MLOS stands for mean level of the sea. It comprises all sea level fluctuations with timescales longer than 1 month. These include:

• Annual cycle - 1 year timescales
• El Niño/La Niña Southern Oscillation - 4 to 8 year timescales - click here to see the latest SOI
• Interdecadal Pacific Oscillation - 20 to 40 year timescales
• Local and global rising sea levels - 100 to 1,000 year timescales

MLOS is different from MSL (mean sea level). MSL is a fixed survey datum. It is usually the average monthly MLOS over a particular year.
In the plots that follow, the monthly MLOS relative to the local MSL datum (from the Nautical Almanac) is plotted for the last 12 months (or less if the record is shorter). If the port's MSL was determined many years ago, the MLOS will be generally above MSL because sea level is rising at 1.8 mm/year on average.

The annual cycle typically has an amplitude of ~ 40 mm and peaks in Autumn, as shown here. It has been removed from MLOS before plotting, so you could say that the plots below are residual MLOS.

Generally, in El Niño conditions, MLOS is depressed by up to 40 mm, while in La Niña conditions it is elevated by up to 40 mm.
You can check whether we are in El Niño or La Niña here.