Coupled HYCOM in CESM and ESPC Alexandra Bozec, Eric P. Chassignet

Overarching goals  To produce a validated benchmark high-resolution coupled ocean-ice-atmosphere with HYCOM as the ocean model => to be compared with GPU code  To evaluate the HYCOM-CESM and HYCOM-ESPC high- resolution configurations; comparison to POP-CESM

Accomplishments  Connection of the CESM ice and atmospheric forcing for OCN-ICE and OCN-ICE-ATM configurations  Update of the HYCOM source code from to  Connection of the CESM river transport model (RTM)  Installation of the HYCOM-CESM source code and ESMF 7.0.0beta on the NAVY machines (IBM DataPlex (Kilrain) and Cray X30 (Shepard))  Ran HYCOM-CESM OCN-ICE (50 years) and OCN-ICE-ATM (50 years) for comparison to POP-CESM (50 years)  Evaluation of HYCOM in CESM in OCN-ICE experiments (comparison to stand-alone and POP) => discrepancy in the ice thickness in Arctic regions  Implementation of the global 0.72º tripolar HYCOM grid in CESM for OCN- ICE and OCN-ICE-ATM experiments

Evaluation of OCN-ICE Comparison of 3 simulations of 50 years:  HYCOM-CESM: HYCOM ocean model coupled with CICE in CESM framework  POP-CESM: POP ocean model coupled with CICE in CESM framework  HYCOM-CICE: HYCOM coupled with CICE as a stand-alone (NOT in the CESM framework, but should give results close to HYCOM-CESM) Experimental set-up:  Bipolar POP 1º global grid  Bathymetry from 2-minute NGDC (full steps)  Initialization from rest with Levitus PHC2.1  Large and Yeager (2004) bulk formulation  CORE-I atmospheric forcing

T and S Surface bias  similar bias in T for HYCOM-CICE, HYCOM-CESM, and POP-CESM  Larger bias in S for POP-CESM in the Arctic, slightly saltier in interior in the HYCOM runs HYCOM-CICEHYCOM-CESMPOP-CESM

Ice cover winter  Similar ice cover in the Arctic for all experiments  Weaker ice cover for the HYCOM experiments, but better in HYCOM- CESM in the Weddell Sea when compared with HYCOM-CICE HYCOM-CESM

Ice thickness winter (m)  Arctic: Overestimation of ice thickness in HYCOM-CESM, HYCOM-CICE comparable with POP-CESM  Antarctic: Under for HYCOM-CICE. OK fro HYCOMCESM, over for POP- CESM HYCOM-CESM

Ice cover summer HYCOM-CICE HYCOM-CESM POP-CESM  Similar ice cover in the Arctic  Weaker ice cover in HYCOM-CICE in the Antarctic

Ice thickness summer (m)  Overestimation of ice thickness in HYCOM-CESM in Arctic  Weaker ice thickness in HYCOM-CICE in the Antarctic HYCOM-CESM POP-CESM HYCOM-CICE

Possible reasons for the discrepancy in ice thickness 1.Exchanged fields slightly different between the stand-alone and CESM 2.Interpolation of the forcing fields through coupler in CESM is different than in the stand-alone 3.CICE in CESM is slightly different from CICE in stand-alone and needs to be check out carefully to see if we can bring the two versions to run identically. 4.Coupling frequency:  In CESM framework:  CICE coupled with HYCOM and ATM every 3 hours  HYCOM coupled with CICE and ATM every 6 hours  In stand-alone:  CICE coupled with HYCOM and ATM every 6 hours  HYCOM coupled with CICE and ATM every 6 hours (N.B.: CICE coupled every 6 hours in CESM framework does not work, have to try every 3 hours in stand-alone )

Exchange Fields Import Fields: Wind stress Net shortwave radiation (ocean+ice) Downward longwave radiation Upward longwave radiation Latent heat flux Sensible heat flux Precipitation (rain+snow) Rivers (ocean+ice) Ice freezing/melting heat flux Ice freshwater flux Ice salt flux Ice fraction Surface ocean current Sea surface temperature Sea surface salinity Ice freeze/melt heat flux potential Export Fields: Ice stress computed in CICE SSH gradient to compute ocean tilt CESM: Stand-alone: Ice stress computer from ice velocity CESM: Ocean current to compute ocean tilt Stand-alone:

Interpolation of the forcing fields CESM Radiative flux (ocean+ice) (W/m2) Stand-alone Radiative flux (ocean only) (W/m2)

Tripolar grid and bathymetry in CESM gx1v6 1º bipolar POP grid glbt º tripolar HYCOM grid

Results with glbt0.72 grid/bathy 2 experiments of 20 years with CORE-I:  HYCOM-CESM with bipolar POP grid  HYCOM-CESM-g72 with tripolar HYCOM grid HYCOM-CESM2HYCOM-CESM-g72 HYCOM-CESM  Similar ice cover and extent between HYCOM-CESM2 and HYCOM-CESM-g72  Similar ice thickness (i.e. same bias)

On-going and Future Work Understand the reason behind this overestimation of the ice-thickness in the Arctic:  Run stand-alone with ocean ice-stress  Run a HYCOM-CICE stand-alone with atmospheric fields interpolated the same way as in CESM and keep forcing constant between coupling cycles  Carefully check the two versions of CICE and see if we can run them identically  Run a stand-alone HYCOM-CICE with a 3 hours coupling frequency Evaluate HYCOM-CESM with active atmosphere (OCN-ICE-ATM):  Bipolar POP grid 1º with 1.9º CAM atmospheric model (50 years done, not validated)  Tripolar HYCOM grid 0.72º with 1.9º CAM atmospheric model (15 years done) => to be compared to HYCOM-ESPC HYCOM-CESM in high resolution:  Biolar POP grid 1/10º with 0.5º CAM atmospheric model (configuration to be provided to B. Kirtman)  Tripolar HYCOM grid 0.08º with 0.5º CAM atmospheric model (A. Bozec)  Possibly Tripolar HYCOM grid 0.25º with 0.5º CAM atmospheric model to evaluate impact of resolution  For comparison with identical experiments with NAVGEM atmospheric model (HYCOM-ESPC)