Easterlies are winds that blow from east to west, and they dominate two critical belts of Earth’s atmosphere: the tropics and the polar regions. In the tropics, these winds are better known as the trade winds, the remarkably steady breezes that carried sailing ships across oceans for centuries and still shape global weather patterns today. Near the poles, a colder and less consistent version of easterly flow hugs the surface, driven by dense air spilling off ice caps. Between these two easterly zones sit the mid-latitude westerlies, which blow in the opposite direction, making Earth’s wind map a kind of layered sandwich with easterlies on both the equatorial and polar ends.
Tropical Easterlies and the Trade Winds
The trade winds are the most famous easterlies on Earth. They blow persistently from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere, converging near the equator in a band called the Intertropical Convergence Zone (ITCZ). The basic driver is straightforward: the sun heats the equatorial region more intensely than anywhere else, causing air to rise. Cooler air flows in from the subtropics to replace it. Because Earth rotates, that incoming air gets deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, turning what would otherwise be a simple north-south breeze into a diagonal, east-to-west flow.
Where the northerly and southerly trade winds collide over warm ocean water, the air rises vigorously, producing the deep convection and towering thunderstorms that define the ITCZ.1Geophysical Research Letters. Double intertropical convergence zones—a new look using scatterometer This convergence zone shifts north and south with the seasons, following the sun’s most direct angle. When it migrates into the Northern Hemisphere during boreal summer, it drags monsoon rains with it across South Asia and West Africa. When it slides south, those regions dry out. The trade winds on either side of the ITCZ are among the most reliable winds on the planet, which is why European sailors relied on them for transatlantic voyages starting in the fifteenth century.
The steadiness of the trade winds also has a less romantic function: they push enormous volumes of warm surface water westward across the tropical Pacific and Atlantic. This piling up of warm water on the western side of ocean basins is a key ingredient in phenomena like El Niño, which we will get to shortly.
Polar Easterlies
At the other end of the globe, easterlies appear again, but with a very different character. Near both poles, extremely cold and dense air sinks from the upper atmosphere toward the surface, then spreads outward. Earth’s rotation deflects this outward-spreading air, creating surface winds that blow roughly from east to west. In the Arctic, these polar easterlies are relatively weak and inconsistent, frequently disrupted by storm systems pushing in from the mid-latitudes. In Antarctica, the picture is more dramatic.
Antarctica’s ice sheet sits on an elevated plateau, and the cold, heavy air that pools over it flows downhill toward the coast under gravity. These katabatic winds can be ferocious, reaching extreme speeds in coastal valleys. Research into Antarctic wind dynamics has revealed that the balance of forces driving these events is surprisingly complex onshore, where katabatic forcing is large in magnitude but actually plays a somewhat passive role, declining as wind speeds increase during the most extreme conditions. Offshore, the balance is simpler, dominated by large-scale atmospheric pressure patterns.2Quarterly Journal of the Royal Meteorological Society. Dynamics of extreme wind events in the marine and terrestrial sectors of coastal Antarctica In other words, the worst Antarctic coastal wind storms are driven less by the downhill drainage of cold air and more by broad synoptic weather systems, even though katabatic flow contributes most of the time.
Polar easterlies also influence sea ice. Around Wrangel Island in the western Chukchi Sea, the prevailing easterly ice motion driven by the Beaufort Gyre pushes sea ice against the island’s eastern shore, where it accumulates, while the western side is left with thinner ice and more open water. This creates a persistent ice-thickness difference across the island that peaks around January, when the eastern side can be roughly a meter thicker than the western side.3Journal of Geophysical Research: Oceans. An Examination of the Wrangel Island Sea Ice Thickness Dipole Small islands and coastlines throughout the Arctic experience similar asymmetries wherever easterly ice drift encounters solid obstacles.
African Easterly Waves and Atlantic Hurricanes
One of the most consequential offspring of tropical easterly flow is the African easterly wave, a ripple in the atmosphere that forms over the heated terrain of North Africa and then propagates westward out over the Atlantic. These waves are embedded in the mid-level easterly flow associated with the African easterly jet, a wind maximum that develops because of the sharp temperature contrast between the Sahara Desert and the cooler, wetter lands to its south. The waves themselves are disturbances within this jet, and they serve as seedlings for Atlantic tropical cyclones.
About 61% of Atlantic tropical cyclones originate directly from African easterly waves, and an additional 11% are indirectly linked to them, making these waves the primary precursor for Atlantic hurricanes.4Geophysical Research Letters. Revisiting the connection between African Easterly Waves and Atlantic tropical cyclogenesis Not every wave becomes a hurricane, of course. Whether a given wave develops depends on sea surface temperatures, wind shear, and the wave’s own internal structure. Research spanning over four decades of data has found a statistically significant relationship between the strength of the wave at genesis and where the resulting storm forms. Weaker waves tend to develop into tropical cyclones closer to the Americas over warmer waters, while stronger waves spawn storms closer to Africa. Because weaker waves produce storms nearer to land, those storms are actually more likely to make landfall.5Journal of Geophysical Research: Atmospheres. African Easterly Wave Strength and Observed Atlantic Tropical Cyclone Genesis and Characteristics
There is also a long-term trend worth noting. Over roughly the past four decades, Atlantic tropical cyclone activity has been increasing, and this trend appears to be primarily driven by an increase in storms with African easterly wave origins. The waves themselves have been growing more frequent and stronger, and anthropogenic aerosols may be playing a role in driving that change.6Quarterly Journal of the Royal Meteorological Society. Characteristics and trends of Atlantic tropical cyclones that do and do not develop from African easterly waves This is an area of active research, and the exact mechanisms linking aerosol pollution, wave strength, and hurricane formation are still being untangled.
Saharan Dust Rides the Easterlies
Hurricanes are not the only thing the tropical easterlies carry across the Atlantic. Every summer, trade winds sweep massive quantities of Saharan mineral dust westward from North Africa, typically between May and September.7E3S Web of Conferences. Using trace metals to quantify long-range transported Saharan dust in Houston, Texas This dust travels in an elevated layer of warm, dry air known as the Saharan Air Layer, which can extend from the surface to altitudes above three kilometers. The transport is facilitated by the African easterly jet at mid-levels and, on the receiving end, by the Caribbean low-level jet, both of which can intensify under certain atmospheric configurations.
A study of the extreme Saharan dust storm of 2015 found that both the African easterly jet and the Caribbean low-level jet had greatly intensified during the event, along with a westward extension of the North Atlantic subtropical high, all of which favored pushing dust deep into the western Atlantic and Caribbean.8Atmospheric Chemistry and Physics. The emission, transport, and impacts of the extreme Saharan dust storm of 2015 The consequences of this dust transport are wide-ranging. On the negative side, dust plumes degrade air quality across the Caribbean and the southeastern United States, sometimes triggering respiratory health warnings. On the positive side, Saharan dust is a major source of iron and phosphorus for the Amazon rainforest and Atlantic marine ecosystems. The minerals fertilize nutrient-poor tropical soils and ocean waters that would otherwise be too depleted to support the productivity they currently do.
Interestingly, the Saharan Air Layer also suppresses hurricane formation. The layer’s warm, dry air and the wind shear it introduces can choke off the moist convection that developing tropical storms need. So the same easterly flow that creates hurricane seedlings through African easterly waves simultaneously carries a dust layer that can kill some of those seedlings before they mature. The net effect in any given hurricane season depends on the timing and intensity of both processes.
The Tropical Easterly Jet and the Indian Monsoon
The easterlies in the tropics are not confined to the lower atmosphere. At altitudes above roughly 12 kilometers, a strong upper-level easterly wind known as the Tropical Easterly Jet (TEJ) develops during the Northern Hemisphere summer. The TEJ is one of the defining features of the Indian Summer Monsoon system, which is itself characterized by a seasonal reversal of winds at lower levels and the presence of this easterly jet aloft.9Geophysical Research Letters. Is the trend in TEJ reversing over the Indian subcontinent? The jet stretches from Southeast Asia across the Indian Ocean and into Africa, and its strength has long been used as an indicator of monsoon vigor. When the TEJ is strong, the monsoon tends to deliver abundant rainfall to the Indian subcontinent. When it weakens, drought years follow more often.
The TEJ exists because of the temperature contrast between the Tibetan Plateau, which heats up enormously in summer, and the cooler air over the Indian Ocean to its south. This temperature gradient drives an upper-level easterly flow that can reach speeds of over 40 meters per second. Changes to this jet’s behavior have implications not just for Indian agriculture, which supports over a billion people, but for rainfall across East Africa as well, since the jet’s western end influences weather systems there.
Trade Winds, El Niño, and Climate Variability
The tropical easterlies are intimately tied to Earth’s most important short-term climate oscillation: the El Niño-Southern Oscillation, or ENSO. Under normal conditions, the trade winds push warm surface water westward across the tropical Pacific, piling it up near Indonesia and Australia. This allows cold, nutrient-rich water to upwell along the coast of South America, feeding productive fisheries. During an El Niño event, the trade winds weaken or even reverse, warm water sloshes back eastward, and the normal pattern of rainfall and temperature shifts dramatically across the Pacific and beyond.
In recent decades, scientists have noticed a shift in ENSO behavior. The early twenty-first century has seen more Central Pacific El Niño events and a strengthening of Pacific trade winds that was not predicted by most climate models.10Geophysical Research Letters. The Effect of Indian Ocean Temperature on the Pacific Trade Winds and ENSO Indian Ocean warming appears to be one factor reinforcing the trade winds and favoring Central Pacific events over the classical Eastern Pacific variety. This has practical consequences: Central Pacific El Niño events tend to have different rainfall and temperature impacts around the world than the Eastern Pacific type, affecting seasonal forecasts for agriculture, water management, and disaster planning.
The puzzle deepens when you look at what climate models project for the future. Under scenarios of rising greenhouse gas concentrations, most models predict a weakening of the Pacific Walker Circulation, the overturning atmospheric loop that the trade winds are part of. Yet over the past four decades, the observed trend has been a strengthening of this circulation, running counter to model expectations. Analysis of state-of-the-art climate models has shown that they are virtually incapable of reproducing the observed strengthening trend during that period.11Communications Earth & Environment. Strengthening atmospheric circulation and trade winds slowed tropical Pacific surface warming This mismatch is one of the more stubborn problems in climate science. Strengthened trade winds during this period have actually slowed the rate of surface warming in the tropical Pacific, acting as a partial brake on global temperature rise. Whether this strengthening will persist or eventually give way to the model-predicted weakening remains an open question with large implications for climate projections.
Easterlies in the Stratosphere
Easterly winds also play a starring role high above the surface in the stratosphere. Near the equator, stratospheric winds alternate between easterly and westerly phases in a cycle known as the Quasi-Biennial Oscillation, or QBO. This oscillation has a period of roughly 28 months and descends through the stratosphere over time, with easterly winds forming at the top and propagating downward to be replaced by westerlies, and vice versa. Analysis of long-term atmospheric data has revealed that the QBO actually has a threefold vertical structure rather than a simple two-layer alternation: in addition to the well-known pattern of easterlies overlying westerlies (or the reverse) in the lower and middle stratosphere, there is a third anomaly in the upper stratosphere whose sign matches the lower stratospheric layer, creating an alternating sandwich of wind regimes through the full depth of the equatorial stratosphere.12Journal of Geophysical Research: Atmospheres. The quasi‐biennial oscillation: Analysis using ERA‐40 data
The QBO matters for surface weather more than you might expect. Its phase influences the strength of the polar vortex, which in turn affects winter weather in the mid-latitudes. An easterly QBO phase tends to weaken the polar vortex, making sudden stratospheric warming events more likely and increasing the odds of cold air outbreaks over Europe and North America. It also affects where tropical cyclones form and how much ozone is transported toward the poles.
Under sustained global warming, however, the QBO may not survive in its current form. Climate model projections suggest that under high-emissions scenarios, the QBO could disappear entirely by around the year 2150, though the stratosphere would continue to exhibit alternating easterly and westerly wind phases, just with a significantly shorter period than the current oscillation.13Nature Communications. The disappearing quasi-biennial oscillation under sustained global warming The loss of this regular oscillation could reshape stratospheric dynamics and the way the stratosphere communicates with surface weather.
Shifting Wind Belts Over Geological Time
The boundaries between easterly and westerly wind zones have not always been where they are today. Over hundreds of thousands of years, these belts expand, contract, and shift latitude in response to changes in ice sheet extent, ocean temperatures, and orbital variations. One way scientists reconstruct past wind patterns is by studying the mineral dust deposited in deep-ocean sediments. Cores drilled from the North Pacific seafloor beneath the modern trade winds and beneath the westerlies have been used to evaluate changes in atmospheric circulation intensity and source-area aridity over the past 700,000 years, tracking variations in the accumulation rate and grain size of wind-blown dust.14Quaternary Research. Quaternary fluctuations in the Northern Hemisphere trade winds and westerlies During ice ages, when temperature gradients between the tropics and poles steepened, both trade winds and westerlies intensified, carrying more and coarser dust.
Even more dramatic shifts have occurred over deeper geological time. During the Miocene epoch, roughly 10 million years ago, Europe’s atmospheric circulation underwent a fundamental transition. Reconstructions of summer precipitation across Tortonian-age Europe show a zonal pattern, with rainfall decreasing from west to east, consistent with dominant westerly moisture transport. This was a departure from earlier periods when trade wind influences extended farther poleward over what is now southern Europe.15Scientific Reports. Miocene shift of European atmospheric circulation from trade wind to westerlies The shift was driven by tectonic changes and the evolving temperature structure of the oceans, which reshaped the subtropical high-pressure systems that define where trade winds end and westerlies begin.
Easterly Winds and Energy Resources
The reliability of tropical easterlies has attracted interest from the renewable energy sector, particularly for islands and coastal regions where trade winds blow almost year-round. On islands exposed to trade winds, the wind resource tends to be dominated by winds from the northeast or east-northeast, with velocities commonly falling in the range of 8 to 14 meters per second, a sweet spot for modern wind turbines.16Elsevier. Wave and offshore wind energy on an island Trade-wind regions offer a significant advantage over many mid-latitude sites: the wind blows from a highly consistent direction and with relatively little variability from day to day, reducing the intermittency problem that plagues wind energy in other locations.
Islands in the Caribbean, the Canary Islands, Hawaii, and parts of the western Pacific all sit within the trade wind belt and have explored or developed wind energy capacity that leverages this consistency. The challenge is that trade-wind zones are often remote, making grid connection and infrastructure costly. Offshore wind farms in these regions must also be engineered to withstand the hurricane risk that comes with the same easterly flow. Still, the capacity factor of turbines in steady trade-wind environments can be impressively high compared to sites where wind speed and direction are less predictable.
The story of understanding easterlies stretches back centuries. In 1735, George Hadley published a paper in the Philosophical Transactions of the Royal Society proposing that the trade winds were caused by the combination of equatorial heating and Earth’s rotation. It took about a hundred years for the scientific community to fully accept his ideas.17History of Meteorology. Hadley’s Principle: Understanding and Misunderstanding the Trade Winds Even now, the global circulation cell that bears his name, the Hadley cell, remains the foundation for how we understand the trade winds. What Hadley could not have anticipated is how many systems those winds connect: hurricanes bred from African easterly waves, dust fertilizing the Amazon, monsoon rains feeding a continent, and stratospheric oscillations reaching down to influence winter cold snaps thousands of kilometers from the equator. The easterlies tie all of these together in a single planetary-scale flow.