Low Mid-level Relative Humidity Limits Atlantic TC Activity

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A stronger-than-normal Bermuda high-pressure system is contributing to the lack of relative humidity in the middle atmosphere across the tropical/subtropical North Atlantic basin. The lack of RH in the mid-level atmosphere is well-correlated to lack of tropical cyclone activity since Elsa earlier this month.

Low mid-atmosphere relative humidity in tropical North Atlantic explains lack of tropical activity in recent weeks likely to continue through first 1/3 of August.

Fig. 1: The July 2021 (so far) mid-atmosphere relative humidity across the North Atlantic tropics/subtropics.

 

Fig. 2: The more recent July 2021 mid-atmosphere relative humidity across the North Atlantic tropics/subtropics.

Discussion: Research has shown a relationship between mid-atmosphere relative humidity (RH) and tropical cyclone (TC) potential and/or intensification across the North Atlantic tropics/subtropics. When RH is above normal, the environment is considerably more capable of producing TC’s and their intensification whereas dry RH limits or prevents TC development.

July 2021 so far has produced moderate to strong dry relative humidity anomalies across most of the North Atlantic tropics at mid-atmosphere level which helps to explain the complete lack of activity since “Elsa” earlier this month (Fig. 1). The more recent mid-level atmosphere RH analysis maintains the dry signature although small areas of wetter RH have been able to develop between 15N and 20N (Fig. 2).

The culprit producing the dry atmosphere and lack of tropical features in recent weeks is a stronger than normal Bermuda high-pressure system centered just east of Bermuda (Fig. 3). The subsidence associated with high-pressure aloft maintains the dry atmosphere and the stronger pressure gradient between the high-pressure center and the tropics is tight and propels strong trade winds also preventing organization of any tropical features.

Fig. 3: July 2021 (so far) sea level pressure anomalies (SLPA) across the tropical/subtropical North Atlantic basin.

The tropical/subtropical poor environment for tropical cyclone development conditions identified appears to last through the first third of August based on the ECM ENS percent of normal rainfall forecast (Fig. 4). The model indicates dry conditions from the Gulf of Mexico to the Caribbean Sea and Bahamas to the Cape Verde Islands. The intra-tropical convergence zone (ITCZ) where most strong tropical systems are born is strengthening during early August.

Fig. 4: The ECM ENS 15-day percent of normal precipitation forecast across the tropical/subtropical North Atlantic basin.  

Summary: Mid-atmosphere relative humidity is an excellent predictor of an ideal environment for tropical cyclone development and/or intensification in the tropical/subtropical North Atlantic basin. Currently, RH is below normal in the middle atmosphere and has been since “Elsa” earlier in July. The 15-day percent of normal rainfall forecast indicates the tropical North Atlantic to the Gulf of Mexico is likely to stay drier than normal and therefore no tropical cyclone risk through the first third of August.