The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. Present study unravels that the ISO intensity increase over the tropical WNP in El Niño developing summers is larger, with the center located eastward, compared with the decrease in La Niña developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Niño developing summers and westward shift of regions of opposite anomalies in La Niña developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Niña developing summers compared to anomalous warming in El Niño developing summers. The 10–20-day and 30–60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Niño and La Niña events and the role of asymmetric atmospheric background field changes. Sensitivity experiments with illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Niño and La Niña events has a small impact on asymmetric ISO intensity changes over the tropical WNP.

The El Niño–Southern Oscillation (ENSO) is the strongest interannual air–sea coupled variability mode in the tropics, and substantially impacts the global weather and climate. Hence, it is important to improve our understanding of the ENSO variability. Besides the well-known air–sea interaction process over the tropical Pacific, recent studies indicated that atmospheric and oceanic forcings outside the tropical Pacific also play important roles in impacting and modulating the ENSO occurrence. This paper reviews the impacts of the atmosphere–ocean variability outside the tropical Pacific on the ENSO variability, as well as their associated physical processes. The review begins with the contribution of the atmosphere–ocean forcings over the extratropical North Pacific, Atlantic, and Indian Ocean on the ENSO occurrence. Then, an overview of the extratropical atmospheric forcings over the Northern Hemisphere (including the Arctic Oscillation and the Asian monsoon systems) and the Southern Hemisphere (including the Antarctic Oscillation and the Pacific–South American teleconnection), on the ENSO occurrence, is presented. It is shown that the westerly (easterly) wind anomaly over the tropical western Pacific is essential for the occurrence of an El Niño (a La Niña) event. The wind anomalies over the tropical western Pacific also play a key role in relaying the impacts of the atmosphere–ocean forcings outside the tropical Pacific on the ENSO variability. Finally, some relevant questions, that remain to be explored, are discussed.

The present study investigates the relationship of tropical cyclone (TC) genesis between the western North Pacific (WNP) and South China Sea (SCS) from 1979 to 2020. A significantly out-of-phase variation is found between spring (MAM) TC genesis over the WNP and the following summer–fall (JJASON) TC genesis over the SCS. More TCs over the WNP in MAM are followed by less TCs over the SCS in succeeding JJASON. Composite analysis and numerical model experiments show that negative sea surface temperature (SST) anomalies during MAM in the tropical central–eastern Pacific (CEP) and southeastern Indian Ocean work together to induce a lower-level cyclonic circulation over the WNP, with the latter more important. The positive specific humidity and ascending motion favor the TC genesis over the WNP in MAM. In following JJASON, the SST anomalies are reversed in the tropical CEP. The positive precipitation anomalies over the western–central Pacific induced by positive SST anomalies further stimulate an anomalous zonal overturning circulation with anomalous descending motion and boundary layer divergence over the SCS. In addition, the persistent negative SST anomalies around the Maritime Continent (MC) induce an anomalous anticyclone to the west. Both processes lead to negative genesis potential index (GPI) anomalies and thus inhibit the TC genesis over the SCS. This out-of-phase relationship of TC genesis between the WNP and SCS also exists when the El Niño–Southern Oscillation (ENSO) years are removed. This finding may be helpful to improve the seasonal prediction of the SCS TC activity over the peak TC season.

Previous studies have revealed the relationship between the Madden-Julian oscillation (MJO) and the Arctic Oscillation (AO). The MJO phase 2/3 is followed by the positive AO phase, and the MJO phase 6/7 is followed by the negative AO phase. This study reveals that the MJO phase 6/7-AO connection is modulated by the Quasi-Biennial Oscillation (QBO) through both tropospheric and stratospheric pathways during boreal winter. The MJO 2/3 phase and AO relationship is favored in both QBO easterly (QBOE) and westerly (QBOW) years because of the MJO-triggered tropospheric Rossby wave train from the tropics toward the polar region. The AO following the MJO 6/7 phase shifts to negative in QBOW years, but the MJO-AO connection diminishes in QBOE years. In QBOW years, the Asian-Pacific jet is enhanced, leading to more evident poleward propagation of tropospheric Rossby wave train, which contributes to the tropospheric pathway of the AO-MJO 6/7 connection. Besides, the enhanced Asian-Pacific jet in QBOW years is favorable for vertical propagation of planetary waves into the stratosphere in MJO phase 6/7, leading to negative AO, which indicates the stratospheric pathway of the AO-MJO 6/7 connection.Atmosphere 2020, 11, 175 2 of 13 response [17,18] that propagates poleward and eastward and modulates the atmospheric Pacific-North America (PNA)-like teleconnection pattern [19] over the North Pacific and North Atlantic. The MJO can also influence the AO [12]. The MJO can also modify the meridional heat flux that is in phase with climatological stationary waves over the North Pacific, which interferes constructively with climatological stationary waves and induces vertical propagation of planetary waves into the stratosphere, contributing to the polar vortex variation [20][21][22][23]. The anomalous signal of the stratospheric polar vortex propagates downward and influences the tropospheric AO [24].Both the MJO and the AO are associated with the Quasi-Biennial Oscillation (QBO), which manifests as alternating easterly and westerly zonal winds over the stratospheric tropics on the time scale of 2.5 years [25][26][27]. During boreal winter, about 40% of the interannual variation of the MJO is contributed by the QBO [28][29][30]. The strength of the MJO can be influenced by the QBO, and thus the MJO-related teleconnection changes in QBO easterly and westerly years [29]. The MJO tends to be stronger in the easterly wind phase of the QBO (QBOE) than the westerly wind phase of the QBO (QBOW), which may be attributed to the change in the static stability at the tropopause caused by the [31][32][33]. The QBO can be linked to the stratospheric polar vortex through upward propagation of planetary waves and the interaction between the stratospheric zonal mean zonal wind and the planetary waves [27]. The variation of the stratospheric polar vortex can influence the AO by downward propagation of stratospheric anomalous signals [24]. The zero-wind line at 50 hPa differs in the easterly and westerly QBO phases, which leads to a difference in the u...

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In this study, we reveal a marked enhanced impact of the early-spring Aleutian Low (AL) on the following winter El Niño and Southern Oscillation (ENSO) after the late-1990s. This enhanced impact of the early-spring AL may have an important contribution to the increased emergence of the central Pacific ENSO during recent decades. After the late-1990s, decrease (increase) in the early-spring AL strength tends to induce an anomalous cyclone (anticyclone) over subtropical North Pacific via wave-mean flow interaction. The associated westerly (easterly) wind anomalies to the south side of the subtropical anomalous cyclone (anticyclone) over the tropical western Pacific contribute to occurrence of central Pacific-like El Niño (La Niña) in the following winter via tropical Bjerknes feedback. Further, the subtropical anomalous cyclone (anticyclone) leads to sea surface temperature (SST) increase (decrease) in the equatorial Pacific in the following summer via wind-evaporation-SST (WES) feedback, which further contributes to succeeding central Pacific-like El Niño (La Niña). Enhanced impact of early-spring AL on ENSO is attributable to enhancement of the mean circulation over the North Pacific, which leads to increased wave-mean flow interaction and strengthened WES feedback after the late-1990s. The results offer the potential to advance our understanding of the factors for the reduced prediction skill of ENSO since the late-1990s.

This work compares the contributions of synoptic, intraseasonal, and interannual components of large-scale parameters to tropical cyclone (TC) genesis over the North Indian Ocean (NIO) from April to December from 1979 to 2020. A composite analysis is employed with respect to TC genesis time and location. It is shown that most TCs occur when the total sea surface temperature (SST) is between 28 and 30 °C and SST anomalies in three time ranges are small (with the magnitude less than 0.2 °C). The TCs form mostly when the anomalies of vertical zonal wind shear are between −6 and 6 m s−1 and total vertical zonal wind shear falls within −12 and −3 m s−1, with the synoptic component being a positive contributor. The intraseasonal component of vorticity and convergence in the low level, vertical motion and specific humidity in the middle level, and convection contributes dominantly to the TC genesis. Synoptic-scale tropical disturbances obtain barotropic kinetic energy from the climatological mean and intraseasonal flows, with the former dominant in the southeastern sector, and the latter dominant in the northwestern sector. The contributions of the three temporal components of environmental factors are compared for TC genesis between the Arabian Sea (AS) and Bay of Bengal (BOB) and between the early season (April through June) and late season (September through December). The relative contributions of the three temporal components of factors are also compared for the TC formation among the NIO, northern tropical Atlantic Ocean (NTA), Northwestern Pacific (WNP), and Northeastern Pacific (ENP).

In this study, we provide a new perspective on the recent increased emergence of the central Pacific type of El Niño and Southern Oscillation (ENSO). Our results indicate that early-spring Aleutian Low (AL) intensity has a remarkable impact on the following winter ENSO especially after the late-1990s. Decrease (increase) in the early-spring AL strength tends to induce an anomalous cyclone (anticyclone) over subtropical North Pacific via wave-mean flow interaction. The anomalous cyclone (anticyclone) leads to sea surface temperature (SST) increase (decrease) in the equatorial Pacific in the following summer via wind-evaporation-SST feedback and trade wind charging mechanism, which further contributes to succeeding central Pacific-like El Niño (La Niña). This remarkable AL’s impact on ENSO is attributable to enhancement of the background trade winds, which is related to changes of both the AMO and NPGO from positive to negative phases around the late-1990s. In contrast, global warming is suggested to have a negative effect on the recent increased connection of the early-spring AL with the following winter ENSO. The results offer the potential to advance our understanding of the factors that explain decrease of the prediction skill of ENSO since the late-1990s.