Earth Systems Science Chapter 5 OCEAN CIRCULATION I: SURFACE Winds, surface currents Flow within gyres: convergence, divergence, upwelling, downwelling, geostrophic flow Boundary currents Sea surface temperatures

Ocean Circulation very different than atmosphere because: it is warmed from above, not below. Thus, convection due to solar heating is not the primary mover of the ocean. Ocean temperatures respond slowly (heat capacity), and therefore the lateral temperature and density gradients are small Solar radiation plays a more indirect role: sea surface temperatures (SSTs) affect the atmospheric temperature and pressure gradients, and the atmospheric winds. Atmospheric winds drive the ocean surface circulation patterns wind stress is the force of the wind on the ocean surface (friction) Coriolis force acts on currents similarly to winds

Idealized winds would result in an idealized ocean circulation, with gyres

actual ocean circulation has gyres similar to ideal, but different due to different arrangement of land masses

Flow within the ocean gyres: Ekman spiral, Ekman transport wind -> friction -> surface current; but coriolis effect deflects current to the right (NH) Surface current -> friction ->current just below surface; moves more slowly (loses energy due to friction), and further displaced to the right The Ekman spiralEkman transport

Gyres: Convergence / Divergence and Downwelling / Upwelling Areas of surface convergence cause downwelling Areas of surface divergence cause upwelling Upwelling brings water from deeper levels, often colder and richer in nutrients

GYRES: GEOSTROPHIC CURRENT Areas of convergence introduce another force: gravity (due to the height difference between the center and the outside). three forces: friction (with the wind), coriolis, and height (i.e. pressure gradient) (should sound familiar to you). The result of these is that the circulation is often geostrophic, and often parallel to the height gradients

BOUNDARY CURRENTS Western boundary currents are narrower, faster, deeper The Gulf Stream -> N. Atlantic drift -> Canary current The Sargasso Sea

WHY ARE WESTERN BOUNDARY CURRENTS DIFFERENT THAN EASTERN? Vorticity: tendency to spin or rotate Conservation of vorticity: absolute vorticity (planetary + relative) does not change Planetary Vorticity: zero at equator, higher at the poles (positive, or counter clockwise, in Northern Hemisphere) Relative Vorticity: tendency to spin from local winds and currents On the western boundary, northward flow increases planetary vorticity, so the relative vorticity has to decrease. On the eastern boundary the decrease in latitude decreases planetary vorticity, so the relative vorticity has to increase So, …

OCEAN CIRCULATION AND SSTs Ocean circulation helps to redistribute energy around the world Warm currents originate near equator, go towards the poles Cold currents originate near the poles, go towards the equator Result is a significant impact on regional climates Mean January Temperature (C)

Earth Systems Science Chapter 5 OCEAN CIRCULATION II: DEEP OCEAN Salinity Thermohaline circulation Ocean circulation and climate

SALINITY NOTE: Some of these will be important when studying the carbon cycle

SALINITY 1.Why is salinity important for ocean circulation? 2.From where does the salt in the ocean originate? 3.To where does the salt in the ocean go? 1. Salinity, in addition to temperature, determines the density of the water, which is the driving force for deep ocean circulation. 2. The source of salts in the ocean is weathering. This is the process whereby water flowing over and through the land picks up dissolved minerals (i.e. salts). Those waters eventually flow out into the ocean.

3. Salts do not evaporate along with the ocean water. So, when ocean water evaporates, it leaves the salts behind. This would lead to ever-increasing salt concentrations if there were no other sinks. Sinks of salt in the ocean include: In shallow seas evaporation leaves such high salt concentrations that the water becomes saturated, and the resulting precipitate forms evaporite deposits. Removal of salt by microorganisms to form shells, some of which are eventually deposited on the ocean floor. Chemical reactions with newly formed sea floor Removal by wind from evaporated sea spray SALINITY

Over long time periods, and averaged over the globe, the amount of salt in the oceans is in dynamic equilibrium. Regionally, there can be differences in salt concentrations due to variations in the salt concentration of river runoff, and the amount of precipitation, runoff, and outflow. For example, where evaporation exceeds precipitation, and there is little runoff, salinity usually increases. SALINITY

THERMOHALINE CIRCULATION Deep ocean circulation. Driven by density variations which are due to temperature and salinity variations. Slower than surface circulation, but important over long time scales (100s to millions of yrs)

SURFACE ZONE Top 50 – 100 meters Exchanges energy with the atmosphere through latent heat, radiation, and kinetic energy Generally well-mixed by the wind, so is often referred to as the mixed layer PYCNOCLINE ~1 km below the mixed layer Region of sharp increase in density, very stable, prevents mixing between mixed layer and deep ocean DEEP OCEAN 1-5 km deep Stable density gradient

Observed temperature distributions Western Atlantic Central Pacific no gradient

Observed salinity distributions no gradient weak gradient no gradient Western Atlantic Central Pacific

DEEP WATER FORMATION Dense water formed in the Weddell Sea (Antarctic Bottom Water, AABW) and in the North Atlantic Ocean near Greenland (North Atlantic Deep Water, NADW) sinks This sinking water initiates a slow moving deep ocean circulation Average residence time in the deep ocean is hundreds of years Why is this water dense? Ice formation: rejection of salt Evaporation from Gulf Stream

DEEP WATER FLOW AT 4000 m DEPTH

THE THERMOHALINE CONVEYOR BELT

SEAWIFS Mean Chlorophyl September 97 - August 2000 Center of gyres – downwelling – few sources of nutrients – little biological activity Areas with nutrient input from rivers – or from upwelling – more biological activity

MEDITERRANEAN OUTFLOW AT 1000m Temperature (C) Salinity 0/00

Remember this from chapter 4 …