Role of Tropical Variability in Driving Decadal Shifts in the Southern Hemisphere Summertime Eddy-Driven Jet

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. Plain Language Summary Atmospheric rivers (ARs) are "rivers" in the sky that carry a huge amount of water vapor equivalent to the world's largest rivers on the ground. Because of the amount of water vapor they carry, ARs can cause extreme rainfall and floods upon landfall when the moisture is condensed out by mountain barriers or other favorable conditions. Most of the studies so far have focused on ARs in the Northern Hemisphere. In this study, we found that ARs in the Southern Hemisphere have been occurring at increasingly higher-latitude regions in the past four decades and that this observed trend is mostly due to winds getting stronger at higher-latitude regions in the Southern Hemisphere. Using results from climate model simulations, we show that the observed AR trends in the Southern Hemisphere are driven by both human activities, such as greenhouse gas increases and ozone depletion, and the internal variability of sea surface temperatures due to the coupled atmosphere-ocean system. The poleward shift of ARs in the Southern Hemisphere may have major implications for the amount of moisture and heat transported into Antarctica, thus may also have profound impacts on the Antarctic and in turn, global climate.

Section: Discussionsupporting

confidence: 65%

Section: Resultsmentioning

confidence: 53%

Section: Introductionmentioning

confidence: 99%

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Poleward Shift of Atmospheric Rivers in the Southern Hemisphere in Recent Decades

Chen

Guan

2020

“…(2015) and D. Yang et al. (2020), who found that a substantial component of recent multidecadal westerly wind variability could be accounted for in model experiments forced by observed tropical SST variations, independent of anthropogenic forcing.…”

Section: Historical Eramentioning

confidence: 97%

Historical and Projected Changes in the Southern Hemisphere Surface Westerlies

Goyal

Gupta

Jucker

et al. 2021

Self Cite

The Southern Hemisphere (SH) surface westerlies fundamentally control regional patterns of air temperature, storm tracks, and precipitation while also regulating ocean circulation, heat transport and carbon uptake. Wind‐forced ocean perturbation experiments commonly apply idealized poleward wind shifts ranging between 0.5 and 10 degrees of latitude and wind intensification factors of between 10% and 300%. In addition, changes in winds are often prescribed ad hoc as a zonally uniform anomaly that neglects important regional and seasonal differences. Here we quantify historical and projected SH westerly wind changes based on examination of CMIP5, CMIP6, and reanalysis data. We find a significant reduction in the location bias of the CMIP6 ensemble and an associated reduction in the projected poleward shift compared to CMIP5. Under a high emission scenario, we find a projected end of 21st Century ensemble mean wind increase of ∼10% and a poleward shift of ∼0.8° latitude, although there are important seasonal and regional variations.

“…Our study extends earlier studies by Schneider et al (2015) and Swart et al (2015) who showed that there are spatial variations in past trends in near‐surface winds, but did not examine the impact of these trends on the strength and position of the peak winds averaged over each ocean basin. Yang et al (2020) have very recently examined zonal variations in eddy‐driven jet trends, but they only considered summer trends. We show here that the winter‐spring (and annual‐mean) trends differ substantially from the summer trends.…”

Section: Introductionmentioning

confidence: 99%

Contrasting Recent Trends in Southern Hemisphere Westerlies Across Different Ocean Basins

Waugh

Banerjee

Fyfe

et al. 2020

Many studies have documented the trends in the latitudinal position and strength of the midlatitude westerlies in the Southern Hemisphere. However, very little attention has been paid to the longitudinal variations of these trends. Here, we specifically focus on the zonal asymmetries in the southern hemisphere wind trends between 1980 and 2018. Meteorological reanalyses show a large strengthening and a statistically insignificant equatorward shift of peak near-surface winds over the Pacific, in contrast to a weaker strengthening and significant poleward shift over the Atlantic and Indian Ocean sectors. The reanalysis trends fall within the ensemble spread for historical climate model simulations, showing that climate models are able to capture the observed trends. Climate model simulations indicate that the differential movement of the peak westerlies is a manifestation of internal variability and is not a forced response. Implications of these asymmetries for other components of the climate system are discussed. Plain Language Summary The band of strong westerly winds in middle latitudes, which are often referred to as the midlatitude jet stream, influence not only temperature, regional storms, and precipitation but also the ocean circulation and amount of carbon and heat entering the oceans. In recent years, much attention has been paid to the observed strengthening and poleward shift of the Southern Hemisphere midlatitude jet. However, nearly all the focus has rested on trends in longitudinally averaged winds. Here we specifically focus on the longitudinal variations in trends over the last four decades. Observationally based data show that while the peak annual-averaged winds over the Atlantic and Indian Oceans have moved toward the South Pole, there has been an insignificant shift toward the equator for the peak winds over the Pacific Ocean. Simulations with climate models indicate that the underlying cause of this differential movement of the peak winds is internal atmospheric variability, and not a response to human activities. The longitudinal variations in the wind trends may have consequences for ocean gyre circulations and associated transport of heat and carbon into the oceans.