2025 Indicators: SST's, MSLP, Shear, SAL, Steering, Instability (Day 16+ Climate Models)

[Team Ghost]

I genuinely have trouble coming up with an explanation for why the West African Monsoon was suppressed during the peak of hurricane season in both 2024 and 2025. These years were both cool-neutral years ENSO wise with record warm Atlantics, which should collectively (a) favor rising air over the Maritime Continent and sinking air over the Pacific per the -ENSO pattern and (b) favor rising air over West Africa per the strong +AMO pattern. 2024 in particular was surprising; it had a nearly identical SST profile to 2010 and 2020 (record-warm Atlantic, -ENSO, -PDO, -IOD), yet those seasons featured rising air across West Africa while 2024 did not.

Pressure patterns across the tropics do not reveal anything noteworthy about 2024 and 2025 either. The tropical Atlantic and West Africa had below-average sea level pressures during ASO 2024, a pattern that mirrors both 2020 and 2010; and 2025’s sea level pressure pattern in these parts mirrored 2021, another year with an active West African Monsoon.

[Category5Kaiju]

I genuinely have trouble coming up with an explanation for why the West African Monsoon was suppressed during the peak of hurricane season in both 2024 and 2025. These years were both cool-neutral years ENSO wise with record warm Atlantics, which should collectively (a) favor rising air over the Maritime Continent and sinking air over the Pacific per the -ENSO pattern and (b) favor rising air over West Africa per the strong +AMO pattern. 2024 in particular was surprising; it had a nearly identical SST profile to 2010 and 2020 (record-warm Atlantic, -ENSO, -PDO, -IOD), yet those seasons featured rising air across West Africa while 2024 did not.

Pressure patterns across the tropics do not reveal anything noteworthy about 2024 and 2025 either. The tropical Atlantic and West Africa had below-average sea level pressures during ASO 2024, a pattern that mirrors both 2020 and 2010; and 2025’s sea level pressure pattern in these parts mirrored 2021, another year with an active West African Monsoon.

I think some good years to probably look at for clues would be 1961, 1996, 1998, and 1999. Those years featured 14 or less named storms and happened during a +AMO era, but nevertheless ended with above-average ACE (just barely or even solidly hyperactive, in fact). I think another year to probably look at would be 2007, with its 2 Category 5s and remaining slop (for 14 NSs total), despite a La Nina.

In terms of exactly why the monsoon was suppressed this year and last year....unless we continue to see something like this happen in the years going forward, I'd have to imagine that it's simply the result of intraseasonal variability.

[Team Ghost]

I genuinely have trouble coming up with an explanation for why the West African Monsoon was suppressed during the peak of hurricane season in both 2024 and 2025. These years were both cool-neutral years ENSO wise with record warm Atlantics, which should collectively (a) favor rising air over the Maritime Continent and sinking air over the Pacific per the -ENSO pattern and (b) favor rising air over West Africa per the strong +AMO pattern. 2024 in particular was surprising; it had a nearly identical SST profile to 2010 and 2020 (record-warm Atlantic, -ENSO, -PDO, -IOD), yet those seasons featured rising air across West Africa while 2024 did not.

Pressure patterns across the tropics do not reveal anything noteworthy about 2024 and 2025 either. The tropical Atlantic and West Africa had below-average sea level pressures during ASO 2024, a pattern that mirrors both 2020 and 2010; and 2025’s sea level pressure pattern in these parts mirrored 2021, another year with an active West African Monsoon.

I think some good years to probably look at for clues would be 1961, 1996, 1998, and 1999. Those years featured 14 or less named storms and happened during a +AMO era, but nevertheless ended with above-average ACE (just barely or even solidly hyperactive, in fact). I think another year to probably look at would be 2007, with its 2 Category 5s and remaining slop (for 14 NSs total), despite a La Nina.

In terms of exactly why the monsoon was suppressed this year and last year....unless we continue to see something like this happen in the years going forward, I'd have to imagine that it's simply the result of intraseasonal variability.

I am fairly confident the low storm counts in the Atlantic of this year and last year are resultant from the suppressed West African Monsoon. Few storms will form if there are few strong tropical waves exiting West Africa in the first place. Seasons with rising air over West Africa tend to produce lots of storms during peak season (ASO), when sea surface temperatures are at their warmest, thereby enabling tropical cyclogenesis. 2019 and 2023 even showed that a strong WAM in conjunction with a warm Atlantic can counteract a warm Equatorial Pacific. Conversely, 2016 and 2017, which had sinking air over West Africa during ASO, had fewer storms form during the peak 3 months of hurricane season.

Intraseasonal variability likely explains some of 2024’s WAM suppression; but the magnitude of 2025’s WAM suppression in spite of the +AMO, -IOD, -ENSO, and -PDO suggests a condition beyond typical intraseasonal variability.

[Category5Kaiju]

I genuinely have trouble coming up with an explanation for why the West African Monsoon was suppressed during the peak of hurricane season in both 2024 and 2025. These years were both cool-neutral years ENSO wise with record warm Atlantics, which should collectively (a) favor rising air over the Maritime Continent and sinking air over the Pacific per the -ENSO pattern and (b) favor rising air over West Africa per the strong +AMO pattern. 2024 in particular was surprising; it had a nearly identical SST profile to 2010 and 2020 (record-warm Atlantic, -ENSO, -PDO, -IOD), yet those seasons featured rising air across West Africa while 2024 did not.

Pressure patterns across the tropics do not reveal anything noteworthy about 2024 and 2025 either. The tropical Atlantic and West Africa had below-average sea level pressures during ASO 2024, a pattern that mirrors both 2020 and 2010; and 2025’s sea level pressure pattern in these parts mirrored 2021, another year with an active West African Monsoon.

I think some good years to probably look at for clues would be 1961, 1996, 1998, and 1999. Those years featured 14 or less named storms and happened during a +AMO era, but nevertheless ended with above-average ACE (just barely or even solidly hyperactive, in fact). I think another year to probably look at would be 2007, with its 2 Category 5s and remaining slop (for 14 NSs total), despite a La Nina.

In terms of exactly why the monsoon was suppressed this year and last year....unless we continue to see something like this happen in the years going forward, I'd have to imagine that it's simply the result of intraseasonal variability.

I am fairly confident the low storm counts in the Atlantic of this year and last year are resultant from the suppressed West African Monsoon. Few storms will form if there are few strong tropical waves exiting West Africa in the first place. Seasons with rising air over West Africa tend to produce lots of storms during peak season (ASO), when sea surface temperatures are at their warmest, thereby enabling tropical cyclogenesis. 2019 and 2023 even showed that a strong WAM in conjunction with a warm Atlantic can counteract a warm Equatorial Pacific. Conversely, 2016 and 2017, which had sinking air over West Africa during ASO, had fewer storms form during the peak 3 months of hurricane season.

Intraseasonal variability likely explains some of 2024’s WAM suppression; but the magnitude of 2025’s WAM suppression in spite of the +AMO, -IOD, -ENSO, and -PDO suggests a condition beyond typical intraseasonal variability.

I definitely agree that the rather low NS count is largely attributed to Africa's WAM being suppressed; however, I also do think that there were other factors that fell in place (for example, a very active EPAC that likely imparted shear over the Atlantic basin, even around the usual September peak) that capped the NS and hurricane potential by an even greater degree. Take 99L and 91L, for example, which happened in late August-early September. They ultimately failed to become NSs, and models greatly busted with 91L. Quite a few models even hinted at the idea of either or both storms becoming strong hurricanes.

One thing that caught my attention when tracking those systems was the amount of dry air and wind shear that were present in the Atlantic, while the EPAC was lighting up with its own activity. I don't think it's unreasonable to say that if there was less shear and dry air, that those systems could've gone on to become what models initially thought they were going to become. But alas, the ambient conditions were simply not favorable during that time. And, combined with the WAM suppression, it's easy to see why we got a period of absolute quiet during what should've been a much more active period. Of course, that didn't stop the rather familiar pattern of a backloaded season from happening, and three major hurricanes occurred in late September and beyond.

So, I guess what I'm trying to say here is, upon closer inspection, I think there were a handful of things that caused the below average NS and hurricane count, with the WAM suppression being a key but not sole culprit in this. As for future years, like next year and beyond, we'll have to see if the WAM suppression is just a 2024/2025 thing or if it really is a trend-setter. My preliminary guess is no, but I could very well be wrong.

[cycloneye]

The 2026 indicators thread is up.