Assessment of a Tropical Transition over the Southwestern South Atlantic Ocean: The Case of Cyclone Akará

Abstract

Tropical cyclones are rare in the South Atlantic Ocean. Hurricane Catarina (2004), developed from a tropical transition, was the first documented case, followed by Iba (2019), which had a purely tropical genesis. In February 2024, the southeastern South Atlantic recorded its third tropical cyclone, Akará, initially a subtropical system. Due to the specific conditions required for tropical cyclones to develop in this ocean basin, the main purpose of this study is to describe the physical mechanisms that triggered the genesis of Akará’s precursor and its tropical transition. Data from various sources and methodologies, including the cyclone phase space diagram, are used in this study. Results show that the passage of a cold front created an environment with horizontal wind shear, contributing to most of the cyclonic relative vorticity in the genesis region. This was the primary driver of cyclogenesis at 1200 UTC on 15 February, along with other secondary processes. The tropical transition occurred as the vertical shear weakened, and turbulent heat fluxes from the ocean to the atmosphere increased, enhancing diabatic processes that warmed the atmosphere. This led to the tropical transition at 0600 UTC on 17 February.

1. Introduction

Several studies have indicated that tropical cyclones develop in environments with 200–850 hPa vertical wind shear lower than 10 m s⁻¹ [1,2,3,4,5] and over sea surface temperatures (SSTs) higher than 26.5 °C [6,7]. Vertical wind shear describes the change in mean large-scale horizontal wind with height. Physically, low vertical wind shear allows for the vertical alignment of a storm’s circulation, enabling the development and intensification of a warm-core structure. When the shear is strong, it generally causes a ventilation effect, contributing to (a) the horizontal advection of heat released during condensation away from the storm center [8]; at the same time, cold, dry air is advected from the outer parts of the cyclone to its center, and (b) the vertical transport of entropy, i.e., it imports low-entropy air (cold and dry air) from the mid-troposphere into the boundary layer. The influx of cold-dry air into the near-surface cyclone’s center suppresses the formation of deep convection, which is essential for its intensification (a review of these processes and other theories is provided by Rios-Berrios et al. [5]). Therefore, the ventilation effect hinders the formation and maintenance of the warm core in tropical systems. In mature tropical cyclones, Frank and Richie [9] showed that the vertical wind shear weakens the system through a sequence of events: (1) the shear causes asymmetry in the structure of the eyewall (tilt of the cyclone’s vortex); (2) with the decoupling of the low-upper level cyclone’s center, the upper-level air is mixed outward rather than into the eye, and, consequently, the shear also acts by ventilating the eye resulting in a loss of the warm core at the upper-levels, causing the central pressure to rise and the weakening of the entire storm; and (3) the shear advects the asymmetric features at the upper-levels, causing the upper portions of the vortex to tilt approximately downshear (in the direction of the shear); the storm weakening is from the top down. Despite the 200–850 hPa vertical wind shear being a good large-scale environmental dynamical control on tropical cyclogenesis, studies across different oceanic basins have indicated that the vertical wind shear calculated between various isobaric levels can have different impacts on tropical cyclogenesis [5,10,11,12]. For instance, Zeng et al. [10] showed in a climatology of the North Atlantic that strong, slow-moving, and low-latitude tropical cyclones are more strongly affected by vertical wind shear throughout a deep layer of the troposphere, while the weak, fast-moving, and high-latitude ones are more influenced by stronger vertical wind shear in the mid-to-lower troposphere.

Regarding warmer SSTs, they are important to help the evaporation, which transports latent heat to the atmospheric layers distant from the surface. This physical process is described by the Wind-Induced Surface Heat Exchange theory (WISHE, [13]). In the WISHE theory, the intensification of tropical cyclones occurs through the exchange of heat and moisture between the ocean surface and the atmosphere, which is driven by surface winds. Strong winds over warm ocean waters enhance evaporation, which, consequently, increases the moisture transport into the cyclone’s core. This moisture is captured by the upward motions and condenses, releasing latent heat, which fuels convection and powers the storm. As the cyclone strengthens, surface winds intensify, creating a positive self-sustaining feedback that leads to further cyclone intensification. WISHE also introduces the concept of potential intensity, which limits the storm’s maximum strength based on factors like sea surface temperature and heat exchange efficiency. Thus, WISHE highlights the crucial role of the air–sea atmosphere interaction through surface heat fluxes to drive tropical cyclone development and intensification.

Latitudes south of 15° S in the South Atlantic Ocean have long been considered unfavorable for tropical cyclogenesis due to strong vertical wind shear (with an average of ~26 m s⁻¹ between 200–850 hPa; [14,15]), and relatively cold SSTs typically below 26.5 °C [15]. However, the development of Hurricane Catarina in 2004 challenged this understanding. Catarina, the first recorded tropical cyclone in the South Atlantic since the beginning of the satellite era [14,16], formed at 29° S and over SSTs around 25 °C [16]. This system developed from a weak extratropical cyclone that interacted with a dipole-blocking pattern in the mid-upper levels of the atmosphere. This blocking pattern led to a weakening in vertical wind shear, allowing the convection to organize. As a result, the extratropical system underwent a tropical transition [16]. As shown by McTaggart-Cowan et al. [17], SST lower 26.5 °C is not a limitant for tropical cyclones when there is a “baroclinic precursor that can alter the formation process sufficiently to promote development over cooler waters”.

Fifteen years later, at ∼16.5° S, the first pure tropical cyclogenesis over the South Atlantic was recorded with the formation of Cyclone Iba [18]. The environment associated with Iba’s development was characterized by a blocking pattern over the southeastern South Pacific Ocean, near the west coast of South America, which disrupted the westerlies and favored the formation of a ridge over Brazil and the eastern South Atlantic Ocean. This helped to decrease the vertical wind shear. Simultaneously, the near-surface environment left by a passing cold front contributed to horizontal wind shear near the surface, facilitating the development of the initial cyclonic circulation. This incipient cyclonic vortex evolved through diabatic processes, eventually becoming Cyclone Iba [18].

Since 2004 the southwestern South Atlantic Ocean has been home to several subtropical cyclones [19,20,21,22,23]. These systems gained attention only after Hurricane Catarina, and those systems that develop near the Brazilian coast, as well as the tropical ones, have been named by the Brazilian Navy. Interest in these systems arises because they have the potential to undergo a tropical transition. Both subtropical and tropical cyclones can develop over the South Atlantic Ocean, as indicated by the Genesis Potential Index (GPI), a dimensionless variable that indicates areas with potential for tropical cyclogenesis. This index was proposed by Emanuel and Nolan [24] and uses in its formulation the vertical component of absolute vorticity at 850 hPa, relative humidity at 700 hPa, vertical wind shear between 850–200 hPa, and potential intensity (i.e., the theoretical maximum intensity that a tropical cyclone can reach). According to Silva and Reboita [15], along the Brazilian coast, a weak signal of GPI appears in October between the coasts of Bahia and Espírito Santo. This signal intensifies, reaching its maximum strength between February and March when it also affects the southern coast of Brazil. However, the presence of GPI is a necessary condition but does not guarantee that a subtropical or tropical cyclone will develop. Another fact is that GPI has a great variation in subtropical cyclogenesis over the South Atlantic. Ribeiro et al. [23] showed that, in this ocean basin, subtropical cyclones occur in an environment where GPI ranges from ~0 to ~21. Therefore, there is no well-established value of GPI indicative of subtropical cyclones. Additionally, Lauton et al. [25] showed that the hybrid phase of Hurricane Catarina had a GPI of ~20, which decreased to around 10 during the tropical storm phase. These results suggest that GPI is variable due to the large-scale environmental factors that influence each cyclogenesis.

In February 2024, the Department of Hydrography and Navigation of the Brazilian Navy recorded the occurrence of a third tropical cyclone in the South Atlantic Ocean, near the Brazilian coast, and named it Akará [26]. Initially, this system had subtropical characteristics and transitioned to tropical. Since tropical cyclones are rare in the South Atlantic Ocean, as they depend on atypical conditions for development, it is important to analyze each episode to provide a physical understanding of the environment that leads to the genesis or transition of such systems. In this context, the objective of this study is to describe the physical mechanisms that led to the genesis of Akará’s precursor and the tropical transition process over the southwestern South Atlantic Ocean in February 2024. It is important to highlight that this is the first study to analyze Cyclone Akará, making it a pioneering study.

2. Materials and Methods

2.1. Study Area

The study area encompasses the southwestern South Atlantic Ocean and the coastlines of South America, between 15° S and 40° S, and 55° W and 25° W (Figure 1). This domain includes two of the three cyclogenetic regions along the eastern coast of South America: southeastern Brazil and extreme south of Brazil and Uruguay [27].

2.2. Data

Data from different sources are used. ERA5 reanalysis [28], provided by the European Centre for Medium-Range Weather Forecasts (ECMWF), was obtained from 14 to 24 February 2024, at standard synoptic times (0000, 0600, 1200, and 1800 UTC), and with a horizontal resolution of 0.25° × 0.25°. We downloaded variables in isobaric levels (horizontal wind components, geopotential, air temperature, and specific humidity), and near-surface (10 m wind components, 2 m air temperature, mean sea level pressure—MSLP, sea surface temperature—SST, and latent and sensible turbulent heat fluxes). To compute a climatology for the period from February 2004 to 2023, we also obtained monthly averages of the geopotential and SST.

Accumulated precipitation every 6 h (0000, 0600, 1200, and 1800 UTC) was obtained from the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN); this dataset was developed by the Center for Hydrometeorology and Remote Sensing (CHRS) at the University of California, Irvine (UCI), with a horizontal resolution of 0.25° × 0.25°, and available at https://chrsdata.eng.uci.edu/ (accessed 3 May 2024).

Brightness temperature from channel 13 (infrared; 10.35 µm) of the GOES-16 satellite, was obtained from the Center for Weather Forecasting and Climate Studies (CPTEC) of the National Institute for Space Research (INPE) at http://ftp.cptec.inpe.br/goes/goes16/retangular/ch13/ (accessed 3 May 2024). The data cover the period between 14 and 24 February 2024 (standard synoptic times), with a horizontal resolution of approximately 0.03° × 0.03°.

2.3. Analyses

2.3.1. Cyclone Tracking, Basic Features, and Classification

The geographical coordinates (latitude and longitude) of the cyclone’s center every 6 h (0000, 0600, 1200, and 1800 UTC) were identified using an algorithm based on MSLP provided by Dr. Frédéric Ferry from Météo-France and are shown in

Table 1. Basic features (geographical position and PNMM) and classification of the phases of the cyclone by CPS and Brazilian Navy. MSLP and V indicate, respectively, mean sea level pressure in hPa and near-surface wind intensity in m s⁻¹ (V was obtained by selecting the maximum wind in a box with borders 2^o away from the cyclone’s center). The acronyms S, T, SD, TS, and TD indicate, respectively, subtropical cyclone, tropical cyclone, subtropical depression, tropical storm, and tropical depression. The green (blue) color line highlights the date in which the cyclone becomes tropical following CPS methodology using moderate warm core (using the classical definition), and the pink color when the cyclone becomes tropical following the classification of the Brazilian Navy. * highlights the dates used to represent the subtropical cyclogenesis, tropical transition, and intense tropical phase in this study.

Day	Hour (UTC)	Lat (°S)	Lon (°W)	MSLP (hPa)	MSLPNavy (hPa)	V (m s−1)	VNavy (m s−1 and knots)	CPS	ClasNavy	Observation
* 15	1200	25	40.5	1014	1016	11		S
15	1800	25	40.25	1013		10		S
16	0000	25	40.75	1013	1010	11		S	SD
16	0600	24.5	40.75	1011		10		S/T		The time when the cyclone transitions to a tropical phase following CPS with a moderate warm core.
16	1200	24	39.75	1013	1010	13		S/T	SD
16	1800	25	39	1011		14		S/T
17	0000	24.5	39.5	1011	1008	14		S/T	SD
* 17	0600	24.5	39.5	1008		15		T		The time when the cyclone transitions to a tropical phase according to the classic CPS.
17	1200	25	39.5	1009	1006	13		T	SD
17	1800	25	39.25	1009		12		T
18	0000	25.25	39.25	1007	1006	13		T	SD
18	0600	26	39.5	1004		14		T
18	1200	26.75	39.75	1005	1006	14		T	TD
18	1800	27.75	40.75	1003		14		T
19	0000	28.75	41.5	1001	1000	17	21 m s−1 40 knot	T	TS	The time when the cyclone transitions to a tropical phase following the Brazilian Navy.
19	0600	29.5	42.25	998		15		T
19	1200	30.25	42	997	998	18	23 m s−1 45 knot	T	TS
* 19	1800	31.5	42	996		18		T
20	0000	31.75	42.25	995	994	16	23 m s−1 45 knot	T	TS
20	0600	32.25	42	994		16		T
20	1200	32.25	42	997	996	15	18 m s−1 35 knot	T	TS
20	1800	32.25	41.5	998		13		T
21	0000	32.25	41.5	1001	1002	15		T	TD
21	0600	32.25	42.25	1002		14		T
21	1200	32.5	42.25	1004	1002	14		S/T	SD
21	1800	32.75	42.75	1004		12		S/T
22	0000	33	43.5	1006	1004	11		S/T	SD
22	0600	33.5	44	1005		10		S/T
22	1200	33.75	44.5	1008	1006	9		S/T
22	1800	34.25	44.75	1008		9		S/T
23	0000	34.75	45.0	1009	1010	7		S

. This algorithm has the advantage of being written in Python and is available on https://github.com/fredericferry/era5_storm_tracking_and_maps (accessed 12 August 2024). Cyclogenesis was identified when the first closed isobar appeared, while cyclolysis was determined at the last time step with a closed isobar. Precipitation along the cyclone’s tracking was accumulated every 6 h within a 5° radius around the cyclone. From the cyclone’s center coordinates recorded every 6 h, its lifecycle, distance traveled, and average speed were calculated. The total distance traveled was computed by summing the distances between the system’s positions at 6-h intervals using the Haversine equation [29]:

$$d=2 r arcsin\left(\sqrt[2]{{sin}^2\left({\displaystyle \frac{{\phi }_2-{\phi }_1 }{2}}\right)+cos\left({\phi }_1\right) cos\left({\phi }_2\right) {sin}^2\left({\displaystyle \frac{{\lambda }_2-{\lambda }_1 }{2}}\right)}\right)$$

(1)

where d is the distance (km), r is the Earth’s radius (6371 km), $\lambda$ is longitude, and $\phi$ is latitude.

As the cyclone evolves throughout its lifecycle, it can transition from one phase to another (e.g., from subtropical to tropical). We used the Cyclone Phase Space (CPS) methodology [30] to classify the phases of the studied cyclone. Based on the geographical coordinates of the cyclone’s center, the CPS algorithm defines a circle with 500 km of radius from the system’s center and computes three thermal parameters (inside this circle) based on geopotential height: (1) thermal asymmetry (B; unit of meters) calculated based on the thickness between the 900 and 600 hPa levels to capture the strength of the cyclone’s frontal nature; (2) thermal wind at the lower troposphere (−|V_T^L|; 900 to 600 hPa); and (3) thermal wind at the upper troposphere (−|V_T^U|; 600 to 300 hPa). The thermal wind is a measure of the vertical shear of the horizontal wind and allows to determine if a cyclone has a cold or warm core (vertical thermal structure). It is obtained through the magnitude of the cyclone’s isobaric height gradient since in a cold (warm) core cyclone, the gradient increases (decreases) with height. A cyclone is classified as extratropical when B << 10 m, −|V_T^L| < 0, and −|V_T^U| < 0, and as tropical when B < 10 m, −|V_T^L| > 0, and −|V_T^U| > 0. For subtropical cyclones over the South Atlantic Ocean, the thresholds are B < 25 m, −|V_T^L| > −50, and −|V_T^U| < −10 [19].

We will also compare the classification obtained using the CPS with those provided by the Brazilian Navy, which uses near-surface wind intensity as one of the main criteria for the cyclone phase classification [31]. Subtropical depression (SD) occurs when the wind intensity is lower than 63 km/h (<34 knots), and a subtropical storm occurs when the wind intensity is higher or equal to 63 km/h (≥34) and lower than 118 km/h (<64 knots). A cyclone is classified as a tropical depression (TD) when the wind intensity is lower than 63 km/h (<43 knots), a tropical storm (TS) when the wind intensity is higher or equal to 63 and lower than 118 km/h (≥34 and <64 m/s), and a hurricane occurs when the wind intensity is higher than 118 km/h (>64 knots). Due to the overlap in wind criteria for subtropical and tropical cyclones, the Brazilian Navy classifies a subtropical cyclone as a system with a hybrid thermal structure and wind and convection patterns with little symmetry. In contrast, tropical cyclones have a well-defined warm core, well-organized deep convection, and a clearly defined wind circulation around the cyclone’s center. The classification of cyclones as subtropical or tropical is provided in synoptic charts, which are produced only for 0000 and 1200 UTC [26]. In the case of tropical cyclones, the navy also includes near-surface wind intensity on the charts.

Additional information about the cyclone’s vertical thermal structure throughout its lifecycle is obtained by calculating a vertical cross-section of the zonal temperature deviation centered on the cyclone. It is obtained by subtracting the mean temperature within a 3° latitude by 6° longitude box from the mean temperature in a wider 3° latitude by 18° longitude box, both centered on the cyclone’s center at each time step [18]. If a box extends over the continent, a mask is applied to exclude the information since there are great temperature differences between land and ocean.

2.3.2. Physical Mechanisms

Dynamic, synoptic, and thermodynamic analyses were performed in the area between 10° N and 70° S and 110° W and 10° E to describe the environment conducive to cyclogenesis and tropical transition. To this end, different quantities were calculated as described below.

• Synoptic charts: they are constructed using MSLP, 1000–500 hPa thickness (i.e., the distance between two pressure levels, it reflects the temperature distribution in the atmosphere, and helps identify fronts), and 250 hPa wind intensity higher than 30 m s⁻¹ (to represent the upper-level jets). The combination of these three variables allows the identification of low- and high-pressure systems and cold, warm, and occluded fronts. After the identification of these systems, they are manually drawn on the charts.
• Vertical shear of the horizontal wind (also referred to in the literature as environmental vertical wind shear): a preliminary analysis of the cyclone’s characteristics showed that this system had a deep core and slow movement aligning with those described by Zeng et al. [10], where deep vertical wind shear (200–850 hPa) has a greater influence on cyclogenesis than shear at other vertical levels. Therefore, we computed the vertical wind shear (defined here as the difference in wind intensity) using 200–850 hPa isobaric levels. Regions with strong shear indicate that the upper-level winds are more intense than those at lower levels (typically characterizing baroclinic regions). In addition, environments with wind shear > 10 m s⁻¹ are not proper for tropical cyclones.
• Relative vorticity due to horizontal wind shear (hereafter shear vorticity): shear vorticity induces rotation through a change in wind speed in the direction normal to the flow. Relative vorticity (ζ) can be defined as the sum of shear vorticity and curvature vorticity as ζ = ${\displaystyle \frac{-\delta V}{\delta n}}$ + ${\displaystyle \frac{V}{R_s}}$, and the shear vorticity can be calculated using Equation B.9 from Schenkel [32] adapted for spherical coordinates:

$${\displaystyle \frac{-\delta V}{\delta n}}=-{\displaystyle \frac{1}{V^2}}\left(u^2{\displaystyle \frac{\partial u}{r \partial \phi }}-v^2{\displaystyle \frac{\partial v}{r cos\phi \partial \lambda }}-uv\left({\displaystyle \frac{\partial u}{r cos\phi \partial \lambda }}-{\displaystyle \frac{\partial v}{r \partial \phi }}\right)\right)$$

(2)

where V is the magnitude of the horizontal wind vector (m s⁻¹), n is in the normal direction of the wind, u and v are the horizontal wind components. In this study, shear vorticity was calculated for the level of 925 hPa. To obtain its contribution to the total ζ, we computed the ratio (%) between the shear vorticity and total ζ.

• Mass divergence: this quantity is computed as the dot product of the del operator ($\overrightarrow{\mathsf{\nabla }}$) and the velocity vector field [33]. In spherical coordinates, mass divergence is expressed as:

$$\overrightarrow{\mathsf{\nabla }}·\overrightarrow{V}=\left({\displaystyle \frac{\partial u}{r cos\phi \partial \lambda }}+{\displaystyle \frac{\partial v}{r \partial \phi }}\right)$$

(3)

where $u$ and $v$ are, respectively, the zonal and meridional horizontal wind components, r is the Earth’s radius, $\phi$ is the latitude and $\lambda$ is the longitude. Mass divergence measures the rate at which mass is exiting or entering a given volume in a fluid. By convention positive (negative) values indicate divergence (convergence). In this study, this quantity was computed for the level of 250 hPa.

• Horizontal temperature advection: it is calculated as the negative dot product of the wind vector and the gradient of temperature [33]. The formula in spherical coordinates is as follows:

$$-\overrightarrow{V}·\overrightarrow{\mathsf{\nabla }}T=-\left(u{\displaystyle \frac{\partial T}{r cos\phi \partial \lambda }}+v{\displaystyle \frac{\partial T}{r \partial \phi }}\right)$$

(4)

Positive (negative) indicates warm (cold) advection. In this study, horizontal temperature advection was computed to the level of 850 hPa.

• Vertically Integrated Moisture Flux (VIMF): this quantity, following Peixoto and Oort [34] (pg. 274), is calculated as follows:

$$VIMF\left(\lambda ,\varphi \right)={\int }_o^{p_0}q\overrightarrow{V}{\displaystyle \frac{dp}{g}}$$

(5)

where $q$ represents specific humidity, $\overrightarrow{V}$ is the horizontal wind vector, $p$ is the pressure, $g$ is the gravity acceleration, and $p_0$ the superior limit of integration. In this study, $VIMF$ is calculated considering the levels between 1000 (0) and 200 hPa (p_o). Additionally, we computed the divergence of VIMF by using its components ($Q_{\lambda }$ and $Q_{\varphi }$) in the expression of the mass divergence (Equation (3)).

$$Q_{\lambda }={\int }_{o }^{p_0}qu{\displaystyle \frac{dp}{g}}$$

(6)

$$Q_{\varphi }={\int }_{o }^{p_0}qv{\displaystyle \frac{dp}{g}}$$

(7)

• Equivalent potential temperature (θ_e): it is the temperature that an air parcel would have if it were lifted from its current level to the upper atmosphere until all the water vapor condenses out (saturated adiabatically), and then brought back down to standard pressure (typically 1000 hPa) along a dry adiabat [35]. Essentially, it accounts for both the temperature and the moisture content of the air [36], making it a useful variable for assessing instability and convective potential. For stability, a positive gradient of θ_e with height (i.e., increasing θ_e with increasing height) means a stable condition, and a negative gradient of θ_e (i.e., decreasing θ_e with increasing height) represents an unstable condition [37,38]. θ_e allows us to understand the vertical structure of the atmosphere without needing to compute deviations, as is required with air temperature. In the latter case, since temperature tends to decrease with height, it becomes difficult to identify which regions in the troposphere are experiencing physical processes such as advection. Here, θ_e is computed using the expression obtained from [33]:

$${\theta }_e=T\left[{\left({\displaystyle \frac{p_s}{p}}\right)}^{{\displaystyle \frac{R}{c_p}}}exp\left({\displaystyle \frac{L_{v }{q}_s}{c_p T}}\right)\right]$$

(8)

where $c_p$ is the specific heat capacity of dry air at constant pressure, $q_s$ is the saturation mixing ratio, $L_v$ is the latent heat of vaporization, $R$ is the gas constant for dry air, T is the temperature in Kelvin, $p_s$ is the surface pressure (1000 hPa), and $p$ is the pressure at a specific vertical level. We computed θ_e from 1000 to 200 hPa and calculated the average in a box 1° away from the cyclone’s center in order to plot vertical profiles.

3. Results and Discussions

3.1. Basic Features and Classification

Based on the MSLP fields, the cyclogenesis of Akará’s precursor occurred near the coast of southeastern Brazil at 1200 UTC on 15 February 2024, with a central pressure of 1014 hPa (Table 1). Its cyclolysis was registered at 0000 UTC on February 23, already away from the Brazilian coast, resulting in a total lifetime of 7 days and 12 h. The cyclone followed an anomalous trajectory towards the southwest over the South Atlantic Ocean (Figure 1), covering a total distance of 1971 km with an average speed of 11 km h⁻¹. Throughout its lifetime, the cyclone produced precipitation that was more intense on the right side of its center, and this rainfall did not affect the continental area (Figure 1).

Table 1 shows the CPS in each time step of the cyclone, with S indicating a subtropical phase and T a tropical phase. Note that the thresholds used in the CPS for classifying cyclone types are not entirely objective. Using the classic CPS definition (−|V_T^L| > 0 and −|V_T^U| > 0, see Section 2.3.1), indicated by the black dashed lines in Figure 2b, the cyclone can be classified as tropical from 0600 UTC on February 17, maintaining this characteristic until 0600 UTC on February 21. During the periods before and after the tropical phase, the system is characterized as subtropical. Some studies, such as Reboita et al. [18] and their references, have shown tropical cyclones with a moderately warm core, even with −60 < −|V_T^U| < 10 (red dashed lines), which means that the vertical wind shear between 300 and 600 hPa is not so weak compared to the classical definition. If this classification is utilized, the system acquires tropical characteristics about 18 h after genesis (at 0600 UTC on 16 February) and keeps it until 1800 UTC on February 22. Note that there is an overlap between the definitions and even with the subtropical phase (green dashed lines; thresholds −|V_T^L| > −50 and −|V_T^U| < −10). For this reason, in Table 1 during the CPS classification, we used the symbol S/T for when both cases can be considered. The intention here is to show that CPS is a sensitive methodology and, in some cases, needs to be analyzed alongside other variables, such as air temperature, to help determine when a specific phase begins and ends. We will perform it to indicate when the subtropical cyclone undergoes a tropical system but first, the Brazilian Navy classification is presented (Table 1).

The synoptic charts of the Brazilian Navy are available from its website [26] and show the cyclogenesis of Akará’s precursor at 1200 UTC on 15 February 2024 and the cyclolysis at 0000 UTC on 23 February, which aligns with the results of our tracking (Figure 1 and Table 1). At 0000 UTC on February 16, the cyclone acquires subtropical depression characteristics but evolves into a tropical depression at 1200 UTC on February 18, a tropical storm at 0000 UTC on February 19, and undergoes again to a subtropical depression at 0000 UTC on 20 February. Regarding the duration of the tropical phase, from 0000 UTC to 1800 UTC on 19 February, it is shorter than the duration obtained using the CPS (Table 1). There are different reasons for the differences between the CPS and the Brazilian Navy’s cyclone phase classification. A key factor is that the Navy works in operational mode, requiring them to make synoptic charts available at specific times. Therefore, at the time of the synoptic analysis, several meteorological products, such as the ERA5 reanalysis, were not yet available, which makes a detailed analysis of the cyclones’ thermal structure more challenging. When a cyclone is re-analyzed after a few days, more meteorological data become available, allowing for a more detailed analysis, as conducted in the present study.

In the case of the issue of tropical phase classification in the CPS, previous studies, such as Reboita et al. [18] and their references, have shown that the vertical cross-section of the zonal deviation of air temperature over time is a useful tool to use alongside the CPS for classifying cyclone types. Therefore, the decision to assume the start of the tropical phase considering the classic or the moderate warm core definitions in the CPS is supported by the cyclone’s vertical thermal structure in a vertical cross-section graphic of zonal air temperature deviation (Figure 3).

Based on its vertical thermal structure, a cyclone is classified as tropical when there is a vertical profile of warm zonal air temperature deviation at the system’s center, extending from the lower to the upper atmosphere. Near the surface, the deviations may be cold due to energy loss from evaporation [18,39,40,41,42,43]. An example of a vertical profile with this cold feature is found in Cyclone Iba, the first documented pure tropical cyclogenesis over the southwestern South Atlantic [18]. Applying the same analysis to the cyclone studied here (Figure 3), until 0000 UTC on 17 February, the mid-troposphere was relatively cold, which is not typical of a tropical cyclone. Afterwards, the expected structure of a tropical cyclone (a warm core throughout the atmosphere) is observed and it remains (intensifies) until 0000 UTC on 21 February. From 0600 UTC on 21 February, the upper troposphere begins to cool, and the tropical cyclone structure dissipates. This vertical pattern of air temperature zonal deviations is consistent with the classification in Table 1 using the classic CPS definition. Using the moderately warm core approach in the CPS, the result does not accurately reflect the cyclone’s real characteristics, leading to a potential misclassification. It is also important to note that the CPS does not use wind intensity criteria. When wind intensity classification is considered, as in the Brazilian Navy’s classification, the tropical phase coincides with the period of greatest warming in the atmospheric column—from 0000 UTC on 19 February to 0000 UTC on 20 February. Therefore, wind intensity is also not a reliable method for classifying cyclones.

In summary, regardless of the methodology, Akará’s precursor transitions into a tropical cyclone. However, the start of the tropical phase as well as the lifetime in this phase occur at different times in the various approaches. Nevertheless, the analysis of zonal deviation of air temperature supports the tropical phase beginning at 0600 UTC on 17 February and ending at 0600 UTC on 21 February, which agrees with the classic CPS approach (and will be used in this study). The periods before and after this phase characterize the system as having a subtropical structure. It is also worth noting that the CPS is a methodology recommended by the World Meteorological Organization (WMO) for application and studies across different ocean basins in an effort to establish threshold guidelines for the scientific community [44], supporting the discussion presented here.

3.2. Physical Processes at Cyclogenesis

This section discusses the physical processes that triggered the cyclone precursor of Akará. We selected 1200 UTC on 14 February 2024, as a pre-cyclogenesis time. Figure 4a shows a cold front, connected with the cyclone located around 55° S and 25° W, reaching the coast of southeast Brazil (~25° S), which will be the place of the cyclogenesis. During the pre-cyclogenesis period, there was ~4 °C of horizontal temperature gradient at 850 hPa between the cold front (~27.5° S) and the cyclogenesis region (~25° S) (figure not shown). Twelve hours later, the cold front was slightly northeastward displaced (figure not shown), and at 1200 UTC on 15 February, this system was away from the Brazilian coast, located between the post-frontal anticyclone and the South Atlantic Subtropical Anticyclone (SASA) (Figure 4b). At the same time, a low-pressure center (1016 hPa) appears near southeastern Brazil (~25° S), characterizing the cyclogenesis. Note that in Table 1, the MSLP is lower (1014 hPa) than in Figure 4b due to the refined methods used in the algorithm to identify the cyclone center.

We analyzed Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 to identify the physical mechanisms that triggered the cyclogenesis. The region of cyclogenesis was initially influenced by the displacement of a cold front towards this region. The proximity of the cold front promoted near-surface convergence (figure not shown) and upper-level mass divergence at 200 hPa (Figure 5a). This divergence persisted in the region, even with the weakening of the upper-level winds, as indicated by the vertical wind shear on the day of cyclogenesis (Figure 5b). Other dynamic processes, such as troughs or cutoff lows at mid-upper levels, which are common drivers of subtropical cyclogenesis in the region ([20,27,44] and their references), were absent from before to the day of cyclogenesis (Figure 5a,b). This is also supported by the anomalies of the geopotential height at 500 hPa (Figure 6a,b) compared to the period of 2004–2023. In the cyclogenesis region, from 1200 UTC on 14 February to 1200 UTC on 15 February, the geopotential height anomalies ranged between 2 and 1 dam, remaining close to the climatological pattern.

The upper-level divergence, although not vigorous, may have coupled with other near-surface processes. Specifically, the passage of the cold front produced near-surface convergence and cyclonic relative vorticity, which remained in the region due to the contribution of horizontal wind shear caused by the interaction between the northeasterly winds from the SASA and the southerly winds from the post-frontal anticyclone (Figure 7). From the pre-cyclogenesis period to the day of cyclogenesis, the contribution of the horizontal wind shear to the total cyclonic relative vorticity ranges from 40% to ~100% in the genesis region (Figure 7a–d). Therefore, it was the main dynamical driver for cyclogenesis. This physical process was also important for Iba’s genesis, the first pure tropical cyclogenesis over the South Atlantic Ocean [18], and for tropical cyclones in other ocean basins [45].

Thermodynamic processes may have a secondary role in cyclogenesis. Since the pre-cyclogenesis period, in the genesis region, predominates warm air advection at 850 hPa promoted by northerly winds (Figure 8a,c), and convergence of the vertically integrated moisture flux, with a main contribution from the tropical ocean (Figure 8b,d). The moisture flux from the north is driven by winds on the western side of the SASA, and it has been reported as important for subtropical cyclogenesis over the South Atlantic Ocean [19,21,46]. Warm and moist air, due to its lower density, thermodynamically contributes to decreasing low-level atmospheric pressure in a specific atmospheric layer. However, as this alone is insufficient to initiate cyclogenesis, it is considered a secondary factor in the process [33]. Indeed, the organization of the low-level cyclonic circulation in the genesis region is carried out by the horizontal wind shear (referred to as shear vorticity), and this area of low pressure helps to channel winds leading to moisture convergence; the convergence acts as an air-lifting mechanism (dynamical process), which is important for cloud development and the near-surface cyclone development [35]. Physically, this process begins with air being lifted by convergence (and in the studied case, further supported by the weak upper-level divergence and convection). Considering warming air, it can be unstable, and further upward motion is encouraged. As the moist air rises, it cools and eventually reaches saturation, leading to condensation. This thermodynamic process releases latent heat (diabatic process), which further warms the rising air parcels, reinforcing their upward movement. This upward motion continues until the air parcels encounter or generate upper-level divergence, allowing the air to evacuate from the region, which in turn leads to a decrease in near-surface pressure, causing cyclogenesis (the described process is known as the “self-development” paradigm for cyclogenesis; [47]). Indeed, Figure 9a,b shows that the cyclogenesis has occurred embedded in a wide area of cloudiness since 14 February, with brightness lower than −40 °C (top cloud temperature), which is a proxy of the atmosphere’s diabatic processes.

The mean vertical profiles of θ_e for different stages of the cyclone are presented in Figure 10a. In the pre-cyclogenesis environment (1200 UTC on 14 February) there is a steeper decrease in θ_e with height, from the near-surface to the mid-troposphere (500 hPa), characterizing an unstable atmosphere for moist convection, which is associated with low-level moisture flux convergence (Figure 8) that can sustain intense deep convection throughout the cyclone’s lifecycle. In the next stages, the decrease in θ_e with the height is restricted from the near-surface to 700 hPa. The strongest instability during the pre- and the cyclogenesis, reaching 500 hPa, is when θ_e increases rapidly with the height, which plays a crucial role in the cyclone’s intensification, as indicated by the drop in central pressure (Figure 11a). As the cyclone moves southwestward and convective activity is established in the later stages, a decrease in θ_e occurs near the surface, along with a weakening of convective instability in the low troposphere (θ_e decreases with height is smaller). At the maturity stage, θ_e does not show a slope from 700 to 250 hPa and presents higher values than the initial, indicating the warming of the atmosphere due to the latent heating releasing associated with convective activity. This process also acts to increase θ_e from mid to upper levels, from the initial to the mature phase. The described pattern of the θ_e vertical profiles is similar to other reported cases of tropical cyclones across various oceanic basins [3,9,48,49].

The air lifting may also have been influenced by convection, as conditions in the region were favorable. For instance, on the day of cyclogenesis, SST reached 27.7 °C, with a positive anomaly of ~1 °C in the genesis region, and a vertical temperature gradient between the sea and air of 2.3 °C (Figure 11c). In terms of total heat fluxes (latent + sensible), the recorded value of 145 W m⁻² (Figure 11b) is comparable to the climatology reported by Reboita et al. [27] and Silva and Reboita [15]. High-intensity total heat fluxes were not observed, possibly due to the weak sea-air vertical humidity gradient (figure not shown) in the presence of a moist atmosphere during the pre-cyclogenesis phase (note that a large vertical humidity gradient is a condition for intense latent heat flux). Later, from 16 to 19 February, the total heat fluxes increase (reaching ~200 W m⁻²) is associated with both a higher vertical temperature gradient and intensification of near-surface winds. From 19 to 20 February, turbulent mixing associated with strong near-surface winds, together with evaporative processes from rainfall, act to reduce the vertical temperature gradient, and contribute to decreasing the total heat fluxes.

3.3. Tropical Transition

At 0600 on 17 February 2024, the cyclone underwent a tropical phase (Figure 2 and Figure 3) was named Akará. The synoptic chart at this time over the Atlantic Ocean (Figure 4c) does not show huge differences in relation to the cyclogenesis (Figure 4b). On the other hand, due to the distancing of the cold front from the continent to the ocean, the upper-level jets over the cyclone region weakened, and the response to this change is a decrease in the vertical wind shear (Figure 5c). Other differences between cyclogenesis and tropical phase are related to the wind shear vorticity, which indicates a well-defined cyclonic circulation (Figure 7c); both warm horizontal advection and convergence of vertically integrated moisture flux, which increase at the eastern of the cyclone center (Figure 8e,f), and cloudiness that begins to assume a spiral pattern around the low-pressure center (Figure 9c). Figure 11d does not show an increase in the convergence of the vertically integrated moisture flux because the areal mean was performed considering a box around the cyclone and the fluxes are concentrated eastern from the cyclone center.

Figure 11 suggests that the cyclone intensification (a decrease in MSLP and an increase in the wind speed, Figure 11a) and its tropical transition are mainly related to the decrease in the vertical wind shear, along with an increase in total heat fluxes (Figure 11b). In Figure 11b, the vertical wind shear changes from positive to negative exactly at 0600 UTC on 17 February, which is the first time that the cyclone exhibits tropical characteristics according to the classical CPS definition (Table 1). The weakening of the vertical wind shear seems to be a result of the displacement of the upper-level jet, associated with the cold front, away from the cyclone region. This weak shear plays an important role in organizing convection in the atmospheric column, which intensifies its heating through the diabatic processes. As a result, the warming of the mid-upper levels in the cyclone region characterizes the system as tropical, as shown by the thermal structure analysis of the cyclone in Figure 2 and Figure 3. On the other hand, as the near-surface cyclone becomes well configured close to the transition phase, its wind intensity increases (Figure 11a), which, in turn, increases the transfer of turbulent heat fluxes from the ocean to the atmosphere (Figure 11b). This physical process is described by the WISHE theory [13] and helps to explain the cyclone strengthening and its tropical transition. During the intense phase, the vertical profile of θ_e indicates large moist instability up to 700 hPa, followed by a well-defined increase in moist stability at higher levels (Figure 10b). As the cyclone progresses, the latent heat release associated with convection intensifies, leading to atmospheric warming. This is evident from the more homogeneous (neutral for moist instability) large area and depth of higher θe at the cyclone center (Figure 10c).

The first time the cyclone exhibits tropical characteristics, at 0600 UTC on 17 February, the cloudiness is still not symmetric, with the highest concentration of clouds southeast of the cyclone center (Figure 9c). The cyclone’s cloudiness seems to be coupled with another cloud band to the north, associated with moisture convergence (Figure 8f). Over time, the cloudiness around the cyclone center becomes more symmetric (Figure 9d), although it remains coupled with another large cloud area (Figure 8h). When viewed on a larger scale (a broader area than in Figure 9), it becomes difficult to distinguish the two cloud bands, one of which is associated with the cyclone. On the other hand, the vertical profiles of cyclonic relative vorticity and θ_e during Akará’s intense phase (Figure 10b,c) clearly highlight its tropical storm characteristics, i.e., the relative vorticity is stronger between the low and mid-levels, and θ_e is higher at the cyclone’s center and decreases outward (as shown by [3]), remains relatively constant through part of the mid-levels, and then begins to decrease with height.

During the period of more symmetric cloud cover, between February 19 and 20, the maximum wind intensity reaches a higher intensity, surpassing the threshold of 17 m s⁻¹ (Figure 11a), corresponding to the Beaufort scale class 8 [50,51,52]. This scale for 10-m winds, ranging from 0 to 12, was developed to help sailors estimate winds via visual observations. For instance, scale class 8 (gale) indicates “Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.” [51]. Hence, Akará produced winds with considerable intensity over the ocean. As the cyclone was far from the continent during its most intense stage, it did not cause any damage to the coastal region. Despite the stronger winds between February 19 and 20, the total heat fluxes begin to decrease. As the cyclone moves southward, colder SSTs, a decrease in the sea-air temperature gradient, and an increase in vertical wind shear (Figure 11b,c) create an unfavorable environment for the tropical cyclone, leading to its transition to a subtropical phase and, posteriorly, cyclolysis (figures not shown).

4. Conclusions

Tropical storms or cyclones are rare in the South Atlantic Ocean. The two recorded cases (in 2004 and 2019) developed through different physical processes. Therefore, when new episodes occur, studies are needed to understand the environment that leads to the formation of these systems. In this context, the present study conducted a dynamic, synoptic, and thermodynamic analysis to understand the genesis of the precursor and the tropical transition of the third recorded tropical cyclone over the South Atlantic Ocean in February 2024. The main conclusions are described below.

Thermal Structure: The analysis of the CPS indicated that the cyclone had genesis with subtropical characteristics, underwent a tropical phase at 0600 UTC on 17 February, and remained with this feature until 0600 UTC on 21 February, when it returned to the subtropical phase. The tropical phase obtained by CPS is longer than that indicated by the Brazilian Navy; as the navy is an operational center, they do not have all the necessary information to proceed with a deeper analysis of the vertical structure of the cyclones during the synoptic times. This is only possible when data, such as reanalysis, are released with two or three days of delay.

Cyclogenesis: Akará’s precursor had genesis in an environment characterized by weakening upper-level winds but with persistent weak divergence. Near the surface, wind shear vorticity appears to be the main mechanism responsible for generating relative cyclonic vorticity, which organizes the cyclogenesis. Thermodynamic processes played a secondary role, contributing to the decrease in atmospheric pressure through warm air advection and moisture convergence in the genesis region.

Tropical transition: From the cyclogenesis onward, the vertical wind shear continues to weaken, supporting the organization of convection, which helps strengthen the near-surface cyclone (lower MSLP and higher wind speeds). As the 10-m winds intensify, the transfer of turbulent heat fluxes from the ocean to the atmosphere increases (WISHE theory), fueling the convective processes. Consequently, these processes enhance the warming of the atmospheric column, contributing to the cyclone’s transition to a tropical structure, which is observed at 0600 UTC on 17 February.

Although Akará’s precursor cyclogenesis was greatly influenced by near-surface shear vorticity, similar to Iba, the other atmospheric conditions, as well as its initial thermal structure, were different. It is also worth mentioning that despite the different environments that led to the development of the three tropical cyclones in the South Atlantic Ocean, in all cases, the atmospheric conditions facilitated the organization of convection, which was crucial to maintaining the warm core of these systems. In summary, this study demonstrates the need for case studies when other tropical cyclones occur over the South Atlantic Ocean.