Wednesday, October 10, 2012

The Global Weather Pattern Shifts

The previous post described how changes in the Arctic were causing changes in the weather and climate.  These changes extended down in the mid-latitudes where the impacts included cold, snowy winters.  However, the impacts of a warming earth do not end there.  Research shows that mankind is shifting the entire global atmospheric circulation.


Meteorologists have developed an idealized view of the global atmospheric circulation over the past century.  It has long been recognized that there is too much heat at the equator and too little at the poles.  The atmosphere acts like “the great equalizer “ by transporting heat from the tropics to the poles.  If the earth were not rotating, this would be a simple straight-forward transfer.

The situation is complicated by the fact that the earth does rotate on its axis.  This has given rise to the three-cell global circulation model.  The three cells from the tropics to the poles are named: Hadley, Ferrel, and Polar.  This model is pictured below with the resulting wind patterns.

The three-cell general circulation model of the atmosphere.  This is an idealized model based on observations.  Image credit: NASA.

The Hadley cell (also known as the tropical cell) extends from the equator to 30 degrees latitude.  Many of the world’s major deserts lie at the poleward extent of the Hadley cell where the air descends from upper levels.  This produces a persistent dry climate with little rainfall.  The North American Desert, Sahara, Australian Desert, and Kalahari Desert all are products of this circulation.


Recent studies show that the Hadley cell has expanded poleward while the polar cell has retreated toward the pole in both hemispheres.  The subtropical jet stream generally marks the boundary between the Hadley and Ferrel cells.

A cross-section of the troposphere from the Equator to the North Pole.  The jet streams are at the intersection of the upper boundaries of the different circulation cells.  Image Credit: NOAA.

A study published just over a month ago found that between 1979 and 2010 the subtropical jet stream moved northward 3.7 +/- 0.3 degrees latitude in the Northern Hemisphere and moved southward 6.5 +/- 0.2 degrees in the Southern Hemisphere.  The author sums up the importance this way:

“The observed poleward movement in both hemispheres over the past thirty years represents a significant change in the position of the sub-tropical jet streams, which should lead to significant latitudinal shifts in the global weather patterns and the hydrologic cycle.”


The study above looked at a number of potential causes of such a shift.  A linear regression analysis was used to try to identify the primary cause.  It was found to be the direct radiative forcing from greenhouse gases.

In other words as greenhouse gases have increased over the past 30 years, the earth has warmed.  This warming of the lowest layer of this atmosphere has led to an expansion of the Hadley cell which in turn has led to a poleward shift in the storm track.  Thus, global warming is leading to a shift in weather and climate patterns.

Could there be other influences that might impact this shift?  The answer is yes.

Another study published in May found that black carbon aerosols and tropospheric ozone played a dominant role in the expansion of the Hadley cell in the Northern Hemisphere.  Their study concluded that these were noticeably better at driving the expansion than greenhouse gases.

The authors used a climate model with detailed aerosol physics.  The model still underestimated the expansion, but the knowledge the true aerosol forcing is poorly understood.  Dr. James Hansen of NASA has been pushing for better understanding of the aerosol forcing for decades.  This is one way to make the climate models better.


All of the studies recently have shown a poleward expansion of the Hadley cell and a shrinking polar cell.  This has resulted in a poleward shift of storm tracks.  Thus, the belts of deserts around 30 degrees latitude are moving poleward and shifting climate zones.

In addition, the rise of greenhouse gases and the presence of black carbon aerosols and tropospheric ozone are products of burning fossil fuels and bio fuels.  This is leading to a warming of the atmosphere which is in turn driving the expansion.

Thus, climate change is happening now and will continue as long as the earth continues to warm.  Only a vast reduction in the emission of greenhouse gases and tropospheric aerosols can halt the ongoing change.