ESS15 Lecture 12. Review, tropical oceans & El Nino, and the thermohaline ocean circulation. Please see new reading material on website.

Transcription

1 ESS15 Lecture 12 Review, tropical oceans & El Nino, and the thermohaline ocean circulation. Please see new reading material on website.

2 Review.

3 I-clicker exercise: In this graph of Earth s energy imbalances the red line is and the blue line is. A. solar heating, terrestrial cooling B. terrestrial cooling, solar heating C. terrestrial heating, solar cooling D. solar cooling, terrestrial heating E. None of the above

4 Energy imbalances between the tropics and poles make the wind want to form a global pattern like this

5 But because of Coriolis, the pattern is multi-cellular. We live underneath a descending branch of The Hadley Cell Actual pattern of global wind circulation on Earth.

6 Coriolis plus energy imbalances makes the actual atmospheric wind patterns look like this.

7 This global wind pattern spins up ocean currents organized as gyres

8 Amazingly, water blows 90 degrees to the right of where the wind is blowing it (left in the southern hemisphere) due to Coriolis forces the Ekman effect. Surface winds & pressure: implied water pileup implied water pileup implied water pileup implied water pile

9 Thus, winds organize the ocean water into giant mounds in the middle of each major ocean basin. (a consequence of Coriolis forces via. the Ekman effect)

10 Water pushes out from under the mound but is deflected on its way to the right by the Coriolis force creating spinny ocean currents called gyres.

11 Map of observed global surface ocean currents. gyre gyre gyre gyre gyre

12 The Coriolis effect is clearly at the heart of planetary patterns of wind and ocean currents What the heck is it again?

13 Coriolis force the apparent force of being in a rotating world.

14 board interlude.

15 That s the basic picture.

16 Near the equator, some exotic things happen. Tropical oceans and El Nino

17 El Nino is changes in location of really warm water. Normal conditions really warm El Nino really warm

18 Sea Surface Temperature Anomalies o C Normal La Niña El Nino

19 Normal (non-el Nino) conditions: Surface water flow from east toward west Deep thermocline and warm water in western Pacific (associated deep convection & rainfall) Shallow thermocline and cool SST s in east Pacific

20 El Nino conditions: Surface water flow toward east Warm SSTs, convection & rainfall shift to central Pacific Warmer SST s in eastern Pacific

21 i-clicker survey. Indonesia is more likely to be having droughts NORMAL: A: During El Nino B: During normal conditions C: No difference EL NINO:

22 El Nino sends ripples around the planet, changing weather.

23 Why do we get more rain in Cali during El Nino? El Nino The jet stream amplifies, is more likely to flow a bit more steadily west-to-east amps our winter storm activity. La Nina

24 Like Friday s crazy winds. Feb 1, 2016 This type of jet configuration can happen any year, but is more likely during El Nino From

25 Thermohaline circulation

26 Sea surface salinity (saltiness) Evaporation increases salinity Precipitation decreases it (dilution) Freshest water at highest latitudes Saltiest water in subtropics (especially in Atlantic)

27 i-clicker question. Salty, cold waters tend to sink because they are denser than their surroundings. Which of the following locations do you think has the saltiest cold water on the planet? C D B A E

28 Thermohaline circulation, the great ocean conveyer belt.

29 i-clicker survey. The thermohaline circulation is what mixes the ocean vertically. How long do you think it takes for the ocean to mix? A: Less than 5 years B: Between 50 and 100 years C: Between 100 and 500 years D: Over 1000 years

30 Underwater planetary-scale flows of water. Deep water formation in North Atlantic Bottom water formation in Antarctic Ekman convergence in subtropical gyres forces water down against buoyancy

31 How do we measure such a slow flow of water? How do we know it exists?

32 Nuclear explosions since 1945 a pulse of radioactive material leaked into the upper ocean, with no natural source. s://

33 Research ships sample water at many depths along a regular grid pattern follow the isotopes!

34 The pattern of where the radioactive water has gone reveals the slow ocean circulations on our planet.

35 Example - Atlantic bottom water masses. (oxygen is also a useful trace on age)

36 Bottom water flow - Deep water formed off Greenland and Norway flows south to fill Atlantic Basin.

37 Antarctic bottom water. Continuous formation of ice along coast and in leads or polynyas forms extremely dense water

38 The thermohaline circulation or the great ocean conveyer belt.

39 It may seem slow, but a lot of energy flows from the thermohaline circulation Upper limb inflow to North Atlantic ~ 10º C Lower limb outflow ~ 3ºC dq = c dt ~ 3 x 10 7 J of heat released by each m 3 of water during conversion from upper limb to lower limb water mass 20 Sv = 20 x 10 6 m 3 s -1 of water makes this transition, releasing 6 x J s -1 (= 0.6 Pw) of heat to the atmosphere This is 35% of solar heating of North Atlantic north of 40º N latitude!

40 Oceans - in a nutshell. Oceans have fast gyres confined to each basin. The atmospheric wind pattern of westerlies-polewardof-easterlies causes water to pile up in the center of ocean basins. Because the Ekman effect pushes water to the right of the wind (left in S.H.). Swirly currents result as water pushes out from the mound but feels a Coriolis deflection. Exceptions are on the equator and in the southern ocean (east-west flow instead of swirly flow).

41 Oceans - in a nutshell. El Nino - a coupled atmosphere/ocean cycle in the equatorial Pacific, every 3-7 years. When El Nino happens, a huge reservoir of very warm water spreads halfway across the Pacific. World wide weather ripples. e.g. changes jet stream direction, promotes Cali winter-storms. Currently happening! A big one.

42 Oceans - in a nutshell. Oceans have a slow global circulation behind the gyres. Much more gradually, salty, cold water sinks in the N. Atlantic, flowing south across the equator to the Antarctic. Thermohaline Part of the great conveyer belt ocean current system. Takes 1000 s of years for water to complete the journey. Thus the adjustment of the ocean to climate change takes 1000 s of years to occur. Water in deepest oceans is inky black, 1000 s of years old, and doesn t know the industrial age is here yet. DEspite being slow, transports a heck of a lot of energy.

43 Thanks. Next time: Weather vs. climate, climate sensitivity & climate feedback