"Pressure systems" refers to areas in the atmosphere where the air pressure is either higher or lower than the surrounding areas. These systems create different weather conditions and affect how air moves around the Earth. Full definition
This energy, which is a measure for the interplay between intensity and frequency of high and low pressure systems in the atmosphere, dropped by roughly one tenth since 1979. (sciencedaily.com)
Are these two mechanisms sufficient for driving the atmospheric pressure system, ecological and biological systems of the whole planet to the best? (ceassessment.org)
Yes, you are correct... these creatures want to prevent the low pressure system off our coast from bringing rain into our region. (geoengineeringwatch.org)