In contrast, the effects of TC “AMPIL” were primarily noted on 17 August, when another notable shift in EPV occurred. To illustrate these changes more clearly, we have provided planar distribution maps of EPV for these specific time periods in Figure 5. From 1 to 7 August, the EPV across the entire study area remained relatively stable, characterized by low negative values that suggested minimal disturbance to the marine environment. However, on 10 August, a high-EPV center emerged, linked to the influence of TC “MARIA”. On 11 August, the high-EPV region not only continued to expand but also began to migrate northward, reflecting the sustained impact of TC “MARIA” as it progressed through the region. Subsequently, on 17 August, another high-EPV center was identified in the central part of the study area, corresponding to the passage of TC “AMPIL”. Finally, during the period from 19 to 25 August, the EPV gradually weakened, indicating a return to pre-typhoon conditions.

3.3. Variations in Phytoplankton Chl-a

From 1 August to 25 August, both surface Chl-a and depth-integrated Chl-a concentrations exhibited notable dynamic fluctuations, as illustrated in Figure 6. The surface Chl-a concentration fluctuated between 0.1 and 0.15 mg·m⁻³, and the depth-integrated Chl-a concentration varied between approximately 43 and 46 mg·m⁻², indicating a relatively stable marine ecological environment from 1 August to 8 August. Before the two typhoons (1–7 August) (Figure 7), the subsurface Chl-a maximum layer in the study area was approximately at a depth of 35 m, with a subsurface Chl-a minimum layer also present at around 10 m. However, the situation began to shift with the influence of TC “MARIA”. Starting on 8 August, the surface Chl-a concentration began to rise dramatically, reaching a peak of 0.35 mg·m⁻³ on 12 August. Meanwhile, the depth-integrated Chl-a concentration began to decline from 8 August. A significant drop occurred between 10 August and 11 August, leading to the lowest recorded value for the entire period, which fell below 40 mg·m⁻². Following this initial decline, the depth-integrated Chl-a concentrations began to rise again after 11 August, signaling a potential recovery or adjustment in the marine ecosystem. Under the influence of TC “MARIA” (10–11 August) (Figure 7), the Chl-a concentration in the subsurface Chl-a maximum layer decreased by 0.2 mg·m⁻³, while the surface Chl-a significantly increased. In addition, the subsurface Chl-a minimum layer clearly disappeared. As TC “AMPIL” approached, the surface Chl-a concentration started to increase once more on 16 August, peaking again at 0.35 mg·m⁻³ on 18 August. During this period, however, the depth-integrated Chl-a concentration experienced another decrease. After the passage of both typhoons, the surface Chl-a concentration and the Chl-a concentration in the subsurface Chl-a maximum layer gradually recovered (Figure 7), returning to levels observed prior to the storms. However, while the depth-integrated Chl-a showed an increase, it remained lower than pre-typhoon levels, indicating that the overall marine ecological environment had not fully returned to its previous state.

4. Discussion

4.1. Comparison of Environmental Factor Changes Caused by Two Typhoons

Due to the destructive nature of severe typhoons, previous studies [27,28,29] have primarily focused on their ecological impacts, while the effects of weaker typhoons have received relatively less attention. When tropical cyclone “MARIA” passed through our study area (Figure 1), its mean MSW was recorded at only 24.73 m·s⁻¹ (Table 1), which classifies it as a strong tropical storm. In stark contrast, when TC “AMPIL” traversed the same region, it exhibited a significantly higher mean MSW of 47.59 m·s⁻¹, nearly double that of TC “MARIA”. This difference in wind speed underscores the considerably greater destructive potential of TC “AMPIL” and its impact on the marine environment. Interestingly, despite TC “AMPIL” having a higher wind speed compared to TC “MARIA” (Figure 4), an important finding of our study is that the EPV generated by TC “MARIA” was actually stronger (Figure 4). This phenomenon can primarily be attributed to the longer duration of TC “MARIA” within our study area, which aligns with the findings of previous research [5,21]. Some studies have indicated that typhoons with weaker intensity and slower movement speeds lead to stronger phytoplankton proliferation [6]. It appears that the intensity of a typhoon plays a less critical role compared to the duration of its forcing effects. This may be due to the lower drag coefficient observed at high wind speeds during tropical cyclones [30,31], which can limit the efficiency of mixing and other ecological impacts. Furthermore, TC “MARIA” had a slower translation speed of 2.79 m·s⁻¹ and a longer duration of 42 h, compared to TC “AMPIL”, which had a translation speed of 5.05 m·s⁻¹ and lasted for only 18 h. This extended presence of TC “MARIA” resulted in more pronounced hydrological changes within the marine environment, highlighting its significant and lasting impact on phytoplankton growth, an aspect that warrants careful consideration in future studies. Previous research has also indicated that both the intensity and translation speed of typhoons are crucial factors in triggering phytoplankton algal blooms [9,19,23,32]. Additionally, the total precipitation associated with TC “MARIA” persisted for 4 days, which was two days longer than the precipitation duration of TC “AMPIL”, and the total rainfall from TC “MARIA” was 50 mm greater than that of TC “AMPIL”. Heavy rainfall can cause the wet deposition of atmospheric aerosols to the sea surface, and the dissolved nutrients it causes can promote the growth of phytoplankton [33,34]. While the MLD during TC “MARIA” was not as deep as that observed during TC “AMPIL”, the mixing processes induced by TC “MARIA” persisted for 5 days, compared to only 3 days for TC “AMPIL” relative to pre-typhoon conditions. This sustained mixing can significantly influence the distribution and availability of nutrients in the water column, thereby affecting phytoplankton dynamics. All of the above suggests that typhoons characterized by slower translation speeds and longer durations exert a more significant impact on changes in the marine environment.

4.2. The Changes in Phytoplankton Chl-a Triggered by Successive Typhoons

Previous studies have focused on the impacts of tropical cyclones occurring at different times in the same marine region. These investigations have consistently demonstrated that tropical cyclones with longer durations tend to have a more pronounced effect on surface phytoplankton bloom phenomena in those areas [5,6,28]. This correlation suggests that the duration of a cyclone’s presence is a critical factor influencing phytoplankton dynamics, as prolonged mixing and nutrient input can enhance growth conditions. In line with these findings, our results also indicated an increase in surface phytoplankton Chl-a concentrations, as illustrated in Figure 6. However, the presence of the subsurface Chl-a maximum layer in the ocean means [35] that intensified mixing during the typhoon can elevate this layer and mix it into the surface, leading to a rise in surface Chl-a concentration [23,29,36]. Recent studies have found that the presence of a subsurface Chl-a minimum layer can lead to a decrease in surface Chl-a concentrations due to mixing caused by typhoons [26]. Although there was a subsurface Chl-a minimum layer (Figure 7) present in our study area prior to the typhoons, the relatively shallow depth of the subsurface Chl-a maximum layer (35 m) (Figure 7) allowed Chl-a from the subsurface Chl-a maximum layer to be mixed into the surface layer. As a result, we observed an increase in surface (Figure 7) concentrations after the passage of both typhoons (Figure 6). Therefore, to determine whether tropical cyclones increase primary productivity in marine waters, it is necessary to evaluate the changes in depth-integrated Chl-a within the euphotic zone before and after the typhoons. We found that the depth-integrated Chl-a exhibited an overall declining trend from 1 August to 25 August (Figure 6). This indicates that, although the consecutive typhoons enhanced mixing and increased surface Chl-a concentrations (Figure 4 and Figure 6), the primary productivity within the euphotic zone actually declined. To accurately assess whether tropical cyclones contribute to increased primary productivity in marine waters, it is crucial to evaluate changes in depth-integrated Chl-a within the euphotic zone both before and after the occurrence of these storms. The decrease in depth-integrated Chl-a indicates that, despite the occurrence of consecutive typhoons which enhanced mixing and elevated surface Chl-a concentrations (Figure 4 and Figure 6), the phytoplankton biomass within the euphotic zone actually decreased during this period. This seemingly paradoxical situation may be attributed to several factors. The instability of the water column induced by the typhoons likely disrupted the originally suitable vertical growth environment for phytoplankton. Additionally, reduced light availability due to increased turbidity from mixing and sediment resuspension could further limit photosynthetic activity [26,37]. Collectively, these factors appear to have constrained phytoplankton productivity in the region, as evidenced by the decline in depth-integrated Chl-a levels.

Just four days after TC “MARIA” passed through our study area, TC “AMPIL” emerged with an even stronger wind speed, with maximum speeds up to 3 m·s⁻¹ stronger than TC “MARIA” (Figure 4). In the marine environment, there is a lagged response in the growth of Chl-a, whereby its increase correlates with changes in environmental conditions with a delay [38]. Following the passage of TC “MARIA”, wind-induced upwelling brought nutrient-rich cold water to the subsurface layer, creating favorable conditions for phytoplankton growth [39,40,41]. As a result, the depth-integrated Chl-a began to rise steadily from 11 to 14 August, reflecting this nutrient enrichment. However, it is important to note that even during this period of growth, the depth-integrated Chl-a concentrations did not fully recover to pre-typhoon levels. The phytoplankton community in the subsurface layer has a greater adaptability to relatively low light conditions and higher nutrient concentrations. Due to the higher latitude of the study area and weaker light intensity, the subsurface Chl-a maximum layer is more likely to occur at shallower depths compared to lower latitude regions. Approximately 30% to 70% of primary productivity in the water column is contributed by the subsurface Chl-a maximum layer [42,43]. As a result, when these two typhoons cause mixing in the upper ocean, they can significantly impact the primary productivity of the water column. This is why the depth-integrated Chl-a concentrations after the two typhoons struggle to return to pre-typhoon levels. Simultaneously, the approach of TC “AMPIL” began to generate rainfall and exert its influence on the area, which led to a subsequent decline in the depth-integrated Chl-a levels once again, as shown in Figure 4 and Figure 6. This decline highlights the compounded effects of successive cyclones on marine ecosystems. After TC “AMPIL” passed, the depth-integrated Chl-a began to rise again; however, the upwelling induced by the winds associated with this second cyclone was not as strong as that caused by TC “MARIA” (Figure 4 and Figure 5). Consequently, this resulted in a less significant increase in depth-integrated Chl-a. In the ocean, the increase in Chl-a is related to changes in environmental conditions, but there is a lagged response [38]. Under the influence of these consecutive typhoons, it became evident that although the first storm initially promoted an increase in surface Chl-a, the inherent growth lag meant that phytoplankton had not fully recovered or reached optimal growth conditions by the time the second typhoon arrived. This timing exacerbated the situation, as the arrival of TC “AMPIL” led to further disturbances and disruptions in the marine environment, ultimately suppressing the anticipated rise in depth-integrated Chl-a. As a result, the overall depth-integrated Chl-a levels in the water remained lower compared to pre-typhoon conditions. This trend reflects the negative impact of consecutive climatic events on marine primary productivity.

4.3. Initial Exploration of Anomalous Sustained Cooling After the Passage of Two Typhoons

The passage of a typhoon typically induces a drop in sea surface temperature (SST) in the open ocean [10,44,45,46], with decreases ranging from less than −1 °C up to −11 °C [7,8,9]. Following the passage of typhoon TC “AMPIL”, the study area experienced an unusual and significant cooling at the surface, with temperatures dropping even lower than those recorded during the previous two typhoons (Figure 2). The cooling process observed in the near-surface ocean following the passage of the first two typhoons is a well-known oceanographic phenomenon, driven by strong wind stress and the mixing of surface waters [47,48]. This cooling effect is predominantly the result of entrainment and mixing processes rather than being directly linked to atmospheric conditions. The intense winds associated with the typhoons create turbulence that facilitates the mixing of warmer surface waters with cooler, deeper waters, resulting in a noticeable decrease in SST. In contrast, the cooling that began on 19 August following TC “AMPIL” was both anomalous and exhibited greater intensity than expected. This unusual cooling trend suggests that while the initial mixing processes post-typhoon contributed to temperature reductions, other factors may have played a significant role. The sustained mixing in the study area that commenced on 19 August could be a contributing factor to the decline in sea surface temperature observed during this period (Figure 4). This ongoing mixing process enhances the upward transport of cooler water from beneath the surface, thereby further reducing the SST. Interestingly, although the mixing that occurred after 19 August was significantly less intense than that experienced during the two preceding typhoons, the temperature decline remained pronounced. This discrepancy indicates that additional factors may have influenced this anomalous drop in temperature. We hypothesize that atmospheric processes could be contributing to these changes. To further investigate this hypothesis, we analyzed the 2 m air temperature (T_(2m)) data for the region from 1 August to 25 August (Figure 8). Our analysis revealed that following the passage of TC “AMPIL”, T_(2m) began to drop sharply until 21 August. This decline in air temperature aligns closely with the observed anomalous decrease in sea surface temperature and is consistent with findings from previous research [49].

5. Conclusions

This study highlights the contrasting impacts of TC “MARIA” and TC “AMPIL” on phytoplankton distributions and environmental factors in the overlapping marine areas during August 2024. TC “MARIA”, despite its lower intensity, had a prolonged presence that facilitated significant increases in surface Chl-a, although depth-integrated Chl-a revealed a declining trend. On the other hand, TC “AMPIL”, characterized by higher wind speeds and faster translation, generated immediate disturbances that initially increased surface Chl-a levels. However, the rapid succession of these storms limited the potential for phytoplankton recovery, as indicated by the lack of significant changes in depth-integrated Chl-a. The increase in surface Chl-a following the two typhoons is primarily influenced by the mixing of the subsurface Chl-a maximum layer. Furthermore, since the subsurface Chl-a maximum layer is relatively shallow, mixing caused by the typhoons can significantly affect the depth-integrated Chl-a concentration in the water column. Additionally, the observed anomalous cooling of sea surface temperatures after both typhoons suggests a complex interplay of mixing processes and atmospheric influences that warrant further investigation. This study underscores the importance of understanding how different cyclone characteristics can affect marine ecosystems, particularly under the pressures of climate change.