Evolution of Dipole-Type Blocking Life Cycles: Analytical Diagnoses and Observations

TL;DR

This paper develops an analytical model based on the VCKdV system to explain the evolution of atmospheric dipole-type blocking life cycles, emphasizing the influence of background westerlies and shear, supported by observational analysis.

Contribution

It introduces an analytical solution from the VCKdV equation to diagnose DB evolution and highlights the roles of westerly strength and shear in blocking life cycles, integrating observational data.

Findings

Weak westerlies promote DB development and longer life cycles.

Cyclonic shear reduces blocking intensity and duration.

Pacific region shows higher DB occurrence linked to weaker westerlies.

Abstract

A variable coefficient Korteweg de Vries (VCKdV) system is derived by considering the time-dependent basic flow and boundary conditions from a nonlinear, inviscid, nondissipative, and equivalent barotropic vorticity equation in a beta-plane. One analytical solution obtained from the VCKdV equation can be successfully used to explain the evolution of atmospheric dipole-type blocking (DB) life cycles. Analytical diagnoses show that background mean westerlies have great influence on evolution of DB during its life cycle. A weak westerly is necessary for blocking development and the blocking life period shortens, accompanied with the enhanced westerlies. The shear of the background westerlies also plays an important role in the evolution of blocking. The cyclonic shear is preferable for the development of blocking but when the cyclonic shear increases, the intensity of blocking decreases and the life period of DB becomes shorter. Weak anticyclonic shear below a critical threshold is also favorable for DB formation. Time-dependent background westerly (TDW) in the life cycle of DB has some modulations on the blocking life period and intensity due to the behavior of the mean westerlies. Statistical analysis regarding the climatological features of observed DB is also investigated. The Pacific is a preferred region for DB, especially at high latitudes. These features may associate with the weakest westerlies and the particular westerly shear structure over the northwestern Pacific.