A record-breaking, long-persisting extreme cold anomaly (ECA) over Southeast Asia, accompanied by an intraseasonal convection over the Maritime Continent, is identi?ed during the La Nin ˜a mature phase in February 2008. The cause of the ECA, in particular the role of the intraseasonal oscillation (ISO) and El Nin ˜o–Southern Oscillation (ENSO) on the ECA, is investigated by diagnosing observations and conducting numerical experiments. The ECA is associated with an enhanced prolonged Siberian high (SH) and a persistent northerly anomaly over Southeast Asia. In contrast to conventional cold surges, which are characterized by a synoptic time scale (less than 10 days), the northerly anomaly associated with the ECA persisted for a month or so. The onset of the northerlyanomalyis concurrentwitha phasechange ofan ISO overSumatra.Unlikethe normalISO that continues its eastward journey, the convection associated with this ISO stationed there during all of February 2008. Numerical experiments with an anomaly atmospheric GCM suggest that the ISO heating over the Maritime Continent is responsible for initiating and maintaining the northerly anomaly. The westward progression of the La Nin ˜a is crucial for blocking the ISO. The circulation and SST anomalies associatedwith the La Nin ˜a movedwestwardat a speedof about 158 longitudeper month. Byearly February, the suppressed convective anomaly had moved to the far western Paci?c. The westward shift of the cold episode prevented the ISO from moving farther eastward. In addition to its blocking effect, the La Nin ˜a also enhanced the heating over the Maritime Continent through the anomalous Walker circulation. Therefore, it is the combined effect of the ISO and ENSO that maintained a prolonged positive heating anomaly, which resulted in a persistent northerly anomaly and thus the ECA.