Q1 (b) Formation of temperate cyclone depends on the condition of axis of dilation. Elucidate.  10 Marks (PYQ/2024)

Climatology – Atmospheric Disturbances - Temperate cyclones

Answer:

Introduction

Temperate cyclones, also known as mid-latitude or extratropical cyclones, are large-scale low-pressure systems that develop in the westerly wind belt between 35°–65° latitudes. A crucial concept in their formation is the axis of dilation, which relates to the kinematic and dynamic structure of the atmosphere during frontal genesis.

Understanding Axis of Dilation

The axis of dilation refers to an imaginary line along which air parcels diverge, resulting in horizontal divergence and vertical ascent. This concept emerges from kinematic analysis of the wind field and is often associated with positive vorticity advection, which encourages cyclogenesis.

• When the axis of dilation is oriented meridionally (north-south), it allows cold polar air and warm tropical air to interact along a stationary front, setting the stage for frontal wave development.

Left 2 diagrams: Confluent wind field applied to the thermal gradient. At a later time (T+1), the wind acts to increase the thermal gradient, thus a frontogenetical situation. Right 2 diagrams: Difluent wind field applied to the thermal gradient. At a later time (T+1), the wind acts to decrease the thermal gradient, thus a frontolytical situation   Axis of dilatation:  axis along which flow is being stretched (deformed) due to convergence in the flow; oriented parallel to the stretching due to the deformation field Axis of contraction: Axis of contraction:axis along which flow is being compacted within the deformation field; oriented perpendicular to the dilatation axis Deformation of the wind can act frontogenetically or frontolytically. One needs to consider the orientation of the isotherms relative to the axis of dilatation. In the case at left, the axis is oriented perpendicular to isotherms, thus convergence and stretching along the axis actually results in frontolysis. The key is the angle between isotherms and the axis of dilatation.  If the angle is < 45º (far left), the wind field is acting frontogenetically (increase in thermal gradient). If the angle is > 45º (near left), the wind field is acting frontolytically (decrease in  thermal gradient). Streamline analysis is a good tool for detecting deformation zones. Normally, the axis of dilatation is associated with a frontogenetical situation.

Cyclogenesis and Axis Orientation

1. Polar Front Theory (Bjerknes, 1918)

• Describes cyclogenesis along the polar front where warm and cold air masses meet.
• A wave develops on this front when upper-air divergence (along the axis of dilation) intensifies, leading to the deepening of a low-pressure centre.

2. Quasi-Geostrophic Theory

• Highlights the role of upper-level divergence and vorticity advection near the axis of dilation.
• Jet streams and associated jet streaks amplify divergence at the surface.

Theoretical and Model Perspectives

• Dynamic Meteorology (Holton): Emphasises the role of vorticity balance and tilting of the axis of dilation in cyclone intensification.
• Rossby Waves and baroclinic instability are amplified where the axis of dilation favours vertical motion.

Examples

• Nor’easters in North America, Cyclones over North Atlantic, and Mediterranean depressions show classic development linked to favourable axis orientation.

Conclusion

The axis of dilation governs the divergence and ascent necessary for the formation of temperate cyclones. Its orientation is crucial in frontal wave development, supporting both classical and modern theories of cyclogenesis, making it a fundamental kinematic control in mid-latitude atmospheric dynamics.