Ocean Circulation Concepts

Surface Ocean Circulation

• Driven primarily by surface winds in the lower atmosphere
• Density differences (dense water sinking) drive deep ocean circulation (thermohaline)
• Friction between ocean layers creates Ekman spirals
• Coriolis effect deflects currents relative to surface winds
• Bathymetry (ocean floor shape) and basin shape significantly impact currents, though these are mostly fixed now but were different in the past due to plate tectonics.

Surface Currents - Schematic

• Largely driven by surface winds
• Warm currents transport heat poleward (moving energy)
• Basin-scale patterns are called "gyres"
• Illustrated with global map showing various ocean currents and gyres (e.g., North Pacific Gyre, South Pacific Gyre, etc.)

Ekman Transport

• Wind-driven surface currents are deflected by the Coriolis effect (right in the Northern Hemisphere).
• Currents weaken with depth due to friction.
• Further deflection with depth by the Coriolis effect.
• Net Ekman transport is perpendicular to surface wind stress.
• Contributes to convergence and higher sea levels at gyre centers.

Surface Currents - Gyres

• Prevailing winds, basin shape, and geostrophic balance, working together, create gyre structures.
• Net Ekman transport is perpendicular to surface wind stress.
• Leads to convergence and higher sea levels in gyre centers.
• Resulting pressure gradient force is balanced via Coriolis effect. Surface flow is geostrophic.
• A weaker Coriolis effect near the equator results in narrow, strong western boundary currents.

Upwelling

• Diverging surface currents cause upwelling along certain coasts (often in the Southern Hemisphere).
• Upwelling brings nutrient-rich water from deeper depths to the surface.
• Associated with high biological productivity due to the increased nutrients.

Past Surface Ocean Currents

• Bathymetry and basin shapes significantly impact ocean currents.
• Maps showing current patterns during the Late Jurassic (150-200 million years ago) and Early Tertiary (~50 million years ago) are provided.
• Changes in map patterns demonstrates the shifting continents.

Deep Ocean Circulation

• Not discussed in detail in the provided images.

Surface Density

• Dense, cold, and salty water sinks.
• Dense waters significantly contribute to deep ocean overturning circulation

Thermohaline Circulation (THC)

• Driven by deep water formation (density-driven).
• Connected to temperature(thermo) and salinity (haline)
• Arctic deep waters enter the global ocean in the North Atlantic.
• Two key deep water masses formed: North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW).
• NADW mainly forms in the North Atlantic, Norwegian, and Greenland Sea regions.
• AABW forms around Antarctica.

THC in Ocean Properties

• Graphs illustrating temperature and salinity variations are provided with depth in the Atlantic Ocean.
• NADW and AABW deep water locations indicated and shown as features of the THC.

The Thermohaline Circulation - Globally

• Circulation patterns are shown on a global map, emphasizing deep and surface currents.
• Deep water masses (NADW, AABW), form and flow throughout the globe.

El Niño Phenomenon

• Characterized by unusual warming in the central and/or eastern Pacific Ocean.
• This impacts the ENSO (El Niño-Southern Oscillation) system.
• This impacts global atmospheric circulation patterns.

Tropical Pacific - Normal Conditions

• East-West sea surface temperature (SST) gradient drives equatorial Walker circulation.
• Trade winds blow westwards.
• Warm surface water piles up in the western Pacific.
• Cold water upwells in the eastern Pacific.
• A coupled ocean-atmosphere system.

El Niño Impacts

• Global surface temperature increase.
• Rainfall anomalies (e.g., droughts in Indonesia).
• Increased fire risk in areas with dry conditions.
• Reduced land sink for atmospheric carbon (meaning more CO2 in the atmosphere).
• Impact on global temperatures are demonstrated by data (like charts).

El Niño and Rainfall Anomalies

• "Tele-connections" in atmospheric circulation are important.
• Maps show rainfall anomalies associated with El Niño events in various locations.

El Niño and Southeast Asian "Haze"

• Lower rainfall combined with land use change leads to fires.
• Atmospheric pollution (haze) increases in Southeast Asia during El Niño events.
• Visibility Reduction data presented in various geographical areas and given for specific times.

ENSO and Carbon Uptake by Land

• ENSO affects Net Primary Production (NPP) in certain regions.
• NPP (larger) during El Niño, and (larger) during La Niña.

The Oceans - Summary

• Ocean density is determined by temp. and salinity.
• Surface currents are driven by wind.
• Westward intensification of subtropical gyres is influenced by Coriolis Effect.
• Deep-ocean overturning thermohaline circulation is density-driven.
• Ocean-atmosphere connections strongly influence climate.

Before the next class

• Students should watch a recording about biogeochemical cycles and outlines the C and N cycles
• Read article "Planetary Boundaries" (Steffen et al. 2015)
• Notes are needed on how human activity impacts C, N, P, and S cycles.

Explore the fundamental concepts of surface ocean circulation, including the roles of wind, density differences, and the Coriolis effect. This quiz covers key terms such as Ekman transport, ocean currents, and the impact of bathymetry. Test your understanding of how these elements shape our oceans.