Energy flows from the tropics to the poles. The tropics absorb most of the sun’s radiation and drives heat flow to polar regions where radiative cooling dominates. In winter the Hadley cell moves heat to ~30N via huge convective currents transporting latent heat from the tropics northwards via a band of powerful thunderstorms . The reverse happens in the northern summer when the tropical Hadley cell moves southwards towards the now winter southern hemisphere and it reverses circulation. David Randall explains this well in his book ‘Atmosphere, Clouds and Climate’ (from which the first 3 figures below are taken).
The rising branch of tropical thunderstorms (ITCZ) is located about 10 deg from the equator in whichever is the summer hemisphere. The release of latent heat warms the troposphere to the moist adiabatic lapse rate. The high warm air moves meridionally and cools by radiating heat, so becoming more dense. It then descends and twists eastward due to the rapidly increasing coriolis forces with latitude. A counter rotating Ferrel cell is driven by the mechanical energy of storms 60 -30 deg and balance mass flow. The JET stream is caused by the coriolis force acting on the descending Hadley circulation accelerated by zonal wind effects. It is concentrated about 12 km up due to lapse rate temperature gradients. The coriolis component of angular momentum M for the earth rotating with angular velocity at latitude and zonal wind u is:
u = 0 at the equator wheras u = 11m/s at 30N
The Jet stream strengthens as the polar night begins. This is closely related to winter storms as the temperature gradient between the tropics and the winter pole increases. This “thermal wind” in mid latitudes is caused by horizontal temperature gradients which increase strongly in winter. Hydrostatic balance vertically becomes unstable
Wind changes rapidly with height when surface temp changes rapidly in the horizontal direction. The Jet stream becomes stronger
If T changes rapidly horizontally then the thermal winds increase strongly with height. This state is termed to be ‘baryclonic’. At 30N in winter the Jet Stream winds starts to increase massively with height. This is because below the Jet Stream the surface temperature gradient increases rapidly towards the ‘night-time’ pole. So the steeper the temperature gradient the stronger the Jet stream becomes.
Given a strong Jet Stream and large temperature gradient, conditions are now critical for the formation of big storms. A strong poleward decrease in temperature causes ‘baryclonic instability’. Such storms are technically called ‘baryclonic waves’ and are composed of cold fronts as polar air moves south under warm tropical air, and warm fronts when simultaneously warm tropical air rises north above cold polar air, all driven by Coriolis forces. Each front causes a rapid change in temperature on the ground. These storms are then accelerated by the release of gravitational potential energy as dense cold air falls downwards feeding kinetic energy to the storm causing strong winds. When conditions are right for baryclonic instability any small external disturbance will be amplified to trigger the formation of a storm. There is clear evidence that strong spring tides are one important trigger of such storms. Tides provide an asymmetric disturbance which act over large northern zones through changing tractional tidal forces acting on the atmosphere. An example of this was the storm on Jan 4-6 last winter.
In a growing perturbation cold air slides under warm tropical air. Cold air moves down causing a front while warm air on the tropical side moves upwards causing a warm front. Warm air rises and cold air descends transporting thermal energy upwards and the center of mass of the total air column descends releasing gravitational energy into kinetic energy. This forms a low pressure vortex at the centre.
Hadley cells do not penetrate into middle latitudes. Likewise Baroclonic waves do not penetrate into the tropics. The energy transported from the tropics by Hadley cells is ‘handed over’ to such baroclonic storms to finish the job by moving the heat onwards to the poles. The Jet stream position defines this boundary between tropical air and polar air. Storms form on the northern side of the Jet Stream at the eastern edge of the Atlantic. The path of these storms is determined by the kinks (Rosby waves) in the Jet Stream. As the earth rotates, an ever changing gravitational field of tractional forces sweep across the Atlantic ocean from west to east about every 12 hours. The effect is similar to a bar magnet sweeping across a sheet of paper covered in iron filings. These two videos show how strongly tractional tidal forces can vary both during the lunar month and yearly with gradual changes in lunar declination. The first is updated 2 hourly for August this year and the second shows the direct tide for a fixed position during 2006 when the moon was at its 18.6 year maximum declination.
The southward swirling tidal force acting on the Jet Stream amounts to about 10 metric tons per km. This gravitational force is varying strongly in strength and position during the lunar month. There are two possible effects of tides.
- They can distort the Jet Stream causing kinks which change the path of mid latitude storms.
- They can seed winter storms by disturbing baroclonic instability at maximum tidal forces. Therefore for lunar tides to effect mid latitude weather we need the following conditions.
- A sharp horizontal temperature gradient leads to a strong Jet Stream and Baryclonic instability.
- Increasing spring tides with strong tractional forces at high latitudes can trigger anticyclonic flow leading to cold/warm fronts and storms moving eastwards.
- These storms are guided across the Atlantic by the Jet Stream.
- The Jet Stream itself is distorted by the same changing tidal ‘tweaks’.
The strength and position of storms is enhances by growing instability and changing atmospheric tides over coming days. The same tractional tidal force field also modifies Rosby waves in the Jet Stream.
Last winter the UK experienced a series of severe storms. The Jet Stream was locked into a pattern whereby the eastern US was extremely cold while warmer air resided over the western Atlantic. Storms were spun off the boundary between very cold air over Newfoundland and warmer tropical air moving up over the western Atlantic. Each major storm coincided with a maximum tide bringing coastal flooding to the UK as large waves were also enhanced by storm surges.
So there is direct evidence that atmospheric tides can trigger storms. Nearly all of last winter’s storms coincided with maximum spring tides. This cannot just be a coincidence. It would be relatively easy to include such tidal forces into GCM weather forecast models. The moon is just one more external force acting on the circulation of the atmosphere. I strongly suspect that medium range weather forecasts would improve significantly if the dynamics of tides were properly taken into account.