Oceanic and Atmospheric Characteristics Associated with Distinct Intensification Scenarios of North Indian Ocean Cyclonic Disturbances

Abstract

Observations and numerical modeling have greatly advanced our understanding of tropical cyclones (or TCs) through both research and operational forecasting. Despite the strides made, challenges persist in achieving precise predictions. The integration of satellite and radar data into Numerical Weather Prediction (NWP) models has proven instrumental in enhancing TC forecasts. However, there is a noticeable gap in the research on cases such as Highly Intensified Cyclonic Storms (HICS), Concurrent Cyclonic Disturbance (CCD) pairs and an entire TC season specific to the North Indian Ocean (NIO). While climate phenomena such as El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Convectively Coupled Equatorial Waves (CCEWs) influence the genesis and intensification of TCs, focused investigations of the NIO TCs lag behind those in other basins. The HICS frequency over the NIO basin exhibits an upward trend in both seasons, accompanied by variability in reaching the LMI (Life-time Maximum Intensity) stage. Conversely, there is a positive correlation between accumulated cyclone energy (ACE) and HICS frequency. Examination of HICS occurrences in the Arabian Sea (AS) and the Bay of Bengal (BOB) during climatological analysis reveals distinct patterns, with mid tropospheric relative humidity (MRH) consistently playing a dominant role, predominantly in the AS. Insights into HICS formation are further provided by anomalies in Genesis potential Index (GPI) and Genesis Potential parameter (GPP), highlighting the significant contributions of MRH and vorticity in the central BOB. The formation of HICS is also influenced by ocean-atmosphere heat exchange and various CCEWs. Spatial analysis indicates that Equatorial Rossby (ER) waves coincide with higher relative vorticity, while Kelvin (KV) waves yield mixed results. Madden Julian Oscillation (MJO) supports HICS genesis over the AS but not in the BOB. MT waves create favorable conditions for HICS genesis in the NIO basin, although MRH values do not consistently support this over the BOB. The variability in filtered vorticity can be considered as a precursor for HICS genesis over the AS, with MRH serving as a secondary precursor, though inconclusive over the BOB. In the case of SuCS, it is observed that the genesis is influenced by weak to moderate vertical wind shear (VWS) in conjunction with low-level relative vorticity and MRH, whereas of the CCEWS, MJO and ER waves predominantly govern the cyclogenesis. Sea Surface Temperature (SST) and Tropical Cyclone Heat Potential (TCHP) also significantly impact the genesis process. It is noted that SST, VWS, vorticity, and MRH play roles in the Rapid Intensification (RI) process for SuCS category TCs, with SST emerging as the primary factor, followed by MRH and vorticity. The SuCS type of TC is furthermore characterized by a slower translational speed, allowing considerable interaction between storms and the underlying ocean, contributing to increased intensity. This underscores the significance of translational speed in the intensification of cyclones like SuCS, ultimately reaching their peak intensity. It is observed that various atmospheric and oceanic factors play a crucial role in influencing the behavior of HICS in the NIO basin. The utilization of NWP models further enhances our comprehension of the dynamic and thermodynamic parameters impacting HICS. When evaluating the predictive performance of the atmospheric component of the model for prediction across scales (MPAS-A) for HICS in the NIO, it becomes evident that the model, initiated with ERA-5 data as the initial condition, consistently outperforms FNL consideration in predicting HICS tracks. This superiority is attributed to the higher resolution of the ERA-5 dataset. However, a detailed comparison of HICS track simulations using GDAS and ERA-5 data reveals intriguing dynamics. While GDAS-simulated tracks closely align with observations and exhibit superiority over ERA-5 beyond 72 hours, quantified track errors expose GDAS's advantage in later forecast periods. Examination of the along-track and cross-track errors indicates a rightward bias and slower movement in both types of simulations. The TC intensity root mean square suggests that the simulation considering GDAS, tends to underestimate stronger TCs and overestimate weaker ones. Indian meteorological department (IMD) error assessments reveal ERA-5's superior intensity forecasting from 42 to 72 hours, with subsequent decline. The model successfully replicates cyclonic wind patterns, vertically integrated moisture transport, and moisture conveyor belts crucial for TC development. Analyzing potential vorticity (PV) at the 320°K isentropic surface underscores the model's ability to capture PV structures, highlighting the significance of moisture transfer from the ocean for intensification. Notably, GPP analysis emphasizes the model's improved predictability with ERA-5 as the initial condition compared to GDAS, especially in forecasting key parameters influencing TC genesis. Delving into the dynamic and thermodynamic aspects of HICS cases with ERA-5 as the initial condition, the analysis identifies moisture concentration in the eyewall, intense mid to upper-level warming, and upward drafts as critical factors influencing HICS intensification. Although the model effectively captures these signatures, challenges are observed in representing distinct characteristics of radial wind, temperature, tangential wind, and diabatic heating. The maximum relative vorticity tendency typically occurs within the 600 to 700-hPa range before peak intensity, highlighting the significant contribution of upper-level positive advection to higher positive vorticity, favoring intensification. However, no substantial convergence offset is noted in any of the cases. The NIO basin further experienced an exceptionally active and record-breaking tropical TC season in 2019, characterized by a significantly higher ACE. The ACE for that season was around 4.5 times the climatological average and nearly double the previous record established in 2007. Notable features of the season included unusual positive anomaly values of potential intensity (PI) and elevated SST across AS, creating favorable environment for increased TC frequency and higher ACE values. Negative anomaly values of VWS were observed in both BOB and AS during specific periods, particularly from April to June and October to December, facilitating the genesis and intensification of TCs. MRH anomalies varied across the NIO throughout different months of the TC season. Unfavorable MRH anomalies were observed over the NIO from April to June, while higher and positive anomaly values were noted over the AS from October to December. Positive anomalies of PI, a crucial factor influencing TC activity through local thermodynamic processes, were limited to the southern NIO from April to June. In October to December, positive PI anomalies were confined to the central part of the NIO. The IOD also exhibited an unprecedented positive value in 2019, contributing to El Niño-like rising and sinking motion across the tropics and elevated SST over the western Indian Ocean. This aided the development of HICS type TCs over the AS. Also, the MJO actively influenced TC genesis and intensification across the NIO in 2019. The thermodynamic conditions over the AS were notably more favorable from October to December due to higher and more prolonged SST, resulting in a unique thermodynamic environment that supported the genesis of highly intense cyclones and led to significantly higher ACE values in the latter half of 2019. CCD pairs in the NIO basin generally develop on one side of the equator, but in both the sub basins. It has been observed that higher convection along with lower-level westerlies can function as an early indicator for the onset of CCD pairs. Among CCEWs, the MJO emerges as a prominent factor influencing the genesis of CCD pairs, while El Niño and a positive IOD plays a secondary role. The formation of CCD pairs is more likely within low-level cyclonic anomalies characterized by strong convective conditions associated with various CCEWs. TD and MRG waves display stronger connections with amplified vorticity anomalies and drier conditions with reasonable thermal conditions, indicating them as crucial precursors for such events in the NIO basin.