Climate Dynamics Lecture11b - El Niño Southern Oscillation
Introduction to Atmospheric Dynamics
https://www.youtube.com/watch?v=LWtNxA2RbDM

Transkript (automatisch erstellt)

0:00 hello well welcome back to climate dynamics in this lecture we're continuing to explore atmosphere ocean interactions we're going to be
0:07 discussing the El Nino Southern Oscillation the reading for this lecture is Marshall and plum chapter twelve point two in this section we're going to
0:16 define Walker circulation burek nice feedback El Nino Southern Oscillation and Southern Oscillation index the questions we'll answer are what are the
0:25 major drivers of zonal tropical circulations how are the ocean atmosphere connected in the tropics what is the normal state of the tropical
0:32 Pacific how does the tropical system swap between warm and cold States and what global impacts are related to El Nino and La Nina conditions this lecture
0:42 is broken up into two parts in the first part we'll discuss the basics of tropical dynamics then we'll discuss the oscillation known as ENSO recall in our
0:52 previous lectures we discussed the global general circulation under this general circulation rising motion in the equatorial regions driven by near
1:01 surface warming from solar insolation drives convection this convection then leads to the generation of the Hadley circulation because of the presence of
1:10 Coriolis force the Hadley circulation cannot extend from equator to pole but instead we end up with strong westerly Jets occurring at about 30 degrees north
1:19 where the Hadley circulation subsides to the near surface convergence in the near surface brings air into this tropical region in the equatorial region the
1:30 zonal flow is primarily easterly that is the returning branch of the circulation brings in air that is turned to the right in the northern hemisphere and to
1:39 the left in the southern hemisphere together this results in an easterly circulation however Coriolis force right at the equator is zero and is relatively
1:48 small in the vicinity of the equator at the right at the equator wind speeds tend to be slower than to the north and to the south because of Coriolis force
1:57 essentially decaying to zero this is only symmetric view of the global system does not account for zonal in Houma in Houma Jenaya T which is important for
2:06 governing flows particularly in the tropics where the flow is essentially symmetric in the Murda onyl direction particularly this is only symmetric
2:15 image does not account for differences between ocean and land of which there is significant variation in the zonal direction so in the tropical atmosphere
2:24 that is in the vicinity of the equator Coriolis force is very small this means that geostrophic balance does not hold so all of the analysis that we
2:32 performed previously that was relevant for the subtropics and the mid latitudes does not immediately apply here and an alternative analysis is necessary this
2:40 also means that baroclinic and stability and thus frontal activity is largely absent the tropics instead features a unique collection of waves and
2:48 disturbances that are not really found in the mid latitudes or the polar regions for instance equatorial roz be waves propagate along the equator as
2:56 well equatorial Kelvin waves do the same thing upward motion in this region is primarily driven through convective activity which is inherently chaotic in
3:04 nature as we discussed for rayleigh bernard convection essentially small inhomogeneities can trigger convective plumes however actually predicting the
3:15 location of that convective event can be quite difficult however we can nonetheless gain some insight into the drivers of this in homogeneity through
3:24 the study of organised convection in particular surface temperature differences play a major role in determining zonal in homogeneity along
3:32 the equatorial band that is surface temperature differences drive where convection and subsidence tend to occur on a large scale again the study of
3:42 convection is primarily statistical in nature so instead of looking at whether or not a convective event will occur in a particular location we usually look at
3:51 the climatological statistics of convection so in particular when sea surface temperatures are warm we have typically more convection when sea
4:00 surface temperatures are cooler convection is typically suppressed these combinations naturally give rise then to differences in surface pressure which
4:08 can then drive the atmospheric dynamics in the region along the equator the primary dynamical circulation is known as the Walker circulation this is in
4:18 addition to the Hadley circulation which operates in the meridian all direction the Walker circulation recall in our last lecture the discussion of the heat
4:26 cities of land and ocean in particular we discovered that the land surface has a much smaller heat capacity than over the ocean and consequently responds much
4:36 faster to solar insolation that is under radiation the land surface will heat up faster than the ocean this leads to temperature differences that can drive a
4:45 circulation such as the Walker circulation but other circulations as well including for instance sea breeze effects in the tropical atmosphere the
4:55 Walker circulation naturally produces rising motion over the land surface and sinking motion over the oceans in particular pay attention to the Pacific
5:04 Ocean here under a strong Walker circulation the rising motion is strong over the maritime continent this then circulates over the Indian Ocean and the
5:14 Pacific Ocean sinking as it does so that is subsiding towards the surface in the cooler regions in the eastern Pacific however this circulation exists over
5:23 essentially all of the continents and oceans in accordance with the picture here although the near surface winds are
5:30 climatologically easterly and along the equator this does not capture the nuance that's present because of the zonal in modernity the surface temperature
5:39 differences end up playing a much greater role in determining the direction of motion all right so the Walker circulation is a conceptual model
5:47 of air flow in the tropics that is caused by differences in heat distribution between ocean and land and it's the dominant description of this
5:55 zonal circulation along the equator warmer land surface temperatures and colder ocean temperatures are driven by differences in the heat capacities of
6:03 these two surfaces the Walker circulation which is then zonal along the equator is perpendicular to the Hadley circulation which describes
6:11 meridian emotions and energy moisture momentum exchanges between the tropics and mid-latitudes okay what about the tropical ocean recall we've discussed
6:21 previously that the easterly wind stress that is dominant in the equatorial near surface in conjunction with the beta plane that is the variation of Coriolis
6:30 force from negative values in the southern hemisphere to positive values in the northern hemisphere gives rise to divergent flow in the location of the
6:37 Equator thus in the ocean we must that there is an induced upwelling along the equator that is waters are pulled from below in order to replace the
6:46 waters that are pushed meridian elite towards either pole because of the Ekman layer stated another way the equatorial strip is a region of upwelling since the
6:57 trade winds naturally drive fluid away from the equator in the surface layer and so they demand a supply of water from below this then produces the
7:06 induced upwelling that occurs within this region and consequently produces this shallower thermocline that tends to occur in the equatorial regions as
7:14 opposed to in the subtropical regions the air-sea interaction then plays a major role in governing the dynamics of both the atmosphere and the ocean since
7:26 the pacific ocean is bounded the westward equatorial current induced by the wind stress then pushes warm water towards the western pacific meanwhile we
7:36 also have upwelling that is bringing cold ocean waters from depth to the surface particularly in the east that is warm waters are pushed from the eastern
7:45 pacific towards the west consequently producing a shallower thermocline in the east because of the shallower thermocline it's naturally easier for
7:54 the upwelling in order to bring colder deep waters to the surface in this region the combination then leads to a deeper thermocline that occurs in the
8:05 west and a shallower thermocline in the east this is reflected in many of the properties of the tropical atmosphere and ocean what we generally see is
8:15 warmer ocean temperatures and accumulated waters leading to higher sea levels in the western Pacific this is in part because of the greater column
8:24 height induced by warmer oceanic waters that is recall that warmer waters 10:00 generally have lower density associated with them and higher surface height this
8:34 is because of thermal expansion of those ocean waters because of this a circulation naturally emerges from this pattern where waters are pushed from
8:43 east to west they then sink in the western Pacific and recirculate so as to rise in the east where upwelling is stronger here we
8:52 see a schematic diagram show the natural circulation that exists within the ocean as well as the Walker circulation which governs the
8:59 atmospheric motions evaporation and solar heat are important in driving both of these processes in the western Pacific we see warmer ocean waters as
9:09 well as warmer surface temperatures wetter conditions and lower surface pressures in the East on the other hand we tend to see cooler temperatures drier
9:20 conditions as well as higher surface pressures recall these images showing the mean climatology of the global oceans here we see the sea surface
9:31 height anomaly as discussed we see that in the western Pacific sea surface Heights tend to be higher than in eastern Pacific again this is in part
9:39 driven by the easterly circulation in the oceans pushing waters from east to west sea surface height is also influenced by thermal expansion of the
9:48 warmer western Pacific Ocean waters near surface temperatures show an analogous pattern that is we see warmer waters in the west and cooler waters in the east
10:00 analogously we see warmer near surface temperatures in the west and cooler temperatures in the east again the easterly flow that dominates the tropics
10:09 in this region pushes the water from the east to the west in the West we get an accumulation of warmer near surface waters that have been warmed through
10:17 radiation in the east we get cooler temperatures because upwelling brings colder fluid from depth to the surface because of the presence of warmer
10:28 temperatures in the western Pacific it also has a propensity towards the generation of convection that is convection will tend to occur because of
10:37 the warmer near surface temperatures convection then vacates air mass that is in the near surface in this region leading to lower pressures overall so
10:47 what we generally see is lower pressures in the western Pacific and higher pressures in the eastern Pacific this atmosphere ocean interaction is known as
10:57 the burek nice feedback the particular phase of the Birkins feedback we've discussed is the positive phase under this phase we obtain warmer sea surface
11:06 temperatures in the western tropical Pacific and cooler sea surface temperatures in the Eastern tropical Pacific again this is driven by easterly
11:13 wind stress that occurs along the equator because of warmer sea surface temperatures in the West we end up generating convection in the West which
11:21 drives a strong Walker circulation the Walker circulation recirculates air across the Pacific that is at higher altitudes we have westerly flow air
11:30 blows from west to east and subsides in the central and eastern Pacific as a consequence this air then drives easterly wind stress that acts on the
11:41 ocean that is the air recirculates in order to replace the air that has been convected upward consequently we obtain flow from east to west the easterly wind
11:52 stress then induces equatorial upwelling that brings up water that is colder than normal this upwelling is again dominant in the eastern Pacific leading to colder
12:01 sea surface temperatures in the East consequently this reinforces the cycle producing warmer sea surface temperatures in the West and cooler sea
12:09 surface temperatures in the east york nice feedback refers to this positive feedback cycle in the tropics is responsible for enhancing the sea
12:18 surface temperature gradient that is enhancing sea surface temperature differences between western Pacific and eastern Pacific as well as amplifying
12:26 the easterly wind stress thus it's a feedback mechanism that works on both atmosphere and ocean and enhances the connectivity between atmosphere and
12:34 ocean the results of this are depicted here in the atmosphere we again have low pressures occurring in the western Pacific which is associated with
12:42 convective activity the convective activity drives the Walker circulation which results in westerly flow at altitude and subsiding both motion in
12:51 the central and eastern Pacific that then drives high surface pressures in the East and Central Pacific the returning branch of the circulation then
13:00 results in air moving from high pressure to low pressure in order to feed into the convection the convective plume the easterly flow along the equator drives
13:09 easterly motion along the tropical ocean surface this pushes warm waters from east to west and produces a warm pool in the western Pacific in order to replace
13:21 those waters upwelling is enhanced in the eastern Pacific sinking motion in the western Pacific then provides water that
13:28 recirculates in order to fuel the upwelling alright now that we've talked about the basics of the tropical circulation let's now look at the El
13:38 Nino Southern Oscillation which has to do with strengthening and weakening of the circulation just discussed so under the positive phase of the Birkins
13:47 feedback we already discussed how warmer sea surface temperatures in the western Pacific drive the Walker circulation which leads to easterly wind stress
13:55 equatorial upwelling and enhances that western - eastern sea surface temperature gradient however anomalous forcing either
14:05 extra-tropical effects in the atmosphere or a relaxation in the tilt of the thermocline in the ocean can potentially interrupt the burek knees feedback this
14:13 can actually result in the feedback being driven in Reverse under the negative phase some extra tropical disturbance may result in a slightly
14:22 weakened Walker circulation if the Walker circulation is weakened then the easterly wind stress is also weakened this allows Western Pacific waters to
14:30 shift to the east consequently moving that warm pool from the western Pacific to the Central Pacific because of higher stratification now occurring in the
14:39 Eastern and Central Pacific this leads to a decrease in upwelling which further enhances the near surface temperatures the sea surface temperatures increase in
14:47 the central and eastern Pacific and decrease in the western Pacific thus weakening the Walker circulation further if there is instead a oceanic driver to
14:57 the reversal of the burek knees feedback such as a change in the tilt of the thermocline then we can go through an analogous cycle except in this case we
15:05 may start at the top where a slight sea surface temperature increase in the central and eastern Pacific and a decrease in the western Pacific then
15:13 drives the weakened Walker circulation as the Walker circulation weakens these feedbacks between atmosphere and ocean drive a tweeker still that is a slightly
15:23 weakened Walker circulation will continue to weaken in accordance with synchronization between atmosphere and ocean under these equatorial El Nino
15:34 conditions namely under the weakening of the Walker circulation and the tendency for the flow to be pushed into a negative phase
15:42 we have Eastern forcing along the oceanic surface that's reduced from under the normal conditions discussed earlier these warmer surface waters then
15:54 are shifted eastwards leading to suppression upwelling with warmer waters now in the center of the Pacific the region of deep convection shifts to this
16:02 region as well and so the low surface pressure region shifts from the western Pacific to the Central Pacific and so we tend to get more convection occurring in
16:10 the Central Pacific region this also shifts the direction of the wind stresses and so there's a tendency for the warmer ocean waters to not
16:19 concentrate in the western Pacific but now to concentrate in the Central Pacific consequently these El Nino conditions or this strong negative phase
16:28 is associated with much warmer Pacific waters in the central and eastern Pacific the circulation itself is also changed with convection occurring
16:37 primarily in the Central Pacific and high pressures occurring in the east and west associated with subsiding motion upwelling is again suppressed in the
16:46 ocean once the ocean has reached an El Nino state some other extra tropical disturbance or some disturbance in the ocean could easily turn the feedback
16:58 cycle back to its positive phase thus enhancing the Walker circulation and driving it back towards the conditions described earlier in this lecture the
17:06 oscillation between these two states that is a weakened Walker circulation and a strengthened Walker circulation or correspondingly between warm sea surface
17:15 temperatures in the West and warm sea surface temperatures in the Central Pacific is known as the El Nino Southern Oscillation formerly the El Nino
17:23 Southern Oscillation is a pattern of variation in winds and sea surface temperatures in the tropical Pacific Ocean with a period of about two to
17:31 seven years that is the ocean atmosphere system tends to switch between El Nino and La Nina States with a period of about two to seven years in the top plot
17:41 here we see the La Nina conditions which is an enhanced state enhance condition analogous to what we discussed earlier in this lecture
17:49 under Lenina conditions we have a strong Walker circulation that again concentrates convective activity in the western Pacific along with warmer ocean
17:57 waters the El Nino conditions are associated with a weak Walker circulation where convection is instead concentrated in the Central Pacific
18:05 along with warmer Pacific Ocean waters the transition process between these two states occurs because of a mutual interaction of the atmosphere ocean
18:15 system that is through the feedbacks that we've discussed in this class regarding how atmosphere and ocean interact with one another the cold Glen
18:25 amia conditions shown in the top here correspond to an enhancement of the neutral state of the equatorial Pacific where the neutral state is again what we
18:33 discussed earlier in this lecture under tropical dynamics it's also associated with a strengthening of the Bjerknes feedback that is the positive phase
18:41 pressure decreases in the West and increases in the Central Pacific during the warm El Nino events the warm pool instead spreads eastwards bringing
18:50 atmospheric convection along with it and weakening the Walker circulation pressure increases in the West and decreases in the mid-ocean here are sea
19:01 surface temperature patterns associated with La Nina conditions on the top that is positive phase and so neutral conditions in the middle and negative
19:10 and so conditions or Lent El Nino conditions on the bottom these are three years in the 1990s where these patterns emerged in the tropical Pacific on the
19:22 top we see colder temperatures a so-called cold Tong of fluids stretching along the eastern Pacific Ocean this is strengthened upwelling that is then
19:31 being carried westwards by the strong easterly wind stress under neutral conditions we still see the presence of cooler sea surface temperatures in the
19:40 east but they're not as strong as they honor under the La Nina conditions seen in the top block and under the El Nino conditions we see no such presence of
19:50 significantly stronger eastern Pacific cold temperatures instead what we see is a fairly uniform temperature gradient between western Pacific and eastern
20:01 Pacific in particular we see as enhancement here of Central Pacific temperatures relative to the Lenina state shown above recall again El Nino
20:10 conditions are associated then with convective activity which tends to occur in the Central Pacific where as Lonnie Nia conditions where we have western
20:19 Pacific temperatures being much higher than eastern Pacific temperatures are associated with stronger convection in the western Pacific
20:27 so in summary climatologically average conditions are not necessarily representative of the persistent climatological state of the Pacific
20:35 these El Nino conditions describe effectively much of the variability that occurs within the Pacific Ocean the tendency is on this two to seven-year
20:46 cycle for the Pacific to swap between El Nino and La Nina States in fact this equatorial Pacific region is subject to this non seasonal variability again
20:55 occurring on with a period of approximately 2 to 7 years associated with enhancement of the normal conditions and suppression of those
21:04 conditions via the Bjerknes feedback the El Nino and La Nina are then warm and cool phases of this recurring climate pattern known as ENSO Nino Southern
21:16 Oscillation is an excellent topic to occur near the culmination of this class because it's so representative of the mutual interactions that occur between
21:24 the atmosphere and ocean a weakening of the Walker circulation and/or eastward movement of the warm pool is the primary triggering mechanism for these little
21:33 Nino events and they lead to a process of steps which drive mutual interaction between the atmosphere ocean system through the processes that we've
21:41 discussed earlier in the class these mutual interactions then drive a feedback loop which can allow the system to swap between these two states under
21:50 the first step of this transition the atmosphere responds naturally to the ocean that is the east-west pressure gradient is reduced to due to changes in
21:59 the lower boundary leading to a weakening of the Walker circulation under the second step the ocean naturally responds to the atmosphere
22:06 that is with the weakening wind stress equatorial upwelling is also weakened and the thermocline deepens this raises the sea surface temperatures in the east
22:15 leading to a more you know form distribution of sea surface temperatures across the equatorial Pacific the transition from El Nino to
22:22 LA anemia it essentially occurs from running this above feedback in Reverse but again occurs because of mutual
22:29 interactions between atmosphere and ocean systems here's another depiction of the conditions of the atmosphere and ocean
22:36 under ENSO again emphasizing some of the key characteristics that we've seen under El Nino conditions convection is dominant in the Central Pacific warm
22:44 water concentrates in the Central Pacific and tends to sink here as well driving weaker upwelling in the eastern Pacific under Lyonya conditions we have
22:52 strong trade winds that reach across the extent of the equatorial Pacific driving higher sea surface temperatures in the West warmer waters in the West as well
23:00 as convection in the West neutral conditions appear more similar to La Nina conditions in terms of the location of convection however they're associated
23:09 with weaker trade winds weaker easterly winds stress and thus a weaker circulation in both the atmosphere and the ocean the state of ENSO is usually
23:19 quantified using an index such as the Southern Oscillation index although other indices are available that rely on variables such as sea surface
23:27 temperature however the Southern Oscillation index takes advantage of the fact that the sea level pressure minima and Maxima shift during La Nina and El
23:36 Nino conditions that is under Lenina conditions we typically have lower sea level pressures in Darwin and higher sea level pressures in Tahiti
23:44 Arwen is our representative location in the western Pacific over the maritime continent Tahiti is our representative location for the Central Pacific being
23:52 located on an island in the Central Pacific these two locations are chosen because of the relatively long sea level pressure record both locations are
24:01 located slightly south of the Equator Darwin at 12 degrees south and Tahiti at 17 degrees south here is a plot of the trans-pacific dipole which is a
24:12 representation of these pressure differences that occur across the Pacific whenever pressures are high in Darwin we
24:19 see low pressures that occur in Tahiti with almost perfect anti-correlation consequently these two locations are very representative of this
24:27 trans-pacific dipole monthly values of the Southern Oscillation index are shown here dating back to before 1880 again positive
24:37 values are associated with a tendency for lower sea level pressures in the western Pacific and negative values are associated with lower sea sea level
24:47 pressures in the Central Pacific whenever the index shows sustained values above +7 we usually refer to that as a La Nina event whenever the
24:56 sustained value is below negative 7 we refer to that as an El Nino event interline any events occurred in 1973 to 74 75 to 76 88 to 89 98 to 99 and 2010
25:11 to 2011 major El Nino events occurred in 1982 to 83 97 to 98 and 2015 to 2016 here we see in the top plot the south and Oscillation index and in the bottom
25:26 plot we see the sea surface temperature differences under the nino 3.4 region which is a measurement region for sea surface temperatures in the Central
25:34 Pacific what we see is near exact anti-correlation between the Southern Oscillation index and sea surface temperatures within this region that is
25:43 whenever an El Nino event occurs corresponding to a negative value of the Southern Oscillation index we see much warmer temperatures through the Central
25:51 Pacific analogously when we see a positive Southern Oscillation index we see much cooler sea surface temperatures than the Central Pacific the time series
26:00 here has been filtered to remove short term oscillations and make it clearer to see these correlations under El Nino conditions there are a number of global
26:11 factors that result because of these much warmer conditions and what we've discussed so far in this class we have learned that changes in the tropics can
26:19 influence the Hadley circulation which can have ramifications for the whole globe under El Nino conditions during the December January February season
26:28 that is the Northern Hemisphere winter El Nino conditions are associated with wetter and warmer conditions along the tropical Central Pacific and tropical
26:37 eastern Pacific but also associated with drier and warmer conditions in the western Pacific however this oscillation also can touch
26:46 regions all over the world in North America El Nino conditions are associated with warmer temperatures along the periphery of the Pacific
26:54 particularly along the Alaskan seaboard and in Japan but we also have wet conditions that occur for instance in Southern California under El Nino
27:03 conditions as well as wet and cool conditions that extend throughout the eastern US along the Gulf of Mexico Coast under the June July August season
27:14 El Nino conditions are associated with dry conditions in the western Pacific and dry and cool in northeastern Australia and New Zealand
27:23 but associated with wet conditions in the Central Pacific in the southern hemisphere El Nino conditions in June July August are associated again with
27:31 warm temperatures along the Pacific periphery as well as warmer temperatures in south eastern Brazil underlining new conditions we see essentially the
27:42 opposite under the Northern Hemisphere winter season La Nina conditions are associated with the wetter whisp a wetter western Pacific a drier and
27:50 cooler central and eastern Pacific cooler temperatures along the Alaskan seaboard and in Canada and cooler temperatures in Japan we also
27:59 see wetter conditions typically in the Pacific Northwest but drier and warmer conditions in the US southeast in the southern hemisphere winter season out La
28:11 Nina conditions are associated with cooler temperatures along in Chile and Peru this is again associated with cooler sea surface temperatures in this
28:20 region as well as wetter and cooler temperatures in wetter and cooler conditions in northern South America we also see drier conditions in the Central
28:30 Pacific and wetter and warmer conditions in the western Pacific El Nino is very closely connected to global temperatures here we see a plot of the global
28:42 temperature anomalies showing a clear anthropogenic signal towards warming conditions warming temperatures as we get closer to present but on top of that
28:51 anthro genic signal we also see strong variations associated with El Nino and new conditions during El Nino events the warm waters spread across much of the
29:01 Pacific and consequently we have a much larger area of warmer waters that can affect global seat surface temperature measurements also results in much warmer
29:10 near surface temperatures in the atmosphere which can enhance the moisture content of the atmosphere also enhance convection because of the warmer
29:18 temperatures throughout the Pacific can enhance the strength of the Hadley circulation and hence the storm track in the mid latitudes under La Nina
29:26 conditions the cold Tong extends further west along the from the eastern Pacific consequently we observe cooler overall global temperatures because there is a
29:36 large extent of colder deep ocean waters that have been brought up along this region El Nino is also strongly connected with extreme weather events
29:45 tropical cyclone activity in the North Atlantic Basin is particularly sensitive to El Nino influences when a moderate to strong El Nino event is present the
29:54 North Atlantic Basin tends to experience substantial reduction in cyclone numbers a 60% reduction in not the number of hurricane days and an overall reduction
30:03 in system intensity this change is due to stronger than normal westerly winds that typically occur during these El Nino years and hence the tendency
30:12 towards greater sheering in the atmosphere alright that's all for today thank you very much next time we'll be talking about Paleo climate