Southwest Tropical Indian Ocean Research Articles

Quantifying climatic suitability for tourism in Southwest Indian Ocean Tropical Islands: Applying the Holiday Climate Index to Réunion Island.

Climate is a key resource for tourists and tourism providers. Varied approaches to quantifying climate resources for tourism have been developed in the last 40years, computing indices from a range of meteorological variables to measure the comparative climatic suitability of different destinations through time. This study provides the first application of a tourism climate index in the tropical southwest Indian Ocean, applying the recently developed Holiday Climate Index (HCI) for Réunion Island. The suitability of this index is evaluated for the case of this French department, with a particular focus on air conditioning availability in tourism accommodation establishments as this index excludes night-time thermal comfort. Both iterations of the HCI (HCIBeach and HCIUrban) are computed with meteorological data from Roland Garros Airport for the period 1991-2020, exploring monthly, annual, and seasonal climatic suitability. Mean monthly HCI scores reveal considerable seasonality in climatic suitability for tourism on the island with scores ranging from 89.3 ('excellent') to 36.9 ('marginal') for the HCIBeach and 85.0 ('excellent') to 27.5 ('unacceptable') for the HCIUrban, with more favourable scores calculated for July and August, displaying a clear austral winter peak seasonal classification. Over the 30-year period, there is no statically significant change in mean annual climatic suitability, and at a monthly scale, only one month of the year for each index displays statistically significant trends. These results are important in informing tourism strategies for the island to maximise visitor satisfaction through targeting advertising more deliberately for peak touristic climate suitability during the winter months.

Rapid Sea Level Rise in the Tropical Southwest Indian Ocean in the Recent Two Decades

AbstractIt has been reported that the sea level falls in the tropical Southwest Indian Ocean (SWIO) from the 1960s to the early 2000s. However, a rising trend of 4.05 ± 0.56 cm/decade has occurred during the recent two decades with our analysis showing that manometric sea level contributes 41% to this sea level rise. 30% of this rise is due to steric sea level (SSL) change in the upper 2,000 m with SSL rise in the upper 300 m of secondary importance. Conversely, thermal expansion below the thermocline (300–2,000 m), likely caused by water mass spread from the Southern Ocean, induces major contribution to SSL changes. Compared to existing studies demonstrating the contribution of thermal variations above the thermocline to sea level variability in the tropical SWIO, this study emphasizes the importance of ocean mass and deeper ocean changes in a warming climate.

High‐frequency variability dominates potential connectivity between remote coral reefs

AbstractCoral larval dispersal establishes connectivity between reefs, but larval fluxes vary over timescales from daily to multidecadal due to oceanographic variability. Using a 2‐km‐resolution ocean model, we simulate daily spawning events from 1993 to 2019 and assess the potential connectivity between all reefs in the tropical southwest Indian Ocean. Although there is a significant seasonal cycle in potential connectivity, day‐to‐day variability generally dominates. Larval dispersal pathways on any particular day provide limited information about the dispersal pathways a few days later. The magnitude of this high‐frequency variability depends on the local geography and oceanography, with small and isolated reefs generally subject to the most variability. Stochastic oceanographic variability introduces considerable uncertainty to dispersal predictions, imposing fundamental limitations on what simulations can tell us about inter‐reef connectivity. Protracted spawning over only a few days can significantly reduce variability associated with the likelihood of a larva settling. The duration of spawning is therefore a more important parameter in modeling coral connectivity than the exact timing of spawning onset. Finally, we find that a small proportion of spawning events account for the majority of settling larvae, particularly at remote islands, and demonstrate that a time‐mean picture of dispersal may be inappropriate for predicting demographic and genetic connectivity. Given the diversity of coral reef environments in the southwest Indian Ocean, we expect that these results will apply to inter‐reef coral connectivity across the tropics more broadly, as well as other weakly swimming reef taxa with the potential for long‐distance dispersal.

Drivers and characteristics of the Indo-western Pacific Ocean capacitor

Coherently coupled ocean-atmosphere variability of the tropical Indo-Pacific Oceans gives rise to the predictability of Asian summer climate. Recent advances in Indo-western Pacific Ocean capacitor (IPOC) theory and the relationship with El Niño-Southern Oscillation (ENSO) are reviewed. The IPOC features tropical Indian Ocean (TIO) warming and an anomalous anticyclonic circulation over the western North Pacific (WNPAAC), the latter driving water vapor transport to East Asia and causing extreme events, e.g., heavy rainfalls from central China to Japan during the boreal summer. IPOC events often occur in post-ENSO summers, but the significant TIO warming could sustain the WNPAAC without a strong El Niño, forced instead by a strong antecedent positive Indian Ocean Dipole (IOD). In latter cases, the Indian Ocean and WNP act as a self-sustaining system, independent of external forcings. El Niño or positive IOD induces the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to SST warming and a “C-shaped” wind anomaly during winter and early spring. Furthermore, the southwest TIO downwelling Rossby waves reflect as oceanic Kelvin waves on the African coast. In the early summer, the resultant southeast TIO SST warming induces a second “C-shaped” wind anomaly. Both southwest and southeast TIO warming contribute to the WNPAAC. The WNPAAC modulates the water vapor pathways to East Asia in the late spring and summer, which mostly converge over the South China Sea and adjacent regions before flowing further to the north. More water vapor is transported from the western Pacific warm pool and less from the southern hemisphere and the Indian Ocean. The enhanced Asian Summer monsoon and moisture content lead to extreme rainfalls in central China and Japan during the boreal summer.

Decadal Cooling Events in the South Indian Ocean During the Argo Era

AbstractDespite the rapid warming in the south Indian Ocean (SIO) during the recent two decades, the upper 300 m of the tropical southwest Indian Ocean (SWIO) experienced significant cooling during 2005–2011. The decreasing rate of the observed temperature reached −0.50 ± 0.1°C/decade. Heat budget analysis reveals that the cooling trend over SWIO results from the competing effect between local air‐sea heat flux and horizontal heat advection. Moreover, all the three observational datasets indicate that in the subtropical SIO, a prolonged cooling event occurred beneath the thermocline during 2010–2016. Although there exist some discrepancies between the observations and estimating the circulation and climate of the ocean (ECCO) reanalysis before 2010, the observations and ECCO reanalysis agree in terms of the post–2010 temperature variability in the intermediate layer of the subtropical region, probably because of the better constraint of the ECCO assimilation product by the increased sampling of Argo floats in the deeper ocean. The heat budget analysis indicates that during the period 2010–2016, the decadal cooling in the 400–1,000 m layer is primarily modulated by vertical ocean advection, and the horizontal ocean advection makes a secondary contribution. This study expands our knowledge of the decadal temperature variability and heat balance in the SIO during the Argo era.

Influence of climate variability on wind‐sea and swell wave height extreme over the Indo‐Pacific Ocean

AbstractIn the Indo‐Pacific Ocean (IPO), extreme significant wave heights (SWHs) can substantially induce coastal erosion, flooding, and devastating impacts on coastal livelihoods. This study examines the seasonal influence of the El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Southern Annular Mode (SAM), and Pacific North American (PNA) pattern on extreme wind‐wave parameters. The climatic extremes are calculated utilizing ERA5 reanalysis datasets from 1979 to 2019 and a nonstationary generalized extreme value distribution. Significant increases in extreme wind‐sea (Hmax) and swell SWH (HmaxSw) response to ENSO occur over the northeast North Pacific (NP) during December–February (DJF) and western NP during June–August (JJA). The PNA influence exhibits a similar pattern to ENSO during DJF yet is inactive in JJA. Hmax and HmaxSw responses to the IOD during September–November (SON) include significant increases over the western Pacific, southern Indian Ocean (IO), and southwest tropical IO, yet decreases over the central tropical IO. The HmaxSw response to the SAM is larger than that for Hmax over the southern IPO during DJF, which extends toward the eastern Pacific in March–May (MAM). Overall, extreme wind‐sea and swell parameter responses are found to be associated with sea level pressure (SLP) and SLP gradients.

Trans-basin influence of southwest tropical Indian Ocean warming during early boreal summer

AbstractThis study examines the climate response to a sea surface temperature (SST) warming imposed over the southwest Tropical Indian Ocean (TIO) in a coupled ocean-atmosphere model. The results indicate that the southwest TIO SST warming can remotely modulate the atmospheric circulation over the western North Pacific (WNP) via inter-basin air-sea interaction during early boreal summer. The southwest TIO SST warming induces a “C-shaped” wind response with northeasterly and northwesterly anomalies over the north and south TIO, respectively. The northeasterly wind anomalies contribute to the north TIO SST warming via a positive Wind-Evaporation-SST(WES) feedback after the Asian summer monsoon onset. In June, the easterly wind response extends into the WNP, inducing an SST cooling by WES feedback on the background trade winds. Both the north TIO SST warming and the WNP SST cooling contribute to an anomalous anticyclonic circulation (AAC) over the WNP. The north TIO SST warming, WNP SST cooling, and AAC constitute an inter-basin coupled mode called the Indo-western Pacific ocean capacitor (IPOC), and the southwest TIO SST warming could be a trigger for IPOC. While the summertime southwest TIO SST warming is often associated with antecedent El Niño, the warming in 2020 seems to be related to extreme Indian Ocean Dipole in 2019 fall. The strong southwest TIO SST warming seems to partly explain the strong summer AAC of 2020 over the WNP even without a strong antecedent El Niño.

Revisiting the 1992 severe drought episode in South Africa: the role of El Ni\xf1o in the anomalies of atmospheric circulation types in Africa south of the equator

During strong El Niño events, below-average rainfall is expected in large parts of southern Africa. The 1992 El Niño season was associated with one of the worst drought episodes in large parts of South Africa. Using reanalysis data set from NCEP-NCAR, this study examined circulation types (CTs) in Africa south of the equator that are statistically related to the El Niño signal in the southwest Indian Ocean and the implication of this relationship during the 1992 drought episode in South Africa. A statistically significant correlation was found between the above-average Nino 3.4 index and a CT that features widespread cyclonic activity in the tropical southwest Indian Ocean, coupled with a weaker state of the south Indian Ocean high-pressure. During the analysis period, it was found that the El Niño signal enhanced the amplitude of the aforementioned CT. The impacts of the El Niño signal on CTs in southern Africa, which could have contributed to the 1992 severe drought episode in South Africa, were reflected in (i) robust decrease in the frequency of occurrence of the austral summer climatology pattern of atmospheric circulation that favors southeasterly moisture fluxes, advected by the South Indian Ocean high-pressure; (ii) modulation of easterly moisture fluxes, advected by the South Atlantic Ocean high-pressure, ridging south of South Africa; (iii) and enhancement of the amplitude of CTs that both enhances subsidence over South Africa, and associated with the dominance of westerlies across the Agulhas current. Under the ssp585 scenario, the analyzed climate models suggested that the impact of radiative heating on the CT significantly related to El Niño might result in an anomalous increase in surface pressure at the eastern parts of South Africa.

Leading patterns of the satellite-era summer precipitation over West Africa and associated global teleconnections

Precipitation patterns over West Africa display a broad range of timescales, from the intraseasonal to decadal timescales, as well as multidecadal shifts. Here we investigate the dominant patterns of the July to September precipitation over the region and the related ocean-atmosphere anomalies during the satellite-era from 1983 to 2017. Using extended empirical orthogonal function analysis of an ensemble of nine precipitation datasets, we identify two dominant modes that together account for about 33% of the variance. The Sahel mode displays spatially coherent increases in precipitation over much of West Africa and a decrease at the Guinea Coast, and is closely reproduced by linear trend analysis. Linear trends explain 25–53% of the Sahel variance from the drier mid-1980s to the wetter mid-1990s. The Guinea Coast mode displays robust precipitation anomalies south of the Sahel, with strong interannual variability and a statistically non-significant trend. The Sahel mode is associated with a northerly displacement of the Inter-Tropical Convergence Zone (ITCZ), warm North Atlantic with cold blob in the subpolar gyre region, warm Mediterranean Sea, warm tropical southwest Indian Ocean and negative Pacific decadal variability pattern. The Atlantic Niño leads the Guinea Coast mode by two months. Both Sahel and Guinea Coast modes are substantially influenced by the interactions between meridional (displacements of the ITCZ) and zonal (variations of the Walker Circulation) atmospheric circulations. The southerly displacement of the ITCZ, convection, upper-level divergence and surface convergence in the equatorial Atlantic associated with the Guinea Coast mode is horizontally compensated by strong surface divergence and upper-level convergence over the equatorial Pacific where anomalous cooling prevails, implying a strong role for the Walker Circulation during this period.

Thermocline Warming Induced Extreme Indian Ocean Dipole in 2019

AbstractThe 2019 positive Indian Ocean Dipole (IOD) was the strongest event since the 1960s which developed independently without coinciding El Niño. The dynamics is not fully understood. Here we show that in March–May, westward propagating oceanic Rossby waves, a remnant consequence of the weak 2018 Pacific warm condition, led to anomalous sea surface temperature warming in the southwest tropical Indian Ocean (TIO), inducing deep convection and anomalous easterly winds along the equator, which triggered the initial cooling in the east. In June–August, the easterly wind anomalies continued to evolve through ocean‐atmosphere coupling involving Bjerknes feedback and equatorial nonlinear ocean advection, until its maturity in September–November. This study clarifies the contribution of oceanic Rossby waves in the south TIO in different dynamic settings and reveals a new triggering mechanism for extreme IOD events that will help to understand IOD diversity.

Biophysical Control on Variability in Phytoplankton Production and Composition in the South-Western Tropical Indian Ocean During Monsoon 2014

The existing oligotrophic conditions in the southwest tropical Indian Ocean (SWTIO) is believed to be one of the causes for low phytoplankton productivity (PP) observed in this area. Though many remote sensing based studies on PP have been carried out in SWTIO; studies on in situ estimation of PP and its cause(s) of variability are scarce. Thus, to understand the controlling environmental forcings on the variability in phytoplankton biomass (chlorophyll-a; Chl-a), community structure and productivity, time series (TS; @6h intervals for 10 days) plus point measurements (RT) were carried out in the SWTIO during southwest monsoon (June) of 2014. Strong thermohaline stratification resulted in shallow (35-40m) mixed layer (ML). Subsurface Chl-a maximum (SCM) was observed to oscillate within 40-60m with majority of peaks at ~50m, and existed just beneath the ML depth. Light availability at the time of sampling was highly conducive for algal growth; whereas nutrient ratios indicated N- and Si-limitation suggesting unfavourable conditions for diatoms and/or silicoflagellates growth within the ML. Furthermore, HPLC-based pigments analysis confirmed dominance of nano-sized plankton (53%) followed by pico-plankton (25%) and micro-plankton (22%). Column integrated production (IPP) varied from 176 - 268 (241+43 mgC m-2 d-1) and was relatively stable during the observation period, except a low value on 11-June, which was ascribed to the drastic dropdown in the daily incident PAR due to overcast sky. Vertical profiles of PP and Chl-a resembled each other and maximum PP usually corresponded with SCM depths. The Chl-a-specific PP (PB) was mostly higher within the ML and showed no surface photoinhibition, due to the dominance of smaller phytoplankton (less prone to pigment packaging effect) in the surface layer. Comparatively, higher PB within the ML is indicative of phytoplankton healthiness during the sampling time, whereas low PB below the SCM was due to light limitation. Highest integrated Chl-a (39 mg m-2) and IPP (328 mgC m-2 d-1) observed at RT-2 was clearly linked to low sea surface height anomaly (SSHA), cyclonic disturbance and associated positive Ekman pumping. Conversely, high SSHA and strong stratification conditions prevailed at TS, RT-4 and RT-6 stations leading to comparatively low IPP.

Moisture sources associated with heavy rainfall over the Limpopo River Basin, southern Africa

Moisture sources and pathways over the Limpopo River Basin (LRB) in southern Africa were identified using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model applied to NCEP II (2.5° × 2.5°) reanalysis data for 1981–2016. The 10-day air parcel backward trajectories were produced for the extended wet season (October–April) as well as the early and late summer. Analysis of a 36-year climatology of air parcel trajectories indicated seven moisture source regions for the LRB; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. The results have shown that important differences in moisture source regions and pathways exist between early (October–December) and late (January–April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. On interannual time scales, there are notable differences in moisture source regions between anomalously wet and dry summers, with the South Indian Ocean moisture contribution and transport over the LRB smaller during dry summers than wet summers. Changes in specific humidity for trajectory linked to heavy daily rainfall events (defined from CHIRPS data) showed that subtropical South Indian Ocean source is more extensive for the heavy rainfall than for the moderate case. Generally, moisture source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic setting.

Evolution of South Tropical Indian Ocean Warming and the Climatic Impacts Following Strong El Niño Events

Abstract The south tropical Indian Ocean (TIO) warms following a strong El Niño, affecting Indo-Pacific climate in early boreal summer. While much attention has been given to the southwest TIO where the mean thermocline is shallow, this study focuses on the subsequent warming in the southeast TIO, where the mean sea surface temperature (SST) is high and deep convection is strong in early summer. The southeast TIO warming induces an anomalous meridional circulation with descending (ascending) motion over the northeast (southeast) TIO. It further anchors a “C-shaped” surface wind anomaly pattern with easterlies (westerlies) in the northeast (southeast) TIO, causing a persistent northeast TIO warming via wind–evaporation–SST feedback. The southeast TIO warming lags the southwest TIO warming by about one season. Ocean wave dynamics play a key role in linking the southwest and southeast TIO warming. South of the equator, the El Niño–forced oceanic Rossby waves, which contribute to the southwest TIO warming, are reflected as eastward-propagating oceanic Kelvin waves along the equator on the western boundary. The Kelvin waves subsequently depress the thermocline and develop the southeast TIO warming.

Influence of Natural Climate Variability on the Extreme Ocean Surface Wave Heights Over the Indian Ocean

AbstractExtreme ocean waves are part of the climate system but responsible for significant impacts on coastal and offshore environments, structures, and populations. In the Indian Ocean (IO), the wind and wave climate can be significantly influenced by natural climate variability, such as the El Niño–Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Southern Annular Mode (SAM), yet our understanding on their regional influence is limited, particularly for seasonal extremes. Here, seasonal extreme significant wave heights (SWHs) and winds in the IO are examined over the period 1957–2010 utilizing ERA‐20C reanalysis data and the nonstationary generalized extreme value distribution to understand climatic extremes, by considering climate indices as covariates. ENSO influence on extreme SWHs include increases in the Bay of Bengal, southwest tropical IO (TIO), southern IO (SIO; a broad extension south of Australia), and South China and Philippine (SCP) Seas, and decreases in the Arabian Sea in boreal summer during El Niño. Extreme SWH responses to the IOD include increases in the eastern TIO, southwest TIO, and SIO in boreal autumn during its positive phase. Lastly, Southern Annular Mode not only significantly affects the SIO year round but has a weak influence in the northern and tropical IO. Composite analysis of ENSO and IOD events further highlight in phase combinations display less significant influence than out of phase combinations during summer, but not autumn. Mean and extreme wind responses are consistent with SWH responses to natural climate variability, and climate mode teleconnection patterns help explain the seasonal variations.

Evidence of organized intraseasonal convection linked to ocean dynamics in the Seychelles–Chagos thermocline ridge

The Madden–Julian oscillation (MJO) is the dominant driver of intraseasonal variability across the equatorial domain of the global ocean with alternating wet and dry bands that propagate eastward primarily between 5°N and 5°S. Past research has shown that MJOs impact the surface and subsurface variability of the Seychelles–Chagos thermocline ridge (SCTR) (55°E–65°E, 5°S–12°S) located in the southwest tropical Indian Ocean (SWTIO), but investigations of how SWTIO internal dynamics may play an important role in producing MJO events remain limited. This study uses Argo, in conjunction with several remote sensing and reanalysis products, to demonstrate that SWTIO oceanic dynamics, particularly barrier layer formation and near surface heat buildup, may be associated with MJO genesis between August and December of most years between 2005 and 2013. A total of eight SWTIO specific MJO events are observed, all occurring between August and December. Four of the eight events are correlated with positive SWTIO total heat content (THC) and barrier layer thickness (BLT) interannual anomalies. Two others formed over the SWTIO during times when only one of the variables was at or above their seasonal average, while two additional events occurred when both variables experienced negative interannual anomalies. Lacking complete 1:1 correlation between the hypothesized oceanic state and the identified SWTIO MJO events, we conclude that additional work is required to better understand when variability in key oceanic variables plays a primary role in regional MJO genesis or when other factors, such as atmospheric variability, are the dominate drivers.

A Comprehensive Survey of Pelagic Megafauna: Their Distribution, Densities, and Taxonomic Richness in the Tropical Southwest Indian Ocean

The distribution and density of pelagic megafauna (marine mammals, seabirds, elasmobranches and sea turtles) are important indicators of marine biodiversity, reflecting the condition of the underlying ecosystems. Taking advantage of their relative accessibility at the sea surface, a dedicated aerial survey was conducted in the tropical Southwest Indian Ocean to map their distribution, the taxonomic diversity and to estimate their densities for a reference standpoint on the area. This large survey across three ecological sub-regions revealed contrasting spatial distributions: maps of taxonomic richness of marine mammals and seabirds revealed different “hotspots” in the area. Densities were estimated for eight cetacean taxa with small and large Delphininae, or small Globicephalinae dominating, and for seven seabird taxa, with terns and noddies dominating. At the community level, the Southwest Indian Ocean megafauna was structured by the marine environment with strong differences between the Mozambique Channel and the Mascarene Islands, or between shelf and slope/oceanic habitats. Our results illustrate how multi-taxa aerial surveys are relevant and cost-effective monitoring tools for marine megafauna, allowing a community-wide approach.

Tropical cyclone activity over the Southwest Tropical Indian Ocean

AbstractThe Southwest Tropical Indian Ocean (SWTIO) is a key region for air‐sea interaction. Tropical cyclones (TCs) regularly form over the SWTIO and subsurface ocean variability influences the cyclogenesis of this region. Tropical cyclone days for this region span from November through April, and peak in January and February during austral summer. Past research provides evidence for more tropical cyclone days over the SWTIO during austral summer (December–June) with a deep thermocline ridge than in austral summer with a shallow thermocline ridge. We have analyzed the Argo temperature data and HYbrid Coordinate Ocean Model (HYCOM) outputs while focusing on the austral summer of 2012/2013 (a positive Indian Ocean Dipole (IOD) year and neutral El Niño Southern Oscillation (ENSO) year) when seven named tropical cyclones developed over the SWTIO region. This study reveals that the climatic events like the IOD and ENSO influence the cyclonic activity and number of TC days over the SWTIO. We ascertain that the IOD events have linkages with the Barrier Layer Thickness (BLT) in the SWTIO region through propagating Rossby waves, and further show that the BLT variability influences the cyclonic activity in this region.

Trends analysis of precipitation data over the tropical South-West Indian Ocean (SWIO) basin using the Ensemble Empirical Mode Decomposition (EEMD) method

In this paper, we propose the use of the Ensemble Empirical Mode Decomposition (EEMD) method in the analysis of trends in climate data. As compared to existing traditional methods, EEMD is simple, fast and reliable. It works by decomposing the time-series data into intrinsic mode functions until a residual component is obtained which represents the trend in the data. The dataset considered consists of satellite precipitation estimates (SPE) obtained from the Tropical Rainfall Measuring Mission (TRMM) for the tropical South-West Indian Ocean (SWIO) basin recorded during the periods January 1998 to December 2013. The SWIO basin spans from the latitudes 5° S to 35° S and the longitudes 30° E to 70° E and comprises of part of the east coast of Africa and some small island developing states (SIDS) such as Comoros, Madagascar, Mauritius and Reunion Island. The EEMD analysis is carried out for summer, winter and yearly time series of the SPE data. The results from the study are presented in terms of intrinsic mode functions (IMFs) and the trends. The analysis reveals that in summer, there is a tendency to have an increase in the amount of rainfall, whereas in winter, from 1998 to 2004 there has been an initial increase of 0.0022 mm/hr/year and from there onwards till 2013 a decrease of 0.00052 mm/hr/year was noted.

The role of salinity on the interannual variability of the Seychelles-Chagos thermocline ridge

Abstract The southwest tropical Indian Ocean (SWTIO) (55°E–65°E, 5°S–12°S) features a seasonal thermocline ridge that has been shown to have important teleconnections with the Asian monsoon. Although several past studies have highlighted the lack of SWTIO salinity variability, vertical stratification may be sufficient to generate a regional barrier layer, a tropical phenomenon found throughout the Indian Ocean with the ability to affect local air-sea interactions. New global, remotely sensed sea surface salinity (SSS) obtained from the ESA's Soil Moisture and Ocean Salinity (SMOS) and NASA's Aquarius satellites gives us reason to revisit the region to see if these datasets can yield new information on salinity's role in SWTIO variability. We use Argo floats data to calculate mixed layer depth (MLD), isothermal layer depth (ILD), and barrier layer thickness (BLT = ILD − MLD). We also use a novel approach to estimate satellite-derived BLT by replacing the Argo SSS estimate with the SMOS and Aquarius “skin” measurement to then recalculate the MLD and thus obtain a different BLT estimate. We find that while BLT estimates from all three products are in phase with one another, Aquarius and SMOS produce BLT estimates that are greater than Argo by 10–20 m on average. An anomalous event in 2011 demonstrates how BLT interannual variability may be created by changes in near surface salinity. Our observations suggest that the event was caused by subduction of surface waters due to increased near surface salinity and an anomalous downwelling Rossby wave generated by positive Indian Ocean dipole. Both mechanisms created a thick barrier layer followed by anomalous precipitation and SSS, a series of events consistent with barrier layer theory. Our results also suggest that new SSS measurements from SMOS and Aquarius provide a different interpretation of near surface salinity stratification that needs to be further explored.

Tropical Indian Ocean response to the decay phase of El Niño in a coupled model and associated changes in south and east-Asian summer monsoon circulation and rainfall

This study investigates the response of tropical Indian Ocean (TIO) sea surface temperature (SST) to El Nino decay phase and its impacts on South and East Asian summer monsoon in the National Centers for Environmental Prediction Climate Forecast System version 2 free run. The TIO basin-wide warming induced by El Nino at its peak phase (winter; DJF) and next spring (MAM + 1) are reasonably well captured by the model but with weak magnitude. This TIO basin-wide SST warming persists until summer (JJA + 1) and exert strong impact on summer monsoon rainfall and circulation as revealed in the observations. However, TIO SST anomalies are very weak in the model during the El Nino decaying summers. Though El Nino decay is delayed by 2 months in the model, decay of TIO SST warming is faster than the observations. Anomalous latent heat loss from ocean and a feeble southern TIO Rossby waves associated with weak wind response to El Nino are mainly accountable for rapid decay of TIO SST warming by mid-summer in the model. This suggests that JJA + 1 TIO SST response to El Nino decay phase in the model is poorly represented. The model is able to capture the SST anomalies associated with the northwest Pacific anticyclone at the peak phase of El Nino but fail to maintain that during the decay phase in MAM + 1 and JJA + 1. It is found that precipitation and circulation anomalies associated with TIO SST warming over the South and East Asian regions are disorganized in the model during the decay phase of El Nino. Rainfall anomalies over the southwest TIO, west coast of India, northern flank of northwest Pacific anticyclone and over Japan in JJA + 1 are poorly represented by the model. Analysis of lower troposphere stream function and rotational wind component reveals that northwest Pacific anticyclone shifted far eastward to the date line in the model during JJA + 1 unlike in the observations. Anomalous divergence observed over the western TIO and convergence in the northwest Pacific are absent in the model during JJA + 1. Extension of anomalous tropospheric warming from TIO region to equatorial western Pacific is also very weak in the model due to poor representation of TIO SSTs and the subsequent absence of any Kelvin wave response. Anomalous Walker circulation persisted from DJF to JJA + 1 due to El Nino late decay in the model unlike in the observations. This is also found to be responsible for the redundant changes in SST, rainfall and circulation over the Indo-western Pacific in the model. This study demonstrates that it is essential to represent the decay phase of El Nino and the associated TIO response accurately to have realistic simulations of summer monsoon in the decaying year.