Upper Tropospheric Cyclonic Vortex - Climatology

Climatology

In the Northern Hemisphere, the tropical upper tropospheric trough (TUTT) normally occurs between May and November, with peak activity between July and September. James Sadler suggested a revised model for the TUTT during the early part of the typhoon season in the western Pacific. Both Sadler and Lance Bosart have shown that the tropical upper tropospheric trough cyclonic cells are caused by the mid-latitude disturbance riding around the western side of the tropical upper tropospheric trough when the subtropical ridge to its south is quite weak. In the north Atlantic, the TUTT is characterized by the semi-permanent circulation pattern that forms in the North Atlantic between August and November. Toby Carlson evaluated data over the eastern Caribbean sea for October 1965 and pinpointed the presence of an upper tropospheric cold-core cyclone. These cold-core cyclones generally form close to the Azores and move south and westward towards a latitude of 20°N. These circulations extend over an area of about 20° of latitude (or 2,220 kilometres (1,200 nmi)) and 40° of longitude. The lowest level of closed circulation underneath the upper level cold-core cyclone is often between the 700 and the 500-hectopascal level (3,000 metres (9,800 ft) to 5,800 metres (19,000 ft) above sea level). Their life cycles span 5 to 14 days.

The upper tropospheric cyclonic centers in the North Atlantic differ from that in the North Pacific. Most of them are detectable in the low tropospheric temperature field as cold troughs in the easterlies. They tend to vertically tilt toward the northeast. Cumulonimbus clouds and rainfall occur in the southeast quadrant, approximately 5° latitude (or 555 kilometres (300 nmi)) from the upper cyclone center. Large variations of cloud cover can exist in different systems. The summer tropical upper tropospheric trough is a dominant feature over the trade wind regions of the North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea, and that the lower tropospheric responses to the tropical upper tropospheric trough in the North Atlantic are differ from those in the North Pacific.