The CHIME coupled climate model Alex Megann, SOC 26 January 2005 (with Adrian New, Bablu Sinha, SOC; Shan Sun, NASA GISS; Rainer Bleck, LANL)  Introduction & model description  Basic model results  Summary  Further plans

The CHIME Project The Coupled Hadley-Isopycnic Model Experiment (CHIME) is a new coupled climate model, which Uses same atmosphere model (HadAM3) and ice model as in HadCM3; Ocean model (HYCOM) has same horizontal resolution as in HadCM3, but uses HYCOM instead of HadCM3’s constant-depth coordinate model. Allows detailed examination of the influence of the vertical coordinate of the ocean component in a coupled system. This is a three-year COAPEC funded project. Two main objectives: (1) Detailed comparison with HadCM3 (2)Analysis of coupled modes of interannual and interdecadal variability (mainly N. Atlantic) in the model

The ocean model HYCOM v0.9. Advects and diffuses T/S on layers. Uses 2000 dbar reference pressure for potential density (  2 ), and applies a correction for thermobaricity. Spherical 1.25° x 1.25° grid south of 55°N, with bipolar grid covering Arctic (poles at 110°W and 70°E). Bering Strait and Gibraltar Strait open. Ice model Semtner thermodynamics, plus drift with ocean surface current (same as in HadCM3). The atmospheric model HadAM3, as used in HadCM3. 2.5° x 3.75° resolution, 19 vertical layers. Pre-industrial greenhouse gas levels. Connected to ocean through OASIS coupler: coupling once per day.

CHIME years Atlantic overturning streamfunction HadCM3 years

The global heat budget Overall heat is conserved to within ~0.3W m -2 : this discrepancy is due to a small residual numerical non-conservation in the advection code. TOA radiation Surface flux Implied flux (heat content tendency)

Ocean “heat content” The mean ocean temperature rises in the first decade, but after year 50 the warming is less than 0.08 K/century, equivalent to a global surface heat flux of ~0.4 Wm -2. c.f. HadCM3 cools at.02K/century (~0.1 Wm -2 ).

Temperature drift by depth CHIME has a warming trend at the ocean surface, with an interannual variability strongly correlated with ENSO. By 80 years overall SST error is ~+2.0K. HadCM3 surface drift is much lower, but below 1000m drift is similar. CHIMEHadCM3

Surface temperature errors CHIME: years mean SST error from SOC climatology HadCM3: years mean SST error from GISST climatology

Model heat fluxes CHIME years mean heat flux SOC climatology mean heat flux HadCM3 years mean heat flux

Model heat fluxes (2) CHIME years minus HadCM3 mean heat flux CHIME years minus SOC climatology mean heat flux

Model heat fluxes (3) Zonal mean heat fluxes Zonal mean heat flux differences Zonal mean SST differences

Heat transport Meridional heat transports are similar to the estimates of Trenberth & Caron (2001), though transport in mid-latitudes seems a little high. CHIME

The model salinity drift The mean ocean salinity increases during the run, except in years where a bug in the ice model causes spurious meltwater production. The overall trend is equivalent to a surface evaporative flux of ~0.07 mm/day, or a global freshwater loss of 0.3 Sv.

Salinity drift by depth In upper 600m salinity in CHIME is rising at 0.17 PSU/century. In deeper water there is a fresh anomaly due to change in Gibraltar outflow but otherwise salinity is stable. In HadCM3 upper water salinity trend is similar to that in CHIME, but below 1000m salinity continues to increase at 0.03 PSU/century. CHIMEHadCM3

Why is the salinity increasing in CHIME? The salinity in the ocean is increasing, at a rate equivalent to a global freshwater loss of 0.3 Sv. The ocean model conserves salt much more tightly than this, so we need a different mechanism. A simple mechanism Model surface temperature is globally too high.  Hydrological cycle will be intensified: more evaporation in tropics & mid-latitudes, more precipitation in high latitudes.  More snow will fall in Arctic and Antarctic, but because iceberg melting is fixed in time, this extra precipitation will not reach the ocean. Existing iceberg melting rate is ~0.2 Sv (and is tuned for HadCM3). If this were 0.3 Sv higher, salt budget would close. Also need to check total water content of atmosphere.

Model wind stress AMIP (atmosphere model) wind stress CHIME years mean wind stress SOC climatological mean wind stress

Model wind stress (2) AMIP (atmosphere model) wind stress curl CHIME years mean wind stress curl HadCM3 years mean wind stress curl (Grey/green shading shows negative curl, white/red shows positive curl. Contour interval is 0.5 N/m 2 )

Annual mean ice extent. HadCM3SSM/ICHIME year 100

North Atlantic Current path CHIME years mean surface elevation and surface velocity HadCM3 years mean surface velocity

Summary We have successfully coupled the HYCOM ocean model (T/S only) to the Hadley Centre’s HadAM3 atmosphere. Completed 120 years. CHIME is close to radiative equilibrium by about 60 years, with TOA residual < 0.3 Wm -2. CHIME has meridional heat transports well within bounds of Trenberth et al. estimates. Mean MOC is similar in spatial structure and amplitude to that of HadCM3. CHIME does not show the unrealistic shift in NAC path observed in HadCM3. CHIME does not show HadCM3’s North Pacific cold anomaly. CHIME shows more near-surface warming than HadCM3 (perhaps need to change cloud parameterisation).

Further work: immediate… Finish quantitative comparisons with HadCM3. Understand SST bias. Carry out detailed analysis of interannual and interdecadal variability. … and in the longer term… Carry out further model tuning and carry out GHG forcing experiments