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  1. Abstract

    The Madden‐Julian Oscillation (MJO) is the dominant intraseasonal variability of the tropical troposphere. Many MJOs originate from the tropical Indian Ocean and propagate into the Pacific via the Maritime Continent (MC). However, 30–50% of the MJO stalls over the MC, while its cause remains unclear. Here, we find that a new interbasin coupled phenomenon dubbed “Warm Pool Dipole” (WPD), which is associated with out‐of‐phase sea surface temperature anomalies (SSTAs) in the southeast Indian Ocean and the western central tropical Pacific, may play an important role in controlling the MJO propagation. During the positive WPD, ~67% of the MJO is stalled over the MC, whereas none is stalled during its negative phase. It is the SSTAs at both poles of the WPD together that produce these effects. The results are robust to cross–data set differences and supported by model experiments, although the small observational sample size limits their level of statistical significance.

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  2. Abstract

    A greater warming trend of sea surface temperature in the tropical Indian Ocean than in the tropical Pacific is a robust feature found in various observational data sets. Yet this interbasin warming contrast is not present in climate models. Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure anomalies, which extend to the western tropical Pacific, strengthening the zonal sea level pressure gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce sea surface temperature in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the greenhouse gas warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic greenhouse gas warming.

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  3. Abstract

    In this study, the Indian Ocean upper-ocean variability associated with the subtropical Indian Ocean dipole (SIOD) is investigated. We find that the SIOD is associated with a prominent southwest–northeast sea level anomaly (SLA) dipole over the western-central south Indian Ocean, with the north pole located in the Seychelles–Chagos thermocline ridge (SCTR) and the south pole at southeast of Madagascar, which is different from the distribution of the sea surface temperature anomaly (SSTA). While the thermocline depth and upper-ocean heat content anomalies mirror SLAs, the air–sea CO2 flux anomalies associated with SIOD are controlled by SSTA. In the SCTR region, the westward propagation of oceanic Rossby waves generated by anomalous winds over the eastern tropical Indian Ocean is the major cause for the SLAs, with cyclonic wind causing negative SLAs during positive SIOD (pSIOD). Local wind forcing is the primary driver for the SLAs southeast of Madagascar, with anticyclonic winds causing positive SLAs. Since the SIOD is correlated with ENSO, the relative roles of the SIOD and ENSO are examined. We find that while ENSO can induce significant SLAs in the SCTR region through an atmospheric bridge, it has negligible impact on the SLA to the southeast of Madagascar. By contrast, the SIOD with ENSO influence removed is associated with an opposite SLA in the SCTR and southeast of Madagascar, corresponding to the SLA dipole identified above. A new subtropical dipole mode index (SDMI) is proposed, which is uncorrelated with ENSO and thus better represents the pure SIOD effect.

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  4. null (Ed.)
    Abstract Multi-time-scale variabilities of the Indian Ocean (IO) temperature over 0–700 m are revisited from the perspective of vertical structure. Analysis of historical data for 1955–2018 identifies two dominant types of vertical structures that account for respectively 70.5% and 21.2% of the total variance on interannual-to-interdecadal time scales with the linear trend and seasonal cycle removed. The leading type manifests as vertically coherent warming/cooling with the maximal amplitude at ~100 m and exhibits evident interdecadal variations. The second type shows a vertical dipole structure between the surface (0–60 m) and subsurface (60–400 m) layers and interannual-to-decadal fluctuations. Ocean model experiments were performed to gain insights into underlying processes. The vertically coherent, basinwide warming/cooling of the IO on an interdecadal time scale is caused by changes of the Indonesian Throughflow (ITF) controlled by Pacific climate and anomalous surface heat fluxes partly originating from external forcing. Enhanced changes in the subtropical southern IO arise from positive air–sea feedback among sea surface temperature, winds, turbulent heat flux, cloud cover, and shortwave radiation. Regarding dipole-type variability, the basinwide surface warming is induced by surface heat flux forcing, and the subsurface cooling occurs only in the eastern IO. The cooling in the southeast IO is generated by the weakened ITF, whereas that in the northeast IO is caused by equatorial easterly winds through upwelling oceanic waves. Both El Niño–Southern Oscillation (ENSO) and IO dipole (IOD) events are favorable for the generation of such vertical dipole anomalies. 
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  5. The southeast Indian Ocean (SEIO) exhibits decadal variability in sea surface temperature (SST) with amplitudes of ~0.2–0.3 K and covaries with the central Pacific ( r = −0.63 with Niño-4 index for 1975–2010). In this study, the generation mechanisms of decadal SST variability are explored using an ocean general circulation model (OGCM), and its impact on atmosphere is evaluated using an atmospheric general circulation model (AGCM). OGCM experiments reveal that Pacific forcing through the Indonesian Throughflow explains <20% of the total SST variability, and the contribution of local wind stress is also small. These wind-forced anomalies mainly occur near the Western Australian coast. The majority of SST variability is attributed to surface heat fluxes. The reduced upward turbulent heat flux ( Q T ; latent plus sensible heat flux), owing to decreased wind speed and anomalous warm, moist air advection, is essential for the growth of warm SST anomalies (SSTAs). The warming causes reduction of low cloud cover that increases surface shortwave radiation (SWR) and further promotes the warming. However, the resultant high SST, along with the increased wind speed in the offshore area, enhances the upward Q T and begins to cool the ocean. Warm SSTAs co-occur with cyclonic low-level wind anomalies in the SEIO and enhanced rainfall over Indonesia and northwest Australia. AGCM experiments suggest that although the tropical Pacific SST has strong effects on the SEIO region through atmospheric teleconnection, the cyclonic winds and increased rainfall are mainly caused by the SEIO warming through local air–sea interactions. 
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