Subtropical Indian Ocean Dipole Research Articles

The influence of tropical and subtropical modes of climate variability on precipitation in Mozambique

AbstractThis 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.

Tropical ocean teleconnections with gross primary productivity of monsoon-Asia

The intricate oceanic climate interactions with terrestrial primary production of Asian ecosystems exert crucial social-economical-environmental repercussions. Yet, a holistic understanding of tropical sea surface temperature (SST) anomalies associated with the gross primary productivity (GPP) variations of monsoon-Asia remains constrained. This study provides a statistical framework demonstrating how SST perturbations in the tropics influence GPP fluctuations in monsoon-Asia by modulating hydrothermal conditions of different climate system components. Observation evidence explicitly illustrated the characteristic anomalous SST signatures of positive and negative GPP anomalies in South and Southeast Asia during June–August. The SST anomalies of the central-eastern tropical Pacific showed a robust negative impact on the GPP variability of South-Asia. The GPP alterations in maritime-Southeast-Asia exhibited strong connections with SST anomalies of the western Pacific (positive) and eastern equatorial Pacific (negative). The oceanic signals in the GPP variability of South-Asia and maritime-Southeast-Asia mirrored canonical El Niño and La Niña patterns. The detected SST–GPP link is feasible through large-scale atmospheric circulation variability and the consequent regional modulation of heat and moisture fluxes. The anomalous strengthening (weakening) of Walker cell enhances (reduces) water availability to plants for photosynthesis during the La Niña (El Niño) phase of the ENSO cycle and thus elevates (lowers) GPP in South-Asia and Maritime-southeast-Asia. In contrast, the enhanced GPP anomaly in mainland-Southeast-Asia depicts signs of canonical La Niña and Indian Ocean subtropical dipole (IOSD) teleconnections. The positive impact of IOSD was through the modulation of the Mascarene High and the consequent impact on the monsoon. Meanwhile, decreased GPP bears the imprint of El Niño Modoki and warm tropical Indian Ocean SSTs. The atmospheric teleconnections demonstrated the delayed impact of El Niño Modoki on GPP variability through the Indian Ocean capacitor effect. Our findings could be instrumental in forecasting the probable effects on vegetation growth in monsoon-Asia associated with high-frequency tropical oceanic changes.

The strengthened role of new predictors of Indian Ocean Dipole (IOD) during the recent decades of weakened ENSO-IOD relationship

Indian Ocean Dipole (IOD) is the major interannual ocean-atmosphere interaction phenomena in the Indian Ocean (IO) and is influenced by external forcing such as El Nino Southern Oscillation (ENSO). Meanwhile, its co-occurrence and stronger relationship with ENSO have decreased during recent decades. The IOD variability also reduced after 2000, accompanied by a shift of the western pole to central IO extending further southward. The study reports that the boreal fall (SON, September, October, November) season IOD has an intensified relationship with the previous winter Subtropical Indian Ocean dipole (SIOD) and spring season equatorial north tropical Atlantic (NTA) SST anomalies., while the ENSO relationship is reduced from its pre-2000 value. It is found that the persistent warming (cooling) in the western side of positive (negative) SIOD during the previous winter induces easterly (westerly) wind anomalies in the equatorial IO during the following summer and fall, leading to positive (negative) IOD events. These IOD events have the western pole shifted to south-central IO instead of the canonical northwestern warming. The spring season NTA SST anomalies induce stronger summer season circulation and SST gradient in the equatorial Pacific, like ENSO. During the SON season, this pattern is associated with cooling and easterly wind anomalies in the tropical eastern IO and IOD. These two patterns explain the major mode of IOD variability after 2000. While the IOD associated with NTA have co-occurring ENSO during summer and fall, the SIOD-induced IOD events are independent of ENSO in the Pacific. Thus, these two predictors provide long-lead predictability (2–3 seasons ahead) of IOD for both ENSO co-occurring and non-ENSO IOD events. However, the IOD predictability of seasonal prediction models mainly depends on the ENSO-IOD relationship, resulting in reduced IOD skills for many of them after 2000. The models such as COLA-CCSM4, which has improved skill have stronger than observed ENSO-IOD relationship than the pre-2000 period. A linear regression model including SIOD and NTA SST indices of the previous winter and spring season respectively as predictors simulates IOD with a skill of around 0.65 during the recent period, indicating the necessity of seasonal prediction models to capture the variability in the southern IO and NTA and their teleconnections for better prediction of IOD in the recent period of reduced ENSO skill.

Impact of climatic oscillations on marlin catch rates of Taiwanese long-line vessels in the Indian Ocean

This study explored the influence of climatic oscillations on the striped, blue, and silver marlin catch rates in the Indian Ocean by using logbook data from Taiwanese large-scale fishing vessels and climate records from 1994 to 2016. Only the Madden–Julian oscillation (MJO) and the subtropical Indian Ocean dipole (SIOD) had immediate effects on the striped and silver marlin catch rates. The positive and negative phases of the IOD at the lags of 7 and 3 years corresponded to increased and decreased catch rates, respectively, for both the striped and blue marlin, contrasting to the reverse pattern for the silver marlin. Similarly, all three marlin species experienced decreased and increased catch rates respectively during the positive and negative phases of the Pacific decadal oscillation. The striped and blue marlin catch rates decreased and increased during the positive and negative phases, respectively, of the SIOD and MJO with various lags. Our results suggest that the impacts of climatic oscillations on fish species are crucial for policymakers and coastal communities for managing marine resources, forecasting changes in marine ecosystems, and developing strategies to adapt to and mitigate the effects of climate variability.

Exploring non-linear modes of the subtropical Indian Ocean Dipole using autoencoder neural networks

The subtropical Indian Ocean Dipole (SIOD) significantly influences climate variability, predominantly within parts of the Southern Hemisphere. This study applies an autoencoder—a type of artificial neural network (ANN)—known for its ability to capture intricate non-linear relationships in data through the process of encoding and decoding—to analyze the spatiotemporal characteristics of the SIOD. The encoded SIOD pattern(s) is compared to the conventional definition of the SIOD, calculated as the sea surface temperature (SST) anomaly difference between the western and eastern subtropical Indian Ocean. The analysis reveals two encoded patterns consistent with the conventional SIOD structure, predominantly represented by the SST dipole pattern south of Madagascar and off Australia’s west coast. During different analysis periods, distinct variability in the global SST patterns associated with the SIOD was observed. This variability underscores the SIOD’s dynamic nature and the challenges of accurately defining modes of variability with limited records. One of the ANN patterns has a substantial congruence match of 0.92 with the conventional SIOD pattern, while the other represents an alternate non-linear pattern within the SIOD. This implies the potential existence of additional non-linear SIOD patterns in the subtropical Indian Ocean, complementing the traditional model. When global temperature and precipitation are regressed onto the ANN temporal patterns and the conventional SIOD index, both appear to be associated with anomalous climate conditions over parts of Australia, with several other consistent global impacts. Nevertheless, due to the non-linear nature of the ANN patterns, their effects on local temperature and precipitation vary across different regions as compared to the conventional SIOD index. This study highlights that while the conventional SIOD pattern is consistent with the ANN-derived SIOD pattern, the climate system’s complexity and non-linearity might require ANN modeling to advance our comprehension of climatic modes.

Paradigm shift in the onset phase of the Indian Summer Monsoon since 2000 and its potential connection to South Indian Ocean

The Indian Ocean basin has undergone significant changes over the past two decades, and this study examines major shifts in the onset characteristics of the Indian Summer Monsoon (ISM) in response to changes in the dynamical and thermodynamic conditions. A thermodynamic index shows that the post-2000 period witnesses an approximately 3-day earlier (May 27) onset of ISM than the pre-2000 period. In the absence of an abrupt and distinct change from the dry to the wet season, the post-2000 era can be distinguished by its early and longer onset process. The Subtropical Indian Ocean Dipole (SIOD) undergoes a phase change during its evolution, which weakens the Mascarene High (MH) and hence the pressure gradient. The weakening of the MH along with the heterogeneity in Sea Surface Temperature (SST) trends have led to a weaker meridional pressure gradient over the Indian monsoon domain, resulting in a weak onset of the ISM. Additionally, warming in the Arabian Sea starts earlier than expected. The prolonged duration of the Indian Summer Monsoon (ISM) onset process in the post-2000 era can be attributed to several factors. These include the broader ascending limb of the Hadley cell over the south equatorial Indian Ocean, reduced wind shear, and the early development of a warmer Arabian Sea prior to the onset of the monsoon.

Antarctic sea ice variability linked to three dipole modes of subtropical sea surface temperature in the Southern Hemisphere

AbstractBased on statistical analyses of gridded data over the past four decades (1979–2020), we examined the teleconnection between the variability of the Antarctic sea ice and the leading modes of subtropical sea surface temperature (SST) variability known as the Indian Ocean subtropical dipole (IOSD), the South Atlantic subtropical dipole (SASD) and the South Pacific subtropical dipole (SPSD). We show that the pattern and strength of the teleconnection differ with the SST modes. For each mode, while the regional distributions of significant Antarctic sea ice anomalies are broadly similar throughout the year, the areal extent and the magnitudes of the anomalies display a strong seasonality. Larger areas and magnitudes of significant sea ice anomalies occur in austral winter for the SASD, autumn and spring for the SPSD and non‐summer seasons for the IOSD. We demonstrate that the spatial and seasonal variations of the sea ice anomalies associated with each of the three subtropical SST variability modes are largely consistent with the patterns of the anomalous temperature advection and sea ice transport by the anomalous atmospheric circulations induced by planetary wavetrains that are triggered by anomalous convective activities over different regions of subtropical oceans. These relationships between subtropical SST modes and Antarctic sea ice may serve as a valuable reference for predicting seasonal to interannual scale variations of Antarctic sea ice concentrations across each austral season.

Synergistic effect of El Niño Southern oscillation and subtropical Indian Ocean Dipole on Southern China winter precipitation

Both the El Niño Southern Oscillation (ENSO) and subtropical Indian Ocean Dipole (SIOD) have significant impacts on the winter precipitation over southern China (SC). In this study, we investigate the synergistic effect of ENSO and SIOD on southern China winter precipitation (SCWP). Our findings indicate that SCWP is closely associated with the out-of-phase configurations of ENSO and SIOD, with precipitation tending to increase (decrease) during El Niño and negative SIOD (La Niña and positive SIOD). During El Niño and negative SIOD, the El Niño leads to an anomalous anticyclone over the northwest Pacific through Walker circulation and Gill-type tropical response. The associated southwesterly winds of the anticyclone transport ample water vapor towards SC. The El Niño and negative SIOD both trigger a wave-like pattern from Europe to SC. This wave train (PC1-like wave train) propagates towards SC and induces anomalous cyclonic circulation and strong upward motion over SC, resulting in enhanced precipitation over SC. During La Niña and positive SIOD, the low-level cyclone over the northwest Pacific results from both La Niña and positive SIOD. The positive SIOD enhances La Niña-related cyclonic circulation response through cross-equator Hadley circulation, weakening the water vapor transport towards SC. The positive SIOD-related wave train (PC3-like wave train) plays the dominant role in inducing anomalous anticyclonic circulation and weak descending motion, leading to reduced precipitation over SC.

Austral summer droughts and their driving mechanisms in observations and present‐day climate simulations over Malawi

AbstractDroughts are a key feature of weather systems over Malawi and southern Africa. Their societal relevance in Malawi underscores the need for improved understanding of drought characteristics and atmospheric processes that drive them. We use the Standardized Precipitation and Evapotranspiration Index with the run theory to identify and characterize droughts in observations (1961–2017) and CMIP5 models across Malawi. We find no major differences in drought duration, severity, and intensity between the northern and southern parts of Malawi. However, circulation patterns associated with droughts in the two regions are different, and typically organized in such a way that droughts in one region coincide with wetter conditions in the other. Anomalous circulation patterns diminishing moisture convergence and convection over the affected region are a typical feature of summer droughts. We show that precipitation variability is principally governed by advected moisture, transported via three main tracks of northwesterly, northeasterly, and southeasterly moisture fluxes. The three tracks interact to form a convergence zone with a peak situated over Malawi. Variability in the respective moisture flux tracks influences the variability in the location and intensity of the convergence zone, and thus the location of the ensuing drought. We note links between variability in the moisture advection tracks and El Nino Southern Oscillations and other modes of variability including the Indian Ocean Dipole and Subtropical Indian Ocean Dipole. Both negative and positive biases in drought frequency are apparent in CMIP5 models but the majority overestimate drought duration and severity. The relationship between precipitation and net total moisture flux is consistently simulated. However, we note significant model inconsistencies in the relationship between precipitation and moisture flux from the southeasterly track, which potentially undermines the confidence in model simulation of drought processes over Malawi.

A new global coupled atmosphere-ocean wavenumber-4 pattern in Southern mid–latitudes during austral summer and its climate linkage

A new pattern referred to as the coupled ocean-atmosphere wavenumber-4 (COAW4) pattern has been identified in the Southern mid-latitudes during austral summer. This pattern consists of a wavenumber-4 pattern in the anomalous 200-hPa meridional wind and a wavenumber-4 pattern of the anomalous sea surface temperature (SST). The SST component of the COAW4 pattern, which includes the Indian Ocean Subtropical Dipole (IOSD), the South Atlantic Subtropical Dipole (SASD) and the South Pacific Subtropical Dipole (SPSD), develops through air-sea interactions involving surface turbulent heat flux, surface wind, and ocean mixed-layer depth. The atmospheric component displays a barotropic structure. The sources for COAW4 pattern include not only the wavenumber-4 pattern of the anomalous SST over Southern oceans, but also the anomalous convective activities over Southern land areas. The COAW4 pattern is associated with increased precipitation over land in southern mid-latitudes, particularly in central Australia. Furthermore, the COAW4 pattern is linked to Antarctic sea ice cover in the Weddell, Amundsen, and northern Ross Seas. Plain language summary In the southern mid-latitudes, there exists a wavenumber 4 pattern in the anomalous 200-hPa meridional wind and a wavenumber-4 pattern of the anomalous sea surface temperature (SST). Previous studies have examined the two wavenumber-4 patterns separately, but their potential linkage remains largely unknown. Through statistical analyses of SST and atmospheric global data, we demonstrated that the two wavenumber 4 patterns could be integrated into a coupled ocean-atmosphere wavenumber-4 (COAW4) pattern in Southern mid-latitudes in austral summer. Additionally, we have investigated the sources of this pattern as well as its effect on precipitation over southern lands and on Antarctic sea ice.

A change in the relation between the Subtropical Indian Ocean Dipole and the South Atlantic Ocean Dipole indices in the past four decades

Abstract. We utilized the global atmospheric reanalysis ERA5 and reconstructed sea surface temperature (SST) data from 1979 through 2020 to examine the stability of the relationship between the SST oscillations in the southern Indian Ocean and the Atlantic Ocean, as described by the Subtropical Indian Ocean Dipole (SIOD) and South Atlantic Ocean Dipole (SAOD) indices, respectively. We note a significant positive correlation between the two indices prior to the year 2000 but practically no correlation afterwards. We show that in the two decades prior to 2000, a positive phase of the SAOD is associated with more convective activities over the subtropical southern Atlantic Ocean and eastern Brazil, which trigger a stronger upper-atmosphere wavetrain. This produces stronger southern subtropical highs and surface anti-cyclonic circulations and therefore a stronger correlation between the two indices. The situation is reversed after 2000. Our results are potentially applicable to predictions of precipitation in southern Africa and South America.

Circulation Patterns Linked to the Positive Sub-Tropical Indian Ocean Dipole

The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. Among the four CTs, the CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; and (iii) low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, over the south-central landmass of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT related to the positive SIOD and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence its rainfall variability. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall over the preferred landmass through enhanced moisture convergence.

How can the positive Indian Ocean Dipole events co‐occur with La Niña?

AbstractPositive/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.

Possible impacts of spring subtropical Indian Ocean Dipole on the summer tropical cyclone genesis frequency over the western North Pacific

AbstractThe linkage between the spring subtropical Indian Ocean Dipole (SIOD) events and tropical cyclone genesis frequency over the western North Pacific (WNP) in the following summer is investigated in this study. It is found that their correlation is positive and statistically significant with a greater number of TCs genesis over the WNP in summer of a positive SIOD year. After removing the ENSO effects, diagnostic analysis reveals that under positive SIOD conditions the summertime WNP monsoon trough becomes intensified. Correspondingly, there are enhanced lower‐level vorticity, reduced vertical zonal wind shear, increased middle‐level relative humidity and vertical velocity over the WNP, conductive to generation of more TCs. Further examination indicates that the Mascarene high and Australian high are strengthened successively from spring to subsequent summer in a positive SIOD year, contributing to the maintenance of SIOD‐related SST dipole anomalies in southern Indian Ocean due to SST–wind–evaporation feedback mechanism. The resulting cyclonic anomaly is formed at lower troposphere over the WNP induced by the prominently enhanced cross‐equator airflows and the related Ekman upwelling. At upper troposphere, there tend to be remarkably WNP divergence anomalies of the interhemispheric vertical circulation forced by the summertime SST patterns closely associated with positive SIOD cases. By contrast, the monsoon trough is weakened and the dynamic and thermodynamic conditions are unfavourable for the WNP TC genesis in summer of a negative SIOD year, in agreement with a lower‐level anticyclonic anomaly and suppressed deep convection over the WNP forced by the SIOD‐related SST anomalies.

Eastward Shift of Interannual Climate Variability in the South Indian Ocean since 1950

Abstract The subtropical Indian Ocean dipole (SIOD) and Ningaloo Niño are the two dominant modes of interannual climate variability in the subtropical south Indian Ocean. Observations show that the SIOD has been weakening in the recent decades, while Ningaloo Niño has been strengthening. In this study, we investigate the causes for such changes by analyzing climate model experiments using the NCAR Community Earth System Model, version 1 (CESM1). Ensemble-mean results from CESM1 large-ensemble (CESM1-LE) show that the external forcing causes negligible changes in the amplitudes of the SIOD and Ningaloo Niño, suggesting a dominant role of internal climate variability. Meanwhile, results from CESM1 pacemaker experiments reveal that the observed changes in the two climate modes cannot be attributed to the effect of sea surface temperature anomalies (SSTA) in either the eastern tropical Pacific Ocean or tropical Indian Ocean. By further comparing different ensemble members from the CESM1-LE, we find that a warm pool dipole mode of decadal variability, with opposite SSTA in the southeast Indian Ocean and the western-central tropical Pacific Ocean plays an important role in driving the observed changes in the SIOD and Ningaloo Niño. These changes in the two climate modes have considerable impacts on precipitation and sea level variabilities in the south Indian Ocean region.

Strengthened Relationship Between Tropical Indian Ocean Dipole and Subtropical Indian Ocean Dipole After the Late 2000s

AbstractThis study identifies a strengthened relationship between the subtropical Indian Ocean dipole (SIOD) during January–February–March (JFM) and the following autumn Indian Ocean dipole (IOD) after the late 2000s. Following the positive SIOD, the northwesterly wind anomalies help to maintain positive sea surface temperature (SST) anomalies in the southwestern tropical Indian Ocean by wind‐evaporation‐SST (WES) feedback during June–July–August (JJA). The anomalous northwesterly winds and the sustained positive SST anomalies promote the development of the zonal wavenumber‐3 pattern (ZW3P) and induce low‐level convergence in the mid‐latitude central southern Indian Ocean, leading to the anticyclone circulation over the northern Indian Ocean by the local Hadley circulation during JJA. The accompanying central equatorial easterly wind anomalies appear in JJA, which is the key to trigger a positive IOD event through Bjerknes feedback. This mechanism for the enhanced SIOD‐IOD relationship is independent from the El Niño‐Southern Oscillation (ENSO).

Role of unusual moisture transport across Equatorial Indian Ocean on the extreme rainfall event during Kerala flood 2018

Southwestern Indian state, Kerala experienced extreme devastating statewide flood event of the century during 2018 monsoon season. In this study, an attempt has been made to bring out the salient dynamical factors contributed to the Kerala flood. There were 3 active spells over Kerala during 2018 Monsoon season. All the three spells were accustomed with the intrinsic factors of low frequency components of the active spells such as strength of monsoon Low Level Jet (LLJ), Monsoon depressions in the Bay of Bengal, favorable Madden-Julian oscillation (MJO) phases and Western Pacific systems. Though all the common ingredients remain same, the third spell is distinct with the less evaporation flux over Western and Central Arabian Sea and unusual moisture transport from maritime continent through South Equatorial Indian ocean (SEIO) towards the Kerala coast across Equator. Strong meridional pressure gradient force created by the combined effect of high pressure anomaly oriented Northwest-Southeast direction across tropical Indian ocean and anomalous low pressure along monsoon trough might have contributed to this unusual moisture transport across SEIO originating from west of Australia. The anomalous high pressure in South Indian ocean was greatly influenced by the position of the Mascarene high. Subtropical Indian ocean dipole (SIOD) also exhibits an influential role by altering tropical Southern Indian ocean dynamics in favor of the unusual moisture transport. The position of the monsoon depression and presence of typhoons in Western Pacific might have aided to this moisture transport. However, the normal moisture transport from Central Arabian sea towards Kerala coast by virtue of the strong LLJ along with additional moisture transport directly from South of maritime continent through SEIO across the Equator towards Kerala coast might have played a dominant role in the historical flood event over entire Kerala state.

Towards using climate to increase lead time of a malaria early warning system in Mozambique

BackgroundMalaria is one of the greatest recurring threats to public health in Mozambique with approximately 10 million cases and thousands of deaths reported annually. Although a malaria early warning system is being established in the country, it is focused on short-term (4–8 week) prediction windows. Increased understanding of the links between quasi-predictable interannual climate variability and malaria could lengthen the lead time for the malaria early warning system and enhance planning and actionable mitigation efforts by public health officials in the country. MethodsTo identify patterns of interannual variability, we did two principal component analyses of processed weekly district-level malaria incidences for the period 2010–17 in Mozambique and southern Africa regional precipitation for the period 1981–2019. We also did linear regression analyses of sea surface temperatures onto a precipitation index, and composites of various climate variables to establish links between precipitation and modes of climate variability. FindingsTwo dominant spatiotemporal patterns collectively account for 81% of the interannual variability of malaria in Mozambique. These patterns consisted of a primary hotspot over the central and southern part of the country and a secondary hotspot over the northern third of the country. We found these patterns to be closely related to precipitation variability driven by climate phenomena: the El Niño-Southern Oscillation and the Subtropical Indian Ocean Dipole. InterpretationQuasi-predictable global and regional climate phenomena influence regional precipitation over southern Africa. Knowledge of these associations can inform an enhancement of the malaria early warning system in Mozambique by lengthening early warning lead times. Similar analyses are possible for other countries whose climate is dominated by tropical modes of climate variability. FundingNone.

The unique mean seasonal cycle in the Indian Ocean anchors its various air-sea coupled modes across the basin

The interannual variability of the sea surface temperature (SST) in the Indian Ocean is complex and characterized by various air-sea coupled modes, which occur around El Niño/La Niña's peak phase (i.e. December–January–February, DJF). Indian Ocean Dipole Mode (IODM) develops over the tropical Indian Ocean and peaks in September–October–November (SON), while Ningaloo Niño, Subtropical Indian Ocean Dipole (SIOD) and Indian Ocean Basin Mode (IOBM) occur respectively over northwest off Australia, subtropical and tropical Indian Ocean, during boreal winter to spring. The apparent contrast between their divergent regionality and convergent seasonality around DJF triggers the present study to examine the interaction between the local mean monsoonal cycle and the anomalous forcing from El Niño/La Niña. The diagnosis confirms that the Indian Ocean’s unique complexity, including the monsoonal circulation over the tropics and the trade wind over the subtropical southern Indian Ocean, plays the fundamental role in anchoring the various regional air-sea coupled modes across the basin. The SST anomalies can be readily explained by the wind-evaporation-SST (WES) mechanism, which works together with other more regional-dependent dynamic and thermodynamic mechanisms. This implies that El Niño/La Niña brings much predictability for the Indian Ocean variations.

Dry Spells, Wet Days, and Their Trends Across Southern Africa During the Summer Rainy Season

AbstractDry spell frequency and wet day occurrence during the summer rainy season across southern Africa are examined using daily rainfall for 1982–2019. Evidence shows two sharp gradients in dry spell frequency, extending southeast from southern Angola to the African south coast near the westernmost boundary of preferred cloud band occurrences (diagonal gradient), and west from the Limpopo River Valley along 22–24°S (meridional gradient) (along 14–16°S in early summer) related to regional topography. Trend analysis suggests that the diagonal gradient has weakened during 1982–2019 along with some other large areas in southern Africa whereas this is only true for part of the meridional gradient. Contrastingly, wet days and Normalized Difference Vegetation Index have significantly increased in several important agricultural areas in southern Africa. Evidence suggests these changes are related to those in the midlevel Botswana High, El Niño Southern Oscillation, Southern Annular Mode, and subtropical Indian Ocean dipole.