INTERACTIONS OF MIDDLE LATITUDE TROUGHS AND TROPICAL DISTURBANCES

ON 2-4 WEEK TIME SCALES

John Molinari and David Vollaro

Department of Earth and Atmospheric Sciences

University at Albany, SUNY

• Upper-tropospheric middle-latitude troughs move equatorward into the subtropics, usually as “equatorward breaking Rossby waves” (Thorncroft et al. 1993)

• These troughs produce extensive convection (Kiladis 1998), in part by creating increased CAPE via lifting in the middle troposphere (Juckes and Smith 2000)

• Subtropical or equatorial disturbances are excited, either by downward energy propagation (Tam and Li 2006) or as a direct response to the enhanced convection (Kiladis 1998)

• The subtropical/equatorial disturbances move westward

• Under the right circumstances, they grow in amplitude and spawn tropical cyclones

• Some tropical cyclones recurve, become extratropical, and amplify the midlatitude trough-ridge pattern

• This affects predictability over North America on a time scale of 2-4 weeks

NH winter: trough penetration common in the eastern Pacific, lower tropospheric equatorial Rossby waves excited (Kiladis 1998)

In winter, TC formation with ER waves is much less likely than in summer

NH summer: Tam and Li (2006) studied subtropical disturbances (probably not equatorial wave modes): excited by midlatitude trough forcing, grow in amplitude between 160ºE and 130ºE, often excite tropical cyclones

Molinari et al. (2006): upper tropospheric midlatitude trough excited equatorial Rossby wave packet that produced 8 tropical cyclones

This talk: subtropical lows (not equatorial wave modes) excited by middle latitude troughs that produce recurving tropical cyclones

Because process is complicated, will show using weather maps

Hovmoeller diagram of the meridional wind at 200 hPa. Cool colors indicate wind from the north, and warm colors from the south.

Wind vectors at 850 hPa and infrared brightness (CLAUS) at 0000 UTC 23 July 1988.

As before, but for 24 July at 0000 UTC.

25 July 1988

26 July 1988

27 July 1988

28 July 1988

29 July 1988

30 July 1988

200 hPa wind vectors and relative vorticity contours at 0000 UTC 1 August 1988. Red shading represents vorticity exceeding 1 X 10-4 s-1.

29 July

IR satellite images of similar situations. A tropical storm is present at the center of each image.

Summary of 1988 event

• Midlatitude lower tropospheric low moves equatorward, from poleward of 40ºN to nearly 20ºN, reinforced by repeated short-wave trough penetrations

• Cyclonic circulation extends 1500 km to the south, 1000 km to the north (broadly consistent with inertial stability arguments)

• Strongly asymmetric convection persists, with virtually clear air to the north and broad, extensive convection to the south

• A tropical cyclone forms near the center of the subtropical low, then recurves, transitions to extratropical, and amplifies middle latitude flow patterns

•Time between the trough penetration and the TC recurving back to middle latitudes: 2 weeks

• In the ER wave packet in 1991, this time scale ranged from 2-4 weeks

• Accuracy of midlatitude forecasts likely depends upon accurately simulating TC genesis and recurvature

Open questions

• What creates the large north-south asymmetry in cloudiness in the subtropical low?

• What processes maintain the intensity of the subtropical low?

• How important are repeated subtropical penetrations of short-wave troughs to generate a broad, equatorward-moving subtropical low, versus a single strong trough?

Open questions (continued)

• What determines whether a subtropical low or an equatorial mode is excited by upper trough penetration?

• What determines whether a recurving tropical cyclone will amplify midlatitude flow? (Bosart, Harr, Evans, etc.)