text: The largest ocean gyres are wind-driven, meaning that their locations and dynamics are controlled by the prevailing global wind patterns: easterlies at the tropics and westerlies at the midlatitudes. These wind patterns result in a wind stress curl that drives Ekman pumping in the subtropics (resulting in downwelling) and Ekman suction in subpolar regions (resulting in upwelling). Ekman pumping results in an increased sea surface height at the center of the gyre and anticyclonic geostrophic currents in subtropical gyres. Ekman suction results in a depressed sea surface height and cyclonic geostrophic currents in subpolar gyres.
  Wind-driven ocean gyres are asymmetrical, with stronger flows on their western boundary and weaker flows throughout their interior. The weak interior flow that is typical over most of the gyre is a result of the conservation of potential vorticity. In the shallow water equations (applicable for basin-scale flow as the horizontal length scale is much greater than the vertical length scale), potential vorticity is a function of relative (local) vorticity , planetary vorticity , and the depth , and is conserved with respect to the material derivative:
  In the case of the subtropical ocean gyre, Ekman pumping results in water piling up in the center of the gyre, compressing water parcels. This results in a decrease in , so by the conservation of potential vorticity the numerator  must also decrease. It can be further simplified by realizing that, in basin-scale ocean gyres, the relative vorticity  is small, meaning that local changes in vorticity cannot account for the decrease in . Thus, the water parcel must change its planetary vorticity  accordingly. The only way to decrease the planetary vorticity is by moving the water parcel equatorward, so throughout the majority of subtropical gyres there is a weak equatorward flow. Harald Sverdrup quantified this phenomenon in his 1947 paper, "Wind Driven Currents in a Baroclinic Ocean", in which the (depth-integrated) Sverdrup balance is defined as:
  Here,  is the meridional mass transport (positive north),  is the Rossby parameter,  is the water density, and  is the vertical Ekman velocity due to wind stress curl (positive up). It can be clearly seen in this equation that for a negative Ekman velocity (e.g., Ekman pumping in subtropical gyres), meridional mass transport (Sverdrup transport) is negative (south, equatorward) in the northern hemisphere (). Conversely, for a positive Ekman velocity (e.g., Ekman suction in subpolar gyres), Sverdrup transport is positive (north, poleward) in the northern hemisphere.