The Madden-Julian Oscillation (MJO) modulates tropical cyclones and easterly waves in the Atlantic, providing the potential for prediction of enhanced Atlantic hurricane activity a couple of weeks in advance. This project will address several major outstanding questions: (i) the teleconnection pathway by which the MJO affects the Atlantic during boreal summer, (ii) how west African and Atlantic precipitation is actually initiated by the MJO once the teleconnection is established, (iii) the mechanism by which the MJO modulates African easterly waves, and (iv) how these easterly waves feed back to influence the MJO.
A hierarchy of models will be used to understand the teleconnection pathways by which the MJO affects the Atlantic and west Africa, including the non-divergent barotropic vorticity equation, a filtered shallow water model on a sphere, and an atmospheric general circulation model. Because it is hypothesized that the MJO first initiates convection anomalies over the Darfur region of the Sudan, several observational products (e.g. Cloud Archive User Service (CLAUS) brightness temperature, Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Climatology Project (GPCP) precipitation) will be used to document the time evolution of precipitation in west Africa as a function of MJO phase. A primitive equation model will be used to test the hypothesis that strong heating variations over the Darfur region forced by the MJO modulate the amplitude of easterly wave activity in west Africa and the Atlantic, and test the hypothesis that MJO-related structural changes in the African easterly jet significantly affect easterly wave growth. Further, the impact of MJO-related easterly wave variability on the column-integrated intraseasonal moisture budget in west Africa and the Atlantic will be examined in satellite and reanalysis datasets.
The broader impacts of this work are educational and in the potential to improve our prospects for medium range prediction of periods of enhanced Atlantic tropical cyclone activity.
Atlantic hurricanes and tropical storms cause significant damage to coastal and inland communities in the United States, Mexico, and Central America. Our past work has shown that periods of active and suppressed hurricane activity in the Atlantic are organized by the Madden-Julian oscillation (MJO) of the tropical atmosphere, a slowly-evolving phenomenon that can be predicted up to 4 weeks in advance. Most individual Atlantic hurricanes originate from atmospheric disturbances that move westward from the west African monsoon region. The following major outcomes have resulted from this research: · We have developed a statistical prediction model for Atlantic and east Pacific hurricanes that shows skill in forecasting hurricane activity up to 3 weeks in advance. This model may provide advanced information on the probability of tropical cyclone formation that informs decisions relevant to industry, public safety, and recreation. This model uses information on the MJO, El Niño, and Atlantic ocean temperatures to provide skillful hurricane predictions several weeks into the future. Figure 1 shows the one-week forecasts of east Pacific tropical cyclone formation probability from our model for the year 2002 (solid black line) compared to the actual tropical storm formation events (gray shading). 2002 is an example of a year where the prediction model performed well, providing successful prediction of several periods of enhanced tropical cyclone formation. The model is not perfect, however, since the details of some events (e.g. June) are not captured well. This indicates that other factors than the MJO and El Niño affect hurricane formation, and we are currently expanding our forecast model to include these other factors and hence build an improved forecast model. · Most Atlantic hurricanes form from African "easterly waves", atmospheric ripples that form over tropical Africa and move westward over the Atlantic. This project has improved our understanding of how the MJO affects the strength and number of African easterly waves from which Atlantic hurricanes form. Our results indicate that the MJO first sparks thunderstorm activity over the mountains of Sudan, which then moves toward the west, forming strong tropical atmospheric waves that may develop into hurricanes in the Atlantic. Improved predictions of Atlantic hurricane activity may result from improved knowledge of African precursor wave disturbances and when and why they form. · We have shown that most climate models do not accurately simulate the impact of El Niño on Atlantic atmospheric conditions that affect hurricanes. The inability of models to represent these El Niño connections between the Atlantic and Pacific bodes poorly for the ability of models to simulate changes in hurricane activity in future climate, since such changes strongly depend on interactions between the Atlantic and Pacific Oceans through the atmosphere.
