Dynamical mechanisms for asymmetric SSTA patterns associated with some Indian Ocean Dipoles

Abstract

Indian Ocean Dipole (IOD) can be divided into two types according to their SST anomaly (SSTA) patterns: the symmetric IODs and the asymmetric IODs. Dynamical mechanisms for the two types of IODs are investigated using an ocean general circulation model, and the numerical experiments indicate that the setup time of the equatorial easterly wind anomalies (EEWA) associated with IODs is crucially important for the symmetrical characteristics of the SSTA patterns. Early setup of the EEWA in spring can induce intense cooling in the east pole but weak warming in the west pole, making the two poles asymmetric. The intense cooling in the east pole is linked to the seasonal variations of thermocline depth, horizontal SST gradient, and unusually long duration of the EEWA, and the weak warming in the west pole is due to unusually cold zonal advection resulted from zonal SST gradient change. The numerical experiments demonstrate that the strength of the EEWA also plays an important role in the symmetrical characteristics of IODs. The SSTA in the two poles both intensify as the EEWA strength increases only when the EEWA speed is below a critical value. Above the critical value, the warming in the west pole starts to decrease while the cooling in the east pole still keeps increasing, leading to an asymmetric SSTA pattern. The mechanism behind this phenomenon is similar to the one in the situation when EEWA sets up earlier.