Does Sea Ice loss create the condition for an emerging permanent El Nino state?
Recently Doug asked: ââ¦my question is, how far could this phenomenon go? What is the âendâ state? Is it possible for example that we could find the jet stream staying in place for months at a time, years, decades? How wavy could this waviness become?â
A main theme of Sea ice loss seems to be that the main northern hemispheric pressure gradient (Ref 1) â the polar vortex collapses (Ref 2), maybe even persistent, in regards to the sea ice state.
This in turn changes the major air oscillation, the jet stream(Ref 2). Which basically means less air flow and/or different air flow – hence profound changes with ripple effects(due to extreme weather events) through out the affected earth systems. A new âmodeâ based on a different atmospheric regime is established, which primary characteristic seems to be persistence of conditions(Ref 3).
This new mode hints especially to a interconnection with the IPO index (Ref 4), which suggests that ocean circulation will be affected. The loss of sea ice and following rapid changes in atmospheric regime could change ocean wave generation which promotes a permanent El Nino configuration. When heat is just hanging in the upper surface of ocean waters(Ref 4), hence ocean dead zones will spread(Ref 5). Though, for the time wave generation seems to promote deep sea warmth (Ref 6).
Further implications arise with problems associated with grain production. This was highlighted in a recent Chapman talk, Richard Alley gave last month(Ref 7). Though he tied it to higher temperatures and persistent conditions, here we make a connection to persistent conditions combined with the possibility for an emerging permanent El Nino.
In the following a study on a past permanent El Nino state, from the Pliocene.
Magnitude of N.American warming during an El NiÃ±o event is comparable to that needed to melt glacial ice sheets (Huybers & Molnar, 2007)
Above image source: NOAA
The Pliocene Paradox (Mechanisms for a Permanent El NiÃ±o)
During the early Pliocene, 5 to 3 million years ago, globally averaged temperatures were substantially higher than they are today, even though the external factors that determine climate were essentially the same. In the tropics, El NiÃ±o was continual (or âpermanentâ) rather than intermittent. The appearance of northern continental glaciers, and of cold surface waters in oceanic upwelling zones in low latitudes (both coastal and equatorial), signaled the termination of those warm climate conditions and the end of permanent El NiÃ±o. This led to the amplification of obliquity (but not precession) cycles in equatorial sea surface temperatures and in global ice volume, with the former leading the latter by several thousand years. A possible explanation is that the gradual shoaling of the oceanic thermocline reached a threshold around 3 million years ago, when the winds started bringing cold waters to the surface in low latitudes. This introduced feedbacks involving ocean-atmosphere interactions that, along with ice-albedo feedbacks, amplified obliquity cycles. A future melting of glaciers, changes in the hydrological cycle, and a deepening of the thermocline could restore the warm conditions of the early Pliocene. Source (2006)
The Pliocene Paradox
Today, a large reduction in the east-west temperature gradient along the equator in the Pacific occurs only briefly during El NiÃ±o which in effect was perennial rather than intermittent up to 3 Ma. Corroborating evidence for a permanent El NiÃ±o is available in land-records that document the distinctive regional climate signatures associated with El NiÃ±o. Up to 3 Ma there was a persistence of mild winters in central Canada and the northeastern United States, droughts in Indonesia, and torrential rains along the coasts of California and Peru, and in eastern equatorial Africa. The onset of dry conditions in the latter region around 3 Ma favored the evolution of African hominids.
Persistent El NiÃ±o conditions would have had a huge impact on the global climate given that, today, even brief El NiÃ±o episodes can have a large influence.The reasons are evident in fig. 2 which shows a remarkably high correlation between tropical sea surface temperature and rainfall patterns. Tall, rain-bearing,convective clouds cover the warmest waters but highly reflective stratus decks that produce little rain cover the cold waters. During El NiÃ±o, the warming of the eastern equatorial Pacific reduces the area covered by stratus clouds thus decreasing the albedo of the planet, while the atmospheric concentration of the powerful greenhouse gas, water vapor, increases. Calculations with a General Circulation Model of the atmosphere indicate that this happened during the early Pliocene and contributed significantly to the warm conditions at that time.
The oceanic heat transport is effected by the meridional overturning of the oceanic circulation. A freshening of the surface waters in the extra-tropics,which increases the buoyancy of the upper ocean and inhibits overturning, can therefore reduce the heat transport. This is true for both the deep, slow thermohaline component of the circulation whose changes affect mainly the climate of the northern Atlantic, and also for the rapid, shallow wind-driven component whose changes affect mostly the tropics. Sufficiently large freshening in the extra-tropics can induce a perennial El NiÃ±o.
A major factor in the warmth of the early Pliocene was the persistence of El NiÃ±o in the Pacific; it contributed to global warming by causing the absence of stratus clouds from the eastern equatorial Pacific, thus lowering the planetary albedo, and by increasing the atmospheric concentration of water vapor, a powerful greenhouse gas. Today the atmospheric concentration of another greenhouse gas, carbon dioxide, is comparable to what it was in the early Pliocene, but the climate of the planet is not yet in equilibrium with those high values. It is possible that a persistence of high carbon dioxide concentrations could result in a return to a globally warm world if it were to melt glaciers and increase temperatures in high latitudes, and as a consequence cause the tropical thermocline to deepen by a modest amount, a few tens of meters. (Near the date line at the equator the thermocline is already so deep that its vertical excursions leave surface temperatures unaffected.)
A deepening of the tropical thermocline requires a reduction in the oceanic heat loss in the extra-tropics. However, in certain atmospheric models, warm conditions in high latitudes depend on the atmosphere gaining heat from the oceans. This is also the case in the coupled ocean-atmosphere climate model that recently was used to simulate the early Pliocene. In that model, the oceanic heat loss in the extra-tropics is balanced by the gain of heat in the eastern equatorial Pacific. This gain is possible in spite of higher sea surface temperatures in low latitudes because temperature gradients along the equator,and presumably the depth of the equatorial thermocline, do not change significantly. This means that, in the model, maximum sea surface temperatures in the western tropical Pacific rise significantly above 30C.
This is inconsistent with observations which indicate that, at no time in the past, were sea surface temperatures much higher than 30C. Are the models at fault, or is there a problem with the observations? More data from the western tropical Pacific (and also from currently warm regions to the west of upwelling zones) are needed to determine the maximum temperatures over the last millions of years, and to determine whether observations of perennial El NiÃ±o are robust. If the information available at present should prove accurate, then temperatures in excess of 30C in some models, and problems in their ability to simulate a perennial El NiÃ±o, could be indicative of flaws in the models, in the parameterization of clouds for example. The models are designed to reproduce the world of today, but it is unclear how much confidence we should have in the simulations of very different climates. Source (2006)
Above image source: NOAA
Supplemental Information about ENSO
3.1 Self-sustained oscillators of ENSO
Bjerknes (1969) first hypothesized that interaction between the atmosphere and the equatorial eastern Pacific Ocean causes El NiÃ±o.In Bjerknesâ view,an initial positive SST anomaly in the equatorial eastern Pacific reduces the east-west SST gradient and hence the strength of the Walker circulation, resulting in weaker trade winds around the equator.The weaker trade winds in turn drive the ocean circulation changes that further reinforce the SST anomaly.This positive ocean-atmosphere feedback leads the equatorial Pacific to a never-ending warm state.A negative feedback is needed to turn the coupled ocean-atmosphere system around.However, during that time, it was not known what causes a turnabout from a warmphase to a cold phase.In search of necessary negative feedbacks for the coupled system, four conceptual ENSO oscillator models have been proposed: the delayed oscillator (Suarez and Schopf,1988; Battisti and Hirst,1989), the recharge oscillator (Jin,1997a, b), the western Pacific oscillator (Weisberg and Wang,1997; Wanget al.,1999), and the advective-reflective oscillator (Picautet al.,1997).These oscillator models respectively emphasized the negative feedbacks of reflected Kelvin waves at the ocean western boundary, a discharge process due to Sverdrup transport, western Pacific wind-forced Kelvin waves, and anomalous zonal advection.These negative feedbacks may work together for terminating El NiÃ±o warming,as suggested by the unified oscillator (Wang,2001).
4. Different flavors of ENSO events
It has been increasingly recognized that at least two different flavors or types of ENSO occur in the tropical Pacific (e.g., Wang and Weisberg, 2000; Trenberth and Stepaniak, 2001;Larkin and Harrison, 2005; Yu and Kao, 2007; Ashok et al., 2007; Kao and Yu, 2009; Kug et al.,2009). The two types of ENSO are the Eastern-Pacific (EP) type that has maximum SST anomalies centered over the eastern tropical Pacific cold tongue region, and the Central-Pacific(CP) type that has the anomalies near the International Date Line (Yu and Kao, 2007; Kao andYu, 2009). The CP El NiÃ±o is also referred to as Date Line El NiÃ±o (Larkin and Harrison, 2005),El NiÃ±o Modoki (Ashok et al., 2007), or Warm Pool El NiÃ±o (Kug et al., 2009). As the central location of ENSO shifts, different influences or signatures may be produced in the eastern Pacific and corals. Therefore, it is important to know how these two types of ENSO differ in their structures, evolution, underlying dynamics, and global impacts.
4.1. Spatial structure and evolution of the Central-Pacific El NiÃ±o
[..] It is interesting to note that at least three of the fourEl NiÃ±o events in the 21stcentury (i.e., the 2002/03, 2004/05, and 2009/10 events) have been of the CP type. Yeh et al. (2009) compared the ratio of the CP to EP type of El NiÃ±o events in Coupled Model Intercomparison Project phase 3 (CMIP3)model simulations and noticed that the ratio is projected to increase under a global warming scenario. They argued that the recent increase in the occurrence of the CP El NiÃ±o is related to a weakening of the mean Walker circulation and a flattening of the mean thermocline in the equatorial Pacific, which might be a result of global warming (Vecchi et al., 2007). However, it was also argued that the increasing occurrence of the CP El NiÃ±o in recent decades could be an expression of natural multidecadal variability and not necessarily a consequence of anthropogenic forcing (Newmann et al., 2011; McPhaden et al., 2011).
4.2.Dynamics of the Central-Pacific El NiÃ±o
[..] Ashok et al. (2007) argued that the thermocline variations induced by this wind anomaly pattern are responsible for the generation of the CP ENSO.The equatorial westerly anomalies induce downwelling Kelvin waves propagating eastward and the equatorial easterly anomalies induce downwelling Rossby waves propagating westward and, together, they deepen the thermocline in the central Pacific to produce the CP El NiÃ±o.Kug et al. (2009) emphasized the fact that the equatorial easterly anomalies can suppress warming in the eastern Pacific during a CP El NiÃ±o event by enhancing upwelling and surface evaporation.However, they also argued that the mean depth of thermocline in the central Pacific is relatively deep and the wind-induced thermocline variations may not be efficient in producing the CP SST anomalies.Instead, they suggested that ocean advection is responsible for the development of the central Pacific warming.
4.3. Distinct climate impacts of the Central-Pacific ENSO
[..] The results shown in Kumar et al. (2006) also imply that the CP El NiÃ±o can reduce the Indian monsoon rainfall more effectively than the EP El NiÃ±o.In the Southern Hemisphere, the CP ENSO has been shown to have a stronger impact on storm track activity than the EP ENSO(Ashok et al., 2007).The 2009 CP El NiÃ±o event was argued to have an influence far south as to contribute to the melting of Antarctica ice by inducing a stationary anticyclone outside the polar continent and enhancing the eddy heat flux into the region (T. Lee et al., 2010). The influence of the CP El NiÃ±o on Atlantic hurricanes may also be different from the conventional EP El NiÃ±o(Kim et al., 2009), but it has been shown that the anomalous atmospheric circulation in the hurricane main development region during the CP El NiÃ±o is similar to that during the EP ElNiÃ±o (S.-K. Lee et al., 2010).
Opposite impacts were noticed for the tropical cyclone activity in the western Pacific: the tropical cyclone frequency in the South China Sea increases during CP El NiÃ±o years but decreases during EP El NiÃ±o years (Chen, 2011). These distinctly climate impacts of the EP and CP ENSOs imply that they may leave different signatures in paleoclimate proxies worldwide including corals, which needs to be explored.
6. ENSO under global warming
6.1.Climate response of the equatorial Pacific to global warming
Paleoclimatic records suggest that the strong east-west SST contrast of the annual-mean conditions in the equatorial Pacific may not be a stable and permanent feature. Average SSTcontrast across the equatorial Pacific was about 2Â°C, much like during a modern El NiÃ±o event(Wara et al.,2005) and during the warm early Pliocene (~4.5 to 3.0 million years ago). This mean state may have occurred during the most recent interval with a climate warmer than today, suggesting that the equatorial Pacific could undergo similar changes as the Earthâs warms up in response to increasing greenhouse gases.
Competing theories anticipate either a stronger or weaker east-west SST contrast in response to warming. The eastern Pacific would warm up more due to cloud feedbacks (Meehl andWashington,1996), evaporation feedbacks (Knutson and Manabe,1995), or a weakening of theWalker circulation (Vecchi and Soden,2007). But, the ocean could also oppose warming in theeast because increased stratification enhances the cooling effect of upwelling (Clement et al.,1996; Seager and Murtugudde,1997). The balance between these processes is not known, therefore it is unclear whether the SST gradient will strengthen or weaken in the future. For instance, the SST signature of these mechanisms has been difficult to detect in the simulations, modern observations, or proxies.
Modern observations do not show a robust pattern of El NiÃ±o-like warming(Vecchi et al.,2008; Deser et al., 2010), despite evidence for a weakening of tropical atmospheric circulation (Vecchi et al.,2006; Zhang and Song,2006). However, there is robust evidence for warming of the eastern equatorial Pacific duringthe 20th Century (Bungeand Clarke,2009). The tropical eastern Pacific SST trend may be also caused by the Atlantic warming (Kucharski et al., 2011) through the mechanisms of the Walker circulation across equatorial South America or inter-basin SST gradient and ocean dynamics (Wang, 2006; Wanget al., 2009; Rodriguez-Fonseca et al., 2009). Climate models project a weak reduction of the SST gradient into the 21stcentury (Knutsonand Manabe,1995; Collins et al.,2005; Meehl et al.,2007).
The lack of robust evidence for El NiÃ±o-like warming in models and observations could be due to cancellation among the mechanisms listed above, especially among the enhanced warming duetoslower currents drivenby a weaker Walker circulation and the enhanced cooling due to a more stratified ocean(DiNezio et al.,2009). Moreover, due to basic equatorial dynamics the adjustment of thethermocline to changes in the trade winds renders the Bjerknes feedback ineffective to amplify an initial El NiÃ±o-like warming (DiNezio et al.,2010; Clarke,2010). For these reasons, aâpermanent El NiÃ±oâ in response to global warming is very unlikely, even if the Walker circulation weakens. Instead, climate models indicate that the equatorial Pacific may just warmup slightly more that the tropics due to the effect of the weakening of the Walker circulation on equatorial currents and a differential in evaporative damping with the off-equatorial tropics (Liu et al.,2006; DiNezio et al.,2009).
6.2.Sensitivity of ENSO to global warming
Paleoclimate records and climate models overwhelmingly indicate that the Pacific will continue to be characterized by large seasonal and interannual variability as the Earth warms up.Seasonally-resolved tropical Pacific paleoclimate records from periods in the Earthâs history thatwere both warmer and colder than today show that interannual variability was present. Available Pliocene records, for example, show that ENSO frequency and amplitude were not significantly different from today (Watanabe et al.,2011;Scroxton et al.,2011).Moreover, glacial climatealso exhibited large seasonal and interannual variability as suggested by isotopic measurements on individual forams at the Last Glacial Maximum (Koutavas and Joanidis,2009), and coral records from prior glacial stages (Tudhope et al.,2001). No climate models have thus far be enable to render ENSO inactive in either warmer (Huber and Caballero,2003; Galeotti et al.,2010,von der Heydt et al.,2011) or cooler climates (Zheng et al.,2008).
Neither climate models and observations nor proxies provide a conclusive answer on whether ENSO is going to become stronger or weaker as the tropics warm up in response to increasing greenhouse gases (GHGs). Climate change simulations coordinated by the CMIP3 simulate a wide range of responses from weaker to stronger. Whether ENSO has changed due to recent observed warming is also controversial according to the observational record (e.g.,Trenberth and Hoar,1997; Harrison and Larkin,1997; Rajagopalan et al.,1997). For these reasons, the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report(AR4) concluded that there is no consistent indication of discernible changes in ENSO amplitude in response to increasing GHGs (Meehl et al.,2007; Guilyardiet al., 2009).
The direct cause of ENSO changes in response to climate changes is generally not straight forward.For instance, the CMIP3 models largely agree in the response of the background ocean conditions over which ENSO variability occurs, but they do not agree on whether or not ENSO will strengthen. The projected changes in the mean climate include a shoaled, less tilted, and sharper thermocline; weaker zonal currents; and weaker upwelling (Vecchi and Soden,2007; DiNezio et al.,2009). ENSO theory indicates that any of these changes in the mean climate can lead to changes in ENSO amplitude.Changes in ENSO amplitude have been attributed to changes in the depth and sharpness of the equatorial thermocline by theoretical, modeling, and observational studies.
For instance, a sharper and deeper thermocline leads to weaker ENSO amplitude in a simple coupled ocean-atmosphere model (Fedorov and Philander,2001). Observations, in contrast, suggest that the strong ENSO events in the 1980s and 1990s could be a result of a deepening of the thermocline after the 1976 climate shift (Guilderson and Schrag,1998) or a sharper thermocline due to GHG related warming (Zhang et al.,2008). However, the observational evidence is not conclusive because:(1) there is evidence of strong ENSO activity before the 20th Century (e.g. Grove,1988)and(2) ENSO has been relatively quiet during the first decade of the 21thCentury despite continued warming.Climate models exhibit a robust relationship between increased ENSO amplitude and a sharper equatorial thermocline (Meehl et al.,2001).
This relationship explains why the previousgeneration ocean models, which had very diffuse thermoclines, simulated much weaker ENSO variability than observed.Conversely, a sharper thermocline has been invoked to explain theincrease ENSO amplitude in some increasing GHG experiments (e.g.,Timmermann et al.,1999;Park et al.,2009). All models participating in CMIP3 simulate a sharper thermocline in response to increasing GHGs, yet not all of them simulate a stronger ENSO. Other physical processes, such as the shoaling of the thermocline, weaker upwelling, or warmer mean SST could also have an amplifying or damping effect on ENSO. Thus, it is reasonable to hypothesize that depending on the balance of these changes, ENSO could strengthen or weaken (e.g.,Guilyardiet al., 2009; Vecchi and Wittenberg,2010;Collins et al.,2010).
Climate model projections do not agree on whether ENSO will increase because the interaction of ENSO and changes in the mean climate lead to subtle changes in the ENSO feedbacks.The shoaling and sharpening of the thermocline enhances ENSO variability, but the warmer mean SST results in stronger atmospheric damping, thus weakening ENSO (van Oldenborgh et al.,2005;Kim and Jin,2010).The weakening of the Walker circulation and the increased thermal stratification associated with the surface intensified ocean warming, both robust features of the climate projections play opposing roles.This interaction results fundamentally from weaker climatological upwelling, driven by a weaker Walker circulation,and a stronger subsurface zonal temperature gradient, associated with the surface intensifiedocean warming.All models simulate these mechanisms, yet their net effect on ENSO is not equal leading to the wide range of ENSO responses (e.g., Fig.13). Overall,these studies show that there is a substantial amount of cancellation among the effect of the changes in the different ENSO feedbacks. As a result,the sensitivity of ENSO simulations to increasing greenhouse gases is much reduced. Source (2012)
1.) A warmer Earth increases the melting of sea ice during summer, exposing more dark ocean water to incoming sunlight. This causes increased absorption of solar radiation and excess summertime heating of the ocean — further accelerating the ice melt. The excess heat is released to the atmosphere, especially during the autumn, decreasing the temperature and atmospheric pressure gradients between the Arctic and middle latitudes. A diminished latitudinal pressure gradient is linked to a weakening of the winds associated with the polar vortex and jet stream. Since the polar vortex normally retains the cold Arctic air masses up above the Arctic Circle, its weakening allows the cold air to invade lower latitudes.
The recent observations present a new twist to the Arctic Oscillation (AO) — a natural pattern of climate variability in the Northern Hemisphere. Before humans began warming the planet, the Arctic’s climate system naturally oscillated between conditions favorable and those unfavorable for invasions of cold Arctic air. Greene says, “What’s happening now is that we are changing the climate system, especially in the Arctic, and that’s increasing the odds for the negative AO conditions that favor cold air invasions and severe winter weather outbreaks. Source (2012)
2.) The N Hemisphereâs Atmospheric Circulation Has Collapsed Creating a Persistent Polar Cyclone
A sudden stratospheric warming split the polar vortex in two in mid-January. Since then, the northern hemisphereâs atmospheric circulation has been behaving very strangely. An area of extreme high pressure formed over the Arctic ocean and lasted for months. It formed in response to prolonged subsidence of air from high above caused by the slow cooling of bubble of warm air that invaded the stratosphere. High pressure at the pole and a very weak polar vortex pushed cold air out of the Arctic towards north America and western Europe. Weather in the U.S. and Europe in February, March and April was exceptionally cold because polar air poured out of the Arctic. Source (2013)
3.) Dr Jennifer Francis, professor of Atmospheric Science at Rutgers University, explained earlier this year how the loss of summer Arctic sea ice has led to the weakening of the polar vortex. Source (2013)
4.) A Looming Climate Shift: Will Ocean Heat Come Back to Haunt us?
Considering the effects of the El NiÃ±o-Southern Oscillation (ENSO) on the vertical distribution of heat in the surface layers of the ocean. During El NiÃ±o heat builds up in the surface layers where it is able to interact with the atmosphere, and therefore raises global surfaces temperatures. During La NiÃ±a much more heat is transported to deeper ocean layers and, with the surface layers cooler-than-normal, global surface temperatures are cooler-than-average. Source (2013)
Breakthrough in El NiÃ±o-Forecasting July 2013: “It is still unclear to which extent global warming caused by humankind’s emissions of greenhouse gases will influence the ENSO pattern,” says Schellnhuber. âYet the latter is often counted among the so-called tipping elements in the Earth system, meaning that at some level of climate change it might experience a relatively abrupt transformation.” Certain data from the Earth’s past suggest that higher mean global temperatures could increase the amplitude of the oscillation
The Pliocene: A Permanent El NiÃ±o-like state? Yes & No. (Has many graphics)
Papers on permanent El NiÃ±o posted by Ari JokimÃ¤ki August 31, 2011
Tropical cyclones and permanent El NiÃ±o in the early Pliocene epoch 2010
Permanent El NiÃ±o during the Pliocene warm period not supported by coral evidence Published online March 9, 2011
Article teaser image source: CIRES