-ENSO seasons with below-average late seasons and +ENSO seasons with above-average late seasons

[WalterWhite]

ENSO is typically strongest in the winter, which corresponds to the final few months of hurricane season, namely OND. Additionally, late season activity typically favors the Western Atlantic, which is the most affected by El Nino shear. This means that there should be a strong correlation between ENSO state (=ENSO or -ENSO) and late season activity (ACE during OND).

Indeed, the data support this hypothesis. The mean OND ACE from 1991 to 2020 is 28.4 square hectoknots. On average, +ENSO seasons have average late-season accumulated cyclone energies below this mean, and -ENSO seasons have late-season accumulated cyclone energies above this mean. However, there are many exceptions.

Note: ENSO type is based on whether or not Nino 3.4 SSTAs are above-average or below-average for OND.

1991 (+ENSO): 8.3 hkt^2
1992 (-ENSO): 8.7 hkt^2
1993* (+ENSO): 0.0 hkt^2
1994 (+ENSO): 19.3 hkt^2
1995 (-ENSO): 46.7 hkt^2
1996 (-ENSO): 31.1 hkt^2
1997 (+ENSO): 2.2 hkt^2
1998 (-ENSO): 48.5 hkt^2
1999 (-ENSO): 40.7 hkt^2
2000 (-ENSO): 23.3 hkt^2
2001 (-ENSO): 39.6 hkt^2
2002 (+ENSO): 15.7 hkt^2
2003 (+ENSO): 35.3 hkt^2
2004 (+ENSO): 7.1 hkt^2
2005 (-ENSO): 74.6 hkt^2
2006 (+ENSO): 3.6 hkt^2
2007 (-ENSO): 6.9 hkt^2
2008 (-ENSO): 20.4 hkt^2
2009 (+ENSO): 11.3 hkt^2
2010 (-ENSO): 31.9 hkt^2
2011 (-ENSO): 35.4 hkt^2
2012** (-ENSO): 28.5 hkt^2
2013 (-ENSO): 10.4 hkt^2
2014 (+ENSO): 29.8 hkt^2
2015 (+ENSO): 30.7 hkt^2
2016 (-ENSO): 79.6 hkt^2
2017 (-ENSO): 20.5 hkt^2
2018 (+ENSO): 42.9 hkt^2
2019 (+ENSO): 18.5 hkt^2
2020 (-ENSO): 74.5 hkt^2
2021 (-ENSO): 26.3 hkt^2
2022 (-ENSO): 15.6 hkt^2

+ENSO mean: 18.1 hkt^2
-ENSO mean: 34.9 hkt^2

*This season had a perfectly neutral ENSO during OND; however, it is considered a +ENSO season because the following NDJ ENSO index is positive.
**This season had a perfectly neutral ENSO during OND; however, it is considered a -ENSO season because the following NDJ ENSO index is positive.

The late-season accumulated cyclone energy means for +ENSO and -ENSO seasons are 18.1 hkt^2 and 34.9 hkt^2. On average, +ENSO seasons have less active late seasons than -ENSO seasons. However, the +ENSO seasons of 2003, 2014, 2015, and 2018 all buck this trend; for they have OND accumulated cyclone energies above 28.4 hkt^2, the 1991-2020 mean. Additionally, the -ENSO seasons of 1992, 2000, 2007, 2008, 2013, 2017, 2021, and 2022 all buck this trend because they have OND accumulated cyclone energies below 28.4 hkt^2, the 1991-2020 mean.

Specific examples are easy to explain: 1992 featured a negative AMO while 1991-2020 features mostly positive AMOs. 2007 and 2021 featured a strong Atlantic Nino, which displaced the ITCZ southward, causing late-season tropical waves to develop in the Pacific, rather than the Atlantic. 2003 featured a very warm Atlantic Ocean, which nullified the impact of a +ENSO. (However, it is worth noting that 2004, which had an even warmer Atlantic, failed to exceed the 1991-2020 late-season mean.)

Generally speaking, however, are there specific broad-scale climate patters that these anomalies share? Are there specific broad-scale climate patterns than 2003, 2014, 2015, and 2018 share that are not present in other +ENSO years? Are there specific broad-scale climate patterns that 1992, 2000, 2007, 2008, 2013, 2017, 2021, and 2022 share that are not present in other -ENSO years?

[zzzh]

ENSO phase locking happens in winter-spring. Not necessarily OND.