Solar-Driven Hadley Cell Expansion Explains Global Brightening Since the Little Ice Age?
In my latest lecture on the Tom Nelson podcast (June 15th), I expanded on the themes introduced in my January 4th, 2026 article titled Beyond Milankovitch: Solar-Driven Atmospheric Dynamics in Neoglacial Evolution. The link to this podcast is included at the end of this article.
Here in this end of June article, I will include elements from my June 15th presentation and specifically will focus on the idea that changes in solar activity indirectly acts on the lower troposphere through a coupling with the Stratospheric Polar Vortext (SPV). The select body of evidence that I will present begins with the zonal mean cloud cover time series for the Atlantic and Pacific Oceans shown above.
The take home messages from this George Tselioudis et al study are:
- The tropical Hadley Cells on either side of the Intertropical Convergence Zone (ITCZ) are expanding poleward.
- The middle latitudes along which the Hadley Cells subsize (aka Horse Latitudes) are moving poleward as the Hadley Cells expand and as they do, marine stratiform cloud cover is declining. Remember, the Horse Latitudes are where the global deserts reside and the causative atmospheric factor is subsiding (anticyclonic) air masses along the higher latitude branches of the tropical Hadley Cells.
- The deeply convective clouds (aka Trade Cumulus clouds) within the ITCZ are contracting.
- The middle latitude Ferrel Cells are narrowing.
I have included in Figure 1, a cross section illustration of a single Hadley Cell bordering the ITCZ (left-hand side), to better visualize how cloud types vary within the low pressure upwelling zone of the ITCZ to the high pressure downwelling zone along the Horselatitude (right-hand side). This illustration conveys the basics of how atmospheric – oceanic circulation are coupled within the Hadley Zone and work to create two wildly different cloud types (i.e., Trade Cumulus vs Stratiform / Marine Boundary Layer clouds).
Figure 1. Cloud cover type variance within a cross section of tropical Hadley Cell.
With this model in mind, it is clear that the ITCZ latitudes are more efficiently ventilating LWR as deeply convective clouds contract, while the sub-tropical / middle latitudes are reflecting less in-coming SWR as marine stratiform cloud cover decreases. In other words, the Cloud Greenhouse Effect within the tropics is decreasing and as this occurs, the ocean is absorbing more sunshine.
The role of Walker Circulation dynamics in modulating the Trade Cumulus clouds and LWR flux out the top of the atmosphere was the topic of the recent article titled Clouds 850% Stronger Than CO₂: Unmasking the True Driver of Changes in Global Average Air Temperatures, where I showed that interannual variabilities of the Pacific Trade Winds is the control knob on heat flux out the top of the atmosphere.
This article will expand on the idea that marine stratiform clouds are declining as the Hadley Cells expand poleward and will argue that the famous Southern Annular Mode (SAM) serves as a proxy for this dynamic process. Specifically, I propose that a positively trending SAM Index implies that the Horse Latitudes in the Southern Hemisphere are migrating poleward and conversely that a negatively trending SAM Index is indicative of the migration of the Horse Latitudes towards the equatorial zone.
Keep in mind that while the ITCZ corresponds to the zone of maximum precipitation, the Horse Latitudes are the zones of minimum precipitation. Thus, why in the neoglacial sub-epoch of the Holocene Interglacial (last 4,200 years), the global deserts reside along the Horse Latitudes.
Once this proxy relationship has been established, we will be in a position to approximate how the Hadley Cell expansion – contraction may be evolved since the Medieval Warm Period (MWP) to present.
Finally, I will emphasize the role of changing solar activity in its influence on the Stratospheric Polar Vortex (SPV), together with the coupling between the SPV and SAM migrations, to introduce a plausible mechanism behind Global Brightening (Warming) versus Global Dimming (Cooling) within the Holocene Neoglacial sub-epoch.
Southern Annular Mode (SAM) Index
The Southern Annular Mode (SAM) index is calculated as the difference in zonal mean sea level pressure (SLP) between 40°S and 65°S.
What a Positive SAM Means:
- •Strengthening of the circumpolar westerly winds (the Roaring 40s, Furious 50s, and Screaming 60s).
- •Poleward shift of the westerlies.
- •More zonal flow — the winds become more west-to-east (zonal) with reduced meridional (north-south) waviness.
- •A deepening (strengthening) of the stratospheric thermal gradient between low latitudes (warmer) and high latitudes (colder) is a well-established causative mechanism for a positive trend in the SAM Index.
- A negative SAM Index implies a reversal of these meteorological effects.
People living in Australia, are well acquainted with the utility of the SAM Index in making sense of climate variability within the southern half of the continent. As shown in Figure 2, when the winter season SAM is positive, the southern coast is droughty, while during a negative winter SAM gives rise to a shift in drought prone regions to the middle eastern coastlines.
Note the zonal shift occurs as a function of the proximity of the westerlies to the southern region of the continent.
The mechanism at play the Southern Hemisphere’s Horse Latitude is migrating in concert with the displacement of the Roaring 40s / Screaming 60s.
Figure 2. Representations of the Southern Annular Mode (SAM) Index and zonal migrations of the Roaring 40s / Screaming 60s.
The left-hand image in Figure 2 is included to help visualize the relationship between the higher pressure and lower pressure zones that exist along the latitudes (40S & 65S) over which the SAM Index is quantified. The red band is the high pressure zone and the blue is the lower pressure field.
I include Figure 3 to further build the mental picture of the cross sectional structure and dynamics of tropospheric circulation over the latitudes that the SAM Index quantifies.
- The left-hand diagram shows the anticyclonic circulation and low cloud cover associated with the high pressure zone that is characteristic of the condition over the 40th parallel when SAM is in a positive state.
- The left-hand diagram likewise shows that the 65th parallel is defined by a prevailing cyclonic-low pressure – higher cloud state.
- The right-hand illustration shows the latitudinal band over which the SAM Index is measured, together with the classical representation of the the Hadley, Ferrel and Polar cells that exist in either hemisphere extending poleward from the equatorial zone.
Thus, Figure 3 helps in understanding what a positive SAM Index equates to in terms of structural changes in tropospheric circulation. As the cover image shows, the poleward migration of the Horse Latitudes, means this higher pressure – lower cloud – anticyclonic state is increasingly advancing towards the 40th parallel at which the SAM Index characterizing.
Figure 3. Cross sectional representation of the anticyclonic (40th parallel) and cyclonic (65th parallel) tropospheric circulation patterns that exist across the SAM Index zone.
Figure 4 shows the modern instrumental Marshall SAM index from 1979 to 2018 and Figure 5 shows the 1,000 year reconstruction of the SAM Index by Abram et al. (2014).
Abram et al used a Composite Plus Scale (CPS) with nesting, a widely used paleoclimate reconstruction technique based on James Ross Island (JRI – 64th parallel) ice core deuterium isotope record (temperature proxy from the Antarctic Peninsula) normalized to the calibration period (1957–1995), which overlaps with and weighted by the instrumental Marshall SAM index.
Figure 4. Modern Marshall SAM Index from 1979 to 2018.
The JRI lies off the northeastern tip of the Antarctic Peninsula in the Weddell Sea, Antarctica. This places it near the Antarctic Circle (66.5°S), in a region known for its relatively mild Antarctic climate and significant glacial and paleontological research.
Likewise included in Figure 5 are proxy surface air temperature reconstructions, using geochemical samples taken from selection locations from JRI and South America. These land based temperature reconstructions coincide with the 40th and 65th parallel latitudes over which SAM characterizes changes in mean sea level pressure.
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