Plume Structures
in Deep Convection of Rotating Fluid

Shoshiro Minobe (1,3), Yoshihito Kanamoto (1)*, Naosuke Okada (2),
Hisashi Ozawa (3), and Motoyoshi Ikeda (2,3)

(1) Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University.
(2) Graduate School of Earth Environmental Science, Hokkaido University.
(3) Frontier Research System for Global Change

Abstract:

Plume structure of open-ocean deep convection is investigated by using a nonhydrostatic numerical model, with 157 experiments for different sets of physical parameters, which are Coriolis parameter, diffusivity, and surface buoyancy (heat) flux. A separation curve between two dimensional (2-D) and three dimensional (3-D) convection regimes is estimated with a higher accuracy than previously examined in a 2-dimensional parameter space of the natural Rossby number and flux Rayleigh number.

For the 2-D and 3-D regime transition, the validity of the maximization hypothesis of entropy increase rate is examined. Across the 2-D and 3-D separation curve, entropy increase rates tend to change their gradient in a consistent manner with the hypothesis. This suggests a possibility that the regime transition of plume behavior is understood from a point of view of macroscopic thermodynamics for a dissipative system.

In the 2-D regime, some experiments exhibited interesting new vorticity structures, which are different from the conventional heton structure. Two structures are closely examined by three-dimensional visualization of vorticity and velocity fields. One structure is characterized by the longer (several kilometers in the fluid of 1 km depth) roll-shape vortexes, with the circulation in a plane that slightly tilts from the plane perpendicular to the roll axis. The roll has a weak negative vertical vorticity (anticyclonic circulation), and simultaneously shorter cyclonic oval-shape vortexes occurs. The other structure is similar to the mushroom consisting of anticyclonic vortex above and cyclonic vortex below, and exhibits the energetic temporal variability of the structure itself. These results indicate that the quasi-stable heton structures do not explain all of the plume structures in the 2-D regime.

Contents:

• Introduction, Fig. 1
• Model
• Analysis method
  • 1. EOF
  • 2. Entropy increase rate
• Results
• 2D & 3D regimes transition
  • 1. 2D/3D separation curve, Fig. 2
  • 2. Entropy increase rate, Fig. 3
• Plume structures in 2D regime
  • 3. Roll structure, top-view animation, Fig. 4
  • 4. Roll structure, 3D close up, Fig. 5
  • 5. Mushroom structure, top-view animation, Fig. 6
  • 6. Mushroom structure, 3D close up & animation, Fig. 7, Fig. 8
• Summary
• Acknowledgements
• References
• Appendixes
  • Table 1. Prescribed parameters, entropy increase rate, and explained variance of the first vertical mode.
  • A1. Heton, top-view animation, Fig. A1
  • A2. Mushroom structure, bottom-view animation, Fig. A2
  • A3. Heton, 3D close-up animation, Fig. A3

corresponding author:
Shoshiro Minobe (minobe@ep.sci.hokudai.ac.jp)
Division of Earth and Planetary Sciences,
Graduate School of Science,
Hokkaido University,
N-10, W-8, Sapporo 060-0810, Japan.

* present affiliation: Japan Information Processing Service Co., Ltd.