How Natural Climate Change Brings Both Drought and Floods.

I love busting myths and the myth that I wish to bust today is the claim that anthropogenic climate change brings floods in some areas and droughts in others.

Whenever I approach trouble shooting exercises like this, I return to the fundamentals of atmospheric physics, by looking to the behavior of things under the Seasonal Cycle. It is from this vantage point that we will find the most likely explanation for how natural climatic changes can give rise to droughts in some regions and to floods in others.

The reason I start with the Seasonal Cycle, is we know from first principles that the Earth's Hydrological Cycle is controlled at a first order by orbital dynamics and the cyclical pattern of summer transitioning to winter in either hemisphere.

Due in part to the tilt of the Earth, the tropical latitudes receives the most solar radiation on average over the entire Seasonal Cycle and thus on average is in a state of energy surplus, while for much the same reasons the polar latitudes exist in an average state of energy deficiency.

This annual average energy gradient between the tropics and polar atmospheres gives rise to the heat pump that drives global circulation patterns.

Due to this geographical energy distribution, the equatorial tropics are were we see the highest rates of precipitation relative to the rates of evaporation. This peculiar feature is shown in Figure 2 as a difference map for evaporation minus precipitation. Note that the tropics are deeply negative, while the subtropical latitudes are conversely positive.

To better understand the mechanics behind the excess state of precipitation that exists along the narrow equatorial zone, one simply has to take a cross section of the atmosphere centered at the equator and to visualize the convective response of the lower troposphere in either hemisphere in response to the excess state of solar radiation in this region.

Figure 3 is absolutely fundamental in understanding the underlying geographical extremes in evaporation versus precipitation that exist tightly centered around the equatorial latitude(s). As shown, due to the maximum state of solar radiation that exists along the equatorial zone, air masses in this region pull in lower tropospheric air masses from either hemisphere as deep convection occurs within this narrow zone.

As this deeply convecting air mass rises as high as 17 km, and as it rises it cools, precipitates (dehydrates) and gradually looses buoyancy, before diverging into either hemisphere along the tropopause.

See how both dehydrated diverging air masses eventually fall under gravity along the subtropical latitudes.

In atmospheric physics we call the zone of converging air masses the Intertropical Convergence Zone (ITCZ) and the zones where dehydrated air masses subside back down into the lower troposphere, the Horse Latitudes. Note that the Horse Latitudes are where we find higher evaporation minus precipitation and thus are were the great global deserts reside (e.g., Sahara).

The Horse Latitudes are droughty by nature, because subsiding dehydrated air masses compresses and heat as they fall under gravity, thereby creating a high pressure / temperature inversion lid over regions where they fall.

This collective process is fundamentally why evaporation exceeds precipitation along either side of the equatorial zone or ITCZ.

Conversely, the absence of this inversion lid within the ITCZ gives rise to deep convection and the precipitation of lower tropospheric water vapor transported from both subtropical zones into the equatorial Firebox. This gives rise to meridional transport from high to low pressures systems in both subtropical zones, and is ultimately why the ITCZ's precipitation rate exceeds its evaporation rate.

By convention, the ITCZ is called the zone of maximum precipitation and the two counter rotating air masses along either side are called the Hadley Cells.

The Hadley Cells are geographically defined by extreme rainfall along their lower latitude limits and by extreme drought along their higher latitude limit.

Now that we have these basic mechanics worked out, it is time to throw in a curve ball to see how orbital mechanics shifts these distinct latitudes as a function of the Seasonal Cycle. Figure 4 shows the annual migration of the Hadley Cells Convergence Zone (or ITCZ); note that its annual path is to always move towards whichever hemisphere is experiencing summer.

This fundamental process is at work behind the Indian Summer Monsoon and why its dry season coincides with winter. During the summer, the ITCZ migrates to higher sub-tropical latitudes in the Northern Hemisphere, while conversely during the winter, the ITCZ migrates towards the Southern Hemisphere.

Note that fundamentally, this orbital forcing gives rise to a cyclical pattern that manifests itself on an annual basis set by the frequency of the Earth's rotation around the Sun.

I propose that we view this as the first order driver behind the migration of the ITCZ into and out of either hemisphere on an annual basis.

This is important, for when we begin to examine inter-annual to decadal migration patterns of the ITCZ, we find evidence that orbital mechanics continue to dominate long term modes of change in the latitudes of maximum (ITCZ) and minimum precipitation (Horse Latitudes).

The best known case in point is the evolution of the North African Sahara region over the Holocene Interglacial period, as shown in Figure 5.

It is well known in paleoclimatology that one of the most defining characteristic of the evolution from the last glacial maximum to the Holocene Thermal Maximum was the net displacement AND intensification of the ITCZ (precipitation) northward over the Sahara region.

Likewise, it is known that the subsequent re-desertification of North Africa coincides with the slow reversal of this orbital (Milankovitch) forcing and the gradual migration of the average position of the ITCZ back towards the Southern Hemisphere.

Again, the ITCZ moves towards whichever hemisphere is warming the fastest and away from whichever hemisphere is cooling the fastest.

This hemispheric thermal gradient dependence in the migration of the ITCZ and Horse Latitudes are why we often see paleo-representations of this long term pattern of change as shown in Figure 6.

While I will not delve into the geochemical analysis that went into creating this 12,000 year record of the global migration pattern of the ITCZ (Horse Latitudes), I will simply say that changes in precipitation give rise to many different geochemical changes in the near surface layer that can be calibrated as a proxy for precipitation.

The red, black and blue data points coincide with distinct sampling points across the global tropical latitudes and are quite often, shallow caves that are susceptible to gradual changes in ground water seepage, but are not directly exposed to precipitation.

See how as this information is represented, the Indian Summer Monsoon strengthens as the hemispheric temperature gradient from the north to the south increases. Conversely, as indicated by the red data points, as the ITCZ moves northwards, the Southern Hemisphere's Horse Latitudes moves northward as well, and it becomes drier.

Note that Figure 6 shows both low and high frequency modes of change.

This same author expands upon the higher frequency modes of change by showing the same representation over data points specific to the 20th century. As shown in Figure 7, the emphasis continues to be on the influence of the changing migration patterns of the ITCZ over sub-tropical North Africa or the Sahel region in particular.

Note the multi-decadal change in precipitation in the Sahel region as the difference in hemispheric air temperature oscillated on an approximate 60 year basis.

Many will recall that the severe droughts within the Sahel region in the late 1970s to early 1990s, abruptly ended. The reason being, was the average position of the ITCZ moved northward - again - just as it had during the 1930s to 1950s.

Of course, as the ITCZ migrated northward towards the Sahara after the 1980s, this also moved northward the corresponding northern Horse Latitudes.

Figure 8 shows the multi-decadal changes in cloudiness and sunshine hours over Europe as the ITCZ moved northward during this latter part of the 20th century to present. Remember, high pressure systems, created by subsiding dehydrated air masses over the Horse Latitudes, creates lower cloud cover and higher sunshine hour meteorological conditions.

When we look at longer term sunshine hours or surface solar radiation (SSR) records for Europe, we find that it shows a matching multi-decadal pattern exhibited by the ITCZ over North Africa over the 20th century.

What is missing is a plot that takes Figure 2 and converts it into a long term time series that shows gradual changes in the relative rates of evaporation versus precipitation by latitude and hemisphere.

I see a dozen Ph.D. thesis in the making!

In conclusions, I hope that my attempt to teach the basics of the ITCZ and Horse Latitude(s) influence on relative evaporation and precipitation is ultimately known to be driven by various modes of orbital forcing processes.

From the Seasonal Cycle to the Milankovitch Cycles, we see that regions experiencing maximum precipitation and maximum doughtiness are in constant flux.

Thank you very much for your attention. Please provide corrections or ask questions in the comment section below and I will do my best to address each on a one by one basis.