At it s most extreme very low pressure off Indonesia, every high off SA, ~8 o C difference over the Pacific and ½ m water level differential) ENSO is

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1 This summer : El Niño (ENSO) and the NAO (Ocean/Atmosphere coupling teleconnections) A teleconnection (as used in the atmospheric sciences) refers to climate anomalies that are related across very large scales Historically, teleconnections teleconnections were first suggested by British meteorologist Gilbert Walker in the late 19 th century, when he reported correlations among time series of atmospheric pressures, temperatures and rainfall His work was a significant building block for understanding climate variability, as it showed that such variability was not random 1

2 El Niño/Southern Oscillation (ENSO) remains the best understood teleconnection El Niños seem to occur ~ every 3-7 7years we re not sure why During an El Niño, cold, dry air moves into places that typically have warm, moist air leading to huge changes in normal weather. Generally we see increases in rainfall in the southern US/Peru with warmer/drier winters in Canada plus drought in the western Pacific We use changes in sea surface temperature (SST) and changes in pressure to predict an ENSO we are currently experiencing a building El Niño!! 2

3 What does a building El Niño mean? Under normal ( neutral neutral ) circumstances: There is lower pressure over Indonesia (due to rising air at the ITC) and high levels of precipitation Higher pressure off Peru (falling air) A strong south equatorial current Strong se trade winds At it s most extreme very low pressure off Indonesia, every high off SA, ~8 o C difference over the Pacific and ½ m water level differential) ENSO is in its La Niña phase 3

4 The neutral case cont. Winds and currents pile up warm water in the western pacific that pushes the colder water deeper. This leads to an upwelling of cold nutrient-rich rich waters off Peru (and California) 4

5 The southern oscillation (ENSO) is a collapse in the Indonesian low The trade winds, precipitation and current patterns in the southern Pacific ocean reverse. The large pool of warm water that was in the western pacific sloshes across the surface of the southern Pacific Ocean The upwelling off Peru ceases There are large changes in global atmospheric circulation, which force changes in weather in regions far removed from the tropical Pacific 5

6 More implications of ENSO Warmer ocean water releases more carbon to the atmosphere (we ll see a correlation between El Niño years and increases in atmospheric CO 2 ) Does ENSO related CO 2 release e foreshadow the behaviour of a generally warmer ocean? There are other implication of ENSO It appears to reinforce the PDO Displaces the northern jet stream (from over Canada, southward over the SE US) the negative phase of the NAO 6

7 NAO While the NAO index varies from year to year, it also exhibits a tendency to remain in one phase for intervals lasting more than a decade. An unusually long period of NAO persisted from the late 40s to the early 70s, followed by a long +NAO between the 70s and It looks like the NAO responsible for the summer will persist. 7

8 What can the past teach us about the future? Uniformitarianism suggests the past behavior of Earth s climate system provides powerful insight into what it may do in the future. This assumes that anthropogenic CO 2 release and accumulation in the atmosphere will behave similarly to natural changes in CO 2 that have occurred in the past! This may not be true as the current rates of CO 2 increase are higher than Earth has experienced. 8

9 Remember this Holocene maximum thing 1999 Mann et al. global data based on satellite And the Little Ice Age An early analysis of data by NOAA: now believed to be biased towards n. hemisphere 9

10 Patterns of warming/cooling depend on the region 1. About 12-14,00014,000 years considered 1. ago, Earth emerged from the most recent ice age and warmed rapidly 2. Rapid ice melting put a huge flux of freshwater in the oceans, disrupting deep ocean circulation. Average temperatures fell, at least in the N. Atlantic 3. This cold period, known as the Younger Dryas, lasted ~1,000 years. 4. Rapid warming again ensued (with the exception of the 8,200 year event) stabilizing about 7000 years ago (the holocene maximum). The medieval warm period allowed the Norse to establish settlements in Greenland and Newfoundland over 1000 ybp. 5. Then the Little Ice Age began The Norse abandoned their settlements when the climate turned abruptly colder. Between 1300 and 1850, severe winters had profound agricultural, economic, and political impacts in Europe and NA 1 R.B. Alley 2000 Greenland ice core data 10

11 Can 1 o C be a big deal? Is the world now warming? Data support that conclusion Global surface temperatures have increased ~ 0.7 C (±0.18 C) since the late-19 th century, and ~ 0.18 ± 0.05 C over the past 25 years (the period with the most credible data). 11

12 How do we understand these trends within the current climate paradigm (I)? Earth s climate system is inherently chaotic, sensitively balanced, and threshold-laden laden 1 Globally derived best estimates (IPCC) project a change of C 5.8 C over the next century. Such a change would be unprecedented in comparison with the best available a ab records from the last several e thousand years. Nevertheless, it is prudent to superimpose on this forecast the potential for even more abrupt regional climate change induced by thermohaline slowing 2. Such a change could cool down selective areas of the globe by 3 to 5 C, while simultaneously causing drought in many parts of the world. The key is to reduce our uncertainty about future climate change, and to improve our ability to predict what could happen and when. It may be the rate more than the amount 3!! 1 Robert B. Gagosian, Director, Woods Hole Oceanographic Institution 2 AR4 predicts MOC will slow down, but is unlikely to undergo any abrupt transition 3 US National Academy of Sciences Abrupt Climate Change: Inevitable Surprises. National Academy Press. 12

13 What are those natural forcing mechanisms (KR 6.2)? 1. changing solar energy output 2. continental drift 3. changes in ocean circulation 4. variation in distance/angles between Earth and the Sun 5. variations in the composition of Earth s atmosphere 13

14 1. Solar output (luminosity) There appears to be an 11 (and 22) year cycle of irradiance. With only ~35 years of reliable measurements it is difficult to deduce d a trend, but the short record suggests a potential increase of ~0.09 W.m during periods of maximum luminosity (compared to 0.4 W.m- Wm 2 from GH gases). ~0.09 W.m- 2 Space-based measurements starting in the late 70s, show luminosity does vary. Conclusion: luminosity is an order of magnitude less significance than increases in gh gases, i.e. it may be a contributory, but less significant factor in short-term term ( y) climate change. 14

15 2. Continental drift and climate Ocean currents change in response to continental movement, changing how the ocean transports warm and cold water When equatorial ocean currents circulate, earth is warmer When equatorial flows are blocked, higher latitude (circum-polar) currents isolate polar continents causing polar temperatures to drop A super continent at the equator would likely lead to a warmer climate as more EMR is absorbed by a large land mass changing precipitation patterns Mountain building affects atmospheric flow Mountains are formed when continents collide. Their locations can affect atmospheric circulation (particularly n/s oriented ranges) Computer models indicate a 2 km uplift can block mid-latitude air flow (i.e. the westerlies) the Himalayas average 6 km, the Rockies 2.5 Circum-polar & Circum-equatorial seaways Conclusion: Interesting but not relevant to the last years 15

16 3. Deep ocean or thermohaline circulation (MOC) During the warming just after the last ice age, large amounts of freshwater were released into the ocean. This freshwater input slowed MOC, apparently leading to strong cooling over the n. Atlantic which lasted for 600 to 1000 years (the Younger Dryas) The Little Ice Age (and an apparently long-term cooling trend) was interrupted by the rapid warming of the past 150 years However this process remains poorly understood (in spite of what you might think if you saw The Day After Tomorrow) Furthermore, slowing of the MOC (as the climate warms, more ice melts and more freshwater pours into the North Atlantic via the Labrador Strait) will slow DOC, cooling rather than warming the climate. Conclusion: A slowing MOC may be a rather than + feedback (globally) ll May contribute significant regional variation Need more data! 16

17 4. Distance/angles between Earth and the Sun In addition to changes in energy from the sun itself (luminosity), the Earth's position and orientation relative to the sun (our orbit) also varies, bringing us closer and farther away in predictable cycles (the so-called Milankovitch cycles). Variations in these 3 cycles are believed to be the cause of Earth's ice-ages (glacials glacials). These are so cool they re worth understanding on their own. 17

18 This leaves forcing mechanism 5 (changes in atmospheric composition) as the strongest candidate to explain overall patterns in Earth s climate and they can account for the changes of the last 150 years 18