Influence Of Indian Ocean Dipole Research Articles

A biological dipole variability in the Indian Ocean

The Indian Ocean Dipole (IOD) is a prominent mode of climate variability in the tropical Indian Ocean (IO). It exerts a significant influence on biological activities in this region. To elucidate the biological response to the IOD, previous research has introduced the biological dipole mode index (BDMI). However, the delineation of the region by the BDMI has limitations in capturing IOD-induced chlorophyll variations in the IO. By analyzing observational data and historical simulations from a Coupled Model Intercomparison Project model, this study shows that chlorophyll anomalies in the IO exhibit a dipole pattern in response to IOD. During the developing and mature phases of the positive IOD, we observe a substantial decrease in chlorophyll in the south-southwest of India, contrasting with a pronounced increase in the southeast of the IO. This response is attributed to anomalous southeasterly winds induced by IOD, which enhance nutrient upwelling in the southeastern IO and suppress it in the south-southwest of India, resulting in corresponding changes in surface chlorophyll blooms. Based on this finding, we propose a new biological dipole index that more robustly explains the surface chlorophyll response to IOD in the tropical IO. This study highlights the profound influence of IOD on oceanic chlorophyll and underscores the importance of a more comprehensive understanding of the associated biophysical interactions.

Identification of Rainfall events on Climate Phenomena in Medan based on Machine Learning

Indonesia has diverse topographical conditions that result in Indonesia having a unique climate. One of the unique climate elements to be studied is rainfall, because rainfall has a different pattern in each region, this different rainfall pattern is caused by several climate phenomena factors that affect the rainfall pattern, including El-Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Madden Julian Oscillation (MJO). Medan City is the capital of North Sumatra province which is one of the areas in the flood-prone category in North Sumatra, where the factor of flooding is due to rainfall events in a long period of time, so the author wants to know which climatic phenomena factors can affect rainfall events in Medan city by using Machine Learning technology through the Matlab application, where in this study has a method by forming four combination models, namely the combination of the influence of IOD, SOI and MJO; second combination of IOD and SOI; third combination of SOI and MJO; and fourth combination of MJO and IOD, these four combinations will be the rainfall value of the four models. Furthermore, the rainfall value of the model is compared with the observed rainfall value and verification test using Mean Absolute Error (MAE) and correlation. Then the calculation of the comparison between the four rainfall models with the observed rainfall obtained the lowest MAE value during the SOI and MJO phenomenon of 15.0 mm and the highest correlation value during the IOD and SOI and SOI and MJO phenomena. So it is concluded that the combination of SOI and MJO has the best verification value. This shows that based on Machine Learning modeling, the model shown as the best predictor in Medan city is when the model combination consists of SOI and MJO.

Interactive effects of the El Niño-Southern Oscillation and Indian Ocean Dipole on the tropical net ecosystem productivity

El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) can exert abiotic stresses on biota to influence regional terrestrial carbon cycling. Here, we investigated their interactive effects on tropical net ecosystem productivity (NEP) when ENSO and IOD simultaneously occur (mainly El Niño & positive IOD [pIOD] and La Niña & negative IOD [nIOD]), based on TRENDYv9 multi-model simulations. Results suggest that NEP anomalies were dominated by IOD over South America near 20°S and Africa south of the Equator, but by ENSO over India and northern South America, and controlled by their amplifying effects over eastern Africa, the Indo-China peninsula, and most of Australia during September-October-November (SON). In the following December-January-February (DJF), IOD legacy effects generally enhanced ENSO-induced NEP anomalies over most of South America, eastern and southern Africa, and Australia. During March-April-May (MAM), the influence of IOD gradually disappeared. Our analysis revealed the dominant role of gross primary productivity (GPP) in these NEP anomalies. We further found asymmetric effects of soil moisture and temperature on NEP anomalies, showing higher correlation coefficients with soil moisture in El Niño & pIOD, but basically with temperature in La Niña & nIOD. Additionally, considering the simultaneous extreme of pIOD and El Niño activity in 1997/98, we calculated their seasonal individual contributions to NEP anomalies. We found that the pIOD event had the greatest influence in SON. During SON in 1997, individual contributions of pIOD and El Niño to NEP caused a land carbon source of −0.34 ± 0.15 and −0.35 ± 0.32 PgC yr−1 over South America, a carbon sink and source of 0.5 ± 0.19 and −0.54 ± 0.14 PgC yr−1 over Africa, and source of −0.08 ± 0.25 and −0.42 ± 0.37 PgC yr−1 over Asia-Pacific, respectively. Understanding interactive effects of IOD and ENSO on regional carbon cycling is crucial due to the more frequent extreme IOD and ENSO events under future climate warming.

Strengthened Impacts of Indian Ocean Dipole on the Yangtze Precipitation Contribute to the Extreme Rainfall of 2020 Meiyu Season

AbstractIn June‐July 2020, the Middle and Lower Yangtze River Region (MLYR) encountered the most severe precipitation event over the past 60 years. The Indian Ocean Dipole (IOD) in the previous autumn of the unusual precipitation event was the main contributing factor. Nonetheless, not every strong IOD event could bring anomalously high precipitation in the MLYR. This study showed significantly enhanced influence of IOD on East Asia climate since 2000. The IOD's effect on MLYR summer precipitation has strengthened significantly with the correlation coefficient up to 0.55 during 2000–2020. The northward movement of East Asia–Pacific pattern associated with IOD since 2000 (as compared with before) was primarily responsible for the enhanced effect. Since 2000, the MLYR has been controlled by a deep baroclinic instability system with anomalous low pressure in the lower troposphere and anomalous high pressure (completely different from before). Furthermore, the western Pacific subtropical high shifted northward and westerly jets were significantly enhanced with eastward expansion. All of these patterns contributed to the water vapor convergence and precipitation generation in the MLYR, which was not the case in the past. The record‐breaking IOD in 2019 autumn and the enhanced IOD's climatic effect in 2000–2020 contribute to the extreme Meiyu in 2020 jointly. The significant change in the influence of IOD and the enhanced climate effect of IOD highlight exciting new avenues for improving climate predictions in the MLYR.

Impact of Indian Ocean Dipole Events on Phytoplankton Size Classes Distribution in the Arabian Sea

Changes in the environmental condition associated with climatic events could potentially influence the PSC dynamics of the regional marine ecosystem. The Indian Ocean dipole (IOD) is one of the critical ocean–atmosphere interactions that affects the climate of the Arabian Sea, and it could be a potential factor influencing the regional PSC distribution. However, the relationship between PSC and IOD remains unclear and less explored. In this study, using the in-situ database acquired from the Arabian Sea, we reparametrized the three−component abundance−based phytoplankton size class model and applied it to reconstructed satellite−derived chlorophyll−a concentration to extract the fractional contribution of phytoplankton size classes to chlorophyll−a concentration. Further, we investigated the influence of IOD on the changes in the biological–physical properties in the Arabian Sea. The results showed that the biological–physical processes in the Arabian Sea are interlinked and the changes in the IOD mode control the physical variables like sea surface temperature (SST), sea surface height (SSH), and mixed layer depth (MLD), which influence the specific PSC abundance. Unprecedented changes in the PSC distribution and physical properties were observed during the extreme positive and negative IOD events, which clearly indicated the potential role of IOD in altering the PSC distribution in the Arabian Sea. This study highlights the impact of extreme climate events on PSC distribution and the need for a better understanding of the associated physical–biological–climate interactions.

On the weakening of northward propagation of intraseasonal oscillations during positive Indian Ocean Dipole events

The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summers, associated with active and break spells of monsoon rainfall over the Indian region. These northward propagations modulate the Indian summer monsoon rainfall. The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. In this study, we investigate the influence of Indian Ocean Dipole (IOD) on northward propagations. We show that northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) events. The “moisture mode” framework is used to understand the processes responsible for the weakening of northward propagations during pIOD years. Our analyses show that moistening caused by the horizontal advection is the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. Further analyses revealed that the reduction in the advection of the background entropy/moisture by zonal wind perturbations during pIOD is primarily responsible for the reduction in the horizontal advection. The mean structure of entropy between 925 and 500 hpa levels remained similar over most of the Asian monsoon region across the contrasting IOD years, and the reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. These weaker zonal wind perturbations during ISO events in pIOD years results from weak Rossby Vortex lobes. The weakening of Rossby wave response owing to cooler than average sea surface temperatures in the South-East Equatorial Indian Ocean and warmer than average West Equatorial Indian Ocean is proposed to be the possible reason for the weakening of northward propagations during pIOD years.

Extreme Indian Ocean dipole and rainfall variability over Central Africa

AbstractUsing both observational and reanalysis datasets, this study examines the influence of extreme Indian Ocean Dipole (IOD) events on Central Africa (CA) rainfall from September to December over the period 1980–2016. It is shown that during extreme positive IOD events (hereafter pIOD), CA experiences enhanced moisture supply from the Indian Ocean that results in increased rainfall while the opposite process is recorded during the negative phase (hereafter nIOD), with the magnitude of pIOD events anomalies stronger than those of nIOD events. Furthermore, the inflow is strongest in the lower troposphere over the ocean while it is strongest in the mid‐troposphere over CA because much of the lower‐level inflow is blocked by the topography to the east of CA. The intensification of mid‐tropospheric moisture transport over CA is associated with the African Easterly Jet which is present during these months. From 1980 to 2016 the IOD and the El Niño‐Southern Oscillation (ENSO) showed a strong positive relationship so that extreme pIOD (nIOD) are mostly coincident with El Niño (La Niña). To assess the possible role of non‐ENSO influences on the IOD influence over CA, the linear relationship between the Niño‐3.4 and dipole mode index (DMI) was removed from the DMI. It appears that the influence of the pIOD on CA rainfall is significant with the non‐El Niño effect and this is characterized by an increase in moisture advection toward CA that contributes to an enhancement of rainfall intensity. However, the IOD seems to contribute strongly to the rainfall variability in the CA regions south of 5°N, especially over the eastern Democratic Republic of Congo, and over the area from Gabon to the Republic of Congo where about 20–32% of the total rainfall variance is associated with the IOD independently of ENSO from November to December.

Revisiting ENSO and IOD Contributions to Australian Precipitation

AbstractTropical modes of variability, such as El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), exert a strong influence on the interannual variability of Australian precipitation. Nevertheless, commonly used indices of ENSO and IOD variability display significant co‐variability that prevents a robust quantification of the independent contribution of each mode to precipitation anomalies. This co‐variability issue is often addressed by statistically removing ENSO or IOD variability from the precipitation field before calculating teleconnection patterns. However, by performing a suite of coupled and uncoupled modeling experiments in which either ENSO or IOD variability is physically removed, we show that ENSO‐only‐driven precipitation patterns computed by statistically removing the IOD influence significantly underestimate the impact of ENSO on Australian precipitation variability. Inspired by this, we propose a conceptual model that allows one to effectively separate the contribution of each mode to Australian precipitation variability.

Seasonal and inter-annual variability of chemotaxonomic marker pigments in the north-eastern Arabian Sea

Using in situ data, the present study explored the seasonal and inter-annual variations of phytoplankton characteristics in the north-eastern Arabian Sea (NEAS). Phytoplankton pigment assessment was carried out at the Arabian Sea Time Series (ASTS) site in the NEAS (17°N 68°E) over 2011–2017 during the late south-west monsoon (SWM) and north-east monsoon (NEM). The site is largely influenced by open ocean upwelling during the SWM and winter convective mixing during the NEM, enhancing the productivity of the region. Winter convective mixing led to high biomass in the euphotic zone during the NEM, while late SWM showed oligotrophic conditions with subsurface chlorophyll maxima (Chlmax). Our measurements showed that the NEM supports larger phytoplankton, whereas the late SWM supports smaller groups, highlighting the crucial role of picoplankton in the productivity of the NEAS. Microscopic observation (cells >10 μm in size) also supported the presence of a higher number of diatoms and dinoflagellates during the NEM as compared to those during the late SWM. Among the picoplankton, Prochlorococcus counts were high during the late SWM than the NEM, which is also reflected in marker pigments. Apart from the seasonal variability, the region also showed large inter-annual variability which was checked with reference to the climate modes such as El-Niña Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Though IOD influence was not distinct on phytoplankton composition in the NEAS during the late SWM, the upper ocean variability during the ENSO resulted in a large contribution of picoplankton in the following NEM. We also compared the pigment data at ASTS with station N9 of US- Joint Global Ocean Flux Study (JGOFS) collected during 1995, as these stations are very close to each other. Though the marker pigments during the recent years were comparable to those of US-JGOFS, large inter-annual variability makes it difficult to interpret long-term changes in the phytoplankton composition. The present study recommends the need for sustained long-term time-series observations in the Arabian Sea to understand the changes in phytoplankton composition under the climate change scenario.

Utilization of remote sensing data for mapping the effect of Indian Ocean Dipole (IOD) and El Nino Southern Oscillation (ENSO) in Sumatra Island

IOD and ENSO are two global phenomena that are quite influencing the territory of Indonesia. Sumatra is one of the islands in Indonesia with unique local characteristics and is located in the westernmost. This study attempts to analyze how much the IOD and ENSO have affected rainfall in the Sumatra Island and which is more dominant, and the distribution that can be used as a reference in forecasting seasons and rainfall. In a period of 37 years (1981-2017), the influence of IOD and ENSO was analyzed using a correlation method which was then mapped. The rainfall data used was obtained from CHIRPS with a scale of 0.05. Data is processed using Climate Data Operator (CDO) and R-statistics to obtain correlation maps. Correlation calculations are carried out on the whole data and in the season period. The effect of IOD was seen to be significant in the southern part of Sumatra, with a maximum correlation of 0.4, while in the north and center, the correlation only -0.25. The effect of ENSO looks more evenly compared to the impact of IOD, with a correlation reaching -0.4 in the northern and southern parts of Sumatra. In contrast, the middle part shows a smaller correlation value with ranges of -0.1 to -0.3. In the DJF and MAM season periods, a positive correlation between rainfall and IOD occurred throughout Sumatra, and in the JJA and SON, a dominant-negative correlation occurred. In comparison, the ENSO shows dominant negative correlation in each period. Based on the correlation, ENSO is more influential on Sumatra compared to IOD in overall data. However, on seasonal data, IOD is more significant than ENSO. The southern part of Sumatra Island is the most affected by ENSO and IOD compared to other parts.

Influence of Indian Ocean dipole on rainfall variability and extremes over southern Africa

Extreme weather events; floods and droughts are common in southern Africa (SA) consisting of 8 countries (Botswana, Namibia, South Africa, Lesotho, Swaziland, Mozambique, Zimbabwe, parts of Angola and Zambia). This study examines the linkage between the SA October-December (OND) rainfall, the Indian Ocean Dipole (IOD) and the South Atlantic Oscillation Dipole (SAOD). Empirical Orthogonal Functions (EOF) technique is used to establish the dominant mode of variability of OND rainfall, as correlation analysis is applied to quantify the relationship between the indices; IOD [Dipole Mode Index (DMI)], SAOD Index (SAODI) and OND rainfall variability. Results show that the dominant mode of variability of OND rainfall exhibits a dipole pattern over SA and there exists a significant correlation at 95% confidence level between the area average OND rainfall (rainfall index (RFI)) and DMI, with a correlation coefficient of -0.3. The relationship between the mean SA OND rainfall and the positive phase of IOD varies greatly in space, ranging from one country to another. Further analysis of the dry and wet of SAOND rainfall years reveal that wet years are associated with convergence at surface level (850 hPa) and divergence at upper level (200 hPa), depicting rising motion in the region, whereas dry years are associated with divergence at low level and convergence at upper level, implying descending motion. The study recommends further research on a reduced spatial scale, for instance at a country level to ascertain the effect of IOD on individual country’s weather. This will help in accurate monitoring of the evolution of IOD events to improve quality of seasonal weather forecasts in the region.

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.

Investigation of the North Atlantic Oscillation and Indian Ocean Dipole Influence on Precipitation in Turkey with Cross-Spectral Analysis

Predicting the future behavior of precipitation is of the utmost importance for planning agriculture or water resource management and in designing water structures. Determining the relationships between precipitation and the oceans may enable more accurate predictions. Therefore, oceanic and other persistent indices called teleconnection patterns can be used, namely the North Atlantic oscillation (NAO) and the Indian Ocean dipole (IOD). The NAO affects the precipitation patterns in the Atlantic Ocean and Mediterranean countries, such as in Turkey. The IOD is related to temperature and precipitation in the Indian Ocean coastal countries and in some areas far from the Indian Ocean. In this study, the effects of the NAO and IOD indices on precipitation in Turkey were investigated by means of cross-spectral analysis between the monthly total precipitation (mm) and monthly NAO and IOD index values. Phase shift values were also calculated for the selected periods and their accuracy was evaluated statistically, using the determination coefficient (R2) and Akaike information criterion (AIC) as performance criteria for the linear model. The results indicated strong correlations for the 13-, 14-, 16-, and 22–23-month periods between the NAO index and precipitation values; and for the 13-, 14-, 16–17-, and 20–21-month periods between the IOD index and precipitation values. After cross-spectral analysis between the NAO and IOD indices and precipitation values, the maximum phase shift values increased as the periods increased, while the maximum phase shift value for each period was almost half of the period value. Moreover, the maximum cross-power spectral density (CPSD) values increased as the periods increased. High CPSD values were observed in the west of Turkey for the NAO and in the east of Turkey for the IOD.

Coral Sr/Ca-based sea surface temperature variability at Seribu and Timor islands waters

Climate phenomena which is happened in the Pacific and Indian oceans i.e. El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) influence Indonesian climate. To understand behaviour of climate phenomena and its impact to Indonesian climate it is required paleoclimate data which provide long time series climate data back to hundred till thousands year. Sea surface temperature (SST) is one of important climate parameter. Coral Sr/Ca is convinced as coral paleo-thermometer. In this study, the available coral Sr/Ca is used to reconstruct past SST variability (Coral SST). Univariate linear regression between coral Sr/Ca and SST is used to reconstruct past SST variability. To understand how the climate phenomena influence the SST variability, the coral Sr/Ca is correlated with the Nino 3.4 and IOD indices. The result shows that the influence of IOD and ENSO is difference between sites. At Seribu island coral SST shows that SST is influenced by IOD rather than ENSO, while from Timor coral SST shows that Indian Ocean Dipole (IOD) and ENSO influences SST.

Hourly maximum individual wave height in the Indian shelf seas—its spatial and temporal variations in the recent 40 years

The spatial and temporal variability of the high individual wave heights, which pose a hazard in India’s shelf seas, has not been investigated. In this study, the spatial and temporal variation of hourly maximum individual wave height (Hmax) in the Indian shelf seas is examined based on 40 years ERA5 Re-Analysis data from European Centre for Medium-Range Weather Forecasts. Waverider buoy data from five locations are used to assess ERA5 Hmax. The variability of Hmax in different temporal scales; daily, monthly and interannual and their linkages with major ocean-atmosphere coupled interaction in the Pacific and the Indian Ocean are presented. Wave climatology throughout India’s western shelf seas shows average Hmax exceeding 2 m during most of the year. Maximum Hmax in the western shelf seas is during May–July, whereas in the southern tip and the eastern shelf seas are during the tropical cyclones and are either in October–December or May. Maximum Hmax is 17.94 m, and it occurred at the northern Bay of Bengal on 25 May 2009 during the passage of tropical cyclone Aila. Spatial variations in Hmax between locations spaced at 1.5° latitude are less than 10% in the northwestern and southeastern shelf seas, but large variations (~ 28%) are in the southwestern and northeastern shelf seas. Annual mean Hmax shows contrasting trends when data for 1979 to 2018 and 1992 to 2018 is considered. Indian Ocean Dipole influence on the Hmax suggests negative correlations in the western shelf seas and positive correlations in the eastern shelf seas. The correlation with climate indices is similar for both significant wave height and Hmax.

Response of global evaporation to major climate modes in historical and future Coupled Model Intercomparison Project Phase 5 simulations

Abstract. Climate extremes, such as floods and droughts, might have severe economic and societal impacts. Given the high costs associated with these events, developing early-warning systems is of high priority. Evaporation, which is driven by around 50 % of solar energy absorbed at surface of the Earth, is an important indicator of the global water budget, monsoon precipitation, drought monitoring and the hydrological cycle. Here we investigate the response of global evaporation to main modes of interannual climate variability, including the Indian Ocean Dipole (IOD), the North Atlantic Oscillation (NAO) and the El Niño–Southern Oscillation (ENSO). These climate modes may have an influence on temperature, precipitation, soil moisture and wind speed and are likely to have impacts on global evaporation. We utilized data of historical simulations and RCP8.5 (representative concentration pathway) future simulations derived from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our results indicate that ENSO is an important driver of evaporation for many regions, especially the tropical Pacific. The significant IOD influence on evaporation is limited in western tropical Indian Ocean, while NAO is more likely to have impacts on evaporation of the North Atlantic European areas. There is high agreement between models in simulating the effects of climate modes on evaporation of these regions. Land evaporation is found to be less sensitive to considered climate modes compared to oceanic evaporation. The spatial influence of major climate modes on global evaporation is slightly more significant for NAO and the IOD and slightly less significant for ENSO in the 1906–2000 period compared to the 2006–2100 period. This study allows us to obtain insight about the predictability of evaporation and hence, may improve the early-warning systems of climate extremes and water resource management.

Surface chlorophyll anomalies associated with Indian Ocean Dipole and El Niño Southern Oscillation in North Indian Ocean: a case study of 2006-2007 event.

North Indian Ocean witnesses varied dynamical response due to independent climate modes such as Indian Ocean Dipole (IOD)/El Niño Southern Oscillations (ENSO) and their co-occurrences. These modes have a significant impact on ocean productivity, which in turn shows feedback for the strengthening of these patterns. Keeping this in view, the present work attempts to analyze the biological activity during the combined influence of positive IOD with El Niño during 2006-2007 event. To divulge the biological variability along with the dynamical response, the study includes intra-annual variability surface chlorophyll anomaly with D20 anomaly using satellite observations. Here, the individual role of IOD and ENSO on both surface chlorophyll and D20 is segregated through partial regression analysis for a period of 25years (1993-2017). By the regression method, it can be seen varied chlorophyll response for the 2006-2007 event with the IOD forcing leads to the major spatial and temporal variability with positive anomalies in Eastern Equatorial Indian Ocean (EEIO) (generally oligotrophic), Northwestern Bay of Bengal (NWBoB), and Northwestern Arabian Sea (NAS2) where production begins in fall intermonsoon and peaks up during November. On the other hand, negative anomalies are observed around the southern tip of India (SBoB) and the Northern Arabian Sea (NAS1). While ENSO depicts the high surface chlorophyll variability in the Western Indian Ocean (WIO1, WIO2) with negative anomalies of surface chlorophyll. This study observed an asymmetric response of chlorophyll variability over the North Indian Ocean during the 1997-1998 and 2006-2007 events with a major influence of IOD mode compared with the El Niño. Therefore, understanding the chlorophyll anomalies during different climate modes will help us to better understand the interannual variability and improve the predictability of chlorophyll productivity regions.

Streamflow Variability in Mahaweli River Basin of Sri Lanka during 1990–2014 and Its Possible Mechanisms

This study investigates the variation of seasonal streamflow and streamflow extremes in five catchments of the Mahaweli River Basin (MRB) Sri Lanka from 1990 to 2014, and the relationship between streamflow and seasonal rainfall in each catchment is then examined. Furthermore, the influence of Indian Ocean Dipole (IOD) and El Nino and Southern Oscillation (ENSO) on the seasonal rainfall and streamflow in the upper (UMRB) and lower reaches (LMRB) of MRB are explored. It’s found that the rainfall amount in southwest monsoon (SWM) season contributes 29.7% out of annual total rainfall in the UMRB, while the LMRB records 41% of the total rainfall during the northeast monsoon (NEM) season. The maximum streamflow of upper (lower) Mahaweli catchments is observed in the SWM (NEM) season. Catchments in the UMRB (LMRB) recorded strong interannual variability of seasonal overall flow (Q50), Maximum 10-day, and 30-day flows during the SWM (NEM) season. It’s further revealed that the catchment streamflow in the UMRB is closely correlated with the SWM rainfall in the interannual time scale, while streamflow of catchments in the LMRB is closely associated with the NEM rainfall. The effects of ENSO and IOD on streamflow are consistent with their impacts on rainfall for all catchments in MRB, with strong seasonal dependent. These suggested that the sea surface temperature anomalies in the both Indian Ocean and tropical Pacific Ocean are important factors affecting the streamflow variability in the MRB, especially during the SWM season.

Covariations between the Indian Ocean dipole and ENSO: a modeling study

The coevolution of the Indian Ocean dipole (IOD) and El Nino-Southern Oscillation (ENSO) is examined using both observational data and coupled global climate model simulations. The covariability of IOD and ENSO is analyzed by applying the extended empirical orthogonal function (EEOF) method to the surface and subsurface ocean temperatures in the tropical Indian Ocean and western Pacific. The first EEOF mode shows the evolution of IOD that lags ENSO, whereas the second mode exhibits the transition from a dipole mode to a basin-wide mode in the tropical Indian Ocean that leads ENSO. The lead-lag relationships between IOD and ENSO are consistent with two-way interactions between them. A comparison between two 500-year model simulations with and without ENSO shows that ENSO can enhance the variability of IOD at interannual time scale. The influence of ENSO on the IOD intensity is larger for the eastern pole than for the western pole, and further, is stronger in the negative IOD phase than in the positive phase. The influence of IOD on ENSO is demonstrated by the improvement of ENSO prediction using sea surface temperature (SST) in the tropical Indian Ocean as an ENSO precursor. The improvement of the ENSO forecast skill is found at both a short lead time (0 month) and long leads (10–15 months). The SST in the western pole has more predictive value than in the eastern pole. The eastward propagation of surface and subsurface temperature signals from the western Indian Ocean that precedes the development of heat content anomaly in the tropical western Pacific is the key for extending the lead time for ENSO prediction. Our results are consistent with previously reported findings but highlight the spatial–temporal evolution of the ENSO-IOD system. It is also illustrated that IOD would have been more helpful in predicting the 1997/98 El Nino than the 2015/16 El Nino.

Influence of Indian Ocean Dipole on Tropical Cyclone Activity over Western North Pacific in Boreal Autumn

In this study, we investigated the influence of the Indian Ocean Dipole (IOD) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific (WNP) during autumn (September–November) from 1961–2015. We found the number of TCs making landfall in China to be significantly negatively correlated with the IOD index, which can be attributed to shifts in the location of TC formation together with the abnormal steering flow at 500 hPa. During negative IOD autumns, TC genesis regions move obviously westward due to the westward retreat of the WNP monsoon trough. The TC activity is remarkably enhanced near South China coastal areas, which is due to a contiguous 500-hPa subtropical ridge. In contrast, during positive IOD autumns, TC genesis positions obviously shift eastward and more TCs tend to exhibit recurvature around 130°E or a westward path south of 15°N led by an equatorward movement of the 500-hPa subtropical ridge with a break near 125°E. In our examination of large-scale circulation, we found a pair of equator-symmetric anticyclones in the lower troposphere resulting from variations in the large-scale Walker circulation induced by the anomalous sea surface temperature (SST) associated with a positive IOD. The resulting Philippines anti-cyclonic anomalies are closely related to the variability of the monsoon trough over the WNP region. Furthermore, the variations in the steering flow can be explained by the suppressed (enhanced) convective activities around the Philippines and the weakened (strengthened) local meridional circulation over East Asia in positive (negative) IOD years.