Low Mid-troposphere Relative Humidity Suppressing North Atlantic Tropics

Low mid-tropospheric relative humidity caused by subsidence in the deep tropics holding back the 2022 season so far and likely to cause less seasonal activity than previously indicated

Executive Summary: Tropical Depression 4 not intensifying to a tropical storm prior to reaching the northeast coast of Mexico yesterday is eye-catching. A tropical depression located across 85-86F water in a relatively light shear (aloft) zone should have enabled this system to strengthen. Dry air entrainment from the Texas drought region is a likely contributor to lack of development. Another contribution is the general low relative humidity at 600 MB across the tropical North Atlantic basin due to strong subsidence in the upper atmosphere. This low RH signature was common in August during 2013-2018 (despite ENSO phase). During that timeframe late August through November produced on average 8.4 tropical storms, 4.2 hurricanes and 2.0 intense hurricanes. The 2022 season is trending in that direction versus the buoyant 15-20 tropical storms, 6-10 hurricanes and 3-5 major hurricanes currently forecast.

Discussion: Due to stronger-than-normal subsidence in the middle troposphere (600 MB) across the deep tropics of the North Atlantic basin during July and August so far (Fig. 1) the tropical cyclone season is off to a slower than normal start for the onset of busiest time of year (AUG/SEP). So far, 2022 has produced just 3 tropical storms and no hurricanes.

During the past 3 tropical cyclone seasons, the middle troposphere was very wet in the eastern North Atlantic tropics where most tropical cyclones are born (Fig. 2). From late August through the remainder of the tropical cyclone season during the past 3 seasons a robust amount of activity was observed averaging 15.0 tropical storms, 6.3 hurricanes and 4.0 intense hurricanes!

Interestingly, the previous 6 seasons (2013-2018) were – on average – very dry in the middle troposphere across the deep tropics during August (Fig. 3) similar to 2022 (so far). During that time there were both La Nina and El Nino years but regardless of ENSO, each year in that timeframe were very dry at the onset of the busiest part of the tropical cyclone season. During 2013-2018 the average number of tropical storms from late August through the remainder of the tropical cyclone season was 8.4 tropical storms, 4.2 hurricanes and 2.0 intense hurricanes or about 58% of the 2019-2021 activity.

Tropical Depression 4 was unable to develop across the very warm southwest Gulf of Mexico waters yesterday…a rarity for mid-August. Operational models are indicating limited tropical cyclone formation for the remainder of August. Consequently, the remainder of 2022 may produce total tropical cyclone activity closer to the 2013-2018 analog versus the 2019-2021 analog.

Summary: Unless the subsidence pattern stretched across the deep tropics of the North Atlantic basin reverses soon, the seasonal activity levels will likely be somewhat lower than outlooks issued late last spring and mid-summer. Due to seasonality, conditions should improve and SEP/OCT looks like the busiest time of the 2022 North Atlantic tropical cyclone season (vs. AUG/SEP of most years). However, due to the late start of the season, the seasonal totals are likely less than the buoyant numbers associated with seasonal outlooks previously issued (Table 1).

Fig. 1: Relative humidity at 600 MB across the deep tropics in August 2022 so far is very dry due to subsidence and preventing tropical cyclone formation.

Fig. 2: During August of 2019-2021 the relative humidity at 600 MB across the eastern North Atlantic tropics (where most tropical cyclones form) was very wet and an exceptionally active late August through November followed.

Fig. 3: During August 2013-2018 the relative humidity at 600 MB across the tropics was quite dry, similar to 2022 (so far). The late August through November tropical cyclone activity was about 58% as active as the 2019-2021 timeframe.

Table 1: Due to the dry deep tropics well into August the core of the most active part of the 2022 tropical cyclone season is delayed likely holding seasonal totals beneath current forecasts.