CCSM Climate Variability Working Group Meeting
24 June 2003
The CCSM Climate Variability Working Group (CVWG) breakout session was held on Tuesday, 24 June 2003 from 1:00 pm to 5:00 pm. Jim Hurrell chaired the session and Mike Alexander served as rapporteur. Prior to the meeting, talks had been solicited from members of the CCSM community to document 1) polar climate variability (joint with Polar Climate Working Group) 2) climate sensitivity to anthropogenic and natural forcing, and 3) model biases in the tropics.
Richard Cullather (NCAR): An appraisal of storm track depictions in coupled and uncoupled simulations of the CCSM, Version 2
Using a storm tracking algorithm, Richard found that in the Northern Hemisphere there were subtle errors in location of wintertime cyclone tracks and that the cyclone densities in major storm tracks NCEP < CAM2.0.1 < CCSM2. CCSM-2. Summer cyclones along the Eurasian/Arctic front were weaker and more zonal compared with observations. In the Southern Hemisphere, the distribution of cyclone density in the circumpolar trough occupied a larger latitudinal distribution in CAM and CCSM2 than in NCEP reanalysis.
Marika Holland (NCAR): Antarctic sea ice variability in the CCSM2 control simulation
As in observations, CCSM2 Antarctic ice variability exhibits a dipole pattern with enhanced Pacific ice associated with reduced Atlantic ice. These are forced by both dynamic and thermodynamic processes, consistent with atmosphere and ocean conditions. Albedo feedback prolongs anomalies in the Pacific. Its influence in the Atlantic is reduced due to transport of anomalous SST to regions where no ice formation occurs. Both ENSO and SAM appear to weakly influence the ice dipole pattern.
Alex Hall (UCLA): Assessing climate feedbacks using models and observations
Alex discussed three methods for examining feedbacks in the climate system, including: 1) Comparing internal variability of the model with and without the feedback to that of observations, 2) comparing the observed response to external forcing to the response of the model with and without the feedback, and 3) using model output to assess which feedbacks might be measurable from observations.
Tom Wigley (NCAR): The effect of climate sensitivity on the response to volcanic forcing
Tom examined the AOGCM (PCM) response to volcanic forcing. The GCM response was well reproduced with a simple energy-balance model (EBM). The EBM was then used to determine the characteristics of volcanic response, which included that the peak cooling is proportional to the peak forcing and varies approximately as the square root of the climate sensitivity. The response has a nearly exponential decay with a relaxation time of 26 - 42 months. Observed peak coolings can be used to estimate the climate sensitivity but uncertainties are large due to internal variability noise in the observations.
Ed Schneider (COLA): Comparing the greenhouse sensitivities of CCM3 and ECHAM4.5
Ed examined the difference in sensitivity between models by isolating those pieces of code (e.g., parameterizations, parameter choices, etc.) that lead to the largest differences in sensitivity between pairs of models (i.e., swap parameterizations between models).
Junichi Tsutsui (CRIEPI): Climate sensitivity of CCM3 to horizontal resolution and interannual variability of tropical cyclones
Junichi examined the climate in CCM3 as a function of four resolutions T42, T85, T170, T341 and found that there was a slight increase (decrease) in globally averaged precipitation (cloud amount) as the resolution increased but little change in the large-scale precipitation and circulation patterns. He also found that a relative humidity threshold of 85% for convection gave the best simulation of tropical cyclones. The model was able to reproduce some of the observed interannual variability in tropical cyclone frequency in an ensemble of AMIP simulations.
Gokhan Danabasoglu (NCAR): Exploration of causes and effects of some CCSM biases
Gokhan explored the impact of biases in the CCSM in key climatic regions by 1) constraining SSTs in the CCSM to track observations off the west coast of subtropical continents and 2) constraining the surface fluxes to track observations along the equatorial Pacific wave guide. The results indicated that including the correct SSTs off the east coast of Africa and North and South America significantly improved the local and remote atmospheric circulation, while the seasonal cycle of SSTs in the eastern tropical Pacific was well simulated only when both the observed zonal and meridional components of the winds were specified along the equator.
Max Suarez (NASA-Goddard) Tropical biases in coupled and uncoupled simulations with the NSIPP model
The NASA Seasonal-to-Interannual Prediction Project (NSIPP) coupled model has many of the same biases as other coupled models. Clear indications of many of these are already apparent in AMIP mode, though some, like the bias in the SPCZ (double ITCZ), are much worse in coupled mode. Both AMIP and coupled models have reasonable stratus clouds, but generic coupled problems in the east equatorial Pacific are still present.
Andrew Wittenberg (GFDL): ENSO in the GFDL coupled model
Andrew highlighted the importance of including a parameterization for cumulus momentum transport (CMT) for obtaining realistic variability in the tropical Pacific. Including CMT broadened the surface wind stress, exciting more off equatorial Rossby waves, which resulted in a less biannual, more realistic ENSO variability. However, the simulated ENSO variability is still too regular in time, located too far west, and has too much westward propagation of SST anomalies.
Ben Kirtman (COLA): A multi-model approach for GCM sensitivity studies
Ben used MOM3 coupled to AGCM combinations of the COLA and CAM heat and momentum fluxes to better understand the process causing SST biases in the tropical Pacific. He found that the COLA heat fluxes were largely responsible for the warm SST bias while the CAM winds were too easterly in the western tropical Pacific.
Dezheng Sun (CDC): Validating and understanding feedback mechanisms in climate models: Results for the tropical Pacific cold-tongue. Part I: Atmospheric feedbacks
Dezheng examined the response of various energy fluxes to El Nino using observations and output from AMIP runs. The observed atmosphere has a strong regulating effect on the underlying SST over the equatorial Pacific cold-tongue region. All of the 5 AGCMs that were analyzed have a weaker regulating effect compared to nature with the exception of the GFDL AM2 model. The lack of sufficient response in short-wave forcing to a SST increase is a major contributor to the weaker regulating effect.
L. Ruby Leung
Community Climate System Model