The Role of the Madden-Julian Oscillation in Transient Equatorial Adjustment

Abstract:

The response of the tropical atmosphere to an equatorial heating is studied using a Lagrangian atmospheric model (LAM) which simulates a realistic Madden-Julian Oscillation (MJO). Idealized Gaussian-shape warm sea surface temperature (SST) anomalies of different magnitudes (0.5K, 1K, 2K) and aspect ratios (5/1, 3/1, 1/1) are added to the central equatorial Indian ocean during the neutral phase of the MJO in March. The SST anomaly is maintained for 15 days. A Gill-type response to the heating develops in all experiments. Initially, a Rossby wave front (easterlies) is seen to propagate westward in the upper troposphere at a speed of about 9° per day, and a Kelvin wave front (westerlies) propagates eastward at a speed of around 25° per day for the aspect ratios of 3 and 5, and 15° per day for the ratio of 1. Further, as the main result of this study, we observe the generation and eastward propagation of an MJO event, seen in precipitation and velocity fields, which becomes the key element of atmospheric adjustment, especially pronounced in the 1K and 2K experiments. The quadrupole circulation typical of the MJO gradually builds up in the upper troposphere in the first week, then starts to propagate eastward at a speed of around 5m/s. The amplitude of the excited MJO is largest for the ratio of 5 and smallest for the ratio of 1. For the 0.5K experiment, no clear signature of Rossby waves, Kelvin waves or MJO is observed except for the aspect ratio of 5, when weak signals are observed. The first induced MJO event is followed by other events, which maintains the MJO for several more cycles. Thus, this study highlights the important potential role of the MJO in the equatorial atmospheric adjustment.