Note: The forecast for Tropical Storm Philippe panned out as expected per last week’s tropical blog, but the focus on the MJO was lacking. This week’s blog will provide full focus on the MJO, as it is the key player of significance in a much quieter tropical environment this week.
Where did all the tropical cyclones go? As we march into November, the tropics have quieted down significantly in the Atlantic and eastern Pacific with the dissipation of Tropical Storms Philippe and Selma. This puts the focus squarely on the Madden-Julian Oscillation (MJO), and its role in inhibiting tropical convection over these basins currently.
Per the MJO Phase Diagram provided by the Australian Bureau of Meteorology, the MJO remains in the active phase over the western Pacific. However, since the last update on this oscillation, the MJO has weakened and appears to be almost doubling back over the western Pacific, as seen by its curve away from the eighth phase.
Figure 1 – The latest MJO Phase Diagram from the Australian Bureau of Meteorology.
The idea of the MJO weakening and becoming less important at this time is further confirmed by the Climate Prediction Center (CPC)’s Global Tropics Hazards Outlook. Here the report references the MJO weakening as it completes its cycle over Africa and returns to the Indian Ocean, which appears to dispute the MJO Phase Diagram above. Perhaps the diagram listed in Figure 1, were it updated for November 1, would show a small propagation of the MJO into the Western Hemisphere as referenced by the CPC.
The influence of an equatorial Rossby wave is also in play, especially with the weakening of the MJO over the Indian Ocean and increased convection over southeast Asia. As seen in the 200 hPa velocity potential, negative anomalies over North America and the eastern Pacific suggest upper-level divergence, or regions of enhanced convection. This is consistent with the propagation of the MJO over the Western Hemisphere. On the other hand, positive anomalies over the maritime continent suggest upper-level convergence, or regions of suppressed convection. This is consistent as well with the eastward propagation of the MJO. Meanwhile, the current convection over this area could be explained by the equatorial Rossby wave.
Figure 2 – 200 hPa velocity potential anomalies on a global scale, noting the location of the MJO and an equatorial Rossby wave.
Outgoing longwave radiation (OLR) is also a good proxy for locating convection in the tropics, and by association waves and oscillations. For OLR, anomalies are used to locate areas of above or below normal output. If OLR is below normal, then this is usually found in an area of convection where cloud cover is restricting the outbound radiation. The opposite is then true for areas where OLR is above normal.
Figure 3 – Weekly (top) and monthly (bottom) averaged Outgoing Longwave Radiation (OLR) on a global scale.
The latest weekly and monthly OLR anomalies shows generally positive anomalies along the western coast of North America, suggesting little convection in this area. However, over the maritime continent of southeast Asia, one can find the strongest area of negative anomalies, consistent with the highest levels of convection currently.
Figure 4 – Outgoing Longwave Radiation (OLR) anomalies, with negative anomalies in cooler colors and positive anomalies in warmer colors.
Hovmoller diagrams can also help in the identification of the MJO and its propagation, as seen in the westerly wind anomalies. As noted by the diagonal pattern between 60 degrees east and 150 degrees east longitude throughout the diagram, it is possible to identify more than one instance of the MJO over time. The latest iteration of the oscillation is seen to the bottom of the diagram, with the diagonal violet colors cutting through the otherwise green meridional signature. The MJO is represented here by a positive westerly wind anomaly, with instances seen between May and June 2017, July and August 2017, and September and October 2017.
Figure 5 – A Hovmoller Diagram describing the westerly wind anomalies throughout time, with the MJO identified by the violet diagonals.
With its continued eastward propagation and interaction with an equatorial Rossby wave, the MJO is expected to weaken. Considering it is nearly at the conclusion of its eight-phase propagation, this does not come as a surprise. However, interaction with the equatorial Rossby wave over the Indian Ocean and maritime Pacific, which will propagate in the opposite direction of the MJO, will cause a convergence of the two events, with one near its beginning and one at its end.
The role of these two events will be to limit tropical convection over the Atlantic and eastern Pacific basins, while enhancing convection over the western Pacific. The former two are near the end of their respective seasons as is, but will also be impacted by the loss of convection from the MJO. In the central Pacific, drier conditions are forecast, as they will be experiencing the downward branch of the MJO, which will inhibit convection. The western Pacific, however, will benefit (in terms of tropical development) first from the propagation of the equatorial Rossby wave, and followed by the final weeks of the MJO.
Figure 6 – The Climate Prediction Center (CPC)’s summary of global tropical hazards over the next two weeks.
Other Tropical Tidbits
While Puerto Rico continues to grab the headlines in the recovery from Hurricane Maria, Dominica’s efforts to rebuild from the storm are even slower. Meanwhile, sports have once again provided a much-needed lift to a devastated city, with the Houston Astros lifting up the World Series title and the city of Houston after Hurricane Harvey. Finally, National Geographic reports that rare species like the famous Key deer could be pushed to the brink of extinction thanks to the recent wave of hurricanes.