Statistical Characteristics Of The South Atlantic Dipole Mode And Its Impact On Summer Precipitation In West Afric

The South Atlantic Ocean Dipole(SAOD)is the northeast-southwest dipole mode of the South Atlantic Sea Surface Temperature anomaly(SSTA)in boreal summer(June-August).Based on revealing the interannual variation,evolution characteristics,and mechanism of the SAOD,this paper analyzes the relationship between SAOD and the Pacific SSTA.The impact of the SAOD on precipitation in West Africa and the modulation by ENSO are discussed.The main conclusions are as follows:(1)Through the Empirical Orthogonal Function(EOF)and composite analysis,SAOD presents a distribution of the SSTA in the northeast-southwest inverse phase,and the intensity of SSTA in the northeast pole is stronger than the southwest pole.Based on wavelet analysis,it has an interannual variation of about 12 years during 1980-2007.The SAOD has an obvious seasonal phase-locking,it occurs in the boreal spring(March-May),peaks in summer(June-August),and decays in autumn(September-November).(2)The mechanism of SSTA in both northeast and southwest poles of the SAOD is different.In the northeast pole,the SSTA is related to the wind speed,latent heat flux loss and shortwave radiation flux.In the southwest pole,the wind and latent heat flux loss contribute to the SSTA.In addition,the variation of shortwave radiation flux,sensible heat flux and mixed layer depth all inhibit the development of the SSTA in southwest pole,resulting in weaker SSTA in the southwest pole than the northeast.(3)According to the results of correlation,partial correlation analysis,and composite analysis,SAOD is related to both El Ni?o-Southern Oscillation(ENSO)and the South Pacific Ocean Dipole(SPOD).There is a two-way interaction between SAOD and ENSO,with positive(negative)SAOD events corresponding to La Ni?a(El Ni?o)in the following winter.In addition,positive(negative)SAOD in boreal summer promotes the negative(positive)SPOD in winter.The regression analysis results show that this impact is transmitted through sea surface winds and sea level pressure anomalies.(4)SAOD affects summer precipitation in West Africa and is modulated by ENSO.The partial correlation results show that SAOD has a significant impact on rainfall along the Guinea coast in West Africa,while it has a weak opposite impact on the Sahel.The regression results show that the Pure South Atlantic Ocean Dipole(P＿SAOD),which is independent of ENSO,influences West African precipitation by affecting the atmospheric circulation.During the P＿SAOD,the air over the Guinea coast converges,rises,and forms strong convection.In addition,there is a sufficient supply of moisture anomalies from the equatorial Atlantic and inland regions,resulting in abundant precipitation.By calculating the meridional mass flow function,it is found that ENSO modulates summer precipitation in West Africa through a remote path during SAOD by influencing the intensity of the upper and middle circulation,including the African Easterly Jet(AEJ)and the Tropical Easterly Jet(TEJ).