The Rodwell–Hoskins mechanism is a hypothesis describing a climatic teleconnection between the Indian/Asian summer monsoon and the climate of the Mediterranean. It was formulated in 1996 by Brian Hoskins and Mark J. Rodwell [d]. The hypothesis stipulates that ascending air in the monsoon region induces atmospheric circulation features named Rossby waves that expand westward and interact with the mean westerly winds of the midlatitudes, eventually inducing descent of the air. Descending air warms and its humidity decreases, thus resulting in a drier climate during the summer months. The interaction of this atmospheric flow with topography (e.g the Atlas and Zagros mountains) further modifies the effect. This hypothesis has been advanced to explain the dry climate of the eastern Mediterranean during the summer months, as other explanations involving the Hadley circulation are not plausible during that season. Together with sea and land surface feedbacks, it may also explain the existence of deserts and subtropical anticyclones elsewhere in the world, as well as changes in the Mediterranean climate that coincide with variations of the monsoon.
Theory The existence of the subtropical anticyclones and of deserts in the subtropics used to be attributed to the descent of air in the Hadley cell, which reduces its humidity. This descent occurs as the air cools through radiation and the energy loss is balanced by adiabatic heating. The lack of vegetation due to the dryness increases the albedo of the ground and thus the cooling, acting as a positive feedback. Air-sea interactions, in particular the upwelling of cold waters east of the subtropical anticyclones induced by their winds and the resulting impediment to convection performs the same role over the oceans. However, the Hadley cell is weak during the summer months when the anticyclones still exist and dryness often reaches its peak in the deserts. In 1996, Mark J. Rodwell and Brian J. Hoskins proposed that instead, a Gill-type response to the Asian monsoon induces a Rossby wave response to the west which triggers descent west of the monsoon. This involves similar processes as the Hadley cell theory, but east–west horizontal advection modifies the energy balance, focusing descent at certain longitudes, unlike in equatorial regions where horizontal advection is less important. The descending air does not originate in the monsoonal regions, thus it is not a Walker circulation; rather, it originates in the mid-latitude westerlies and descends along atmospheric isentropes. In the Rodwell and Hoskins 1996 simulation, the location of the descent is controlled by orography just west of the descending region; heating over the topography induces anticyclonic (clockwise) flow and thus southward movement of cold air to its east, although the direction of the mean wind modulates the longitude direction of the forcing. The Etesian winds over Greece can be interpreted as the southward flow linked to the Rossby wave. In 2019, Ossó et al. showed that coupled sea surface temperature responses are important in inducing the descent west of the eastern Mediterranean as otherwise the Indian monsoon is located too far south to induce the Rossby wave response. Later research has indicated that the Rodwell–Hoskins mechanism can be induced by monsoons other than the Indian monsoon. For example, the South American monsoon may induce subsidence in the Southeastern Pacific and on the western slope of the Andes, and the North Pacific High may be a product of the North American monsoon. The subtropical anticyclones are subsequently strengthened by cooling over the oceans and cloud feedbacks, and according to Miyasaka and Nakamura 2005 by solar (sensible) heating of the dry landmass under the descent region. Thus, the Rodwell–Hoskins mechanism may play a role over most of the global subtropics and tropics, especially over the Mediterranean where the subtropical anticyclones are less influential than in other Mediterranean climates. It may play a lesser role in Southern Hemisphere anticyclone dynamics according to Seager et al. 2003, and only a minor role in intensifying the North Pacific High. According to Kelly and Mapes 2013, in the Community Atmosphere Model a strong Asian monsoon can extend the Rodwell–Hoskins mechanism to North America, resulting in drying of the western Atlantic. Smaller scale features such as the Thar Desert may also result from this mechanism.
Evidence In June to August, ascent occurs over Africa and Asia, with centres over the northern Bay of Bengal and equatorial Africa. Descent occurs to the west of the Asian monsoon, that is over the Kyzylkum Desert, the eastern Mediterranean including southeastern Europe and eastern Sahara, and in the eastern Atlantic. The longitude of the descent is connected to the underlying orography of the Zagros and Atlas Mountains, and summer precipitation is negligible in the descent areas. The Rodwell–Hoskins mechanism appears to be less important for the Arabian Desert, where the cooling through radiation and subsequent descent may instead be the key factor. The "monsoon-desert" mechanism has been identified both in climate reanalyses, idealized simulations and climate models. Other phenomena linked to the Rodwell–Hoskins mechanism are:
Oxygen isotope variations in corals of the northern Red Sea appear to correlate with the intensity of the Indian Monsoon. Near East atmospheric circulation changes are synchronous with the monsoon. After the onset of the Indian monsoon, dry air intrusions cause fluctuations in the intensity of the African monsoon, including a temporary weakening of precipitation.
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