How does the Indian Ocean subtropical dipole trigger the tropical Indian Ocean dipole via the Mascarene high?

Abstract

The variation in the Indian Ocean is investigated using Hadley center sea surface temperature (SST) data during the period 1958-2010. All the first empirical orthogonal function (EOF) modes of the SST anomalies (SSTA) in different domains represent the basin-wide warming and are closely related to the Pacific El Nino-Southern Oscillation (ENSO) phenomenon. Further examination suggests that the impact of ENSO on the tropical Indian Ocean is stronger than that on the southern Indian Ocean. The second EOF modes in different domains show different features. It shows a clear east-west SSTA dipole pattern in the tropical Indian Ocean (Indian Ocean dipole, IOD), and a southwest-northeast SSTA dipole in the southern Indian Ocean (Indian Ocean subtropical dipole, IOSD). It is further revealed that the IOSD is also the main structure of the second EOF mode on the whole basin-scale, in which the IOD pattern does not appear. A correlation analysis indicates that an IOSD event observed during the austral summer is highly correlated to the IOD event pealdng about 9 months later. One of the possible physical mechanisms underlying this highly significant statistical relationship is proposed. The IOSD and the IOD can occur in sequence with the help of the Mascarene high. The SSTA in the southwestern Indian Ocean persists for several seasons after the mature phase of the IOSD event, likely due to the positive wind-evaporation-SST feedback mechanism. The Mascarene high will be weakened or intensified by this SSTA, which can affect the atmosphere in the tropical region by teleconnection. The pressure gradient between the Mascarene high and the monsoon trough in the tropical Indian Ocean increases (decreases). Hence, an anticyclone (cyclone) circulation appears over the Arabian Sea-India continent. The easterly or westerly anomalies appear in the equatorial Indian Ocean, inducing the onset stage of the IOD. This study shows that the SSTA associated with the IOSD can lead to the onset of IOD with the aid of atmosphere circulation and also explains why some IOD events in the tropical tend to be followed by IOSD in the southern Indian Ocean.