These are the original geostrophic adjustment papers. Rossby starts out by asking what,if anything, limits the ability for a flow to be in geostrophic balance (to precisely follow pressure contours on a rotating planet). This topic is now a classroom standard, but we often forget how insightful the original articles are...
Rossby, C.-G.. 2020. "Editor's Commentary: On the mutual adjustment of pressure and velocity distributions in certain simple current systems." Journal of Marine Research 78, (1). https://elischolar.library.yale.edu/journal_of_marine_research/482