A Comparison of Wave Spectra during Pre-Monsoon and Post-Monsoon Tropical Cyclones under an Intense Positive IOD Year 2019

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.

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.

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.

Non-linear behavioral links with atmospheric teleconnections were identified between the Indian Ocean Dipole (IOD) mode and seasonal precipitation over East Asia (EA) using statistical models. The analysis showed that the lower the lag time, the higher the correlation; more than a two-fold correlation for non-linear regression with a kernel density estimator than for the linear regression method. When the IOD peaked, a pattern of significant reductions in seasonal precipitation during the negative IOD period occurred throughout the Korean Peninsula (KP). The occurrence of the positive IOD was in line with the El Niño phenomenon and generated greater seasonal precipitation than only the positive IOD, which takes place from March to May. This change occurred more in the cold tongue El Niño than the warm pool El Niño, inducing much higher spring precipitation throughout the KP. When negative IODs and La Niña coincided, there was slightly greater precipitation from March to May compared to the sole occurrence of negative IODs. In positive (negative) IOD years, there was anti-cyclonic (cyclonic) circulation in the South China Sea (SCS), helping to transport moisture to EA. The composite precipitation anomalies in the positive (negative) IOD years show above (below) normal precipitation in southern China. In contrast, other parts of the EA experienced drier (humid) signals than normal years. In positive IOD years, the anti-cyclonic circulation strength of the Bay of Bengal and the SCS continued until autumn and spring of the following year. This shows possible remote connections between climate events related to the tropical Indian Ocean and variations in precipitation over EA.

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 Dipole Mode Index (DMI), an Index used to represent the Indian Ocean Dipole (IOD), shows a back to back occurrence of positive and negative IOD events during the period 1994 to 1998. The year 1994 was a strong positive IOD year and was followed by a strong negative IOD in 1996. The positive IOD in 1997 and negative IOD in 1998 were co-occurred with the strong El-Nino and strong La-Nina, respectively. In the present study, we have looked into the evolution of consecutive IOD events during the period 1994 to 1998 and analyzed the differences in their formation, features and association with Indian Summer Monsoon Rainfall (ISMR) for various homogenous regions of the Indian subcontinent. We have found that the El-Nino and La-Nina enhances the IOD-induced positive SST anomalies in the western and eastern parts of the Indian Ocean during the co-occurred years, respectively. The evolution of positive IOD in 1997 is driven by the strong easterly wind anomalies associated with strong El-Nino. The positive anomalies of SST in the western equatorial Indian Ocean is maintained by the El-Nino which results in the long life span of positive IOD in the year 1997 than that of pure positive IOD year 1994. The westward propagating Rossby wave generated due to PIOD is maintained by strong El-Nino in the year 1997. The vertical dynamics, represented by D20, showed minor differences during the pure and co-occurred IOD events. The positive IOD years with El-Nino had good correlation with the Peninsular India and Central North East India regions rainfall. Since we have used only limited samples or cases under the different categories, our present results are preliminary in nature.

[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.

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.

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.

The intraseasonal variability (ISV) of sea level anomalies (SLAs) along the southern coast of Java and its interannual modulation were studied based on a gridded SLA product produced from the Archiving, Validation, and Interpretation of Satellite Oceanography dataset. This ISV is induced by the propagation of intraseasonal Kelvin waves derived from the central equatorial Indian Ocean (EIO). Wavelet analysis and empirical mode decomposition of intraseasonal SLAs along the southern coast of Java showed interannual variability, with weaker ISV events during El Nino years and positive Indian Ocean Dipole (IOD) years than during normal years. This interannual modulation of the ISV is influenced by the El Nino-Southern Oscillation teleconnection via the Walker Circulation and eastern Indian Ocean upwelling connected to IOD events. The anomalously weaker Walker Circulation during El Nino events generates anomalous surface easterlies over the central-eastern tropical Indian Ocean that produce upwelling Kelvin waves in the EIO and offshore water transport along the southern coasts of Sumatra and Java, resulting in negative SLAs along the southern coast of Java. These negative SLAs damp the positive SLAs induced by the eastward propagation of downwelling Kelvin waves from the central EIO during the following March–May of El Nino years. Similar features of SLAs and sea surface wind anomalies also occur during positive IOD years. Consequently, the sea level ISV along the southern coast of Java is weaker in El Nino and positive IOD years.

The influence of the Indian Ocean dipole (IOD) on the poleward propagation of boreal summer intraseasonal oscillations (BSISOs) is examined using observed datasets. This study finds that coherent (incoherent) poleward propagation of precipitation anomalies from 5°S to 25°N are observed during negative (positive) IOD years. Disorganized poleward propagation of BSISO in the south equatorial Indian Ocean is observed during positive IOD years. The rationale behind such an anomaly in the poleward propagation of BSISO in contrasting IOD years is identified based on the theory of northward-propagating BSISO, which suggests the influential role of air–sea interaction on the genesis and propagation of BSISO. It is found that the mean structure of moisture convergence and meridional specific humidity distribution undergoes radical changes in contrasting IOD years, which in turn influences the meridional propagation of BSISO. This study assumes significance, considering the critical role of BSISO in modulating the seasonal mean summer monsoon rainfall.

<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>

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.

Abstract. Gross primary production (GPP), a crucial component in the terrestrial carbon cycle, is strongly influenced by large-scale circulation patterns. This study explores the influence of the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on China's GPP, utilizing long-term GPP data generated by the Boreal Ecosystem Productivity Simulator (BEPS). Partial correlation coefficients between GPP and ENSO reveal substantial negative associations in most parts of western and northern China during the September–October–November (SON) period of ENSO development. These correlations shift to strongly positive over southern China in December–January–February (DJF) and then weaken in March–April–May (MAM) in the following year, eventually turning generally negative over southwestern and northeastern China in June–July–August (JJA). In contrast, the relationship between GPP and IOD basically exhibits opposite seasonal patterns. Composite analysis further confirms these seasonal GPP anomalous patterns. Mechanistically, these variations are predominantly controlled by soil moisture during ENSO events (except MAM) and by temperature during IOD events (except SON). Quantitatively, China's annual GPP demonstrates modest positive anomalies in La Niña and negative IOD years, in contrast to minor negative anomalies in El Niño and positive IOD years. This outcome is due to counterbalancing effects, with significantly larger GPP anomalies occurring in DJF and JJA. Additionally, the relative changes in total GPP anomalies at the provincial scale display an east–west pattern in annual variation, while the influence of IOD events on GPP presents an opposing north–south pattern. We believe that this study can significantly enhance our understanding of specific processes by which large-scale circulation influences climate conditions and, in turn, affects China's GPP.