Little Ice Age displaced the tropical rain belt

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The ITCZ is the central band of bright white clouds in this composite image [credit: NASA]

This seems to add weight to the idea that the moving position of the ITCZ can be a useful indicator of natural climate change, in conjunction with other data sources.

The tropical rain belt, also known as the Intertropical Convergence Zone (ITCZ), is in a state of constant migration, as Phys.org reports.

It continuously changes position in response to the seasons and follows the sun’s zenith, with a slight delay. This in turn determines the wet and dry periods in the tropics and subtropics over the course of the year.

The tropical rain belt therefore effectively controls the climate in most of the tropical and subtropical regions, such as the monsoon season in Southeast Asia and Central America.

An international team of researchers led by Franziska Lechleitner from the Geological Institute at ETH Zurich has proven for the first time that the migration of the tropical rain belt is quite sensitive to even small changes in global temperatures.

The team’s findings have been published in the journal Scientific Reports, where they present the most comprehensive reconstruction of rainfall patterns within the Intertropical Convergence Zone for the past 2000 years.

Lower temperatures worldwide

In the past, scientists have only studied the migration of the tropical rain belt over very long timespans, such as glacial and interglacial cycles over tens of thousands of years, with correspondingly sizeable temperature differences of several degrees.

“So far, however, scientists have not investigated the past two millennia on a global scale, when temperature changes have been far less pronounced,” explains the climate geologist.

The ETH researcher and her colleagues have now managed to demonstrate how the tropical weather system shifted a good way south between 1450 and 1850, a period known as the Little Ice Age. “This migration is linked to the lower global temperatures during this time,” explains Lechleitner.

The latest climate reconstructions show that the average temperatures during this period were around 0.4 degrees Celsius lower than before and after the Little Ice Age. The migration of the tropical rain belt also caused substantial changes in the tropical and subtropical climate during this time, affecting the areas of drought and heavy rainfall.

Link to Europe’s weather system

The scientists also found that the climates in the tropical rain belt and the mid-latitudes are interconnected through the North Atlantic Oscillation (NAO). This phenomenon dictates the weather in western and central Europe, and is the result of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high over the North Atlantic.

The NAO is a numeric index: if it is positive, both the Icelandic low and the Azores high are very strong, which generally causes wet weather and strong westerly winds in central Europe, or in extreme cases winter storms and hurricanes, such as the violent Lothar storm in 1999.

However, the NAO index is negative when both the Icelandic low and Azores high are only weak. The west winds die down and shift sideways. This creates damper weather conditions in the Mediterranean region. Central Europe, on the other hand, is more prone to cold air masses sweeping in from the northeast, which can cause icy winters and dry springs.

Continued here.