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Eisenman, I, Yu LS, Tziperman E.  2005.  Westerly wind bursts: ENSO's tail rather than the dog? Journal of Climate. 18:5224-5238.   10.1175/jcli3588.1   Website
Eisenman, I.  2005.  Non-normal effects on salt finger growth. Journal of Physical Oceanography. 35:616-627.   10.1175/jpo2716.1   Website
Eisenman, I, Untersteiner N, Wettlaufer JS.  2007.  On the reliability of simulated Arctic sea ice in global climate models. Geophysical Research Letters. 34   10.1029/2007gl029914   Website
Gebbie, G, Eisenman I, Wittenberg A, Tziperman E.  2007.  Modulation of westerly wind bursts by sea surface temperature: A semistochastic feedback for ENSO. Journal of the Atmospheric Sciences. 64:3281-3295.   10.1175/jas4029.1   Website
Eisenman, I, Untersteiner N, Wettlaufer JS.  2008.  Reply to comment on "On the reliability of simulated Arctic sea ice in global climate models". Geophysical Research Letters. 35   10.1029/2007gl032173   Website
Eisenman, I, Wettlaufer JS.  2009.  Nonlinear threshold behavior during the loss of Arctic sea ice. Proceedings of the National Academy of Sciences of the United States of America. 106:28-32.   10.1073/pnas.0806887106   Website
Eisenman, I, Bitz CM, Tziperman E.  2009.  Rain driven by receding ice sheets as a cause of past climate change. Paleoceanography. 24   10.1029/2009pa001778   Website
Eisenman, I.  2010.  Geographic muting of changes in the Arctic sea ice cover. Geophysical Research Letters. 37   10.1029/2010gl043741   Website
Ashkenazy, Y, Eisenman I, Gildor H, Tziperman E.  2010.  The Effect of Milankovitch Variations in Insolation on Equatorial Seasonality. Journal of Climate. 23:6133-6142.   10.1175/2010jcli3700.1   Website
Abbot, DS, Eisenman I, Pierrehumbert RT.  2010.  The Importance of Ice Vertical Resolution for Snowball Climate and Deglaciation. Journal of Climate. 23:6100-6109.   10.1175/2010jcli3693.1   Website
Armour, KC, Eisenman I, Blanchard-Wrigglesworth E, McCusker KE, Bitz CM.  2011.  The reversibility of sea ice loss in a state-of-the-art climate model. Geophysical Research Letters. 38   10.1029/2011gl048739   Website
Finnegan, S, Bergmann K, Eiler JM, Jones DS, Fike DA, Eisenman I, Hughes NC, Tripati AK, Fischer WW.  2011.  The Magnitude and Duration of Late Ordovician-Early Silurian Glaciation. Science. 331:903-906.   10.1126/science.1200803   Website
Eisenman, I, Schneider T, Battisti DS, Bitz CM.  2011.  Consistent Changes in the Sea Ice Seasonal Cycle in Response to Global Warming. Journal of Climate. 24:5325-5335.   10.1175/2011jcli4051.1   Website
Eisenman, I.  2012.  Factors controlling the bifurcation structure of sea ice retreat. Journal of Geophysical Research-Atmospheres. 117   10.1029/2011jd016164   Website
Merlis, TM, Schneider T, Bordoni S, Eisenman I.  2013.  Hadley Circulation Response to Orbital Precession. Part I: Aquaplanets. Journal of Climate. 26:740-753.   10.1175/jcli-d-11-00716.1   AbstractWebsite

The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with an aquaplanet slab-ocean lower boundary. Contrary to expectations, the simulated monsoonal Hadley circulation is weaker when perihelion occurs at the summer solstice than when aphelion occurs at the summer solstice. The angular momentum balance and energy balance are examined to understand the mechanisms that produce this result. That the summer with stronger insolation has a weaker circulation is the result of an increase in the atmosphere's energetic stratification, the gross moist stability, which increases more than the amount required to balance the change in atmospheric energy flux divergence necessitated by the change in top-of-atmosphere net radiation. The solstice-season changes result in annual-mean Hadley circulation changes (e.g., changes in circulation strength).

Merlis, TM, Schneider T, Bordoni S, Eisenman I.  2013.  Hadley Circulation Response to Orbital Precession. Part II: Subtropical Continent. Journal of Climate. 26:754-771.   10.1175/jcli-d-12-00149.1   AbstractWebsite

The response of the monsoonal and annual-mean Hadley circulation to orbital precession is examined in an idealized atmospheric general circulation model with a simplified representation of land surface processes in subtropical latitudes. When perihelion occurs in the summer of a hemisphere with a subtropical continent, changes in the top-of-atmosphere energy balance, together with a poleward shift of the monsoonal circulation boundary, lead to a strengthening of the monsoonal circulation. Spatial variations in surface heat capacity determine whether radiative perturbations are balanced by energy storage or by atmospheric energy fluxes. Although orbital precession does not affect annual-mean insolation, the annual-mean Hadley circulation does respond to orbital precession because its sensitivity to radiative changes varies over the course of the year: the monsoonal circulation in summer is near the angular momentum-conserving limit and responds directly to radiative changes; whereas in winter, the circulation is affected by the momentum fluxes of extratropical eddies and is less sensitive to radiative changes.

Merlis, TM, Schneider T, Bordoni S, Eisenman I.  2013.  The Tropical Precipitation Response to Orbital Precession. Journal of Climate. 26:2010-2021.   10.1175/jcli-d-12-00186.1   AbstractWebsite

Orbital precession changes the seasonal distribution of insolation at a given latitude but not the annual mean. Hence, the correlation of paleoclimate proxies of annual-mean precipitation with orbital precession implies a nonlinear rectification in the precipitation response to seasonal solar forcing. It has previously been suggested that the relevant nonlinearity is that of the Clausius-Clapeyron relationship. Here it is argued that a different nonlinearity related to moisture advection by the atmospheric circulation is more important. When perihelion changes from one hemisphere's summer solstice to the other's in an idealized aquaplanet atmospheric general circulation model, annual-mean precipitation increases in the hemisphere with the brighter, warmer summer and decreases in the other hemisphere, in qualitative agreement with paleoclimate proxies that indicate such hemispherically antisymmetric climate variations. The rectification mechanism that gives rise to the precipitation changes is identified by decomposing the perturbation water vapor budget into "thermodynamic" and "dynamic" components. Thermodynamic changes (caused by changes in humidity with unchanged winds) dominate the hemispherically antisymmetric annual-mean precipitation response to precession in the absence of land-sea contrasts. The nonlinearity that enables the thermodynamic changes to affect annual-mean precipitation is a nonlinearity of moisture advection that arises because precession-induced seasonal humidity changes correlate with the seasonal cycle in low-level convergence. This interpretation is confirmed using simulations in which the Clausius-Clapeyron relationship is explicitly linearized. The thermodynamic mechanism also operates in simulations with an idealized representation of land, although in these simulations the dynamic component of the precipitation changes is also important, adding to the thermodynamic precipitation changes in some latitudes and offsetting it in others.

Pistone, K, Eisenman I, Ramanathan V.  2014.  Observational determination of albedo decrease caused by vanishing Arctic sea ice. Proceedings of the National Academy of Sciences of the United States of America. 111:3322-3326.   10.1073/pnas.1318201111   AbstractWebsite

The decline of Arctic sea ice has been documented in over 30 y of satellite passive microwave observations. The resulting darkening of the Arctic and its amplification of global warming was hypothesized almost 50 y ago but has yet to be verified with direct observations. This study uses satellite radiation budget measurements along with satellite microwave sea ice data to document the Arctic-wide decrease in planetary albedo and its amplifying effect on the warming. The analysis reveals a striking relationship between planetary albedo and sea ice cover, quantities inferred from two independent satellite instruments. We find that the Arctic planetary albedo has decreased from 0.52 to 0.48 between 1979 and 2011, corresponding to an additional 6.4 +/- 0.9 W/m(2) of solar energy input into the Arctic Ocean region since 1979. Averaged over the globe, this albedo decrease corresponds to a forcing that is 25% as large as that due to the change in CO2 during this period, considerably larger than expectations from models and other less direct recent estimates. Changes in cloudiness appear to play a negligible role in observed Arctic darkening, thus reducing the possibility of Arctic cloud albedo feedbacks mitigating future Arctic warming.

Eisenman, I, Meier WN, Norris JR.  2014.  A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated? The Cryosphere. 8:1289-1296.: Copernicus Publications   10.5194/tc-8-1289-2014   AbstractWebsite

Recent estimates indicate that the Antarctic sea ice cover is expanding at a statistically significant rate with a magnitude one-third as large as the rapid rate of sea ice retreat in the Arctic. However, during the mid-2000s, with several fewer years in the observational record, the trend in Antarctic sea ice extent was reported to be considerably smaller and statistically indistinguishable from zero. Here, we show that much of the increase in the reported trend occurred due to the previously undocumented effect of a change in the way the satellite sea ice observations are processed for the widely used Bootstrap algorithm data set, rather than a physical increase in the rate of ice advance. Specifically, we find that a change in the intercalibration across a 1991 sensor transition when the data set was reprocessed in 2007 caused a substantial change in the long-term trend. Although our analysis does not definitively identify whether this change introduced an error or removed one, the resulting difference in the trends suggests that a substantial error exists in either the current data set or the version that was used prior to the mid-2000s, and numerous studies that have relied on these observations should be reexamined to determine the sensitivity of their results to this change in the data set. Furthermore, a number of recent studies have investigated physical mechanisms for the observed expansion of the Antarctic sea ice cover. The results of this analysis raise the possibility that much of this expansion may be a spurious artifact of an error in the processing of the satellite observations.

Zhu, J, Liu ZY, Zhang X, Eisenman I, Liu W.  2014.  Linear weakening of the AMOC in response to receding glacial ice sheets in CCSM3. Geophysical Research Letters. 41:6252-6258.   10.1002/2014gl060891   AbstractWebsite

The transient response of the Atlantic Meridional Overturning Circulation (AMOC) to a deglacial ice sheet retreat is studied using the Community Climate System Model version 3 (CCSM3), with a focus on orographic effects rather than meltwater discharge. It is found that the AMOC weakens significantly (41%) in response to the deglacial ice sheet retreat. The AMOC weakening follows the decrease of the Northern Hemisphere ice sheet volume linearly, with no evidence of abrupt thresholds. A wind-driven mechanism is proposed to explain the weakening of the AMOC: lowering the Northern Hemisphere ice sheets induces a northward shift of the westerlies, which causes a rapid eastward sea ice transport and expanded sea ice cover over the subpolar North Atlantic; this expanded sea ice insulates the ocean from heat loss and leads to suppressed deep convection and a weakened AMOC. A sea ice-ocean positive feedback could be further established between the AMOC decrease and sea ice expansion.

Ewing, RC, Eisenman I, Lamb MP, Poppick L, Maloof AC, Fischer WW.  2014.  New constraints on equatorial temperatures during a Late Neoproterozoic snowball Earth glaciation. Earth and Planetary Science Letters. 406:110-122.   10.1016/j.epsl.2014.09.017   AbstractWebsite

Intense glaciation during the end of Cryogenian time (similar to 635 million years ago) marks the coldest climate state in Earth history - a time when glacial deposits accumulated at low, tropical paleolatitudes. The leading idea to explain these deposits, the snowball Earth hypothesis, predicts globally frozen surface conditions and subfreezing temperatures, with global climate models placing surface temperatures in the tropics between -20 degrees C and -60 degrees C. However, precise paleosurface temperatures based upon geologic constraints have remained elusive and the global severity of the glaciation undetermined. Here we make new geologic observations of tropical periglacial, aeolian and fluvial sedimentary structures formed during the end-Cryogenian, Marinoan glaciation in South Australia; these observations allow us to constrain ancient surface temperatures. We find periglacial sand wedges and associated deformation suggest that ground temperatures were sufficiently warm to allow for ductile deformation of a sandy regolith. The wide range of deformation structures likely indicate the presence of a paleoactive layer that penetrated 2-4 m below the ground surface. These observations, paired with a model of ground temperature forced by solar insolation, constrain the local mean annual surface temperature to within a few degrees of freezing. This temperature constraint matches well with our observations of fluvial deposits, which require temperatures sufficiently warm for surface runoff. Although this estimate coincides with one of the coldest near sea-level tropical temperatures in Earth history, if these structures represent peak Marinaon glacial conditions, they do not support the persistent deep freeze of the snowball Earth hypothesis. Rather, surface temperatures near 0 degrees C allow for regions of seasonal surface melting, atmosphere-ocean coupling and possible tropical refugia for early metazoans. If instead these structures formed during glacial onset or deglaciation, then they have implications for the timescale and character for the transition into or out of a snowball state. (C) 2014 Elsevier B.V. All rights reserved.

Li, L, Miller AJ, McClean JL, Eisenman I, Hendershott MC.  2014.  Processes driving sea ice variability in the Bering Sea in an eddying ocean/sea ice model: anomalies from the mean seasonal cycle. Ocean Dynamics. 64:1693-1717.: Springer Berlin Heidelberg   10.1007/s10236-014-0769-7   AbstractWebsite

A fine-resolution (1/10°) ocean/sea ice model configured in the Community Earth System Model framework is compared with observations and studied to determine the basin-scale and local balances controlling the variability of sea ice anomalies from the mean seasonal cycle in the Bering Sea for the time period 1980–1989. The model produces variations in total Bering Sea ice area anomalies that are highly correlated with observations. Surface air temperature, which is specified from reanalysis atmospheric forcing, strongly controls the ice volume variability in this simulation. The thermodynamic ice volume change is dominated by surface energy flux via atmosphere-ice sensible heat flux, except near the southern ice edge where it is largely controlled by ocean-ice heat flux. While thermodynamic processes dominate the variations in ice volume in the Bering Sea on the large scale, dynamic processes are important on the local scale near ice margins (both oceanic and land), where dynamic and thermodynamic ice volume changes have opposite signs and nearly cancel each other. Ice motion is generally consistent with winds driving the flow, except near certain straits in the north where ice motion largely follows ocean currents. Two key climate events, strong ice growth with cold air temperature and northerly wind anomalies in February 1984 and weak ice growth with warm air temperature and southerly wind anomalies in February 1989, are studied here in detail. While the processes controlling the ice changes are generally similar to those in other years, these large events help reveal the characteristic spatial patterns of ice growth/melt and transport anomalies.

Li, L, McClean JL, Miller AJ, Eisenman I, Hendershott MC, Papadopoulos CA.  2014.  Processes driving sea ice variability in the Bering Sea in an eddying ocean/sea ice model: Mean seasonal cycle. Ocean Modelling. 84:51-66.   AbstractWebsite

The seasonal cycle of sea ice variability in the Bering Sea, together with the thermodynamic and dynamic processes that control it, are examined in a fine resolution (1/10°) global coupled ocean/sea-ice model configured in the Community Earth System Model (CESM) framework. The ocean/sea-ice model consists of the Los Alamos National Laboratory Parallel Ocean Program (POP) and the Los Alamos Sea Ice Model (CICE). The model was forced with time-varying reanalysis atmospheric forcing for the time period 1970–1989. This study focuses on the time period 1980–1989. The simulated seasonal-mean fields of sea ice concentration strongly resemble satellite-derived observations, as quantified by root-mean-square errors and pattern correlation coefficients. The sea ice energy budget reveals that the seasonal thermodynamic ice volume changes are dominated by the surface energy flux between the atmosphere and the ice in the northern region and by heat flux from the ocean to the ice along the southern ice edge, especially on the western side. The sea ice force balance analysis shows that sea ice motion is largely associated with wind stress. The force due to divergence of the internal ice stress tensor is large near the land boundaries in the north, and it is small in the central and southern ice-covered region. During winter, which dominates the annual mean, it is found that the simulated sea ice was mainly formed in the northern Bering Sea, with the maximum ice growth rate occurring along the coast due to cold air from northerly winds and ice motion away from the coast. South of St Lawrence Island, winds drive the model sea ice southwestward from the north to the southwestern part of the ice-covered region. Along the ice edge in the western Bering Sea, model sea ice is melted by warm ocean water, which is carried by the simulated Bering Slope Current flowing to the northwest, resulting in the S-shaped asymmetric ice edge. In spring and fall, similar thermodynamic and dynamic patterns occur in the model, but with typically smaller magnitudes and with season-specific geographical and directional differences.

Wagner, TJW, Eisenman I.  2015.  How climate model complexity influences sea ice stability. Journal of Climate. 28:3998-4014.   10.1175/jcli-d-14-00654.1   AbstractWebsite

Record lows in Arctic sea ice extent have been making frequent headlines in recent years. The change in albedo when sea ice is replaced by open water introduces a nonlinearity that has sparked an ongoing debate about the stability of the Arctic sea ice cover and the possibility of Arctic "tipping points.'' Previous studies identified instabilities for a shrinking ice cover in two types of idealized climate models: (i) annual-mean latitudinally varying diffusive energy balance models (EBMs) and (ii) seasonally varying single-column models (SCMs). The instabilities in these low-order models stand in contrast with results from comprehensive global climate models (GCMs), which typically do not simulate any such instability. To help bridge the gap between low-order models and GCMs, an idealized model is developed that includes both latitudinal and seasonal variations. The model reduces to a standard EBM or SCM as limiting cases in the parameter space, thus reconciling the two previous lines of research. It is found that the stability of the ice cover vastly increases with the inclusion of spatial communication via meridional heat transport or a seasonal cycle in solar forcing, being most stable when both are included. If the associated parameters are set to values that correspond to the current climate, the ice retreat is reversible and there is no instability when the climate is warmed. The two parameters have to be reduced by at least a factor of 3 for instability to occur. This implies that the sea ice cover may be substantially more stable than has been suggested in previous idealized modeling studies.

Wagner, TJW, Eisenman I.  2015.  False alarms: How early warning signals falsely predict abrupt sea ice loss. Geophysical Research Letters. 42   10.1002/2015gl066297   AbstractWebsite

Uncovering universal early warning signals for critical transitions has become a coveted goal in diverse scientific disciplines, ranging from climate science to financial mathematics. There has been a flurry of recent research proposing such signals, with increasing autocorrelation and increasing variance being among the most widely discussed candidates. A number of studies have suggested that increasing autocorrelation alone may suffice to signal an impending transition, although some others have questioned this. Here we consider variance and autocorrelation in the context of sea ice loss in an idealized model of the global climate system. The model features no bifurcation, nor increased rate of retreat, as the ice disappears. Nonetheless, the autocorrelation of summer sea ice area is found to increase in a global warming scenario. The variance, by contrast, decreases. A simple physical mechanism is proposed to explain the occurrence of increasing autocorrelation but not variance when there is no approaching bifurcation. Additionally, a similar mechanism is shown to allow an increase in both indicators with no physically attainable bifurcation. This implies that relying on autocorrelation and variance as early warning signals can raise false alarms in the climate system, warning of "tipping points" that are not actually there.