Blog Archive

Tuesday, February 25, 2014

Does Sea Ice loss create the condition for an emerging permanent El Nino state?

Does Sea Ice loss create the condition for an emerging permanent El Nino state?
Source: El Nino Wikipedia page.
Source: El Nino Wikipedia page.
Recently Doug asked: “…my question is, how far could this phenomenon go? What is the “end” 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?”
A main theme of Sea ice loss seems to be that the main northern hemispheric pressure gradient (Ref 1) – 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 “mode” 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 Niño 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 Niño)

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 Niño was continual (or “permanent”) 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 Niño. 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 Niño which in effect was perennial rather than intermittent up to 3 Ma. Corroborating evidence for a permanent El Niño is available in land-records that document the distinctive regional climate signatures associated with El Niño. 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 Niño conditions would have had a huge impact on the global climate given that, today, even brief El Niño 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 Niño, 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 Niño.
A major factor in the warmth of the early Pliocene was the persistence of El Niño 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 30 C.
This is inconsistent with observations which indicate that, at no time in the past, were sea surface temperatures much higher than 30 C. 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 Niño are robust. If the information available at present should prove accurate, then temperatures in excess of 30 C in some models, and problems in their ability to simulate a perennial El Niño, 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

El Niño and Southern Oscillation (ENSO): A Review (2012)
3.1 Self-sustained oscillators of ENSO
Bjerknes (1969) first hypothesized that interaction between the atmosphere and the equatorial eastern Pacific Ocean causes El Niño.In Bjerknes’ 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 & Schopf, 1988; Battisti & Hirst, 1989), the recharge oscillator (Jin, 1997a, b), the western Pacific oscillator (Weisberg & Wang, 1997; Wang et 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 Niño 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 & Weisberg, 2000; Trenberth & Stepaniak, 2001; Larkin a& Harrison, 2005; Yu & Kao, 2007; Ashok et al., 2007; Kao & 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 & Kao, 2007; Kao &Yu, 2009). The CP El Niño is also referred to as Date Line El Niño (Larkin & Harrison, 2005), El Niño Modoki (Ashok et al., 2007), or Warm Pool El Niño (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 Niño
[..] It is interesting to note that at least three of the four El Niño events in the 21st century (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 Niño 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 Niño 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 Niño 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 Niño
[..] 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 Niño. Kug et al. (2009) emphasized the fact that the equatorial easterly anomalies can suppress warming in the eastern Pacific during a CP El Niño 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 Niño can reduce the Indian monsoon rainfall more effectively than the EP El Niño. 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 Niño 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 Niño on Atlantic hurricanes may also be different from the conventional EP El Niño (Kim et al., 2009), but it has been shown that the anomalous atmospheric circulation in the hurricane main development region during the CP El Niño is similar to that during the EP El Niño (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 Niño years but decreases during EP El Niño 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 SST contrast across the equatorial Pacific was about 2 °C, much like during a modern El Niño 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 Earth 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 & Washington, 1996), evaporation feedbacks (Knutson & Manabe, 1995), or a weakening of the Walker circulation (Vecchi & Soden, 2007). But, the ocean could also oppose warming in the east because increased stratification enhances the cooling effect of upwelling (Clement et al., 1996; Seager & 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 Niño-like warming (Vecchi et al., 2008; Deser et al., 2010), despite evidence for a weakening of tropical atmospheric circulation (Vecchi et al., 2006; Zhang & Song, 2006). However, there is robust evidence for warming of the eastern equatorial Pacific duringthe 20th century (Bunge & 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; Wang et al., 2009; Rodriguez-Fonseca et al., 2009). Climate models project a weak reduction of the SST gradient into the 21st century (Knutson & Manabe, 1995; Collins et al., 2005; Meehl et al., 2007).
The lack of robust evidence for El Niño-like warming in models and observations could be due to cancellation among the mechanisms listed above, especially among the enhanced warming due to slower currents driven by 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 the thermocline to changes in the trade winds renders the Bjerknes feedback ineffective to amplify an initial El Niño-like warming (DiNezio et al., 2010; Clarke, 2010). For these reasons, a “permanent El Niño” in response to global warming is very unlikely, even if the Walker circulation weakens. Instead, climate models indicate that the equatorial Pacific may just warm up 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....

No comments: