Variability and trends of circulation zonal asymmetries in the Southern Hemisphere

ELIO CAMPITELLI; CAROLINA VERA; LEANDRO DIAZ

Madrid

Workshop; DynVarMIP workhop on Atmospheric circulation in a changing climate; 2019

Trends in extratropical zonally-symmetric mean circulation in the Southern Hemisphere (SH)have received considerable attention by the scientific community and have been associatedwith a poleward migration of the mean westerlies. On the other hand, changes in thecorresponding circulation zonal asymmetric component have not received the sameattention and they are not as well understood as its Northern Hemisphere counterpart. Whilein the SH circulation zonal asymmetries are less pronounced than in the NorthernHemisphere, they are still important large-scale drivers of regional variability, includingblocking activity and regional patterns of temperature, precipitation, wind, and sea-iceanomalies. Therefore, the purpose of the study is to describe the variability and change ofthe circulation zonal asymmetries in the SH by using reanalysis data (ERA interim, ERA20C,NCEP/NCAR) and climate model simulations (CMIP5, CMIP6).Circulation zonal asymmetries are defined by subtracting the zonal mean from seasonalgeopotential heights at 200 and 50 hPa. The standard deviation of the zonal asymmetriesderived from ERA Interim over the period 1978-2010 at 50 hPa is larger near 60°S, with twomaximum centers located in the vicinity of South America and the southern Indian Oceanrespectively. They are more pronounced in austral winter and spring. At 200 hPa, a singlemaximum is only evident south of South America. Applying complex EOF analysis (CEOF) tothe circulation zonal asymmetries at each level, two leading modes (CEOF1 and CEOF2)are identified that explain approximately 90% and 5% of the variance respectively. CEOF1corresponds to a wave 1-like spatial pattern at both levels, indicating an equivalentbarotropic structure. CEOF2 at 50hPa has a wave-2 pattern with maximum amplitude in thesouth Pacific Ocean and evidences of meridional Rossby wave propagation from tropicalregions. At 200hPa, CEOF2 is similar but associated with a stronger wave meridionalpropagation. The year-to-year variability of both CEOFs is large without evidencing anysignificant trend.Similar results were found over the same period using NCEP Reanalysis and ERA-20C.We extended the analysis to the 1900-2010 period using ERA20C data. Long-term trends ofthe zonal asymmetries are largest at 50 hPa and in austral spring with a wave-1 structure.The CEOFs derived from the long period is consistent with those from the shorter one.Spectral analysis of the CEOF?s absolute magnitude timeseries show interannual andinterdecadal periodicities. CEOF2 exhibits two maximums around 3-5 and 20 years at bothlevels, which are more evident in autumn and winter. CEOF1 has similar periodicities at200hPa and in summer at 50hPa without a clear signal in the other seasons. In additionCEOF1?s magnitude has significant trends in spring at both levels, while trends are notsignificant for CEOF2.A similar analysis but using CMIP5/CMIP6 model simulations will be presented at theconference.