A Comparison of Atmospheric Blockings over the Southeast and Southwest Pacific Ocean

Abstract

Several earlier studies showed that the southwest Pacific near New Zealand is a region of blocking in the Southern Hemisphere. In this paper a second region of frequent blocking over the southeast Pacific is confirmed using 25 years of daily data. A comparison between the characteristics of blockings over the southwest and southeast Pacific revealed several interesting differences. Although the highest frequency of blocks occurs in the austral winter in both the regions, the frequency of blocks is greater over the southwest Pacific. However, these blocks are short lived and no block is found that lasts beyond 11 days duration. While over the southeast Pacific blocks are longer lasting with several blocks lasting for more than 11 days duration.
Local energetics of two blocks, one with a 21 day duration over the southeast Pacific and another with a 10 day duration over the southwest Pacific are analysed using Mak’s formulation. The contributions from various temporal scale interactions (seasonal, intraseasonal and high frequency components) to the episodal average local energetics of these two blocks are evaluated. It is found that blocking disturbances in both the cases extract barotropically, kinetic energy from the seasonal diffluent jet at almost the same rate. But the redistribution of kinetic energy within the block region is wider for the southeast Pacific block than for the southwest Pacific blocks this seems to maintain the longer duration of the southeast Pacific block. The synoptic eddy straining mechanism proposed by Shutts in manifested in three energetic terms, of which one (V^→₁.A^→_2,2) is found to be particularly large for both blocks. The pressure work process and the baroclinic conversion are also important, for both the blocks and the magnitudes are higher for the southeast Pacific block.