When you look at a surface pressure chart e.g.:
you can see low pressure zones over the North Atlantic ocean. How are these zones tracked? I cannot imagine there are many monitoring stations in the Atlantic ocean.
Ships - but I doubt there are enough to get readings over a wide enough area.
Comparison of pressure readings from coastal monitoring stations. Are there enough points for accurate interpolation of the readings?
Aircraft - comparison of GPS altitude to barometric pressure readings could give the surface pressure in a given area. That said, most aircraft travelling over the Atlantic are on specified tracks which may not cover a given area.
Monitoring stations on buoys at well spaced locations in the ocean. Potentially impractical and a hazard to shipping.