# Thermocline A thermocline is the layer where water temperature drops sharply with depth, separating a warm wind-mixed surface layer from the cold deep. It is a [[Density]] barrier as much as a temperature one: crossing it costs [[Energy]], so it suppresses vertical exchange and effectively divides the ocean — or a lake — into two weakly-communicating reservoirs. ## Microsim — p5.js Sidecar: `Thermocline.p5.js` · route leaf: `microsim/p5js/Thermocline__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `season slider` | time through the year | | `wind slider` | mixing energy at the surface | | `[o]` | force an overturn | *What to watch:* Push the season forward and watch the warm layer build, then deepen, then collapse into a full overturn when cooling wins. ## Seasonal and permanent Summer heating builds a shallow seasonal thermocline in weeks; autumn cooling and storms erode it, and in temperate lakes the whole column overturns. Beneath that, a permanent thermocline persists year-round in the open ocean at a few hundred metres. The barrier matters because nutrients sink and light does not penetrate: the sunlit surface runs short of nutrients while the dark deep has plenty, which is why the ocean's most productive regions are exactly where [[Upwelling]] breaks the barrier. ## Stratification as a switch The strength of the stratification decides whether shear can mix the layers, which is the Richardson-number criterion behind [[Kelvin–Helmholtz_instability]] — a thermocline holds until the shear across it wins. [[Langmuir_circulation]] and storm [[Turbulence]] deepen the mixed layer from above; internal waves riding the thermocline break and mix it from within. In [[Properties_of_water|fresh water]] the density maximum at 4 °C adds a twist that makes lake overturn a seasonal event rather than a gradual one. **Reads with:** *Introduction to Physical Oceanography (Robert H. Stewart, 2008)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/introduction-to-physical-oceanography) · [download](https://github.com/introocean/introocean-en/releases/tag/v20200229) · CC BY-NC-SA. Section 7 of the [[PORTAL_Thury_Hydrodynamics_Apex_Spine|Apex Spine]] book shelf. **On the spine:** [[Thermohaline_circulation]] · [[Upwelling]] · [[Kelvin–Helmholtz_instability]] · [[Properties_of_water]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Thermocline) : [Wikitube](https://en.wikitube.io/wiki/Thermocline) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]]. --- *Book-section wave · 2026-09-10 · article + p5 microsim shipped together.*