From AMS Glossary
In meteorology, this term usually refers to the effect of surface friction. The eastward component of the frictional torque is of primary interest because, together with the eastward component of the mountain torque, it constitutes the only significant mechanism by which the atmosphere can gain or lose absolute angular momentum about the earth's axis. This torque may be expressed by the integral
where a is the earth's radius, φ is the latitude, τx is the eastward component of the frictional force per unit area, and ds is an areal element of the earth's surface. The frictional torque serves to impart the earth's angular momentum to the air, thus decreasing or increasing the (relative) wind speed.