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1. The Recent Decline and Recovery of Indian Summer Monsoon Rainfall: Relative Roles of External Forcing and Internal Variability

   https://doi.org/10.1175/JCLI-D-19-0833.1  

   Huang, Xin ; Zhou, Tianjun ; Turner, Andrew ; Dai, Aiguo ; Chen, Xiaolong ; Clark, Robin ; Jiang, Jie ; Man, Wenmin ; Murphy, James ; Rostron, John ; et al ( June 2020 , Journal of Climate)

   null (Ed.)

   Abstract The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the interdecadal Pacific oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950–99 and 1999–2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950 to 1999 reduces what would have been an externally forced weak upward rainfall trend of 0.01 to −0.15 mm day −1 decade −1 during that period, while the rainfall trend from 1999 to 2013 increases from the forced value of 0.42 to 0.68 mm day −1 decade −1 associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100-member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia. 

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2. 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)

   null (Ed.)

   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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3. Near-Surface Salinity Reveals the Oceanic Sources of Moisture for Australian Precipitation Through Atmospheric Moisture Transport

   https://doi.org/10.1175/JCLI-D-19-0579.1  

   Rathore, Saurabh ; Bindoff, Nathaniel L. ; Ummenhofer, Caroline C. ; Phillips, Helen E. ; Feng, Ming ( May 2020 , Journal of Climate)

   The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on inter-seasonal to inter-annual time scales, and to locate the source of moisture. Seasonal composites during El Niño Southern Oscillation/Indian Ocean Dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies towards Australia. During co-occurring La Niña and negative-IOD events, salty anomalies around the maritime continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, there is the moisture transport divergence anomaly over Australia and results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean-atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g. 2010-11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall. 

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4. Temporal Variability of Air‐Sea CO 2 flux in the Western Tropical North Atlantic Influenced by the Amazon River Plume

   https://doi.org/10.1029/2020GB006798  

   Mu, Linquan ; Gomes, Helga do Rosario ; Burns, Shannon M. ; Goes, Joaquim I. ; Coles, Victoria J. ; Rezende, Carlos E. ; Thompson, Fabiano L. ; Moura, Rodrigo L. ; Page, Benjamin ; Yager, Patricia L. ( June 2021 , Global Biogeochemical Cycles)

   The partial pressure of carbon dioxide (pCO₂) was surveyed across the Amazon River plume and the surrounding western tropical North Atlantic Ocean (15–0°N, 43–60°W) during three oceanic expeditions (May–June 2010, September–October 2011, and July 2012). The survey timing was chosen according to previously described temporal variability in plume behavior due to changing river discharge and winds.In situsea surfacepCO₂and air‐sea CO₂flux exhibited robust linear relationships with sea surface salinity (SSS; 15 < SSS < 35), although the relationships differed among the surveys. Regional distributions ofpCO₂and CO₂flux were estimated using SSS maps from high‐resolution ocean color satellite‐derived (MODIS‐Aqua) diffuse attenuation coefficient at 490 nm (K_d490) during the periods of study. Results confirmed that the plume is a net CO₂sink with distinctive temporal variability: the strongest drawdown occurred during the spring flood (−2.39 ± 1.29 mmol m⁻² d⁻¹in June 2010), while moderate drawdown with relatively greater spatial variability was observed during the transitional stages of declining river discharge (−0.42 ± 0.76 mmol m⁻² d⁻¹in September–October 2011). The region turned into a weak source in July 2012 (0.26 ± 0.62 mmol m⁻² d⁻¹) when strong CO₂uptake in the mid‐plume was overwhelmed by weak CO₂outgassing over a larger area in the outer plume. Outgassing near the mouth of the river was observed in July 2012. Our observations draw attention to the importance of assessing the variable impacts of biological activity, export, and air‐sea gas exchange before estimating regional CO₂fluxes from salinity distributions alone.

    

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5. Enhanced ENSO-Unrelated Summer Variability in the Indo–Western Pacific under Global Warming

   https://doi.org/10.1175/JCLI-D-22-0450.1  

   Wang, Chuan-Yang ; Zheng, Xiao-Tong ; Xie, Shang-Ping ( March 2023 , Journal of Climate)

   El Niño–Southern Oscillation (ENSO) is an important but not the only source of interannual variability over the Indo–western Pacific. Non-ENSO forced variability in the region has received recent attention because of the implications for rainy-season prediction. Using a 35-member CESM1 Large Ensemble (CESM-LE) and 30 CMIP6 models, this study shows that the ensemble means project intensified interannual variability for precipitation, low-level winds, and sea level pressure under global warming, associated with the enhanced large-scale anomalous anticyclone (AAC) over the tropical northwestern (NW) Pacific after the ENSO signal is removed. A decomposition based on the column water vapor budget reveals that enhanced precipitation variability is due to the increased background specific humidity. The resultant anomalous diabatic heating intensifies the AAC, which further strengthens the precipitation anomalies. Over the tropical NW Pacific, the wind-induced evaporative cooling on the southeastern flank of the AAC is countered by the increased shortwave radiation due to the strengthened precipitation reduction. Tropospheric temperature anomalies in the ensemble means show no significant change, suggesting no apparent change of the interbasin positive feedback between the AAC and northern Indian Ocean SST. Intermodel analysis based on CMIP6 reveals that models with a larger increase in ENSO-unrelated precipitation variability over the NW Pacific are associated with stronger background warming in the eastern equatorial Pacific, due to the modulated Walker and Hadley circulations.

    

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