Chlorophyll bloom triggered by tropical storm chedza at the southern tip of madagascar island

A phytoplankton bloom during the passage of Tropical Storm (TS) Chedza was observed at the southern tip of Madagascar on January 28, 2015. The mechanisms of the chlorophyll bloom were researched with satellite remote sensing data, reanalysis data and Argo buoy data. The results show that there was horizontal transport of chlorophyll-a (Chl-a) with the western coastal current of Madagascar and the South East Madagascar Current (SEMC). At the southern tip of Madagascar, there was a tilted anticyclonic eddy moving westward, which promoted the flow of nitrate at depths from 60 m to 100 m northward into Box A for a month. Simultaneously, the SEMC formed another anticyclonic eddy entering Box C. Following the passage of TS Chedza, cyclone vorticity in Box A increased continuously, and the upwelling became stronger with Ekman pumping. At the same time, the barrier layer weakened, the mixed layer deepened, and the thermocline decreased, which can uplift the nitrate to the upper ocean. After the passage of TS Chedza, sufficient photosynthetically active radiation (PAR) facilitated the Chl-a bloom in Box A. This study contributes to the assessment of the ecological impact of ocean eddies at the southern tip of Madagascar.


Introduction
Mesoscale eddies with a radius of approximately 100 km are known to be one of the most important characteristics of the global ocean (Chelton et al 2011).Meanwhile, the troposphere is the main layer in which our weather phenomena occur, including typhoons.Therefore, a correct understanding and calculation of tropospheric parameters (e.g.temperature, wind speed, etc) are essential for the accurate prediction of typhoons (Guan et al 2019, Liu et al 2020).After a tropical cyclone (TC), cyclonic eddies from TCs typically trigger a chlorophyll bloom in various sea areas such as the East China Sea (Lü et al 2020), South China Sea (Zhao et al 2009), Bay of Bengal (Xia et al 2022) and Arabian Sea (Lu et al 2020).Because of the hazardous sea conditions, measurements along tropical cyclone (TC) tracks are impractical for long-term time series observations (McGillicuddy 2015).Therefore, satellite remote sensing and reanalysis data are extensively used to analyze the dynamics of ocean eddies under TCs (Ke et al 2019).Remote sensing imagery is not only useful for extracting land use and land cover information (He et al 2022), but also can be used for monitoring and predicting the path of marine typhoons (Sun et al 2021).Godo et al highlight the importance of eddies in the patchiness of marine biomass (Balaguru et al 2012), and demonstrate that they provide rich foraging habitat for higher trophic marine organisms (Godo et al 2012).In marine ecosystems, ocean eddies are considered the main element leading to phytoplankton blooms with changes in biological properties such as nutrients and chlorophyll-a (Chl-a) (Raj et al 2010, Tew-Kai andMarsac 2009).Three main processes by which mesoscale ocean eddies affect marine phytoplankton are emphasized: the horizontal advection of phytoplankton, the vertical fluxes of nutrients and phytoplankton, and the effects of eddies on stratification and ocean mixing from upwelling (Gaube et al 2014).Fehmi (2018) described the effect of nitrate from deep ocean (upwelling) and nitrate from the coast of Madagascar (advection) on phytoplankton blooms.In addition, near-inertial internal waves were also observed as the TS passed over Madagascar (Vianello et al 2020, Zheng et al 2020), which may exert force and torque on the cylindrical tendon leg (LÜ et al 2016).TCs can enhance turbulent mixing and cause sediment resuspension, providing favorable conditions for nearshore Chl-a blooms (Li et al 2021).
Studies on Chl-a blooms in the southwest Indian Ocean have mainly focused on the large dendritic phytoplankton blooms in eastern and southeastern Madagascar (Dilmahamod et al 2019, Longhurst 2001, Srokosz and Quartly 2013, Uz 2007).Halo researched mesoscale vortices in the southern extension of the East Madagascar Stream (Halo et al 2014).Based on field measurements in eastern Madagascar, Chl-a blooms were confined to shallow mixed layers (approximately 30 m) with water temperatures above 26.5 °C (Srokosz and Quartly 2013).That study found that the blooms may have been triggered by warming of the mixed layer, with iron advecting eastward from Madagascar providing ample trace element supply during the blooming period (Srokosz et al 2015).A large number of eddies and upwelling cells were found in the South East Madagascar Current (SEMC) (DiMarco et al 2000, J R E Lutjeharms 2000).Increased mesoscale eddy activity in the southwestern Indian Ocean, including southern Madagascar, could lead to more southeast Madagascar blooms (Ramanantsoa et al 2021).Upwelling can stimulate surface phytoplankton production by transporting nutrients to the sea surface.
The ecological dynamics at the southern tip of Madagascar during TCs have received little attention from marine scientists.Additionally, Tropical Storm (TS) Chedza over Madagascar led to increased precipitation along the east coast of Madagascar and river runoff (Uz 2007).These iron-rich waters are brought ashore by the SEMC and spread further offshore by eddies, which can trigger Chl-a blooms (Dilmahamod et al 2019).Anticyclonic eddies generated at the southern tip of Madagascar drift westward (Beal et al 2011, de Ruijter et al 2005, Halo et al 2014).This paper documents a phytoplankton bloom on January 28, 2015, that was observed at the southern tip of Madagascar during the passage of TS Chedza.What caused the Chl-a bloom at the southern tip of Madagascar?What is the effect of coastal currents and anticyclonic eddies on the Chl-a bloom?To date, few studies have been conducted to address these questions.
In this study, we studied the mechanism of the Chl-a bloom that occurred on January 28, 2015, in southern Madagascar.The data and approaches used are explained in section 2. The results of the study are shown in section 3. The mechanism of Chl-a blooms is considered in section 4, and finally, the conclusions are presented in section 5.

Study area
TCs are categorized depending on the near-center maximum wind speed (MWS).They can be divided into tropical depressions (10.8 ∼ 17.1 m s −1 ), tropical storms (17.2 ∼ 24.4 m s −1 ), severe tropical storms (24.5 ∼ 32.6 m s −1 ), typhoons (32.7 ∼ 41 m s −1 ), severe typhoons (42 ∼ 51 m s −1 ) and super typhoons (52 m s −1 or above) (Lu et al 2020).TS Chedza originated in the Mozambique Channel as a tropical depression at 19.1°N, 41.2°E at 12:00 UTC on 15 January 2015 and transformed into a tropical storm at 19.1°N, 41.8°E at 18:00 UTC on January 15 and made landfall off the southwest coast of Madagascar at 15:00 UTC on January 16.The TS moved eastward over Madagascar before returning to the Southern Indian Ocean (figure 1).
Three boxes were selected in the study area, as shown in figure 1. Box A (25.3°S-27.2°S,44.8°E-46.7°E),located in the southeast corner of Madagascar, was used to observe the chlorophyll explosion on January 28.Box B (26.5°S-29.8°S,41.4°E-44.7°E)(Srokosz and Quartly 2013) and Box C (25.8°S-28°S, 46.8°E-49°E), located off the southern tip of Madagascar, were used to observe the movement of anticyclonic eddies.

Data
Daily Chl-a and photosynthetically active radiation (PAR) at 4 km spatial resolution were derived using GlobColor L3 products (http://hermes.acri.fr/index.php?class=archive).The GlobColor project was initiated in 2005 as an ESA Data User Element (DUE) project, which offers a continuous dataset of merged L3 marine color products.Consolidating outputs from different sensors ensures data continuity, increases spatial and temporal coverage, and decreases data noise.Since May 2015, the GlobColor project has also contributed environmental monitoring services to the Copernicus Marine Servive.
As an effective climate variable, CMEMS has been extensively used in climate change research.(Greiner and Benkiran 2008).Near real-time daily sea surface height (SSH) (1/12)°× (1/12)°, current velocity, seawater temperature and salinity data from the Copernicus Marine and Environmental Monitoring Service Global Monitoring and Forecasting Centre(CMEMS, https://resources.marine.copernicus.eu/products)were merged into a gridded product, which delivers in situ temperature and salinity profiles, chlorophyll fluxes and nitrate fluxes, sea surface temperature (SST) and sea level data.
The track of TS Chedza was acquired by the Joint Storm Warning Centre (JTWC) (https://www.metoc.navy.mil/jtwc/jtwc.htm),which offers typhoon forecasts for the Western Pacific and Indian Ocean basins.The data provided for Chedza were at 6-h intervals, including specific times, wind speeds and center locations.
January 1°× 1°resolution nitrate gas climatology profiles were obtained from the World Ocean Atlas 2018 (WOA18, https://www.nodc.noaa.gov/OC5/woa18/woa18data.html).WOA18 provides monthly mean nitrate data for the 0 to −1500 m layer.The climatological map of nitrate concentrations in the study area is shown in figure 6.
Temperature and salinity profiles up to 100 m below sea level were obtained from the Argo project in India

Brunt-väisälä frequency
In a stable temperature laminar junction, the fluid mass is disturbed and moves in a vertical direction, always being brought back to its equilibrium position by the combination of gravity and buoyancy, and oscillating due to inertia, and its frequency of oscillation is called the buoyancy frequency.
In atmospheric dynamics and oceanography, the Brunt-Väisälä frequency can be used to measure the stability of a fluid caused by convection.The Brunt-Väisälä frequency (N) can be computed as where density (ρ) depends on temperature and salinity, g is the local acceleration of gravity, z is the geometric height and d(ρ)/d(z) denotes the derivative of the potential energy density with reference to the vertical coordinate z.The depth of the maximum Brunt-Väisälä frequency is specified as the location of the thermocline (Lu et al 2020).

Mixed layer depth (MLD) and isothermal layer depth (ILD)
The MLD, which is defined as a layer of homogenous density and temperature, acts as the air-sea interface (Balaguru et al 2012).MLD is defined as the increase in potentiometric density in relation to the surface (∆sq) when the SST decreases by ∆ T =−0.5 °C, equal to the depth of the increase in surface potential density (D) (de Boyer Montégut 2024).

∆ ( )
T T S P T S P , , , , 3 10 10 0 10 10 0 where T 10 and S 10 are the temperature and salinity values, respectively, at the reference surface (10 m) and P 0 is the pressure at the ocean surface.The ILD is specified as the depth (D) at which the temperature is 0.5 °C below the surface (10 m).
The curl of the current vector (u, v) can be described by equation (7) (Lu et al 2020), where u(v) is the velocity component along the x(y) direction.

Variation in chl-a concentration
Figure 1 shows the distribution of Chl-a concentrations before, during and after the passage of the TS.After the passage of TS Chedza, a Chl-a bloom appeared in Box A from January 26 to January 31 and reached a maximum Chl-a concentration of 0.68 mg m −3 on January 28, which is 17 times that of the Chl-a concentration (0.04 mg m −3 ) on January 26.However, the concentration of Chl-a in Box B and Box C was 0.1 mg m −3 , and there was no significant fluctuation (figure 2).One of the causes of the Chl-a bloom in Box A was horizontal Chl-a transport along the western coastal current of Madagascar (figure 1(b)) and the SEMC (figure 1(c)).The same Argo buoy followed the eastern coastal current southward and eventually entered Box A, suggesting that the eastern coastal current was stronger during this period and transported chlorophyll horizontally along the eastern coast of the island into Box A. The mechanism of the Chl-a bloom in Box A will be explained in detail in the next section.
3.2.Sea surface height (SSH), sea current and ∆ SST Prior to TS Chedza, a persistent and strong anticyclonic eddy has been detected in the south of Madagascar, surpassing the intensity observed in normal anticyclonic patterns in the region (figure 3 After the passage of the TS, SST in Box A, Box B and Box C all dropped sharply from January 22; the TS remained for half a month in Box A (figure 4), where the cyclonic eddy accompanied by upwelling brought cold water from the bottom to the surface.Upwelling can bring nutrients and Chl-a from the lower layer to the upper ocean, which is then prone to Chl-a blooms at the surface.

Photosynthetically available radiation (PAR)
Figure 5 shows the time series of the average daily light intensity in Box A, Box B and Box C from January 13 to February 13.Three days after the passage of the TS, the daily PAR of Box A increased significantly after January 24 and remained stable for four days.Enhanced light entering the euphotic layer can provide sufficient light conditions for a phytoplankton bloom, which may be an important factor influencing the Chl-a bloom in Box A on January 28.

Climatology nitrate distribution in January
Adequate nutrients are an essential condition for phytoplankton blooms.Ding used the distribution of nitrate in the Agulhas Reflection Zone to investigate the mechanism of phytoplankton blooms (Ding et al 2023).Phytoplankton blooms in the southern part of the Indian Peninsula (Tan et al 2022), the southeastern Arabian Sea (Lu et al 2020),the Bay of Bengal (Xia et al 2022), and other seas have also been used to explore the mechanism of the bloom by using the nitrate distribution in the study area as an important indicator.Srokosz explores the phytoplankton bloom in eastern Madagascar through nitrate concentrations (Srokosz and Quartly 2013).As shown in figure 6, nitrate was mainly distributed on the western side of Madagascar at depths of 30 m and 80 m.However, large amounts of nitrate were distributed at a depth of 100 m (figure 6(d)), particularly the nitrate concentration in southern Madagascar, which reached 5.5 μmol l −1 .Nutrient-rich upwelling can provide sufficient nitrate for surface Chl-a blooms (Xia et al 2022).
The western, southern and eastern Chl-a fluxes (nitrate fluxes) of Box A from January 13 to February 13 are shown in figure 7 (figure 8).The Chl-a flux (nitrate flux) on the south side is obtained by multiplying its Chl-a concentration (nitrate concentration) by v, and the Chl-a flux (nitrate flux) on the east and west sides is obtained by multiplying the Chl-a concentrations (nitrate concentration) by u.The Chl-a flux (nitrate flux) on the south side is obtained by multiplying its Chl-a concentration (nitrate concentration) by u (Tan et al 2022, Xia et al 2022).The flux on the south and west sides is positive for inflow, and the flux is negative for outflow.In the east, the negative value represents the inflow, and the positive value represents the outflow.From January 13 to January 31, a large amount of nutrients entered the south side of Box A at depths of 60-100 m (figure 7(b)) and flowed out in an east-west direction (figures 7(a) and (c)).From January 13 to January 31, there was significant chlorophyll entry on the south side at water depths of 60-100 m in Box A (figure 8(b)) and outflow from the  east-west (figures 8(a) and (c)), and there was a small amount of chlorophyll entry from the south side and outflow from the east-west at water depths above 60 m.This may be due to the entrainment of northward flow from the east side of the anticyclonic eddy in Box B, leading to inflow into Box A; then, upwelling brings nutrients and chlorophyll from the lower layer to the upper ocean.

The role of stratification
In the Indian Ocean, salinity has a predominant role in near-surface stratification with an additional interlayer, the barrier layer, located between the base of the mixed layer and the top of the thermocline (Sprintall andTomczak 1992, You 1998).Figure 9 shows the corresponding buoyancy frequency values for January 13, 2015 before the TS passage and January 23, 2015 after the TS passage.Figure 10 shows the analysis of two time points before and after the Chl-a bloom based on formulas (2-6).During summer in the Southern Hemisphere, the depth of the mixed layer in the entire Southern Indian Ocean is 25-50 m (Longhurst 2001). On January 20, 2015(February 15, 2015), the water depth of the mixed layer was 23.41 m (27.33 m), and the depth of the thermocline was 37.39 m (31.94 m).Maximum buoyancy frequency changed from 0.0341 s −1 to 0.0233 s −1 (figure 9).The thickness of the barrier layer changed from 13.98 m to 4.61 m.The deepened mixed layer, the weakened thermocline, and the thinned barrier layer can facilitate the uplift of nutrients from the deep sea to the upper layer (Wang et al 2022).

The influence of ocean cyclonic eddies
The cyclonic eddy in Box A was significantly stronger after the passage of TS Chedza (figures 3(c), (e) and (g).
Figure 11 shows the strength of the cyclonic eddy based on the vorticity calculated by equation (7).Enhanced cyclonic eddies will have a greater effect on the depth of the mixing layer (Gaube et al 2019).Phytoplankton is more likely to proliferate when sufficient nutrients (figure 6) and light conditions (figure 5) are available.There is a hysteresis in Chl-a bloom.Figure 12 fully illustrates the mechanism of chlorophyll bloom on the January 28, 2015.From January 21 to January 25, the upper vorticity of Box A increased with a maximum absolute value of 1.3 s −1 , such that vertical mixing was strengthened.The absolute vorticity decreased to approximately 0.9 s −1 , which was conducive to Chl-a blooms.As the barrier layer weakened and the cyclonic eddy strengthened, nitrate at a depth of 60-100 m (figure 7) was lifted into the mixed layer during upwelling.The horizontal submarine transport of Chl-a and nitrate from January 13 to January 31 accompanied by upwelling caused a surface Chl-a

Mechanism exploration
Ramanantsoa et al confirmed that the southern coastline of Madagascar is conducive to the generation of upwelling, particularly in the southeast corner of Madagascar (Ramanantsoa et al 2018).South of 24°S, phytoplankton enrichment extends outward from southern Madagascar and coastal upwelling is well characterized(Tew-Kai and Marsac 2009).For Box A, the upper cyclonic eddy flow field diverged horizontally, and strong upwelling lifted the nutrient-rich cold water into the mixed layer, fostering phytoplankton growth (figure 12(a)).The surface horizontal transport of Chl-an occurred due to the western Madagascar coastal current and the SEMC (figure 1).Simultaneously, a large submarine influx of nutrients at depths of 60-100 m also appeared due to easterly flow of the anticyclonic eddy in Box B (figures 3(c) and 12(b)).Vertically, after the passage of TS Chedza, the mixed layer deepens, the thermocline weakens, and the thermocline thins, which is more favorable for upwelling to lift nutrients from the bottom to the surface.The superposition of upwelling and horizontal transport (figure 12(c)) brought Chl-a and nutrients to the upper ocean (Tan et al), and high PAR values in Box A sustained for 3-4 days (figure 5), which contribute to a major Chl-a bloom at the southern tip of Madagascar.

Conclusions
The mechanism of Chl-a blooms at the southern tip of Madagascar was studied using remote sensing data, reanalysis data and Argo buoy data.The following conclusions can be drawn.
(1) Horizontal surface chlorophyll transport was observed with the western Madagascar coastal current and the SEMC.Additionally, the nutrients and chlorophyll at depths of 60-100 m entered Box A via an easterly flow of the anticyclonic eddy in Box B, which were brought to the upper ocean via upwelling.
(2) With the passage of the TS, the cyclonic eddy in Box A is strengthened, which can trigger stronger upwelling.Due to the deepened mixed layer and the weakened barrier layer and thermocline, nutrients and chlorophyll can be lifted to the upper ocean by upwelling in Box A.
(3) The daily PAR in box A increased significantly after January 24 and remained stable for four days.Adequate PAR provided sufficient light for the Chl-a bloom in Box A on January 28.
(https://dataselection.euro-argo.eu/).The Argo platform number 1901333 used in this paper followed the South East Madagascar Current (SEMC), which flowed into Box A on 11 February 2015 (its location is indicated by a red asterisk in figure 1); this buoy point has a period number of 45 and a position of 46.696°E, 26.614°S and provides temperature and salinity data on subsurface stratification for use in calculating buoyancy frequencies.Data are for the period 03/01/2015 to 11/02/2015.

Figure 1 .
Figure 1.The resultant sea surface Chl-a and flow field are indicated by black arrows.The concentration of Chl-a on the surface before, during and after the passage of TS Chedza: (a) January 8, (b) January 18, (c) January 28, and (d) February 08.The black asterisk represents different locations of the same Argo buoy, and the red asterisk indicates that the Argo buoy fell in Box A.
(a)).During the passage of Chedza, the anticyclonic eddy kept moving westward, with SSH remaining relatively stable at approximately 1.4 m, and entered Box B (figures 3(e) and (g)).A similar phenomenon was found in which the cyclonic eddy south of Madagascar moved west to the coast of South Africa(Noyon et al 2019).The Southeast Madagascar Current flowed into Box C along the southern Madagascar shelf, forming an anticyclonic eddy (figure 3(e)).Barlow et al also described the apparent extension of the SEMC flowing from the southern Madagascar shelf along the southern edge of the cyclonic eddy(Barlow et al 2017).Anticyclonic eddies in Box B and Box C triggered downwelling with higher SSH and upwelling around the anticyclonic eddies(Chelton et al 2011, Xia et al 2022).Hence, the upper seawater in Box A diverged with the sea level over Box A being lower.The flow field at a depth of approximately 70 m was stronger than that of the sea surface flow in Box B (figure3).There was an easterly flow of the anticyclonic eddy in Box B that entered Box A at a depth of 70 m (figures 3(c)-(f)), and more nutrients were carried into Box A.

Figure 2 .
Figure 2. Time series of mean Chl-a concentrations from January 13 to February 13 in Box A, Box B and Box C (the vertical dotted line indicates the start time of the TC).

Figure 3 .
Figure 3.The left side is the SSH, the black line indicates the path of TS Chedza, and the black arrow signifies the flow field: (a) January 8, (c) January 18, (e) January 28, and (g) February 8.The right side is the flow field at a depth of 70 m: (b) January 8, (d) January 18, (f) January 28, and (h) February 8.

Figure 4 .
Figure 4. Time series of D SST in Box A, Box B and Box C from January 13 to February 13.

Figure 5 .
Figure 5.Time series of average daily light intensity in Box A, Box B and Box C from January 13 to February 13 (the vertical dotted line indicates the start time of the TC).

Figure 6 .
Figure 6.The climatic nitrate concentration (μmol l −1 ) in January 2015 was derived from WOA (a, b, c and d are representative of nutrient concentrations at water depths of 30, 50, 80 and 100 m, respectively).

Figure 7 .
Figure 7. Time series of nitrate fluxes on the west side (a), south side (b) and east side (c) of Box A from January 13 to February 13.

Figure 8 .
Figure 8.Time series of Chl-a fluxes on the west side (a), south side (b) and east side (c) of Box A from January 13 to February 13.

Figure 10 .
Figure 10.The temperature (black line), salinity (blue line) and density (red line) profiles at the southern tip of Madagascar on January 13, 2015 (a) and February 15, 2015 (b).The red dashed lines indicate the mixed layer depth (MLD), and the black dashed lines indicate the isothermal layer depth (ILD).The space between them outlines the barrier layer thickness.

Figure 11 .
Figure 11.Time series of the negative spatial mean vorticity of Box A with depth from January 13 to February 13.