Expansion of the Tropical Belts and Poleward Migration of the Storm Tracks Shown by Variations in Surface-Observed Cloud Cover

Abstract:

Human-made cloud observations from a subset of exceptionally consistent, land-based weather stations are used to show the global distribution of cloud cover versus latitude. Distinct peaks and valleys in the cloud distribution coincide with features of the general circulation, including cloud-cover maxima in the tropics and in the mid-latitude storm tracks and cloud-cover minima in subtropical subsidence regions.

Time series of the yearly-average latitude of the ‘center’ of these features show a significant poleward migration of the clouds associated with the storm tracks in each hemisphere, suggesting a widening of the Hadley Cells. Seasonal versions of these time series correlate significantly with the ENSO index, agreeing with previous work, showing that a positive ENSO index is associated with equatorward shifts in the storm track.

The study is repeated using cloud observations from both land-based weather stations and ships. The combined land-ocean study shows less significant poleward migration of the storm tracks, but more significant poleward migration of the sub-tropical subsidence regions.