Physical oceanography Study Guide

📖 Core Concepts Physical oceanography – study of oceanic physical conditions (temperature, salinity, density, currents) using observations & numerical models; rooted in thermodynamics & fluid mechanics. Descriptive vs. Dynamical – Descriptive: what the ocean looks like (profiles, maps). Dynamical: why it moves (forces, equations). Thermohaline circulation – global “conveyor belt” driven by density differences created by temperature (thermal) and salinity (haline) variations. Coriolis effect – apparent deflection of moving water: right‑hand turn in the Northern Hemisphere, left‑hand turn in the Southern Hemisphere; strongest at poles, zero at the equator. Ekman transport – net water movement 90° to the right of wind (NH) or left (SH) due to wind‑driven shear and Coriolis. Pycnocline / Thermocline / Halocline – layers of rapid change in density, temperature, and salinity, respectively; usually coincide. Western‑boundary vs. Eastern‑boundary currents – narrow, fast poleward flows (e.g., Gulf Stream) vs. broad, slow equatorward flows (e.g., California Current). 📌 Must Remember Surface mixed layer depth: ≈50–200 m (varies seasonally). Typical open‑ocean salinity: 34–35 ppt. Deep‑water temperature range: 0 °C–3 °C (75 % of ocean volume is 0–5 °C). Coriolis deflection direction: right (NH), left (SH). Ekman transport direction: 90° to the right of wind (NH) / left (SH). Heat transport split: 75 % by atmosphere, 25 % by ocean. Sea‑level rise (observed): 1.5–3 mm yr⁻¹; projected 260–820 mm by 2100. Major wind‑driven gyre pattern: Subtropical interior flow equatorward (Sverdrup balance) + fast poleward western‑boundary return. 🔄 Key Processes Formation of the thermocline/pycnocline Solar heating → warm surface → mixed layer → rapid cooling with depth → thermocline → nearly uniform cold deep water. Ekman spiral & net transport Wind stress → surface layer moves 45° to the right (NH). Successive deeper layers are further rotated → spiral. Integrate over depth → net transport 90° right of wind. Thermohaline (global conveyor) circulation Polar cooling & sea‑ice formation → dense water sinks → flows along ocean floor toward equator → upwells elsewhere → completes loop. Wind‑driven gyre circulation Trade winds + westerlies → generate Sverdrup interior flow → western‑boundary intensification (geostrophic balance) → fast poleward currents. Kelvin wave propagation Wind shift (e.g., El Niño onset) → non‑dispersive gravity wave trapped along coast or equator → moves eastward (equatorial) or with coast on right (NH). 🔍 Key Comparisons Thermocline vs. Halocline Thermocline: rapid temperature drop with depth. Halocline: rapid salinity change; often coincident with thermocline in tropics. Western‑boundary vs. Eastern‑boundary currents Western: narrow, fast (≈1.5 m s⁻¹), poleward, high transport. Eastern: broad, slow, equatorward, lower transport. Kelvin vs. Rossby waves Kelvin: trapped by boundary, non‑dispersive, eastward (equatorial) or coast‑right (NH). Rossby: arise from latitudinal variation of Coriolis, slower, westward‑propagating. ⚠️ Common Misunderstandings “Ocean is heated from below.” – True for the atmosphere; the ocean is heated from above by solar radiation, which suppresses convection. “Coriolis force creates currents.” – It deflects motion; primary driver is wind stress (surface) and density gradients (deep). “All tides are caused by the Moon.” – The Sun also contributes; lunar tides dominate the monthly pattern, but solar tides are significant. “Sea‑level rise equals thermal expansion only.” – It also includes added water from melting ice and other processes (not detailed in outline). 🧠 Mental Models / Intuition “Layered cake” model: Imagine a cake with three layers – fluffy frosting (mixed layer), a thin dense frosting (thermocline/pycnocline), and solid cake (deep ocean). The thin dense layer blocks mixing, just as the pycnocline limits vertical exchange. “Coriolis as a steering wheel”: Water parcels behave like cars on a road that turns more sharply near the poles (stronger Coriolis) and goes straight at the equator (no turn). “Conveyor belt” analogy: Cold, salty water sinks in the north, slides along the ocean floor like a belt, and eventually rises elsewhere, transporting heat globally. 🚩 Exceptions & Edge Cases Polar thermocline absence: In polar regions the thermocline is weak or absent because surface water is already near deep‑water temperatures. Salinity increase by evaporation: Regions like the Mediterranean experience higher surface salinity due to strong evaporation, contrary to the general rule that evaporation cools water. Ekman transport near the equator: Because Coriolis is near zero, Ekman transport weakens, and the classic 90° transport breaks down. 📍 When to Use Which Predicting surface current direction: Use wind direction + Ekman transport rule (90° right/left). Assessing vertical mixing potential: Check for strong pycnocline/thermocline; weak layers → deeper mixing possible. Estimating heat transport contribution: If question asks for relative roles, remember 25 % of poleward heat is oceanic, 75 % atmospheric. Identifying wave type: If disturbance is trapped along a coastline or the equator and moves eastward → Kelvin wave; if it involves latitude‑dependent restoring force → Rossby wave. 👀 Patterns to Recognize “Warm surface + cold deep = strong thermocline.” Look for large temperature gradients in tropical/temperate profiles. “High wind + strong Coriolis → strong western‑boundary current.” Presence of westerlies + poleward flow indicates Gulf Stream‑type currents. “Freshwater input → reduced surface salinity, possible deep‑water formation suppression.” Meltwater in polar regions lowers surface salinity. “Sea‑level rise + thermal expansion = higher SSTs.” Rising sea level often accompanies warmer surface layers. 🗂️ Exam Traps Distractor: “Ocean circulation transports most of the Earth’s heat.” – Wrong; only 25 % is oceanic. Distractor: “Coriolis force is strongest at the equator.” – Opposite; it’s weakest at the equator. Distractor: “Ekman transport moves water in the same direction as the wind.” – It moves perpendicular (90°) to the wind. Distractor: “Thermohaline circulation is driven primarily by wind.” – It is driven by density differences (temperature & salinity), not wind. Distractor: “All tides have the same amplitude worldwide.” – Tidal amplitude varies with lunar/solar alignment and local basin geometry. --- If any heading lacked sufficient source material, a placeholder line would be used, but the outline supplied enough detail for all sections.