The Second Mode Of The Southern Hemisphere Extratropical Circulation And Its Influence On Summer Precipitation In China

Based on the Empirical Orthogonal Functions(EOF)decomposition of the Sea level pressure(SLP)anomalies over the southern extratropic,the second pattern is defined as the southern second mode(SMD2),and its corresponding principal component is named the southern second mode index(SMD2I).Using the diagnostic analysis of station observations and reanalysis data,combined with numerical simulations and other methods,this dissertation analyzes the spatiotemporal variability of the SMD2 from1979-2017.The connection between the SMD2 and the Antarctic “sea-ice-atmosphere"coupled system is established,and the connection between SMD2 and Antarctic sea ice,and sea surface temperature(SST)of the Antarctic Ocean is also analyzed.This dissertation revealed that the SMD2 in March is negatively associated with the June precipitation in southern China.Focus on the physical mechanisms connecting SMD2 to summer climate in China through sea ice and SST in key regions,and validate the diagnostic analysis findings using numerical simulations.Explanate the modulation mechanisms of Antarctic sea ice and extratropical SST in the cross-equatorial and crossseasonal impacts.This research contributes to understanding interhemispheric interactions and the “sea-ice-atmosphere” coupled system of the Southern Hemisphere affecting the summer climate mechanism in China and deepens the understanding of the circulation system of the extratropical Southern Hemisphere.This work may provide a new source of predictability for the summer precipitation in southern China and a new perspective on understanding seasonal or interannual changes in the Antarctic climate.The main findings of this paper are summarized as follows:(1)The spatial and temporal structure of SMD2.The most significant feature of SMD2 in spatial structure is the seesaw pattern of SLP,which indicates the opposite phase of the Amundsen Low and its surroundings.The dominant signal of SMD2 is in the western hemisphere.There are two inversion relationships bounded by 60°S and 45°S,one is the pressure inverse between high and mid-latitudes,and the other is the pressure inversion variation between mid-latitudes and the subtropics.The SMD2 has different spatial manifestations in different months,it depicts degenerated wave-3 patterns in the monthly mean sea level pressure field at mid-latitude,which corresponds to the PacificSouth American telecorrelation(PSA)pattern.The SMD2 is not deep,and the anomalous signal is mainly located in the troposphere.In terms of temporal variability,SMD2 is dominated by interannual variability with two significant periods of 2 and 4 years,among which the 4-year is the strongest.(2)The interaction links between the SMD2 and Antarctic sea ice.The SMD2 is closely related to the contemporaneous Antarctic sea ice dipole,with the SMD2 having a significant negative correlation with sea ice concentration in the AmundsenBellingshausen Sea,and a significant positive correlation with sea ice concentration in the Ross Sea.The SMD2 affects sea ice generation and melt by affecting zonal wind anomalies,altering Ekman drift,which in turn causes meridional transport anomalies.The SMD2 can cause meridional wind anomalies that drive sea ice motion through wind stress,and can also affect sea ice production by changing SST and air temperature.The anomalous winds in the east and west ice areas are opposite,leading to opposite driving directions and the resulting in the dipole of sea ice.(3)The interaction between the SMD2 and SST in the Southern Ocean.The SST anomaly mode is similar to and corresponds to the atmospheric PSA pattern,with circumpolar positive and negative alternation on the correlation field of SMD2 and the Southern Ocean SST in simultaneous.In the SST anomaly field,there is a pair of SST dipoles significantly correlated with SMD2.The SST anomaly in the Ross Sea is warm(cold),while the SST anomaly in the Bellingshausen Sea and Drake Passage is cold(warm).Similar to the process of sea ice dipole formation,the SMD2 can cause wind field anomalies,and through wind stress dragging effect drives the changes of shallow surface temperature,resulting in the inverse phase on the east and west sides.The SMD2 can also cause the changes of heat fluxes.The SMD2 is significant correlated with latent heat,sensible heat and heat fluxes at the ocean surfaces of the Ross Sea,Bellingshausen Sea and Drake Passage.The sensible heat flux on the west(east)side is a negative(positive)anomaly,with the latent heat flux is also a negative(positive)anomaly.Negative(positive)sensible heat and latent heat anomalies indicate weakened(enhanced)heating of the ocean to the atmosphere.A negative(positive)heat flux anomaly,where the ocean transports less(more)heat to the atmosphere,resulting in higher(lower)SST here.The uneven distribution of heat transport from the east and west oceans,consistent with the results of wind fields,which leads to a zonal dipole distribution of SST anomaly.(4)The mechanism of “coupled ice-atmospheric bridge” in which the March SMD2 is negatively correlated with June precipitation in southern China.The SMD2 can influence the lower wind field and modulate the sea ice production and extinction in the Amundsen-Bellingshausen Sea(AS)ice area through interactions between ice and air.Due to the positive feedback effect of “icecover-albedo”,atmospheric anomalous signals are stored,and can presist into late summer.Sea ice act as a “bridge”,to store the atmospheric anomalous signal through ice-air interactions.The extent of sea ice expanded,affecting the meridional SST gradient in June.Changes in the meridional SST gradient lead to the changes in vertical circulation through diabatic heating.The anomalous vertical motion changes the strength of the subtropical high,affecting the precipitation in southern China.(5)The mechanism of “coupled oceanic-atmospheric bridge” in which the March SMD2 is negatively correlated with June precipitation in southern China.The low-level wind anomalies associated with SMD2 can affect transport of the shallow water and heat flux anomalies,then trigger the SST anomaly.The SMD2 in March is negatively correlated with SST on the east side of Australia.The SST anomaly acts as the “oceanic bridge” to preserve the March SMD2 signal and persists into late summer.In summer,the key area of SST is associated with Maritime Continent and the Philippine Peninsula convective activity anomalies,which can excite and maintain the Pacific-Japan(PJ)pattern resulting in precipitation anomalies in South China.Through sea-air interactions,the SMD2 can affect the precipitation in southern China across seasons.