Abstract

The local influence of mountains upon large- and synoptic-scale low-level atmospheric circulations is investigated in this study. The sea-level pressure associated with low-frequency fluctuations exhibit phase propagation of monopolar structures around mountains in an anticyclonic sense, while the corresponding 500 mb height patterns are relatively stationary and evolve in a manner consistent with the concept of Rossby wave dispersion on a sphere. The sea-level pressure patterns in the high-pass filtered data exhibit characteristics of synoptic-scale baroclinic waves and are steered around mountain ranges in an anticyclonic sense, while the corresponding 500 mb height patterns propagate nearly parallel to the time-mean flow in the middle troposphere.

It is hypothesized that the phase propagation of lower tropospheric circulation patterns is a reflection of the conservation of potential vorticity in flows over variable terrain. Most of the observations presented in this study are interpreted as the evidence of topographic Rossby waves in the atmosphere. However, the features observed to the north of the Tibetan Plateau exhibit some characteristics of Kelvin waves induced by the blocking effects of the orography on the lower tropospheric flow.

Because of the strong stratification during wintertime, the steering effect of mountains upon atmospheric circulations is restricted to the lower troposphere. Lower tropospheric waveguides exist in the vicinity of the major mountain ranges in the Northern Hemisphere. These regions are located (i) along the eastern slopes of the Rockies (ii) along the west and north coasts of Greenland, (iii) along the eastern slopes of the mountain ranges in Mongolia and northern China, (iv) to the north and east of the Tibetan Plateau, and (v) to the north of the mountains in northern Iran and Afghanistan.

There appear to be only minor differences between the structure and evolution of cyclonic and anticyclonic circulation anomalies, even though the corresponding sequences of synoptic maps may appear quite different. Deviations of the static stability field associated with the anomalies from the climatological mean static stability field are relatively small in comparison to the mean static stability. These observations suggest that the behavior of these features is relatively linear.

Abstract

Teleconnections of the streamfunction in the global domain based on ECMWF 250-mb winds for the 11 northern winters from 1978/79 through 1988/89 are documented in this study. A zonal structure with a node near the equator, indicating an out-of-phase relationship between the streamfunctions in the Northern and Southern hemisphere, appears to mask the fluctuations of the asymmetric components of streamfunction. After removing zonal means, a global pattern emerges as the dominant structure in the low-frequency band. This pattern consists of several dipoles straddling either the exit region of midlatitude jets or the equator, indicating the existence of teleconnections not only between the midlatitudes and the tropics but also between the Northern and Southern hemispheres.

Teleconnection patterns in the intermediate-frequency band are predominantly wavelike. Seven waveguides are identified based on the one-point lag-correlation maps for base points near the maximum teleconnectivity. Among them are three waveguides that have not been identified in previous studies. One originates in Europe, skirts the southern Eurasian continent, and spreads into the western Pacific. The other two originate in the northern central Pacific and the North American continent, respectively, and cross the equatorial regions of the westerlies into the Southern Hemisphere. The existence of cross-equatorial waveguides indicates the possibility of interhemispheric interaction and is in agreement with the hypothesis of Webster and Holton. Squared refractive indices are calculated based on the climatological flow and are found to be consistent with the existence of waveguides.

Abstract

This study examined the impact of northward- and westward-propagating summertime intraseasonal oscillations (ISOs) on submonthly wave patterns and tropical cyclones (TCs) in the subtropical western North Pacific. In the ISO westerly phase, submonthly wave patterns associated with the northward-propagating ISO appeared to be more energetic and most of the corresponding TCs maintained their wind speed for a relatively long period. Perturbation kinetic energy exhibited a stronger maximum in the ISO northward mode than in the westward mode. The analysis of barotropic conversion in the ISO northward mode revealed that an increase in barotropic conversion can be attributed to a strong association between the perturbation zonal wind component and the background flow. Therefore, submonthly wave patterns moving in a direction similar to that of the northward-propagating ISO continuously extracted energy from the background flow to the south of the submonthly base region. However, in the westward mode, the ISO propagating in a direction almost perpendicular to the submonthly wave pattern tracks not only altered the direction of the wave pattern but also created a background environment that was detached from submonthly perturbations. Thus, the background flow transferred less energy to submonthly wave patterns, resulting in shorter TC durations in the ISO westward mode than in the northward mode.