Tropical Cyclones That Are Non-Tropical In Origin

[Ptarmigan]

I wonder if there have been tropical cyclones that are non-tropical in origin outside of the North Atlantic. I know many hurricanes have formed from non-tropical lows in the past in the North Atlantic. I wonder if this happens in other basins.

[1900hurricane]

It's not particularly common, but it does happen in the WPac on occasion. Three pretty extreme examples have occurred in the last decade. Soulik '13, Lionrock '16, and Noru '17 all worked their way down from upper lows and ended up becoming category 4s. Gordon '89 might be the most extreme example, transitioning from an upper low to a category 5.

[J_J99]

I believe Joaquin (2015) was from non-tropical origins and was the strongest such storm in the Atlantic that we know of to form from non-tropical origins.

[AJC3]

The primary (nearly exclusive) mode of "tropical" (deep warm core) and "subtropical" (shallow warm core) cyclone formation in the Mediterranean Sea comes from mid level cold core lows, whose surface reflections are initially "non-tropical".

The degree of warm core transition ranges from "almost none" to "100 percent", and is dependent on the upper level pattern, location and forward motion of the initial low, and degree of instability/lapse rate which is a function of the difference in SSTs vs mid level temperatures.

There are a few members on here who study these types of cyclones intently. There are several GREAT Masters theses and Ph.D. dissertations, etc. waiting to be written an presented on this subject. All that's needed is some folks to rigorously document and study these MEDSEA cyclones in a formal academic setting.

[Hypercane_Kyle]

It does seem like the NATL produces substantially more subtropical cyclones than any other basin. I wonder why?

[Monsoonjr99]

It does seem like the NATL produces substantially more subtropical cyclones than any other basin. I wonder why?

Might just be a matter of the NHC choosing to designate them unlike other basins' RSMCs.

[Hypercane_Kyle]

It does seem like the NATL produces substantially more subtropical cyclones than any other basin. I wonder why?

Might just be a matter of the NHC choosing to designate them unlike other basins' RSMCs.

True, but the NHC also monitors the East Pacific and it seems very rare to see a subtropical storm there too.

[Iune]

It does seem like the NATL produces substantially more subtropical cyclones than any other basin. I wonder why?

Might just be a matter of the NHC choosing to designate them unlike other basins' RSMCs.

True, but the NHC also monitors the East Pacific and it seems very rare to see a subtropical storm there too.

Other than Omeka, has there ever been a subtropical storm in the Central or Eastern Pacific?

[EquusStorm]

There was a possibly subtropical storm in 2006 in the CPac (https://en.m.wikipedia.org/wiki/2006_Central_Pacific_cyclone) and I'm sure there are other examples (an upper level low absorbing the remnants of Lane last year became a probable subtropical stom as well in the CPac) but probably less commonly than in the Atlantic. The waters are colder at high latitudes in the EPac/CPac than the Atlantic, so I'd wager warm core systems of any sort would find the environment a bit unpleasant. Am extremely fascinated with high latitude nontropical origin tropical/subtropical cyclones so it's definitely a subject I like to keep an eye on. October was certainly a great month for that lol.

Joaquin indeed being the strongest nontropical origin storm on Atlantic record - then two other category fours on record (Diana 1984 and Claudette 1991) and at least three category threes (Alicia 1983, Bob 1991, Michael 2012) Fascinating stuff. Whoops, forgot about Ophelia. Four then.

[CrazyC83]

In the eastern Pacific, the environment in the subtropical latitudes (i.e. 25-40N) is incredibly hostile, and in the north Indian, you run into land before you can get into the subtropics. The WPAC may have had some that went unnamed though that may be worth looking into, along with the southern hemisphere.

[1900hurricane]

In terms of mid-latitude lows transitioning to tropical cyclones, shear/mid-latitude westerlies may also play a role. WPac waters can also be somewhat warm in the upper subtropics, but the mid-latitude jet tends to be lower in latitude in the WPac, which is observable in the further south TCs transition to extratropical cyclones. And while I don't currently have the data to back this up, I do believe it is in general more zonal and stronger, further eliminating opportunities for mid-latitude cyclones to transition to TCs.

As far a TCs that are non-tropical in origin, there appears to be three main types to me.
1) TCs that develop from decaying frontal boundaries/MCSs as an initial disturbance. Alicia '83 is a good example of this kind.
2) TCs that develop from TUTT cells/lows. Colder upper level temperatures from these that wander over warmer waters for a sustained time can result in convection and eventually a developing system. Joaquin '15 was one of these, as well as the WPac examples I mentioned above. This case might actually be more common in the WPac than elsewhere, and perhaps has the highest potential intensity ceiling based on observed cases.
3) TCs that develop from occluding mid-latitude cyclones. This might be the rarest case overall. This is where you get your Pablo '19s and Epsilon '05s. Ophelia '17 is the only major that has come from this case that I can think of offhand.