Modeled response of snow cover- atmosphere-ocean interactions in the Northern Hemisphere. Gina Henderson, Daniel J. Leathers and Brian Hanson Department of Geography, University of Delaware

Outline: 1. Introduction & Background: climate modeling and snow cover 2. The Model: atmospheric, land and ocean components 3. Experiment Design: observed snow datasets 4. Preliminary Results: high vs. low snow experiments for North America and Eurasia 5. Future work

Introduction: Significance of Snow  Snow cover identified as the most variable land surface condition in both time and space (Cohen, 1994; Gutzler and Rosen, 1992) Snow cover is a major factor in modulating climate variability and change January ~ 46.8 x 10 6 km 2 August ~ 3.4 x 10 6 km 2

Snow cover forcing climate:  Cold dense air above snow surfaces affect local climate and can propagate anomalies to neighboring regions. Modeled snow forcing: Snow cover over western Siberia found to be negatively correlated with the leading EOF of sea level pressure in the North Atlantic (Gong, G., D. Entekhabi, and J. Cohen, (2002): A large-ensemble model study of the wintertime AO-NAO and the role of interannual snow perturbations. Journal of Climate, 15). Snow cover in the northern Great Plains in the U.S. was found to be linked to downstream tropospheric circulation (Klingaman, N. P., B. Hanson, D. J. Leathers, (2008): A teleconnection between forced Great Plains snowcover and European winter climate. Journal of Climate, 21).

This study: How does an interactive slab ocean affect the land-atmosphere forcing under a forced snow scenario?

The Model:  Employs 3 modules of the NCAR Community Climate System Model, version 3.1 (CCSM3)  The Community Atmospheric Model (CAM3.1) ✔  The Community Land Model (CLM) ✔  The Data Ocean Model (DOM) ✗  The Slab Ocean Model (SOM) ✔  The GCM has 26 vertical levels and a standard baseline spherical truncation at wavenumber 42 (T42).  Grid cells are approximately 2.8º latitude by 2.8º longitude (~ 200 km by 300 km in middle latitudes)

Slab Ocean Model (SOM):  SOM is a mixed-layer slab ocean model where mixed layer temperature is the prognostic output variable.  Seasonal deep water exchange is simulated by an internal heat source, Q, set from a control run.  Mixed layer depths vary seasonally and geographically. F = net atmosphere to ocean heat flux FS = net downward solar flux absorbed FL = net upward longwave flux SH = upward sensible heat flux LH = upward latent heat flux T 0 = ocean mixed layer temperature  0 & C 0 = density and heat capacity of ocean water h 0 = annual mean ocean mixed layer depth A = fraction of the ocean covered by sea ice F = net atmosphere to ocean heat flux Q = internal ocean mixed layer heat flux F oi & F frz = sea ice energy flux terms  0 C 0 h 0 (  T 0 /  t) = (1-A)F + Q + AF oi + (1-A)F frz

Control Run Max North American Snow Max Eurasian Snow Min North American Snow Min Eurasian Snow Model prescribes snow 58 year run, equilibrium after yr 20 Last 38 years used for analysis Branch model runs Snow is prescribed not predicted Snow depth based on observations Ensemble size of 20 Experiment Design:

Snow Data:  1° X 1° interpolated snow depth data (Dyer and Mote, 2006) from U.S. NWS cooperative stations an the Canadian daily surface observations.  Period of record with daily resolution  Gridded 2.5° X 2.5° snow depth data.  NSIDC’s Historical Soviet Daily Snow Depth (HSDSD)  Period of record is , we are using  Gridded by Hengchun Ye, CSU.

Experiment Name DescriptionLengthFixed/Free sea surface Snow condition Ctl_freeControl run38 yearsSlab Ocean (SOM) Free to vary Ctl_domControl run, prescribed SSTs 200 years Data Ocean (DOM) Free to vary Ctl_climControl run, prescribed snow to model climatology 20 yearsSlab Ocean (SOM) Prescribed Max_eurSnow prescribed everywhere, max over Eurasia 20 yearsSOM and DOM Prescribed Min_eurSnow prescribed everywhere, min over Eurasia 20 yearsSOM and DOM Prescribed Max_naSnow prescribed everywhere, max over North America 20 yearsSOM and DOM Prescribed Min_naSnow prescribed everywhere, min over North America 20 yearsSOM and DOM Prescribed

Snow Data: prescribed North American experiment

Snow Data: prescribed Eurasian experiment

Results: Max – Min Eurasian Experiment  Student’s t-test performed to test for significance. Dark gray = 95% confidence Lighter gray = 90% confidence  Cooler sea surface temperatures associated with maximum snow.  Anomalies of -1.5 to -2 K in the North Pacific and Atlantic.

Results: Max – Min Eurasian Experiment Latitudinally averaged SST difference

Results: Max – Min Eurasian Experiment  Cooler 2 m temperature with maximum snow prescription  Temperature depressions up to -5 K over Northern Eurasia  Diabatic cooling over North Pacific and Atlantic

Results: Max – Min Eurasian Experiment

 Sea level pressure shows an organized pattern but is not significant.  Negative NAO pattern over North Atlantic during maximum snow conditions.  Lack of significance may be an ensemble size problem.

The North Atlantic Oscillation (NAO): Positive NAO:  Steep pressure gradient  Strong westerlies  Warm & wet N Europe  Cold & dry Mediterranean Negative NAO:  Weakened pressure gradient  Cold & dry N Europe  Warm & wet Mediterranean  Cold air outbreaks in E U.S.

Results: Max – Min North American Experiment

 Once again, pattern is organized but not significant.  Maximum snow conditions associated with weakened pressure gradient over the North Atlantic.  Lower pressures over Aleutian Low area under maximum snow conditions.

Summary of Findings:  Both experiments show response to prescribed snow forcing, Eurasian response larger.  Significant negative sea surface temperature response is evident in both the Eurasian and North American maximum snow forcing experiments.  Surface and lower atmosphere temperature, and 500 hPa heights show negative response to maximum snow forcing.  Although sea level pressure response is organized and suggests the excitement of modes of Northern Hemisphere atmospheric circulation, results are not statistically significant.

Experiment Name DescriptionLengthFixed/Free sea surface Snow condition Ctl_freeControl run38 yearsSlab Ocean (SOM) Free to vary Ctl_domControl run, prescribed SSTs 200 years Data Ocean (DOM) Free to vary Ctl_climControl run, prescribed snow to model climatology 20 yearsSlab Ocean (SOM) Prescribed Max_eurSnow prescribed everywhere, max over Eurasia 20 yearsSOM and DOM Prescribed Min_eurSnow prescribed everywhere, min over Eurasia 20 yearsSOM and DOM Prescribed Max_naSnow prescribed everywhere, max over North America 20 yearsSOM and DOM Prescribed Min_naSnow prescribed everywhere, min over North America 20 yearsSOM and DOM Prescribed Future work:

Questions?

Extra

Background: Characteristics of snow:  High reflectivity  Thermal insulator  Sink for latent heat  Frozen storage term in hydrologic cycle Snow cover is a major factor in modulating climate variability and change

The Community Land Model (CLM): CLM subgrid hierarchy, land biogeophysical and hydrologic processes.

Results: Max – Min North American Experiment  Similar response to Eurasian experiment, weaker values.  Anomalies of -0.5 to -1 K in the North Pacific and Atlantic.

Results: Max – Min North American Experiment  Cooler surface temperatures over North Atlantic and Pacific of -1 K.  Cooler surface temperatures over southern North America of -1 to -5 K.

Max - Min Eurasian Max - Min North American