Ocean Circulation: Module 7 Part 2 PDF

UPV, College of Fisheries and Ocean Sciences, Institute of Marine Fisheries and Oceanology Ocean circulation Module 7, Part 2 Prepared by: Keryll Bastien Sison Water Hills Ekman transport deflects water to the right in the Northern Hemisphere Ekman transport...

UPV, College of Fisheries and Ocean Sciences, Institute of Marine Fisheries and Oceanology Ocean circulation Module 7, Part 2 Prepared by: Keryll Bastien Sison Water Hills Ekman transport deflects water to the right in the Northern Hemisphere Ekman transport adds to the rightward clockwise motion of northern subtropical gyres a subtropical convergence of water forms in the middle of the gyre, causing water to pile up at the center the middle of the gyre can be up to 2 m higher in water level compared to the edges. Sometimes termed as water hills Geostrophic currents gravity pulls harder on the higher Coriolis effect opposes gravity mass of water at the center and and curves it rightward into the hill makes it flow downhill again Geostrophic currents the net effect of Coriolis and it is a current that moves in a gravity when balanced is a circular path around the water hill geostrophic current as illustrated by the ideal geostrophic flow North Pacific Gyre the apex of the hill is closest to the western boundary current compared to the center this causes western boundary currents of subtropical gyres to be faster, narrower, and deeper than their eastern boundary counterparts. the Kuroshio current is 15 times faster, 20 times narrower, and 5 times deeper than the California current Western intensification this phenomenon is western intensification currents affected by it (i.e. western boundary currents) are called western intensified both the Northern and Southern Hemisphere are affected by it Coriolis effect strengthens toward the poles, making eastward flowing water from high latitudes towards the equator turn more strongly compared to westward flowing water toward high latitudes. Western intensification This causes a wide, slow, and shallow flow of water toward the equator across most of each subtropical gyre, leaving only a narrow band through which the poleward flow can occur along the western margin of the ocean basin. If a constant volume of water rotates around the apex of the hill then the velocity of the water along the western margin will be much greater than the velocity around the eastern side bottleneck effect! Upwelling and Downwelling occurs through divergence or convergence of surface waters; also through coastal winds (Ekman spiral) Divergence pathway/Equatorial upwelling occurs when surface waters move away from each other Ekman transport causes the north surface water to move to the right (up north) and the south water to move to the left (down south) the divergence leaves a gap at the center, causing cold, nutrient-rich water to rise and fill in the gap left behind this makes the equator a prolific fishing ground with high productivity is called equatorial upwelling Convergence pathway/Downwelling surface waters move toward each other when surface waters converge, the water piles up and has no choice but to sink downward the excess mass of water must equalize by sinking down rather than exist as a water hill brings oxygen-rich water into deeper depths downwelling areas are not good fishing water is not nutrient-rich grounds due to low productivity Convergence pathway/Downwelling the convergence of 2 currents, the Gulf Stream and Labrador Current meet at their end points water piles up at their convergence point the piled up water sinks downward and downwells oxygen-rich water is brought to the deep sea Coastal wind pathway coastal winds can cause upwelling/downwelling due to Ekman transport  coastal winds blow parallel to shore (NH) and has 2 different effects: 1. Ekman transport brings water away from the shoreline, causing upwelling 2. Ekman transport brings water towards the shoreline, causing downwelling Other pathways can also be caused by: a) offshore winds b) seafloor obstructions c) coastal geometry bends Upwelling and downwelling driven by the need to fill in gaps in water or the need to equalize in volume water lost = upwelling water gained = downwelling El Nino Southern Oscillation (ENSO) leads to extreme conditions such as droughts or extreme rainfall caused by the weakening of Pacific winds and belts pressure decreases in the east and center, increases in the west ENSO warm phase high pressure along South America weakens causes trade winds to weaken or even reverse depending on the strength temperature increases by up to 10 degrees the diminished trade winds cause the western water level increases Pacific Warm Pool to flow back eastward due to thermal expansion the equatorial countercurrent aids in the eastward upwelling and mixing movement of the warm water reduced (thermocline) ENSO cool phase air pressure in the west is lower than normal pressure gradient or difference is higher air from the east blows more strongly to the west the western Pacific Warm Pool remains in the air circulation and trade west winds are strengthened upwelling becomes more common due to the weaker thermocline and the trade winds Frequency of ENSO El Nino and La Nina occur every 2 to 10 years, then being replaced by the opposite phase after. can occur for as short as 12 months, but can also be for several years