ESS15 Lecture 13. End of the oceans (tropical / El Nino, thermohaline circulation) Weather vs. climate.

Transcription

1 ESS15 Lecture 13 End of the oceans (tropical / El Nino, thermohaline circulation) Weather vs. climate.

2 Please give me feedback through the EEE midterm evaluation survey. I promise to adapt to it.

3 Review

4 The pattern of the Hadley Cell explains why the deserts are where they are. Pattern: 30S dry EQ wet 30N dry Increasing latitude

5 Winds on the rotating Earth. Deep convective cells confined to tropics wavy westerlies doldrums easterly Trade Winds Condensation heating in rising branch of Hadley Cell lifts the center of mass of the atmosphere (latent -> potential energy) Downhill slope toward winter pole produces jet streams in middle latitudes Jet is unstable to small perturbations, breaks down in waves we call winter storms

6 Wind-driven ocean circulation the gyre pattern. gyre gyre gyre gyre gyre

7 Ekman flow Force balance within the upper ocean. Combined effects of Coriolis and friction on stack of thin layers Each layer moves more slowly and further right than layer above; Average motion of upper ocean is 90º to the right of wind (in NH)

8 Ekman pumping. Ekman flow in NH is 90º to the right of the wind stress Cyclonic wind forces divergence in water, and upwelling Anticyclonic wind forces convergence and downwelling

9 Idealized gyre Convergence of Ekman flow raises sea surface Rotating dome results

10 Asymmetric gyres. Real gyres aren t symmetrical! Boundary currents are strong in west, weak in east

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

12 El Nino is changes in location of really warm water near the equator. Normal conditions really warm El Nino really warm

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

14 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

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

16 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:

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

18 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

19 Like crazy winds last winter Feb 1, 2016 This type of jet configuration can happen any year, but is more likely during El Nino From

20 Thermohaline circulation

21 Map of sea surface salinity (saltiness) Evaporation increases salinity Precipitation decreases it (dilution) Freshest water at highest latitudes Saltiest water in subtropics (especially in Atlantic)

22 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

23 Thermohaline circulation, the great ocean conveyer belt.

24 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

25 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

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

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

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

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

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

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

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

33 The thermohaline circulation or the great ocean conveyer belt.

34 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!

35 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).

36 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.

37 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.

38 Next: Weather vs. climate, climate sensitivity & climate feedback

39 Weather vs. climate What s the difference?

40 Climate vs. weather Climate is what you expect weather is what you get! Climate is an envelope of possibilities within which the weather bounces around Climate is determined by the properties of the Earth system itself (the boundary conditions), whereas weather depends very sensitively on the evolution of the system from one moment to the next

41 Predictability If they can t predict the weather, how can they possibly hope to predict the climate? Weather forecasts are only useful for a few days, maybe a week at best Forecasting is limited by modeling skill and inadequate observations, but even if these were perfect, the limit of predictability would be about 2 weeks This limit is a property of the atmosphere itself, not a failure of our science!

42 Limits to predictability. The butterfly effect Instability and scale interactions make longrange weather forecasting impossible (not just hard!) This is not true for climate!

43 Limits to predictability. The flow around an airplane wing is governed by the same strongly nonlinear Navier-Stokes equations that govern the atmosphere For the same reasons we will never forecast the weather a month in advance, we can never predict the details of the flow around the wing But given boundary values and parameters, we can predict with confidence the statistics of this flow, or flight would be impossible!

44 Long-term forecasting. Can t forecast the weather in Irvine on the day of the ESS15 final exam in March (Cloud? Sunshine? 60 F? 70 F?) Can forecast with complete confidence that 100 C < T max < +100 C, or even that March ocean will be colder than October Why? Boundary conditions! Solar constant, position of Earth in orbit Atmospheric composition Tilt of Earth s axis, Irvine latitude, Heat capacity, thermal inertia of ocean.

45 Weather Depends on time weather nearby (especially upwind!) weather yesterday which way the wind blows Changes a lot! from day to day from season to season from place to place on a given day Unpredictable more than a few days ahead

46 Climate Depends on where you live: Latitude! Altitude (mountains vs plains) What s upwind (ocean vs land) Location relative to global circulation features. Changes very slowly Very predictable We can predict that Miami is warmer than Minneapolis for precisely the same reasons that we can predict a warmer future!

47 Next time Climate sensitivity & feedback. Please read Chapter 7 in Archer Textbook