Dipole Events Research Articles

On the Second-Year Warming in Late 2019 over the Tropical Pacific and Its Attribution to an Indian Ocean Dipole Event

On the Second-Year Warming in Late 2019 over the Tropical Pacific and Its Attribution to an Indian Ocean Dipole Event

Long‐Lasting Marine Heatwaves Instigated by Ocean Planetary Waves in the Tropical Indian Ocean During 2015–2016 and 2019–2020

AbstractMarine heatwaves (MHWs) in the tropical Indian Ocean (TIO) showed remarkable increases in duration and frequency during the satellite observing era, responding to rising sea surface temperature. Long‐lasting MHWs were found in three upwelling regions of the TIO in 2015–2016 and 2019–2020, closely related to persistent downwelling oceanic planetary waves. In 2015, a prolonged MHW (149 days) in the western TIO was initiated by the downwelling Rossby waves associated with the co‐occurring super El Niño and positive Indian Ocean dipole (IOD) events. In the following year, the negative IOD sustained the longest MHW (372 days) in the southeastern TIO, prompted by the eastward‐propagating equatorial Kelvin waves. In 2019–2020, the two longest MHWs recorded in the southwestern TIO (275 days in 2019 and 149 days in 2020) were maintained by the downwelling Rossby waves associated with the 2019 extreme IOD. This study revealed the importance of ocean dynamics in long‐lasting MHWs in the TIO.

Observed of Equatorial Currents in the Indian Ocean during Two Contrasting IOD Events: 2006 and 2010

The evolution of Indian Ocean Dipole (IOD) events in 2006 and 2010 is investigated using observational data products that are made to understand several processes in the positive (negative) phase of IOD events. Two Acoustic Doppler Current Profiler (ADCP) moorings mounted at 90°E and 80.5°E along the equator were used to evaluate the zonal current variation during two contrasting Indian Ocean Dipole (IO) events. Westward anomalies of the zonal current were observed at 0°, 80.5°E during the peak phase of the positive IOD event from October to December 2006. Meanwhile, the observed zonal currents at 0°, 90°E only showed the short-term westward anomalies during October 2006. On the other hand, during the negative IOD event in 2010, the observed zonal current at both mooring locations indicated strong intraseasonal variations of the eastward anomalies from August to December 2010. Strong easterly (westerly) anomalies of the surface zonal winds were observed during the peak phase of the positive (negative) IOD event in 2006 (2010). These easterly (westerly) anomalies forced upwelling (downwelling) equatorial Kelvin waves indicated by the negative (positive) sea surface height anomalies. Strengthening (weakening) of upwelling (downwelling) along the equatorial Indian Ocean would be a significant factor for further understanding of IOD evolution.

How Does El Niño Affect Predictability Barrier of Its Accompanied Positive Indian Ocean Dipole Event?

The effects of El Niño on the predictability of positive Indian Ocean dipole (pIOD) events are investigated by using the GFDL CM2p1 coupled model from the perspective of error growth. The results show that, under the influence of El Niño, the summer predictability barrier (SPB) for pIOD tends to intensify and the winter predictability barrier (WPB) is weakened. Since the reason for the weakening of WPB has been explained in a previous study, the present study attempts to explore why the SPB is enhanced. The results demonstrate that the initial sea temperature errors, which are most likely to induce SPB for pIOD with El Niño, possess patterns similar to those for pIOD without El Niño, whose dominant errors concentrate in the tropical Pacific Ocean (PO), with a pattern of negative SST errors occurring in the eastern and central PO and subsurface sea temperature errors being negative in the eastern PO and positive in the western PO. By tracking the development of such initial errors, it is found that the initial errors over PO lead to anomalous westerlies in the southeastern Indian Ocean (IO) through the effect of double-cell Walker circulation. Such westerly anomalies are inhibited by the strongest climatological easterly wind and the southeasterlies related to the pIOD event itself in summer, while they are enhanced by El Niño. This competing effect causes the intensified seasonal variation in latent heat flux, with much less loss in summer under the effect of El Niño. The greater suppression of the loss of latent heat flux favors the positive sea surface temperature (SST) errors developing much faster in the eastern Indian Ocean in summer, and eventually induces an enhanced SPB for pIOD due to El Niño.

The impact of ocean data assimilation on the simulation of mesoscale eddies at São Paulo plateau (Brazil) using the regional ocean modeling system

This work presents simulations of ocean circulation in the southwestern South Atlantic Ocean with emphasis on the São Paulo Plateau region, a unique physiographic feature at the foot of the continental slope offshore southeast Brazil, where the flow is dominated by ocean eddies and dipoles. Surface dynamics south of 23°S presents a heterogeneous circulation pattern intimately related to the presence of mesoscale features, such as meanders of the Brazil Current along the continental slope, and eddies that interacts with deep ocean features. The reproduction of these eddies at the correct time and spatial structure are quite challenging in the context of a non-assimilative model, considering the limitations imposed by the model’s numerical schemes, physical parameterizations, boundary conditions and surface forcing. In this sense, the Regional Ocean Modeling System (ROMS) 4-dimensional Variational Data Assimilation (4D-Var) module was used to increase model skill and minimize deviations from oceanic observations. Weekly observations of Sea Surface Height (SSH), daily Sea Surface Temperature (SST) fields and in situ vertical profiles of temperature and salinity from multiple sources were used in consecutive assimilation cycles to produce an accurate representation of ocean features and their variability. Both the assimilative and free run were evaluated against observations, including in situ current measurements. Results showed significant improvements of the assimilation procedure on both assimilated and non-assimilated variables, with RMSE reductions of 27% and 47% for SST and SSH, respectively, and 51% for current speed measurements during a dipole event in the São Paulo Plateau. The assimilative run was able to reproduce a remarkable event of mesoscale eddies interaction (dipole) and consequent current intensification, leading to important improvements on model skill. The correct use of observational information of open ocean surface features through data assimilation is a key factor for a proper simulation of such eddy dipole events.

Identifying Metocean Drivers of Turbidity Using 18 Years of MODIS Satellite Data: Implications for Marine Ecosystems under Climate Change

Turbidity impacts the growth and productivity of marine benthic habitats due to light limitation. Daily/monthly synoptic and tidal influences often drive turbidity fluctuations, however, our understanding of what drives turbidity across seasonal/interannual timescales is often limited, thus impeding our ability to forecast climate change impacts to ecologically significant habitats. Here, we analysed long term (18-year) MODIS-aqua data to derive turbidity and the associated meteorological and oceanographic (metocean) processes in an arid tropical embayment (Exmouth Gulf in Western Australia) within the eastern Indian Ocean. We found turbidity was associated with El Niño Southern Oscillation (ENSO) cycles as well as Indian Ocean Dipole (IOD) events. Winds from the adjacent terrestrial region were also associated with turbidity and an upward trend in turbidity was evident in the body of the gulf over the 18 years. Our results identify hydrological processes that could be affected by global climate cycles undergoing change and reveal opportunities for managers to reduce impacts to ecologically important ecosystems.

Positive relationship between seasonal Indo-Pacific Ocean wave power and SST

The influence of increasing sea surface temperatures (SSTs), in response to greenhouse warming, on wave power (WP) remains uncertain. Here, seasonal relationships between SST anomalies and mean and extreme WP over the Indo-Pacific Ocean are examined. Overall, seasonal WP has significantly increased over much of the Pacific, Indian, and Southern Ocean by 1.21–3.10 kW/m dec−1 over 1979–2019. Contributions from wave characteristics, namely significant wave height (SWH) and peak wave period (PWP), to changes in WP show that SWH contributes most in extra-tropical regions, and PWP most in tropical regions. Further, seasonal relationships between SST anomalies and WP indicate that increases in WP are also seen during strong El Niño years in December–February, and in-phase combinations of El Niño and positive Indian Ocean Dipole (IOD) events during June–August and September–November. Results highlight both long-term increasing SSTs and climate variability roles for inducing large-scale seasonal WP changes throughout the Indo-Pacific.

Evaluation of intraseasonal wind rectification on recent Indian Ocean dipole events using LICOM

Evaluation of intraseasonal wind rectification on recent Indian Ocean dipole events using LICOM

Interannual variability of chlorophyll-a and impact of extreme climatic events in the South Eastern Arabian Sea

South Eastern Arabian Sea (SEAS) is an upwelling dominated region, where the distribution of chlorophyll-a (Chl-a) is mainly controlled by basin-scale to mesoscale processes associated with seasonal reversal of winds and currents. The analysis of long-term satellite-derived Chl-a data revealed interannual variability in association with extreme climatic events. Chl-a data were obtained from Ocean Colour Climate Change Initiative (OC-CCI) which provides high resolution cloud free data for the Arabian Sea during summer monsoon season (June to September). Satellite measurements of Sea Level Anomaly (SLA), Sea Surface Temperature (SST) and sea surface wind data were also compiled from various sources. The monthly data for the above environmental variables during 1998 to 2016 were analysed as representatives of the possible causative physical processes influencing the variability in Chl-a. The time series and Hovmöller analyses shows that the maximum variability occurred during summer monsoon. This study shows that the interannual variability in Chl-a was prominently influenced by the underlying processes involved in coastal upwelling. The relative variability in Chl-a and associated environmental variables with these events was remarkable during strong El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events. Cross Correlation Function (CCF) analysis points out that Chl-a content is negatively correlated with IOD to an extent of one month lag or less whereas, ENSO does not indicate such direct relations. Further, the variability associated with IOD was more conspicuous than ENSO. The study elucidates the interannual variability of Chl-a and the overall influence of associated extreme events in the SEAS.

Evolution of Subsurface Temperatures in West Sumatra - Southern Java Waters During 2010–2014 Indian Ocean Dipole Events

The temperature anomaly formation in the West Sumatra and South Java Waters plays an important role in the formation of the Indian Ocean Dipole (IOD). There have not been many detailed studies on the evolution of temperature anomalies in the subsurface layers in the area during the IOD events. In this study, temperature data from the HYCOM were used to examine the evolution of temperature anomalies on the surface and subsurface in the event of negative IOD (nIOD) 2010 and positive IOD (pIOD) 2012). The analysis was done using a cross-section plot and a Hovmöller diagram. It has shown that in the negative IOD 2010, a positive temperature anomaly in the subsurface layer was started four months earlier than the surface layer and ended six months after the IOD event. In contrast to positive IOD 2012, a negative temperature anomaly formed in the surface layer seven months earlier, and then move to the deeper layer coincide with the onset of the positive IOD event. The negative anomaly in both layers was simultaneously over two months after the positive IOD event over. The La-Niña phase that coincides with the positive or negative IOD event, influences the process of forming temperature anomalies in the subsurface layer, which in this case supports (inhibits) the formation of positive (negative) temperature anomalies in negative (positive) IOD event. The temperature anomaly in the subsurface layer can be an alternative indicator in identifying and predicting IOD events.

Impacts of the 2019 Strong IOD and Monsoon Events on Indian Ocean Sea Surface Salinity

The impact of the 2019 super positive Indian Ocean Dipole (PIOD) event, the strongest in the last four decades, which also co-occurred with an El Niño and a strong summer monsoon, on Indian Ocean sea surface salinity (SSS) is examined using the Soil Moisture Active Passive satellite measurements. Salt budget estimation suggests a predominant, nearly ocean-wide influence by surface freshwater flux and horizontal advective terms. Subsurface ocean influence on the salt budget occurs mainly in the southeastern tropical Indian Ocean (SETIO). The PIOD event suppressed the influence of the El Niño, thereby causing anomalous high precipitation in western India, and leading to an unusual freshening in the southeastern Arabian Sea (AS), which is subsequently advected towards the equatorial Indian Ocean (EIO). In the western Bay of Bengal (BoB), following the waning of monsoon-influenced precipitation in the fall, SSS becomes anomalously salty and traverses towards the AS against the flow of anomalous surface currents. During the peak of the summer monsoon in August–September and the peak of the PIOD event in September–November, SSS in the EIO exhibited tendency for freshening, mainly driven by westward advection of freshwater from the eastern BoB. Conversely, in the SETIO, there was tendency for salinification due to suppression of precipitation, enhanced upwelling of high subsurface salinity, and northward advection of salty water. During December to January of the following year, these salinity tendencies reversed, with salinification in the EIO and freshening in the SETIO.

Vegetation dynamics of Kisima Ngeda freshwater spring reflect hydrological changes in northern Tanzania over the past 1200 years: Implications for paleoenvironmental reconstructions at paleoanthropological sites

Kisima Ngeda (KN), a spring on the northern margin of saline Lake Eyasi, Tanzania, sustains an Acacia-Hyphaene palm woodland and Typha swamps, while the surrounding vegetation is semi-desert. To study the vegetation changes associated with this spring, which represents a plausible modern analog for the fossil springs documented in the nearby paleoanthropological and archaeological sites of Olduvai Gorge, we analyzed the pollen content of a 43 cm-long sediment core that documents vegetation changes since the last ~1200 years (from cal yrs. C.E. 841 to 2011). Our results show that (1) Hyphaene palms, which require meso-halophytic soil conditions were most abundant in the area of the coring site until cal yrs. C.E. ~1150 when the groundwater supplying the KN spring was likely lower than at present, allowing intrusions of saline lake waters. (2) From cal yrs. C.E. ~1200, a peat began to develop, the palm woodland was replaced by a Mimosaceae woodland, and the increased presence of Typha pollen indicates the presence of more wetlands. (3) From cal yrs. C.E. 1600, the groundwater level of the KN spring increased and reached its highest level in the last 1200 years. (4) Peaks of wetland expansion, which reflect increased groundwater flow and level in response to amplified rainfall in the recharge area (Mt Oldeani, Ngorongoro Highlands), occurred at cal yrs. C.E. ~1200–1400 and ~1650–2011. These outflows of groundwater at Kisima Ngeda were linked to the intensity and frequency of positive Indian Ocean Dipole (IOD) events, which trigger heavy rains in eastern Africa. We conclude that the Kisima Ngeda hydrological system, which has been active for more than 1200 years, responds rapidly to regional climate change driven by changes in the sea surface temperatures (SSTs) of the Indian Ocean. Yet, it is also capable of remaining active during dry intervals as inferred from the Kisima Ngeda record prior to cal yrs. C.E. 1200. Our results support the hypothesis that this type of system helped to maintain Plio-Pleistocene hominin populations and activities in the arid lowlands of the rift on a multi-decennial scale.

Effect of Indian Ocean Dipole (IOD) Events on the Second Inter-monsoonal Rainfall in the Wet Zone of Sri Lanka

The climatic variations in the Indian Ocean have a strong relationship with the rainfall anomaly of Sri Lanka. Indian Ocean Dipole (IOD) is an ocean-atmospheric coupled phenomenon associated with an east-west gradient in the tropical Indian Ocean Sea Surface Temperature (SST) anomalies. Identifying the impact of IOD phenomenon on the spatial and temporal variation of the rainfall pattern is a useful tool for seasonal climate forecasting. The present study examined the influence of IOD on the Second Inter-monsoon (SIM) rains of the Wet zone (WZ) of Sri Lanka. Considering 16 rain-gauge locations scattered in the WZ of Sri Lanka covering nine Agro-ecological Regions (AERs), daily rainfall time series for 44-years (1976-2019) were collected. The IOD-positive and IOD-negative years were extracted based on the Dipole Mode Index (DMI) over the neutral years. Five rainfall indices, namely, cumulative rainfall, the maximum rainfall received within a day, number of wet days, heavy rainfall events and the maximum consecutive dry days were statistically analysed. The results revealed a significant positive anomaly of mean cumulative rainfall in the SIM season during the IOD-positive years while in IOD-negative years this anomaly was negative (p&lt;0.05). An apparent increase or decrease of number of wet days and heavy rainfall events was observed during IOD-positive or IOD-negative years, respectively. However, the mean maximum consecutive dry days showed a distinct negative anomaly with the positive IOD events and positive anomaly with the negative IOD events. Further investigations are suggested to elucidate the <em>El Niño </em>Southern Oscillation (ENSO)-induced anomalous variation over IOD impacts.

Cloud Radar Observations of Diurnal and Seasonal Cloudiness over Reunion Island

In November 2016, a 95 GHz cloud radar was permanently deployed in Reunion Island to investigate the vertical distribution of tropical clouds and monitor the temporal variability of cloudiness in the frame of the pan-European research infrastructure Aerosol, Clouds and Trace gases Research InfraStructure (ACTRIS). In the present study, reflectivity observations collected during the two first years of operation (2016–2018) of this vertically pointing cloud radar are relied upon to investigate the diurnal and seasonal cycle of cloudiness in the northern part of this island. During the wet season (December–March), cloudiness is particularly pronounced between 1–3 km above sea level (with a frequency of cloud occurrence of 45% between 12:00–19:00 LST) and 8–12 km (with a frequency of cloud occurrence of 15% between 14:00–19:00 LST). During the dry season (June–September), this bimodal vertical mode is no longer observed and the vertical cloud extension is essentially limited to a height of 3 km due to both the drop-in humidity resulting from the northward migration of the ITCZ and the capping effect of the trade winds inversion. The frequency of cloud occurrence is at its maximum between 13:00–18:00 LST, with a probability of 35% at 15 LST near an altitude of 2 km. The analysis of global navigation satellite system (GNSS)-derived weather data also shows that the diurnal cycle of low- (1–3 km) and mid-to-high level (5–10 km) clouds is strongly correlated with the diurnal evolution of tropospheric humidity, suggesting that additional moisture is advected towards the island by the sea breeze regime. The detailed analysis of cloudiness observations collected during the four seasons sampled in 2017 and 2018 also shows substantial differences between the two years, possibly associated with a strong positive Indian Ocean Southern Dipole (IOSD) event extending throughout the year 2017.

Seasonal prediction skills in the CAMS-CSM climate forecast system

The seasonal prediction skills in the CAMS-CSM (the acronym stands for the Chinese Academy of Meteorological Sciences Climate System Model) climate forecast system is evaluated with a set of retrospective forecast experiments during the period of 1981–2019. The CAMS-CSM, which has been registered for the sixth phase of the coupled model intercomparison project (CMIP6), is an atmosphere–ocean–land–sea ice fully coupled general circulation model. The assimilation scheme used in the forecast system is the 3-dimentional nudging, including both the atmospheric and oceanic components. The analyses mainly focus on the seasonal predictable skill of sea surface temperature, 2-m air temperature, and precipitation anomalies. The analyses revealed that the model shows a good prediction skill for the SST anomalies, especially in the tropical Pacific, in association with El Niño-Southern Oscillation (ENSO) events. The anomaly correlation coefficient (ACC) score for ENSO can reach 0.75 at 6-month lead time. Furthermore, the extreme warm/cold Indian Ocean dipole (IOD) events are successfully predicted at 3- and even 6-month lead times. The whole ACC of IOD events between the observation and the prediction can reach 0.51 at 2-month lead time. There are reliable seasonal prediction skills for 2-m air temperature anomalies over most of the Northern Hemisphere, where the correlation is mainly above 0.4 at 2-month lead time, especially over the East Asia, North America and South America. However, the seasonal prediction for precipitation still faces a big challenge. The source of precipitation predictability over the East Asia can be partly related to strong ENSO events. Additionally, the anomalous anticyclone over the western North Pacific (WPAC) which connects the ENSO events and the East Asian summer monsoon (EASM) can be well predicted at 6-month lead time.

Tanzanian rainfall responses to El Niño and positive Indian Ocean Dipole events during 1951–2015

The relative impacts of Indian and Pacific Ocean processes on Tanzanian rainfall was evaluated using composite and correlation analyses. It was found that the seasonal responses of rainfall to positive Indian Ocean Dipole (pIOD) and El Niño events are substantial from September–October–November (SON) to December–January–February (DJF), whereas the Indian Ocean Dipole (IOD) exerts more control than El Niño–Southern Oscillation (ENSO) in both seasons. The associated relationship with the sea surface temperature (SST) and large-scale atmospheric circulations revealed distinct features. For the pure pIOD years, there is above-normal rainfall over the entire country. A strong rainfall condition is evident over the Lake Victoria basin and coastal and northeastern highland parts of the country during SON, while areas of the central and southern highlands exhibit substantial rains during DJF. For the pure El-Niño events, Tanzania has suffered from insignificant, weak, and non-coherent rainfall conditions during SON. However, a contrasting insignificant rainfall signature is found between the northern and southern parts of the country during the subsequent DJF season. For the co-occurrence of pIOD and El Niño, significant, excessive rainfall conditions are restricted to over the northern coast and northeastern areas of the country during SON, consistent with the rainfall pattern for pIOD. A weak, positive rainfall condition is observed over the entire country in the following season of DJF. Generally, in terms of Tanzanian rainfall, the IOD/ENSO variability and the associated impacts can be explained by the anomalous SST and circulation anomalies.

The Strong Upwelling Event off the Southern Coast of Sri Lanka in 2013 and Its Relationship with Indian Ocean Dipole Events

ABSTRACTMultisource satellite remote sensing data have been used to analyze the strong upwelling event off the southern coast of Sri Lanka in 2013 and its relationship with Indian Ocean dipole (IOD) events. The upwelling area in 2013 is 5.7 times larger than that in a normal year and lasts from June to August, with the peaks of the cooling anomaly reaching −1.5°C and the positive chlorophyll a concentration anomaly exceeding 3.1 mg m−3. In 2013, the negative unseasonable IOD (IODJJA) event enhances the southwest monsoon, while the blocking of the monsoon wind by the island results in a stronger westerly/northwesterly wind stress off the southern coast of Sri Lanka and a weaker westerly/northwesterly wind stress over the eastern Sri Lanka waters. This causes stronger offshore transport and positive Ekman pumping off the southern coast, forming a strong upwelling event there. Further analysis indicates that the interannual variability of the upwelling, as represented by a newly constructed index based on satellite observations, is primarily caused by the variations of local wind associated with the IOD. The upwelling off the southern coast of Sri Lanka weakens (strengthens) in the positive (negative) IOD years. However, an analysis based on 21 IOD events during 1982–2019 demonstrates that the effects of the three types of IOD events, including IODJJA, prolonged IOD (IODLONG), and normal IOD (IODSON), on the upwelling are different. Compared to the IODSON events, the IODJJA and IODLONG events tend to have stronger influences due to their earlier developing phases.

Indispensable Role of the Madden‐Julian Oscillation in the 2019 Extreme Autumn Drought Over Eastern China

AbstractEastern China suffered an extensive drought during autumn (September–October 2019) with precipitation deficits reaching as high as 70% of the climatology, the worst on record since the 1980s. Concurrent with this extreme drought, the tropical Indo–Pacific oceans experienced a central–Pacific (CP) El Niño event with the westernmost location, and the third–ranking positive Indian Ocean Dipole (IOD) event. However, the CP El Niño associated convection anomalies are unexpectedly absent over the western equatorial Pacific, leading to the decoupling of traditional atmospheric–oceanic connections between the tropical Pacific and Indian Oceans. A strong Madden–Julian oscillation (MJO) event stayed in the western Indian Ocean for 32 days during September–October 2019, nearly triple the frequency of climatology. This extraordinary long–lasting MJO activity suppressed the western Pacific convection associated with the CP El Niño and simultaneously enhanced the local IOD–related sea surface temperature anomalies via the atmospheric forcing. The MJO associated convection over the Indian Ocean can excite an anomalous cyclone in the western North Pacific, producing persistently extensive precipitation deficiencies over eastern China. It highlights that the abnormal MJO activity plays a crucial role on the Indo–Pacific atmospheric–oceanic features and the extreme precipitation deficits over eastern China.

IOD-ENSO interaction with natural coccolithophore assemblages in the tropical eastern Indian Ocean

Coccolithophores, a calcifying phytoplankton group, are a major component of the oligotrophic ocean. The tropical eastern Indian Ocean (EIO) possesses a complex regional hydrological system that also impacts the global climate. Coccolithophores are a major indicator of these oceanographic and air-sea processes. Understanding coccolithophore population dynamics associated with extreme climate events is significant for predicting future ocean biogeochemical studies, and contributing to the regional and global climatic model. Therefore, we used a consecutive 7 years (2011–2018) dataset of coccolithophore assemblages and their organic carbon biomass from the tropical EIO during the spring premonsoon period to interpret the aforementioned climatic changes. Among the 33 identified species, the ecologically important species Gephyrocapsa oceanica, Emiliania huxleyi, Algirosphaera robusta, Florisphaera profunda, Umbilicosphaera sibogae, and Umbellosphaera irregularis dominated the coccolithophore assemblages. In the EIO, regional environmental factors including high-level eddy kinetic energy, equatorial jets and upwelling lead to the patchy distribution of surface coccolithophores, but no signs of interannual spatial variations were observed. Furthermore, this study revealed that variability in euphotic coccolithophore abundance and diversity indices were correlated with global climate anomalies. The variations in the interannual coccolithophore abundance and estimated coccolithophore organic carbon could be a result of global warming and other climatic variabilities. Particularly, an apparent increase in coccolithophore abundance was observed during El Niño and positive Indian Ocean Dipole (IOD) events due to their favorable thermal regimes. In contrast, the abundance of coccolithophores reduced during La Niña and negative IOD events. Overall, our pilot findings would encourage further studies on coccolithophore responses to regional EIO environments and global climatic anomalies.

Inter-basin and Multi-time Scale Interactions in generating the 2019 Extreme Indian Ocean Dipole

AbstractAn unprecedented extreme positive Indian Ocean Dipole event (pIOD) occurred in 2019, which has caused widespread disastrous impacts on countries bordering the Indian Ocean, including the East African floods and vast bushfires in Australia. Here we investigate the causes for the 2019 pIOD by analyzing multiple observational datasets and performing numerical model experiments. We find that the 2019 pIOD is triggered in May by easterly wind bursts over the tropical Indian Ocean associated with the dry phase of the boreal summer intraseasonal oscillation, and sustained by the local atmosphere-ocean interaction thereafter. During September-November, warm sea surface temperature anomalies (SSTA) in the central-western tropical Pacific further enhance the Indian Ocean’s easterly winds, bringing the pIOD to an extreme magnitude. The central-western tropical Pacific warm SSTA is strengthened by two consecutive Madden Julian Oscillation (MJO) events that originate from the tropical Indian Ocean. Our results highlight the important roles of cross-basin and cross-timescale interactions in generating extreme IOD events. The lack of accurate representation of these interactions may be the root for a short lead time in predicting this extreme pIOD with a state-of-the-art climate forecast model.