Analysis of Primary Madden–Julian Oscillation Events in the Indian Ocean Using Satellite Observations

The Madden-Julian Oscillation (MJO) is a dominant mode of tropical variability on intraseasonal timescales, notably across the equatorial Indian and Pacific oceans, with global implications. As a highly coupled phenomenon, the MJO is not exclusively an atmospheric event but rather the product of coupled air-sea interaction throughout its life cycle. The coupled nature of the MJO can be observed in synchronous atmosphere and oceanic signals alike, especially sea surface salinity (SSS). SSS changes as a direct result of variability in MJO precipitation between convectively active to suppressed phases, where negative SSS anomalies are associated with the MJO’s active phase and positive anomalies with the suppressed phase. These anomalies propagate in a similar manner to that of the MJO itself across the equatorial Indian and Pacific oceans. Using satellite-derived observations, we find that all three of the current satellite salinity missions (ESA’s Soil Moisture Ocean Salinity (SMOS); NASA’s Aquarius/SAC-D, and NASA’s Soil Moisture Active Passive (SMAP)) are capable of capturing the MJO convective signal on an unprecedented observational scale. We also find that the near-equatorial MJO signal improves when processed with improved parameterized algorithms, such as the combined active passive (CAP), especially near land. While the broad-scale MJO convective structure is observable in satellite-derived SSS, so is that of primary MJO events. Primary MJO events are those which are the more dominant events not preceded by coherent MJO activity. These spontaneous events are relatively isolated and, as such, are difficult to discern specific initiation mechanisms. Being the coupled air-sea phenomenon that it is, evidence exists for both atmospheric and ocean triggers of primary MJO events over the equatorial Indian Ocean. We examine the intraseasonal signals in both the atmosphere and ocean in an attempt to bridge the connection between observed primary MJO triggers.Understanding how the atmospheric component of the MJO interacts with the ocean is critically important for modelling efforts, as SSS is, along with sea surface temperature (SST), a component of sea surface height (SSH), which has been shown to be related to the initiation of primary MJO events. As such, SSS may contribute to mixed-layer variability and thus, ocean heat content (OHC) related to primary MJO initiation, which requires further research.

Although the MJO has been an active subject of research for decades, specific mechanisms for its initiation are poorly understood. In this research, we further examine the coupled nature of MJO initiation. The western Indian Ocean is observed to exhibit an increase in both SSH, SST, and OHC prior to the initiation of the convective signal over the Indian Ocean. The warming of SST acts to warm and moisten the near-surface air above, thus destabilizing the lower troposphere. An increase in moisture flux convergence (MFC) acts to moisten the lower troposphere prior to MJO convection, which leads to an increase in moist static energy (MSE) conducive to MJO convective initiation. Synchronously, the mid-troposphere exhibits a negative temperature anomaly, which acts to further destabilize the atmosphere such that the combined influences of SST, MFC, and MSE precede the MJO signal and prime the atmosphere for MJO convection.

The conditions preceding MJO initiation and its subsequent development are also influenced by longer term climate variability,such as the Indian Ocean Dipole (IOD) and the El Niño-Southern Oscillation (ENSO). MJO development and propagation is favored under negative IOD conditions across the eastern Indian Ocean and suppressed during the positive IOD phase. Similarly, MJO growth and development is favored over the central and eastern Pacific under El Niño conditions and suppressed under La Niña. Understanding the interaction of the MJO with IOD and ENSO is critically important for MJO prediction and improved model simulations. As initiation of the MJO is not perfectly understood and may be better explained by an analysis of concurrent ENSO and IOD effects on intraseasonal oceanic and atmospheric parameters.