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In this study, the joint effects of the El Niño-Southern Oscillation (ENSO) and Arctic Oscillation (AO) on winter net surface heat flux (Q net) anomalies over the tropical Indian Ocean (TIO) are investigated for the 1979/1980–2017/2018 period. The results show that, in multisource datasets, the Q net pattern for El Niño plus positive AO cases is the most robust. The major feature of Q net is dominated by a significant dipole pattern, with oceanic heat gain anomalies appearing over the southeastern TIO and oceanic heat loss near the western TIO. Analysis of the flux components shows that the Q net anomalies are mainly contributed by changes in latent heat flux and shortwave radiation (Q LHF and Q SWR), which are tightly associated with the regional atmospheric and oceanic conditions. In association with positive AO, northerly wind anomalies on the eastern flanks of a low-level anomalous anticyclone in the Arabian Sea enhance the intertropical convergence zone (ITCZ) in the western TIO. Meanwhile, the El Niño-related SST warming occurs in the western TIO. Both enhanced ITCZ and anomalous SST warming favour in situ increased low and middle cloud cover and a reduced Q SWR. Furthermore, under El Niño's effect, a low-level anomalous anticyclone located in the southeastern TIO leads to a positive Q LHF through decreased near-surface wind speed and increased near-surface air humidity. As a result, a dipole Q net pattern tends to be observed for the combination of El Niño and positive AO. • In multisource datasets, the Q net dipole pattern for El Niño plus positive AO cases is the most robust. • The Q net anomalies are contributed by Q LHF in the southeastern TIO and Q SWR in the western TIO. • The El Niño-related anticyclone and SST warming anomalies and positive AO-related enhanced ITCZ affect Q net changes. [ABSTRACT FROM AUTHOR]

Observations have shown that the hydroclimatic variations in East Central Asia (ECA) are a result of the interaction of high-, middle- and low-latitude atmospheric circulation systems. This study aims to explore the potential mechanisms of hydroclimate variability in ECA over a longer timescale than in previous studies. Based on tree-ring stable oxygen isotope (δ18O), we reconstructed the May–July relative humidity variations in the central Tianshan Mountains from 1661 to 2016, which can represent the dry/wet variations over a large region of ECA. The reconstruction revealed several dry periods that occurred in 1674–1691, 1705–1740, 1750–1770, 1806–1826, the 1940s, and 1960–1980. The reconstructed relative humidity presented an obvious upward trajectory from the 18th to the early 20th century and humidification characteristics in the mid- to late 1980s. The summer North Atlantic Oscillation (SNAO) had a nonlinear impact on the hydroclimate of ECA by modulating the position of the subtropical westerly jet stream (SWJS). Positive sea surface temperature anomalies in the tropical Indian Ocean (TIO), which may enhance the cross-equatorial jet, transport additional water vapour into ECA, and the South Asian summer monsoon (SASM) provides favourable circulation conditions for water vapour transport. Summer hydroclimatic variations in ECA are jointly affected by the North Atlantic and the TIO. The impact of the TIO on the hydroclimate in ECA cannot be ignored. • Summer relative humidity in ECA was reconstructed by tree-ring δ18O during the past three centuries. • Summer hydroclimatic variations in ECA were jointly affected by the North Atlantic and the Indian Ocean. • The negative phase of the SNAO favours the transport of southerly water vapour to ECA through the SWJS. • Summer hydroclimatic variations in ECA correspond well with the climate variability of tropical Indian Ocean. [ABSTRACT FROM AUTHOR]

Inter-decadal modulation of El Niño–Southern Oscillation (ENSO) teleconnections to tropical Indian Ocean (TIO) is investigated in the coupled general circulation model Climate Forecast System (CFS) using a hundred year integration. The model is able to capture the periodicity of El Niño variability, which is similar to that of the observations. The maximum TIO/north Indian Ocean (NIO) SST warming (during spring following the decay phase of El Niño) associated with El Niño is well captured by the model. Detailed analysis reveals that the surface heat flux variations mainly contribute to the El Niño forced TIO SST variations both in observations and model. However, spring warming is nearly stationary throughout the model integration period, indicating poor inter-decadal El Niño teleconnections. The observations on the other hand displayed maximum SST warming with strong seasonality from epoch to epoch. The model El Niño decay delayed by more than two seasons, results in persistent TIO/NIO SST warming through the following December unlike in the observations. The ocean wave adjustments and persistent westerly wind anomalies over the equatorial Pacific are responsible for late decay of El Niño in the model. Consistent late decay of El Niño, throughout the model integration period (low variance), is mainly responsible for the poor inter-decadal ENSO teleconnections to TIO/NIO. This study deciphers that the model needs to produce El Niño decay phase variability correctly to obtain decadal-modulations in ENSO teleconnection. [ABSTRACT FROM AUTHOR]

Modern observations and model simulations suggest a strong influence of tropical processes on high-latitudinal climate. The evidence of similar tropics-high latitude climate link during glacial termination is, however, elusive. Here we show a strong influence of tropical processes in terminating the last glacial period based on multi-decadal seawater temperature, upwelling intensity and runoff records from the tropical Indian Ocean. We report a well-defined warming (1.4 °C) starting at ~20 kyr BP, coeval with abrupt increase in upwelling intensity and local CO 2 outgassing, long before the beginning of increase in global atmospheric CO 2 at 18.2 kyr BP. We suggest that the tropical Indian Ocean warming prior to the increase in global atmospheric CO 2 was modulated by intense regional hydrographic changes caused by local summer insolation. The warming prior to Termination I (~18–11 kyr BP), at our site was synchronous with a small but evident strengthening of the Atlantic Meridional Overturning Circulation (AMOC). We conclude that the early warming in the tropical northern Indian Ocean led to stronger AMOC and the subsequent melting of the northern high latitude sea ice, which ultimately triggered the Heinrich Stadial during the last glacial termination. • Mg/Ca–SST and upwelling/CO2 outgassing from the tropical Indian Ocean during Termination I was reconstructed. • A well-defined warming starting at ~20 kyr BP was found. • The warming was coeval with intensified local solar insolation and abrupt increase in upwelling intensity. • An early warming in the tropical Indian Ocean led to a stronger AMOC and caused HS1. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the mechanisms governing the teleconnections associated with the long-lived La Niña variability in the tropical Indian Ocean (TIO) sea surface temperature (SST) anomalies using observational and reanalysis products. Two long-lived La Niña events (1973 to 1976 and 1998 to 2001) are observed in the recent years, one falling before and the other after the mid 1970''s climatic shift. The winter (boreal) and spring (November to April) TIO SST is highly influenced by long-lived La Niña forcing. Climatic shift in mid 1970s contributes to the changes in TIO SST pattern during these two long-lived La Niña events. Surface heat flux variations due to long-lived La Niña contribute to the SST changes except in the southwest TIO. The upwelling favorable local surface wind stress curl and upwelling Rossby waves originating from the east are the dominant mechanisms responsible for the La Niña related winter time SST cooling over the southwest TIO. Long-lived La Niña induced surface wind anomalies enhance the fall Wyrtki Jet in the equatorial Indian Ocean resulting large scale anomalous heat transport. Local SST cooling reduces convection and contributes to the low rainfall over southwest TIO and the northern parts of Madagascar Island. [Copyright &y& Elsevier]

The long-term change in the sea surface temperature (SST) of the Tropical Indian Ocean (TIO) and the underlying mechanisms are examined in detail by analysing multiple reanalysis datasets and historical simulations of a global climate model. A robust basin-wide warming signal was found in SST, with the highest long-term warming trend (~0.09 ± 0.015 °C decade−1) seen over the north-western and equatorial Indian Ocean regions. The robust warming trend of TIO SST is a dominant warming signal of the global tropical oceans and is largely in phase with similar warming in western Pacific and Atlantic Oceans. The central equatorial Indian Ocean and south TIO regions went through significant warming during recent decades. The weakened cross-equatorial flow and the associated cyclonic winds over the Arabian Sea region have resulted in equatorial westerlies with an enhanced loss of latent heat over the central and eastern equatorial Indian Ocean. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south-eastern Indian Ocean to the western Indian Ocean and reduced oceanic cooling by vertical processes, overcome the cooling by the net loss of surface heat fluxes, and favor surface warming the TIO. • The robust basin-wide warming of the TIO is largely in phase with the warming of the western Pacific and Atlantic Oceans. • The cyclonic circulation of the Arabian Sea basin resulted in equatorial westerlies and enhanced latent heat loss. • The oceanic processes reinforce the warming tendency of TIO, which overcomes the cooling by net loss of surface heat flux. [ABSTRACT FROM AUTHOR]