Linear And Nonlinear Winter Atmospheric Responses To Extreme Phases Of Low Frequency Pacific Sea Surface Temperature Variability

Based on the Surface Air Temperature(SAT)datasets by NOAA,500-hPa geopotential height(Z500)datasets by NCEP/NCAR Reanalysis and 20th Century Reanalysis,precipitation datasets by GPCC,the observed linear and nonlinear winter atmospheric responses to extreme phases of low frequency Pacific sea surface temperature(SST)variability(LFPSSTV,LF)are examined using the methods of composite analysis during 1949-2014 and 1904-2014.Based on the LF anomaly pattern simulations by GFDL AM2.1 and CAM4,the simulated linear and nonlinear winter atmospheric responses to LF forcing are examined and compared with the observed.The LF pattern is divided into tropical Pacific(LFTP),tropical India(LFI),North Pacific(LFNP),and South Pacific(LFSP)of four parts.The simulated linear and nonlinear winter atmospheric responses to those four parts SST anomaly pattern are examined using CAM4.The mechanisms of the winter nonlinear atmospheric response to LF are explored by CAM4 double forcing and single forcing simulations.The main conclusions are as follows:First,the observed composites for the extreme phases of LF revealed that the winter Z500 linear responses including a significant positive Pacific-North America(PNA)pattern over PNA sector,a significant positive NAO-like pattern over North Atlantic-Europe sector,and a significant dipole-like pattern over Eurasia.The dipole like pattern with positive anomalies centered near Lake BaiKal and negative anomalies over Eurasia high latitudes and adjacent to Arctic Ocean is the cause of the significant linear warming over central Eurasia.The winter nonlinear Z500 responses including the PNA-like pattern over PNA sector with positive anomaly centered the eastern North America and negative anomaly centered costal of western America,and a significant NAO-like pattern over North Atlantic-Europe sector.Second,the GFDL AM2.1 and CAM4 generally reproduced the main feature of the observed linear and nonlinear winter responses to LF over the land areas in North America and South Hemisphere(SH),including a significant positive linear and nonlinear PNA pattern over PNA sector,linear warming over northern North America and linear wetting over southern North America and significant linear warming over the land areas in SH.However,the significant positive linear and nonlinear NAO-like pattern over North Atlantic,dipole-like pattern over Eurasia and a significant linear warming over central Eurasia are not reproduced in neither GFDL AM2.1 nor CAM4.The LFTP simulations reproduced the significant linear and nonlinear PNA-like pattern over PNA sector and forced a significant positive NAO-like nonlinear response over North Atlantic-Europe sector.Same as the LF simulations,the LFTP simulations are not able to reproduce the significant positive linear NAO-like pattern.The results from the analysis of CAM4 200 years LF simulations suggest that at least 100-200 years simulations may be needed in order to force a significant stable nonlinear PNA-like pattern.The inter-model difference may be related to the length of the GFDL AM2.150 years simulations.Third,CAM4 LF simulations suggests that the LFTP may plays a dominant role in the LF.The LFI mainly has influence on Eurasia,Africa and Australia but has little influence on other areas.Although the North Pacific SST anomaly is the prominent signal in LF,but the LFNP and LFSP have negligible impacts on the extratropical atmosphere.Fourth,CAM4 double forcing and single forcing simulations show that the western tropical Pacific rainfall response is quasi-linear with the magnitude of the LF+ forcing,but not linearly with the LF-forcing.The CAM4 LF simulations suggests that the nonlinear PNA-like pattern over PNA sector results from both nonlinearity of western tropical Pacific rainfall response and extratropical synoptic transients eddy feedback to the LF forcing.The western tropical Pacific rainfall response are stronger associated with the LF+ forcing.The extratropical synoptic transients eddy feedback over PNA sector are stronger associated with the LF+ forcing.The CAM4 LFTP simulations suggests that the negative linear NAO-like pattern and the positive nonlinear NAO-like pattern over North Atlantic-Europe sector results from both nonlinearity of western tropical Pacific rainfall response and extratropical synoptic transients eddy feedback to the LFTP forcing.The western tropical Pacific rainfall response are stronger associated with the LFTP+ forcing.The extratropical synoptic transients eddy feedback over North Atlantic-Europe sector are stronger associated with the LFTP-forcing.