STORM2K

What is the scientific reasoning for when a 55 knot TS gets caught up in the ITCZ that it begins to weaken? What is it exactly that causes it to falter? This is what I asked about Hurricane Carlos when the NHC said this:

NHC Forecast Discussion 14 for Carlos wrote: wrote:CARLOS' INVOLVEMENT WITH THE ITCZ COMPLICATES THE INTENSITY FORECAST. IF THE CYCLONE CONTINUES TO MOVE WEST-SOUTHWESTWARD ALONG THE AXIS...RE-STRENGTHENING WOULD BE HIGHLY UNLIKELY.

I might understand how a weak TD that doesn't have a well formed LLC can be absorbed by it, but a 55 knot TS or mature hurricane that is "small"? What mechanism is it that can do this? My guess is that the LLC (once connected) gets ripped apart from the rest of the cyclone.

Cyclenall wrote:What is the scientific reasoning for when a 55 knot TS gets caught up in the ITCZ that it begins to weaken? What is it exactly that causes it to falter? This is what I asked about Hurricane Carlos when the NHC said this:

NHC Forecast Discussion 14 for Carlos wrote: wrote:CARLOS' INVOLVEMENT WITH THE ITCZ COMPLICATES THE INTENSITY FORECAST. IF THE CYCLONE CONTINUES TO MOVE WEST-SOUTHWESTWARD ALONG THE AXIS...RE-STRENGTHENING WOULD BE HIGHLY UNLIKELY.

I might understand how a weak TD that doesn't have a well formed LLC can be absorbed by it, but a 55 knot TS or mature hurricane that is "small"? What mechanism is it that can do this? My guess is that the LLC (once connected) gets ripped apart from the rest of the cyclone.

Just throwing out an educated guess from what I have read here, but the closer a system is to the equator, the less chance of the 'coriolis' effect has to induce the necessary cyclonic spin. Since the ITCZ is relatively close to the equator, this may explain it. Although, in my lack of understanding of such items, I am not sure the 'coriolis' effect is required only during the formation of a tropical storm, or required to continually assist in sustaining the cyclonic spin.

Just another amateur fumbling in the dark for the light switch too....

TYNI wrote:Just throwing out an educated guess from what I have read here, but the closer a system is to the equator, the less chance of the 'coriolis' effect has to induce the necessary cyclonic spin. Since the ITCZ is relatively close to the equator, this may explain it. Although, in my lack of understanding of such items, I am not sure the 'coriolis' effect is required only during the formation of a tropical storm, or required to continually assist in sustaining the cyclonic spin.

Just another amateur fumbling in the dark for the light switch too....

I don't think that's the reason since:

1. The ITCZ fluctuates throughout the season (and year) and can be well above the latitude where storms can form.
2. I don't think the Coriolis has anything to do with it since once a tropical cyclone is created, the Coriolis effect isn't a factor (that's the answer to your question).
3. When storms weaken or be absorbed into the ITCZ, the cyclone getting too close to the equator has never been an issue (that I know of).

Thanks for answering my question! Hopefully one of the Pro Mets can help answer the rest...

I'm not sure specifically what they were referring to, but I do know that with the ITCZ (and in all honesty, I would classify where Carlos was as the NETWC) you get a East-Southeasterly wind on the southern side of the convergence zone, right where you would expect northwesterly winds around a tropical cyclone. This counterflow "cancels out" the flow on the south side of the system and opens up weak storms into waves or reverse oriented troughs. Once the circulation opens up, convection starts to suffer and then the system is no longer self-sustaining.

theavocado wrote:I'm not sure specifically what they were referring to, but I do know that with the ITCZ (and in all honesty, I would classify where Carlos was as the NETWC) you get a East-Southeasterly wind on the southern side of the convergence zone, right where you would expect northwesterly winds around a tropical cyclone. This counterflow "cancels out" the flow on the south side of the system and opens up weak storms into waves or reverse oriented troughs. Once the circulation opens up, convection starts to suffer and then the system is no longer self-sustaining.

This is one of the most convincing answers to the question and it did cross my mind as to why they cannot continue. I think of it as wind shear but strangely enough this doesn't seem to be a problem when TC's first develop while being connected to the ITCZ.

Cyclenall wrote: I think of it as wind shear but strangely enough this doesn't seem to be a problem when TC's first develop while being connected to the ITCZ.

Actually, in the North Pacific, very few storms form in the NETWC. Most form from the trough extending west out of Mexico or from the Monsoon Trough extending across the South China Sea, over the Philippines, and east across Micronesia. There has been some wave development, but it's rare and usually associated with a westerly wind burst. It's the westerly wind burst near the equator (sometimes associated with the MJO) that relaxes the easterlies enough to kick off a system.

I have little experience in the Atlantic, so I'm not comfortable enough to apply this outside of the Pacific.

theavocado wrote:Actually, in the North Pacific, very few storms form in the NETWC. Most form from the trough extending west out of Mexico or from the Monsoon Trough extending across the South China Sea, over the Philippines, and east across Micronesia. There has been some wave development, but it's rare and usually associated with a westerly wind burst. It's the westerly wind burst near the equator (sometimes associated with the MJO) that relaxes the easterlies enough to kick off a system.

I have little experience in the Atlantic, so I'm not comfortable enough to apply this outside of the Pacific.

In the eastern Pacific I have seen several systems start off from the ITCZ within the last couple of years. What does "NETWC" stand for?

Cyclenall wrote:In the eastern Pacific I have seen several systems start off from the ITCZ within the last couple of years. What does "NETWC" stand for?

The NETWC is the Near Equatorial Trade Wind Convergence Zone. It's most commonly used in the Central and Eastern Pacific referring to the areas where East-Northeast flow from the northern hemisphere and East-Southeast flow from the southern hemisphere converge. It's essentially a sub-division of the ITCZ, like the SPCZ (south pacific convergence zone) that describes a specific type of pattern. The NETWC tends to be mostly a convergence feature that rarely sees westerlies on it's equatorward side.

For clarification, right now the pattern in the East Pac that extends from Mexico to about 110W is a monsoon trough. This area is characterized by the the cross equatorial flow becoming westerlies with cyclonic turning around the trough axis. The flow west of 115W is ENE/ESE convergent with very little cyclonic flow. Most the activity we have seen this year, and most from previous years has been from the trough extending out of Mexico or troughing along the Mexican coast and not from the NETWC further west.

Additionally, the cyclonic horizontal shear within the WNP Monsoon Trough can kickstart storm development at latitudes significantly below 10N despite the lack of Planetary Vorticity so close to the equator.

Steve