Interannual variability of the tropical Indian Ocean: impact of the tropical Atlantic

2009/2010La variabilita' interannuale dell'Oceano Indiano (OI), con particolare attenzione agli effetti delle teleconnessioni dall'Atlantico SubTropicale (AST), e' stata studiata tramite l'utilizzo sia del modello oceanico regionale Regional Ocean Modeling System (ROMS) che con i risultati tratti da cinque diversi modelli accoppiati di circolazione generale (MACGs) che fanno parte del progetto CMIP3 (Coupled Model Intercomaprison Project version 3). Il modello ROMS e' capace di riprodurre fedelmente molti aspetti fondamentali della variabilita' del OI e si e' dimostrato che la variabilita' interna del OI ha un ruolo importante lungo l'Equatore e nella parte occidentale del OI, e che la variabilita' interna contribuisce significativamente alla variabilita' annuale ed interannuale del bacino. Inoltre, l'analisi della Temperatura Oceanica Superficiale (TOS) di una simulazione forzata mensilmente per un periodo di 24 anni ha rivelato che la variabilita' interannuale e' principalmente determinata da due fenomeni: il Nino - Southern Oscillation (ENSO), che e' un forzante esterno al bacino, e il modo Dipolo del OI (DOI), che puo' essere determinato sia da forzanti interni che esterni. L'OI tropicale si surriscalda gradualmente nel corso di un anno che vede la presenza di ENSO. Al contrario, il periodo in cui il DOI e' presente, e' caratterizzato da un gradiente zonale di temperatura nella parte tropicale del OI. Inoltre, studi recenti hanno confermato che l'ENSO e il DOI non sono i soli ad avere una particolare influenza sulla variabilita' del OI, ma che anche le anomalie di TOS nel AST modulano sia la variabilita' interannuale delle piogge dei monsoni africani e indiani sia la stessa TOS del OI. Questa teleconnessione puo' essere spiegata fisicamente dal meccanismo di Gill-Matsuno. In questo studio, e per la prima volta, si e' utilizzato il modello ROMS accoppiato al modello di ecosistema NPZD (Nutriente-Fitoplancton-Zooplancton-Detrito) per analizzare l'effetto delle anomalie di TOS dell'Atlantico tropicale sulla variabilita' della fisica e degli ecosistemi del OI. Anomalie fredde (calde) di TOS nella zona dell'Atlantico tropicale scaturiscono un rafforzamento (indebolimento) del jet Somalo, generando quindi anomalie fredde (calde) della TOS nella parte Nord del OI durante l'estate boreale. In generale, i risultati di questo studio confermano l'effetto del ATS sul OI precedentemente identificati attraverso simulazioni idealizzate con modelli di circolazione generale e dati osservazionali. Simultaneamente alle anomalie ti TOS si osservano cambi nella profondita' del termoclino dovuti a venti che favoreggiano l'affioramento o sprofondamento delle acque nell'area del jet di Findlater, la regione di Sri-Lanka e la parte occidentale della baia di Bengal. La diminuzione (incremento) della profondita' del termoclino e' accompagnata da un aumento (diminuzione) della concentrazione di fitoplancton in superficie. Abbiamo inoltre studiato la rappresentazione della teleconnessione ATS-OI nei MACGs, riscontrando che quattro dei cinque modelli simulano una teleconnessione fra il ATS e il OI piu' debole di quella osservata. Abbiamo dimostrato grazie ad una serie di simulazioni con un modello atmosferico di circolazione globale che le differenze in amplitudine e forma sono dovute a forti errori di rappresentazione dell'Atlantico tropicale e della sua variabilita' nei MACGs. Inoltre il segnale nei modelli e' ulteriormente ridotto dovuto alle diverse parametrizzazione di processi fisici non risolti.The interannual variability of the Indian Ocean (IO), with a particular focus on the effect of the South Tropical Atlantic (STA) teleconnection is investigated using both Regional Ocean Modeling System (ROMS), and output from five different models chosen within the CMIP3 (Coupled Model Intercomparison Project version 3) ensemble of coupled general circulation models (CGCMs). ROMS successfully reproduces many fundamental characteristics of the IO variability. It is found that the IO internal variability plays an important role along the equator and in the western IO. The internal variability also contributes significantly to the annual and interannual variability of the basin. Furthermore, an analysis of the Sea Surface Temperature (SST) from a 24-year monthly-forced model run suggests that the prominent phenomena affecting the interannual variability of the Indian Ocean are the El-Nino Southern Oscillation (ENSO), which is a remote forcing to the basin and the Indian Ocean Dipole (IOD) mode, which is caused by both internal and remote forcing. During an ENSO year, the tropical IO gradually warms. Whereas, IOD is characterized by a zonal temperature gradient in the tropical IO. Moreover, a series of recent papers showed that not only the ENSO and IOD affect the Indian Ocean variability but also SST anomalies in the STA modulate the interannual variability of the African and Indian monsoon rainfall, as well as the IO SST. Physically, such a teleconnection can be explained by a simple Gill-Matsuno mechanism. In this work, we used ROMS coupled with the NPZD (Nutrient-Phytoplankton-Zooplankton-Detritus) ecosystem model to analyze for the first time the effect of the tropical Atlantic SSTs anomaly on the IO physics and ecosystem variability. A cold (warm) SST anomaly in the tropical Atlantic area triggers a strengthening (weakening) of the Somali jet and therefore cold (warm) SST anomaly in the northern IO during boreal summer. Overall the response found in this study confirms the STA effect onto the IO identified in previous studies using idealized experiments with an atmospheric General Circulation Model and observational data. Along with the SST anomaly, changes in thermocline depth due to the upwelling/downwelling favorable winds are seen in the Findlater jet area, the Sri-Lankan region and western Bay of Bengal. The shoaling (deepening) of the thermocline is accompanied by an increase (decrease) in phytoplankton concentration at the surface. An investigation of how CGCMs represent the STA-IO teleconnection is also carried out. Four out of the five models display a teleconnection between STA and the Indian region which is generally weaker than in the observations. With a suite of atmospheric-only GCM integrations, it is shown that the differences in amplitude and pattern are due to strong biases and reduced variabilities of the CGCMs over the tropical Atlantic. In addition, different physical parameterizations used in the models contribute to the weakening of the signal.XXIII Cicl

Representation of the Mozambique channel trough and its link to southern African rainfall in CMIP6 models

The topography of Madagascar and the strength of the Mozambique Channel Trough (MCT) modulate summer rainfall over southern Africa. A strong MCT hinders the penetration of moisture bearing easterlies from the South Indian Ocean into the mainland, thus reducing rainfall there and vice versa for weak MCT summers. Given the link between the MCT and rainfall, it is important to analyse how climate models represent the trough. Here, output from 20 models within the CMIP6 ensemble of Coupled General Circulation Models (CGCMs) are analyzed to investigate how state-of-the-art CGCMs represent the MCT and its link to southern African rainfall. Overall, the ensemble mean insignificantly underestimates the observed MCT. There is a large spread among the models, with the strength of the MCT significantly correlated with the Froude number based on the mountain height over Madagascar. In models, the vorticity tendency in the MCT area is dominated by the stretching and friction terms, whereas the vertical advection, tilting and residual terms dominate in the ERA5 reanalysis. The link between MCT and rainfall in the southern African subcontinent is missing in the models. Large rainfall biases are depicted over mainland even in models with a very strong MCT. It is found that the impacts of the MCT in the models could be masked by a complex mix of processes such as the strength of the Angola low, moisture fluxes from the Indian and South Atlantic Oceans as well as overestimated convection in the Mozambique Channel area.Representation of the Mozambique channel trough and its link to southern African rainfall in CMIP6 modelspublishedVersio

The 2019-21 drought in southern Madagascar

Two consecutive failed rainy seasons in the southern part of Madagascar in 2019–21 had devastating impacts on the population, including an amplification of the ongoing food insecurity in the area. The drought events were second in severity only to the 1990–92 drought and were estimated in a previous study to have a return period of 135 years. In this study, the physical mechanisms that led to these consecutive drought events are investigated. We found that the anomalously cold sea surface temperatures (SSTs) that persisted to the south of Madagascar between December 2019 and December 2020 led to a decrease in the transport of moist air over land. These cold SST anomalies were the most negative anomalies in the past four decades and intensified the rainfall deficit resulting from a negative Subtropical Indian Ocean Dipole (SIOD) mode during the rainy season of December 2019 to March 2020 and during December 2020. We also found that the rainfall response to the SST anomaly south of Madagascar was three times greater than that of a canonical SIOD. A weak Mozambique Channel Trough and a strong Angola low system, on the other hand, modulated the expected above-normal rainfall from a La Niña event in January–February 2021. Our study demonstrates how local factors can modulate the impacts of large-scale drivers, and that both local and global drivers, and their interactions, should be considered when producing seasonal forecasts and advisories, as well as climate change adaptation and mitigation plans for southern Madagascar

Effects of the Congo Basin Rainforest on Rainfall Patterns

Large-scale deforestation in the Congo Basin has an impact on rainfall patterns, both in the Basin and beyond. Factors like socio-economic drivers contribute to ongoing deforestation, and forest loss rates are expected to increase. The mechanisms linking deforestation and rainfall are complex. On a local scale, deforested areas might experience increased rainfall, but adjacent forests could see reduced rainfall. On larger scales, widespread deforestation can reduce overall rainfall in large areas. These changes can impact agriculture, with delayed rainfall and shorter rainy seasons affecting crop yields. By 2100, projected forest loss in the Congo Basin may reduce annual rainfall by 8-10%. However, uncertainties remain due to limited data and understanding of rainfall drivers and interactions in the region.publishedVersio

A unified English term that best reflect the people, the culture, and other things from Madagascar: 'Malagasy' instead of 'Madagascan'

Two words, Malagasy and Madagascan, have emerged in the literature as an English term to refer to the people, the culture, and other animate and inanimate objects from Madagascar, the choice of which has left room for confusions for users. This article has two aims: (1) understanding the sources of such confusions, and (2) subsequently proposing a unified English term (noun and adjective) that will avoid further misperceptions in written and verbal communications accounting for Madagascar. The results from investigating the Web of Science, from historical documents, and from viewpoints from online survey combine to suggest that the term 'Malagasy' must have unintentionally been substituted by the term 'Madagascan' from the earliest documented usage until now. These two words have been used interchangeably, and the usage has never been applied consistently. This could have been influenced by the simple assumption that 'Madagascan' is an intuitive form, derivative of Madagascar. However, 'Malagasy' is an irregular word and is unique to Madagascar, thus its usage as a word qualifier for different objects, animals, plants, and the people of Madagascar in the English language would suffice

Upscaling impact of wind/sea surface temperature mesoscale interactions on southern Africa austral summer climate

International audienceMesoscale sea surface temperature (SST) variability plays an important role in shaping local atmospheric boundary layers through thermodynamic processes. This study focuses on the upscaling effects of mesoscale SST gradients in sensitive areas on the southern Africa regional atmospheric circulation. Using regional atmospheric model sensitivity experiments which differ only in the mesoscale SST forcing characteristics (either the full spectrum of SST variability or only its large‐scale components are included), we first quantify the importance of SST gradients on regional atmospheric conditions. Agulhas eddies and meanders influence the vertical air column up to the troposphere, and mesoscale ocean patterns significantly modify incoming landwards moisture fluxes. The austral summer mean state is then modified in terms of air temperature, cloud cover and mean rainfall, with notable differences in tropical rainbands over southwestern Africa. Mesoscale SST variability favours tropical–extra‐tropical interactions and cloudband development over the continent. These results stress the importance of high‐resolution ocean forcing for accurate atmospheric simulations

Human Influence on the Climate System. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

The Working Group I contribution to the Sixth Assessment Report is the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science, and combining multiple lines of evidence from paleoclimate, observations, process understanding, and global and regional climate simulations

Limited role of climate change in extreme low rainfall associated with southern Madagascar food insecurity, 2019-21

Southern Madagascar recently experienced a severe food security crisis, made significantly worse by well below average rainfall from July 2019 to June 2021. This exceptional drought has affected a region with high pre-existing levels of vulnerability to food insecurity (subsistence agriculture and pastoralism in the region is rain-fed only), while impacts have been compounded further by COVID-19 restrictions and pest infestations. The rainy seasons of both 2019/20 and 2020/21 saw just 60% of normal rainfall across the Grand South region and was estimated as a 1-in-135 year dry event, only surpassed in severity by the devastating drought of 1990–92. Based on a combination of observations and climate modelling, the likelihood of experiencing such poor rains in the region was not significantly increased due to human-caused climate change: while the observations and models combine to indicate a small shift toward more droughts like the 2019–2021 event as a consequence of climate change, these trends remain overwhelmed by natural variability. This result is consistent with previous research, with the Intergovernmental Panel on Climate Change (IPCC)'s Sixth Assessment Report concluding that any perceptible changes in drought will only emerge in this region if global mean temperatures exceed 2 °C above pre-industrial levels.publishedVersio