Sea Ice, Solar Radiation, and SH High-latitude Climate Sensitivity Alex Hall UCLA Department of Atmospheric and Oceanic Sciences SOWG meeting January 13-14, 2005

climate sensitivity parameter change in outgoing longwave with SAT change in net incoming shortwave with SAT classical climate sensitivity framework

surface albedo feedback has been thought for quite some time to be a possible contributor to high-latitude climate sensitivity… --Budyko (1969), Sellers (1969) --Manabe and Stouffer (1980) --Robock (1983) --Ingram et al. (1989) --Meehl and Washington (1990) --Bitz and Holland (2003)

surface albedo feedback to dQ/dT s. Climate sensitivity parameter Change in outgoing longwave with SAT Change in net incoming shortwave with SAT change in solar radiation with surface albedo change in surface albedo with SAT

Simulated reduction in reflected solar due to CO 2 doubling (Hall, 2004)

Geographical and seasonal distribution of the quasi- equilibrium SAT response to CO 2 -doubling.

Geographical and seasonal distribution of the SAT sensitivity when surface albedo feedback is suppressed.

Surface albedo feedback to dQ/dT s. Climate sensitivity parameter Change in outgoing longwave with SAT Change in net incoming shortwave with SAT change in solar radiation with surface albedo change in surface albedo with SAT

Could clouds neutralize ice albedo feedback in the Southern Hemisphere? (Qu and Hall 2005) To address this question, we examine the controls on SH planetary albedo in the recent climate record.

We assess the controls on SH planetary albedo by examining the ISCCP D2 data set ( ). For both clear and all-sky cases, the ISCCP data set (D2) contains (1) surface radiation fluxes (2) TOA radiation fluxes These were generated based on observations at 3 different channels (visible, near IR, and IR) and a radiative transfer model. Rossow and Gardner (1993a and b) J Clim. Rossow and Schiffer (1999) BAMS.

climatological planetary albedo (%) by season, ISCCP D2 data set There is a visible increase in planetary albedo with latitude in the southern hemisphere. Whether the atmosphere or the surface is responsible for this is particularly mysterious in the SH sea ice zone, as atmospheric albedo increases substantially with zenith angle.

The ratio of photons reflected by sea ice or atmosphere to the total number of photons reflected by the planet in the ISCCP data set. In the SH sea ice zone, the atmosphere accounts for about 80% of the photons reflected by the planet during most of the year. This demonstrates the importance of clouds in determining the mean solar radiation budget, even in this area of high surface albedo. SH sea ice zone DJF MAM JJA SON

What is the surface contribution to planetary albedo variations in the Southern Hemisphere cryosphere zones? standard deviation of seasonal-mean planetary albedo (%) in the ISCCP D2 data set broken down by season

SURFACE CLOUD RESIDUAL COVARIANCE Planetary albedo variability can be divided into contributions from four components: (1) SURFACE: the portion unambiguously related in linear fashion to surface albedo variability (2) CLOUD: The portion unambiguously related in linear fashion to cloud cover and optical depth variability (3) RESIDUAL: The portion that cannot be linearly related to either surface or cloud variability (4) COVARIANCE: The portion linearly related to surface and cloud variability but not unambiguously attributable to either. (1) (2) (3) (4)

SURFACE CLOUD RESIDUAL COVARIANCE The magnitude of the covariance term is generally small, indicating interannual variability in surface albedo and cloud are largely uncorrelated. However, in the SH sea ice zone, it is not completely negligible, suggesting some ice - cloud interaction. In particular, ice and cloud seem to be weakly anti- correlated. SH sea ice zone fraction of total variance DJF MAM JJA SON

SURFACE CLOUD RESIDUAL COVARIANCE The magnitude of the residual term is larger than the covariance term but is also generally small, but becomes larger as the sun becomes lower in the sky. In the winter, the residual term accounts for about 25% of the variance. This may be due to nonlinear dependence of planetary albedo on surface albedo and cloud when zenith angle is large. SH sea ice zone fraction of total variance DJF MAM JJA SON

SURFACE CLOUD RESIDUAL COVARIANCE Clouds account for 10-20% of the variance of planetary albedo in the SH sea ice zone throughout the calendar year. SH sea ice zone fraction of total variance DJF MAM JJA SON

SURFACE CLOUD RESIDUAL COVARIANCE The surface accounts for 60-75% of the variance in planetary albedo throughout the year in the SH sea ice zone. In the critical springtime months, when insolation and ice extent are large, sea ice accounts for about 3/4 of the variance. SH sea ice zone fraction of total variance DJF MAM JJA SON

Comparison to a climate model To allow for as direct a comparison with the ISCCP data as possible, we used a simulated time series with approximately the same mix of internal variability and externally-forced climate change:  a recent CCSM3 scenario run (T85)  data taken from the same time period as ISCCP ( ).

SURFACE CLOUD RESIDUAL COVARIANCE Controls on planetary albedo variability in CCSM3 SH sea ice zone fraction of total variance DJF MAM JJA SON

SH sea ice zone fraction of total variance ISCCPCCSM The model does a reasonable job simulating the relative contributions to planetary albedo in the SH sea ice zone. As in ISCCP, the covariance term is small but slightly negative, the residual term is largest when zenith angles are high, and the cloud term accounts for less variance than the surface term. However, in ISCCP, sea ice accounts for more variance in planetary albedo than CCSM through most of the year. DJF MAM JJA SON

Why is the contribution of sea ice smaller in CCSM3? --Is it that clouds are more variable, increasing the relative contribution of clouds? --Or is it that the CCSM3 atmosphere is more opaque to solar radiation, attenuating the effect of surface albedo anomalies? --Or is sea ice albedo itself less variable in CCSM3?

ISCCP CCSM3 Clear-sky surface albedo standard deviation (%) in ISCCP and CCSM3, broken down by season. ISCCP has consistently more sea ice albedo variability.

ISCCP CCSM3 Clear-sky surface albedo standard deviation (%) in ISCCP and CCSM3, broken down by season. The larger surface albedo variability in ISCCP is particularly apparent in the interior of the SH ice pack in JJA.

ISCCP CCSM3 Clear-sky surface albedo standard deviation (%) in ISCCP and CCSM3, broken down by season. In MAM a similar pattern is visible.

Conclusions (part I)  Though SH sea ice only accounts for about 20% of climatological planetary albedo in the SH sea ice zone, it accounts for about 70% of the variance in planetary albedo.  This demonstrates that in spite of large cloud amounts seen in the SH sea ice in the ISCCP data, clouds are not opaque enough to obscure the signature of sea ice albedo variations in top-of-the-atmosphere fluxes.  Because it dominates the variability in solar fluxes, sea ice must be the dominant factor in the interannual climate variability of the southern hemisphere.  These results also suggests that if SH sea ice albedo were to vary in cryosphere regions in the future, the planet’s shortwave radiation absorption would be significantly affected.

Conclusions (part II)  CCSM3 has substantially less sea ice albedo variability than ISCCP, particularly in the interior of the ice pack, resulting in a smaller simulated contribution of sea ice to planetary albedo variability.  This may suggest improvements to the model’s sea ice model and sea ice albedo parameterization.