Great paper that I have found here which helps explains. http://www.reading.a...on_Driscoll.pdf Firstly torque definition: 2.2 Torque A Torque, deï¬ned as T, is given by T = r × F (2.5) If a torque, or a ‘turning force’, is applied to a rotating object it will slow down or speed up the object’s rate of rotation about the origin, or even change its direction of rotation. Through friction and pressure the atmosphere applies these rotational forces to the solid Earth which cause the rotation rate of the solid earth to either speed up or slow down, and in doing so the atmosphere’s angular momentum decreases or increases as it imparts or receives angular momentum to or from the earth, thus torques represent the flux of angular momentum across the surface of the earth. Mountain torques: Mountain Torque is a function of pressure and orography and is the ‘turning force’ exerted due to a difference in pressure across any raised surface on the earth, but most signiï¬cantly, mountains or mountain massifs. Consider a mountain with a high pressure on the west side of a mountain and low pressure on the east. The pressure system will exert an eastward torque that causes the earth to increase it’s rate of rotation, imparting angular momentum from the atmosphere to the solid earth. The opposite case, where there is higher pressure on the east side of the mountain, will slow the earth’s rotation down, reducing the solid earth’s angular momentum, and imparting it to the atmosphere. The first above is a positive MT as I understand it.