The Pangean megamonsoon refers to the paleoclimatological hypothesis that the ancient supercontinent Pangaea had experienced a distinct seasonal reversal of winds (monsoons), which resulted in extreme transitions between dry and wet periods throughout the year. Pangaea was a conglomeration of all the global continental land masses, which lasted from the late Carboniferous to the mid-Jurassic. The megamonsoon intensified as the continents continued to shift toward one another and reached its maximum strength in the Triassic, when the continental surface area of Pangaea was at its peak. The megamonsoon would have led to immensely arid regions with extremely hot days and frigid nights around the interior of the supercontinent, making those areas nearly uninhabitable to terrestrial ecosystems. The coastal regions experienced seasonality, however, and transitioned from rainy weather in the summer to dry conditions during the winter.
Monsoon circulation Monsoon circulations, defined as a seasonal reversal of winds, exhibit large shifts in precipitation patterns across the impacted region. Monsoons are therefore characterized by two primary seasons: rainy and dry. They are induced by the presence of at least one large land mass and large body of water in close proximity to each other. The most commonly studied present-day monsoon circulation is the East Asian Monsoon.
Discovery The concept of a Pangean monsoon circulation was first proposed in 1973. The evaporites in the geologic record suggest vast and extensive regions of persistent dry conditions near the Pangean centre, serving as the initial evidence for the theory’s dissemination. The interior of the supercontinent, especially the eastern portion, would have been extremely dry as the hemispheric pressure systems driving the circulation would have diverted nearly all atmospheric moisture away from the region. The later indication of a monsoon-driven climate was acquired via the examination of coal deposits along the exterior portions of the continent. The presence of both features in the geologic record suggest monsoonal circulations. As the theory of the Pangean megamonsoon began to increase in credibility, paleoclimatologists predicted the climatological impacts of the circulation to ascertain whether observations and models supported the hypothesis. The general consensus listed four primary signs that needed to be present to validate the existence of megamonsoon.
The lithologic indicators of seasonality should span broad distances along the Pangean coasts. Evidence depicting a deviation from zonal flow regimes needed to be identified. Records should indicate that the equatorial regions of Pangea would have been plagued by persistent aridity. Models and geologic observations would need to demonstrate that this circulation peaked during the Triassic.
Monsoon climate on Pangea In the Northern Hemisphere's summer, when Earth’s axial tilt was directed toward the sun, Laurasia would have received the most direct solar insolation, which would have yielded a broad area of warm, rising air and low surface pressure over the continent. Models have suggested that this seasonal low was positioned at 35° latitude, relatively near the Tethys Ocean. In Gondwana, high pressure would have dominated, as the land would have been receiving less solar radiation and therefore experiencing cooler temperatures. The pressure-gradient force dictates that air will travel from regions of high to low pressure. That would have driven the atmospheric flow from the Southern Hemisphere toward Laurasia during which it would cross the Tethys Ocean. Water from the Tethys would evaporate into the air mass. Eventually, the air mass would reach the coast of Laurasia and resulted in immense amounts of precipitation. Models estimate the globally-averaged precipitation to equal roughly 1,000 mm per year, with coastal regions receiving upwards of 8 mm of rain each day during the rainy season. As the atmospheric flow was directed away from the Gondwana high pressure system, surface winds would have diverged, producing clear and very dry conditions across the Southern Hemisphere. Several studies have indicated that the circulation was so intense during the Triassic, it would have been capable of reversing part of the predominantly-easterly global wind flow and so westerly winds impacted the western coast. That worked to maximize surface convergence and increased seasonality along the western coasts of each continent. During the Northern Hemisphere winter, when Earth’s tilt was directed away from the Sun, the circulation reversed as the area of maximum solar insolation shifted toward the Southern Hemisphere. Air then traveled from Laurasia (region of high pressure), across the Tethys Ocean to Gondwana (region of low pressure). Moisture advection toward the Southern Hemisphere would have fueled heavy precipitation along the Gondwana coasts, while Laurasia remained very dry.
… excerpt ends here. Continue reading the full article.
