Easterlies Winds

Winds are defined by their origins. The “easterly” descriptor refers to winds with an easterly zonal component (coming from the east). These include northeasterly and southeasterly winds. Easterly winds occur at all atmospheric scales, including local and synoptic. However, the term easterly winds generally refers to large-scale belts of winds operating within the global circulation of the atmosphere.

In the atmosphere’s general circulation, two distinct bands of easterly winds exist: the trade winds and the polar easterlies. They are found at low and high latitudes, respectively, and arise from the dynamics of air flow among pressure systems.

• Trade Winds

Among the most consistent winds on Earth, the trade winds (or trades) are part of the Hadley cell circulation found from approximately 0 degrees to 30 degrees north and south of the equator. Air rises near the equator as a result of a combination of convection spurred by intense solar radiation and the low-level convergence of wind in a circumpolar zone called the Intertropical Convergence Zone. As the rising air approaches the tropopause, it turns. At approximately 30 degrees, the air subsides, or sinks, resulting in the belt of persistent subtropical highs. Air diverges out of these anticyclones and flows toward the equatorial low. This outflow of air gives rise to the trades. As the air moves toward the equatorial low, it is deflected as a consequence of the Coriolis force (or effect). This deflection results in northeasterly trade winds in the Northern Hemisphere and southeasterly trades in the Southern Hemisphere. The trade winds are also referred to as tropical easterlies, particularly when the associated vertical wind shear is large.

• Polar Easterlies

The polar easterlies, a belt of winds found from approximately 60 to 90 degrees in each hemisphere,

constitute another component of the general circulation. These winds originate from dynamics similar to those of the Hadley cell. Thermally driven high pressure exists at the poles; similarly, a circumpolar zone of low pressure is found near 60 degrees. As air flows from the polar high to the subpolar low (as a result of the presence of a pressure gradient), it is deflected. This deflection leads to a belt of easterly winds in both Northern and Southern Hemispheres .

Rising air over the western tropical Pacific, with sinking air over the eastern tropical Pacific, comprises the Walker air current. The trades, as part of the Walker circulation, push warm surface waters toward the west. Any weakening of the trades would disrupt this oceanic transport of water. This dynamic is similar to an El Niño, in which the trade winds slow and even break down. Recent

research suggests that climate changes, specifically global warming, would weaken or slow the trade winds. The Walker circulation has already diminished by 3.5 percent since the 1800s. This slowing is projected to continue, and much of this change is attributed to anthropogenic activity.

The culprit in the slowing trade winds is the balance between evaporation and precipitation. To maintain a balance, the atmospheric absorption of moisture must be balanced by its release by precipitation. Water vapor, transported west by the trade winds, is precipitated out over the western Pacific. Warmer temperatures, then, spur the absorption of additional water vapor by enhancing evaporation rates. However, precipitation rates increase more slowly. To balance these processes, wind flow must decrease.

Weakened trade winds would result in numerous consequences, including the disruption of normal weather and climate patterns, as well as suppressed oceanic upwelling and potentially reduced biological productivity. The latter would have important economic ramifications, particularly for the Pacific fishing industries.