The Bay of Bengal (BOB), though not the most active TC breeding ground, experiences significant TC activity, particularly during the pre-monsoon (April-May) and post-monsoon (October-December) seasons. Post-monsoon TCs constitute 64% of the annual BOB TC frequency. These storms cause severe coastal damage in densely populated, low-lying areas of South Asia. The South China Sea (SCS), separated from the BOB by the Indochina Peninsula, is also a major TC breeding ground with distinct seasonal patterns. Late-season SCS TCs (31% of annual frequency) can potentially affect the BOB. Previous research indicates the influence of the tropical Indo-Pacific Oceans, specifically ENSO and IOD, on TC activity in these regions. ENSO, particularly La Niña events, increases TC frequency in both the BOB and SCS due to warmer SSTs, reduced wind shear, and increased moisture. The Indian Ocean Dipole (IOD) also influences BOB TCs; negative IOD events increase TC frequency. However, the relationship between IOD and SCS TCs and the overall inter-basin covariability of BOB-SCS TC activity remain largely unexplored. This study investigates this covariability and the relative contributions of the Pacific and Indian Oceans to TC anomalies in the BOB and SCS.

Existing studies have individually explored the impact of ENSO and IOD on TC activity in the Bay of Bengal and South China Sea. ENSO, particularly La Niña, is known to increase TC frequency and intensity in both regions due to favorable thermodynamic and dynamic conditions. The IOD, a zonal dipole mode in the tropical Indian Ocean, also significantly impacts BOB TC activity, with negative IOD phases leading to increased TC frequency. However, research on the interplay between IOD and SCS TCs has been limited. Furthermore, while the impact of SST anomalies in the Indian and Pacific Oceans on the atmospheric circulation over the BOB-SCS region is acknowledged, the coherent changes in TC activity and the relative contributions of each ocean remain poorly understood. This study aims to fill this gap by examining the inter-basin covariability of TC activity and the underlying mechanisms.

This study analyzed TC track data from the Joint Typhoon Warning Center (JTWC) best-track data (IBTRACS) for the period 1979-2019, focusing on TCs with maximum sustained winds exceeding 33 knots. TC genesis and track density were calculated in 5°x5° grid boxes, with spatial smoothing applied. Different TC types were defined: local TCs (LTCs) and migrated TCs (MTCs). Maximum covariance analysis (MCA) and empirical orthogonal function (EOF) analysis were used to identify the dominant modes of covariability in TC track density between the BOB and SCS. The relationship between TC activity and sea surface temperature (SST) anomalies in the Indo-Pacific region was explored using MCA. SST data were obtained from NOAA ERSSTv5 and the Hadley Centre Global SST. The Indo-Pacific Tripole (IPT) index was used to represent SST variability. Atmospheric data from ECMWF ERA5 reanalysis were used to analyze atmospheric circulation patterns. Numerical simulations using the Community Atmosphere Model version 4 (CAM4) were conducted to investigate the relative contributions of the Pacific and Indian Ocean SST anomalies to the atmospheric circulation anomalies over the BOB-SCS region. The dynamic genesis potential index (DGPI) was used to assess the environmental conditions for TC genesis. Statistical methods such as the two-tailed Student's t-test were used for significance testing.

The study revealed a strong, in-phase covariability in TC activity between the BOB and SCS during October-December, 1979-2019. The leading MCA mode explained 74.8% of the covariance, while the leading EOF mode accounted for 35% of the total variance in TC track density. This covariability was strongly linked to the Indo-Pacific Tripole (IPT) mode, with a negative IPT phase (warmer western North Pacific and eastern Indian Ocean, cooler western Indian Ocean and central-eastern Pacific) leading to enhanced TC activity in both basins. This is driven by anomalous ascending motions and cyclonic circulation over the BOB-western WNP, favorable for TC genesis. The IPT primarily influenced the genesis location, not the frequency, of migrating TCs. Excluding the influence of migrating TCs from the SCS to the BOB, the significant covariability remained. Numerical simulations using CAM4 confirmed that the Pacific SST anomalies played a more significant role in modulating the atmospheric circulation than Indian Ocean SST anomalies (roughly twice as strong). The analysis also revealed that the IPT primarily affected the frequency of locally generated TCs, with significant differences observed between years with strong positive and negative IPT anomalies. The frequency of migrating TCs was not significantly affected by IPT phases. Although the steering flow changed significantly with IPT, it didn't significantly change migrating TC frequency.

The findings highlight a significant inter-basin teleconnection in TC activity between the BOB and SCS, driven by large-scale atmospheric circulation changes modulated by the Indo-Pacific Tripole. The strong correlation between IPT and TC activity suggests that forecasts incorporating IPT indices may improve prediction of TC activity in these regions. The fact that the Pacific Ocean's influence is more substantial than the Indian Ocean's provides crucial insight into the regional climate dynamics. The differing impacts on local versus migrating TCs suggest that forecasting models need to account for these nuanced differences. Further research should investigate the relative contributions of various timescales of variability (e.g., interannual, interdecadal, intraseasonal) to the overall TC covariability.

This study demonstrates a strong, IPT-modulated covariability in late-season TC activity between the BOB and SCS. The Pacific Ocean exerts a stronger influence than the Indian Ocean on this inter-basin relationship. The IPT primarily affects local TC genesis frequency, while the effect on migrating TCs is mainly on their genesis location. This research enhances understanding of regional TC activity and underscores the importance of considering large-scale climate modes in TC forecasting. Future work should investigate the complex interactions between various climate modes and their influence on BOB and SCS TC activity across multiple timescales.

The study relied on JTWC best-track data, which may have limitations regarding the accuracy of TC track positions and intensities, particularly for weaker storms. The numerical simulations used a relatively coarse resolution, which might not capture all the fine-scale processes influencing TC genesis and migration. The study primarily focused on the interannual variability associated with IPT, while other timescales of variability could also play a significant role.