DooM,
Per the latest warning on Typhoon Maysak issued by the JTWC on March 31, at 2100z, hurricane force winds (roughly 64 kt and above) in the southern flank of Maysak's wind field only extend outward up to 40 nautical miles (nm) from the center, and tropical storm force winds (34 kt) only extend outward up to 110 nm from the center. Wind speeds begin to drop off quickly beyond the 34 kt wind field. While Maysak is a record breaking early season CAT 5/super typhoon, it also a relatively small cyclone (size wise) based on the wind field estimates (NOTE: The outflow of a TC in satellite imagery can make it look much larger than it really is at the surface). Each degree of latitude is roughly 60 nm. So if Maysak is currently centered right around or just above 10N (as of the latest warning), tropical storm force winds in the southern flank of Maysak only extend down to about 8N. Any winds that would be of decent benefit to Kelvin wave activity, likely do not extend any further south than about 6N. The forecast suggests that Maysak will not get any closer to the equator as it's projected to continue tracking towards the west-northwest. (NOTE: I'm NOT saying that we need a tropical cyclone's 34 kt wind field to be right on the equator, but generally a tropical cyclone's 34 kt wind field should not be any further north than 5-6N, if one is hoping for a beneficial WWB or westerly winds). If Maysak was a large cyclone, say maybe twice as big, then we would probably see some beneficial westerly winds. In order for westerly wind to aid in any kind of respectable Kelvin wave activity, we typically like to see those winds between 5N-5S and 130E to about the Dateline. This is roughly the area that some refer to as the Kelvin wave generation area. (NOTE: If cross equatorial cyclones were present then it would be a different story as we saw with Bavi and Pam, but those cyclones were also larger in size. It generally seems that when there’s only one cyclone present, it needs to be larger and/or closer to the equator than cross equatorial cyclones to provide beneficial westerly winds).
In short, just because a tropical cyclone is within 10 degrees north or south of the equator, doesn’t always mean it's generating beneficial westerly winds or a WWB (although tropical cyclones within 10N-10S and between 130E and the Dateline typically generate at least some beneficial westerly wind). It's important to monitor a tropical cyclone's wind field. Look at the southern flank in the northern hemisphere and the northern flank in the southern hemisphere. This will allow one to get a good idea of how much beneficial westerly may actually be occurring, but one could also just look at the earth wind map. Sometimes a tropical cyclones size can more important than the strength, as is the case with Maysak.
Thanks,
BigB