Editing Surface layer (section)

===Diurnal cycle===
The depth of the surface mixed layer is affected by solar insolation and thus is related to the diurnal cycle. After nighttime convection over the ocean, the turbulent surface layer is found to completely decay and restratify. The decay is caused by the decrease in solar [[insolation]], divergence of turbulent flux and relaxation of lateral gradients.<ref name = "Caldwell">{{cite journal|last1=Caldwell|first1=D. R.|last2=Lien|first2=R-C.|last3=Moum|first3=J. N.|last4=Gregg|first4=M. C.|title=Turbulence Decay and Restratification in the Equatorial Ocean Surface Layer following Nighttime Convection|journal=Journal of Physical Oceanography|volume=27|issue=6|year=1997|pages=1120–1132|issn=0022-3670|doi=10.1175/1520-0485(1997)027<1120:TDARIT>2.0.CO;2|bibcode=1997JPO....27.1120C|url=https://journals.ametsoc.org/jpo/article/27/6/1120/9282/Turbulence-Decay-and-Restratification-in-the|doi-access=free}}</ref>
During the nighttime, the surface ocean cools because the atmospheric circulation is reduced due to the change in heat with the setting of the sun each day. Cooler water is less buoyant and will sink. This buoyancy effect causes water masses to be transported to lower depths even lower those reached during daytime. During the following daytime, water at depth is restratified or un-mixed because of the warming of the sea surface and buoyancy driving the warmed water upward. The entire cycle will be repeated and the water will be mixed during the following nighttime.<ref name="Talley">{{cite book |last=Talley |first=Lynne |title=Descriptive Physical Oceanography: An Introduction |edition=6th |year=2011 |publisher=Elsevier Academic Press |location=Burlington, MA |pages=74–76 |chapter=Chapter 4 - Typical Distributions of Water Characteristics }}</ref>

In general, the surface mixed layer only occupies the first 100 meters of the ocean but can reach 150 m in the end of winter. The diurnal cycle does not change the depth of the mixed layer significantly relative to the seasonal cycle which produces much larger changes in [[sea surface temperature]] and buoyancy. With several vertical profiles, one can estimate the depth of the mixed layer by assigning a set temperature or density difference in water between surface and deep ocean observations – this is known as the “threshold method”.<ref name="Talley">{{cite book |last=Talley |first=Lynne |title=Descriptive Physical Oceanography: An Introduction |edition=6th |year=2011 |publisher=Elsevier Academic Press |location=Burlington, MA |pages=74–76 |chapter=Chapter 4 - Typical Distributions of Water Characteristics }}</ref>

However, this diurnal cycle does not have the same effect in midlatitudes as it does at tropical latitudes. Tropical regions are less likely than midlatitude regions to have a mixed layer dependent on diurnal temperature changes. One study explored diurnal variability of the mixed layer depth in the Western Equatorial Pacific Ocean. Results suggested no appreciable change in the mixed layer depth with the time of day. The significant precipitation in this tropical area would lead to further stratification of the mixed layer.<ref name = "Lukas">{{cite journal | last = Lukas | first = Roger |author2=Lindstrom, Eric  | title = The Mixed Layer of the Western Equatorial Pacific Ocean | journal = Journal of Geophysical Research | volume = 96 | issue = S01 | pages = 3343–3357 | year = 1991 | doi=10.1029/90jc01951|bibcode = 1991JGR....96.3343L }}</ref> Another study which instead focused on the Central Equatorial Pacific Ocean found a tendency for increased depths of the mixed layer during nighttime.<ref name = "Gregg">{{cite journal | last = Gregg | first = M. C. |author2=PETERS H. |author3=WESSON J. C. |author4=OAKEY N. S. |author5=SHAY T. J.  | title = Intensive measurements of turbulence and shear in the equatorial undercurrent | journal = Nature | volume = 318 | issue = 6042 | pages = 140–144 | year = 1985 | doi=10.1038/318140a0|bibcode = 1985Natur.318..140G | s2cid = 4230219 }}</ref> The extratropical or midlatitude mixed layer was shown in one study to be more affected by diurnal variability than the results of the two tropical ocean studies. Over a 15-day study period in Australia, the diurnal mixed layer cycle repeated in a consistent manner with decaying turbulence throughout the day.<ref name = "Caldwell"/>