FY 2022

A global ocean surface mixed layer monthly climatology: Means, percentiles, skewness, and kurtosis

Johnson, G.C., and J.M. Lyman

J. Geophys. Res., 127, e2021JC018219, doi: 10.1029/2021JC018219, View online (2022)

Here we discuss a global ocean surface mixed layer statistical monthly climatology (GOSML) of depth, temperature, and salinity that includes means; variances; 5th, 50th, and 95th percentiles; as well as skewness and kurtosis. Ocean surface mixed layer properties are influenced by gravity and a wide variety of factors that operate over a wide variety of time scales. Mixed layer depths can shoal very quickly as a result of surface heating, precipitation, or “slumping” of horizontal density gradients. However, deepening the mixed layer in the presence of a strong pycnocline requires substantial buoyancy loss or strong wind mixing, which often takes more time. This pattern is clear in the annual cycle monthly mixed layer depth values, with deepening in the fall much slower than shoaling in the spring. The 95th percentile values are chosen as a reasonable indicator of ventilation depth, robust to extreme outliers. Mean mixed layer depths are on average 0.56 of 95th percentile mixed layer depths, with only 1% of values below 0.31% and 1% above 0.81. Over 71% of mixed layer depth distributions are skewed positive, usually when there are more shallow mixed layer depths than not and deep mixed layer tails are strong. Comparing 95th percentile depth conditions to mean values shows in late winter temperatures are generally lower in the subtropics and salinities generally higher in the subpolar regions, consistent with the importance of temperature in the midlatitudes and salinity in the higher latitudes in setting stratification.

Plain Language Summary. The ocean surface mixed layer is key to exchanges of heat, freshwater, momentum, and dissolved gasses between atmosphere and ocean. Hence, it affects marine life, weather, and climate. Mixed layer depths shoal suddenly with warming from the sun, rainfall, riverine outflow, or currents that slide lighter water over denser. However, mixed layer deepening requires loss of heat or freshwater to the atmosphere or strong wind mixing to overcome ocean density stratification below. The monthly mixed layer climatology presented here illustrates that asymmetry, with slow deepening from summer to winter, and fast shoaling during the spring. Likewise, shallower mixed layers often predominate over deeper ones for many months and locations. Since the deeper mixed layers determine the local exchange of surface properties with the ocean interior (known as ventilation), mean values are not a good indicator of those processes. Here we choose 95th percentile mixed layer depths in a given month (and the associated temperature and salinity values) as an indicator of the ventilation conditions that is not impacted by extreme outliers. These 95th percentile depths are on average about 85% deeper than the means, but can reach over five times deeper in a few locations at a few times.