While West Antarctica has experienced the most significant warming in the world, a profound cooling trend in austral summer was observed over East Antarctica (30°W to 150°E, 70° to 90°S) from 1979 to 2014. Previous studies attributed these changes to high-latitude atmospheric dynamics, stratospheric ozone change, and tropical sea surface temperature anomalies. We show that up to 20 to 40% of the observed summer cooling trend in East Antarctica was forced by decadal changes of the Madden-Julian oscillation (MJO). Both observational analysis and climate model experiments indicate that the decadal changes in the MJO, characterized by less (more) atmospheric deep convection in the Indian Ocean (western Pacific) during the recent two decades, led to the net cooling trend over East Antarctica through modifying atmospheric circulations linked to poleward-propagating Rossby wave trains. This study highlights that changes in intraseasonal tropical climate patterns may result in important climate change over Antarctica.

science & technology, nanjing, china; c collaborative innovation center on Forecast and evaluation of meteorological disasters, nanjing University of information science & technology, nanjing, china; d Guangzhou meteorological observatory, Guangzhou, china

This study examined multi-decadal changes in the amplitude of the boreal-winter Madden-Julian Oscillation (MJO) over the 20th century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability as other state-of-the-art reanalysis datasets. We detected pronounced multi-decadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two 20th century reanalysis datasets and the Oliver-Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–1999) compared to the earlier decades (1920–1949). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western-Pacific MJO precipitation amplitude over the 20th century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture [−(𝑞̅∇ ∙ 𝑉′)]. Further diagnosis showed that the effect of anomalous circulation (∇ ∙ 𝑉′) change on the MJO precipitation amplitude change over the 20th century was about an order larger than that of mean moisture (𝑞̅) change, different from the mechanisms (i.e., increased gradient of 𝑞̅) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–1999 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

Meiyu onset marks the beginning of the rainfall season in the densely populated Yangtze River Basin, whether the Meiyu initiates early or late in June, and thus has a profound effect on the several hundred million people living there. Applying a Bayesian change-point analysis to data from 1960–2014, we objectively detected an abrupt change of Meiyu onset around 2002. The Meiyu onset date averaged over 2002–2014 was 19 June, delayed by about two weeks compared to that of 1989–2001 (6 June). This decadal change is attributable to the distinct amplitude of moisture transport toward the Yangtze River Basin induced by the changes in climatological intraseasonal oscillation (CISO). The CISO emerges as the annual cycle interacts with the transient intraseasonal perturbations. The wet/dry phases of the CISO are consistent with the climatological active/break stages of the East Asian summer monsoon. In early June, the northwestward-propagating CISO convective/cyclonic anomalies over the western North Pacific (WNP) show weaker amplitude during the earlier-onset epoch compared to the delayed-onset epoch. Thus, relative to the delayed onset epoch, a quasi-barotropic anticyclonic CISO anomaly appears over the WNP in early June during the earlier-onset years. This anticyclonic anomaly was conducive to the westward extension of the WNP subtropical high, conveying warm, moist air from the tropics toward the Yangtze River Basin for the rainy season onset. Model experiments suggest that the decadal changes in WNP CISO intensity were associated with the epochal changes in large-scale background circulation and sea surface temperature over the WNP.

The Madden-Julian Oscillation (MJO) exhibits the most vigorous activity over the Indo-Pacific warm pool during the boreal winter. In this study, we find the inter-basin connection between the tropical Pacific MJO activity and North Atlantic sea-surface temperature at the interdecadal timescale. Accompanied with the negative phase of the Atlantic Multidecadal Oscillation (AMO), the enhanced MJO variability (i.e., larger amplitude of 20–90-day convection) tends to extend eastward into the central tropical Pacific (170°E−150°W, 20°S−0°), while the vigorous MJO variability is confined to the Indo-Pacific warm pool during the positive phase of AMO. The eastward displacement of MJO convection into the central tropical Pacific during the negative AMO phase is supported by the increased moist static energy and low-level moisture tendencies there when the low-frequency background (>90 days) westerly transports the moisture perturbation [−( ∙ ∇q′)′] and the MJO-associated easterly perturbation transports the background moisture [−(V′∙ ∇)′] toward the central tropical Pacific. This suggests that the interdecadal changes in zonal displacement of MJO activity over the tropical Pacific are closely linked with the impacts of AMO on the background flow and moisture condition over the central tropical Pacific. The results of coupled model sensitivity experiments and pre-industrial simulations of the Coupled Model Intercomparison Project Phase 6 support the observational findings.