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1. Decadal Modulation of the ENSO–Indian Ocean Basin Warming Relationship during the Decaying Summer by the Interdecadal Pacific Oscillation

   https://doi.org/10.1175/JCLI-D-20-0457.1  

   Liu, Fangyu; Zhang, Wenjun; Jin, Fei-Fei; Hu, Suqiong (April 2021, Journal of Climate)

   null (Ed.)

   Abstract Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. 

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2. Tropical Pacific Modulation of the Asian Summer Monsoon Over the Last Millennium in Paleoclimate Data Assimilation Reconstructions

   https://doi.org/10.1029/2023JD039207  

   Hu, Jun; Dee, Sylvia; Parajuli, Grant; Thirumalai, Kaustubh (October 2023, Journal of Geophysical Research: Atmospheres)

   Abstract Large uncertainties exist in climate model projections of the Asian summer monsoon (ASM). The El Niño‐Southern Oscillation (ENSO) is an important modulator of the ASM, but the ENSO‐ASM teleconnection is not stationary. Furthermore, teleconnections between ENSO and the East Asian versus South Asian subcomponents of the ASM exhibit distinct characteristics. Therefore, understanding the variability of the ENSO‐ASM teleconnection is critical for anticipating future variations in ASM intensity. To this end, we here use paleoclimate records to extend temporal coverage beyond the instrumental era by millennia. Recently, data assimilation techniques have been applied for the last millennium, which facilitates physically consistent, globally gridded climate reconstructions informed by paleoclimate observations. We use these novel data assimilation products to investigate variations in the ENSO‐ASM relationship over the last 1,000 years. We find that correlations between ENSO and ASM indices are mostly negative in the last millennium, suggesting that strong ASM years are often associated with La Niña events. During periods of weak correlations between ENSO and the East Asian summer monsoon, we observe an El Niño‐like sea surface temperature (SST) pattern in the Pacific. Additionally, SST patterns associated with periods of weak correlations between ENSO and South Asian summer monsoon rainfall are not consistent among data assimilation products. This underscores the importance of developing more precipitation‐sensitive paleoclimate proxies in the Indian subcontinental realm over the last millennium. Our study serves as a baseline for future appraisals of paleoclimate assimilation products and an example of informing our understanding of decadal‐scale ENSO‐ASM teleconnection variability using paleoclimate data sets. 

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3. Diverse impacts of Indian Ocean Dipole on El Niño-Southern Oscillation

   https://doi.org/10.1175/JCLI-D-21-0085.1  

   Zhang, Lei; Han, Weiqing; Meehl, Gerald A.; Hu, Aixue; Rosenbloom, Nan; Shinoda, Toshiaki; McPhaden, Michael J. (September 2021, Journal of Climate)

   Abstract Understanding the impact of the Indian Ocean Dipole (IOD) on El Niño-Southern Oscillation (ENSO) is important for climate prediction. By analyzing observational data and performing Indian and Pacific Ocean pacemaker experiments using a state-of-the-art climate model, we find that a positive IOD (pIOD) can favor both cold and warm sea surface temperature anomalies (SSTA) in the tropical Pacific, in contrast to the previously identified pIOD-El Niño connection. The diverse impacts of the pIOD on ENSO are related to SSTA in the Seychelles-Chagos thermocline ridge (SCTR; 60°E-85°E and 7°S-15°S) as part of the warm pole of the pIOD. Specifically, a pIOD with SCTR warming can cause warm SSTA in the southeast Indian Ocean, which induces La Niña-like conditions in the tropical Pacific through interbasin interaction processes associated with a recently identified climate phenomenon dubbed the “Warm Pool Dipole”. This study identifies a new pIOD-ENSO relationship and examines the associated mechanisms. 

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4. Teleconnections and relationship between the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) in reconstructions and models over the past millennium

   https://doi.org/10.5194/cp-16-743-2020  

   Dätwyler, Christoph; Grosjean, Martin; Steiger, Nathan J.; Neukom, Raphael (January 2020, Climate of the Past)

   null (Ed.)

   Abstract. The climate of the Southern Hemisphere (SH) is stronglyinfluenced by variations in the El Niño–Southern Oscillation (ENSO) andthe Southern Annular Mode (SAM). Because of the limited length ofinstrumental records in most parts of the SH, very little is known about therelationship between these two key modes of variability over time. Usingproxy-based reconstructions and last-millennium climate model simulations,we find that ENSO and SAM indices are mostly negatively correlated over thepast millennium. Pseudo-proxy experiments indicate that currently availableproxy records are able to reliably capture ENSO–SAM relationships back to atleast 1600 CE. Palaeoclimate reconstructions show mostly negativecorrelations back to about 1400 CE. An ensemble of last-millennium climatemodel simulations confirms this negative correlation, showing a stablecorrelation of approximately −0.3. Despite this generally negativerelationship we do find intermittent periods of positive ENSO–SAMcorrelations in individual model simulations and in the palaeoclimatereconstructions. We do not find evidence that these relationshipfluctuations are caused by exogenous forcing nor by a consistent climatepattern. However, we do find evidence that strong negative correlations areassociated with strong positive (negative) anomalies in the InterdecadalPacific Oscillation and the Amundsen Sea Low during periods when SAM andENSO indices are of opposite (equal) sign. 

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5. Placing the AD 2014–2016 ‘protracted’ El Niño episode into a long-term context

   https://doi.org/10.1177/0959683619875788  

   Allan, Robert J; Gergis, Joëlle; D’Arrigo, Rosanne D (January 2020, The Holocene)

   Although extended or ‘protracted’ El Niño and La Niña episodes were first suggested nearly 20 years ago, they have not received the attention of other ‘flavours’ of the El Niño–Southern Oscillation (ENSO) or low-frequency ‘ENSO-like’ phenomena. In this study, instrumental variables and palaeoclimatic reconstructions are used to investigate the most recent ‘protracted’ El Niño episode in 2014–2016, and place it into a longer historical context. Although just reaching the threshold for such an episode, the 2014–2016 ‘protracted’ El Niño had very severe societal, agricultural, environmental and ecological impacts, particularly in western Pacific regions like eastern Australia. We show that although ‘protracted’ ENSO episodes of either phase cause similar, near-global modulations of weather and climate as during more ‘classical’ events, impacts associated with ‘protracted’ episodes last longer, with strong influences in eastern Australia. The latter is a response to the dominance of Niño 4 sea surface temperature (SST) and associated atmospheric teleconnection anomalies during ‘protracted’ ENSO episodes. Importantly, while Niño 4 SST anomalies recorded during the austral summer of 2016 were the highest values on record, an analysis of long-term palaeoclimate records indicates that there may have been episodes of greater magnitude and duration than seen in instrumental observations. This suggests that shorter instrumental observations may underestimate the risks of possible future ENSO extremes compared with those observed from multi-century palaeoclimate records. Improved knowledge of ENSO and the potential to forecast ‘protracted’ episodes would be of immense practical benefit to communities affected by the severe impacts of ENSO extremes. 

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