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Atlantic Niño is the Atlantic equivalent of El Niño-Southern Oscillation (ENSO), and it has prominent impacts on regional and global climate. Existing studies suggest that the Atlantic Niño may arise from local atmosphere-ocean interaction and is sometimes triggered by the Atlantic Meridional Mode (AMM), with overall weak ENSO contribution. By analyzing observational datasets and performing numerical model experiments, here we show that the Atlantic Niño can be induced by the Indian Ocean Dipole (IOD). We find that the enhanced rainfall in the western tropical Indian Ocean during positive IOD weakens the easterly trade winds over the tropical Atlantic, causing warm anomalies in the central and eastern equatorial Atlantic basin and therefore triggering the Atlantic Niño. Our finding suggests that the cross-basin impact from the tropical Indian Ocean plays a more important role in affecting interannual climate variability than previously thought. The Atlantic Niño is an important mode of tropical Atlantic variability that influences the climate conditions in surrounding areas. Here, the authors use observational data and model simulations to show that positive phases of the Indian Ocean Dipole can trigger Atlantic Niño events.

This paper investigates interannual variability of the tropical Indian Ocean (IO) upwelling through analyzing satellite and in situ observations from 1993 to 2016 using the conventional Static Linear Regression Model (SLM) and Bayesian Dynamical Linear Model (DLM), and performing experiments using a linear ocean model. The analysis also extends back to 1979, using ocean–atmosphere reanalysis datasets. Strong interannual variability is observed over the mean upwelling zone of the Seychelles–Chagos thermocline ridge (SCTR) and in the seasonal upwelling area of the eastern tropical IO (EIO), with enhanced EIO upwelling accompanying weakened SCTR upwelling. Surface winds associated with El Niño–Southern Oscillation (ENSO) and the IO dipole (IOD) are the major drivers of upwelling variability. ENSO is more important than the IOD over the SCTR region, but they play comparable roles in the EIO. Upwelling anomalies generally intensify when positive IODs co-occur with El Niño events. For the 1979–2016 period, eastern Pacific (EP) El Niños overall have stronger impacts than central Pacific (CP) and the 2015/16 hybrid El Niño events, because EP El Niños are associated with stronger convection and surface wind anomalies over the IO; however, this relationship might change for a different interdecadal period. Rossby wave propagation has a strong impact on upwelling in the western basin, which causes errors in the SLM and DLM because neither can properly capture wave propagation. Remote forcing by equatorial winds is crucial for the EIO upwelling. While the first two baroclinic modes capture over 80%–90% of the upwelling variability, intermediate modes (3–8) are needed to fully represent IO upwelling.

The equatorial eastern Indian Ocean (EIO) upwelling occurs in the Indian Ocean warm pool, differing from the equatorial Pacific and Atlantic upwelling that occurs in the cold tongue. By analyzing observations and performing ocean model experiments, this paper quantifies the remote versus local forcing in causing interannual variability of the equatorial EIO upwelling from 2001 to 2011 and elucidates the associated processes. For all seasons, interannual variability of thermocline depth in the EIO, as an indicator of upwelling, is dominated by remote forcing from equatorial Indian Ocean winds, which drive Kelvin waves that propagate along the equator and subsequently along the Sumatra–Java coasts. Upwelling has prominent signatures in sea surface temperature (SST) and chlorophyll- a concentration but only in boreal summer–fall (May–October). Local forcing plays a larger role than remote forcing in producing interannual SST anomaly (SSTA). During boreal summer–fall, when the mean thermocline is relatively shallow, SSTA is primarily driven by the upwelling process, with comparable contributions from remote and local forcing effects. In contrast, during boreal winter–spring (November–April), when the mean thermocline is relatively deep, SSTA is controlled by surface heat flux and decoupled from thermocline variability. Advection affects interannual SSTA in all cases. The remote and local winds that drive the interannual variability of the equatorial EIO upwelling are closely associated with Indian Ocean dipole events and to a lesser degree with El Niño–Southern Oscillation.

The Indian Ocean has received increasing attention for its large impacts on regional and global climate. However, sea surface temperature (SST) variability arising from Indian Ocean internal processes has not been well understood particularly on decadal and longer time scales, and the external influence from the tropical Pacific has not been quantified. This paper analyzes the interannual-to-decadal SST variability in the tropical Indian Ocean in observations and explores the external influence from the Pacific versus internal processes within the Indian Ocean using a linear inverse model (LIM). Coupling between Indian Ocean and tropical Pacific SST anomalies (SSTAs) is assessed both within the LIM dynamical operator and the unpredictable stochastic noise that forces the system. Results show that the observed Indian Ocean basin (IOB)-wide SSTA pattern is largely a response to the Pacific ENSO forcing, although it in turn has a damping effect on ENSO especially on annual and decadal time scales. On the other hand, the Indian Ocean dipole (IOD) is an Indian Ocean internal mode that can actively affect ENSO; ENSO also has a returning effect on the IOD, which is rather weak on decadal time scale. The third mode is partly associated with the subtropical Indian Ocean dipole (SIOD), and it is primarily generated by Indian Ocean internal processes, although a small component of it is coupled with ENSO. Overall, the amplitude of Indian Ocean internally generated SST variability is comparable to that forced by ENSO, and the Indian Ocean tends to actively influence the tropical Pacific. These results suggest that the Indian–Pacific Ocean interaction is a two-way process.

Generation and development mechanisms of the Ningaloo Niño are investigated using ocean and atmospheric general circulation model experiments. Consistent with previous studies, northerly wind anomalies off the West Australian coast are critical in generating warm sea surface temperature (SST) anomalies of the Ningaloo Niño, which induce SST warming through reduced turbulent heat loss toward the atmosphere (by decreasing surface wind speed), enhanced Leeuwin Current heat transport, and weakened coastal upwelling. Our results further reveal that northerly wind anomalies suppress the cold dry air transport from the Southern Ocean to the Ningaloo Niño region, which also contributes to the reduced turbulent heat loss. A positive cloud–radiation feedback is also found to play a role. Low stratiform cloud is reduced by the underlying warm SSTAs and the weakened air subsidence, which further enhances the SST warming by increasing downward solar radiation. The enhanced Indonesian Throughflow also contributes to the Ningaloo Niño, but only when La Niña co-occurs. Further analysis show that northerly wind anomalies along the West Australian coast can be generated by both remote forcing from the Pacific Ocean (i.e., La Niña) and internal processes of the Indian Ocean, such as the positive Indian Ocean dipole (IOD). Approximately 40% of the Ningaloo Niño events during 1950–2010 co-occurred with La Niña, and 30% co-occurred with positive IOD. There are also ∼30% of the events independent of La Niña and positive IOD, suggesting the importance of other processes in triggering the Ningaloo Niño.

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 windforced 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 (QT ; 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 QT 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.

Zonal currents in the equatorial Indian Ocean (EIO) are typically considered to exhibit strong semiannual seasonality influenced by the monsoon. However, our study reveals that the Equatorial Undercurrent (EUC) exhibits an unexpected seasonality with prominent annual variability. Constructive (destructive) interference between annual and semiannual components strengthens (weakens) the EUC during early boreal spring (fall). We identify two key mechanisms underlying this behavior: (a) resonant second‐baroclinic‐mode waves enhance equatorial semiannual variability, but their first zero‐crossing near the depth of the EUC limits their contribution to the EUC seasonality; and (b) fourth‐baroclinic‐mode waves profoundly contribute to the annual variability, with equatorial Kelvin and Rossby waves resonating under annual forcing, enhancing the annual response of the EUC. Sensitivity experiments using a linear continuously stratified ocean model indicate that wind forcing in the western EIO (40°−55°E) plays a dominant role in generating the three‐dimensional structure of annual variability of the EUC.

Ocean heat uptake is the primary heat sink of the globe and modulates its surface warming rate. In situ observations during the past half century documented obvious multidecadal variations in the upper-ocean heat content (0–400 m; OHC400) of the Indian Ocean (IO). The observed OHC400 showed an increase of (5.9 ± 2.5) × 1021 J decade−1 during 1965–79, followed by a decrease of (−5.2 ± 2.5) × 1021 J decade−1 during 1980–96, and a rapid increase of (13.6 ± 1.1) × 1021 J decade−1 during 2000–14. These variations are faithfully reproduced by an Indo-Pacific simulation of an ocean general circulation model (OGCM), and insights into the underlying mechanisms are gained through OGCM experiments. The Pacific wind forcing through the Indonesian Throughflow (ITF) was the leading driver of the basin-integrated OHC400 increase during 1965–79 and the decrease during 1980–96, whereas after 2000 local wind and heat flux forcing within the IO made a larger contribution. The ITF heat transport is primarily dictated by Pacific trade winds. It directly affects the south IO, after which the signatures can enter the north IO through the meridional heat transport of the western boundary current. The prevailing warming of the western-to-central IO for 2000–14 was largely induced by equatorial easterly wind trends, Ekman downwelling off the equator, and northeasterly wind trends over the west Asia–East Africa coastal region. The increasing downward longwave radiation, probably reflecting anthropogenic greenhouse gas forcing, overcame the decreasing surface shortwave radiation and also made a significant contribution to the rapid upper-IO warming after 2000.

Modes of climate variability can affect weather extremes, posing intractable challenges to our environment. However, to what extent climate modes can modulate heatwaves in China under a warming background remains poorly understood. Here, we examine the changes in heatwave intensity in seven distinct regions: three East, two middle, and two west regions over China and systematically explore the impacts of climate modes, by analyzing observations and performing model experiments using a Bayesian dynamic linear model and an atmospheric general circulation model (AGCM). Abrupt increases in heatwave intensity are detected across China during a transition period of 1993–2000, and the intensification remains robust in northern and western China after the warming trend being removed. The combined impacts of the El Niño–Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO), and Indian Ocean Dipole (IOD) explain 62.35–70.01% of the observed heatwave intensification in East I, Middle I, West I, and West II regions. Decadal changes of atmospheric circulations associated with the negative phase transition of the Interdecadal Pacific Oscillation (IPO), which is highly correlated with the decadal variability of ENSO, combined with the positive phase transition of the AMO around the mid-1990s increase surface air temperature and enhance atmospheric internal variability and climate modes’ impacts, resulting in the abrupt increase of heatwaves in the past two decades. These results highlight the importance of the concurrent phase transitions of decadal climate modes in regulating heatwaves.

The international scientific community has highlighted decadal and multidecadal climate variability as a priority area for climate research. The Indian Ocean rim region is home to one-third of the world's population, mostly living in developing countries that are vulnerable to climate variability and to the increasing pressure of anthropogenic climate change. Yet, while prominent decadal and multidecadal variations occur in the Indian Ocean, they have been less studied than those in the Pacific and Atlantic Oceans. This paper reviews existing literature on these Indian Ocean variations, including observational evidence, physical mechanisms, and climatic impacts. This paper also identifies major issues and challenges for future Indian Ocean research on decadal and multidecadal variability.