Objective: Summarize mechanisms for Nuclear Nino during nuclear winter in CESM-WACCM4. Perform a literature review on the subject of El Nino as a result of volcanic eruptions.

1. African Kelvin Waves Due to Continental Cooling Initiate Westerly Wind Anomalies in Western Pacific (Khodri et al., 2017)

Khodri et al. (2017) found a 30-60% increase in the chance of an El Nino following a volcanic eruption in CMIP5 simulations, relative to climatological El Nino probability within 2 years. After a major tropical volcanic eruption, continental cooling over tropical Africa led to an extreme reduction in precipitation, triggering a Matsuno-Gill response where atmospheric equatorial Rossby and Kelvin waves induce easterly wind anomalies over the Atlantic and westerly wind anomalies over the Indian Ocean and western Pacific. Eastward propagating equatorial Kelvin waves that deposit westerly momentum tropical troposphere help to kickstart an El Nino following volcanic eruptions. Anomalous easterly wind anomalies in the Atlantic contribute to the Atlantic Nina, which has the potential to feedback on westerly wind anomalies in the Pacific. To develop evidence for this mechanism as a cause of the increased likelihood of El Nino, they performed a number of model experiments:

1. ATM – volcanic aerosol forcing on vertical temperature profile only
2. OCEAN – horizontal SST gradients changed and atmosphere responds
3. LAND – land surface albedo modification to simulate surface cooling
______a) LAND-T: all tropical regions cool
______b) LAND-ET: extratropical regions cool
______c) LAND-AFRICA: tropical Africa cools
______d) LAND-SEA: southeast Asia cools
______e) LAND-MC: maritime continent cools

LAND-AFRICA was responsible for ~80% of the initial westerly wind stress in the Pacific, with contributions from LAND-MC. After the forcing from the continental cooling is diminished, OCEAN is responsible for a larger portion of the westerly wind stress as the ocean side of the Bjerknes feedback kicks in.


2. Altered Horizontal SST Gradients Across the Pacific Due to Cooling Causes Westerly Wind Anomalies (Gabriel and Robock, 2015; Zebiak and Cane, 1987; Clement et al. 1996)

Clement et al., (1996) found that uniformly reducing solar radiation in the Pacific would reduce the SST gradient because the Western Pacific Ocean is more sensitive to direct sunlight for its heat budget compared to the Eastern Pacific. Therefore, this reduced basin-wide SST gradient would weaken the Walker Circulation and weaken the trade winds, starting the Bjerknes feedback.

3. Southward Shift in Hadley Circulation Shifts Westerlies Southward, Kickstarts El Nino Conditions
(Pausata et al., 2015; Stevenson et al. 2016; McGregor et al., 2010; Stevenson et al., 2017)

Cooling of NH is rapid during the summer, which leads to a southward shift in the mean position of the ITCZ (as defined by the maximum wind convergence per longitude band). A southward shift in the ITCZ increases westerly winds over the equatorial Pacific, kickstarting an El Nino. Surface easterlies are weakest in the proximity of the ITCZ, so an equatorward shift of the ITCZ implies a weakening of the easterly winds along the equator. Via Bjerknes feedback, a reduction in the east-west temperature contrast occurs, favoring an El Nino. However, in our Nuclear War simulations the westerly wind anomalies begin in the Indian Ocean and propagate eastward. Need to reconcile this with this theoretical mechanism.




ENSO Literature Review

Zebiak, S. and Cane, M., (1987). A Model El Nino-Southern Oscillation. Monthly Weather Review, 115, 2262-???, http://doi.org/10.1175/1520-0493(1987)115<2262:AMENO>2.0.CO;2.

Similarities between model and observed: interannual variability, spatial structure, temporal evolution.
Differences: SST propagation during warm events, eastward migration of wind anomalies. Interperiod events where easterly anomalies propagate westward from the eastern model region.

Clement, A. C., Seager, R., Cane, M. A., and Zebiak, S. E.: An ocean dynamical thermostat, J. Climate, 9, 2190–2196, 1996.

Uses the Zebiak-Cane model to investigate role of ocean dynamics in SSTs.
Ramanthan and Collins (1991) found that cirrus clouds act as a brake on positive feedback of water vapor in tropical atmosphere as a destablizing effect on tropical climate. (Super Greenhouse Effect)
a uniform solar dimming is likely to result in a muted zonal SST gradient across the equatorial tropical Pacific because the western Pacific mixed layer's heat budget is almost exclusively from solar heating. In the east, both horizontal divergence and strong upwelling contribute to the mixed layer heat budget.
A diminished zonal SST gradient weakens the trade winds and results in less upwelling, an elevated thermocline, leading to the Bjerknes feedback.
La Nina event probability peaks in the third year post-eruption (Maher et al., 2015).

Deser, C., Capotondi, A., Saravan, R., Phillips, A., (2006). Tropical Pacific and Atlantic Climate Variability in CCSM3. Journal of Climate, 19, 2451-24.. https://doi.org/10.1175/JCLI3759.1

'Recharge oscillator paradigm has been proposed (Jin 1997a,b) that is dynamically consistent with the delayed oscillator but emphasizes meridional mass transports rather than wave processes. The interannual evolution of equatorial Pacific heat content anomalies is consistent with the recharge oscillator mechanism. Theory explains deepening of thermocline in eastern equatorial Pacific and growth phase of instability in terms of downwelling eq Kelvin waves forced by anomalous wind stress. The recharge oscillator paradigm can be summarized as follows: during warm events, equatorial easterlies are weakened, and the equatorial thermocline is flatter than average. Reduced zonal gradient of thermocline depth is associated with anomalous poleward Sverdrup transport, shoaling equatorial thermocline and progressively reducing original SST anomaly in eastern eq Pacific as well as wind stress anomaly. This acts as a brake on the original El Nino, slowly reducing the SST anomaly in the equatorial Pacific, reversing the phase of ENSO. A broader zonal wind stress anomaly is characterized by a wind stress curl anomaly farther from the equator, generating off-eq Rossby waves w/ slower velocities than a narrower wind stress anomaly. This promotes longer-duration ENSO events.

Emile-Geay, J., (2007). Volcanoes and ENSO over the Past Millennium. Journal of Climate, 21, 3134-???. https://doi.org/10.1175/2007JCLI1884.1

Climate model of intermediate complexity used to draw diagram of El Nino likelihood as a function of intensity of volcanic forcing. Shows that El Nino events tend to occur in the year subsequent tropical eruptions, including Tambora (1815) and Krakatau (1883). Used 200 ZC simulations lasting 10000 years each, showed the probability of an El Nino event in the year after a simulated volcanic forcing never exceeded 43%.





McGregor, S., and A. Timmermann, 2011: The effect of explosive tropical volcanism on ENSO. J. Climate, 24, 2178–2191, doi:10.1175/2010JCLI3990.1.

Uses CCSM3 to determine the initial tropical Pacific Ocean response to volcanic forcing is determined by four different mechanisms: Dynamical Thermostat Mechanism and others are related to zonal equatorial gradients of mean cloud albedo, Newtonian cooling, and mixed layer depth.
Following ENSO recharge dynamics (Jin 1998), curl of off-equatorial wind changes accompanying initial volcanically forced equatorial regional cooling acts to recharge equatorial region heat. Recharging of the equatorial region (has been shown to lead the eastern equatorial Pacific SSTA) switches equatorial region La Nina cooling to El Nino conditions six months after zonal mean thermocline depth reaches maximum depth.

Zheng, F., Fang, X., Yu, J., Zhu, J., (2014). Asymmetry of the Bjerknes positive feedback between the two types of El Nino. Geophysical Research Letters,41, 7651-7657, doi:10.1002/2014GL062125.

Modoki El Nino (CP El Nino) exhibits a weaker relationship between the zonal wind anomaly and zonal gradient of SST. Caused by cancelation induced by neg SST-cloud thermo feedback to pos dyn feedback for CP El Nino and off-equator shift of the max SLP anomalies.
Bjerknes (1969) pointed out the initial positive SST anomaly in eq eastern Pacific reduces east-west SST gradient and weakens the Walker circulation, resulting in weaker trade winds. The Bjerknes positive feedback strength can be measured as the correlation or regression between zonal gradient of SST anomaly (dSST/dx) and zonal wind stress (dSST/dx).

Pausata, F., Chafik, L., Caballero, R., Battisti, D., (2015). Impacts of high-latitude volcanic eruptions on ENSO and AMOC, PNAS, 112, 13784-13788. doi:10.1073/pnas.1509153112.

High latitude eruptions cause a southward shift in the ITCZ, which is where the weakest surface easterly winds are. Thus, if the ITCZ moves south into the region where winds are important to driving ENSO dynamics, then it can favor El Nino development. Via Bjerknes feedback, a reduction in the east-west temperature contrast occurs, favoring an El Nino.





Stevenson, S., Otto-Bliesner, B., Fasullo, J. & Brady, E. 2016, “El Nino-Like Hydroclimate Responses to Last Millennium Volcanic Eruptions”, Journal of Climate, 29, 2907-2921. doi:10.1175/JCLI-D-15-0239.1.

Hydroclimate anomalies in monsoonal Asia and western US resemble ENSO teleconnection pattern after tropical and NH high latitude volcanic eruptions even when ENSO neutral conditions are present. Claim: shift in the ITCZ causes monsoon to be suppressed, but couldn't it be the land-sea gradient that is suppressing the monsoon? Doesn't the temperature of continental Asia determine the strength of the ITCZ, not that the strength of the ITCZ determines the amount of precipitation in Asia? Chicken v. egg argument here, but I think cooling of the continents causes the reduction of precipitation due to reduction in evaporation, and then the monsoon shuts down because the temperature gradient just isn't there.
Proxy evidence indicates the initiation of El Nino is favored by volcanic eruptions in the year after the eruption (Adams et al. 2003; Li et al. 2013) which has been documented in climate models (Timmreck 2012)
In climate models, generally attributed to the oceanic dynamical thermostat mechanism (Clement et al., 1996). This mechanism explains westerly winds during zonal global dimming because the Western Pacific is more sensitive to direct sunlight compared to the East Pacific. The heat budget of the Eastern Pacific is far more complicated. Therefore, uniform zonal dimming of sunlight will decrease the temperature gradient across the Pacific Ocean, weakening the Walker Cycle and promoting El Nino conditions via the Bjerknes feedback. However, Khodri et al., (2017) found that this mechanism was not the main mechanism responsible for starting the El Nino after volcanic eruptions. Khodri et al., (2017) found that eastward propagating Kelvin waves coming off of the coast of Africa weakened the surface trade winds and acted as an atmospheric forcing onto the ocean to shift the Pacific Ocean into El Nino conditions. The cooling of the Maritime Continent and Southeast Asia played a minor role, as well as the Ocean Dynamic Thermostat Mechanism played a role.



Sun, W., Liu, J., Wang, B., Chen, D., Liu, F., Wang, Z., Ning, L., (2018). A La Nina-like state occurring in the second year after large tropical volcanic eruptions during the past 1500 years. Climate Dynamics



Predybaylo, E., Stenchikov, G., Wittenberg, A., Zeng, F. 2017. Impacts of a Pinatubo-size volcanic eruption on ENSO. Journal of Geophysical Research: Atmospheres. doi:10.1002/2016JDO25796

Discusses importance of initial conditions on evolution of ENSO. Mentions equatorward Sverdrup transport of warm water following wind stress curl due to changes in ITCZ.



notes:

Main findings: Very little change to frequency or amplitude of ENSO in AGW or stratospheric geoengineering scenarios. ENSO typically exhibits a 2-7 year periodicity with warm (El Nino) and cold (La Nina_ events, each lasting 9-12 months and peaking during DJF. (McPhadden, 2006).

Zebiak and Cane, 1987) – ZC model describes the coupled ocean-atmospheric dynamics of the tropical Pacific.

Emile-Geay et al. (2007) – showed that El Nino events tend to occur in the year subsequent to major tropical eruptions, including Tambora (1815) and Krakatau (1883). Used 200 ZC simulations lasting 1000 years each, showed the probability of an El Nino event in the year after a simulated volcanic forcing never exceeded 43%.


La Nina event probability peaks in the third year post-eruption (Maher et al., 2015).

Held et al. (2010): dynamical thermostat mechanism