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ABSTRACT Flash drought events can be characterised by the quick depletion of crop root zone soil moisture (rapid intensification) and hence can be termed as agricultural flash droughts. These events can have devastating impacts, such as increasing the risk of agricultural yield loss, heatwaves and increased wildfire risk, which further have cascading impacts on the socio‐economic conditions. The regional hotspots of flash droughts are analysed for winter, pre‐monsoon, monsoon and post‐monsoon seasons over India from 1981 to 2020. We assess the impact of the El Niño‐Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) on the flash drought frequency (FDF: number of flash drought events). The causal connection of the FDF with the ENSO and IOD is analysed using the PCMCI (Peter and Clark's algorithm combined with the Momentary Conditional Independence) algorithm. The monsoon season (June–September) is found to be more prone to flash droughts with higher spatial/regional average values of total per pixel FDF during the 40‐year period over the Central Northeast (~54) and West Central (~41) regions. It is observed that the fraction of the total number of flash droughts during the El Niño years (38.8%) is higher as compared with that in La Niña years (25.7%). It is also found that the co‐occurrence of positive/negative IOD with the El Niño phase can alter the seasonal fraction of FDF over India, highlighting the high complexity in the ENSO–IOD interactions. The causal analysis shows that only the Southern Peninsula and West Central regions have significant direct and lagged causal links of average per pixel FDF with IOD. Whereas, similar (direct and lagged) causal connections are observed between the ENSO and IOD. This study reveals that flash droughts and their teleconnections vary greatly among the seasons and regions in India, limiting its accurate predictions and increasing the risk to agricultural communities. [ABSTRACT FROM AUTHOR]

In this study, we applied the conditional nonlinear optimal perturbation (CNOP) method to generate nonlinear fast‐growing initial perturbations for ensemble forecasting, aiming to assess the effectiveness of the CNOP method in improving the forecast skill of climate events. Our findings reveal a significant improvement in the forecast skill of the Indian Ocean Dipole (IOD) within the CNOP ensemble forecast, particularly at long lead times, thereby extending the skilful forecast lead times. Notably, this improvement is more prominent for strong IOD events, with skilful forecast lead times exceeding 12 months, outperforming many current state‐of‐the‐art coupled models. The high forecast skill of the CNOP method is primarily attributed to its ability to capture the uncertainties in the wind anomaly field in the eastern Indian Ocean (EIO) closely associated with IOD evolution. Consequently, CNOP ensemble members exhibit significant deviations from the control forecast, resulting in a large ensemble spread encompassing IOD evolution. Furthermore, a comparison with the climate‐relevant singular vectors (CSV) method in terms of IOD and El Niño–Southern Oscillation (ENSO) predictions reveals the superior performance of the CNOP ensemble forecast. Despite the initial perturbations for ensemble forecasting being generated aimed at improving IOD forecast skill, the CNOP method significantly improves the forecast skill of both IOD and ENSO events, with a greater improvement for ENSO. Additionally, the CNOP ensemble forecast system provides more reliable estimates of forecast uncertainties and exhibits higher reliability with increasing lead times. In conclusion, the CNOP method effectively captures the nonlinear physical processes of climate events and improve the forecast skill. [ABSTRACT FROM AUTHOR]

This study investigated relationships between year‐to‐year variability in precipitation in the rainy season in Mozambique and major modes of climate variability in the Tropics and subtropics. The Niño3.4 index was strongly negatively correlated with precipitation in Mozambique's southern and central regions. We suggest that Rossby wave propagation reaching Southern Africa from the tropical Pacific is key to the relationship between precipitation in Mozambique and El Niño–Southern Oscillation. Subtropical Indian Ocean Dipole did not lead rainy‐season precipitation, but showed a simultaneous correlation with precipitation in southern, central and northeastern regions. Benguela Niño was found to have a significant positive lead correlation by 6 months with precipitation in the southern, central and northwestern regions. In contrast, Indian Ocean Dipole led precipitation in the southern, central and northeastern regions by 3 months. Overall, the modes of climate variability exerted stronger control over precipitation variability in southern and central Mozambique, and weaker control in northern Mozambique, particularly in the northwestern region. [ABSTRACT FROM AUTHOR]

The extreme wind climate and its trends and variability of the southwest coast of India, particularly the Kerala coast, are investigated using hourly reanalysis wind speeds for the period 1979–2021. The observed maximum wind speed and 99th percentile wind speed off Kerala coast are 21.0 and 11.3 m·s−1, respectively. The predominant direction of high wind speeds is in the SW–WNW sector, which accounts for 93.5% of the total high winds. Extreme wind speeds in this region are not due to tropical cyclones alone, but also due to seasonal high winds. Seasonally, the premonsoon and SW monsoon extreme winds are predominantly in the WSW–WNW directional sector, while postmonsoon strong winds are present in all directional sectors, but predominantly in the SSW–WNW sector, followed by the E–SSE sector. The 99th percentile wind speeds during premonsoon, SW monsoon and postmonsoon are 8.8, 12.3 and 8.9 m·s−1, respectively. A significant decreasing trend is observed in the 99th percentile wind speeds during 1979–2021, ranging from 3 to 6 cm·s−1·year−1 off Kerala coast. This is in contrast with global increasing extreme wind speeds, and it can be attributed to regional variability as well as weakening of the land–sea thermal gradients associated with rising surface temperature in the Arabian Sea in the light of global warming. In addition to the long‐term declining trend and monthly as well as seasonal features, distinct interannual variations are also noticed in the 99th percentile wind speeds. Analysis shows that the atmospheric teleconnections associated with El Niño–La Niña events and Indian Ocean Dipole have an important role in the variability of extreme wind speeds. The co‐occurrence of positive (negative) IOD with El Niño (La Niña) results in a sharp decline (increase) in extreme wind speeds. Significant decadal variations are also noticed in the 99th percentile wind speeds. [ABSTRACT FROM AUTHOR]

The seasonal evolution and interannual variability of latent heat flux (LHF) over the tropical Indian Ocean (TIO) region is examined for the period 1980–2019. The seasonal distribution of LHF is dominated by the winter pattern of each hemisphere in the TIO. Climatologically high LHF is mostly confined to the Arabian Sea and Bay of Bengal regions in the north Indian Ocean and the zonal belt between 10oS and 25oS in the south Indian Ocean. Warm and cold phases of El‐Niño Southern Oscillation, Indian Ocean Dipole (IOD), and their co‐occurrence events leave distinct LHF signatures over the TIO. The LHF anomalies during these events are more profound than pure events with enhanced latent heat loss when the La Niña and negative IOD events coincide. The LHF anomalies associated with the wind‐evaporation‐SST feedback is prominent when positive IOD co‐occur with the El‐Niño event. Both the vertical humidity gradient and wind speed changes play substantial roles in driving LHF anomalies during the co‐occurring events of NIOD and La Niña. [ABSTRACT FROM AUTHOR]

Positive/negative Indian Ocean Dipole (IOD) tends to co‐occur with El Niño/La Niña. Five positive IODs, however, are observed under La Niña condition in 1961, 1966, 1967, 2007, and 2011. This study uses the monthly ocean temperature and atmospheric reanalysis datasets from ECMWF to investigate how these positive IOD events occurred. It is found that the five positive IODs are consistently accompanied by a moderately strong positive subsurface IOD (Sub‐IOD) event that initiated earlier. The Sub‐IOD‐related oceanic dynamical processes including oceanic Kelvin and Rossby waves help to promote the development of the positive IODs. More specifically, the equatorial easterly anomalies in the Tropical Indian Ocean (TIO) that are crucial for the development of the positive Sub‐IODs and IODs in 1961/2011 are triggered by an anticyclonic curl over South Indian Ocean that is induced by a Southern Hemispheric Indian Ocean Subtropical Dipole mode. The crucial equatorial easterly anomalies in 1967 are triggered by a thermocline warming in the southwestern TIO induced by westward downwelling oceanic Rossby waves, while in 1966/2007, the equatorial easterly anomalies are connected with the anomalous Walker circulation resulting from the certain sea surface temperature anomaly pattern over the equatorial Pacific. On the other hand, the remote effects of La Niña on TIO are relatively weaker, being overwhelmed by the above triggering processes. [ABSTRACT FROM AUTHOR]

As two pronounced anomaly patterns over the tropical Indian Ocean, the Indian Ocean basin mode (IOBM) and the Indian Ocean Dipole (IOD) feature phase‐locking with seasonal cycle. We find that IOBM and IOD exhibit significant connection without the influence of the preceding El Niño–Southern Oscillation (ENSO). The tropical IOBM cooling in preceding winter, which is accompanied by anomalous westerlies in the lower troposphere of the tropical eastern Indian Ocean, can transit to a positive IOD in subsequent seasons. In boreal spring following a IOBM cooling, the equatorial wind anomalies over the Indian Ocean turn to easterlies due to the Rossby wave response to the decreased precipitation over the Maritime Continent region. The increased zonal gradient of the sea surface temperature anomalies in the tropical Indian Ocean favours the intrinsic Bjerknes feedback to build up a positive IOD. With the reinforcing process of the weakened Walker circulation in the tropical Indian Ocean, the positive IOD events develop during boreal summer and then reach the peak in following autumn. These results demonstrate that the IOD is transited from the preceding IOBM, which is independent of the influence of the preceding ENSO in boreal winter. [ABSTRACT FROM AUTHOR]

Wind climate along the southwest coast of India (Kerala coast) has been analysed using 41 years of Climate Forecast System Reanalysis (CFSR) winds to delineate long‐term trends and variability. The study reveals significant decreasing trends in annual mean wind speeds, of the order of −2.0 to −2.5 cm·s−1·year−1, for the period 1979–2019. Southwest monsoon has contributed the highest weakening trends (−3.0 to −4.0 cm·s−1·year−1) due to significant decrease in the southwesterly/westerly wind speeds. The wind climate and trends during the northeast monsoon and premonsoon seasons have been characterized largely by the prevalence of sea breeze and shamal–makran wind systems. We find that the intensity of both sea breeze and shamal–makran wind systems reduces from north to south along the Kerala coast, which is in consistent with the earlier studies. Decadal oscillations in wind speeds, driven by the decadal variability in large‐scale atmosphere–ocean circulations, are evident. Recent changes in the frequency of occurrence of Indian Ocean Dipole in the Indian Ocean have determined the overturning trends since 2010, and resulted in increasing trends in the current decade along the central and northern coasts of Kerala. Furthermore, interannual variability in wind speeds has been linked to the El Niño–Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD). [ABSTRACT FROM AUTHOR]

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large‐scale tropical climate processes such as the El Niño–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow‐up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build‐up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience 'false onsets' with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co‐occur with a flash drought. [ABSTRACT FROM AUTHOR]

Although climate models are important for making projections of future climate, little attention has been devoted to model simulation of the complex climate of Central Africa (CA). This study investigates rainfall biases through processes in three versions of the Met Office Unified Model over CA with both coupled and atmosphere‐only formulations for each version. The study shows that the models depict a wet (dry) bias over the eastern (coastal western) CA in the September–November season with the wet (dry) bias stronger in coupled (atmosphere‐only) models. Here, we explore potential regional to large‐scale causes of these biases. Results reveal that the overestimation of the simulated sinking branch of the Atlantic‐Congo zonal overturning cell is associated with a strong near‐surface temperature and pressure gradient between CA and the eastern Atlantic Ocean. This leads to strong low‐level westerlies (LLWs) which dry the coastal western CA and strengthen the intensity of the Congo basin cell. Over eastern CA, the wet bias is partially due to the misrepresentation in the strength of both African easterly jet components that shift the mid‐tropospheric moisture flux convergence southward favouring more convection south of the equator. Furthermore, the overestimation in the simulated width and intensity of the Congo basin cell is associated with a strong low‐level moisture convergence over eastern CA which contributes to more precipitation. Remote contributions to the wet bias come from both the Atlantic and the Indian Oceans. The simulated Atlantic‐Congo zonal overturning circulation dries the coast through its overestimated lower branch (LLWs) which moves further into the continent and advects more moisture to the eastern CA. In the meantime, during positive Indian Ocean Dipole years, the advected moisture from the Indian Ocean to the CA region is overestimated in models, much more in coupled models and contributes to the eastern CA wet bias. [ABSTRACT FROM AUTHOR]

The respective influences of the Indian Ocean dipole (IOD) and El Niño‐Southern Oscillation (ENSO) on the thermocline depth (represented by the 20°C isotherm depth, D20) variability in the south‐central Indian Ocean (SCIO) are investigated for the period of 1960–2020. The results show that pronounced interannual variations in D20 mainly occur in the SCIO and display a close relationship with the two tropical climate modes, with positive (negative) phase being associated with SCIO D20 deepening (shoaling). The D20 anomalies (D20A) induced by IOD and ENSO events have the same intensities but present different spatial–temporal features, as demonstrated by partial regression analysis. The IOD‐related D20A occurs during the mature phase of the event and behaves as a local response to anomalous wind stress curl induced by IOD, while the ENSO signature, dominated by the westward‐propagating oceanic Rossby waves, lasts much longer and extends further west. [ABSTRACT FROM AUTHOR]

The synergetic effect of Indian Ocean Dipole (IOD) and El Niño‐Southern Oscillation on the particulate matter (PM2.5) concentration in Guangdong for October–November–December 1980–2016 has been investigated. Results indicate that, during a typical polluted episode, Guangdong is usually under the influence of a high‐pressure system behind the cold front, which is characterized by weakened southwesterly, subsidence/divergence, and stable atmospheric conditions. Persistent low values of PM2.5 are found to prevail during the years when concurrent IOD positive phase and El Niño occur. However, separate occurrence of El Niño and the positive phase of IOD had only marginal effect on PM2.5. The remarkable synergetic effect of El Niño and the positive phase of IOD is caused by a significantly strengthened ascending motion associated with anomalous southwesterly, which facilitates the transport of abundant moisture, increasing rainfall and thus suppressing PM2.5 pollution in Guangdong. [ABSTRACT FROM AUTHOR]

The present study examines the relationship between the tropical Indian Ocean (TIO) significant wave height (SWH) and Indian Ocean dipole (IOD) during boreal summer season (June through August) in the latest version of European Centre for Medium‐Range Weather Forecasts reanalysis (ERA5) and wave simulations forced with surface winds and sea‐ice fields from the Coupled Model Intercomparison Project version‐5 (CMIP5) models. The interannual variability of SWH shows a significant negative correlation with the IOD over TIO. SWH anomalies display meridional tripole pattern with significant negative (positive) anomalies over eastern equatorial Indian Ocean caused by anomalous easterlies (westerlies), and positive (negative) anomalies over southeastern TIO and the north Bay of Bengal during positive (negative) phase of IOD. The strong wave heights along the east coast of India during positive IOD and the south and southwest coast of India during negative IOD are noticed. CMIP5 models GFDL‐CM3, MRI‐CGCM3 and the multi‐model mean display considerable skill in capturing these teleconnections with substantial magnitude differences. A thorough understanding of the teleconnections between IOD and TIO wave heights is a significant prerequisite for the accurate forecast of surface waves in the Indian Ocean. Hence, this study advocates the importance evaluating the ability of models in representing the SWH and IOD interactions and its implications on Indian coastal regions in the form of inundation, coastal flooding and other vulnerabilities in a changing climate scenario. [ABSTRACT FROM AUTHOR]

Daily values of precipitation in southwestern parts of Iran along with the corresponding Madden–Julian oscillation (MJO) phase are analysed to introduce some auxiliary indicators that can identify the wet and dry spells within a particular MJO phase. As a case study, the identification procedure focuses on phase 1 during the rainy season (Nov–Apr). The study shows that this phase contains two opposite spells, referred to as spells with pervasive precipitation (SWPP) and spells with widespread dryness (SWWD). Although the precipitation characteristics are essentially different between SWPP and SWWD, their associated MJO indices are found statistically identical. Therefore, the MJO–precipitation relationship fails to identify the wet/dry spells within an MJO phase. To resolve this deficiency, other climate indicators, which behave differently between these two spells, are introduced. The westward moisture flux over equatorial parts of the Indian Ocean and the northeastward moisture flux over the North African tropics are up to seven times stronger in the SWPP compared to SWWD. The dominance of a strong cyclonic (anticyclonic) circulation over the eastern side of the Mediterranean Sea and the negative (positive) west–east temperature gradient over southern parts of Iran are introduced as SWPP (SWWD) indicators. The study reveals that the SWPP (SWWD) coincides with a strong (weak) convection dipole that resembles the positive Indian Ocean dipole. The intensified (weakened) wind speed in the subtropical jet stream and persistence of the jet‐exit‐regions above Iran (Egypt and Saudi Arabia) are other SWPP (SWWD) indicators. Negative (positive) anomalies of sea surface temperature over the North Atlantic Ocean, the Black, Mediterranean, and Caspian Seas, as well as the Persian Gulf, signify the prevalence of the SWPP (SWWD). To the best of our knowledge, this is the first time that auxiliary indices are utilized to improve the MJO–precipitation relationship in southwestern Iran. [ABSTRACT FROM AUTHOR]

The coupled atmosphere–ocean phenomenon in the Indian Ocean, known as the Indian Ocean Dipole (IOD) mode, shows various influences on regional tropical climates. East Africa (EA) experiences enhanced precipitation during October–December (the annual "short rains"), and therefore it was hypothesized that a shift from wetter to drier conditions in EA after 1880 documented by proxy data (e.g., lake levels, glacier size) was favoured by a decline in positive IOD frequency. However, this proposed century‐scale pattern of the IOD emerged from single data sets, while the ensemble perspective from the Coupled Model Intercomparison Project phase 5 (CMIP5) global climate model (GCM) archive is still missing. Based on rigorous test criteria that capture crucial IOD and EA climate characteristics, the present study identifies 15 runs from the CMIP5 data for the ensemble analyses. A key result of the test is that the ensemble is dominated by the GISS E2 H model (13 realizations) with a little influence from HadCM3 (2 realizations), and therefore most CMIP5 models seem unable to capture the long‐term variability of EA precipitation. The selected model realizations show a significant increase in the frequency of positive IOD events in the late 20th century, in agreement with previous research and most likely due to external climate forcing. Further frequency changes are absent in the available time series, yet we find that the precipitation excess during positive IODs was stronger in the 19th century (1860–1909) than in the recent decades (1956–2005). Both thermodynamic factors on the regional scale (convection, evaporation) and dynamical processes on the large scale (strength of the zonal tropical circulation) emerge as physical basis of this changing IOD property. The higher precipitation intensity in EA during positive IOD events in the 19th century period provides a consistent mechanism that could have contributed to the moister regional climate of that time, irrespective of IOD frequency. [ABSTRACT FROM AUTHOR]

The intra‐seasonal variation of precipitation over Iran is examined in terms of the combined effects of the Madden–Julian Oscillation (MJO), Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO), using daily observations during the wet season (October–May) from 1961 to 2015. We have examined how the probability of daily rainfall above the upper tercile varies across MJO phases during positive and negative IOD and ENSO states. The results indicate that the response of Iran's wet‐season rainfall to the MJO is affected more by large‐scale atmospheric variations associated with the IOD than by ENSO. The negative (positive) IOD strengthens (suppresses) the MJO–rainfall relationship in the wet and dry MJO phases. The variation in the atmospheric variables (relative humidity and vertical velocity) indicates more (less) humidity and upwards (downwards) motion which increases (decreases) precipitation in wet (dry) MJO phases during the negative IOD. The rainfall relationship with the MJO during the negative IOD is statistically significant, while the relationship during the positive IOD is weak; and no significant relationship is found during either phase of ENSO. [ABSTRACT FROM AUTHOR]

There is a large discrepancy between studies on how the Indian Ocean Dipole (IOD) is defined. Indeed, the definition of historical events varies substantially, resulting in conflicting classifications, hampering hydroclimatic impact assessment of this phenomenon. Therefore, this paper aims to compare the various methods that have previously been adopted to define IOD events and demonstrate an alternative method that specifically captures different "flavours" of IOD. It is shown that the choice of sea surface temperature (SST) data set and climatological baseline has significant impact on the resulting classification of IOD events. Furthermore, not all events display the traditional dipole signature pattern of warming/cooling occurring at opposite poles. Rather, some events could be considered "pseudo dipoles," whereby the SST anomalies in the two poles are not of opposite sign. Importantly, the different types of IOD (i.e., "pseudo and true") are associated with varying impacts on rainfall for surrounding regions. To overcome these issues, this study treats Indian Ocean SST anomalies as a continuum of a finite maximum number of statistically distinguishable patterns using a neural network‐based cluster analysis. Ten individual SST patterns were identified with distinct rainfall teleconnections. In particular, four strong IOD patterns (two positive and two negative) were identified, alongside three pseudo dipoles representing anomalously warm conditions across the entire Indian Ocean Basin, with a SST gradient towards the east (i.e., a pseudo negative dipole) or west (i.e., a pseudo positive dipole). The occurrence of each of the 10 self‐organizing mapping (SOM) patterns is shown to be variable over the past 130 years, with some types more common during certain phases of the El Niño/Southern Oscillation or during the first (or second) half of the 20th century. The implications of these findings are particularly significant for attribution studies, development of proxy climate reconstructions, climate model evaluation and future climate change assessment. The Indian Ocean Dipole (IOD) is known to impact hydroclimate across many Indian Ocean rim counties. However, there is a growing body of research that confirms the IOD is a complex phenomenon with multiple variants, which cannot be captured by a simple index such as the dipole mode index (DMI). This paper uses a novel technique (SOM) to identify 10 individual flavours of the IOD with distinct rainfall teleconnections. Figure shows 10 different Indian Ocean SST "types" derived using SOM. Note that numbers in brackets show the number of occurrences of each type. [ABSTRACT FROM AUTHOR]

Using the monthly reanalysis data sets of National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) for 1961–2015, the inter‐decadal change in inter‐annual water vapour transport over the tropical Indian Ocean–western Pacific during summer and the related mechanisms were analysed in this article. The results show that two major modes of anomalous water vapour transport over the tropical Indian Ocean–western Pacific (60°–140°E, 10°S–30°N) experienced a significant inter‐decadal change in the mid‐1980s. The first mode (EOF1) shows that the anticyclonic water vapour transport over the northwestern Pacific moves significantly southwards after the mid‐1980s. Before the mid‐1980s, the anomalous water vapour originates from the subtropical western Pacific and moves through the South China Sea to eastern China. However, the anomalous water vapour mainly originates from the tropical western Pacific and moves through the South China Sea to eastern China after the mid‐1980s. The second mode (EOF2) reflects the enhanced effect of the anomalous water vapour transport over the tropical Indian Ocean on East Asia after the mid‐1980s. Before the mid‐1980s, the distribution of the water vapour anomalies is uneven over the tropical Indian Ocean and the magnitude of water vapour is relatively small, so it has a weak effect on rainfall over East Asia. After the mid‐1980s, anomalous anticyclonic water vapour transport originates from the tropical Indian Ocean and moves along the Arabia Sea, Indian Peninsula and Indo‐China Peninsula and then forms an anomalous cyclonic water vapour transport over south China. Furthermore, the possible causes of the inter‐decadal transition in anomalous water vapour transport are investigated for the two major modes. The significant inter‐decadal transition in anomalous water vapour transport is associated with the enhanced effect of the sea surface temperature (SST). Before the mid‐1980s, the relationships between the two major modes and the SST of the tropical oceans are relatively weak. After the mid‐1980s, the EOF1 becomes significantly regulated by the El Niño‐Southern Oscillation (ENSO) in the previous winter, and the positive (negative) phase of the principal component of the first mode (PC1) corresponds to significantly positive (negative) SST anomalies over the tropical northern Indian Ocean and Maritime Continent during the same period. For EOF2, the anomalous water vapour transport over the tropical Indian Ocean is related to the enhanced inter‐annual variability of tropical Indian Ocean dipole (TIOD) after the mid‐1980s. [ABSTRACT FROM AUTHOR]

Limited coverage of tide gauge observations and limited length record of altimeter observations remain inadequate for the Indian Ocean sea level variability studies especially for longer periods. This study takes advantage of Ocean General Circulation Model (OGCM) simulations and ocean reanalysis to explore the interannual variability of sea level in the Indian Ocean. The model simulated sea level anomalies display good correlation with altimetry and reanalysis products. This consistency in model simulations with observations and reanalysis highlights the potential use of this OGCM in understanding the sea level variability. Analysis reveals that co-occurrence years (years when Indian Ocean Dipole (IOD) and El Niño events co-occur) contribute significantly towards the interannual variability in the Indian Ocean compared to pure IOD or El Niño events, and are characterized by persistent generation of open ocean downwelling, with maximum signals in the 10° to 5°S region. The OGCM experiments are carried out by modulating the forcing fields over the Pacific and the Indian Ocean to understand the physical mechanisms causing sea level variability. The experiments suggest that the remote forcing from the Pacific Ocean via Indonesian ThroughFlow (ITF) induce significant sea level changes in the southern Indian Ocean only with variability mostly in the decadal time scales and the northern and equatorial Indian Ocean are not affected by ITF. This study also shows that El Niño and IOD-induced variations in zonal wind stress and wind stress curl mainly contribute the interannual sea level variations in the Indian Ocean. The forcing associated with the co-occurrence years are multi-phased with coastal and open ocean forcing. Whereas the pure IOD and El Nino forcing is mainly found only over the open ocean. The decadal variability in the ENSO is primarily responsible for the decadal sea level variability in the Indian Ocean by modulating ITF transport. [ABSTRACT FROM AUTHOR]

ABSTRACT The climate shift in 1976-1977 sets a clear distinction in the teleconnection between Indian summer monsoon rainfall ( ISMR) and sea surface temperatures ( SST) in the tropical oceans, particularly in the tropical Indian Ocean ( TIO). Before the climate shift (Pre-77), the observed correlation between ISMR and SST was positive in the eastern equatorial Indian Ocean ( EIO) and negative correlation prevailed in the western equatorial Indian Ocean ( WIO), whereas after the climate shift (Post-77), the correlation pattern has reversed (i.e. positive correlation in WIO and negative correlation in EIO). The major reason for this shift is the consistent warming of SSTs throughout the tropics and frequent occurrence of IOD events. Further, it is noticed that during the earlier decades, the Indian Ocean was responding to the atmospheric forcing, particularly along the axis of strong cross equatorial flow, whereas in the recent decades, Indian Ocean dynamics are playing a crucial role in determining the SST in major parts of the Indian Ocean particularly in the EIO. Seasonal re-forecasts from six state-of-the-art global coupled atmosphere-ocean climate models which participated in ENSEMBLES faithfully capture the pre-77 dipole structure of correlations. But the post-77 correlation pattern is not simulated by any of the six models considered in this study. Two possible reasons are pointed out for the models' inability to capture the reversal of teleconnection: (1) in the coupled models, unchanged mean state of the winds inhibits ISMR to be governed by coupled dynamics in the TIO; (2) the strong, local SST-rainfall relationship in models even in post-77 period limits the influence of the teleconnections on the ISMR. [ABSTRACT FROM AUTHOR]

This paper presents key results from analysis of surface meteorological observations collected in the Northern Arabian/Persian Gulf (N Gulf; Kuwait, Bahrain, and NE Saudi Arabia), which spans a 40-years period (1973-2012). The first part of this study analyzes climate variability in the N Gulf, and relates them to teleconnection patterns (North Atlantic Oscillation, El Nino Southern Oscillation, and Indian Ocean Dipole). Results of the analysis indicate that during the study period the climate in the region experienced a general trend of increase in temperature (0.8°C), decrease in barometric pressure (1 mbar), reduction in humidity (6%), and decrease in visibility (9%). Significant correlations were found between the three teleconnection patterns and the meteorological conditions suggesting that seasonal variabilities in air temperature, barometric pressure, and precipitation are closely related to the teleconnection patterns. The second part of this study examines the 40-year variability of Shamal events (strong NW winds that commonly generate significant dust storms). The data suggests that on average Shamal events occur at a rate of 10 events year-1 with 85% of the events occurring during the summer and winter. The number of these events has increased in the past 14 years of the study period. These events resulted in abrupt changes in meteorological conditions: an increase in wind speed of 2.7 m s-1, a decrease in visibility of 1.7 km, and reduction in humidity of 4.3%. Seasonal variations in temperature (an increase in temperature during summer of 0.8°C, and a decrease of 1.5°C during winter) and barometric pressure (a decrease in barometric pressure during summer of 0.6 mbar and an increase of 7.8 mbar during winter) were observed during Shamal events. [ABSTRACT FROM AUTHOR]

ABSTRACT Despite earlier studies over various parts of the world including equatorial Eastern Africa (EEA) showing that intraseasonal statistics of wet and dry spells have spatially coherent signals and thus greater predictability potential, no attempts have been made to identify the predictors for these intraseasonal statistics. This study therefore attempts to identify the predictors (with a 1-month lead time) for some of the subregional intraseasonal statistics of wet and dry spells (SRISS) which showed the greatest predictability potential during the short rainfall season over EEA. Correlation analysis between the SRISS and seasonal rainfall totals on one hand and the predefined predictors on the other hand were initially computed and those that were significant at 95% confidence levels retained. To identify additional potential predictors, partial correlation analyses were undertaken between SRISS and large-scale oceanic and atmospheric fields while controlling the effects of the predefined predictors retained earlier. Cross-validated multivariate linear regression (MLR) models were finally developed and their residuals assessed for independence and for normal distribution. Four large-scale oceanic and atmospheric predictors with robust physical/dynamical linkages with SRISS were identified for the first time. The cross-validated MLR models for the SRISS of wet spells and seasonal rainfall totals mainly picked two of these predictors around the Bay of Bengal. The two predictors combined accounted for 39.5% of the magnitude of the SST changes between the July-August and October-November-December periods over the Western Pole of the Indian Ocean Dipole, subsequently impacting EEA rainfall. MLR models were defined yielding cross-validated correlations between observed and predicted values of seasonal totals and number of wet days ranging from 0.60 to 0.75, depending on the subregion. MLR models could not be developed over a few of the subregions suggesting that the local factors could have masked the global and regional signals encompassed in the additional potential predictors. [ABSTRACT FROM AUTHOR]

ABSTRACT The variability of intense tropical cyclone ( TC) in the Southern Hemisphere and its relation to large-scale environmental parameters are examined through temporal and spatial principal component analysis ( PCA). The results show that there was a shift from low intense TC activity during 1976/1977-1987/1988 (period 1) to high activity during 1988/1989-2007/2008 (period 2). Accumulated cyclone energy maps indicate that this shift is related to more occurrences of intense TCs in the southwestern Indian Ocean ( SWIO) and northwestern Australian region during period 2. The spatial-mode PCA identifies distinct sea surface temperature anomaly ( SSTA) patterns dominating in periods 1 and 2. The four dominating patterns in period 1 have low or negative SSTA in the TC development regions (especially over SWIO), whereas the five dominating ones in period 2 have much higher SSTA in the same regions. The temporal variability of the vertical wind shear ( VWS) over the Indian Ocean ( IO) is examined through the temporal-mode PCA. The first mode is El Niño Southern Oscillation ( ENSO)-related, but there is no clear interdecadal variability identified. The second mode shows high VWS during the 1970s to early 1980s that is similar in length to period 1, followed by low VWS afterwards during period 2. This change in VWS thus may be responsible for the shift in intense TC activity over the IO. Linear correlative analysis shows that the mode of variability in VWS is significantly related to subtropical dipole events in the IO. Potential linkages of these findings with ENSO and Interdecadal Pacific Oscillation are discussed. [ABSTRACT FROM AUTHOR]

ABSTRACT El Niño/La Niña Southern Oscillation ( ENSO), Indian Ocean Dipole ( IOD) and aerosols have important roles to play in relation to Southeast Asian rainfall anomalies. This study investigates connections between these factors and local seasonal rainfall anomalies at 17 locations throughout Thailand using ground measurements and satellite observations available from 1980 to October 2011. This research investigates further the usefulness of incorporating these factors into rainfall forecasts. Results from canonical correlation analysis indicate that strong ENSO signals from October to March may affect rainfall anomalies in the nonsummer monsoon months, whereas IOD signals in the previous summer monsoon months were deemed to be of greater responsibility for current summer monsoon rainfall. Comparison of rainfall levels with different aerosol loadings suggests rainfall-decreasing aerosol from December to February for the South East Coast and in pre-summer monsoon months for deeper inland areas having less coastal influences. Rainfall-increasing aerosol was observed in post-summer monsoon months at the inland stations. The use of ENSO/ IOD signals as predictors yielded better rainfall forecast model performance than that of the aerosol loading. However, model performance significantly improved when both ENSO/ IOD signals and seasonal aerosol loadings were introduced. [ABSTRACT FROM AUTHOR]

A highlight of the 2006 boreal summer monsoon season was the enhanced activity of long-lived monsoon depressions and low-pressure systems (LPS) over the Indian region. Another important phenomenon during this period was the evolution of a positive Indian Ocean Dipole (PIOD) event. Although previous studies have investigated the impact of PIOD on the large-scale monsoon response, their influence on monsoon LPS activity is not well understood. Based on detailed diagnostic analyses of monsoon LPS during 2006, as well as those associated with other PIOD events during 1958-2007, the present work addresses two specific issues concerning the roles of (a) PIOD-induced large-scale circulation changes and (b) internal feedbacks between latent heating and dynamics, in sustaining the monsoon LPS activity. The results show that PIOD conditions generally favour increased propensity of long-lived (>5 days) LPS with long westward tracks extending into northwest India. The average contribution of long-lived monsoon LPS to the total is found to be approximately 12% higher during PIOD episodes as compared to non-PIOD. The PIOD events showed two important large-scale elements conducive for enhancement of LPS activity: (a) strengthening of cross-equatorial moisture transport from south-eastern tropical Indian Ocean into the Bay of Bengal and (b) enrichment of barotropic instability of monsoon flow. Estimates of latent-heating profiles from TRMM-satellite products during the 2006 LPS showed heating in the mesoscale updrafts above 600 hPa with maximum approximately 400 hPa; while cooling prevailed in lower levels. Stratiform precipitation covered approximately 70-85% of rain area during the prolonged LPS; and the large-scale monsoon Hadley-type circulation was found to be intensified with strong mid-level inflows entering the stratiform rain region. The overall findings suggest that the PIOD-induced background circulation together with internal feedbacks between mesoscale convective systems and large-scale circulation can effectively enhance the longevity of monsoon LPS. These results should serve as important inputs for numerical weather forecasting of extreme rainfall events associated with the regional monsoon phenomenon. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article presents a study on the atmospheric balance over South Peninsular India and Sri Lanka during the northeast monsoon season. The study uses computed moisture convergence (C) and residual evaporation (E) from the National Centers for Environment Prediction or National Center for Atmospheric Research (NCEP/NCAR) reanalysis data and Global Precipitation and Climatology Project (GPCP) precipitation data. It discusses the spatial and temporal characteristics of the hydrological cycle and the contribution of evaporation (E) dominates during the entire monsoon months. It notes that the positive northeast monsoon rainfall (NEMR) is associated with El Niño coupled with Indian Ocean dipole, but negative NEMR is associated with only La Niña.

The article explores the spatiotemporal features of the Indian Ocean Dipole (IOD) on the sea surface and the subsurface ocean. It was found that the previous positive IOD is associated with the normal South China Sea summer monsoon. Also, a stronger 100-hPa South Asian High is induced by a positive IOD in the summer after an IOD year. Summer rainfall in China is said to exhibit various patterns corresponding to different phases of the IOD, as influenced by the 850-hPa Asian monsoon circulations, as well as the position of the 500-hPa subtropical high ridge.

The article presents a study on the level and source of predictability of seasonal rainfall anomalies in Malaysia using canonical correlation analysis. It states that this research examines the level and origin of seasonal forecast skills of rainfall anomalies in Malaysia. It is also mentioned that the forecast models are grounded on an empirical technique addressed as the canonical correlation analysis (CCA). Moreover, the station rainfall in preceding seasons, local sea surface temperature (SST) over the western Pacific sector, quasi-global SST, and northern hemisphere 700 hPa geopotential height are also investigated.

The article presents a research that evaluates the significance of combining an index of Indian Ocean sea surface temperature (SST) and an index of equatorial Pacific Ocean SST to obtain a model of Sept-Dec drought (PDSI) in Java, Indonesia from 1997-1998. In the said period, the country experienced severe droughts which have long been linked with El Niño events that caused major food shortages. Researchers noted that the combined models can be used to generate warning forecasts of drought and crop failure risk across the region, particularly the western part which has been influenced by the Indian Ocean's climate variability.

The article observes basin-wide wintertime surface warming in the Indian Ocean during El Nino years. The basin-wide warming is found to be stronger when El Nino and Indian Ocean Dipole co-occur. The mechanisms responsible for the basin-wide warming are different for the years with El Nino only and for the co-occurrence years. Strong westward propagation of downwelling Rossby waves is observed in the Southern Indian Ocean during the Indian Ocean Dipole years. It suggests that the variability related to surface circulation is due to the local Indian Ocean Dipole forcing rather than El Nino induced wind anomaly.