Thermocline oscillation and warming event in the tropical Indian Ocean

In this paper, an indicator that shows the maximum subsurface temperature anomalies ( MSTA) is first defined to represent the thermocline variations through a case analysis of 1997/98. By using the past 46-year (1955-2000) subsurface temperature data, thermocline oscillation and wave-signal propagation in the tropical Indian Ocean are revealed. The thermocline oscillation is robust in the tropical Indian Ocean in spite of the coincidence with the strong El Nino/Southern Oscillation (ENSO) cycle as well as the anomalous Walker circulation in the tropical Pacific and the anomalous monsoon circulation in the tropical Indian Ocean. Due to the nature of the propagation of subsurface wave signals, the oscillation may be a potential factor for the predictability of climate and for the dynamical study of the air-sea interaction in the Indian Ocean region. The warming of sea surface temperature in the tropical Indian Ocean is a basin-wide phenomenon and results from the reflection of the tropical Rossby waves. By defining an MSTA dipole mode index (M-DMI), some features of the interannual climate anomalies that correspond to the different phases of M-DMI obviously exist in the tropical Indian Ocean and tropical central-western Pacific Ocean. In the positive phase of the M-DMI, strong 850-hPa easterly winds appear in the equatorial central-eastern Indian Ocean while strong 850-hPa westerly winds occur over the equatorial central western Pacific. Also, for the positive M-DMI, above-normal precipitation is located in the tropical central-western Indian Ocean while below-normal precipitation is situated over the tropical eastern Indian Ocean and the tropical western Pacific Ocean. The reverse is noted for the negative M-DMI phase. A scenario for the warming event and the MSTA dipole event in the tropical Indian Ocean is also proposed.