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This study conducted a survey of the correlation coefficient between sea surface temperature (SST) and reference evapotranspiration (ETo) at stations in the south of Vietnam in order to find a new factor for ETo warning. The data included in the survey are meteorological variables related to evaporation of 40 stations in the southern provinces and the field of SST from the NCEP/NCAR Reanalysis for the period between 1978 and 2018. The methods used in the study are spatial correlation analysis and principal component analysis. This study identified three regions of the tropical Pacific Ocean whose major components are best related to ETo. The third major component of SST in the area surrounding the Niño 3.4 area has a fairly good relationship with ETo in southern Vietnam. Using this component to build an new index to monitor El Niño and La Niña activity, the results show that the new index can be used for ETo warning in southern Vietnam. Compared with ocean Niño index, the new index has more advantages in ETo warning for the period from October to February. It is possible to combine this index and Ocean Niño index to warn of evapotranspiration for the South of Vietnam.