Quasi-geostrophic theory models large-scale atmospheric and oceanic flow as an approximation to geostrophic balance, with ageostrophic components representing departures from this balance; this is a foundational principle of geophysical fluid dynamics that does not require citation.
The claim states a foundational definition and textbook concept in geophysical fluid dynamics (quasi-geostrophic motion and ageostrophic deviations), which is common knowledge in atmospheric and oceanic sciences.