Intraseasonal rainfall variability in the Bay of Bengal during the Summer Monsoon: coupling with the ocean and modulation by the Indian Ocean Dipole

This study investigates the interannual variability (IAV) of the Bay of Bengal (BoB) thermohaline structure in terms of mixed layer depth (MLD), isothermal layer depth (ILD), and barrier layer thickness (BLT) over a 65-year period spanning from 1958 to 2022. This study offers a comprehensive understanding of the mentioned IAV and underlying mechanism using the ORAS5 and ERA5 reanalysis data products. Although previous studies have explored seasonal and year-to-year variability in this region, this study delves into unexplored dynamics like differential spatial response to the plausible drivers of the IAV. An empirical orthogonal function (EOF) analysis conducted on monthly anomalies of MLD, ILD, and BLT reveals that the first EOF accounts for 44.5% of the variance in ILD, 23.7% in MLD, and 22.3% in BLT, and the first principal component (PC) shows a good correlation to Niño3.4 index and dipole mode index (DMI). This analysis further reveals that the influence of El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) is restricted to the southern and eastern boundaries of the bay. The composite analysis shows that the ILD exhibits negative (positive) anomalies in the equatorial and the eastern BoB during El Niño (La Niña) years, whereas the MLD does not show a distinct response to the ENSO events. The negative (positive) ILD anomalies are also prominent in the eastern BoB during positive (negative) IOD events. Unlike the ENSO years, negative (positive) MLD anomalies are visible in the Southern BoB during positive (negative) IOD years. The above anomalous variation in the MLD and ILD results in an anomalous decrease (increase) in BLT in the eastern side during El Niño and positive IOD years (La Niña and Negative IOD years). The response mentioned above in the MLD, ILD, and BLT is linked to the interannual response of the Kelvin waves and associated Rossby wave radiation to the ENSO and IOD forcing. In the northern, central, and western BoB, salinity exerts a strong influence on barrier layer formation, likely driven by the freshwater influx through evaporation, precipitation, and river runoff; however, the exact role of river discharge in modulating the IAV of BLT remains poorly understood. Since southern BoB acts as a persistent heat source, meridional heat transport (MHT) via the eastern boundary plays a critical role in enhancing the deepening of ILD, with heat being advected northward by oceanic currents. The role of the East India Coastal Current (EICC) is also evident, with freshwater advection contributing significantly to MLD variability along the east coast of India. The influence of monsoon current is observed in the second EOF of both MLD and ILD, capturing 9% and 8.5% of the variance respectively. A strong monsoon current brings high-saline, relatively cooler Arabian Sea water into the southwest BoB, resulting in deeper MLD and shallow ILD.

Fourteen years of monthly repeated expendable conductivity–temperature–depth sections across the northeastern Bay of Bengal were used to examine the interannual variability of temperature inversions during Indian Ocean dipole (IOD) years. Data averaged over three regions at the latitude band of 15°–19°N along the Port Blair–Kolkata shipping transect reveal occurrences of temperature inversions during November–February. Notably, inversions with higher thickness (∼92 m) and intensity (3.24°C) occurred at relatively shallow depths (∼50 m) during the 2012 positive IOD compared to the 2016 negative IOD (∼25-m thickness, 1.91°C intensity, at 86-m depth). The intensity of the inversion layer gradually weakened from Kolkata to Port Blair during the 2012 and 2019 positive IOD years and vice versa during the co-occurrence of IOD with El Niño–Southern Oscillation (ENSO) in 2015. The observed stratification was higher during 2012 and 2019 positive IOD years. Salt budget analysis revealed that the advection of freshwater plumes was the primary reason for this observed high stratification. Interestingly, on average, the inversion layer occupied 73%–86% of the barrier layer during the positive IOD years (2012 and 2019) and 40%–55% during the 2016 negative IOD (nIOD) year, from Kolkata to Port Blair. Heat budget analysis revealed that the net surface heat flux and penetrative shortwave radiation were the governing factors responsible for the observed inversion characteristics in the study region during different IOD phases. Our analysis indicates that undulations from westward-propagating Rossby waves were the driving mechanism behind the shallow and deep occurrences of inversions in our study region during the positive (2012 and 2019) and negative (2016) phases of IOD. The changes in mixed layer temperature caused by these wave processes were notably more dominant than the effect of the heat trapped in the inversion layer. Significance Statement The increase of temperature with depth in the ocean, known as temperature inversion, forms over the northern Bay of Bengal regularly during winter. The heat trapped in the inversion layer is believed to impact the sea surface temperature and, hence, the region’s climate. Therefore, using 14 years of expendable conductivity–temperature–depth data collected along the Kolkata–Port Blair transect through passenger ships, the year-to-year variability of temperature inversion is studied. Due to excess freshwater plumes, weak downwelling Rossby wave propagation, and conducive heat fluxes, intense temperature inversions formed near the surface in the 2012 positive Indian Ocean dipole year compared to the 2016 negative Indian Ocean dipole year. The heat trapped in the inversion does not seem to impact sea surface temperature as it is eroded due to the propagation of the upwelling Rossby waves.

The characteristics and variability of the East India Coastal Current (EICC), the western boundary current in the Bay of Bengal (BoB) during the Indian Ocean Dipole (IOD) years between 2006 and 2012 have been investigated using the high-resolution Regional Ocean Modeling System (ROMS). The evolution of temperature, mixed layer depth (MLD), and seasonal basin scale circulation in the upper ocean simulated by the model agrees well with the observations. The EICC in BoB is characterized by a seasonal reversal flow: the poleward EICC during February−May and the equatorward EICC during August−December. A long-term simulation from 2006 to 2012 suggest that the circulation pattern, boundary current structure, and transport in the western BoB are completely different in positive and negative IOD years. As IOD is mainly phase-locked to the seasonal cycle with most significant influence in the Borel autumn, the equatorward EICC is affected during the IOD years. It is found that the strength of this EICC is ~ 5 Sv in October 2010 and a weaker EICC dominated by the presence of eddies is observed in October 2006. We also quantified the local and remote forcing effects on the variability of EICC and found that the seasonal coastal Kelvin waves (KWs) play a dominant role in the development of the EICC. During positive IOD year 2006, due the absence of second downwelling KW, the EICC is completely disorganized and dominated by the eddies, whereas in the negative IOD year 2010, the strong second downwelling KW plays a key role in developing organized and stable EICC in the western BoB.

<p>Based on SODA reanalysis data set from 1980 to 2016, this paper combined with a variety of mathematical statistical methods to study the intraseasonal variability characteristics of barrier layer thickness and its physical correlation with climate modes in the Bay of Bengal, and quantitatively explored the dynamic mechanism of intraseasonal variability of barrier layer in different sea areas in the Bay of Bengal by means of Marine dynamic diagnosis method. The relative contributions of different physical processes, such as oceanic advection, Kelvin waves, Rossby waves and freshwater fluxes (rainfall and river runoff), to the barrier layer were evaluated. The physical relationship between the seasonal variation of barrier layer thickness and the Indian Ocean dipole (IOD) is also discussed. The results show that the thickness of the barrier layer varies most obviously in the northern coast of the bay of Bengal and the western coast of Sumatra, and the maximum value of the barrier layer occurs in November ~ December every year, while the variation of the barrier layer in the northern coast is more regular than that in the southern coast. Horizontal advance and entrainment affect the thickness of barrier layer by affecting the salinity of the mixed layer. However, the thickness of barrier layer is mainly caused by the change of isothermal layer due to the obvious stratification of sea surface salinity in the Bay of Bengal. In the southern part of the Bay of Bengal near the equator, during the positive IOD events, the isothermal layer shallowness was caused by the negative anomaly of equatorial zonal wind stress from October to December. In negative IOD events, the equatorial zonal wind stress appears positive abnormality after June, which leads to the increase of isothermal layer in this period. As a result, the thickness of barrier layer In positive IOD years is smaller than that in normal years from October to December, and that in negative IOD years is greater than that in normal years from June to September. However, in the northern Bay of Bengal, the seasonal variability of barrier layer caused by different IOD events was not obvious. At the same time, the net heat flux upward at the air-sea interface will lead to instability and deepen the local mixed layer.</p>

ABSTRACTThe influence of the biweekly sea surface temperature (SST) in the South China Sea (SCS) on the SCS summer monsoon, especially during the Indian Ocean Dipole (IOD) is presented using the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST and rainfall data for April to June from 1999 to 2013. During positive IOD (PIOD) years the biweekly SST anomalies over the SCS lead the rain anomalies by three days, with a significant correlation (r = 0.8, at the 99% confidence level), whereas during negative IOD (NIOD) years, the correlation is only 0.2. The biweekly SST is observed to influence the westward and northward propagating rainfall anomalies over the SCS and, hence, affect the SCS summer monsoon, especially during PIOD years. No such propagation was seen during NIOD years. The biweekly intraseasonal oscillation of SST in the SCS results in enhanced sea level pressure and surface shortwave radiation, especially during PIOD years. The potential findings here indicate that the biweekly SST in the SCS is strongly (weakly) influenced during PIOD (NIOD) years. Further, it is observed that SST in the SCS has a strong (weak) effect on the SCS summer monsoon by westward and northward propagation of rainfall, especially during PIOD (NIOD) years. When a PIOD or NIOD exists over the tropical Indian Ocean, the SCS SST will be strongly (r = 0.6, at the 99% confidence level) or weakly correlated with the residual index, respectively.

The influence of climate variability events on the mesoscale eddies in the Bay of Bengal

Interannual variations of the monsoon onset over Kerala (MOK) have been studied using data from over 60 years (1948–2009) of NCEP/NCAR reanalysis and outgoing long-wave radiation. The sea surface temperature fields over the North Indian Ocean associated with the MOK have been examined in association with El Nino and Indian Ocean Dipole (IOD) events which originate in the Pacific and Indian Ocean, respectively. An analysis of the tropical convective maximum showed significant differences in its strength and location during the El Nino, IOD, early, normal, and delayed MOK composites. Further, we also looked into the role of the convective systems formed over the Arabian Sea and Bay of Bengal on MOK. The most significant features during early (delayed) MOK years is the abnormal persistence of westerlies (easterlies) several days prior to MOK and enhanced (suppressed) deep convection over the southeastern Arabian Sea and the southern Bay of Bengal. Moisture builds up over peninsular India several pentads prior to MOK during La Nina, negative IOD, and concurrent La Nina and negative IOD years as compared to the El Nino, positive IOD, and concurrent El Nino and positive IOD years, indicating its significant role on MOK. The monsoon Hadley cell and Walker circulations are weaker (stronger) during a delayed (early) MOK. Further, the sea surface temperature anomalies in the western Pacific are negative (positive) during delayed (early) MOK.

Episodic phytoplankton bloom events in the Bay of Bengal triggered by multiple forcings

The variation in the vegetation pattern reflects the change in the regional environment. Normalized Difference Vegetation Index (NDVI) data from 2000 to 2022 for the Upper Bhima sub-basin in Western India has been used to identify the response of vegetation to the El Niño-Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD) events. As a novelty, the present study identifies the ENSO-sensitive and IOD-sensitive vegetation areas within the watershed using vegetation mean to difference anomalies. Monthly NDVI anomalies are used to determine sensitive pixels of vegetation using mean monthly NDVI. Local spatial autocorrelation (LISA) is performed to analyze the pattern of the NDVI anomalies and cluster maps are generated. The results of spatial variation show that NDVI is adversely affected in El Niño years. During La Niña years, the percentage area covered by dense vegetation is more than 80%, which is significantly higher than that of El Niño years in the monsoon and post-monsoon periods. Positive IOD years show significantly more sparse vegetation cover than negative IOD years. The results of LISA analysis show that the rainfall shadow zone in the study area has a cluster of negative sensitive pixels even in the monsoon and post-monsoon period except in negative IOD year.

This study investigates air-sea flux variability associated with the ENSO and Indian Ocean dipole years in the Tropical and northern Indian Ocean. Objectively Analysed (OA) surface fluxes, and altimeter derived sea surface height anomaly datasets for a period of thirty years from 1986 to 2015 were utilised. First, the El-Nino and La-Nina years were separated and the difference between them was analyzed to understand the effect of ENSO. A similar approach was also followed for positive and negative Indian Ocean dipole years. It was found that the upwelling caused by planetary Rossby waves during El-Nino and positive Indian Ocean dipole years suppress the latent heat flux from ocean closer to the Somali coast. Ocean dynamics and thermodynamics both were observed to play significant roles in flux variation in this region.

[1] Analysis of long time series of current meter data from a mooring at 77°E and the equator during 2003–2007, along with mean sea level anomaly data, throws light on the occurrence of the lower-frequency (24 to 40 day) Yanai waves in the upper water column of the central equatorial Indian Ocean (EIO) during the positive Indian Ocean dipole (IOD) years of 2003, 2004, 2006, and 2007 and its absence during the negative IOD year 2005. This result is in contrast with the earlier studies that observed only the higher-frequency (biweekly period) Yanai wave in this region. We propose a new notion for the generation of the lower-frequency Yanai wave in the upper central EIO owing to the positive IOD phenomenon. The strong meridional current shear created by the northward shifting and strengthening of the westward flowing south equatorial current associated with positive IOD and the eastward flowing southwest monsoon current provides energy for the generation of lower-frequency Yanai waves. Vertical stratification of the water column appears to be responsible for the trapping of the different frequency of Yanai waves, with only the higher-frequency Yanai wave in the region of lower pycnocline. During positive IOD the strongly stratified upper water column responds to the lower-frequency Yanai wave, while the deeper ocean (4000 m) exhibited a longer-period (47 day) oscillation. The expected surface signature of Madden-Julian oscillation seems to be suppressed by strong easterlies during the positive IOD years.

The impact of Indian Ocean Dipole (IOD) events on the generation and intensity of tropical cyclones under the influence of monsoons is explored. The standardized sea surface temperature (SST) anomalies are computed for the pre-monsoon and post-monsoon months for the Bay of Bengal (BOB) and Arabian Sea (AS) from 1971 to 2020 and relationships are analyzed with the frequency of tropical cyclones for the neutral, positive and negative IOD years. Ocean states are sensitive to cyclonic conditions exhibiting a complex spectral distribution of the wave energy. Due to a tropical cyclone, the surface waves remain under high wind forcing conditions for prolonged periods generating a huge amount of energy. The spectral wave model SWAN (Simulating WAves Nearshore) is used to generate the energy density spectra during FANI (26 April–5 May 2019), which was a pre-monsoon extreme severe cyclonic storm, and BULBUL (5–12 November 2019), which was a post-monsoon very severe cyclonic storm in the BOB region. This study aims to estimate the intensity of wave energy during tropical cyclones in the pre- and post-monsoon months for 2019 (an extremely positive IOD year).

The low-latitudinal cyclones (LLCs, originating between 5°N–10°N) constitute ≈40% of tropical cyclones (TCs) formed in the Bay of Bengal (BoB). We investigate the interannual variability of post-monsoonal (October to December) BoB LLCs and their teleconnection with El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). It is found that the years with the fewer number of BoB LLCs are associated with anomalous equatorial easterlies that are largely connected with the El Niño and positive IOD. Likewise, equatorial westerly phases, often associated with the La Niña and negative IOD years, favour the LLC formation by providing the initial spin-up required for cyclogenesis. This teleconnection between ENSO/IOD and BoB TC frequency is confined in the low-latitudinal region with little influence for cyclogenesis north of 10°N during ENSO and IOD except during negative IOD. These results may help extend the lead time and improve the seasonal prediction of BoB TCs.

The impacts of El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on tropical cyclone (TC) activity (intensity, frequency, genesis location, track and average lifetime) in the Bay of Bengal (BoB) are studied for the period 1891–2007 using cyclone e-Atlas of India Meteorological Department, Nino3.4 Index, Oceanic Nino Index and Dipole Mode Index (DMI). TCs in the present study include cyclones with maximum sustainable wind (MSW) ≥34 knots (referred as cyclonic storms) and severe cyclonic storms with MSW ≥48 knots. The study shows a total of 502 TCs over BoB during the 117-year study period at the rate of 4.29 TC per year. Seven-year running mean of TCs for the period 1891–2007 shows a decreasing trend. Correlation between Nino3.4 Index and DMI for the 117-year period is significant and positive and the significance level is higher (lower) for the period with higher (lower) TC frequencies. One-third monthly interval analysis for the 117-year period indicates first third (1–10) of November as the most favoured period of TC formation over BoB. 117-year study period is divided into years of ENSO (El Nino, La Nina and neutral ENSO) and IOD (+ve IOD, −ve IOD and no IOD) categories. Maximum frequency of TC is observed during La Nina years, −ve IOD years and also when La Nina co-occurred with −ve IOD. More severe cyclones are formed during La Nina and +ve IOD years. Genesis location of TCs indicates that during La Nina (El Nino) years, the TCs are oriented in the south-east–north-west (south-west–north-east) direction. TCs in no IOD and −ve (+ve) IOD years are more (less) in northern BoB (north of 15° N), while in southern BoB (south of 15° N), TCs are more (less) during no IOD and +ve (−ve) IOD condition. BoB is divided into four quadrants, and number of TCs in each quadrant is computed under different ENSO–IOD events. Peak direction of track movement is observed as north-east followed by north–north-west which is corroborated from the dissipation of TCs in the specific quadrant. Total TC tracks in the peak direction of track movement are maximum during El Nino and no IOD years. The study reveals that TCs with shorter lifetime are observed during El Nino and −ve IOD years, while TCs with longer lifetime are observed during La Nina, neutral ENSO and +ve IOD years. The decade with maximum TC formation is observed as 1921–1930, and the impacts of ENSO and IOD on decadal variability are distinctly observed.

The El Niño–Southern Oscillation (ENSO) influence on tropical cyclone (TC) activity (frequency, genesis location, and intensity) in the Bay of Bengal (BoB) during the primary TC peak season (October–December) are studied for the period of 1993–2010. The study shows that during primary TC peak season, accumulated cyclone energy in the BoB is negatively correlated with Niño3.4 sea surface temperature anomaly. Under La Niña regime number of extreme TC cases (wind speed >64 kt) increases significantly in the BoB during the primary TC peak season. The analysis further shows that negative Indian Ocean dipole year is also favorable for extreme TC activity in the BoB during the primary TC peak season. The existence of low‐level cyclonic (anticyclonic) vorticity, enhanced (suppressed) convection, and high (low) tropical cyclonic heat potential (TCHP) in the BoB provides favorable (unfavorable) conditions for the TC activity under La Niña (El Niño) regimes together with weak vertical wind shear and high sea surface temperature (SST). The genesis location of TC shifts to the east (west) of 87°E in the BoB during La Niña (El Niño) regime due to the variability in convective activity. The probable reason for the intense TC during a La Niña regime is likely explained in terms of longer track for TCs over warm SST and high TCHP due to eastward shifting of genesis location together with other favorable conditions. The variability of Madden‐Julian Oscillation and its influence on TC activity in the BoB during La Niña and El Niño regime are also examined.