The relationship between thermal gradients, atmospheric circulation, and sea-level air pressure variations is a fundamental, well-established principle of meteorology that requires no formal citation.
The claim states that thermal gradients and atmospheric circulation cause spatial variations in sea-level air pressure. This is a basic, universally accepted physical principle of meteorology and atmospheric science, taught as foundational knowledge (differential heating drives pressure gradients, which in turn drive winds and circulation). Because it is a foundational textbook fact of everyday scientific observation and definition, it falls under the COMMON_KNOWLEDGE category where no citation is required.