Moderating Effects Of MJO On Blocking Frequency In The Pacific Ocean In Winter

Based on ERA-interim reanalysis daily data during 1979–2019,the spatial-temporal evolution of the Pacific blocking frequency(PBF)and the impact of Madden-Julian Oscillation(MJO)on PBF in individual winter-month are investigated,utilizing a two-dimensional blocking index which proposed by previous study.The impact of Madden-Julian Oscillation(MJO)on PBF during two types of El Ni?o(Eastern and Central El Ni?o)mainly is examined in this study.Finally the performance of 11 climate models from phase 6 of the Coupled Model Intercomparison Project(CMIP6),are assessed in terms of spatial distribution of PBF in the winter time.Some conclusions were concluded as below:(1)Pacific blocking occurs at all latitudes north of 50°N in the whole winter.Pacific blocking is the most active in January.The maximum of PBF in the winter-mean and individual winter-month are both located in the region(140°E–160°W,50°–70°N).Thus this area is defined as the key-area.It is found that Pacific blocking in January is more active during the first(1980–1988)and the third(2009–2019)periods than that during the second period(1989–2008)over the key-area.The 500h Pa-geopotential height(Z₅₀₀)anomaly between the first and the second period(between the second and the third period)connect to a positive 2-meter air temperature(T2m)anomaly over the northeastern Asia and mid-western Pacific(the high-latitude area),and a negative one over the high-latitude area(the mid-latitude area of Asia).(2)Comparing datasets between from ERA-interim reanalysis and from 11 CMIP6models,it is found that there are both better performances of 11 CMIP6 models in simulating horizontal distribution of PBF during the winter-mean.Among of them,Can ESM5(Can),EC-Earth3(EC),GFDL-CM4(GFD),IPSL-CM6A-LR(IPS),and MPI-ESM-1-2-HAM(MPI)are better than others in simulating PBF from the assessment result.These models are chosen the key models.The assessed result of simulating PBF in individual months shows that performance of simulating PBF during January in the mostly models are better than others winter month from two objective measures.Among of key models,MPI ranks top three of two measures(NRMSE and TS)in 11 CMIP6 models.GFD,IPS,and EC ranks top three of one measures among of them(the NRMSE and TS),but the overall performance of GDF are better than that of IPS and EC.Thus MPI is identified as the best model on the simulation PBF during winter in 11 CMIP6 models,following by GFD model.(3)Phase 3 and 7 with the higher frequency and the stronger amplitude are selected in this study.The differences between individual months of PBF in MJO phases are shown and the reasons are examined in this study.It is found that the PBF increases in the winter months during MJO phase 3.During the 7 phase of the MJO,Pacific blocking is the most active in January.This result is caused by the different location of monthly MJO convection,leading to the different influence on PBF.(4)It is found that the locations of MJO teleconnections are similar in MJO phase 3during the Eastern and Central El Ni?o years(EP3 and CP3),which increase the Pacific blocking frequency.The blocking frequency anomalies over the mid-high-latitudinal Pacific are significantly positive in MJO Phase 7 during the Eastern El Ni?o years(EP7),but are not largely significant in MJO Phase 7 during the Central El Ni?o years(CP7).Because the MJO-related anomalous Rossby wave source locates in the northwestern of the subtropical jet core area during the EP7,the MJO teleconnection locates in the north of 50°N.This teleconnection corresponds to the geopotential height anomalies,which increase the Pacific blocking frequency,in the Pacific sector.However,the MJO-related anomalous Rossby wave source locates in the subtropical jet core area during the CP7.The corresponding teleconnection propagates in the subtropical jet stream,resulting in weak influence to geopotential height in the Pacific region.So,there are not largely significant blocking frequency anomalies over the Pacific region in the CP7.