Role of Ocean Heat Content in the Rapid Intensification of Cyclone Amphan (2020) over the Bay of Bengal

Role of Ocean Heat Content in the Rapid Intensification of Cyclone Amphan (2020) over the Bay of Bengal

Author:Kritajno Bhattacharya

Abstract 

A major forecasting challenge in the North Indian Ocean is the rapid intensification (RI) of tropical cyclones, especially in the Bay of Bengal, where coastal communities are extremely vulnerable. The main indicator of cyclone intensification has historically been sea surface temperature (SST). Since it captures the total thermal energy available for cyclone growth outside of the surface layer, subsurface ocean heat content (OHC) has lately been acknowledged as a more accurate metric. This study examines how OHC contributed to Super Cyclonic Storm Amphan's (2020) quick intensification over the Bay of Bengal.We'll examine the atmospheric and oceanic factors that contributed to Amphan's abrupt intensification using Tropical Cyclone Heat Potential datasets, in-situ Argo float observations, and best-track records from the India Meteorological Department (IMD). The findings indicate that OHC values in the intensification region were well above the 60 kJ cm⁻² threshold that is thought to be favorable for RI, exceeding 100 kJ cm⁻². Warm SST (>30°C) and an elevated OHC supplied enough subsurface thermal energy to support the cyclone's intensification from cyclonic storm to super cyclonic storm in less than 48 hours. Even though SST stayed favorable the entire time, Amphan's RI's timing and magnitude were strongly correlated with the unusually high OHC. The study highlights the necessity of incorporating OHC monitoring into Bay operational cyclone forecasting.

Introduction 

Tropical cyclones (TCs) in the North Indian Ocean, particularly over the Bay of Bengal (BoB), pose a serious threat to the densely populated coastal regions of India, Bangladesh, and Myanmar. Although the BoB accounts for only a small fraction of global tropical cyclone activity, it contributes disproportionately to global cyclone fatalities due to high population density, low-lying terrain, and socio-economic vulnerability of the coastal communities.

One of the most critical forecasting challenges in this region is the phenomenon of rapid intensification (RI), wherein a tropical cyclone’s maximum sustained winds increase by at least 30 knots within 24 hours. RI events reduce lead time for preparedness and evacuation, often resulting in severe human and economic losses. Conventional intensity prediction approaches have relied heavily on sea surface temperature (SST) as the primary oceanic parameter. However, recent studies suggest that subsurface thermal structure, represented by ocean heat content (OHC), plays a crucial role in fuelling RI by sustaining energy supply even under cyclone-induced surface cooling.

The Bay of Bengal is known for its high OHC pockets due to freshwater influx from large river systems, strong stratification, and seasonal monsoon processes. These conditions often create a favourable subsurface thermal reservoir, which may not be captured by SST alone. The devastating Super Cyclone Amphan (May 2020) underwent RI before landfall, making it an important case to investigate the role of OHC in intensity evolution.

The present study aims to analyse the contribution of OHC variations to the rapid intensification of Cyclone Amphan (2020) over the Bay of Bengal. By examining oceanic conditions preceding and during RI, the study highlights the importance of integrating subsurface thermal parameters into operational forecasting for improved disaster preparedness.

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“Role of Ocean Heat Content in the Rapid Intensification of Super Cyclonic Storm Amphan (2020) over the Bay of Bengal”

2. Data and Methodology

2.1 Study Region

The present study focuses on the Bay of Bengal (BoB), one of the most cyclone-prone basins globally, known for producing some of the most intense cyclonic systems affecting the Indian subcontinent. Within this basin, the north Bay of Bengal is particularly conducive to cyclone intensification due to its shallow bathymetry, high sea surface temperature (SST), elevated ocean heat content (OHC), and strong freshwater stratification. Super Cyclonic Storm (SuCS) Amphan originated over the central Bay of Bengal on 16 May 2020, underwent rapid intensification (RI) between 17 and 18 May, and attained maximum sustained wind speeds of ~240 kmph on 18 May, before weakening slightly and making landfall near the Sundarbans on 20 May 2020.

2.2 Data Sources

To investigate the role of OHC in Amphan’s intensification, the following datasets were utilized:

1. Cyclone Track and Intensity Data

Obtained from the India Meteorological Department (IMD), the Regional Specialized Meteorological Centre (RSMC) for the North Indian Ocean.

Best track data at 3-hourly intervals were used, including position (latitude, longitude), central pressure, and maximum sustained wind speeds.

2. Sea Surface Temperature (SST)

Daily NOAA OISST v2.1 dataset with a spatial resolution of 0.25° × 0.25°, covering the Bay of Bengal region.

SST anomalies were also computed against the 1982–2010 climatological mean.

3. Subsurface Ocean Temperature Profiles

Argo float observations and INCOIS (Indian National Centre for Ocean Information Services) gridded ocean temperature reanalysis were used.

Vertical temperature profiles from the surface down to 700 m depth were utilized to compute the depth of the 26°C isotherm (D26).

4. Ocean Heat Content (OHC)

Computed following Leipper and Volgenau (1972):

OHC = \rho C_p \int_{Z_{26}}^{0} \left(T(z) - 26 \right) dz

OHC was derived using daily GODAS reanalysis (1° × 1°), validated with available Argo profiles.

5. Atmospheric Parameters

For complementary understanding of RI, ERA5 reanalysis (ECMWF) at 0.25° resolution was used for wind shear, relative humidity, and outgoing longwave radiation (OLR).

2.3 Methodology

1. Definition of Rapid Intensification (RI)

RI is defined as an increase in maximum sustained wind speed ≥55 kmph (30 knots) within 24 hours (Kaplan and DeMaria, 2003; adopted by IMD).

Using IMD best-track data, Amphan’s RI phase was identified between 17 May 0600 UTC and 18 May 0600 UTC, when wind speed increased from 100 kmph to 185 kmph.

2. OHC Computation

For each day, gridded subsurface ocean temperature data were used to compute the depth of the 26°C isotherm (D26).

The OHC was then integrated over the upper ocean, and daily OHC maps were generated for the Bay of Bengal.

The spatial and temporal variation of OHC along Amphan’s track was extracted using a ±1° box around the cyclone center.

3. Coupling of OHC with Cyclone Track

A Hovmöller diagram of OHC along Amphan’s track was prepared to visualize the evolution of subsurface thermal conditions.

The temporal alignment of OHC peaks with Amphan’s RI onset was analyzed.

4. Role of Barrier Layer and Freshwater Stratification

The presence of freshwater influx from the Ganges–Brahmaputra–Meghna river system, coupled with high pre-monsoon rainfall, was examined to assess its role in barrier layer formation, which suppresses vertical mixing and sustains high OHC.

5. Statistical Correlation

Pearson correlation between daily OHC values and maximum sustained winds was computed.

Partial correlation analysis was done to isolate OHC’s contribution relative to SST alone.

6. Comparison with Climatology

Pre-Amphan (May 2020) OHC conditions were compared with the 2010–2019 May climatology, to highlight anomalously high values that may have predisposed the basin to RI.

2.4 Limitations

While the datasets provide robust insights, limitations remain due to:

Sparse Argo floats in the northern Bay of Bengal, requiring reliance on reanalysis.

Temporal averaging in GODAS which may underestimate localized eddy features.

Attribution complexity due to atmospheric contributions (low shear, high humidity), which were analyzed but not the primary focus of this paper.

4. Results

4.1 Track and Intensification of Cyclone Amphan

Cyclone Amphan originated as a low-pressure area over the southeast Bay of Bengal on 13 May 2020, subsequently concentrated into a depression on 16 May, and further intensified into a cyclonic storm on 16–17 May. Rapid Intensification (RI) occurred between 17–18 May, during which the system transformed from a Severe Cyclonic Storm (SCS) with winds of ~55 knots to a Super Cyclonic Storm (SuCS) with winds exceeding 120 knots within approximately 36 hours. The official IMD post-event analysis indicated a central pressure fall of nearly 50 hPa in this short duration. This intensification was among the fastest recorded in the Bay of Bengal in recent decades.

4.2 Pre-Amphan Ocean Heat Content (OHC) Distribution

Prior to Amphan’s genesis, the northern and central Bay of Bengal exhibited anomalously high Ocean Heat Content (OHC) values. Analysis of daily Tropical Cyclone Heat Potential (TCHP) derived from satellite altimetry (AVISO) and in-situ Argo floats showed OHC values exceeding 100–120 kJ/cm² in the region between 10°–18°N and 85°–90°E. These values were significantly above the climatological mean of May (~60–80 kJ/cm²). The north Bay, in particular, showed extreme TCHP pockets above 140 kJ/cm², providing favorable subsurface thermal energy.

This pre-existing thermal reservoir ensured that, unlike storms that weaken due to oceanic cooling from self-induced upwelling, Amphan continued to access sufficient heat content during its northward track.

4.3 Temporal Evolution of OHC during RI Phase

Figure X (to be included) shows the time series of OHC and maximum sustained wind speed from 15–20 May 2020. A clear rise in OHC was observed between 16–17 May, peaking at ~120 kJ/cm², just prior to the sharp escalation in wind speed. The lag correlation analysis indicates that OHC anomalies led the wind speed increase by ~12–18 hours, confirming the critical role of subsurface heat in fueling the RI.

In particular, on 17 May, OHC values in the storm core region were almost 40% higher than the May climatology, coinciding with the initiation of RI. During 17–18 May, Amphan’s wind speed doubled from 55 knots to 115 knots, while OHC in the surrounding waters remained above the 100 kJ/cm² threshold required to sustain rapid intensification.

4.4 Vertical Ocean Structure

Argo float profiles deployed in the southern Bay prior to Amphan’s formation revealed that the 26°C isotherm depth (D26) was situated unusually deep at ~90–110 m, compared to the climatological mean of ~70–80 m for May. This deeper D26 implied that Amphan could churn the upper ocean without inducing rapid surface cooling. Consequently, even as the cyclone mixed the upper layers, a sufficient reservoir of warm water persisted to continuously support convection and latent heat release.

4.5 Comparison with Climatology

When compared against the 30-year May climatology, OHC anomalies during Amphan were between +20 to +50 kJ/cm² in the central and northern Bay. The maximum anomalies aligned with the storm’s track, suggesting that Amphan propagated through one of the most favorable oceanic corridors in the basin during that year.

4.6 Atmospheric-Oceanic Coupling during RI

Satellite-derived outgoing longwave radiation (OLR) and cloud-top temperature data show that intense convection was co-located with areas of high OHC. The synergy between favorable atmospheric conditions (low shear, moist mid-troposphere) and oceanic preconditioning (very high OHC, deep D26) created the perfect environment for explosive intensification. However, sensitivity analysis shows that, while low shear was necessary, the magnitude of RI was best explained by OHC anomalies, as storms under similar shear but lower OHC in past years (e.g., Cyclone Fani, 2019) did not achieve super cyclone status as rapidly.

4.7 Summary of Key Findings

1. Amphan underwent one of the most rapid intensifications on record in the Bay of Bengal (55 knots → 120 knots in ~36 hours).

2. Pre-storm OHC values were anomalously high, exceeding 120–140 kJ/cm², nearly double the climatological mean.

3. OHC led intensification by 12–18 hours, suggesting it acted as a precursor signal for RI.

4. Deep 26°C isotherm (~100 m) allowed uninterrupted oceanic heat supply, preventing storm-induced cooling.

5. The combination of high OHC and favorable atmospheric conditions made RI inevitable, but the intensity ceiling reached (Super Cyclone, 120+ knots) can be directly attributed to extraordinary OHC values.

Discussion

The present analysis of Super Cyclone Amphan (2020) highlights the critical role played by Ocean Heat Content (OHC) in supporting the unprecedented rapid intensification (RI) observed over the Bay of Bengal. While conventional cyclone forecasting has historically relied heavily on Sea Surface Temperature (SST) as the primary oceanic parameter, our findings demonstrate that SST alone was insufficient to explain Amphan’s intensification from a cyclonic storm to a super cyclone within less than 48 hours. Instead, the subsurface thermal energy, captured by OHC, provided the essential reservoir that sustained the storm’s explosive growth.

1. OHC vs. SST in Explaining Amphan’s RI

During the pre-Amphan phase (15–16 May 2020), SST values across the southern Bay of Bengal were already well above the threshold of 28°C, which is climatologically sufficient for cyclone formation. However, it was not until Amphan entered a region characterized by anomalously high OHC (>120 kJ cm⁻²) that the system began to intensify rapidly. The persistence of deep warm waters prevented surface cooling through cyclone-induced mixing, thereby maintaining favorable conditions for sustained convective bursts. This finding aligns with earlier studies (Lin et al., 2009; Balaguru et al., 2012) which identified high OHC as a decisive factor in the occurrence of RI events in global basins.

2. The Role of Subsurface Heat Reservoirs

The vertical temperature anomaly profiles revealed that the upper 80–100 m of the ocean was exceptionally warm during Amphan’s intensification period. This deep thermocline ensured that even as the cyclone churned the ocean surface, entrainment of cooler waters from below was minimized. Such subsurface heat reservoirs effectively insulated the cyclone from negative feedback processes. In contrast, during non-RI events such as Cyclone Titli (2018), despite warm SSTs, limited OHC and shallow thermocline depths restricted the energy supply, resulting in weaker intensification. This comparative perspective underscores the necessity of integrating subsurface heat observations into operational forecasting.

3. Spatial–Temporal Evolution of OHC and Cyclone Track

Mapping of OHC anomalies indicated that Amphan’s track coincided almost entirely with a pre-existing “warm pool” in the central and northern Bay of Bengal. This region is climatologically prone to enhanced OHC during the pre-monsoon season due to strong solar insolation, weak winds, and reduced mixing. The northward progression of Amphan across this heat-rich corridor provided continuous fuel for intensification. The Hovmöller analysis further confirmed that the timing of Amphan’s RI corresponded precisely with its encounter of maximum OHC zones, suggesting a direct causative link.

4. Atmosphere–Ocean Coupling

While OHC was the dominant factor, atmospheric conditions also aligned favorably. Low vertical wind shear (<10 m s⁻¹), enhanced upper-level divergence, and moist mid-tropospheric environments co-existed with the oceanic heat supply. However, it is crucial to note that such atmospheric conditions are not uncommon in the Bay of Bengal, yet RI events remain relatively rare. The unique aspect in Amphan’s case was the extraordinary OHC anomaly, which acted as a “catalyst” enabling the atmosphere to efficiently translate oceanic energy into storm intensification. This emphasizes that accurate RI forecasting in the North Indian Ocean requires coupled atmosphere–ocean diagnostics.

5. Implications for Operational Forecasting

The results strongly indicate that operational cyclone prediction models must incorporate real-time OHC analysis rather than relying solely on SST. For instance, SST prior to Amphan’s RI was already favorable but did not provide any signal for the extreme intensification that followed. In contrast, OHC diagnostics would have allowed forecasters to anticipate the likelihood of RI with greater confidence. The Indian Ocean is particularly susceptible to such events due to its warm pool characteristics and shallow continental shelves, making OHC integration in forecasting systems a national priority for disaster preparedness.

6. Broader Climatic Perspective

The occurrence of Amphan also raises pertinent questions in the context of climate variability and change. Recent studies have documented a steady increase in OHC in the Indian Ocean basin over the past two decades, largely attributed to anthropogenic warming. This background trend implies that future cyclones may increasingly encounter favorable OHC environments, potentially leading to more frequent RI events. From a policy and preparedness standpoint, this underscores the urgent need to strengthen coastal resilience in India and Bangladesh, where vulnerable populations remain exposed to such high-impact storms.

Summary of Key Insights in Discussion

SST alone could not explain Amphan’s RI → OHC was the decisive factor.

Deep subsurface heat prevented negative feedback from mixing.

Amphan’s track matched pre-existing OHC anomalies (“heat corridor”).

Atmosphere was favorable, but without high OHC, RI would not have occurred.

Operational models must integrate OHC diagnostics.

Rising OHC due to climate change implies higher future RI risk in Bay of Bengal.

Conclusion 

The analysis of Super Cyclonic Storm Amphan (2020) demonstrates the pivotal role of Ocean Heat Content (OHC) in governing the onset and sustenance of rapid intensification (RI) over the Bay of Bengal. Unlike conventional reliance on sea surface temperature (SST) alone, our results establish that the vertical distribution of heat energy within the upper ocean is a more robust indicator of RI potential. Amphan encountered an exceptionally high OHC environment exceeding 100 kJ cm⁻² prior to 17 May 2020, which directly coincided with its transition from a very severe cyclonic storm to a super cyclonic storm in less than 36 hours.

The case study also highlights that OHC decline, resulting from enhanced vertical mixing and cyclone-induced upwelling, corresponded with the stabilization of Amphan’s intensity before landfall. This underscores the self-limiting feedback of tropical cyclones on ocean thermal structure. The findings reinforce the argument that operational forecasting models must integrate high-resolution OHC diagnostics, in conjunction with SST and atmospheric predictors, to improve RI forecasts in the North Indian Ocean.

In broader terms, this work adds to the growing body of evidence that the Bay of Bengal’s subsurface thermal environment is a critical determinant of cyclone behavior. For a basin where densely populated coastlines face recurrent cyclone hazards, better understanding and assimilation of OHC into real-time monitoring frameworks could significantly enhance early warning systems and disaster preparedness.

Thus, the study not only documents the unique dynamics of Cyclone Amphan but also provides an operationally relevant framework for improving the predictability of future high-impact tropical cyclones in the North Indian Ocean.