Development of a One-Month-Long, Westward-Propagating Subtropical Low in Boreal Summer (Invited Presentation)

A strong MJO event produced an upper tropospheric jet streak in northeast Asia and repeated wavebreaking in the jet exit region along 150°E during July 1988. A midlatitude low intensified and moved equatorward in the presence of strong bandpass-filtered (15-100 d) Q vector forcing for upward motion associated with the wave breaking. The midlatitude forcing moistened the atmosphere enough to increase the column water vapor to above 55 mm. This value was sufficiently large to allow a self-sustaining low even after the upper forcing weakened. The horizontal scale of the Q vector convergence was about 1500 km, consistent with the scale of most favorable convective response to quasi-geostrophic forcing in the subtropics described by Nie and Sobel. The low lasted one month as it moved southwestward, then westward, north of 20°N.

This case study will be used to address one of David Raymond’s key research topics: the role of balanced dynamics versus localized convection in the tropics. In addition, I will address where such long-lasting lows fit in the hierarchy of northwest Pacific cyclones in the subtropics.