A Diagnostic Study of Upper Tropospheric Cold Lows Over the Western North Pacific

Abstract

Four summers (1967, 1969, 1970, 1971) of rawinsonde data from four western North Pacific island stations (Guam, Midway, Johnston and Wake) were used to form a three-dimensional composite of the subtropical upper-tropospheric cold-core lows commonly found in the mid-Pacific trough. The basic meteorological parameters of wind, height, temperature and humidity were utilized to form an initial composite from the 117 lows identified during the period of study. Satellite mosaics were used to define a composite of horizontal cloud cover. The basic composite was harmonically smoothed and dynamically adjusted to generate a consistent set of derived fields that emphasize the important large-scale characteristics of the phenomenon. North-south and east-west vertical cross sections through the center of the composite low are used to illustrate the prominent features of the zonal and meridional winds, temperature and height anomalies, relative vorticity, divergence, vertical motion and relative humidity fields. The composite low is mostly confined to the region between 700 and 100 mb. The maximum circulation around the low occurs at 200 mb. The cold anomaly is strongest at 300 mb and lies primarily north of the vortex center. A warm anomaly centered at 125 mb is vertically aligned over the lower-level feature. The cold anomaly extends weakly to the surface. The region to the northwest of the low center is characterized by subsidence and a minimum in the cloudiness of the composite while the region to the southeast is characterized by ascent and a maximum in the cloudiness. Maximum vertical motion occurs near 300 mb. A calculation of the potential to kinetic energy conversion within the composite low reveals a net direct circulation (positive conversion). The vertical profile of the conversion shows positive contributions from below 200 mb and negative contributions from above. The upper-level disturbance with its associated pattern of low-level convergence to the south and east was found to move at nearly the same speed and direction as the surface easterlies. This lends support to the hypothesis that the upper tropospheric lows can induce a separate surface disturbance. The mechanism involves the amplification of the surface relative vorticity field by a prolonged exposure to the imposed large-scale convergence.