What is the Madden-Julian Oscillation (MJO)?
The MJO is the dominant mode of intraseasonal (∼30-90 days) variability in tropical circulation, cloudiness, and precipitation. The phenomenon is named after the two scientists at NCAR, Roland A. Madden and Paul R. Julian, who had first discovered its existence in the 1970s.
As shown in the schematic on the left, the MJO produces an enhanced area of cloudiness and precipitation over several thousands of kilometers in the tropics, which propagates slowly eastward at about 5 m/s. It is also often paired with an area of suppressed cloudiness and precipitation to its east or west. The enhanced/suppressed areas of cloudiness in the MJO are also associated with an atmospheric circulation that spans the entire tropics.
The influence of the MJO extends beyond the tropics, into higher latitude weather and climate. Because of its potential influence on long-range forecasts, various forecasting centers monitor the MJO. The information on the recent evolution of the MJO and its potential impact on temperature and precipitation over the United States can be found at the NOAA Climate Prediction Center website.
My research focuses on understanding the large-scale dynamics of the MJO, characteristics of cloud and rainfall activity embedded within the MJO, and the interactions between the MJO and the extratropical circulation.
Sources:
, 1994: Observations of the 40–50-Day Tropical Oscillation—A Review. Mon. Wea. Rev., 122, 814–837.
2005: Madden-Julian Oscillation, Rev. Geophys., 43, RG2003, doi:10.1029/2004RG000158.
Some of My Ongoing/Previous Work:
The interactions between the MJO and diurnal cycle of rainfall
The diurnal cycle of rainfall and cloudiness in the tropics had been known to be influenced by the MJO, but previous studies were often limited in the number of MJO events or geographical location. Sakaeda et al. (2017) provided a general overview of how the diurnal cycle of rainfall and cloudiness varies with the MJO using satellite-based data over a long time period in the Indian and Pacific basins. This study found that the enhanced convective envelopes of the MJO tends to increase the diurnal variability of rainfall over the ocean and also delays the peak timing of diurnal rainfall due to enhanced growth of stratiform clouds. The relationship between the MJO and the diurnal cycle over the Maritime Continent islands is complicated. Ongoing work is continuing to understand how the MJO and the diurnal cycle interact over the Maritime Continent and the importance of such interactions on MJO propagation.
The upper-tropospheric circumnavigating circulation associated with MJO initiation
The MJO often initiates its convection over the Indian basin. The initiating convection is often accompanied by an eastward propagating circulation from the Western Hemisphere. This circumnavigating (eastward-propagating) circulation had been traditionally thought of as an atmospheric Kelvin wave that is triggered by MJO convection. However, my study emphasized that this circumnavigating circulation cannot be thought as a pure Kelvin wave. By examining the structure of the intraseasonal circumnavigating circulation, we found that the circulation over the Western Hemisphere deviates from the theoretical Kelvin wave structure, while the circulation over the Eastern Hemisphere does resemble a Kelvin wave. Rather than a pure Kelvin wave, my research suggests that the equatorward propagation of midlatitude waves modulates the intraseasonal circulation, especially over the eastern Pacific and Atlantic basin, influencing the subsequent development of MJO convection over the Indian basin.