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The influence of Atlantic Niño on the following El Niño–Southern Oscillation becomes significant since mid‐1970s. However, exact mechanisms for this inter‐decadal change are still unclear. Here, we perform a set of model pacemaker experiments to probe the relative contributions of the changes in the Atlantic Niño itself and the mean‐state under global warming. The results suggest that the warmer background of the tropical Atlantic plays an essential role in enhancing local mean precipitation, inducing stronger divergence and low‐level easterlies in the Pacific. Under a favorable condition in the Pacific, even a weak Atlantic Niño‐related warming could promote the development of La Niña through cross‐basin Walker circulation and the Indian Ocean‐relayed Kelvin wave response. In contrast, the Atlantic Niño pattern change itself induces feeble convection anomalies in the western Atlantic, which cannot induce significant atmospheric response in the Pacific. These results imply an important modulation of global warming on the inter‐basin connection. Plain Language Summary Climate phenomena in the three tropical oceans are tightly inter‐connected through atmospheric and oceanic pathways. The observations show that the influence of equatorial Atlantic sea surface temperature (SST) on the evolution of the following El Niño–Southern Oscillation displays a noticeable multi‐decadal change. The inter‐basin influence is negligible in early decades but becomes statistically significant since mid‐1970s. To understand this multi‐decadal change, the changes in the remote impacts of the tropical Atlantic Niño are divided into those related to the changes in the Atlantic Niño itself and the background mean SST. By comparing their relative contributions, we find the warmer climatological mean SST excites incremental precipitation around the western tropical Atlantic and Intertropical Convergence Zone (ITCZ), amplifying inter‐basin Walker circulation and the Indian Ocean relayed effect, and thus can more easily promote the evolution of La Niña in the following seasons. In contrast, despite the Atlantic Niño pattern broadens westwards in the recent decades, its induced‐feeble positive convection anomalies in the western Atlantic scarcely induce the atmospheric response over the tropical Pacific. Our results stress the vital role of the multi‐decadal SST warming in the tropical Atlantic and provide a plausible explanation for the multi‐decadal strengthening of this inter‐basin connection in recent decades. Key Points The correlation between summer Atlantic Niño and the following El Niño–Southern Oscillation (ENSO) becomes significantly negative since 1976 The warming‐induced tropical Atlantic mean‐state change, rather than the Atlantic Niño change itself, dominantly modulates the inter‐basin impact on ENSO The climatological sea surface temperature/precipitation strengthened in the tropical Atlantic is important to exert the influence on ENSO

China has been experiencing increased concentrations of aerosols, commonly attributed to the large increases in emissions associated with the rapid economic development. We show by using a chemical transport model driven by the assimilated meteorological fields that the observed decadal‐scale weakening of the East Asian summer monsoon also contributed to the increases in aerosols in China. We find that the simulated aerosol concentrations have strong negative correlations with the strength of the East Asian Summer monsoon. Accounting for sulfate, nitrate, ammonium, black carbon, and organic carbon aerosols, the summer surface‐layer PM2.5 concentration averaged over eastern China (110°–125°E, 20°–45°N) can be 17.7% higher in the weakest monsoon years than in the strongest monsoon years. The weakening of the East Asian Summer monsoon increases aerosol concentrations mainly by the changes in atmospheric circulation (the convergence of air pollutants) in eastern China. Key Points The weakening of the East Asian summer monsoon increases aerosol in China Monsoon circulation is the dominant factor that explains the increase in aerosol Climate change increases aerosol concentrations in China

Regional temperature changes are a crucial factor in affecting agriculture, ecosystems and societies, which depend greatly on local temperatures. We identify a nonuniform warming pattern in summer around the mid-1990s over the Eurasian continent, with a predominant amplified warming over Europe-West Asia and Northeast Asia but much weaker warming over Central Asia. It is found that the nonuniform warming concurs with both the phase shift of the Atlantic Multi-decadal Oscillation (AMO) and the decadal change in the Silk Road Pattern (SRP), which is an upper-tropospheric teleconnection pattern over the Eurasian continent during summer. We suggest that the AMO may modulate the decadal change in SRP and then induce the zonal asymmetry in temperature changes. Our results have important implications for decadal prediction of regional warming pattern in Eurasia based on the predictable AMO.

The tropical cyclone (TC) genesis number in the western North Pacific (WNP) exhibits a pronounced decadal decrease around the mid-1990s, with prominent seasonal and spatial inhomogeneity. This decadal shift of TC activity is mostly confined to the southeastern part of the WNP and occurs mainly during the second half of the calendar year. Accordingly, westward and northeastward TC recurving movements strongly decreased in recent decades after 1995 compared with TC tracks in the earlier period (1979–1994). We find that this TC activity decadal change is associated with tropical Pacific decadal variability, which is measured here by a low-pass filtered Niño3.4 index. In contrast to the earlier period, the anomalous cold mean state in the tropical Pacific during recent decades favored the enhancement of zonal vertical wind shear (UVWS) and suppressed TC activity. This tropical Pacific mean state change is possibly related to decadal changes of El Niño–Southern Oscillation (ENSO) properties (i.e., more La Niña events occurred during recent decades). This relationship between tropical Pacific mean state change and the UVWS in the southeastern WNP on decadal timescales is further validated based on longer observations (1951–2017) and control simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The statistical relationships between TC activity and the Pacific Decadal Oscillation (PDO) or Atlantic Multidecadal Oscillation (AMO) are weaker and insignificant, both for the observations and for simulations. Our results imply that decadal variations of the tropical Pacific mean state should be taken into account when predicting WNP TC activities on decadal timescales.

A pronounced decadal change in tropical cyclone (TC) activity over the western North Pacific (WNP) in the late 1990s was identified. Based on a comparison of the two epochs that occurred before and after the late 1990s, the TC genesis number exhibited an evident decrease over the southern WNP (S-WNP: 5°–20°N, 105°–170°E) and an increase over the northern WNP (N-WNP: 20°–25°N, 115°–155°E), which partly corresponded to a significant northward migration in the seasonal mean latitudinal location of TC genesis, i.e., from 17.2°N to 18.7°N. After the late 1990s, the northwestward-moving track became the most dominant track mode, accompanied by the weakening of both the westward-moving track and the northeastward-recurving track. Meanwhile, the TC occurrence frequency (TCF) experienced evident increases over southeastern China and the Okinawa islands, while prominent decreases occurred over the South China Sea, the Philippine Sea, Japan and east of Japan. Changes in the TCF were determined by TC genesis changes, TC track shifts and variations in regional TC durations, which were all ascribed to the decadal change in tropical Indo-Pacific sea surface temperature. The full picture on the decadal changes in the WNP TC activity revealed in this study may provide useful guidance for regional TC seasonal forecasts and future projections.

The propagation of sea surface temperature (SST) anomalies along the equatorial Pacific Ocean during El Niño shifted around the late 1970s, previously propagating westward from the eastern Pacific and shifting to eastward propagation from the central Pacific. This shift has been attributed to decadal changes in the mean state of the equatorial Pacific. This study examined the influence of the subtropical South Pacific on the decadal variation in El Niño zonal propagation through the utilization of observational data and extensive simulations with coupled climate models. We showed that El Niño induced by the positive South Pacific Subtropical Dipole mode (pSPSD) started in the eastern Pacific and developed westward. The mean trade winds in the southeastern Pacific were stronger before the late 1970s, leading to a stronger pSPSD through enhanced wind–evaporation–SST feedback. Then, the pSPSD could trigger westward propagating El Niño. Contrarily, weaker pSPSD did not induce El Niño events after the late 1970s. El Niño initiated from other processes and propagated eastward. Our study reveals the modulation of the subtropical South Pacific on the decadal shift of El Niño propagation.

The Pacific–Japan (PJ) pattern, which is characterized by meridional alternating anomalous cyclone and anticyclone along the East Asian coast, is the key circulation system modulating the East Asian summer climate. It is essential to explore the oceanic origins of the PJ pattern and examine their fidelity, with the aim to identify reliable seasonal predictability sources and therefore improve the seasonal prediction of the East Asian climate. This study reveals a decadal change in the connection between the PJ pattern and the Indian Ocean SST basin mode (IOBM) around 2006. Before 2006, the PJ pattern is closely associated with the IOBM, with a correlation coefficient of 0.59 ( p  < 0.05), whereas this tight connection disappears after 2006. We further find that the weakened relationship between the PJ pattern and the IOBM is mainly contributed by the decoupling of the Northeast Asian anomalous cyclone of the PJ pattern (PJ_C) and the IOBM after 2006. From the first to the second epoch (before and after the decadal change), the oceanic forcing of PJ_C changes from the IOBM to the tropical central/eastern Pacific (CEP) SSTA. Numerical experiments reproduce nicely the influence of the alternate oceanic drivers (i.e., the IOBM and CEP SSTA) on the formation of the PJ pattern. The shortened period of ENSO in the second epoch weakens the IOBM’s capacitor effect, which possibly leads to the decoupling of PJ_C and the IOBM.

Observational analysis indicates significant changes in some temperature extremes over China across the mid-1990s. The decadal changes in hot extremes are characterized as a rise in annual hottest day and night temperature (TXx and TNx) and an increase in frequencies of summer days (SU) and tropical night (TR). The decadal changes in cold extremes are distinguished by a rise in annual coldest day and night temperature (TXn and TNn) and a decrease in frequencies of ice days (ID) and frost days (FD). These decadal changes manifest not only over China as a whole, but also over individual climate sub-regions. An atmosphere-ocean-mixed-layer coupled model forced by changes in greenhouse gases (GHG) concentrations and anthropogenic aerosol (AA) emissions realistically reproduces the general spatial patterns and magnitudes of observed changes in both hot and cold extremes across the mid-1990s, suggesting a pronounced role of anthropogenic changes in these observed decadal changes. Separately, changes in GHG forcing lead to rise in TXx, TNx, TXn and TNn, increase in frequencies of SU and TR and decrease in frequencies of ID and FD over China through increased Greenhouse Effect with positive clear sky longwave radiation and play a dominant role in simulated changes of both hot and cold extremes over China. The AA forcing changes tend to cool Southern China and warm Northern China during summer via aerosol-radiation interaction and AA-induced atmosphere-cloud feedback and therefore lead to some weak decrease in hot extremes over Southeastern China and increase over Northern China. Meanwhile, AA changes lead to warming over China during winter through cloud feedbacks related to aerosol induced cooling over tropical Indian Ocean and western tropical Pacific, and also induce changes in cold extremes the same sign as those induced by GHG, but with weak magnitude.

The density stratification in the ocean is directly related to the diapycnal mixing, which drives the abyssal cell of the Meridional Overturning Circulation (MOC). It is important to understand how stratification has been changing in the world's deep and abyssal oceans under climate change. Using repeat hydrographic data obtained since the 1990s, we find a decreasing stratification associated with changes in the source Antarctica Bottom Water (AABW) properties in its formation basins as well as in basins along its dispersal pathways. Averaged south of 60°S, the squared buoyancy frequency N2 shows a negative trend of −6% per decade in waters deeper than 4,000 m. The observed decadal reduction in stratification is associated with large spatial variability, especially in the Southern Ocean basins with multiple AABW sources. Additionally, there are also significant differences between neighboring basins that are related to the blocking effect of topography. Plain Language Summary Cold and dense Antarctica Bottom Water (AABW) forms near the coastline of Antarctica, sinks to the ocean bottom, moves northward in deep branches of the Meridional Overturning Circulation, becomes lighter through mixing, and eventually upwells to shallower depths. The AABW properties change under the changing climate. Repeat hydrography sections occupied approximately once per decade since the 1990s have revealed continuously warming and freshening of the AABW. In many regions of the ocean, the warming and freshening trends are strongest at the bottom. This causes a decrease in the vertical gradient of density, namely stratification. As the level of mixing is strongly related to stratification, it is important to quantify its trend and understand its spatial structure. We quantify the trend in stratification globally and find a reduction in stratification in the Southern Ocean and along the spreading pathways of the AABW at lower latitudes. Key Points Three decades of repeat hydrographic sections reveal decreasing density stratification in the deep and abyssal layers of the Southern Ocean South of 60°S, N2 has reduced by 6% per decade in the ocean below 4,000 m, with a peak reduction rate of 15% per decade at 4,800 m The decreasing stratification is found along the pathway of Antarctica Bottom Water and its vertical structure depends on the topography of basin boundaries

In this study, the relationship between the summer North Atlantic Oscillation (SNAO) and the East Asian summer rainfall was statistically diagnosed based on the European Centre for Medium‐Range Weather Forecasts (ECMWF) 40‐year and interim reanalysis data (ERA‐40 and ERA‐Interim) as well as precipitation data from the Global Precipitation Climatology Centre (GPCC). The results show that the decadal change of the SNAO pattern around the late 1970s significantly enhanced its connection with summer rainfall over central and northern East Asia. Over the period before the late 1970s, the SNAO‐related circulations were dominant over the North Atlantic region. Consequently, there was a weak connection between the SNAO and the East Asian summer rainfall. However, over the period after the late 1970s, the SNAO pattern experienced a decadal change, with the southern center shifting eastward. Such changes in the SNAO pattern can alter the stationary wave activity over the Eurasian Continent, producing an anomalous meridional dipole pattern over East Asia. This dipole pattern can then change the divergence circulation, vertical motion, water vapor, and total cloud cover, which would consequently provide beneficial conditions for more (less) summer rainfall over central (northern) East Asia in a positive (negative)‐phase SNAO year. Key Points There is a connection between the SNAO and the East Asian summer rainfall The connection between the SNAO and the East Asian summer rainfall is unstable The decadal change of the SNAO enhances its impact on East Asian summer rainfall