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Future projections of precipitation at regional scales are vital to inform climate change adaptation activities. Therefore, is it important to quantify projected changes and associated uncertainty, and understand model processes responsible. This paper addresses these challenges for southern Africa and the adjacent Indian Ocean focusing on the local wet season. Precipitation projections for the end of the twenty-first century indicate a pronounced dipole pattern in the CMIP5 multimodel mean. The dipole indicates future wetting (drying) to the north (south) of the climatological axis of maximum rainfall, implying a northward shift of the ITCZ and south Indian Ocean convergence zone that is not consistent with a simple "wet get wetter" pattern. This pattern is most pronounced in early austral summer, suggesting a later and shorter wet season over much of southern Africa. Using a decomposition method we determine physical mechanisms underlying this dipole pattern of projected change, and the associated intermodel uncertainty. The projected dipole pattern is largely associated with the dynamical component of change indicative of shifts in the location of convection. Over the Indian Ocean, this apparent northward shift in the ITCZ may reflect the response to changes in the north-south SST gradient over the Indian Ocean, consistent with a "warmest get wetter" mechanism. Over land subtropical drying is relatively robust, particularly in the early wet season. This has contributions from dynamical shifts in the location of convection, which may be related to regional SST structures in the southern Indian Ocean, and the thermodynamic decline in relative humidity. Implications for understanding and potentially constraining uncertainty in projections are discussed. [ABSTRACT FROM AUTHOR]

Using daily rain gauge records for Madagascar and nearby islands, this paper investigates rainfall intraseasonal variability at local and regional scales during the austral summer season (November-February), as well as the respective influences of recurrent convective regimes over the southwest Indian Ocean (SWIO) and the Madden-Julian oscillation (MJO). The results show a general consistency between local-scale rainfall variability in Madagascar and regional-scale features of climate variability. The influence of tropical temperate troughs in their mature phase and/or their easternmost locations is first underlined. The development of such systems over southern Africa and the Mozambique Channel can be considered as precursors for Malagasy wet spells, especially over the southern part of the island. Regional and local effects of the MJO are weaker on average, and only concern the northwest of the island and the north of the Mozambique Channel. MJO and convective regimes are finally shown to explain distinct fractions of regional rainfall variability. [ABSTRACT FROM AUTHOR]

This paper investigates the structure and dynamics of the Equatorial Undercurrent (EUC) of the Indian Ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model. The results show that the EUC regularly occurs in each boreal winter and spring, particularly during February and April, consistent with existing studies. The EUC generally has a core depth near the 20°C isotherm and can be present across the equatorial basin. The EUC reappears during summer-fall of most years, with core depth located at different longitudes and depths. In the western basin, the EUC results primarily from equatorial Kelvin and Rossby waves directly forced by equatorial easterly winds. In the central and eastern basin, however, reflected Rossby waves from the eastern boundary play a crucial role. While the first two baroclinic modes make the largest contribution, intermediate modes 3-8 are also important. The summer-fall EUC tends to occur in the western basin but exhibits obvious interannual variability in the eastern basin. During positive Indian Ocean dipole (IOD) years, the eastern basin EUC results largely from Rossby waves reflected from the eastern boundary, with directly forced Kelvin and Rossby waves also having significant contributions. However, the eastern basin EUC disappears during negative IOD and normal years because westerly wind anomalies force a westward pressure gradient force and thus westward subsurface current, which cancels the eastward subsurface flow induced by eastern boundary-reflected Rossby waves. Interannual variability of zonal equatorial wind that drives the EUC variability is dominated by the zonal sea surface temperature (SST) gradients associated with IOD and is much less influenced by equatorial wind associated with Indian monsoon rainfall strength. [ABSTRACT FROM AUTHOR]

Observations show that decadal (10–20 yr) to interdecadal (>20 yr) variability of the tropical Indian Ocean (TIO) sea surface temperature (SST) closely follows that of the Pacific until the 1960s. Since then, the TIO SST exhibits a persistent warming trend, whereas the Pacific SST shows large-amplitude fluctuations associated with the interdecadal Pacific oscillation (IPO), and the decadal variability of the TIO SST is out of phase with that of the Pacific after around 1980. Here causes for the changing behavior of the TIO SST are explored, by analyzing multiple observational datasets and the recently available large-ensemble simulations from two climate models. It is found that on interdecadal time scales, the persistent TIO warming trend is caused by emergence of anthropogenic warming overcoming internal variability, while the time of emergence occurs much later in the Pacific. On decadal time scales, two major tropical volcanic eruptions occurred in the 1980s and 1990s causing decadal SST cooling over the TIO during which the IPO was in warm phase, yielding the out-of-phase relation. The more evident fingerprints of external forcing in the TIO compared to the Pacific result from the much weaker TIO internal decadal–interdecadal variability, making the TIO prone to the external forcing. These results imply that the ongoing warming and natural external forcing may make the Indian Ocean more active, playing an increasingly important role in affecting regional and global climate. [ABSTRACT FROM AUTHOR]

The variation of the tropical cyclone (TC) season in the western North Pacific (WNP) was analyzed based on the percentiles of annual TC formation dates. The results show that the length of the TC season is highly modulated by the TC season's start rather than its end. The start of the TC season in the WNP has large interannual variation that is closely associated with the variation of the sea surface temperature (SST) in the Indian Ocean (IO) and the central-eastern Pacific (CEP). When the SSTs of the IO and CEP are warm (cold) in the preceding winter, anomalous high (low) pressure and anticyclonic (cyclonic) circulation are induced around the WNP TC basin the following spring, resulting in a late (early) start of the TC season. These results suggest that a strong El Niño in the preceding winter significantly delays the TC season start in the following year. [ABSTRACT FROM AUTHOR]

Climate models consistently project reduced surface warming over the eastern equatorial Indian Ocean (IO) under increased greenhouse gas (GHG) forcing. This IO dipole (IOD)-like warming pattern, regarded as robust based on consistency among models by the new Intergovernmental Panel on Climate Change (IPCC) report, results in a large increase in the frequency of extreme positive IOD (pIOD) events, elevating the risk of climate and weather disasters in the future over IO rim countries. These projections, however, do not consider large model biases in both the mean state and interannual IOD variance. In particular, a 'present-future relationship' is identified between the historical simulations and representative concentration pathway (RCP) 8.5 experiments from phase 5 of the Coupled Model Intercomparison Project (CMIP5) multimodel ensemble: models with an excessive IOD amplitude bias tend to project a strong IOD-like warming pattern in the mean and a large increase in extreme pIOD occurrences under increased GHG forcing. This relationship links the present simulation errors to future climate projections, and is also consistent with our understanding of Bjerknes ocean-atmosphere feedback. This study calibrates regional climate projections by using this present-future relationship and observed IOD amplitude. The results show that the projected IOD-like pattern of mean changes and frequency increase of extreme pIOD events are largely artifacts of model errors and unlikely to emerge in the future. These results illustrate that a robust projection may still be biased and it is important to consider the model bias effect. [ABSTRACT FROM AUTHOR]

Based on models from phase 5 of the Coupled Model Intercomparison Project (CMIP5), the present study investigates the South Asian high (SAH) change in response to global warming. Under global warming, the selected 16 coupled general circulation models all feature an elevation of geopotential height at 100 hPa to the south of the SAH climatological position; an easterly response is found over the northern Indian Ocean in all the models, while a westerly response is found over subtropical Asia. The ridges of the SAH shift equatorward in 75% of models. Using the linear baroclinic model, it is found that the combined effects of latent heating and the mean advection of stratification change (MASC) are mainly responsible for those responses. The MASC mainly leads to the aforementioned easterly and westerly responses; the latent heating contributes to the geopotential height response and the easterly response over the northern Indian Ocean. The most important intermodel diversity is found in the 100-hPa circulation change under global warming, accounting for more than half of the total intermodel variance. The intermodel spread of latent heating and the MASC are important factors in driving the 100-hPa circulation diversity. Furthermore, analysis shows that the projected uncertainties in humidity, vertical velocity, and global mean temperature change are the three most important sources of intermodel diversity for the 100-hPa circulation change. [ABSTRACT FROM AUTHOR]

Recent studies have pointed out an increased warming over the Indian Ocean warm pool (the central-eastern Indian Ocean characterized by sea surface temperatures greater than 28.0°C) during the past half-century, although the reasons behind this monotonous warming are still debated. The results here reveal a larger picture-namely, that the western tropical Indian Ocean has been warming for more than a century, at a rate faster than any other region of the tropical oceans, and turns out to be the largest contributor to the overall trend in the global mean sea surface temperature (SST). During 1901-2012, while the Indian Ocean warm pool went through an increase of 0.7°C, the western Indian Ocean experienced anomalous warming of 1.2°C in summer SSTs. The warming of the generally cool western Indian Ocean against the rest of the tropical warm pool region alters the zonal SST gradients, and has the potential to change the Asian monsoon circulation and rainfall, as well as alter the marine food webs in this biologically productive region. The current study using observations and global coupled ocean-atmosphere model simulations gives compelling evidence that, besides direct contribution from greenhouse warming, the long-term warming trend over the western Indian Ocean during summer is highly dependent on the asymmetry in the El Niño-Southern Oscillation (ENSO) teleconnection, and the positive SST skewness associated with ENSO during recent decades. [ABSTRACT FROM AUTHOR]