scholarly journals A Linkage Observed between Austral Autumn Antarctic Oscillation and Preceding Southern Ocean SST Anomalies

2016 ◽  
Vol 29 (6) ◽  
pp. 2109-2122 ◽  
Author(s):  
Chundi Hu ◽  
Qigang Wu ◽  
Song Yang ◽  
Yonghong Yao ◽  
Duo Chan ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Turbulent Heat Flux ◽  
Turbulent Heat ◽  
Antarctic Oscillation ◽  
Maximum Covariance Analysis ◽  
Gradual Evolution ◽  
Dominant Feature ◽  
Austral Autumn

Abstract In this study, the authors apply a lagged maximum covariance analysis (MCA) to capture the cross-seasonal coupled patterns between the Southern Ocean sea surface temperature (SOSST) and extratropical 500-hPa geopotential height anomalies in the Southern Hemisphere, from which Niño-3.4 signals and their linear trends are removed to a certain extent. Statistically significant results show that the dominant feature of ocean–atmosphere interaction is likely the effect of atmosphere on SOSST anomalies, with a peak occurring when the atmosphere leads the SOSST by 1 month. However, the most eye-capturing phenomenon is that the austral autumn atmospheric signal, characterized by a negatively polarized Antarctic Oscillation (AAO), is significantly related to the gradual evolution of preceding SOSST anomalies, suggesting that the SOSST anomalies tend to exert an effect on the Southern Hemisphere atmospheric circulation. A regression analysis based on SOSST anomaly centers confirms these features. It is also demonstrated that the gradual evolution of changes in SOSST is mainly driven by internal atmospheric variability via surface turbulent heat flux associated with cold or warm advection and that the atmospheric circulation experiences a change from a typical positive AAO to a negative phase in this process. These findings indicate that such a long lead cross-seasonal covariance could contribute to a successful prediction of AAO-related atmospheric circulation in austral autumn from the perspective of SOSST anomalies, with lead times up to 6–7 months.

scholarly journals Using data assimilation to investigate the causes of Southern Hemisphere high latitude cooling from 10 to 8 ka BP

2013 ◽  
Vol 9 (2) ◽  
pp. 887-901 ◽  
Author(s):  
P. Mathiot ◽  
H. Goosse ◽  
X. Crosta ◽  
B. Stenni ◽  
M. Braida ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Southern Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Sea Surface ◽  
Freshwater Flux ◽  
West Antarctic ◽  
The Impact

Abstract. From 10 to 8 ka BP (thousand years before present), paleoclimate records show an atmospheric and oceanic cooling in the high latitudes of the Southern Hemisphere. During this interval, temperatures estimated from proxy data decrease by 0.8 °C over Antarctica and 1.2 °C over the Southern Ocean. In order to study the causes of this cooling, simulations covering the early Holocene have been performed with the climate model of intermediate complexity LOVECLIM constrained to follow the signal recorded in climate proxies using a data assimilation method based on a particle filtering approach. The selected proxies represent oceanic and atmospheric surface temperature in the Southern Hemisphere derived from terrestrial, marine and glaciological records. Two mechanisms previously suggested to explain the 10–8 ka BP cooling pattern are investigated using the data assimilation approach in our model. The first hypothesis is a change in atmospheric circulation, and the second one is a cooling of the sea surface temperature in the Southern Ocean, driven in our experimental setup by the impact of an increased West Antarctic melting rate on ocean circulation. For the atmosphere hypothesis, the climate state obtained by data assimilation produces a modification of the meridional atmospheric circulation leading to a 0.5 °C Antarctic cooling from 10 to 8 ka BP compared to the simulation without data assimilation, without congruent cooling of the atmospheric and sea surface temperature in the Southern Ocean. For the ocean hypothesis, the increased West Antarctic freshwater flux constrainted by data assimilation (+100 mSv from 10 to 8 ka BP) leads to an oceanic cooling of 0.7 °C and a strengthening of Southern Hemisphere westerlies (+6%). Thus, according to our experiments, the observed cooling in Antarctic and the Southern Ocean proxy records can only be reconciled with the reconstructions by the combination of a modified atmospheric circulation and an enhanced freshwater flux.

scholarly journals Using data assimilation to investigate the causes of Southern Hemisphere high latitude cooling from 10 to 8 ka BP

2012 ◽  
Vol 8 (6) ◽  
pp. 5545-5581
Author(s):  
P. Mathiot ◽  
H. Goosse ◽  
X. Crosta ◽  
B. Stenni ◽  
M. Braida ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Southern Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Sea Surface ◽  
Freshwater Flux ◽  
Modeling Framework ◽  
Using Data

Abstract. Paleoclimate records show an atmospheric and oceanic cooling in the high latitudes of the Southern Hemisphere from 10 to 8 ka BP. In order to study the causes of this cooling, simulations covering the early Holocene period have been performed with the climate model of intermediate complexity LOVECLIM constrained to follow the signal recorded in climate proxies using a data assimilation method based on a particle filtering. The selected proxies represent oceanic and atmospheric surface temperature in the Southern Hemisphere derived from terrestrial, marine and glaciological records. Using our modeling framework, two mechanisms potentially explaining the 10–8 ka BP cooling pattern are investigated. The first hypothesis is a change in atmospheric circulation. The state obtained by data assimilation displays a modification of the meridional atmospheric circulation around Antarctica, producing a 0.6 °C drop in atmospheric temperatures over Antarctica from 10 to 8 ka BP without congruent cooling of the atmospheric and sea-surface temperature in the Southern Ocean. The second hypothesis is a cooling of the sea surface temperature in the Southern Ocean, simulated here as the response to a higher West Antarctic Ice Sheet melting rate. Using data assimilation, we constrain the fresh water flux to increase by 100 mSv from 10 to 8 ka BP. This perturbation leads to an oceanic cooling of 0.5 °C and a strengthening of Southern Hemisphere westerlies (+6%). However, the observed cooling in Antarctic and the Southern Ocean proxy records can only be reconciled with the combination of a modified atmospheric circulation and an enhanced freshwater flux.

Linking submesoscale fronts and air-sea heat fluxes in the Southern Ocean: Results from the first Saildrone circumnavigation of Antarctica

2021 ◽  
Author(s):  
Hanna S. Rosenthal ◽  
Louise C. Biddle ◽  
Sebastiaan Swart ◽  
Sarah T. Gille ◽  
Matthew R. Mazloff
Keyword(s):  
Heat Flux ◽  
Southern Ocean ◽  
Global Climate ◽  
Global Scale ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Fine Scale ◽  
Atmospheric Conditions ◽  
The Impact

<p>The role of the Southern Ocean in the global heat and carbon cycle is fundamental towards our climate, but observational data to quantify air-sea fluxes, such as surface  heat  fluxes, are  still  scarce. In  order  to  investigate  the  effects  of  fine- scale oceanic fronts (0.1 km–10 km) on air-sea fluxes in the Southern Ocean, high-resolution  hydrographic  and  meteorological  data  collected  by  three  un-crewed surface vehicles (Saildrones) during their first Circumnavigation of Antarctica in 2019 was assessed. Comparisons of key variables from the in situ Saildrones datasets with those from ERA5 and a stationary mooring show good  agreement.  Temperature-driven density fronts were detected in the Saildrone data and their impact on the turbulent heat flux was quantified during steady atmospheric conditions.  Over 2000 surface ocean temperature dominated density fronts were detected at length-scales (i.e.  front width) ranging from sub-kilometer to mesoscale (order of 0.1 km–100 km). <br>Temperature-driven density fronts with a length scale (as seen from the Saildrones perspective ) smaller than 1 km contributed 75% and 51% of the sensible and latent heat flux changes, respectively. The direct link between the fronts and the impact on the heat fluxes decreases sharply  when the front length increases. This suggests that smaller (submesoscale) fronts have a larger impact on heat flux variability than larger (balanced) fronts . The parametrization of  these  fine-scale ocean-atmospheric processes  in  global climate  models  could  lead to more accurate  representations  of  the  heat  flux  variability both at local and global scale.</p>

scholarly journals Hydrographic shifts south of Australia over the last deglaciation and possible interhemispheric linkages

2021 ◽  
pp. 1-12
Author(s):  
Matthias Moros ◽  
Patrick De Deckker ◽  
Kerstin Perner ◽  
Ulysses S. Ninnemann ◽  
Lukas Wacker ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Critical Role ◽  
Temperature Response ◽  
South Australia ◽  
Last Deglaciation ◽  
Tight Coupling ◽  
Oceanic Fronts ◽  
The Last Deglaciation ◽  
Circumpolar Current

Abstract Northern and southern hemispheric influences—particularly changes in Southern Hemisphere westerly winds (SSW) and Southern Ocean ventilation—triggered the stepwise atmospheric CO2 increase that accompanied the last deglaciation. One approach for gaining potential insights into past changes in SWW/CO2 upwelling is to reconstruct the positions of the northern oceanic fronts associated with the Antarctic Circumpolar Current. Using two deep-sea cores located ~600 km apart off the southern coast of Australia, we detail oceanic changes from ~23 to 6 ka using foraminifer faunal and biomarker alkenone records. Our results indicate a tight coupling between hydrographic and related frontal displacements offshore South Australia (and by analogy, possibly the entire Southern Ocean) and Northern Hemisphere (NH) climate that may help confirm previous hypotheses that the westerlies play a critical role in modulating CO2 uptake and release from the Southern Ocean on millennial and potentially even centennial timescales. The intensity and extent of the northward displacements of the Subtropical Front following well-known NH cold events seem to decrease with progressing NH ice sheet deglaciation and parallel a weakening NH temperature response and amplitude of Intertropical Convergence Zone shifts. In addition, an exceptional poleward shift of Southern Hemisphere fronts occurs during the NH Heinrich Stadial 1. This event was likely facilitated by the NH ice maximum and acted as a coup-de-grâce for glacial ocean stratification and its high CO2 capacitance. Thus, through its influence on the global atmosphere and on ocean mixing, “excessive” NH glaciation could have triggered its own demise by facilitating the destratification of the glacial ocean CO2 state.

scholarly journals Centennial-scale variability of the Southern Hemisphere westerly wind belt in the eastern Pacific over the past two millennia

2014 ◽  
Vol 10 (3) ◽  
pp. 1125-1144 ◽  
Author(s):  
B. G. Koffman ◽  
K. J. Kreutz ◽  
D. J. Breton ◽  
E. J. Kane ◽  
D. A. Winski ◽  
...  
Keyword(s):  
Particle Size ◽  
Wind Speed ◽  
Atmospheric Circulation ◽  
Southern Hemisphere ◽  
Zonal Wind ◽  
Ice Core ◽  
Eastern Pacific ◽  
The Past ◽  
West Antarctic ◽  
Lower Elevation

Abstract. We present the first high-resolution (sub-annual) dust particle data set from West Antarctica, developed from the West Antarctic Ice Sheet (WAIS) Divide deep ice core (79.468° S, 112.086° W), and use it to reconstruct changes in atmospheric circulation over the past 2400 years. We find a background dust flux of ~4 mg m−2 year−1 and a mode particle size of 5–8 μm diameter. Through comparing the WAIS Divide record with other Antarctic ice core particle records, we observe that coastal and lower-elevation sites have higher dust fluxes and coarser particle size distributions (PSDs) than sites on the East Antarctic plateau, suggesting input from local dust sources at these lower-elevation sites. In order to explore the use of the WAIS Divide dust PSD as a proxy for past atmospheric circulation, we make quantitative comparisons between both mid-latitude zonal wind speed and West Antarctic meridional wind speed and the dust size record, finding significant positive interannual relationships. We find that the dust PSD is related to mid-latitude zonal wind speed via cyclonic activity in the Amundsen Sea region. Using our PSD record, and through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, we infer that the SH westerlies occupied a more southerly position from circa 1050 to 1400 CE (Common Era), coinciding with the Medieval Climate Anomaly (MCA). Subsequently, at ca. 1430 CE, the wind belt shifted equatorward, where it remained until the mid-to-late twentieth century. We find covariability between reconstructions of El Niño–Southern Oscillation (ENSO) and the mid-latitude westerly winds in the eastern Pacific, suggesting that centennial-scale circulation changes in this region are strongly influenced by the tropical Pacific. Further, we observe increased coarse particle deposition over the past 50 years, consistent with observations that the SH westerlies have been shifting southward and intensifying in recent decades.

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