IX. Upper Ocean Circulation Covers 71% of Earth’s surface “World Ocean” Contains 97% of surface water Arctic Ocean NH: 61% ocean, 39% land Pacific Atlantic Ocean Ocean Indian Ocean SH: 81% ocean, 19% land Southern Ocean global satellite image (NOAA/NGDC) false colors = elevation (blue is below sea level, avg. dep. ~4000m) generalized vertical structure •upper ocean is less salty •upper ocean is warmer •upper 1-200m is mixed seasonally by winds generalized vertical structure upper ocean is thus partly separated from deep ocean by a large density gradient (pycnocline) more next class on the role of density (neg. buoyancy) forcing in the global circulation upper ocean circulation • upper ocean circulation is largely a wind-driven circulation • i.e wind pushes on ocean surface water (this is called wind stress) • which way will the water move? review • differential heating leads to variations of pressure at surface • air moves from areas of high to low pressure • Coriolis effect causes moving air to deflect to the right in NH (left in SH) • this leads to clockwise movement of air around areas of high pressure in the NH (anti-clockwise in the SH) surface winds wind-driven circulation • which way will the water move? • there are 3 main factors • Coriolis effect • Ekman transport • geostrophic balance • westward intensification Ekman transport • Ekman transport occurs in response to the balance between • wind stress • Coriolis effect • friction Ekman layer and Ekman spiral •imagine wind pushing on surface •surface water moves off to right due to Coriolis, but only by about 20- 45° ~100 m Ekman layer and Ekman spiral •imagine wind pushing on surface •surface water moves off to right •this water now pushes the water below, which now ~100 m moves off to the right •now THIS layer pushes the water below, which moves off to the right and so on and so on.... Ekman layer and Ekman spiral •averaged over the top 100m or so (the Ekman layer) the water moves ~100 m EXACTLY 90° to the right of the wind direction Ekman layer and Ekman spiral NET here you can see the velocity and amount of friction diminish downward- by ~100m depth friction has greatly diminished the velocity and reversed the direction of flow - NET flow is 90° RT of the wind consider winds over N. Atlantic.... clicker question: • water seems to be moving in from all around • What must happen? ????? a) convergence b) makes a pile c) divergence d) makes a hole e) both a) & b) Ekman transport leads to convergence convergence leads to pile! convergence creates a “pile” water Sea surface height (North Atlantic Subtropical Gyre) X X is ~80 cm higher than surroundings convergence creates a “pile” water Sea surface height (North Atlantic Subtropical Gyre) i.e. an area of H HIGH pressure geostrophic flow • geostrophic flow occurs when the movement due to pressure gradient balances that from Coriolis effect • geostrophic flow is thus “balanced” flow • (geostrophic = “earth turning”) geostrophic flow top of pile initial movement is down the pressure gradient (i.e. down hill) but turns until pressure gradient force and Coriolis balance, the balanced flow follows lines of equal pressure in a circular pattern sea surface height (Feb ‘95) IGOS TOPEX UTCSR clicker question: b a c c d d e e clockwise flows occur at: a), b), c), d), or e) wind-driven surface “gyres” c c e e large scale circular balanced flows or currents wind-driven surface “gyres” • large scale circular flows or currents • mostly <400 m, i.e. 10% of ocean volume • dominated by the great subtropical gyres (up to 1000 m) • suptropical gyres • clockwise in NH (i.e. high pressure) • anti-clockwise in SH (i.e. high pressure) Western Boundary Currents • fast (~2 m/s, 7 km/hr), deep (~1000 m), narrow (~70 km wide) • warm, nutrient-poor, clear • N Atl: Gulf Stream • S Atl: Brazil Current • N Pac: Kuroshio Current • S Pac: East Australian Current • Ind: Agulhas Current Gulf Stream meanders Eastern Boundary Currents • slow (~2 km/hr), shallow (~100 m), wide (to ~1000 km) • cold, nutrient-rich • N Atl: Canary Current • S Atl: Benguela Current • N Pac: California Current • S Pac: Peru Current • Ind: West Australian Current Antarctic Circumpolar Current (West Wind Drift) • Southern Ocean, south of ~40°S • flows completely around globe • not very fast (~3 km/hr) but largest volume flux • ~2-4 km deep, up to 2000 km wide • driven by westerlies (geostrophic) subtropical gyres c c e e e the western boundary currents are warm the eastern boundary currents are cold clicker question: c c e e e the subtropical gyres produce a net heat transport that is: a) equatorward, b) poleward, c) E-W d) in all directions, e) negligible subtropical gyres pick up and transport heat from tropical ocean toward the poles global heat transport northward southward great subtropical gyres reach and carry heat to ~40 °lat. (exception is N. Atl as we will see next class) up- and down- welling • so far talked about lateral motions arising from wind stress • what about vertical motions • these are required by mass balance convergence v. divergence + subtropical gyre centers convergence of mass mass balance requires sinking subpolar gyre centers, equator divergence of mass mass balance requires upwelling upwelling = biological productivity • upwelled water is rich in nutrients • when nutrients enter sunlit upper 1-200m, photosynthesis can occur (in the so-called photic zone) • upwelling regions are therefore biologically rich and diverse • very active marine ecosystems • good fishing visible light in seawater • blue penetrates farthest • ~1% reaches 250 m • “photic zone” upwelling and cholorophyll SeaWiFS Sep. ‘97 - Aug. ‘98 equatorial upwelling normal equatorial upwelling •easterly trades straddle equator •Ekman divergence to N and S •divergence balanced by upwelling direction of coriolis force changes at equator upwelled water is cold water sea surface temp. °C normal equatorial upwelling •easterly trades straddle equator •Ekman divergence to N and S •divergence balanced by upwelling direction of coriolis force changes at equator and rich in nutrients chlorophyl (mg/m3) El Nino or La Nina? coastal upwelling • equatorward winds parallel eastern boundaries • Ekman transport away from shore • surface waters replaced by sub- surface waters upwelling and cholorophyll SeaWiFS Sep. ‘97 - Aug. ‘98 coastal upwelling CA summer coastal upwelling Winds blowing south along coast move water in the Ekman layer to the right. Upwelling occurs along the coast to replace water moved offshore….. CA summer coastal upwelling convergence and downwelling recall: subtropical gyre centers convergence of mass mass balance requires sinking dynamic height + convergence and downwelling chlorophyll dynamic height nutrient starved open- ocean desert, why? coastal upwelling, why? sea surface temperature (Sun.) divergence and upwelling vs. convergence (and downwelling) upwelling of cold deep water produces cooling: downwelling permits heating El Niño • the leading source of natural, year-to year variations in weather and climate around the world • driven by changes in atmosphere-ocean circulation over equatorial Pacific Equatorial Pacific mean state • along equator: water flows in direction of wind (Coriolis = 0) • warm waters pile up in west • cold waters upwell in east (just south of equator) “warm pool” “cold tongue” sea surface temperatures La Niña El Niño Strong easterlies Weak easterlies Strong eastern upwelling (cold SSTs) Weak eastern upwelling (warm SSTs) Heavy rainfall in western warm pool Eastward shift in rainfall • both states recur every 2-7 yr • usually develop April-June • reach max Dec-Feb • last ~9-12 months (or more) coupled with widespread changes in atmospheric circulation Walker Circulation (tropical E-W linkages) la Nina (“normal”) PERU cold low surface pressure area of convection el Nino moves east warm weak low surface monsoon pressure Walker Circulation (tropical E-W linkages) la Nina (“normal”) PERU warm cold low surface pressure area of convection el Nino moves east what might be the atmospheric response to warm surface water here warm Walker Circulation (tropical E-W linkages) la Nina (“normal”) PERU cold low surface pressure area of convection el Nino moves east warm weak low surface monsoon pressure El Niño precipitation anomalies (vs. “normal”) (La Niña pattern approx. opposite) NOV-APR DRIER cm/yr WETTER notice anomaly pattern alternates E-W in association with changes in the Walker Circulation 1997/8 El Niño drought, rainforest burning Wet (Amazon, Indonesia), and Dry monsoon weakening, California storms are major impacts 1999 La Niña flooding in South America, Indonesia, SE Asia and Wet Africa, drying of central Pacific islands and Dry American SW : “extreme tropical weather” • El Nino is the leading source of natural, year-to-year variation in weather and climate around the world •How will it change in response to global warming? •Different models give different results, none are particularly reliable (this is a good example of physics that is difficult to represent in models subject to changing conditions) summary points •upper ocean circulation is largely a wind driven circulation, i.e. winds push on surface (providing surface wind stress) •pushing occurs layer-by-layer in upper ocean leading to Ekman spiral •Ekman transport is exactly 90° to the right of the wind in the NH (or, to the left in SH) •This allows water to pile up near middle of the ocean basins •Higher height provides pressure gradient and permits balanced geostrophic flow in large circular gyres