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1. A Multidecadal-Scale Tropically Driven Global Teleconnection over the Past Millennium and Its Recent Strengthening

   https://doi.org/10.1175/JCLI-D-20-0216.1  

   Feng, Xiaofang ; Ding, Qinghua ; Wu, Liguang ; Jones, Charles ; Baxter, Ian ; Tardif, Robert ; Stevenson, Samantha ; Emile-Geay, Julien ; Mitchell, Jonathan ; Carvalho, Leila M. ; et al ( April 2021 , Journal of Climate)

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   Abstract In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this “wavy” pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades. 

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2. Walker circulation response to extratropical radiative forcing

   https://doi.org/10.1126/sciadv.abd3021  

   Kang, Sarah M. ; Xie, Shang-Ping ; Shin, Yechul ; Kim, Hanjun ; Hwang, Yen-Ting ; Stuecker, Malte F. ; Xiang, Baoqiang ; Hawcroft, Matt ( November 2020 , Science Advances)

   null (Ed.)

   Walker circulation variability and associated zonal shifts in the heating of the tropical atmosphere have far-reaching global impacts well into high latitudes. Yet the reversed high latitude–to–Walker circulation teleconnection is not fully understood. Here, we reveal the dynamical pathways of this teleconnection across different components of the climate system using a hierarchy of climate model simulations. In the fully coupled system with ocean circulation adjustments, the Walker circulation strengthens in response to extratropical radiative cooling of either hemisphere, associated with the upwelling of colder subsurface water in the eastern equatorial Pacific. By contrast, in the absence of ocean circulation adjustments, the Walker circulation response is sensitive to the forcing hemisphere, due to the blocking effect of the northward-displaced climatological intertropical convergence zone and shortwave cloud radiative effects. Our study implies that energy biases in the extratropics can cause pronounced changes of tropical climate patterns. 

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3. A role for orbital eccentricity in Earth’s seasonal climate

   https://doi.org/10.1186/s40562-023-00313-7  

   Chiang, John C. H. ; Broccoli, Anthony J. ( December 2023 , Geoscience Letters)

   The seasonality of Earth’s climate is driven by two factors: the tilt of the Earth’s rotation axis relative to the plane of its orbit (hereafter thetilt effect), and the variation in the Earth–Sun distance due to the Earth’s elliptical orbit around the Sun (hereafter thedistance effect). The seasonal insolation change between aphelion and perihelion is only ~ 7% of the annual mean and it is thus assumed that the distance effect is not relevant for the seasons. A recent modeling study by the authors and collaborators demonstrated however that the distance effect is not small for the Pacific cold tongue: it drives an annual cycle there that is dynamically distinct and ~ 1/3 of the amplitude from the known annual cycle arising from the tilt effect. The simulations also suggest that the influence of distance effect is significant and pervasive across several other regional climates, in both the tropics and extratropics. Preliminary work suggests that the distance effect works its influence through the thermal contrast between the mostly ocean hemisphere centered on the Pacific Ocean (the ‘Marine hemisphere’) and the hemisphere opposite to it centered over Africa (the ‘Continental hemisphere’), analogous to how the tilt effect drives a contrast between the northern and southern hemispheres. We argue that the distance effect should be fully considered as an annual cycle forcing in its own right in studies of Earth’s modern seasonal cycle. Separately considering the tilt and distance effects on the Earth’s seasonal cycle provides new insights into the workings of our climate system, and of direct relevance to paleoclimate where there are outstanding questions for long-term climate changes that are related to eccentricity variations.

    

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4. The Origin of Systematic Forecast Errors of Extreme 2020 East Asian Summer Monsoon Rainfall in GloSea5

   https://doi.org/10.1029/2021GL094179  

   Ham, Yoo‐Geun ; Kim, Ji‐Gwang ; Lee, Jeong‐Gil ; Li, Tim ; Lee, Myong‐In ; Son, Seok‐Woo ; Hyun, Yu‐Kyung ( August 2021 , Geophysical Research Letters)

   This study examined the origin of the systematic underestimation of rainfall anomalies over East Asia during July–August 2020 in operational forecasts. Through partial nudging experiments, we found that the East Asian rainfall anomalies were successfully predicted in GloSea5 with corrected tropical sea surface temperature (SST) forcing. Once the observed SST is applied over the Indian Ocean and tropical central‐eastern Pacific, a low‐level anticyclonic anomaly over the subtropical western Pacific, which transports warm‐moist air from the tropics to increase the East Asian precipitation, is well reproduced as observed. By further separating the SST into climatological and anomalous components, we revealed that the cold and dry mean state bias over the Indian Ocean and central‐eastern Pacific is responsible for the weak anomalous atmospheric teleconnection patterns from the tropics to East Asia. This implies that correcting the model mean climatological fields can directly impact the operational seasonal forecast skill.

    

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5. Anthropogenic aerosol effects on tropospheric circulation and sea surface temperature (1980–2020): separating the role of zonally asymmetric forcings

   https://doi.org/10.5194/acp-21-18499-2021  

   Diao, Chenrui ; Xu, Yangyang ; Xie, Shang-Ping ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Anthropogenic aerosols (AAs) induce global and regionaltropospheric circulation adjustments due to the radiative energyperturbations. The overall cooling effects of AA, which mask a portion ofglobal warming, have been the subject of many studies but still have largeuncertainty. The interhemispheric contrast in AA forcing has also beendemonstrated to induce a major shift in atmospheric circulation. However,the zonal redistribution of AA emissions since start of the 20th century, with anotable decline in the Western Hemisphere (North America and Europe) and acontinuous increase in the Eastern Hemisphere (South Asia and East Asia),has received less attention. Here we utilize four sets of single-model initial-condition large-ensemblesimulations with various combinations of external forcings to quantify theradiative and circulation responses due to the spatial redistribution of AAforcing during 1980–2020. In particular, we focus on the distinct climateresponses due to fossil-fuel-related (FF) aerosols emitted from the Western Hemisphere (WH) versus the Eastern Hemisphere (EH). The zonal (west to east) redistribution of FF aerosol emission since the1980s leads to a weakening negative radiative forcing over the WHmid-to-high latitudes and an enhancing negative radiative forcing over theEH at lower latitudes. Overall, the FF aerosol leads to a northward shift of the Hadley cell and an equatorward shift of the Northern Hemisphere (NH) jet stream. Here, two sets of regional FF simulations (Fix_EastFF1920and Fix_WestFF1920) are performed to separate the roles ofzonally asymmetric aerosol forcings. We find that the WH aerosol forcing,located in the extratropics, dominates the northward shift of the Hadley cell by inducing an interhemispheric imbalance in radiative forcing. On the other hand, the EH aerosol forcing, located closer to the tropics, dominates the equatorward shift of the NH jet stream. The consistent relationship between the jet stream shift and the top-of-atmosphere net solar flux (FSNTOA) gradient suggests that the latter serves as a rule-of-thumb guidance for the expected shift of the NH jet stream. The surface effect of EH aerosol forcing (mainly from low- to midlatitudes)is confined more locally and only induces weak warming over the northeastern Pacific and North Atlantic. In contrast, the WH aerosol reduction leads to a large-scale warming over NH mid-to-high latitudes that largely offsets the cooling over the northeastern Pacific due to EH aerosols. The simulated competing roles of regional aerosol forcings in drivingatmospheric circulation and surface temperature responses during the recentdecades highlight the importance of considering zonally asymmetric forcings(west to east) and also their meridional locations within the NH (tropicalvs. extratropical). 

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