The Westerlies or the Prevailing Westerlies are the prevailing winds in
the middle latitudes between 35 and 65 degrees latitude. These prevailing
winds blow from the west to the east,[35][36] and steer extratropical
cyclones in this general manner. The winds are predominantly from the
southwest in the Northern Hemisphere and from the northwest in the
Southern Hemisphere.[28] They are strongest in the winter when the
pressure is lower over the poles, and weakest during the summer and when
pressures are higher over the poles.[37]
Together with the trade winds, the westerlies enabled a round-trip trade
route for sailing ships crossing the Atlantic and Pacific Oceans, as the
westerlies lead to the development of strong ocean currents on the western
sides of oceans in both hemispheres through the process of western
intensification.[38] These western ocean currents transport warm,
sub-tropical water polewards toward the polar regions. The westerlies can
be particularly strong, especially in the southern hemisphere, where there
is less land in the middle latitudes to cause the flow pattern to amplify,
which slows the winds down. The strongest westerly winds in the middle
latitudes are within a band known as the Roaring Forties, between 40 and
50 degrees latitude south of the equator.[39] The Westerlies play an
important role in carrying the warm, equatorial waters and winds to the
western coasts of continents,[40][41] especially in the southern
hemisphere because of its vast oceanic expanse.