More Like this

1. A global poleward shift of atmospheric rivers

   https://doi.org/10.1126/sciadv.adq0604  

   Li, Zhe; Ding, Qinghua (October 2024, Science Advances)

   Atmospheric rivers (ARs) are key agents in distributing extratropical precipitation and transporting moisture poleward. Climate models forced by historical anthropogenic forcing suggest an increase in AR activity in the extratropics over the past four decades. However, reanalyses indicate a ~6° to 10° poleward shift of ARs during boreal winter in both hemispheres, featuring a rise along 50°N and 50°S and a decrease along 30°N and 30°S. Our analysis demonstrates that low-frequency sea surface temperature variability in the tropical eastern Pacific exhibits a cooling tendency since 2000 that plays a key role in driving this global AR shift, mostly over extratropical oceans, through a tropical-driven eddy-mean flow feedback. This mechanism also operates on interannual timescales, controlled by the El Niño–Southern Oscillation, and is less pronounced over the Southern Ocean due to weaker eddy activity during austral summer. These highlight the sensitivity of ARs to large-scale circulation changes driven by both internal variability and external forcing in current and upcoming decades. 

   more » « less
2. Changes in the Global Climate: Atmospheric Angular Momentum and Pacific Ocean Temperatures

   https://doi.org/10.1175/JCLI-D-22-0322.1  

   Weickmann, Klaus; Berry, Edward; Gensini, Victor; Gold, David; Petroski, Thomas (October 2023, Journal of Climate)

   Abstract Atmospheric angular momentum (AAM) is used to study the variability of Earth’s atmospheric circulation during the past 45 years, a time of considerable climate change. Using global AAM, two interdecadal states are defined covering the periods 1977–98 (hereinafter P1) and 1999–2022 (P2). Global AAM decreased from P1 to P2 and was accompanied by weakened subtropical jet streams in both hemispheres, strong convection around the northern Maritime Continent, and a strengthened sea surface temperature (SST) gradient across the tropical Pacific Ocean. The period differences project onto 1) internal interdecadal Pacific variability (IPV), 2) a postulated transient ocean thermostat response to greenhouse gas and aerosol emissions, and 3) circulation anomalies related to the ozone hole. During 1977–2023, the first two processes are forcing the climate toward larger Pacific Ocean SST gradients and a poleward expansion of the Indo-Pacific warm pool (IPWP), especially into the Northern Hemisphere. The ozone hole produces its own distinct pattern of anomalies in the Southern Hemisphere that tend to become persistent in the early 1990s. The zonal and vertical mean AAM variations during P1 have frequent westerly wind anomalies between 40°N and 40°S with poleward propagation on interannual time scales. During P2, the circulation is dominated by subtropical easterly wind anomalies, poleward-shifted jets, and weaker propagation. Locally, the zonal mean anomalies manifest as midlatitude ridges that lead to continental droughts. Case studies illustrate the weakened subtropical jet streams of P2 and examine the factors behind a transition to La Niña in early 2020 that maintains the P2 pattern. 

   more » « less
3. Reconciling Roles of External Forcing and Internal Variability in Indian Ocean Decadal Variability Since 1920

   https://doi.org/10.1029/2021GL097198  

   Hua, Wenjian; Dai, Aiguo; Qin, Minhua (May 2022, Geophysical Research Letters)

   Abstract On decadal time scales, Indian Ocean sea surface temperatures (SSTs) exhibit coherent basin‐wide changes, but their origins are not well understood. Here we analyze observations and model simulations from Coupled Model Intercomparison Project Phase 6 and Community Earth System Model Version 1 to quantify the roles of external forcing and internal climate variability in causing Indian Ocean decadal SST variations. Results show that both external forcing and internal variability since 1920 have contributed to the observed decadal variations in linearly detrended Indian Ocean SSTs, and they exhibit an out‐of‐phase relationship since the 1950s. The internally‐generated variations arise from remote influences from the tropical Pacific and possible contributions from internal local processes, while the influence from the Atlantic Multidecadal Oscillation is opposite to that of the Interdecadal Pacific Oscillation. Decadal SST changes caused by nonlinear variations in greenhouse gases and aerosols are roughly out‐of‐phase with the internal variability, thus dampening observed SST variations since the 1950s. 

   more » « less
4. Poleward Shift of Atmospheric Rivers in the Southern Hemisphere in Recent Decades

   https://doi.org/10.1029/2020GL089934  

   Ma, Weiming; Chen, Gang; Guan, Bin (October 2020, Geophysical Research Letters)

   Abstract The atmospheric river (AR) frequency trends over the Southern Hemisphere are investigated using three reanalyses and two Community Earth System Model (CESM) ensembles. The results show that AR frequency has been increasing over the Southern Ocean and decreasing over lower latitudes in the past four decades and that ARs have been shifting poleward. While the observed trends are mostly driven by the poleward shift of the westerly jet, fully coupled CESM experiments indicate anthropogenic forcing would result in positive AR frequency trends over the Southern Ocean due mostly to moisture changes. The difference between the observed trends and anthropogenically driven trends can be largely reconciled by the atmosphere‐only CESM simulations forced by observed sea surface temperatures: Sea surface temperature variability characteristic of the negative phase of the Interdecadal Pacific Oscillation strongly suppresses the moisture‐driven trends while enhances the circulation‐induced trends over the Southern Ocean, thus bringing the simulated trends into closer agreement with the observed trends. 

   more » « less
5. Decadal Modulation of the ENSO–Indian Ocean Basin Warming Relationship during the Decaying Summer by the Interdecadal Pacific Oscillation

   https://doi.org/10.1175/JCLI-D-20-0457.1  

   Liu, Fangyu; Zhang, Wenjun; Jin, Fei-Fei; Hu, Suqiong (April 2021, Journal of Climate)

   null (Ed.)

   Abstract Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. 

   more » « less