More Like this

1. Hydroclimatic trends during 1950–2018 over global land

   https://doi.org/10.1007/s00382-021-05684-1  

   Dai, Aiguo ( February 2021 , Climate Dynamics)

   null (Ed.)

   Global hydroclimatic changes from 1950 to 2018 are analyzed using updated data of land precipitation, streamfow, and an improved form of the Palmer Drought Severity Index. The historical changes are then compared with climate model-simulated response to external forcing to determine how much of the recent change is forced response. It is found that precipitation has increased from 1950 to 2018 over mid-high latitude Eurasia, most North America, Southeast South America, and Northwest Australia, while it has decreased over most Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Pacifc coasts of Canada. Streamfow records largely confrm these precipitation changes. The wetting trend over Northwest Australia and Southeast South America is most pronounced in austral summer while the drying over Africa and wetting trend over mid-high latitude Eurasia are seen in all seasons. Coupled with the drying caused by rising surface temperatures, these precipitation changes have greatly increased the risk of drought over Africa, southern Europe, East Asia, eastern Australia, Northwest Canada, and southern Brazil. Global land precipitation and continental freshwater discharge show large interannual and inter-decadal variations, with negative anomalies during El Niño and following major volcanic eruptions in 1963, 1982, and 1991; whereas their decadal variations are correlated with the Interdecadal Pacifc Oscillation (IPO) with IPO’s warm phase associated with low land precipitation and continental discharge. The IPO and Atlantic multidecadal variability also dominate multidecadal variations in land aridity, accounting for 90% of the multidecadal variance. CMIP5 multi-model ensemble mean shows decreased precipitation and runoff and increased risk of drought during 1950–2018 over Southwest North America, Central America, northern and central South America (including the Amazon), southern and West Africa, the Mediterranean region, and Southeast Asia; while the northern mid-high latitudes, Southeast South America, and Northwest Australia see increased precipitation and runoff. The consistent spatial patterns between the observed changes and the model-simulated response suggest that many of the observed drying and wetting trends since 1950 may have resulted at least partly from historical external forcing. However, the drying over Southeast Asia and wetting over Northwest Australia are absent in the 21st century projections. 

   more » « less
2. Contributions of Different Combinations of the IPO and AMO to Recent Changes in Winter East Asian Jets

   https://doi.org/10.1175/JCLI-D-18-0218.1  

   Huang, Danqing ; Dai, Aiguo ; Yang, Ben ; Yan, Peiwen ; Zhu, Jian ; Zhang, Yaocun ( March 2019 , Journal of Climate)

   Recent concurrent shifts of the East Asian polar-front jet (EAPJ) and the East Asian subtropical jet (EASJ) in the boreal winter have raised concerns, since they could result in severe weather events over East Asia. However, the possible mechanisms are not fully understood. In this study, the roles of the interdecadal Pacific oscillation (IPO) and the Atlantic multidecadal oscillation (AMO) are investigated by analyzing reanalysis data and model simulations. Results show that combinations of opposite phases of the IPO and AMO can result in significant shifts of the two jets during 1920–2014. This relationship is particularly evident during 1999–2014 and 1979–98 in the reanalysis data. A combination of a negative phase of the IPO (−IPO) and a positive phase of the AMO (+AMO) since the late 1990s has enhanced the meridional temperature gradient and the Eady growth rate and thus westerlies over the region between the two jets, but weakened them to the south and north of the region, thereby contributing to the equatorward and poleward shifts of the EAPJ and EASJ, respectively. Atmospheric model simulations are further used to investigate the relative contribution of −IPO and +AMO to the jet shifts. The model simulations show that the combination of −IPO and +AMO favors the recent jet changes more than the individual −IPO or +AMO. Under a concurrent −IPO and +AMO, the meridional eddy transport of zonal momentum and sensitive heat strengthens, and more mean available potential energy converts to the eddy available potential energy over the region between the two jets, which enhances westerly winds there.

    

   more » « less
3. An Investigation of Extreme Cold Events at the South Pole

   https://doi.org/10.1175/JCLI-D-21-0404.1  

   Keller, Linda M. ; Maloney, Kathryn J. ; Lazzara, Matthew A. ; Mikolajczyk, David E. ; Battista, Stefano Di ( March 2022 , Journal of Climate)

   Abstract From 5 July to 11 September 2012, the Amundsen–Scott South Pole station experienced an unprecedented 78 days in a row with a maximum temperature at or below −50°C. Aircraft and ground-based activity cannot function without risk below this temperature. Lengthy periods of extreme cold temperatures are characterized by a drop in pressure of around 15 hPa over 4 days, accompanied by winds from grid east. Periodic influxes of warm air from the Weddell Sea raise the temperature as the wind shifts to grid north. The end of the event occurs when the temperature increase is enough to move past the −50°C threshold. This study also examines the length of extreme cold periods. The number of days below −50°C in early winter has been decreasing since 1999, and this trend is statistically significant at the 5% level. Late winter shows an increase in the number of days below −50°C for the same period, but this trend is not statistically significant. Changes in the southern annular mode, El Niño–Southern Oscillation, and the interdecadal Pacific oscillation/tripole index are investigated in relation to the initiation of extreme cold events. None of the correlations are statistically significant. A positive southern annular mode and a La Niña event or a central Pacific El Niño–Southern Oscillation pattern would position the upper-level circulation to favor a strong, symmetrical polar vortex with strong westerlies over the Southern Ocean, leading to a cold pattern over the South Pole. Significance Statement The Amundsen–Scott South Pole station is the coldest Antarctic station staffed year-round by U.S. personnel. Access to the station is primarily by airplane, especially during the winter months. Ambient temperature limits air access as planes cannot operate at minimum temperatures below −50°C. The station gets supplies during the winter months if needed, and medical emergencies can happen requiring evacuations. Knowing when planes would be able to fly is crucial, especially for life-saving efforts. During 2012, a record 78 continuous days of temperatures below −50°C occurred. A positive southern annular mode denoting strong westerly winds over the Pacific Ocean and a strong polar vortex over the South Pole contribute to the maintenance of long periods of extremely cold temperatures. 

   more » « less
4. The role of interdecadal climate oscillations in driving Arctic atmospheric river trends

   https://doi.org/10.1038/s41467-024-45159-5  

   Ma, Weiming ; Wang, Hailong ; Chen, Gang ; Leung, L. Ruby ; Lu, Jian ; Rasch, Philip J. ; Fu, Qiang ; Kravitz, Ben ; Zou, Yufei ; Cassano, John J. ; et al ( December 2024 , Nature Communications)

   Atmospheric rivers (ARs), intrusions of warm and moist air, can effectively drive weather extremes over the Arctic and trigger subsequent impact on sea ice and climate. What controls the observed multi-decadal Arctic AR trends remains unclear. Here, using multiple sources of observations and model experiments, we find that, contrary to the uniform positive trend in climate simulations, the observed Arctic AR frequency increases by twice as much over the Atlantic sector compared to the Pacific sector in 1981-2021. This discrepancy can be reconciled by the observed positive-to-negative phase shift of Interdecadal Pacific Oscillation (IPO) and the negative-to-positive phase shift of Atlantic Multidecadal Oscillation (AMO), which increase and reduce Arctic ARs over the Atlantic and Pacific sectors, respectively. Removing the influence of the IPO and AMO can reduce the projection uncertainties in near-future Arctic AR trends by about 24%, which is important for constraining projection of Arctic warming and the timing of an ice-free Arctic.

    

   more » « less
5. Empirical–Statistical Downscaling of Austral Summer Precipitation over South America, with a Focus on the Central Peruvian Andes and the Equatorial Amazon Basin

   https://doi.org/10.1175/JAMC-D-20-0066.1  

   Sulca, Juan ; Vuille, Mathias ; Timm, Oliver Elison ; Dong, Bo ; Zubieta, Ricardo ( January 2021 , Journal of Applied Meteorology and Climatology)

   null (Ed.)

   Abstract Precipitation is one of the most difficult variables to estimate using large-scale predictors. Over South America (SA), this task is even more challenging, given the complex topography of the Andes. Empirical–statistical downscaling (ESD) models can be used for this purpose, but such models, applicable for all of SA, have not yet been developed. To address this issue, we construct an ESD model using multiple-linear-regression techniques for the period 1982–2016 that is based on large-scale circulation indices representing tropical Pacific Ocean, Atlantic Ocean, and South American climate variability, to estimate austral summer [December–February (DJF)] precipitation over SA. Statistical analyses show that the ESD model can reproduce observed precipitation anomalies over the tropical Andes (Ecuador, Colombia, Peru, and Bolivia), the eastern equatorial Amazon basin, and the central part of the western Argentinian Andes. On a smaller scale, the ESD model also shows good results over the Western Cordillera of the Peruvian Andes. The ESD model reproduces anomalously dry conditions over the eastern equatorial Amazon and the wet conditions over southeastern South America (SESA) during the three extreme El Niños: 1982/83, 1997/98, and 2015/16. However, it overestimates the observed intensities over SESA. For the central Peruvian Andes as a case study, results further show that the ESD model can correctly reproduce DJF precipitation anomalies over the entire Mantaro basin during the three extreme El Niño episodes. Moreover, multiple experiments with varying predictor combinations of the ESD model corroborate the hypothesis that the interaction between the South Atlantic convergence zone and the equatorial Atlantic Ocean provoked the Amazon drought in 2015/16. 

   more » « less