From Glossary of Meteorology
The westerly winds occasionally found in the equatorial trough and separated from the midlatitude westerlies by the broad belt of easterly trade winds.
As the air flow in the lower atmosphere is mostly easterly in and about the equatorial trough, the existence of westerlies on mean charts in some areas has been a subject of much interest and speculation. In some regions, this abnormality can be explained as the result of limited areas of west winds on the equatorward side of frequent westward moving cyclones in the equatorial trough. Elsewhere (notably over the Indian Ocean during the Northern Hemisphere summer), the equatorial westerlies may result from the deflection of Southern Hemisphere air as it flows northward across the geographical equator as part of the monsoon. Equatorial westerlies can also be induced on the western side of a large scale localized heating near the equator, such as in the maritime continent.
Palmer, C. E. 1951. Compendium of Meteorology. p. 876.
Riehl, H. 1954. Tropical Meteorology. 3–4.