Phases Of Indian Ocean Dipole Research Articles

Comparative Analysis of Characteristics and Physical Mechanisms for Typical Summer Extreme Precipitation in Pakistan

Abstract The 2022 floods in Pakistan resulted in severe losses and garnered global attention. This study aims to enhance the understanding of extreme precipitation (EP) events in Pakistan by examining the characteristics and mechanisms behind the persistent EP during summer, utilizing daily precipitation data from the Climate Prediction Center (CPC). Results showed that the monsoon precipitation in 2010, 2020 and 2022 are the highest three years on record. Notably, these peak events in 2010 (concentrating in the north) and 2022 (concentrating in the south) spanned from July through August. Conversely, the extreme precipitation in August 2020 was concentrated in northern Pakistan. For the circulation patterns, the intensification of the South Asian High and the western Pacific subtropical high with a strong Indian monsoon is a unifying feature, but the Iranian high and monsoon low-pressure system on the south of Pakistan was different. Additionally, the EP in July 2010 and August 2022 were also influenced by the teleconnection associated with European Blocking. La Niña events and the negative-phase Indian Ocean Dipole (IOD) also played a role in affecting summer EP, with the strongest La Niña occurring in 2010 and a notable triple-dip La Niña coinciding with a significant negative IOD phase in 2022. La Niña contributed to the formation of an anomalously strong anticyclone over the northwest Pacific and easterly winds along the southern Himalayas, impacting moisture transport to Pakistan. Conversely, the negative IOD phase amplified EP in Pakistan by enhancing the northward movement of convective systems and westerly winds over the Indian Ocean. Furthermore, reduced snow cover on the Tibetan Plateau in the springs of 2010 and 2022 likely induced a stronger thermal dynamical effect, acting as a heat source in summer and increasing precipitation in Pakistan.

Investigation of Tropical Cyclones in the North Indian Ocean and the Linkage to Extreme Weather Events over Sri Lanka

Heavy rainfall due to tropical cyclones (TCs) in the North Indian Ocean (NIO) adversely impacts nations frequently. Though extensive research has focused on TCs in the NIO, less attention has been given to the connection between TCs and extreme events in Sri Lanka. This study examined atmospheric characteristics during sixteen extreme events, focusing on linkages between TCs, the Indian Ocean Dipole (IOD), and mechanisms behind heavy rainfall associated with TCs over Sri Lanka. The results showed that in the pre-monsoon period, TCs move northward with high water vapor (WV) content accumulated in the Southern Hemisphere. This main WV flow over the equatorial Indian Ocean (EIO) is connected with TCs, causing considerable damage in the southwestern part of Sri Lanka. During negative IOD years, strong westerly winds create cyclonic circulations on either side of the equator. Conversely, during the post-monsoon period, the IOD phase has no significant effect. TCs generally followed westward tracks, supported by winds from the Northern Hemisphere, and caused heavy rainfall in the Eastern, Northern, and Northcentral provinces in Sri Lanka. These TCs are isolated from the main WV flow over EIO. Such observed common characteristics during pre-monsoon and post-monsoon seasons are key factors contributing to extreme rainfall in Sri Lanka.

Interpreting Negative IOD Events Based on the Transfer Routes of Wave Energy in the Upper Ocean

Abstract The present study adopts an energy-based approach to interpret the negative phase of Indian Ocean dipole (IOD) events. This is accomplished by diagnosing the output of hindcast experiments from 1958 to 2018 based on a linear ocean model. The authors have performed a composite analysis for a set of negative IOD (nIOD) events, distinguishing between independent nIOD events and concurrent nIOD events with El Niño–Southern Oscillation (ENSO). The focus is on investigating the mechanism of nIOD events in terms of wave energy transfer, employing a linear wave theory that considers the group velocity. The proposed diagnostic scheme offers a unified framework for studying the interaction between equatorial and off-equatorial waves. Both the first and third baroclinic modes exhibit interannual variations characterized by a distinct packet of eastward energy flux associated with equatorial Kelvin waves. During October–December, westerly wind anomalies induce the propagation of eastward-moving equatorial waves, leading to thermocline deepening in the central-eastern equatorial Indian Ocean, a feature absent during neutral IOD years. The development of wave energy demonstrates different patterns during nIOD events of various types. In concurrent nIOD–ENSO years, characterized by strong westerly winds, the intense eastward transfer of wave energy becomes prominent as early as October. This differs significantly from the situation manifested in independent nIOD years. The intensity of the energy-flux streamfunction/potential reaches its peak around November and then rapidly diminishes in December during both types of nIOD years. Significance Statement The present study provides an interpretation of wave energy transfer episodes in the upper ocean during the negative phase of the Indian Ocean dipole (IOD) based on the diagnosis of hindcast experiments. The results suggest that the reflection of Kelvin and Rossby waves at the eastern and western boundaries of the Indian Ocean (IO), respectively, accompanied by variations in thermocline depth, plays a crucial role in the development process of IOD events. Specifically, during the negative phase of the IOD, the tropical IO exhibits positive signals of energy-flux streamfunction in the Northern Hemisphere, along with positive signals of energy-flux potential associated with westerly wind anomalies occurring in October–December. These findings highlight the significance of these factors in shaping the characteristics of negative IOD events.

Oceanographic characteristics of Lampung Bay and its relationship to the Indian Ocean Dipole (IOD) period 2013-2021

The Indian Ocean Dipole (IOD) may affect the distribution pattern of the sea surface temperature (SST) in the Indonesian seas. Understanding this impact is crucial for comprehending the dynamics of marine ecosystems, particularly in Lampung Bay. This study evaluates IOD effects on SST and associated upwelling/downwelling in Lampung Bay. 2014 fish catch data analysis reveals during negative IOD phases, SST increases (0.2°C to 2°C) with subsequent downwelling, coinciding with reduced catches (609.18 tons vs. 3,079 tons normal). Conversely, positive IOD phases show SST anomalies (-1°C to -2.5°C) with upwelling, correlating with increased catches to 3,349 tons in 2018. IOD-SST relationship primarily observed in Lampung Bay’s open connection to Sunda Strait (r=0.64 to 0.69), while weak correlations in coastal waters (r < 0.2) suggesting the influence of other factors on SST in Lampung Bay.

A strange negative Indian Ocean Dipole (IOD) in 2022

The Indian Ocean Dipole (IOD) is a prominent pattern of climatic mode in the Indian Ocean that contributes to the changes in Rainfall pattern, Sea Surface Temperature (SST), and Surface Salinity (SSS) in the Indian Ocean. Recently in the scenario of Climate Change, this IOD may experience shifting in the phases development that link to the Indo-Pacific teleconnection. Here, we analyse the changes of negative IOD patterns in 2022 through the SST, SSS, rainfall pattern, and surface current system. This study showed the negative IOD was initiated earlier and coincided with the Asian monsoon during May 2022. The IOD is faster ended in October 2022 when the Australian Monsoon was generated. The Rainfall pattern at the Eastern Indian Ocean (EIO) and the Indonesian Archipelago experienced an intensive pattern from May up to December 2022. During the negative IOD phases, the precipitation at the EIO is more concentrated in Java and less precipitation in Sumatra. The warmer SST and fresh surface salinity in the Indonesian Archipelago are wider following the Pacific SST. This reveals the Indo-Pacific teleconnection is stronger during 2022 via the surface ocean patterns and the coincidence of stronger westerly wind and easterly wind in the maritime continent.

Identifying important drivers of East African October to December rainfall season

Monsoon rainfall plays a crucial part in Africa's socio-economic structure and its year-to-year variability has profound implications for agricultural, energy, and other societal sectors. The current study focuses on two of the major climate drivers of the east African rainy season during October-November-December (OND), which is when the season starts for a large portion of east Africa (e.g. Tanzania and Malawi). Such drivers could be different in early austral summer from the rest of the year, due to the relative positioning of the Intertropical convergence zone, which passes through this region - hence regions of east Africa and OND season are the focus here.The two drivers of Monsoon viz. El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are considered: both independently indicate strong connections with monsoon OND rain. Not only is there a strong significant positive correlation in the OND season as a simultaneous relation, but the signal is also there even with the lag of a few months. This has been tested using various data sources, detrending data beforehand, analysing either recent time periods or earlier time periods - covering two decades each, and using regression analyses. To further strengthen the results, a compositing approach is applied that can additionally identify strong rainfall signals to that from years when opposite combinations of ENSO and IOD phases act as confounding factors.Results of precipitation anomaly for OND for compositing, when IOD and ENSO are both negative (positive) in July–August-September(JAS) indicate a deficit (excess) in rainfall in that region. The Walker circulation seems to play a major part via altering ascending to descending branch in two situations, when both drivers are in same phase. In the last near thirty years period, a total of 9 years matched the criteria when both drivers were negative and suggested a deficit in OND rainfall; more recently, that criteria occurred also in 2022 (JAS) and was again associated with a rainfall deficit in OND 2022. Based on this analysis, it is possible to deliver an estimation of cumulative rain in terms of median value, range and distribution, one season in advance, at a point location or average over a region.Some results of compositing are confirmed for longer record (1940–2021) too and further classifications based on threshold of drivers are tested. Rainfall (OND) variability at intra-decadal, decadal and multi-decadal scales are studied by applying the method of centered moving averages of 5-year, 11-year and 21-year respectively. Our results have implications for future planning in optimizing agricultural and energy outputs, mitigating severe consequences and losses associated with droughts and excess rain and will impact the livelihoods of millions of east Africans.

Compound drought and heat waves variation and association with SST modes across China

Compound drought and heatwaves (CDH) have garnered increasing attention because concurrent extreme events can exacerbate the harmful impacts caused by univariate extremes. However, various severities in CDH events and their relationships with sea surface temperature (SST) variations in China remain little understood. Here, we accurately identify CDH events and multi-aspect of characteristics using the standardized precipitation evapotranspiration index (SPEI) and the excess heat factor (EHF) during the extended summer (May–September) of 1961–2017. The evolution of multifaceted characteristics of CDH and their association with SST variation are further explored. The results suggest that the number, frequency, duration and intensity of regional CDH events show heterogeneous spatial patterns, with a significant increasing trend. A consistent abrupt transition in CDH characteristics averaged over China occurred in the period of 1993–1996. Mild and moderate CDHs occur more commonly in Northwest and North China, whereas severe CDHs are mainly found in central and eastern regions. Mild and moderate CDHs are more susceptible to SST modes than severe CDH, and there are strong positive correlations between mild and moderate CDH characteristics and SST variations in the northwest and northern regions. Compared to El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) plays a dominant role in the intensifications of mild and moderate CDH events. Regionally, the northwest and north have experienced longer, more frequent and severe CDH events during the positive phase of IOD. These findings reveal the divergent evolutions in CDH characteristics with various severities and inconsistent impacts of different SST modes on the compound events.

Important drivers of East African monsoon variability and improving rainy season onset prediction

Monsoon rain and its year-to-year variability have a profound influence on Africa’s socio-economic structure by heavily impacting sectors such as agricultural and energy. This study focuses on major drivers of the east African monsoon during October-November-December (OND) which is the standard time window for the onset of the rainy season, be it unimodal or bimodal. Two drivers viz. Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) both separately indicate very strong positive connections with monsoon (OND) rain not only in the OND season with zero seasonal lag, but the signal is also present even taking IOD and ENSO a season ahead. A compositing approach is applied that can additionally identify strong signals when different combinations of ENSO and IOD phases act as confounding factors. Results of precipitation anomaly suggest that when IOD and ENSO are both on the same phase in July-August-September (JAS), a significant OND rainfall anomaly occurs around the east African sector: A deficit (excess) of OND monsoon rain occurs when both drivers are in a negative (positive) phase during JAS. A location Kibaha in Tanzania, for which station data are available, is considered for a more in-depth analysis. The uncertainty range in cumulative OND rainfall is also reduced to a large degree when IOD and ENSO phases are both negative in JAS. These results can be used for prediction purposes and interestingly, that criterion of IOD and ENSO being of same phase in JAS was again matched in 2022 (both negative) and hence it was possible to deliver early warnings for a deficit in rainfall a season ahead. Techniques to compute the monsoon onset as determined by meteorological services such as the Tanzania Meteorological Authority rely on various thresholds, which may also vary by country. To overcome some of the issues with thresholds-based techniques, other definitions of ‘onset’ take into account cumulative rainfall amount and such technique has also been tested and compared. In both approaches, late (early) onsets dominate in years when ENSO and IOD are both negative (positive) during JAS. In these cases, it is therefore possible to provide an estimation of cumulative rainfall and onset for OND in terms of average, median value, range and distribution of rainfall one season in advance. Such results have implications for optimizing agricultural, water and energy management, also mitigating possible severe production losses, which would impact the livelihoods of millions of Africans.

Causes of 2022 summer marine heatwave in the East China Seas

Recent occurrences of marine heatwaves (MHWs) in coastal China seas have caused serious impacts on marine ecosystem services and socio-economics. Nevertheless, the underlying physical process, including local drivers and remote associations, remains poorly understood, thereby hindering accurate predictability. In this study, we reported an extreme MHW event in the East China Seas (ECSs, including the Bohai, Yellow, and East China Sea), lasting for 75 d with a maximum intensity of 1.96 °C relative to 1982–2011 during the summer 2022. This ECSs MHW event was triggered by a combination of anomalous atmospheric and oceanic conditions, including enhanced insolation, weakened surface wind speed, suppressed latent heat loss from ocean, a shallower mixed layer, and upper ocean current anomaly. Mixed-layer temperature budget diagnosis suggested that changes in the ECSs temperature were dominated by the surface net heat flux, largely due to strong shortwave radiation flux, during the development and decay of the MHW event. Oceanic advection also created favorable conditions for the maintenance of the MHW. These physical drivers were further regulated by the westward expanded and intensified western Pacific subtropical high (WPSH), potentially linked to the negative phase of Indian Ocean Dipole (IOD). Despite the three years (2020–2022) consecutive La Niña events, the ECSs summer MHWs appeared to be more closely linked to negative IOD events, with a lagging period of 1–3 mon. The seasonal precursor signals of the negative IOD have the potential to affect local physical drivers of ECSs MHWs through regulating the strength and position of WPSH, thus serving as a promising predictor for the ECSs MHWs. The future likelihood and intensity of the ECSs MHWs are projected to increase substantially in the coming decades, largely due to broad-scale warming attributed to anthropogenic climate change. Consequently, there is an urgent need to develop MHW forecasting and early warning systems, and robust approaches to address climate change.

Significant Precipitation Anomalies over Indonesia in the Aftermath of iod Events

Indian Ocean Dipole (IOD) is one of the prominent climate phenomena affecting Indonesia. The well-documented impacts of IOD are dry and wet anomalies during positive and negative IOD events, respectively, which are clearly detected during the development and mature phases of IOD in the boreal summer to autumn (June-November). IOD impacts on other seasons were questionable because of unclear associated signals. Here, we find significant precipitation anomalies in the aftermath of IOD events, which reach a peak in the following spring (March-May). The delayed pattern is distinctive with the typical impact of IOD. During positive IOD, the quick recovery and warming of IOD-induced sea surface temperature (SST) in the eastern Indian Ocean due to increasing shortwave radiation is suggested to play a role. When the influence of El Niño-Southern Oscillation (ENSO) is linearly removed, the aftermath impacts of IOD in the western regions appear to be suppressed because of weaker positive (SST) anomalies. While the eastern regions show anomalous increase on precipitation due to anomalous increase of convergence. This finding reveals the complex impacts of IOD and ENSO and may benefit to developing a better seasonal forecast over Indonesia.

Wind Variability and its Impact on IOD and MJO Phenomena in Western Waters


 
 
 
 
 
 This study investigates the interplay between wind variability and the Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO) phenomena in the western waters of Sumatra during the period 1982-2021. Utilizing monthly average wind data from satellite images, the research employs a comprehensive approach involving GrADS, Microsoft Excel, Surfer, Matlab, and Panoply software for data processing. The analysis reveals distinct wind patterns and varying IOD and MJO phases, with notable instances such as a positive IOD in October 1982 and a negative IOD in March 1983, coupled with strong MJO activity in February 1982 and weak activity in October 1983. This research underscores the significant influence of wind conditions on the dynamics of IOD and MJO phenomena in the studied region, contributing to a deeper understanding of ocean-atmosphere interactions.
 Highlight:
 
 
 
 
 
 
 
 
 
 
 
 Temporal Analysis: This study examines wind variability's interplay with IOD and MJO phenomena over three decades, revealing distinct patterns and phases in the western waters of Sumatra.
 Comprehensive Approach: Utilizing advanced software tools, the research processes satellite-derived wind data, shedding light on the complex dynamics of ocean-atmosphere interactions.
 Phenomena Dynamics: Notable occurrences, including positive and negative IOD events and varying MJO strengths, emphasize the profound impact of wind conditions on shaping the behavior of these critical climate phenomena, contributing to enhanced understanding and prediction.
 
 
 
 
 
 
 
 
 Keyword: Wind Variability, Indian Ocean Dipole (IOD), Madden-Julian Oscillation (MJO), Ocean-Atmosphere Interactions, Western Sumatra
 
 
 
 
 
 
 
 

A satellite-observation based study on responses of clouds to aerosols over South Asia during IOD events of south-west monsoon season

The present study investigated the possible effects of aerosols on cloud properties during positive and negative Indian Ocean Dipole (IOD) months over South Asia using 20 years (2002–2021) of long-term satellite-based observations and reanalyses datasets. Positive aerosol optical depth (AOD) differences between the two IOD phases are found mainly over the land areas and some parts of the Indian Ocean, with the maximum value observed over Indonesia, Malaysia, Northern China, Western India, and the Northern Arabian Sea. The negative AOD differences are seen over the equatorial Indian Ocean, some regions of Indian land mass, the North Indian Ocean, and some parts of the South China Sea, with the highest dip over the central Indo-Gangetic Plain region. The highest increase (decrease) in rainfall is observed over the western (eastern) pole. The reduced rainfall is responsible for the increased AOD over Indonesia, Malaysia, and neighboring regions. The strong negative relationships of cloud fraction (CF) with cloud top pressure and cloud optical depth (COD) suggest persisting optically thick middle and high-level clouds during the IOD months. Twomey effect is noticed over the study region during both the IOD phases irrespective of the meteorological conditions. CF and COD both showed an upsurge with increased AOD in the presence of high relative humidity and stronger updrafts, suggesting the role of prevailing meteorology in forming thicker clouds. However, the height of the cloud tops is getting lowered in polluted conditions. The aerosol-cloud relationships observed during IOD scenarios could be attributed to the prevailing meteorology.

Characteristics of Marine Heatwaves in the Indonesian Waters during the PDO, ENSO, and IOD Phases and Their Relationships to Net Surface Heat Flux

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics.

Spatial and Temporal Trends in the Timing of Budburst for Australian Wine Regions

Background and Aims. This research investigates spatial and temporal trends in budburst timing across Australian wine regions from 1910–2019. The potential drivers of these observed trends were then identified, including anthropogenic climate change and large-scale climate drivers (El Nino–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM)). Methods and Results. The timing of budburst was approximated using accumulation measurements applied to Australia wide gridded temperature data. We show that the modelled budburst date has been gradually shifting to earlier in the year for most (95%) Australian wine regions, at an average rate of one day every 24 years. This linear trend in budburst timing is likely to be associated with steadily increasing air temperatures due to anthropogenic climate change. Significant interannual variability was also observed and was correlated with IOD and SAM; however, no significant relationship was found with ENSO. Positive IOD phases result in budburst occurring on average four days earlier than the long-term average; however, this can be as high as eight days. Conclusions. The results of this study highlight that budburst timing for wine grapes is not a stationary phenomenon and is influenced by both natural and anthropogenic conditions. Significance of the Study. Understanding variability and trends in modelled budburst timing will assist tactical and strategic management practices and improve phenological modelling and adaptation planning for climate change.

Responses of Satellite Chlorophyll-a to the Extreme Sea Surface Temperatures over the Arabian and Omani Gulf

Extreme events such as Marine Heat Waves (MHWs) and Low Chlorophyll-a (LChl-a) in the ocean have devastating impacts on the marine environment, particularly when they occur simultaneously (i.e., the compound of MHWs and LChl-a events). In this study, we investigate the spatiotemporal variability of MHWs and LChl-a events in the Arabian and Omani Gulf. For this purpose, we used satellite-based high-resolution observations of SST (0.05° × 0.05°; from 1982 to 2020) and chlorophyll-a concentration data (0.04° × 0.04°; from 1998 to 2020). Hourly air temperature, wind, and heat flux components from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) were used to explain the link between these extreme events and atmospheric forcings. Moreover, our results revealed that the annual frequency of MHW and LChl-a is related to the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). The results revealed an average SST warming trend of about 0.44 ± 0.06 °C/decade and 0.32 ± 0.04 °C/decade for the Arabian Gulf (AG) and the Gulf of Oman (OG), respectively. This warming rate was accompanied by MHW frequency and duration trends of 0.97 events/decade and 2.3 days/decade, respectively, for the entire study region from 1982 to 2020. The highest annual MHW frequencies were recorded in 2010 (6 events) and 2020 (5 events) associated with LChl-a frequency values of 4 and 2, respectively. La Niña events in 1999, 2010, 2011, and 2020 were associated with higher frequencies of MHW and LChl-a. The positive phase of IOD coincides with high MHW frequency in 2018 and 2019. The longest compound MHW and LChl-a event with a duration of 42 days was recorded in 2020 at OG. This extreme compound event was associated with wind stress reduction. Our results provide initial insights into the spatiotemporal variability of the compound MHW and LChl-a events that occurred in the AG and OG.

Spatio-temporal signature of surface biophysical parameters in response to extreme IOD events using remote sensing

The Indian Ocean (IO) evidenced significant effects on regional and global ocean-atmospheric coupling variability in recent decades. Anomalous negative (positive) Indian Ocean Dipole (IOD) events witnessed along tropical IO during 2016 (2019) are one of them. The spatial signature of anomalous IOD events was studied using sea surface temperature anomaly (SSTA), sea surface height, sea surface salinity, chlorophyll-a, zonal and meridional wind components obtained from remote sensing data. The strong easterly wind anomaly blowing along the Sumatra coast causes strong coastal upwelling pioneer for the positive IOD event in 2019. The westward horizontal advection of seawater leads to low (high) sea surface height anomaly along eastern (western) tropical IO. The eastern (western) tropical IO becomes gradually cold (warm) during the mature phase (September–October–November) of positive IOD. Due to strong upwelling high and low sea surface salinity were observed along the eastern and western tropical IO respectively. The prolonged stronger upward (downward) Ekman pumping velocity in east (west) results in divergence (convergence) of seawater responsible for the increase (decrease) of chlorophyll-a and marine primary productivity. The reverse phenomena were characterized by warmer (cooler) SSTA, higher (lower) sea surface height, low (high) sea surface salinity, and decrease (increase) of chlorophyll-a in the eastern (western) tropical IO in the 2016 negative IOD year. The westward (eastward) zonal Ekman mass transport is possible fueling the extreme condition of positive (negative) IOD events. A strong eastward propagating downwelling Kelvin wave was observed due to the reflection of the upwelling Rossby wave from the African coast. Dominated picoplankton (microplankton) to other phytoplankton size classes are responsible for the reoccurrence of open ocean bloom along western (eastern) tropical IO during positive IOD.

Changes of Oceanic Conditions Drive Chagos Whale Migration Patterns in the Central Indian Ocean

Marine ecosystems are experiencing rapid shifts under climate change scenarios and baleen whales are vulnerable to environmental change, although not all impacts are yet clear. We identify how the migration behaviour of the Chagos whale, likely a pygmy blue whale (Balaenoptera musculus brevicauda), has changed in association with shifts in environmental factors. We used up to 18 years of continuous underwater acoustic recordings to analyse the relationships between whale acoustic presence and sea surface temperature (SST), chlorophyll-a concentration, El-Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). We compared these relationships between two independent sites Diego Garcia southeast (DGS) and Diego Garcia northwest (DGN) where Chagos whales are detected and are suspected to move interannually across the Chagos-Laccadive ridge. We showed that the number of whale songs detected increased on average by 7.7% and 12.6% annually at DGS and DGN respectively. At the DGS site, Chagos whales shifted their arrival time earlier by 4.2 ± 2.0 days/year ± SE and were detected for a longer period by 7.3 ± 1.2 days/year ± SE across 18 years. A larger number of songs were detected during periods of higher chlorophyll-a concentration, and with positive IOD phases. At the DGN site, we did not see an earlier shift in arrival and songs were not detected for a longer period across the 13 years. Whale presence at DGN had a weaker but opposite relationship with chlorophyll-a and IOD. The oceanic conditions in the Indian Ocean are predicted to change under future climate scenarios and this will likely influence Chagos whale migratory behaviour. Understanding how environmental factors influence whale movement patterns can help predict how whales may respond to future environmental change. We demonstrate the value of long-term acoustic monitoring of marine fauna to determine how they may be affected by changing environmental conditions.

Demonstrating the asymmetry of the Indian Ocean Dipole response in regional earth system model of CORDEX-SA

An accurate representation of the Indian Ocean Dipole (IOD) is crucial for the reliable projection of Indian summer monsoon rainfall, making it necessary to improve the understanding of the response of the IOD in the warming climate. For the first time, a high-resolution regional earth system model (RESM) over the CORDEX-SA domain is used to investigate the IOD characteristics. The model performance is evaluated in simulating the IOD and associated mechanism. RESM shows a good resemblance in simulating IOD phases (positive and negative). However, the systematic discrepancy is observed in magnitude. Additionally, RESM well represented the positive IOD's inter-event variability. For example, the stronger event dominated by significant cold anomalies over Sumatra with enhanced westward-extended while a moderate event shows weak cooling confined to the region of Sumatra. Additionally, RESM shows potential to distinguish the ENSO and non-ENSO years with more remarkable skill in representing the spatial pattern of SST over IOD region during non-ESNO years than ENSO years. The RESM realistically simulated the IOD amplitude with greater skill than CMIP5/6 models reported in the earlier studies, indicating reliability towards the projection of the Indian summer monsoon. The weaker IOD-ENSO relationship is caused by producing the more significant number of IOD during non-ENSO years. Despite this reliable fidelity, IOD's slightly earlier peak is driven by the early establishment of low-level equatorial easterly wind. This study provided valuable insight into the IOD's different phases, responsible forcings, and limitations of the RESM in accounting for the role of internal climate variability that can be useful for further improvement in the model physics.

Wildfires enhance phytoplankton production in tropical oceans

Wildfire magnitude and frequency have greatly escalated on a global scale. Wildfire products rich in biogenic elements can enter the ocean through atmospheric and river inputs, but their contribution to marine phytoplankton production is poorly understood. Here, using geochemical paleo-reconstructions, a century-long relationship between wildfire magnitude and marine phytoplankton production is established in a fire-prone region of Kimberley coast, Australia. A positive correlation is identified between wildfire and phytoplankton production on a decadal scale. The importance of wildfire on marine phytoplankton production is statistically higher than that of tropical cyclones and rainfall, when strong El Niño Southern Oscillation coincides with the positive phase of Indian Ocean Dipole. Interdecadal chlorophyll-a variation along the Kimberley coast validates the spatial connection of this phenomenon. Findings from this study suggest that the role of additional nutrients from wildfires has to be considered when projecting impacts of global warming on marine phytoplankton production.

Indian Ocean Dipole : Assessing its impacts on the Indian Summer Monsoon Rainfall (ISMR) across North East India

The Indian Ocean Dipole (IOD), a climatic anomaly, results in sustained sea surface temperature (SST) variations between tropical western and eastern Indian Ocean temperatures. In this study, we studied the variations to inculcate the teleconnections between IOD and Indian summer monsoon rainfall (ISMR) distribution across the country for the period 1960-2020 for all the three phases of ISMR. We analyzed rainfall, SST and low-level wind circulation anomalies for the above mentioned time horizon. Positive IOD events noticeably resulted in increase in summer monsoon rainfall distribution across the country respectively while its negative counterpart led to decrease in rainfall except for the commencement phase of ISMR. The variations in SST, wind circulation and moisture movement processes across the Indian Ocean characterize significant changes in rainfall during the positive and negative phases of IOD especially during the recent decades (1991-2020). The recent time horizon also witnesses enhanced low-level equatorial jets (LEJ) across the equatorial Indian Ocean and the Arabian Sea during the positive IOD events as compared to the prior decades (1960-1990). The effect of moisture convergence zone is also analyzed which results in above rainfall conditions across northeastern and central India. Conversely, negative IOD events were found to subdue any such moisture movement mechanisms. Furthermore, and additional investigation to analyze the effect of IOD on the retreating/withdrawal monsoon across northeast India has been done and it has been observed that a stronger positive IOD is detrimental to the seasonal rainfall (May- September) over North East India (-0.7 one month lag correlation). Furthermore, the DMI index of April-May presented a clear indication of monsoon activity over the area during the withdrawal or retreating phase of the summer monsoon, i.e., during September.