Why was November 2020 so active?

[Hypercane_Kyle]

We saw a 130 knot + Category 4 and a 140 knot Category 5 in a month that often doesn't even see a named storm (2018, 2014, 2012 as recent examples). The only comparison I can think of to this was July 2005 with two storms of similar intensity. What atmospheric conditions were in place that made such an incredibly unlikely chain of events happen?

[weeniepatrol]

moderate-strong la nina

[CyclonicFury]

Significantly reduced wind shear as a result of La Niña, combined with well above normal sea surface temperatures in the Caribbean and a perfectly timed MJO pulse.

This month may not be completely done yet either. That 0/20 AOI has a chance to develop before month's end, and if it gets named during November, it would break the record.

[Hypercane_Kyle]

moderate-strong la nina

We've certainly seen strong storms in November in the past (Lenny in 1999, Paloma in 2008), both in La Nina years, but still... two of the most intense November hurricanes in the matter of days?

[Category5Kaiju]

To sum it all up:

1. We were in a moderate La Nina by then that reduced the wind shear especially in the Caribbean late season

2. Tropical waves kept being spit out of Africa late into the season due to the extremely active monsoon train and wet Western Africa this year

3. The SST anomalies in the Caribbean were quite high in November, with the actual temperatures on the surface being roughly 28-30 degrees C and virtually untapped ocean heat content (with the possible exceptions of Delta, Rina, and Paloma, the WCAR has recently experienced a dearth in very strong cyclones for some years; like the last high end Cat 4/5 hurricane in this area was Gustav in 2008).

Many of these waves struggled to develop in the open Atlantic due to typical November shear, only for some of them to encounter the La Nina enhanced low shear environment/rocket fuel surface temperatures and high ocean heat content levels in the Caribbean Sea. As a result, you got storms like Eta and Iota whose precursor waves were "ghosts" and did not develop much until they reached the WCAR's extremely favorable conditions and then exploded into the monster storms we remember them as. In addition, perhaps more relating to the highly specific atmospheric conditions then, Eta and Iota were boosted by not only the generic low shear in the WCAR but also anticyclones that surrounded them, making the shear near them virtually 0 knots at certain times and allowing them to swell in intensity rapidly.