Indian Ocean Dipolelike Variability in the CSIRO Mark 3 Coupled Climate Model

Seventeen coupled general circulation models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are employed to assess the relationships of interannual variations of sea surface temperature (SST) between the tropical Pacific (TP) and tropical Indian Ocean (TIO). The eastern/central equatorial Pacific features the strongest SST interannual variability in the models except for the model CSIRO-Mk3-6-0, and the simulated maximum and minimum are produced by models GFDL-ESM2M and GISS-E2-H respectively. However, It remains a challenge for these models to simulate the correct climate mean SST with the warm pool-cold tongue structure in the equatorial Pacific. Almost all models reproduce El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole mode (IOD) and Indian Ocean Basin-wide mode (IOB) together with their seasonal phase lock features being simulated; but the relationship between the ENSO and IOD is different for different models. Consistent with the observation, an Indian Ocean basin-wide warming (cooling) takes place over the tropical Indian Ocean in the spring following an El Nino (La Nina) in almost all the models. In some models (e.g., GFDL-ESM2G and MIROC5), positive ENSO and IOB events are stronger than the negative events as shown in the observation. However, this asymmetry is reversed in some other models (e.g., HadGEM2-CC and HadGEM2-ES).

The NCEP Climate Forecast System version 2 (CFSv2) provides important source of information about the seasonal prediction of climate over the Indo-Pacific oceans. In this study, the authors provide a comprehensive assessment of the prediction of sea surface temperature (SST) in the tropical Indian Ocean (IO). They also investigate the impact of tropical IO SST on the summer anomalous anticyclonic circulation over the western North Pacific (WNPAC), focusing on the relative contributions of local SST and remote forcing of tropical IO SST to WNPAC variations. The CFSv2 captures the two most dominant modes of summer tropical IO SST: the IO basin warming (IOBW) mode and the IO dipole (IOD) mode, as well as their relationship with El Nino-Southern Oscillation (ENSO). However, it produces a cold SST bias in IO, which may be attributed to deeper-than-observed mixed layer and smaller-than-observed total downward heat flux in the tropical IO. It also overestimates the correlations of ENSO with IOBW and IOD, but underestimates the magnitude of IOD and summer IOBW. The CFSv2 captures the climate anomalies related to IOBW but not those related to IOD. It depicts the impact of summer IOBW on WNPAC via the equatorial Kelvin wave, which contributes to the maintenance of WNPAC in July and August. The WNPAC in June is mostly forced by local cold SST, which is better predicted by the CFSv2 compared to July and August. The mechanism for WNPAC maintenance may vary with lead time in the CFSv2.

In this study, we have investigated contributions of tropical Indian Ocean (IO) sea surface temperature (SST) warming and El Niño—Southern Oscillation (ENSO) to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) between 1948 and 2010 and the involved physical mechanisms. Both ENSO and tropical IO Basin Mode (IOBM) warming are found to play important roles in modulating the WNP TC genesis frequency, but their effects are significantly different. The time series of seasonal empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multi-decadal variability removed are defined as the IOBM index and ENSO index, respectively. The results show that the IO warming year is usually the El Niño decaying year. The number of total TCs, especially weak TCs, decreases during the tropical IO warming year when an anomalous anticyclonic circulation is observed over the tropical Northwest Pacific off the equator. On the other hand, the number of intense TCs increases in the El Niño developing year because of the eastward shifts of both the western Pacific monsoon trough and the cyclonic shear of the equatorial westerlies, and thus the eastward shift of the main TC genesis region. It is also found that the relationship between ENSO and the frequency of intense TCs has a decadal variation. During 1968–1987, the number of intense TCs was not related to ENSO with a correlation coefficient of only 0.19, while the correlation coefficient is 0.63 and 0.73 during 1948–1967 and 1988–2007, respectively. The extent to which the WNP anomalous anticyclone is forced by IO warming is investigated by using the global climate model (European Centre Hamburg Model, ECHAM) with imposed SST anomalies over the tropical IO. The results suggest that the WNP anomalous anticyclone that develops from June to September results mainly from the northward shift of the tropical IO warming from boreal spring to boreal summer.

Sea surface temperature (SST) variability in the equatorial Pacific plays a crucial role in shaping global climate patterns. As El Niño–Southern Oscillation (ENSO)‐related extremes intensify, understanding SST fluctuations and their connection to ENSO becomes increasingly important. However, limited instrumental records hinder the ability to simulate SST variability and distinguish natural variability from anthropogenic trends. Tree rings provide annual data essential for reconstructing past SST and ENSO dynamics. While previous research has successfully reconstructed SST variability in the central equatorial Pacific, capturing signals from the eastern Pacific remains more challenging. This study analyzes P . rodolfovasquezii trees from the central Peruvian Andes (11.72°S, 75.14°W) using ring width and stable oxygen and carbon isotopes to assess their sensitivity to Pacific SST variability. We focus on the two leading modes of SST variability derived from Empirical Orthogonal Functions: EOF1 representing SST variability in the Niño 3.4 region, and EOF2 depicting a west‐east dipole between the Niño 4 and 1 + 2 regions, explaining 66.22% and 10.79% of the variance in tropical Pacific SSTs, respectively. We developed a 129‐year multiproxy tree‐ring record. Trees' sensitivity to local and regional hydroclimate is reflected in the temporal variability of their isotopic content. Tree‐ring oxygen isotopes showed significant correlations with both SST modes, although their ability to capture EOF2 variability was limited—likely due to the trees' distance from the influence region of the Niño 1 + 2. Overall, oxygen isotopes outperformed ring width and carbon isotopes, underscoring their potential for future SST reconstructions in the tropical Andes.

The Indian Ocean basin (IOB) mode is the dominant mode of the interannual sea surface temperature (SST) variability in the Indian Ocean, with the Indian Ocean dipole (IOD) as the second mode. An IOB event normally occurs after an El Niño or a concurrent IOD–El Niño event, the dynamics of which are traditionally believed as forced by ENSO through the Walker circulation anomalies over the tropical Indian Ocean. A strong IOB in 2020 took place after the strongest 2019 IOD on record but independent of El Niño, which challenges the traditional atmospheric bridge dynamics of the IOB event. In this study, the dynamics of the 2020 IOB event are investigated using the numerical seasonal climate prediction system of the National Marine Environmental Forecasting Center of China. It is found that the initialization of the Indian Ocean subsurface temperature during the 2019 IOD event has led to the outburst of the 2020 IOB event successfully, the dynamics of which are the propagation and the western boundary reflection of the equatorial and off-equatorial Rossby waves, inducing heat content recharge over the tropical Indian Ocean upper thermocline. In comparison, experiments of SST initialization over the tropical Indian Ocean, with the subsurface temperature in a climatological state, were unable to reproduce the onset of the 2020 IOB event, suggesting that the local air–sea interaction within the Indian Ocean basin is of secondary importance. The numerical experiments suggest that the thermocline ocean wave dynamics play an important role in forcing the IOB event. The revealed thermocline dynamics are potentially useful in climate prediction associated with IOB events.

The El Niño–Southern Oscillation (ENSO) signal in Indian Ocean sea-surface temperatures has weakened since the late-1970s, resulting in important changes in the association between tropical western Indian Ocean sea-surface temperatures and December–February rainfall over South Africa and Namibia. Prior to the late-1970s, warm events in the tropical Indian Ocean occurred frequently in association with ENSO warm events, and both were typically associated with anomalously dry conditions over much of South Africa and Namibia. Over the most recent two decades, however, sea-surface temperature variability in the tropical western Indian Ocean has become significantly less dependent upon ENSO. At the same time, warm (cold) events in the tropical western Indian Ocean have become associated with wet (dry) conditions over the north-eastern half of South Africa and northern Namibia since the late-1970s. These changes in rainfall–sea-surface temperature associations can be successfully simulated using a general circulation model. Such changes in the climate system have important implications for the predictability of southern African rainfall, and highlight the need for an operational forecast system to predict Indian Ocean sea-surface temperatures. Copyright © 1999 Royal Meteorological Society

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.

Understanding decadal changes in monsoon rainfall is essential for mitigation policies in the next few decades, but comparatively less is known about the Southern Hemisphere monsoon. We find that decadal variations of the Southern Hemisphere land monsoon (SHLM) have a zonal dipole pattern with a significant periodicity of 10–15 years. This spatial pattern is characterized by wet southern African (SAF) monsoon and Australian monsoon (AUM) but a dry southeastern South American monsoon (SESAM). Moisture budget analysis shows that changes in monsoon circulation explain over 70% of the decadal variation. Tropical dynamics dominate the monsoon circulation change, and extratropical Rossby waves also have a contribution. The wet SAF and AUM but dry SESAM pattern is mainly driven by a tropical sea surface temperature (SST) gradient, specifically warm SST over the western Pacific and cold SST over the tropical Indian, central Pacific, and eastern Pacific Oceans. This gradient enhances the Walker circulation and subtropical highs over the South Indian and South Pacific Oceans, increasing moisture convergence over the SAF and AUM. The resultant Walker circulation change decreases ascending motion over the SESAM. The enhancement of AUM rainfall triggers the Pacific–South American pattern of the Rossby wave train, which propagates eastward along the SH subpolar jet, generating descending motion over the SESAM. The tropical and extratropical mechanisms are verified in a coupled climate model with nudged SST and a simplified model with artificial atmospheric heating imposed over eastern Australia, respectively. Our findings help unify the understanding of decadal variations of the regional monsoons and may add confidence for decadal prediction of SHLM changes in the following decades.

The subsurface ocean temperature response to El Niño–Southern Oscillation (ENSO) is examined based on 31-yr (1981–2011) simulations with the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) coupled model. The model sea surface temperature (SST) in the tropical Pacific is relaxed to observations to ensure realistic ENSO variability in the simulations. In the tropical Pacific, the subsurface temperature response to the ENSO SST is closely related to the variability of thermocline. The subsurface response is stronger and deeper in the tropical Indian Ocean than in the tropical Atlantic. The analysis at three selected locations reveals that the peak response of the subsurface temperature to ENSO lags the Niño-3.4 SST by 3, 6, and 6 months, respectively, in the southern tropical Indian Ocean, the northern tropical Atlantic, and the North Pacific, where SSTs are also known to be strongly influenced by ENSO. The ENSO-forced temperature anomalies gradually penetrate to the deeper ocean with time in the North Pacific and the tropical Atlantic, but not in the tropical Indian Ocean where the subsurface response at different depths peaks almost at the same time (i.e., at about 3–4 months following ENSO). It is demonstrated that the ENSO-induced surface wind stress plays an important role in determining the time scale and strength of the subsurface temperature response to ENSO in the North Pacific and the northern tropical Atlantic. Additionally, the ENSO-related local surface latent heat flux also contributes to the subsurface response to ENSO in these two regions.

In this paper, an indicator that shows the maximum subsurface temperature anomalies (MSTA) is first defined to represent the thermocline variations through a case analysis of 1997/98. By using the past 46‐year (1955–2000) subsurface temperature data, thermocline oscillation and wave‐signal propagation in the tropical Indian Ocean are revealed. The thermocline oscillation is robust in the tropical Indian Ocean in spite of the coincidence with the strong El Niño/Southern Oscillation (ENSO) cycle as well as the anomalous Walker circulation in the tropical Pacific and the anomalous monsoon circulation in the tropical Indian Ocean. Due to the nature of the propagation of subsurface wave signals, the oscillation may be a potential factor for the predictability of climate and for the dynamical study of the air‐sea interaction in the Indian Ocean region. The warming of sea surface temperature in the tropical Indian Ocean is a basin‐wide phenomenon and results from the reflection of the tropical Rossby waves. By defining an MSTA dipole mode index (M‐DMI), some features of the interannual climate anomalies that correspond to the different phases of M‐DMI obviously exist in the tropical Indian Ocean and tropical central‐western Pacific Ocean. In the positive phase of the M‐DMI, strong 850‐hPa easterly winds appear in the equatorial central‐eastern Indian Ocean while strong 850‐hPa westerly winds occur over the equatorial central western Pacific. Also, for the positive M‐DMI, above‐normal precipitation is located in the tropical central‐western Indian Ocean while below‐normal precipitation is situated over the tropical eastern Indian Ocean and the tropical western Pacific Ocean. The reverse is noted for the negative M‐DMI phase. A scenario for the warming event and the MSTA dipole event in the tropical Indian Ocean is also proposed.

The Western Pacific Warm Pool (WPWP) large warm water mass contributes to atmospheric heat and moisture, modulating El Niño-Southern Oscillation (ENSO) phase changes, potentially leading to global climate extremes. Researching WPWP climate variability will improve understanding of ENSO and the behavior of longer timescale climate drivers. However, the lack of long, continuous and high-resolution instrumental climate observations within the WPWP limits understanding of its past climate changes. Massive corals from the WPWP allows past climate reconstruction characterizing the warm pool spatiotemporal and hydrology changes beyond instrumental timescales. A better understanding of these processes and their underlying climate drivers allows for better evaluation of regional trends, climate connections to global regions, and climate predictions in response to global warming. Porites spp. coral cores sampled from Kosrae island (KOS) and Woleai atoll (WOL), located on separate sites of Federated States of Micronesia, capture the opposite dipole signal of ENSO. Each coral core was slabbed and x-rayed to reveal images of annual density banding to assist in determining sampling axis and age assignment. The slabbed coral cores were analyzed for δ18Oc and Sr/Ca proxies for sea surface temperature (SST) and salinity (SSS), respectively. Monthly resolved reconstructed SST and SSS spanning 165 and 277 years were generated from KOS and WOL, respectively. The Sr/Ca-based reconstructed SST from KOS and WOL display the expected ENSO dipole signal. The difference between these records generates the zonal SST gradient, KOSWOLSST, capturing the warm pool's east-west zonal migration during ENSO events. The extended KOSWOLSST century exhibit ENSO’s spatiotemporal SST influence on the WPWP dating back to the mid-19th century. At decadal timescales, separate KOS SST displays consistent variability to the IPO. However, WOL SST exhibit strong influence driven by the Northern Hemisphere SST. Extending the WOL SST record further ~110 years allow examination of centennial-scale SST variability. Significant power and centennial relationship between extended WOL SST to a South china sea paleo SST record indicates simulated Pacific Centennial Oscillation (PCO) observed in controlled coupled global climate simulation (cGCMs) exists in nature. These findings collectively establish a centennial-scale natural SST forcing in the WPWP that potentially impacts global teleconnections.The δ18Oc-based reconstructed KOS and WOL SSS reflect each site’s precipitation driven changes. The averaged SSS records generate KOSWOLSSS, an index representative of the regional WPWP precipitation. Separate KOS and WOL SSS records dating back to the mid-19th century indicates WPWP experienced increasing precipitation changes leading to freshening trends driven by global warming. Extending WOL SSS records further ~110 years allows examination of WPWP centennial driven SST variability (PCO) influence on WOL SSS. Significant WOL SSS and SST relationship indicate during SST increases saw a decline in precipitation. This and antiphase centennial SST relationships between Micronesia and Vietnam suggest a robust precipitation footprint of the PCO. This thesis derived a WPWP zonal SST gradient, regional hydrological records, established WPWP climate SST and SSS climate drivers and verified the presence of PCO and its precipitation footprint based on coral proxy records. These key findings underscore the need to investigate natural climate driver(s) variability within the WPWP. Furthermore, incorporating the PCO variability and its precipitation footprint in cGCMs is crucial to modelling future climate as climate changes within this region impact global teleconnections and tropical Pacific regional hydrology.

In recent decades, wave power (WP) energy from the ocean is one of the cleanest renewable energy sources associated with oceanic warming. In Indo-Pacific Ocean, the WP is significantly influenced by natural climate variabilities, such as El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Pacific Decadal Oscillation (PDO). In this study, the impact of major climate variability modes on seasonal extreme WP is examined over the period 1979–2019 using ERA5 reanalysis data and the non-stationary generalized extreme value analysis is applied to estimate the climatic extremes. Independent ENSO influence after removing the IOD impact (ENSO|IOD) on WP are evident over the northeast and central Pacific during December–February, and March–May, respectively, which subsequently shifts towards the western Pacific in June–August (JJA) and September–November (SON). The ENSO|PDO impact on WP exhibits similar yet weaker intensity year round compared to ENSO. Extreme WP responses due to the IOD|ENSO include widespread decreases over the tropical and eastern Indian Ocean, with localized increases only over South China and Philippine seas and Bay of Bengal during JJA, and the Arabian Sea during SON. Lastly, for the PDO|ENSO, the significant increases in WP are mostly confined to the Pacific, and most prominent in the North Pacific. Composite analysis of different phase combinations of PDO (IOD) with El Niño (La Niña) reveals stronger (weaker) influences year-round. The response patterns in significant wave height, peak wave period, sea surface temperatures, and sea level pressure help to explain the seasonal variations in WP.

Climate variability has distinct spatial patterns with the strongest signal of sea surface temperature (SST) variance residing in the tropical Pacific. This interannual climate phenomenon, the El Niño-Southern Oscillation (ENSO), impacts weather patterns across the globe via atmospheric teleconnections. Pronounced SST variability, albeit of smaller amplitude, also exists in the other tropical basins as well as in the extratropical regions. To improve our physical understanding of internal climate variability across the global oceans, we here make the case for a conceptual model hierarchy that captures the essence of observed SST variability from subseasonal to decadal timescales. The building blocks consist of the classic stochastic climate model formulated by Klaus Hasselmann, a deterministic low-order model for ENSO variability, and the effect of the seasonal cycle on both of these models. This model hierarchy allows us to trace the impacts of seasonal processes on the statistics of observed and simulated climate variability. One of the important outcomes of ENSO’s interaction with the seasonal cycle is the generation of a frequency cascade leading to deterministic climate variability on a wide range of timescales, including the near-annual ENSO Combination Mode. Using the aforementioned building blocks, we arrive at a succinct conceptual model that delineates ENSO’s ubiquitous climate impacts and allows us to revisit ENSO’s observed statistical relationships with other coherent spatio-temporal patterns of climate variability—so called empirical modes of variability. We demonstrate the importance of correctly accounting for different seasonal phasing in the linear growth/damping rates of different climate phenomena, as well as the seasonal phasing of ENSO teleconnections and of atmospheric noise forcings. We discuss how previously some of ENSO’s relationships with other modes of variability have been misinterpreted due to non-intuitive seasonal cycle effects on both power spectra and lead/lag correlations. Furthermore, it is evident that ENSO’s impacts on climate variability outside the tropical Pacific are oftentimes larger than previously recognized and that accurately accounting for them has important implications. For instance, it has been shown that improved seasonal prediction skill can be achieved in the Indian Ocean by fully accounting for ENSO’s seasonally modulated and temporally integrated remote impacts. These results move us to refocus our attention to the tropical Pacific for understanding global patterns of climate variability and their predictability.

The boreal spring relationship between variabilities of East China precipitation (ECP) and tropical Ocean sea surface temperature (SST) during the period 1951–2014 is investigated in this study. The results show that the leading mode of the ECP variability exhibits an enhanced response to the anomalous El Nino–Southern Oscillation (ENSO)-like SST after the late 1970s, when the SST underwent a decadal change, with two positive centers over the eastern tropical Pacific (ETP) and tropical Indian Ocean (TIO). To further understand the relative roles of the ETP and TIO SST anomalies (SSTAs) in the variability of ECP after the late 1970s, partial regression and correlation methods are used. It is found that, without the contribution of the TIO, ETP SSTA plays a limited role in the variability of ECP after the late 1970s; comparatively, a significant correlation between TIO SST and ECP is identified during the same period, when the ETP signal is linearly removed. Physical analyses show that, after the late 1970s, the TIO SSTA affects East Asian atmospheric circulation in two ways: by exciting a zonal wave-train pattern over the mid-latitude Eurasian Continent and by inducing anomalous convection over the Maritime Continent. Via these two mechanisms, the TIO SST variability results in an anomalous East Asian trough and vertical motion over East China and consequently leads to anomalous precipitation over the region. The physical processes linking the ECP and TIO SST are confirmed by an atmospheric general circulation model experiment forced with idealized TIO warming.

Both the tropical Indian and tropical Pacific Oceans are active atmosphere-ocean interactive regions with robust interannual variability, which also constitutes a linkage between the two basins in the mode of variability. Using a global atmosphere-ocean coupled model, we conducted two experiments (CTRL and PC) to explore the contributions of Indian Ocean interannual sea surface temperature (SST) modes to the occurrence of El Nio events. The results show that interannual variability of the SST in the Indian Ocean induces a rapid growth of El Nio events during the boreal autumn in an El Nio developing year. However, it weakens El Nio events or even promotes cold phase conversions in an El Nio decaying year. Therefore, the entire period of the El Nio is shortened by the interannual variations of the Indian Ocean SST. Specifically, during the El Nio developing years, the positive Indian Ocean Dipole (IOD) events force an anomalous Walker circulation, which then enhances the existing westerly wind anomalies over the west Pacific. This will cause a warmer El Nio event, with some modulations by ocean advection and oceanic Rossby and Kelvin waves. However, with the onset of the South Asian monsoon, the Indian Ocean Basin (IOB) warming SST anomalies excite low level easterly wind anomalies over the west tropical Pacific during the El Nio decaying years. As a result, the El Nio event is prompted to change from a warm phase to a cold phase. At the same time, an associated atmospheric anticyclone anomaly appears and leads to a decreasing precipitation anomaly over the northwest Pacific. In summary, with remote forcing in the atmospheric circulation, the IOD mode usually affects the El Nio during the developing years, whereas the IOB mode affects the El Nio during the decaying years.