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The spatial and temporal distribution of tropospheric water vapour in the South Indian Ocean (SIO) basin is investigated using observations collected from twelve GNSS stations spanning the basin. The comparison of GNSS‐derived integrated water vapour (IWV) content against radiosoundings and satellite‐borne microwave radiometer data shows good agreement, with global uncertainties ranging from 0.76 to 1.17 kg·m−2, depending on GNSS station locations. GNSS‐derived IWV contents show a strong seasonal cycle, characterized by higher water vapour content during the austral summer, when the InterTropical Convergence Zone (ITCZ) is located in the Southern Hemisphere. At the seasonal time‐scale, the observed annual IWV amplitude varies from 10 to 15 kg·m−2 near the Equator to 20 to 30 kg·m−2 in the Subtropics. The GNSS IWV signature of the Madden–Julian Oscillation (MJO) is hardly noticeable during the Austral winter, but varies from 1–2 to 4 kg·m−2 between the active and suppressed phases of the MJO during austral summer. At diurnal time‐scales, GNSS IWV shows larger diurnal amplitude over land (2–3 kg·m−2) than over open ocean (1–2 kg·m−2), with highest amplitudes (up to 7 kg·m−2) observed over large and mountainous islands. The phase analysis of the IWV diurnal cycle indicates that the diurnal maximum (minimum) is reached in the late afternoon/evening (morning) over land, at night (mid‐day) over ocean and in the early morning (late afternoon) at coastal locations. A comparison of GNSS‐derived IWV contents against fifth‐generation European Centre for Medium‐range Weather Forecasts (ECMWF) Reanalysis (ERA5) data shows that ERA5 generally correctly reproduces the IWV content at both seasonal, intra‐seasonal and diurnal time‐scales, although some discrepancies can be noticed over small islands characterised by steep orography. The signature of the MJO in ERA5 also shows good agreement with GNSS observations at most studied locations. [ABSTRACT FROM AUTHOR]

Based on initial errors of sea temperature in the tropical Indian Ocean that are most likely to induce spring predictability barrier (SPB) for the El Niño prediction, the sensitive area of sea temperature in the tropical Indian Ocean for El Niño prediction starting from January is identified using the CESM1.0.3 (Community Earth System Model), a fully coupled global climate model. The sensitive area locates mainly in the subsurface of eastern Indian Ocean. The effectiveness of applying targeted observation in the sensitive area is also evaluated in an attempt to improve the El Niño prediction skill. The results of sensitivity experiments indicate that if initial errors exist only in the tropical Indian Ocean, applying targeted observation in the sensitive area in the Indian Ocean can significantly improve the El Niño prediction. In particular, for SPB-related El Niño events, when initial errors of sea temperature exist both in the tropical Indian Ocean and the Pacific Ocean, which is much closer to the realistic predictions, if targeted observations are conducted in the sensitive area of tropical Pacific, the prediction skills of SPB-related El Niño events can be improved by 20.3% in general. Moreover, if targeted observations are conducted in the sensitive area of tropical Indian Ocean in addition, the improvement of prediction skill can be increased by 25.2%. Considering the volume of sensitive area in the tropical Indian Ocean is about 1/3 of that in the tropical Pacific Ocean, the prediction skill improvement per cubic kilometer in the sensitive area of tropical Indian Ocean is competitive to that of the tropical Pacific Ocean. Additional to the sensitive area of the tropical Pacific Ocean, sensitive area of the tropical Indian Ocean is also a very effective and cost-saving area for the application of targeted observations to improve El Niño forecast skills. [ABSTRACT FROM AUTHOR]

The spatial and temporal decorrelation scales of sea surface salinity (SSS) have been calculated in the tropical Indian ocean from the satellite measurements including Soil Moisture and Ocean Salinity (SMOS), Aquarius, Soil Moisture Active Salinity (SMAP), and the model output data for the period of 2011–2017. The differences in spatial and temporal scales from different products are discussed and the physical interpretations of the scales of SSS variability are analysed. The results show that, despite the differences in spatial and temporal resolution, there is good agreement between the spatial and temporal scales of SSS field among all products. Large zonal scales (> 2000 km) and temporal scales with strong anisotropy appear in the central of the equatorial Indian Ocean (8° S–15 °S). In addition, the large meridional decorrelation lengths (~ 800 km) and temporal scales with low anisotropy are found in the southern region of the Arabian Sea (0–12 °N) for all products. The decorrelation scales of SSS in these two areas are mainly caused by freshwater flux and salinity advection, respectively. Our results provide new insights into the controlling mechanisms of SSS variability in different regions. [ABSTRACT FROM AUTHOR]

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; @6 h intervals for 10 days; 1 station), plus point measurements (RT; 3 stations) were carried out in the SWTIO during the southwest monsoon (June) of 2014. Strong thermohaline stratification resulted in shallow (35–40 m) mixed layer (ML). Subsurface Chl-a maximum (SCM) was observed to oscillate within 40–60 m with majority of peaks at ∼50 m, and existed just beneath the ML depth. Light availability during sampling period was highly conducive for algal growth; nutrient ratios indicated N- and Si-limitation (N:P < 10; N:Si < 1 and SiO4 < 5 μM) suggesting unfavorable 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 to 268 (241 ± 43 mgC m–2 d–1) and was relatively stable during the observation period, except a low value (19.4 E m–2 d–1) 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 suction. Conversely, high SSHA and strong stratification conditions prevailed at TS, RT-4, and RT-6 stations leading to comparatively lower IPP (176-268, 252, and 243 mgC m–2 d–1), respectively.

Contrasting effects of the tropical Indian and Pacific Oceans on the atmospheric circulation and rainfall interannual variations over South America during southern winter are assessed considering the effects of the warm Indian Ocean basin-wide (IOBW) and El Niño (EN) events, and of the cold IOBW and La Niña events, which are represented by sea surface temperature-based indices. Analyses are undertaken using total and partial correlations. When the effects of the two warm events are isolated from each other, the contrasts between the associated rainfall anomalies in most of South America become accentuated. In particular, EN relates to anomalous wet conditions, and the warm IOBW event to opposite conditions in extensive areas of the 5° S–25° S band. These effects in the 5° S–15° S sector are due to the anomalous regional Hadley cells, with rising motions in this band for the EN and sinking motions for the warm IOBW event. Meanwhile, in subtropical South America, the opposite effects of the EN and warm IOBW seem to be due to the presence of anomalous anticyclone and cyclone and associated moisture transport, respectively. These opposite effects of the warm IOBW and EN events on the rainfall in part of central South America might explain the weak rainfall relation in this region to the El Niño–Southern Oscillation (ENSO). Our results emphasize the important role of the tropical Indian Ocean in the South American climate and environment during southern winter.

Based on the fifth generation reanalysis of European Centre for Medium–Range Weather Forecasts (ECMWF), Hadley Centre sea ice and sea surface temperature data, and ENSEMLES hindcasts, the impact of north tropical Atlantic (NTA) sea surface temperature (SST) anomalies in the ensuing spring of El Niño on the tropical Indian Ocean (TIO) and Northwest Pacific (NWP) is investigated using composites, singular value decomposition, and correlations. The anomalous SST warming in the NTA region in the decay spring of El Niño leads to a Gill‐type response of the overlying atmosphere in spring–summer. The Kelvin wave over the TIO stimulated by the NTA warming causes the anomalous easterlies in the regions of TIO and NWP, thereby the strengthening of the NWP subtropical anticyclone (NWPSA). The NTA SST warming also forces a local ascending motion, leading to a downward branch over the NWP region. This descent gives rise to the TIO‐NWP low‐level easterly anomalies and an intensification of the NWPSA. The anomalous easterly weakens the background southwesterly monsoon, accompanied by the increased atmospheric boundary layer specific humidity caused by the troposphere warming, suppressing upward latent heat fluxes, finally contributing to the North Indian Ocean (NIO) warming in summer. This indicates that the second warming of the NIO region in the following summer of El Niño may be partially attributed to the NTA warming in the ensuing spring of El Niño. This study improves our understanding of the impact of the NTA SST anomalies in the El Niño decay spring on the TIO and NWP regions, and emphasizes the importance of the NTA region in the pantropical inter‐basin interactions. [ABSTRACT FROM AUTHOR]

This study analyzes the thermodynamic response of an ocean model to two different flux parameterizations. We compared two experiments, a control run (CR) with the flux formulation proposed by Kara et al. [Journal of Atmospheric and Oceanic Technology, 17(10):1421–1438, 2000] with relative wind effect, and an experimental run (ER) with the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment version 3.0 [COARE3.0, Fairall et al. (J Geophys Res Oceans 101(C1):1295–1308, 1996; J Geophys Res Oceans, 101(C2):3747–3764; J Clim 16(4):571–591, 2003)] flux algorithm in the tropical Indian Ocean. Both experiments are performed for the period 2014–2017. The model is forced with daily analyzed fields of winds, radiation and freshwater fluxes from ERA-Interim. The performance of the CR and ER with respect to in situ and satellite observations is examined for the year 2015 in the Bay of Bengal (BoB). COARE3.0 weakens the surface wind stress by ~ 20% and increases the basin-averaged net heat flux by ~ 14%, and makes the sea surface temperature (SST) warmer by around 0.3–0.9 °C in the BoB in the ER. SST simulations were compared with observations, which revealed that in the ER, the SST errors were reduced by 5–40%, and errors in the temperature profile were significantly reduced by ~ 10 to 40% up to a depth of 80 m. BoB heat budget analysis showed that COARE3.0 significantly increased the upper ocean heat content, caused by a reduction in meridional heat transport across the 10° N latitude. This reduction in meridional heat transport is attributed to the reduced strength of upper ocean circulation resulting in the weakening of meridional volume transport (~ 25%). These findings indicate that COARE3.0 derived fluxes better simulate upper ocean thermal structure in the BoB. [ABSTRACT FROM AUTHOR]

A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden-Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean. [ABSTRACT FROM AUTHOR]

By conducting sensitivity experiments with the Whole Atmosphere Community Climate Model, this study reveals the interference of the sea surface temperature (SST) forcing in the southern winter stratosphere between the tropical Indian Ocean (TIO) and tropical Pacific Ocean (TPO) during ENSO. Similar to the variance pattern of zonal-mean zonal winds with two maximum centers on both flanks of the southern polar night jet, the southern circumpolar zonal wind responses to TPO and TIO SST forcings also exhibit a dipole pattern, indicating their meridional shifting effect on the polar jet. However, a warm TPO SST favors an equatorward shift of the polar jet, while a warm TIO SST induces a poleward shift. This has resulted in an observed weaker ENSO signal in the southern winter stratosphere than that purely forced by the ENSO TPO SST. Further diagnosis indicates that the interference between the TIO and TPO is dynamically associated with the destructive interference of the wave train forced by the TIO with the Pacific–South America-like wave train pattern forced by the TPO. As a result, the southern extratropical wavenumber-1 response to the TPO is nearly out of phase with that to the TIO throughout the troposphere and stratosphere. While upward propagation of planetary waves is significantly suppressed in midlatitudes but enhanced in high latitudes when warm SST anomalies are confined over the TPO, the opposite pattern is observed when warm anomalies are confined over the TIO. Furthermore, the significant destructive interference between the TIO and TPO in their forcings in the southern winter stratosphere is also confirmed in reanalysis data. [ABSTRACT FROM AUTHOR]

The present study investigates the interdecadal modulations of interannual variability in the Indo-western Pacific Ocean Capacitor (IPOC) mode and its influence on the Indian summer monsoon (ISM) rainfall during 1886–2014. The IPOC mode is extracted using singular value decomposition (SVD) analysis of tropical Indian Ocean (TIO) sea surface temperature (SST) and western North Pacific (WNP) 850 hPa relative vorticity anomalies during the boreal summer season (June to August; JJA). Analysis suggests that the IPOC mode induces a southwest-northeast dipole pattern in the Indian subcontinent rainfall with significant positive anomalies over the southern peninsular India and negative anomalies over the eastern parts of the Indo-Gangetic plains during the positive phase of IPOC. The 21-year sliding correlation coefficients between rainfall dipole index and leading principal component of SVD analysis (SVD-PC1) suggest that IPOC and ISM rainfall relationship exhibits significant interdecadal variability during the study period. The entire period has been divided into three distinct epochs as 1895–1926 (epoch-1), 1932–1972 (epoch-2) and 1976–2007 (epoch-3) based on 21-year sliding correlation. It is found that the IPOC mode strongly affect ISM rainfall in epoch-1 and epoch-3, whereas this relationship between the IPOC mode and ISM rainfall is inadequate in epoch-2 pertaining to the absence of the dipole rainfall pattern. The mechanisms responsible for such epochal changes in relationship between IPOC mode and ISM rainfall are discussed. In epoch-1 and epoch-3, anomalous TIO SST and the Pacific–Japan (PJ) pattern/WNP circulation both are well organized as part of the IPOC mode. However, in epoch-2 vigorous warm SSTs over the equatorial central Pacific and cyclonic circulation over the north Pacific accompanied by SST cooling around 40°N are noticed. The north Pacific cyclonic circulation accelerates the southwesterlies emerging from north-western flank of WNP anticyclone. On the other hand, positive SST anomalies and associated low-level convergence in central equatorial Pacific limit the extension of northeasterlies from the southern edge of WNP anticyclone to TIO. As a result, WNP anticyclone is squeezed/twisted between them, thereby weakening and shifting it northeastward. This in turn weakens the PJ mode and the inter-basin interaction of WNP circulation and TIO SSTs, causing disorganized IPOC pattern in epoch-2. [ABSTRACT FROM AUTHOR]

The variability of boreal spring Hadley circulation (HC) over the Asian monsoon domain over the last four decades is explored. The climatological distribution of the regional HC is symmetric of the equator, with the ascending branch around the equator and sinking branch around the subtropics in each hemisphere. The first dominant mode (EOF1) of the regional HC is equatorial asymmetric, with the main body in the Southern Hemisphere (SH) and the ascending branch to the north of the equator. This mode is mainly characterized by interannual variation and is related to El Niño-Southern Oscillation (ENSO). Significant negative sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean (TIO) along with the development of La Niña events; however, the magnitude of SST anomalies in the southern Indian Ocean is greater than that in the northern counterpart, contributing to EOF1 formation. The spatial distribution of the second dominant mode (EOF2) is with the main body lying in the Northern Hemisphere (NH) and the ascending branch located to the south of the equator. The temporal variation of this mode is connected to the warming of the TIO. The warming rate of the southern TIO SST is faster than that in the northern counterpart, resulting in the southward migration of the rising branch. The above result indicates the critical role of the meridional distribution of SST on the variability of the regional HC. [ABSTRACT FROM AUTHOR]

The present study examined the ability of Coupled Model Intercomparison Project phase 5 (CMIP5) models in representing the differences in decaying phases of El Niño and their impact on south Asia and east Asia monsoon during June and July (JJ), the early summer monsoon months. El Niño decay is classified into three categories, based on the timing of decay with respect to the summer season after the peak phase of El Niño. Analysis suggests that many CMIP5 models are able to capture the differences in the decaying phase of El Niño. Observed rainfall anomalies are positive over most parts of south and east Asia regions during the early decay (ED) years mainly due to anomalous Tropical Indian Ocean (TIO) Sea Surface Temperature (SST) warming and La Niña like forcing from the eastern Pacific. Most models show significant skill in capturing the positive rainfall anomalies over south Asia and TIO SST warming in ED years. However, many models have failed to represent east Asian rainfall anomalies due to weak Western North Pacific (WNP) anticyclone and its association with El Niño‐Southern Oscillation. In case of mid‐decay (MD) years, observed rainfall anomalies over south Asia is negative especially in the monsoon trough region and positive over east Asian region. Low‐level divergence induced by anomalous WNP anticyclone extending to head Bay of Bengal and Gangetic Plain region in MD years caused low rainfall over south Asia. These features are however not well organized in many CMIP5 models. In no decay (ND) years, the rainfall anomalies over south and east Asia regions are negative in almost all the CMIP5 models, which is consistent with the observations. This study highlights the importance of proper representation of differences in the decaying phase of El Niño and associated teleconnections in CMIP5 models. [ABSTRACT FROM AUTHOR]

In this study, we for the first time observed two subsurface cyclonic eddies (SCEs) in the southeastern Tropical Indian Ocean. One SCE (SCE‐C) was observed along 8°S transect during the cruise conducted in July 2016. Another SCE (SCE‐A) was also observed near 8°S but by an Argo float in November 2005. The SCE‐C had a maximum thickness of about 500 m (300–800 m) and a radius of ~90 km, while the SCE‐A has a vertical extent of 700 m (200–900 m) and a radius larger than 110 km. The water trapped by SCE shows a three‐compartment structure. Both the two SCEs are characterized by stratified water with negative lens structure and cause prominent temperature/salinity/spiciness anomalies in a wide density range. As the water carried by SCEs is stratified, we can trace the SCE origin by finding an area sharing the same water property with the SCE on different density surfaces. The result suggests that the two SCEs originate from the southwest coast of the Sumatra Island. Considering the topography and hydrologic characteristics of the SCE origin, we speculate that the SCEs are generated due to topography‐induced South Java Undercurrent instability. Plain Language Summary: Subsurface eddies usually have long lifespans and can carry fluid parcels and transport them far away from the eddy origin. However, compared with surface eddies, subsurface eddies are poorly observed because they usually have small or no surface signatures. In this study, we for the first time observed two subsurface cyclonic eddies (SCEs) in the southeastern tropical Indian Ocean. The SCEs were characterized by stratified water with negative lens structure. Further analysis suggested that the SCEs should be generated off southwest coast of the Sumatra Island due to topography‐induced South Java Undercurrent instability and move southwestward. These results provide new insight into the SCEs in the southeastern tropical Indian Ocean. Key Points: Two subsurface cyclonic eddies are observed in the southeastern Tropical Indian OceanThe subsurface cyclonic eddies are characterized by stratified water and cause prominent water property anomalies in a wide density rangeThe subsurface cyclonic eddies should be generated off southwest coast of the Sumatra due to topography‐induced undercurrent instability [ABSTRACT FROM AUTHOR]

The tropical Indian Ocean consists of three basins, namely the Arabian Sea (AS), Bay of Bengal (BoB) and Southern Indian Ocean (SIO), with relatively nutrient-rich waters in the former two basins. It is hypothesized that the excess carbon produced in the northern Indian Ocean may support heterotrophic carbon demand in the SIO. In order to test this hypothesis, deck incubation experiments were conducted during the spring intermonsoon under the aegis of the Indian-GEOTRACES program. Nutrients in the mixed layer were low in the SIO compared to AS and BoB due to strong thermal stratification in the former region. Dominant net autotrophy was noticed in the AS whereas net heterotrophy in the BoB and SIO. High community respiration (CR) was observed in the BoB which may be supported by riverine organic carbon, whereas in situ produced and advected excess carbon from the northern Indian Ocean may support in AS and SIO respectively. Net community production (NCP) displayed an inverse (linear) relationship with temperature (salinity) in the euphotic zone in the BoB and SIO suggesting that stratification driven by river discharge and equatorial currents, respectively, reduced nutrients inputs through vertical mixing in the upper ocean resulting in the formation of the strong net heterotrophy and contrast to this was found in the AS due to increase in primary production due to nitrogen fixation. The euphotic zone integrated nutrients displayed a linear relationship with NCP and Gross Primary Production (GPP) indicating that the availability of nutrients controlled the plankton metabolic rates in the tropical Indian Ocean. The threshold of GPP for plankton metabolic balance in the tropical Indian Ocean (1.9 mmol O 2 m−3 d−1) was close to that of the global mean (2.2 mmol O 2 m−3 d−1). The slope of the log-log relationship between GPP and CR was 0.5 and it is close to that of the global mean value of 0.60. [ABSTRACT FROM AUTHOR]

This paper examines the relative role of the Indian Ocean basin-wide (IOBW) mode and El Niño–Southern Oscillation (ENSO) in the atmospheric circulation and rainfall interannual variations over South America (SA) during southern summer of the 1951‒2016 period. The effects of the warm IOBW and El Niño (EN) events, and of the cold IOBW and La Niña (LN) events are examined using partial correlations. The ENSO and IOBW modes, through the associated large-scale and regional anomalous circulation patterns, induce contrasting effects on the rainfall in northeastern SA. The EN without the warm IOBW effect induces anomalously dry conditions over eastern Amazon and part of northeastern Brazil (NEB) through anomalous sinking motions of the EN-related anomalous Walker and Hadley cells and strong moisture divergence associated with a vigorous anticyclone over tropical South Atlantic (TSA) and SA. The warm IOBW without the EN effect induces anomalously wet conditions in NEB, which is marginally related to the anomalous Walker and Hadley cells but is modulated by an anticyclone over SA between the equator and 20° S, and a cyclone in the southwestern Atlantic between 20° S and 40° S. The results here might be relevant for climate monitoring and modeling studies.

The effects of air–sea coupling over the tropical Indian Ocean (TIO) on the eastward propagating boreal winter intraseasonal oscillation (MJO) are investigated by comparing a fully coupled and a partially decoupled Indian Ocean experiment using the SINTEX-F coupled model. Air–sea coupling over the TIO significantly enhances the intensity of the eastward propagations of the MJO along the 5°–10°S zonal areas. The zonal asymmetry of the SST anomaly (SSTA) is responsible for the enhanced eastward propagation. A positive SSTA appears to the east of the MJO convection, which results in the boundary layer moisture convergence and positively feeds back to the MJO convection. In addition, the air–sea interaction effect on the eastward propagation of the MJO is related to the interannual variations of the TIO. Air–sea coupling enhances (reduces) the eastward-propagating spectrum during the negative Indian Ocean dipole mode and positive Indian Ocean basin mode. Such phase dependence is attributed to the role of the background mean westerly in affecting the wind–evaporation–SST feedback. Air–sea coupling (decoupling) enhances (reduces) the zonal asymmetry of the low-level specific humidity, and thus the eastward propagation spectrum of the MJO.

The influence of Indo-Western Pacific Ocean Capacitor (IPOC) mode on South Asian summer monsoon rainfall variability is investigated in the present study. It is found from the observations that the IPOC mode induces a tripole pattern in precipitation anomalies over the South Asian region. Strong positive precipitation anomalies over some parts of western Ghats and Sundarbans and Bangladesh region separated by negative precipitation anomalies over the monsoon trough region are found to be the main characteristic features closely associated with the IPOC mode. It is noted that anomalous western north Pacific (WNP) anticyclone and tropical Indian Ocean (TIO) warming maintain a tripole precipitation pattern over the South Asian region. Anomalous low level convergence in the northern flank of WNP anticyclone and orographic lift cause enhanced rainfall over the Sundarbans and Bangladesh region. Meanwhile, westward propagating atmospheric cold Rossby wave as a response to suppressed convection over the WNP region induced moisture divergence and downward motion play important role in maintaining negative rainfall anomalies over the monsoon trough. At the same time, TIO SST warming supported by easterly wind anomalies associated with WNP anticyclone has contributed to the enhanced rainfall over parts of eastern Arabian Sea and western Ghats. Coupled General Circulation Model sensitivity experiments are performed to investigate individual and combined impact of the TIO warming and WNP circulation on south Asian summer monsoon rainfall variability associated with IPOC mode. The model experiments revealed that WNP anticyclone influences the rainfall over the monsoon trough region and Sundarbans-Bangladesh region. Further the moisture convergence induced by local SST warming is mainly helping to sustain enhanced Western Ghats rainfall. Sub-seasonal variability of ISM rainfall associated with the IPOC modes is also discussed. This study advocates the importance of IPOC mode in exerting strong impact on regional summer rainfall variability over the Indian/South Asian summer monsoon region on the interannual time scale. Thus, it is important to consider the possible roles of TIO warming and WNP circulation in predicting the South Asian summer monsoon rainfall. [ABSTRACT FROM AUTHOR]

The response of the eastern tropical Indian Ocean (ETIO) to heat fluxes of equal amplitude but opposite sign is investigated using the Community Earth System Model (CESM). A significant positive asymmetry in sea surface temperature (SST) is found over the ETIO—the warming responses to the positive forcing exceeds the cooling to the negative forcing. A mixed layer heat budget analysis is carried out to identify the mechanisms responsible for the SST asymmetry. Results show that it is mainly ascribed to the ocean dynamical processes, including vertical advections and diffusion. The net surface heat flux, on the contrary, works to reduce the asymmetry through its shortwave radiation and latent heat flux components. The former is due to the nonlinear relationship between SST and cloud, while the latter is resulted mainly from Newtonian damping and air-sea stability effects. Changes in the SST skewness are also evaluated, with more enhanced negative SST skewness over the ETIO found for the cooling than heating scenarios due to the asymmetric thermocline-SST feedback. [ABSTRACT FROM AUTHOR]

The relationship between the tropical Indian Ocean (TIO) and East Asian summer monsoon/precipitation has been documented in many studies. However, the precursor signals of summer precipitation in the TIO sea surface temperature (SST), which is important for climate prediction, have drawn little attention. This study identified a strong relationship between early-spring TIO SST and subsequent early-summer precipitation in Northeast China (NEC) since the late 1980s. For 1961-1986, the correlations between early-spring TIO SST and early-summer NEC precipitation were statistically insignificant; for 1989-2014, they were positively significant. Since the late 1980s, the early-spring positive TIO SST anomaly was generally followed by a significant anomalous anticyclone over Japan; that facilitated anomalous southerly winds over NEC, conveying more moisture from the North Pacific. Further analysis indicated that an early TIO SST anomaly showed robust persistence into early summer. However, the early-summer TIO SST anomaly displayed a more significant influence on simultaneous atmospheric circulation and further affected NEC precipitation since the late 1980s. In 1989-2014, the early-summer Hadley and Ferrell cell anomalies associated with simultaneous TIO SST anomaly were much more significant and extended further north to mid-latitudes, which provided a dynamic foundation for the TIO-mid-latitude connection. Correspondingly, the TIO SST anomaly could lead to significant divergence anomalies over the Mediterranean. The advections of vorticity by the divergent component of the flow effectively acted as a Rossby wave source. Thus, an apparent Rossby wave originated from the Mediterranean and propagated east to East Asia; that further influenced the NEC precipitation through modulation to the atmospheric circulation (e.g., surface wind, moisture, vertical motion). [ABSTRACT FROM AUTHOR]

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

This study investigates observed and modeled contributions of global sea surface temperature (SST) to China winter climate trends in 1960-2014, including increased precipitation, warming through about 1997, and cooling since then. Observations and Atmospheric Model Intercomparison Project (AMIP) simulations with prescribed historical SST and sea ice show that tropical Indian Ocean (TIO) warming and increasing rainfall causes diabatic heating that generates a tropospheric wave train with anticyclonic 500-hPa height anomaly centers in the TIO or equatorial western Pacific (TIWP) and northeastern Eurasia (EA) and a cyclonic anomaly over China, referred to as the TIWP-EA wave train. The cyclonic anomaly causes Indochina moisture convergence and southwesterly moist flow that enhances South China precipitation, while the northern anticyclone enhances cold surges, sometimes causing severe ice storms. AMIP simulations show a 1960-1997 China cooling trend by simulating increasing instead of decreasing Arctic 500-hPa heights that move the northern anticyclone into Siberia, but enlarge the cyclonic anomaly so it still simulates realistic China precipitation trend patterns. A separate idealized TIO SST warming simulation simulates the TIWP-EA feature more realistically with correct precipitation patterns and supports the TIWP-EA teleconnection as the primary mechanism for long-term increasing precipitation in South China since 1960. Coupled Model Intercomparison Project (CMIP) experiments simulate a reduced TIO SST warming trend and weak precipitation trends, so the TIWP-EA feature is absent and strong drying is simulated in South China for 1960-1997. These simulations highlight the need for accurately modeled SST to correctly attribute regional climate trends. [ABSTRACT FROM AUTHOR]

High-resolution ocean general circulation model (OGCM) simulations are employed to investigate interannual variability of the upper-ocean temperature in the tropical Indian Ocean (20°S–20°N). The seasonal cycle and interannual variability in the upper-ocean temperature in the tropical Indian Ocean in the forced ocean simulation are in good agreement with available observation and reanalysis products. Two further sensitivity OGCM simulations are used to separate the relative contributions of heat flux and wind stress. The comparison of the model simulations reveals the depth-dependent influences of heat flux and wind stress on the ocean temperature variability in the tropical Indian Ocean. Generally, heat flux dominates the temperature variability in the top 30 m, while wind stress contributes most strongly to the subsurface temperature variability below 30 m. This implies that a transition depth should exist at each location, where the dominant control of the ocean temperature variability switched from heat flux to wind stress. We define the depth of this transition point as the ‘crossing depth’ and make use of this concept to better understand the depth-dependent impacts of the heat flux and wind stress on the upper-ocean temperature variability in the tropical Indian Ocean. The crossing depth tends to be shallower in the southern tropical Indian Ocean (20°S-EQ), including the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IOD), suggesting the dominance of forcing due to wind stress and the resulting ocean dynamical processes in the temperature variability in those regions. The crossing depth also shows prominent seasonal variability in the southern tropical Indian Ocean. In the SCTR, the variability of the subsurface temperature forced by the wind stress dominates largely in boreal winter and spring, resulting in the shallow crossing depth in these seasons. In contrast, the intensified subsurface temperature variability with shallow crossing depth in the eastern part of the IOD is seen during boreal autumn. Overall, our results suggest that the two regions within the tropical Indian Ocean, the SCTR and the eastern part of the IOD, are the primary locations where the ocean dynamics due to wind-stress forcing control the upper-ocean temperature variability.

This study investigates the influences of tropical Indian Ocean (TIO) warming on tropical cyclone (TC) genesis in different regions of the western North Pacific (WNP) from July to October (JASO) during the decaying El Ni no. The results show significant negative TC frequency anomalies localized in the southeastern WNP. Correlation analysis indicates that a warm sea surface temperature anomaly (SSTA) in the TIO strongly suppresses TC genesis south of 21°N and east of 140°E in JASO. Reduced TC genesis over the southeastern WNP results from a weak monsoon trough and divergence and subsidence anomalies associated with an equatorial baroclinic Kelvin wave. Moreover, suppressed convection in response to a cold local SSTA, induced by the increased northeasterly connected by the wind-evaporation-SST positive feedback mechanism, is found unfavorable for TC genesis. Positive TC genesis anomalies are observed over higher latitudinal regions (at around 21 ° N, 140° E) and the western WNP because of enhanced convection along the northern flank of the WNP anomalous anticyclone and low-level convergence, respectively. Although local modulation (e.g., local SST) could have greater dominance over TC activity at higher latitudes in certain anomalous years (e.g., 1988), a warm TIO SSTA can still suppress TC genesis in lower latitudinal regions of the WNP. A better understanding of the contributions of TIO warming could help improve seasonal TC predictions over different regions of the WNP in years of decaying El Nino. [ABSTRACT FROM AUTHOR]

This study investigated the effects of sea surface temperature (SST) in different tropical regions on tropical cyclone (TC) genesis over the western North Pacific (WNP) in May. The results revealed that positive SST anomalies over the tropical Pacific (Indian) Ocean in March may lead to increased (decreased) numbers of TCs over the WNP in May by affecting the atmospheric circulation anomalies over the WNP. Warmer SSTs over the tropical Pacific Ocean (TPO) may lead to stronger low-level southwesterly winds and enhanced convection. However, warmer SSTs over the tropical Indian Ocean (TIO) may enhance the western part of the western Pacific subtropical high (WPSH) and suppress convection. Further analysis suggests that changes in the atmospheric circulation anomalies over the WNP are related to changes in environmental factors. Concomitantly with the positive SST anomalies over the TPO, a favorable environment for TC genesis in May is present, with stronger low-level relative vorticity and upper-level divergence, smaller vertical wind shear and abundant water vapor. In contrast, the positive SST anomalies over the TIO might lead to an unfavorable environment for TC genesis over the WNP. This study also investigated the joint contributions of the TPO and TIO, and the results indicate that positive SST anomalies over the TPO and negative SST anomalies over the TIO may lead to an increased number of TC geneses. The analysis of the energy budget suggests that the joint activity of the TPO and TIO influences the barotropic eddy kinetic energy conversions and is mainly attributed to the contributions from the meridional shear of the mean zonal winds and the zonal wind convergence. [ABSTRACT FROM AUTHOR]

Using the NCEP/NCAR reanalysis and HadISST sea surface temperature (SST) data, the joint effects of the tropical Indian Ocean and Pacific on variations of area of the summertime western Pacific subtropical high (WPSH) for period 1980-2016 are investigated. It is demonstrated that the central tropical Indian Ocean (CTI) and central equatorial Pacific (CEP) are two key oceanic regions that affect the summertime WPSH. During autumn and winter, warm SST anomalies (SSTAs) in CEP force the Walker circulation to change anomalously, resulting in divergence anomalies over the western Pacific and Maritime Continent (MC). Due to the Gill-type response, the abnormal anticyclonic circulation is generated over the western Pacific and South China Sea (SCS). In the subsequent spring, the warm SSTAs in CEP weaken, while the SST over CTI demonstrates a lagged response to Pacific SSTA. The warm CTISSTA and CEP-SSTA cooperate with the eastward propagation of cold Kelvin waves in the western Pacific, leading to the eastward shift of the abnormal divergence center that originally locates at the western Pacific and MC. The anticyclone forced by this divergence subsequently moves eastward, leading to the intensification of the negative vorticity there. Meanwhile, warm SSTA in CTI triggers eastward propagating Kelvin waves, which lead to easterly anomalies over the equatorial Indian Ocean and Indonesia, being favorable for maintenance and intensification of the anticyclone over the SCS and western Pacific. The monsoonal meridional-vertical circulation strengthens, which is favorable for the intensification of the WPSH. Using SSTA over the two key oceanic regions as predictors, a multiple regression model is successfully constructed for prediction of WPSH area. These results are useful for our better understanding the variation mechanisms of WPSH and better predicting summer climate in East Asia. [ABSTRACT FROM AUTHOR]

Delayed impact of El Niño on Tropical Indian Ocean (TIO) Sea Surface Temperature (SST) variations and associated physical mechanisms are well documented by several studies. However, TIO SST evolution during the decay phase of La Niña and related processes are not adequately addressed before. Strong cooling associated with La Niña decay over the TIO could influence climate over the Indian Oceanic rim including Indian summer monsoon circulation and remotely northwest Pacific circulation. Thus understanding the TIO basin-wide cooling and related physical mechanisms during decaying La Niña years is important. Composite analyses revealed that negative SST anomalies allied to La Niña gradually dissipate from its mature phase (winter) till subsequent summer in central and eastern Pacific. In contrast, magnitude of negative SST anomalies in TIO, induced by La Niña, starts increasing from winter and attains their peak values in early summer. It is found that variations in heat flux play an important role in SST cooling over the central and eastern equatorial Indian Ocean, Bay of Bengal and part of Arabian Sea from late winter to early summer during the decay phase of La Niña. Ocean dynamical processes are mainly responsible for the evolution of southern TIO SST cooling. Strong signals of westward propagating upwelling Rossby waves between 10°S to 20°S are noted throughout (the decaying phase of La Niña) spring and summer. Anomalous cyclonic wind stress curl to the south of the equator is responsible for triggering upwelling Rossby waves over the southeastern TIO. Further, upwelling Rossby waves are also apparent in the Arabian Sea from spring to summer and partly contributing to the SST cooling. Heat budget analysis reveals that negative SST/MLT (mixed layer temperature) anomalies over the Arabian Sea are mostly controlled by heat flux from winter to spring and vertical advection plays an important role during early summer. Vertical and horizontal advection terms primarily contribute to the SST cooling anomalies over southern TIO and the Bay of Bengal cooling is primarily dominated by heat flux. Further we have discussed influence of TIO cooling on local rainfall variations. [ABSTRACT FROM AUTHOR]

The inter-annual relationship between the boreal winter Arctic Oscillation (AO) and summer sea surface temperature (SST) over the western tropical Indian Ocean (TIO) for the period from 1979 to 2015 is investigated. The results show that the January-February-March AO is significantly correlated with the June-July-August SST and SST tendency. When both El Niño/Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) variance are excluded, the winter AO is significantly correlated with the regional mean SST of the western TIO (40 $$^\circ$$ - $$60^\circ$$ E and $$10^\circ$$ S- $$10^\circ$$ N), $$r=0.71$$ . The multi-month SST tendency, i.e., the SST difference of June-July-August minus April-May, is correlated with the winter AO at $$r=0.75$$ . Composite analysis indicates similar warming over the western TIO. Two statistical models are established to predict the subsequent summer's SST and SST tendency. The models use the winter AO, the winter ENSO and the autumn-winter IOD indexes as predictors and explain 65 and 62 % of the variance of the subsequent summer's SST and SST tendency, respectively. Investigation of the regional air-sea fluxes and oceanic dynamics reveals that the net surface heat flux cannot account for the warming, whereas the oceanic Rossby wave plays a predominant role. During positive AO winters, the enhanced Arabian High causes stronger northern winds in the northern Indian Ocean and leads to anomalous cross-equatorial air-flow. The Ekman pumping in association with the anomalous wind stress curl in the central TIO generates a significantly deeper thermocline and above-normal sea surface height at 60 $$^\circ$$ -75 $$^\circ$$ E and 5 $$^\circ$$ - $$10^\circ$$ S. The winter AO-forced Rossby wave propagates westward and arrives at the western coast in summer, resulting in the significant SST increase. Forced by the observed winter AO-related wind stress anomalies over the Indian Ocean, the ocean model reasonably reproduces the Rossby wave as well as the resulting surface ocean warming over the western TIO in the subsequent summer. Observational analysis and numerical experiments suggest the importance of the oceanic dynamics in connecting the winter AO and summer SST anomalies. [ABSTRACT FROM AUTHOR]

The characteristics of droughts and floods in China during the summers (May-August) of 2016 and 1998 were compared in great detail, together with the associated atmospheric circulations and external-forcing factors. Following results are obtained. (1) The precipitation was mostly above normal in China in summer 2016, with two main rainfall belts located in the Yangtze River valley (YRV) and North China. Compared with 1998, a similar rainfall belt was located over the YRV, with precipitation 100% and more above normal. However, the seasonal processes of Meiyu were different. A typical 'Secondary Meiyu' occurred in 1998, whereas dry conditions dominated the YRV in 2016. (2) During May-July 2016, the Ural high was weaker than normal, but it was stronger than normal in 1998. This difference resulted from fairly different distributions of sea surface temperature anomalies (SSTAs) over the North Atlantic Ocean during the preceding winter and spring of the two years. (3) Nonetheless, tropical and subtropical circulation systems were much more similar in May-July of 2016 and 1998. The circulation systems in both years were characterized by a stronger than normal and more westward-extending western Pacific subtropical high (WPSH), a weaker than normal East Asian summer monsoon (EASM), and anomalous convergence of moisture flux in the mid and lower reaches of the YRV. These similar circulation anomalies were attributed to the similar tropical SSTA pattern in the preceding seasons, i.e., the super El Niño and strong warming in the tropical Indian Ocean. (4) Significant differences in the circulation pattern were observed in August between the two years. The WPSH broke up in August 2016, with its western part being combined with the continental high and persistently dominating eastern China. The EASM suddenly became stronger, and dry conditions prevailed in the YRV. On the contrary, the EASM was weaker in August 1998 and the 'Secondary Meiyu' took place in the YRV. The Madden-Julian Oscillation (MJO) was extremely active in August 2016 and stayed in western Pacific for 25 days. It triggered frequent tropical cyclone activities and further influenced the significant turning of tropical and subtropical circulations in August 2016. In contrast, the MJO was active over the tropical Indian Ocean in August 1998, conducive to the maintenance of a strong WPSH. Alongside the above oceanic factors and atmospheric circulation anomalies, the thermal effect of snow cover over the Qinghai-Tibetan Plateau from the preceding winter to spring in 2016 was much weaker than that in 1998. This may explain the relatively stronger EASM and more abundant precipitation in North China in 2016 than those in 1998. [ABSTRACT FROM AUTHOR]

The Oceansat-2 scatterometer (OSCAT) of the Indian Space Research Organization (ISRO), provides surface wind speed and direction with a spatial resolution of 50 km × 50 km. With a revisit time of 2 days it had provided ocean surface wind vectors over the global oceans. In the present work, an attempt has been made to generate two day composite of OSCAT wind vectors using Data-Interpolating Variational Analysis (DIVA) and compare them with daily composite winds to check how better is the two day composites in comparison to daily composites. The daily and two days composite wind vectors of zonal ( U) and meridional ( V) components have been validated with wind measurements from in situ buoys and Advanced Scatterometer (ASCAT) for the year 2012 over the tropical Indian Ocean region. The statistical comparison with the in situ measurements and ASCAT has shown that the two-day OSCAT wind composites are slightly better than the daily composite winds. The improvement in the statistics can be attributed to the use of ascending and descending passes pertaining to two days which results in fewer gaps between passes, thereby reducing the interpolation errors. [ABSTRACT FROM AUTHOR]

Based on the 10 years sea surface salinity (SSS) data from Argo, we identified a salinity dipole mode in the tropical Indian Ocean, termed S-IOD: a pattern of interannual SSS variability with anomalously low-salinity in the central equatorial and high-salinity in the southeastern tropical Indian Ocean. The S-IOD matures in November-December, lagging the Indian Ocean dipole (IOD) mode derived from sea surface temperature (SST) by 2 months. For the period of observations, the S-IOD persists longer than the IOD, until the following September-October. Oscillations of the two S-IOD poles are governed by different processes. Ocean advection associated with equatorial current variability dominates the SSS anomalies of the northern pole, while surface freshwater flux variability plays a key role in the SSS anomalies of the southern pole, where anomalous precipitation is sustained by preformed sea surface temperature anomalies. The S-IOD concurs with the strong IOD, reflecting an ocean-atmosphere coupling through the SST-precipitation-SSS feedback. [ABSTRACT FROM AUTHOR]

The bio-optical characteristics of southwestern tropical Indian Ocean (SWTIO) and Equatorial Indian Ocean (EIO). [Display omitted] • Contrasting biophysical and bio-optical environments were observed in SWTIO and EIO. • DCM shows biomass maxima at SWTIO and photoacclimation maxima at EIO. • Surface absorption characteristics suggested phytoplankton are in the growth phase. • Backscattering properties explained carbon fixation by phytoplankton. • CDOM in this region are from biological and photochemical sources. The bio-optical characteristics of the surface ocean play a pivotal role in radiative transfer and photosynthetic carbon fixation. To examine spatiotemporal variability of bio-optical properties in the tropical Indian Ocean, a 10-day time-series comparative study was made at one fixed and 3 variable stations in the southwestern tropical Indian Ocean (SWTIO), and 4-stations in the Equatorial Indian Ocean (EIO) during June 2014. A total of 24 hyperspectral radiometer profiles were collected along with physicochemical parameters and phytoplankton pigments analysis. A negative sea level anomaly (SLA), higher precipitation, a more stable mixed layer, and higher buoyancy frequency (N2) were observed in SWTIO compared to EIO. The deep chlorophyll maxima (DCM) in SWTIO were characterised by deep biomass maxima (DBM) whereas in the EIO it was due to deep photoacclimation maxima (DAM). The photosynthetically available radiation (PAR) profiles were obtained by integrating downwelling irradiance from 400 to 700 nm suggesting that the SWTIO had a greater abundance of optically active substances (OAS) than the EIO. Surface chlorophyll a (Chl-a) was found to have significant relationships with phytoplankton absorption coefficient (a ph (443)). Chl-a-specific phytoplankton absorption coefficients (a* ph (443)), total absorption coefficient (a t (443)), and particulate backscattering (b bp (555)) were also related to Chl-a implying that the phytoplankton was in growth/active phase. The time-series variability of the phytoplankton and changes in the photosynthetic pigments (PSP) to photoprotective pigments (PPP) ratio at SWTIO suggested phytoplankton community shift during 5th to 7th days. Microplankton percent-abundances showed distinct linear relationships with Chl-a and b bp (555) implying that carbon fixation per unit Chl-a was higher in microplankton than in nano and picoplankton at SWTIO. The in situ Chl-a, a ph (443), a dg (443), K d (490) and C ¯ pd were used to validate the various empirical algorithms available as well as the satellite-retrieved data. The MODIS and VIIRS data correlated moderately. This study would help towards improving regional optimized algorithms for OAS and provide a baseline understanding of the scale of variability of primary production and carbon flux from space. In addition, this would contribute valuable bio-optical data from an under-sampled geographical area. [ABSTRACT FROM AUTHOR]

Based on the historical and RCP8.5 experiments from 25 Coupled Model Intercomparison Project phase 5 (CMIP5) models, the impacts of sea surface temperature (SST) warming in the tropical Indian Ocean (IO) on the projected change in summer rainfall over Central Asia (CA) are investigated. The analysis is designed to answer three questions: (1) Can CMIP5 models reproduce the observed influence of the IO sea surface temperatures (SSTs) on the CA rainfall variations and the associated dynamical processes? (2) How well do the models agree on their projected rainfall changes over CA under warmed climate? (3) How much of the uncertainty in such rainfall projections is due to different impacts of IO SSTs in these models? The historical experiments show that in most models summer rainfall over CA are positively correlated to the SSTs in the IO. Furthermore, for models with higher rainfall-SSTs correlations, the dynamical processes accountable for such impacts are much closer to what have been revealed in observational data: warmer SSTs tend to favor the development of anti-cyclonic circulation patterns at low troposphere over north and northwest of the Arabian Sea and the Bay of Bengal. These anomalous circulation patterns correspond to significantly enhanced southerly flow which carries warm and moisture air mass from the IO region up to the northeast. At the same time, there is a cyclonic flow over the central and eastern part of the CA which further brings the tropical moisture into the CA and provides essential moist conditions for its rainfall generation. In the second half of twenty-first century, although all the 25 models simulate warmed SSTs, significant uncertainty exists in their projected rainfall changes over CA: half of them suggest summer rainfall increases, but the other half project rainfall decreases. However, when we select seven models out of the 25 based on their skills in capturing the dynamical processes as observed, then the model projected changes are much closer. Five out of the seven models predicted more rainfall over CA. Such a result is helpful for allowing us to attribute part of the observed upward rainfall trend in the CA region in the last several decades to the IO SST warming. [ABSTRACT FROM AUTHOR]

In the present study sea surface salinity (SSS) biases and seasonal tendency over the Tropical Indian Ocean (TIO) in the coupled models [Climate Forecasting System version 1 (CFSv1) and version 2 (CFSv2)] are examined with respect to observations. Both CFSv1 and CFSv2 overestimate SSS over the TIO throughout the year. CFSv1 displays improper SSS seasonal cycle over the Bay of Bengal (BoB), which is due to weaker model precipitation and improper river runoff especially during summer and fall. Over the southeastern Arabian Sea (AS) weak horizontal advection associated with East Indian coastal current during winter limits the formation of spring fresh water pool. On the other hand, weaker Somali jet during summer results for reduced positive salt tendency in the central and eastern AS. Strong positive precipitation bias in CFSv1 over the region off Somalia during winter, weaker vertical mixing and absence of horizontal salt advection lead to unrealistic barrier layer during winter and spring. The weaker stratification and improper spatial distribution of barrier layer thickness (BLT) in CFSv1 indicate that not only horizontal flux distribution but also vertical salt distribution displays large discrepancies. Absence of fall Wyrtki jet and winter equatorial currents in this model limit the advection of horizontal salt flux to the eastern equatorial Indian Ocean. The associated weaker stratification in eastern equatorial Indian Ocean can lead to deeper mixed layer and negative Sea Surface Temperature (SST) bias, which in turn favor positive Indian Ocean Dipole bias in CFSv1. It is important to note that improper spatial distribution of barrier layer and stratification can alter the air-sea interaction and precipitation in the models. On the other hand CFSv2 could produce the seasonal evolution and spatial distribution of SSS, BLT and stratification better than CFSv1. However CFSv2 displays positive bias in evaporation over the whole domain and negative bias in precipitation over the BoB and equatorial Indian Ocean, resulting net reduction in the fresh water availability. This net reduction in fresh water forcing and the associated weaker stratification lead to deeper (than observed) mixed layer depth and is primarily responsible for the cold SST bias in CFSv2. However overall improvement of mean salinity distribution in CFSv2 is about 30 % and the mean error has reduced by more than 1 psu over the BoB. This improvement is mainly due to better fresh water forcing and model physics. Realistic run off information, better ocean model and high resolution in CFSv2 contributed for the improvement. Further improvement can be achieved by reducing biases in the moisture flux and precipitation. [ABSTRACT FROM AUTHOR]

Correlations between sea surface temperature (SST) and outgoing longwave radiation (OLR) anomalies and their implication for seasonal prediction are discussed. The observed correlations exhibit robust seasonality with the maximum (minimum) in boreal spring (autumn) in the eastern equatorial Indian Ocean (EEIO). This relationship is investigated in initial conditions that have been produced with SST nudging for seasonal predictions by a coupled model. It is found that the observed positive correlations in the EEIO cannot be reproduced by the model, despite its success in the Pacific. Thus, the SST nudging is not suitable for initialization of dynamical seasonal forecast for the Indian Ocean. [ABSTRACT FROM AUTHOR]

The characteristics and causes of inhomogeneous warming of the Tropical Indian Ocean (TIO) sea surface temperature during 1900-2005 are investigated based on observations and 16 Coupled Model Intercomparison Project phase 5 (CMIP5) models. Over the TIO, the observed warming trend has more than doubled since 1965, which is well simulated by the CMIP5 historical runs. However, as to spatial warming pattern, observations manifest a double-peak pattern during 1900-1940 and a non-uniform Indian Ocean Mode (IOBM)-like pattern during 1965-2005, which is not captured by the CMIP5 historical runs. Herein, an optimal detection analysis is employed, which indicates that the double-peak warming pattern can be explained well by a combination of Greenhouse Gas (GHG) and natural forcing, and the non-uniform IOBM-like pattern is mostly attributable to anthropogenic forcing. Further, a mixed-layer heat budget analysis shows that atmospheric and oceanic processes, especially latent heat flux from atmospheric forcing part associated with GHG forcing, are beneficial for the warming patterns formation. Our study supports the claim that intrinsic ocean-atmosphere interaction within the TIO is the key mechanism for maintaining the TIO warming. From the model perspective, during 1900-1940, the weak anti-symmetric atmospheric circulation with easterly (northwesterly) anomalies north (south) of the equator helps to sustain the double-peak warming pattern. During 1965-2005, the intensified anti-symmetric wind pattern is in favor of the non-uniform IOBM-like warming pattern. [ABSTRACT FROM AUTHOR]

In this study role of upper ocean processes in the evolution of sea surface temperature (SST) seasonal variations over the tropical Indian Ocean (TIO) is investigated in climate forecast system version1 (CFSv1) and version2 (CFSv2). Analysis reveals that CFSv2 could capture seasonal evolution of SST, wind speed and mixed layer depth better than CFSv1 with some biases. Discrepancy in reproducing the evolution of seasonal SST in coupled models leads to bias in the spatial and temporal distribution of precipitation. This has motivated to carry out mixed layer heat budget analysis in determining seasonal evolution of TIO SST. Spatial pattern of mixed layer heat budget from observations and models suggest that the processes responsible for SST tendency differ from region to region over the TIO. Further it is found that models underestimated SST tendency compared to the observations. Misrepresentation of advective processes and heat flux (HF) over the TIO is mainly responsible for the distortion of seasonal SST change in the coupled models. Sub-regional heat budget analysis reveals that CFSv1 is unable to reproduce the annual cycle of mixed layer temperature (MLT) tendency over the Arabian Sea, while CFSv2 captured the annual cycle of SST with systematic cold bias. Misrepresentation of the annual cycles of net HF and horizontal advection (Hadv) are accountable for the low rate of change of MLT during most of the year. Hadv during summer season is underestimated by 50 and 25 % respectively in CFSv1 and CFSv2. Further, CFSv1 fails to simulate MLT tendency due to improper evolution of HF annual cycle over the Bay of Bengal. Though annual cycle of HF in CFSv2 is well represented over the Bay of Bengal, its contribution to MLT change is underestimated compared to observations. Over the southern TIO region, MLT tendency is dominated by HF and Hadv terms in both observations and models. Contribution of HF to the annual cycle of MLT tendency is underestimated in CFSv1 whereas it is overestimated in CFSv2. Contribution of Hadv to MLT change is underestimated by about 50 % in CFSv1 and 10-20 % in CFSv2 over southern TIO. These errors in HF and Hadv are associated with biases in HF components and surface wind representation. Evolution of lead-lag relationship between HF and MLT/SST in both the observations and models suggest the importance of HF in SST evolution over the TIO region. Over all, CFSv2 produced better SST seasonal/annual cycle in spite of having cold bias. This improvement in CFSv2 may be attributed to better cloud-aerosol-radiation physics, which reduces radiation biases. Updated land-surface, ocean and sea ice processes and ocean component may be responsible for improved circulation and annual cycle of ocean-atmospheric components (winds and ocean circulation). However, there is a requirement for improved parameterization of turbulent HF and radiation estimates in CFSv2 to reduce the cold SST bias. [ABSTRACT FROM AUTHOR]