The Role of Synoptic-Scale Flow during Tropical Cyclogenesis over the North Atlantic Ocean

Abstract

The synoptic-scale flow during tropical cyclogenesis and cyclolysis over the North Atlantic Ocean is investigated using compositing methods. Genesis and lysis are defined using the National Hurricane Center (NHC, now known as the Tropical Prediction Center) best-track data. Genesis (lysis) occurs when NHC first (last) identifies and tracks a tropical depression in the final best track dataset. Storm-centered composites are created with the Analysis of the Tropical Oceanic Lower Level (ATOLL; ∼900 hPa) and 200-hPa winds for June–November produced by NHC for the years 1975–93. Results show that significant regional differences exist in 200-hPa flow during genesis across the Atlantic basin. Composites of genesis in the western part of the basin show a 200-hPa trough (ridge) located to the west (east) of the ATOLL disturbance. In the eastern half of the basin composites of genesis show a sprawling 200-hPa ridge centered northeast of the ATOLL disturbance. The major axis of this elliptically shaped 200-hPa anticyclone extends zonally slightly poleward of the ATOLL level disturbance. Another composite of relatively rare genesis events that are associated with the equatorward end of frontal boundaries show that they generally occur in the equatorward entrance region of a jet streak in conjunction with an ATOLL cyclonic vorticity maximum in a region where vertical shear is minimized. An approximation of the Sutcliffe–Trenberth form of the quasigeostrophic omega equation is used to estimate the forcing for vertical motion in the vicinity of developing tropical cyclones. Forcing for ascent is found in all three genesis composites and is accompanied by a nonzero minimum in vertical shear directly above the ATOLL cyclonic vorticity maximum. Vertical shear over developing depressions is found to be near 10 m s⁻¹, suggestive that weak shear is necessary during tropical cyclogenesis to help force synoptic-scale ascent. Composites of tropical cyclone lysis show much weaker ATOLL cyclonic vorticity when compared to the genesis composites. The magnitude of the vertical shear and the forcing for ascent above the lysis ATOLL disturbance are stronger and weaker, respectively, than in the genesis composites. These differences arise due to the presence of a jet-streak and a longer half-wavelength between the trough and ridge axes in the lysis 200-hPa flow composite. The genesis flow patterns are decomposed by crudely removing the signature of the developing cyclone and its associated convection. Two separate and very different flow patterns commonly observed during genesis over the eastern and western Atlantic Ocean are found to be very similar once the flows are decomposed. Both flows are characterized by strong deformation at low levels and at 200 hPa with an upper-level jet exit region near the developing depression.