Monsoon Cyclonic Gyre - FLHurricane.com

OK. I'll give it a try. Although not known for my brevity, I will attempt to explain it in a way so everyone may understand. My apologies to the Mets if this is too simplified.

As you may know, " ... a monsoon is a seasonal prevailing wind which lasts for several months and has lasting climatic effects. It most commonly refers to the "wet monsoon", or rainy season, in many parts of the world. Most summer monsoons have a dominant westerly or southwesterly component and a strong tendency to ascend and produce copious amounts of rainfall ..."

For an excellent intro article from Wikipedia on the Monsoon in general, check out:

http://en.wikipedia.org/wiki/Monsoon

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I Googled "Monsoon Cyclonic Gyre" and came up with quite a few rather technical articles that go into great meteorological detail, often similar to the link to the excellent PDF file that Climate Aficionado ( thank you ) shared with us. More on that later.

But I did come across a nice 'nutshell' version in the form of Mark Lander's (University of Guam) abstract to his December, 1994 article in the American Meteorological Society Publication.

So, I'll try and paraphrase Mark's abstract, and you may begin to understand this interesting phenomena, occurring about once a year.

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"The specific configuration of the monsoon circulation, which is called a monsoon gyre, is an episodic eventâ€”occurring roughly once per year, for two or three weeks during July, August, or September.

As a monsoon gyre, the low-level circulation of the Western North Pacific becomes organized as a large cyclonic vortex associated with a nearly circular 2500-km-wide (!) depression in the contours of the sea level pressure."

>> So we have the normal monsoon flow which evolves, over time, into a huge, gently curved band of cloud and thunderstorms stretching across thousands of miles of ocean, where lower surface pressures form an enormous, sometimes elongated depression, kind of like the red spot on Jupiter, but on a smaller scale of course.

"A cyclonically curved band of deep convective clouds rims the southern through eastern periphery of this large vortex. Once this pattern is established, it becomes a prolific generator of mesoscale vortices (large clusters of thunderstorms that tend to spin about some low-level center) that emerge from the downstream end of the major peripheral cloud band. These mesoscale vortices form the seed disturbances for midget or small-sized tropical cyclones."

>> So we have within this enormous, gently curved band of stormy weather, clusters of thunderstorms that tend to start spinning about some center, which then sometimes go on to evolve into small to midget-sized tropical storms or typhoons.

>> Mark goes onto say in his abstract that this particular monsoon gyre ...

"... was the site of cyclogenesis for two tropical depressions, two tropical storms, and two typhoons during its 20-day westward journey. Initially, small tropical cyclones formed in the peripheral circulation of the gyre ..."

>> And, remarkably ...

" ... later, the gyre itself evolved into a VERY large tropical cyclone." !! (Exclamation my own.)

"This is suggestive of two distinct modes of tropical cyclogenesis: one mode operates to produce small tropical cyclones in the eastern periphery of the gyre, and the other mode operates to accelerate the winds of the monsoon gyre until it becomes a giant tropical cyclone."

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So the 'take away' here is that the monsoon flow itself (a huge feature) slowly evolves into an enormous, cyclonic feature with a prominent band of gently curved, deep convection on its southern and eastern sides that tends to spawn small and midget sized tropical cyclones.

And, most remarkably, the whole gyre itself can sometimes evolve into a *huge* cyclone in itself, akin to the red spot on Jupiter, although the winds do not attain typhoon (hurricane) strength; usually only depression or tropical storm force, but *enormous* in size.

It would appear the the Atlantic basin sometimes exhibits its' own version of a 'monsoon cyclonic gyre', now developing as we now head into the peak of hurricane season, and the large wave being watched near 30W appears to be still embedded within the monsoon gyre that is now setting up shop in the far eastern Atlantic.

The latest tropical weather discussion (2am EDT, as I write) indicates " ...BROAD ELONGATED LOW / MID LEVEL CYCLONIC ROTATION COVERING THE AREA FROM 11N-19N BETWEEN 25W-36W WITH SEVERAL POSSIBLE ILL-DEFINED LOW LEVEL CIRCULATIONS.", which would be entirely consistent with the behavior of a monsoon gyre, as described above.

Thank you for bringing this interesting phenomena to everyone's attention.

For those interested in learning more, I heartedly recommend the PDF article that 'Climate Aficionado' contributed (actually, this is the entire publication of the abstract I paraphrased above), which although a little tough-going for the lay person, but potentially rewarding, presents a fascinating story, (and profusely illustrated!), of a western pacific monsoon gyre, that over a 3 week period, spun up not less than 6 tropical cyclones and itself eventually evolved into an enormous tropical cyclone, encompassing thousands of miles of ocean! Pretty cool stuff for a tropical weather buff. Enjoy!

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"Don't Get Stuck on Stupid" - General Honore, following Hurricane Katrina

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