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Rodwell–Hoskins mechanism is a hypothesis about a climatic teleconnection between the Indian/Asian summer monsoon and the climate of the Mediterranean. It stipulates that ascending air in the monsoon region induces atmospheric circulation features named
Rossby wave Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby. They are observed in the atmospheres and ...
s 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 the topography such as the
Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...
and
Zagros mountains The Zagros Mountains ( ar, جبال زاغروس, translit=Jibal Zaghrus; fa, کوه‌های زاگرس, Kuh hā-ye Zāgros; ku, چیاکانی زاگرۆس, translit=Çiyakani Zagros; Turkish: ''Zagros Dağları''; Luri: ''Kuh hā-ye Zāgro ...
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
subtropic The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° nort ...
s used to be attributed to the descent of air in the Hadley cell, which reduces its
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
. This descent occurs because the air is cooling through radiation and the energy loss is balanced by adiabatic heating. The lack of vegetation due to the dryness increases the
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
of the ground and thus the cooling, acting as a
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
. Air-sea coupling, in particular the
upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...
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, and the latitudes of the
Sahara , photo = Sahara real color.jpg , photo_caption = The Sahara taken by Apollo 17 astronauts, 1972 , map = , map_image = , location = , country = , country1 = , ...
coincide with these of wet climates.
Mark J. Rodwell Mark may refer to: Currency * Bosnia and Herzegovina convertible mark, the currency of Bosnia and Herzegovina * East German mark, the currency of the German Democratic Republic * Estonian mark, the currency of Estonia between 1918 and 1927 * Finn ...
and
Brian J. Hoskins Professor Sir Brian John Hoskins, Commander of the Order of the British Empire, CBE Fellow of the Royal Society, FRS, (born 17 May 1945) is a British atmospheric dynamics, dynamical meteorologist and climatologist based at the Imperial College L ...
in 1996 proposed that instead, a Gill-type response to the
Asian monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscil ...
induces a
Rossby wave Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby. They are observed in the atmospheres and ...
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 In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is a ...
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 The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere). According to this model, parcels of air follow a closed circulation in the zonal and vertical dir ...
; rather it originates in the mid-latitude
westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend t ...
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, which induce anticyclonic (clockwise) flow and thus southward movement of cold air to their east through the heating over the topography, 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. Ossó ''et al.'' 2019 showed that coupled
sea surface temperature Sea surface temperature (SST), or ocean surface temperature, is the ocean temperature close to the surface. The exact meaning of ''surface'' varies according to the measurement method used, but it is between and below the sea surface. Air ma ...
responses are important in inducing the descent west of the eastern Mediterranean as otherwise the Indian monsoon is located too far south to induce a Rossby wave train. 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 the
North Pacific High The North Pacific High is a semi-permanent, subtropical anticyclone located in the northeastern portion of the Pacific Ocean, located northeast of Hawaii and west of California. It is strongest during the northern hemisphere summer and shifts towa ...
may be a product of the
North American monsoon The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern Uni ...
. 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 climate A Mediterranean climate (also called a dry summer temperate climate ''Cs'') is a temperate climate sub-type, generally characterized by warm, dry summers and mild, fairly wet winters; these weather conditions are typically experienced in the ...
s. 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 The Thar Desert, also known as the Great Indian Desert, is an arid region in the north-western part of the Subcontinent that covers an area of and forms a natural boundary between India and Pakistan. It is the world's 20th-largest desert, a ...
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 The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line bet ...
and equatorial Africa. Descent occurs to the west of the Asian monsoon, that is over the
Kyzylkum Desert The Kyzylkum Desert ( uz, Qizilqum, Қизилқум, قىزىلقۇم; kk, Қызылқұм, Qyzylqūm, قىزىلقۇم) is the 15th largest desert in the world. Its name means ''Red Sand'' in Turkic languages. It is located in Central Asia, ...
, 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 The Zagros Mountains ( ar, جبال زاغروس, translit=Jibal Zaghrus; fa, کوه‌های زاگرس, Kuh hā-ye Zāgros; ku, چیاکانی زاگرۆس, translit=Çiyakani Zagros; Turkish: ''Zagros Dağları''; Luri: ''Kuh hā-ye Zāgro ...
and
Atlas Mountains The Atlas Mountains are a mountain range in the Maghreb in North Africa. It separates the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean; the name "Atlantic" is derived from the mountain range. It stretches around through Moroc ...
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 model Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the ...
s. Other phenomena linked to the Rodwell–Hoskins mechanism are oxygen isotope variations in
coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secre ...
s of the northern
Red Sea The Red Sea ( ar, البحر الأحمر - بحر القلزم, translit=Modern: al-Baḥr al-ʾAḥmar, Medieval: Baḥr al-Qulzum; or ; Coptic: ⲫⲓⲟⲙ ⲛ̀ϩⲁϩ ''Phiom Enhah'' or ⲫⲓⲟⲙ ⲛ̀ϣⲁⲣⲓ ''Phiom ǹšari''; ...
which appear to correlate with the intensity of the Indian Monsoon, the synchrony of Near East atmospheric circulation changes with the monsoon and fluctuations in the intensity of the African monsoon after the onset of the Indian one which are mediated by dry air intrusions and include a temporary weakening of precipitation.


Implications

The intensity of the Rodwell–Hoskins mechanism is a function of the latitude of the monsoon; ascent close to the equator (such as during the pre-monsoon season) does not induce it effectively. According to Rodwell and Hoskins 2001, the African monsoon being a tropical monsoon does not induce a substantial Rodwell–Hoskins effect. The air forced to descend by the Rodwell–Hoskins mechanism can in turn flow into the convergence zones of the monsoon region and alter the monsoon behaviour; this is known as the "interactive Rodwell–Hoskins mechanism" and it reduces the precipitation in the western sector of a monsoon by importing dry/low energy air into the monsoon region. It appears to play a role in restricting the westward extent of the
North American monsoon The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern Uni ...
, inducing dryness along the West Coast, and likewise in limiting the southward extent of the South American monsoon. Enomoto 2003 recognized that the descent forced by the Rodwell–Hoskins mechanism over the Mediterranean and
Aral Sea The Aral Sea ( ; kk, Арал теңізі, Aral teñızı; uz, Орол денгизи, Orol dengizi; kaa, Арал теңизи, Aral teńizi; russian: Аральское море, Aral'skoye more) was an endorheic lake lying between Kazakh ...
coincide with the "inlet" region of the Asian jet stream and consequently Rossby waves could enter the jet stream through these regions ("Silk Road pattern"). Enomoto 2004 proposed that the Rossby waves travelling through this jet stream eventually accumulate over Japan and induce the formation of a second anticyclone there – a process they called the "Monsoon-Desert-Jet mechanism" and which they suggested may induce
heat wave A heat wave, or heatwave, is a period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary, a heat wave is usually measured relative to the usual climate in the ...
s in Japan. This structure of the atmospheric circulation has also been described by other researchers. Additionally, vorticity anomalies originating directly from the monsoon outflow may enter into the jet stream as Rossby waves. Di Capua ''et al.'' 2020 noted that
La Niña La Niña (; ) is an oceanic and atmospheric phenomenon that is the colder counterpart of as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name ''La Niña'' originates from Spanish for "the girl", by ...
in the Pacific Ocean boosts the monsoon-desert mechanism through a coupling with the Walker circulation. Wu and Shaw 2016 proposed that the Rodwell–Hoskins mechanism alters the
potential temperature The potential temperature of a parcel of fluid at pressure P is the temperature that the parcel would attain if adiabatically brought to a standard reference pressure P_, usually . The potential temperature is denoted \theta and, for a gas well-a ...
of the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...
by shifting its monsoon-related maxima.


Paleoclimate

The strength of the Rodwell–Hoskins-induced descent is a function of the strength of the monsoon and
climate variability Climate variability includes all the variations in the climate that last longer than individual weather events, whereas the term climate change only refers to those variations that persist for a longer period of time, typically decades or more ...
impacting monsoon intensity can thus alter the descent as well. Through the Rodwell–Hoskins mechanism the development of the
Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the T ...
during the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
to
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...
and its effect on the Indian monsoon may have had remote effects on Africa and the Mediterranean, and the same mechanism may be responsible for the drying of northwestern India between 11–7 million years ago. The climate effects of the Rodwell-Hoskins mechanism may have influenced oceanic climate during the existence and breakup of
Pangea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million ...
in the last 250 million years. During maxima in
precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
, a key
Milankovitch cycle Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hyp ...
, and minima in global ice area, an intensified Indian monsoon may have induced a more intense summer drought over the eastern Mediterranean, although increased autumn/winter rainfall may have negated the drying. An anticorrelation between wetter Northern Hemisphere monsoons during the
Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togeth ...
and drier subtropics may also be explained by the Rodwell–Hoskins effect, as is drying in
Oman Oman ( ; ar, عُمَان ' ), officially the Sultanate of Oman ( ar, سلْطنةُ عُمان ), is an Arabian country located in southwestern Asia. It is situated on the southeastern coast of the Arabian Peninsula, and spans the mouth of ...
and the Near East during the Medieval Climate Anomaly and the Early Holocene. Conversely, wetter subtropics during the
Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Teleconnections associated with the
Indian Ocean Dipole The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern ...
(IOD), in particular the development of Rossby wave trains. A positive IOD would tend to intensify the monsoon and the Rodwell–Hoskins mechanism while a negative IOD would tend to weaken them. * During years where the monsoon is unusually wet over
Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a U.S. state, state in the Southwestern United States. It is the list of U.S. states and territories by area, 6th largest and the list of U.S. states and territories by population, 14 ...
, the Great Plains, Midcontinent and
Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...
are unusually dry. * Monsoon breaks in India and the low level
temperature inversion In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. ...
over the
Arabian Sea The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channel ...
. * When the Indian monsoon is weaker, e.g. after the 1912 Mount Katmai eruption, cloudiness and precipitation increase over the Mediterranean. * Volcanic eruptions resulting in wetter climates over the dry regions. * The wet summers 2002 and 2014 in Southern Europe when the Indian monsoon was weaker than normal. * Bollasina and Nigam 2011 proposed that the subsidence northwest of the
Hindu Kush The Hindu Kush is an mountain range in Central and South Asia to the west of the Himalayas. It stretches from central and western Afghanistan, Quote: "The Hindu Kush mountains run along the Afghan border with the North-West Frontier Prov ...
interacts with the topography to produce the Indo-Pakistani
heat low Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments.Glossary of Meteorology (2009 ...
. * Yang 2021 proposed that the
Sahel drought The Sahel region of Africa has long experienced a series of historic droughts, dating back to at least the 17th century. The Sahel region is a climate zone sandwiched between the Sudanian Savanna to the south and the Sahara desert to the north, ...
stemmed from an intensified Rodwell–Hoskins descent over Africa and ended when warming Atlantic sea surface temperatures decreased the Africa-Asia temperature gradients. * Anticorrelations between
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabia (including the Arabian Peninsula and Bahrain), Asia Minor (Asian part of Turkey except Hatay Province), East Thrace (Europ ...
precipitation and the Asian monsoon. * Maximum temperatures occur in the northern Arabian Peninsula. * Dipole-like precipitation biases in certain climate models. * The development of tropical upper-tropospheric troughs over the Northern Hemisphere oceans in summer. * Increased precipitation in the eastern Mediterranean after
sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of ant ...
release. Atmospheric waves similar to these of the Rodwell–Hoskins mechanism are also found in climate simulations where the monsoons have been modified by vegetation changes induced through man-made
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
increases or increased condensational heating. They may play a role in altering European climate according to Douville ''et al.'' 2000 and Gregory, Mitchell and Brady 1997, such as causing drying in the Mediterranean. Studying the output of some climate models, Cherchi ''et al.'' 2016 found both increased descent and a westward shift of the descent in response to increased monsoon precipitation during the 21st century.


Alternative processes

Chen ''et al.'' 2001 proposed that upstream monsoon heating can induce
Rossby wave Rossby waves, also known as planetary waves, are a type of inertial wave naturally occurring in rotating fluids. They were first identified by Sweden-born American meteorologist Carl-Gustaf Arvid Rossby. They are observed in the atmospheres and ...
s that generate the subtropical
anticyclone An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from abo ...
s through an eastward-directed forcing, which acts in the opposite direction to the westward-directed Rodwell–Hoskins hypothesis. A Hadley cell-like interaction between the Mediterranean and the African monsoon intensifies Mediterranean drought. Eastward propagating
Kelvin wave A Kelvin wave is a wave in the ocean or atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., t ...
s can induce descent to the east of the monsoon region, which is important in inducing the
North Pacific High The North Pacific High is a semi-permanent, subtropical anticyclone located in the northeastern portion of the Pacific Ocean, located northeast of Hawaii and west of California. It is strongest during the northern hemisphere summer and shifts towa ...
in response to the Asian and the South Atlantic High in response of the South American monsoon and are also connected to the low-level inflow of moisture into the monsoons.


References


Sources

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