Status of CORE forcing fields. Are there any known problems or planned updates beyond Large and Yeager (2008)? CORE version 2 datasets have been released through the GFDL web site. In this version, the IAF data sets cover the period. The NYF data sets are the same as in version 1 except for the new corrections. NCAR is committed to update / extend the CORE data sets as new data or corrections become available. An example is the time varying river runoff constructed to cover the period. We would like to get input from the groups using the CORE data sets. However, these inputs should go beyond just identifying problems: Provide us with possible fixes.

There are undoubtedly errors (Q7), in particular with precipitation. Polar forcings are very uncertain and could be improved (e.g., precipitation). Interannual variability in all fields start only after Some issues have been addressed based on the CORE-I experience, e.g., wind speed and rotation corrections, the adjustment of specific humidity, the latitudinally dependent albedo (see Large and Yeager 2008).

ANNUAL-MEAN SEA ICE THICKNESS IAFNYF

Q8: Would it be possible to construct key atmospheric forcing fields for the first half of the 20 th Century and merge this (constructed) forcing with CORE-II to generate a continuous, albeit not fully consistent, data set from year 1900 (or 1880)? Possible, but very problematic: no credibility in either the mean or the interannual variability. NCAR has no interest to do this. Suggestion: The output from the 20 th Century coupled simulations can be used for this purpose. Output from one (any) coupled model can be used to force other models in an ocean—sea-ice coupled mode.

Summarize the efforts of CORE-II at NCAR, including POP and HYCOM. We do not have any HYCOM simulations using CORE-II forcing at NCAR. We have several simulations using CORE-II (IAF) data sets with POP: - Sensitivity of the North Atlantic gyre circulations to high latitude buoyancy forcing is explored. - Sensitivity of the oceanic CFC-11 uptake to physical initial conditions and surface dynamical forcing (NYF vs IAF) is investigated. The solutions remain sensitive to the choice of salinity restoring and coupling to a sea-ice model, i.e., the same issues raised in the CORE-I paper.

What papers have been published using the interannual forcing? (in addition to the ones already listed on the CORE web site) Yeager, S. G., and W. G. Large, 2007: Observational evidence of winter spice injection. J. Phys. Oceanogr., 37, , doi: /2007JPO Danabasoglu, G., S. Peacock, K. Lindsay, and D. Tsumune, 2009: Sensitivity of CFC-11 uptake to physical initial conditions and interannually varying surface forcing in a global ocean model. Ocean Modelling, doi: /j.ocemod (in press). Yeager, S. G., and M. Jochum, 2009: The connection between Labrador Sea buoyancy loss, Deep Western Boundary Current strength, and Gulf Stream path in an ocean circulation model. Ocean Modelling (submitted).

McWilliams, J.C., and G. Danabasoglu, 2002: Eulerian and eddy-induced meridional overturning circulations in the tropics. J. Phys. Oceanogr., v32, Yeager, S., and W.G. Large, 2004: Late-winter generation of spiciness on subducted isopycnals. J. Phys. Oceanogr., v34, Capotondi, A., M. Alexander, C. Deser, and M.J. McPhaden, 2005: Anatomy and decadal evolution of the Pacific subtropical-tropical cells (STCs). J. Climate, v18, Capotondi, A., M. Alexander, C. Deser, and A.J. Miller, 2005: Low frequency pycnocline variability in the Northeast Pacific, J. Phys. Oceanogr., v35, Gent, P.R., F.O. Bryan, G. Danabasoglu, K. Lindsay, D. Tsumune, M.W. Hecht, and S.C. Doney, 2006: Ocean chlorofluorocarbon and heat uptake during the twentieth century in CCSM3. J. Climate, v19, Deser, C., A. Capotondi, R. Saravanan, and A. Phillips, 2006: Tropical Pacific and Atlantic climate variability in CCSM3. J. Climate, v19, Other earlier studies that used interannual forcing:

Additional papers using the CORE NYF data sets: Danabasoglu, G., and J. Marshall, 2007: Effects of vertical variations of thickness diffusivity in an ocean general circulation model. Ocean Modelling, 18, , doi: /j.ocemod Danabasoglu, G., R. Ferrari, and J. C. McWilliams, 2008: Sensitivity of an ocean general circulation model to a parameterization of near- surface eddy fluxes. J. Climate, 21, , doi: /2007JCLI Eden, C., M. Jochum, and G. Danabasoglu, 2009: Effects of different closures for thickness diffusivity. Ocean Modelling, 26, 47-59, doi: /j.ocemod For more references see and search for Large and Yeager.

Propose a protocol for baseline CORE-II simulations, based on the experiences at NCAR. Q10:How should the ocean and sea-ice models be initialized? What about spin-up time prior to the focused analysis period? What portion of the simulation should be analyzed, and what should be ignored (due to initialization shock)? IAF protocol is the same as the one used in the CORE-I experiments except for the 59-year repeat forcing cycle covering the period instead of the repeated single annual cycle used in NYF. Ocean model is initialized using the January-mean PHC2 temperature and salinity data with zero velocities. Sea-ice model is initialized with a “spun-up” state from a simulation forced with either NYF or IAF. (There are other options.) Weak salinity restoring (50m/4yr, global) with its horizontal-mean subtracted and a precipitation factor are used. (Q9)

OCEAN – SEA-ICE COUPLED WITH NYF ATLANTIC MERIDIONAL OVERTURNING CIRCULATION CONTROLWITH OVERFLOWS FULLY COUPLED

OCEAN – SEA-ICE COUPLED WITH NYF ATLANTIC MERIDIONAL OVERTURNING CIRCULATION CONTROLWITH OVERFLOWS

No S restoring, No precip adjust No S restoring Weak S restoring (V p = 50m/4 years) Weak S restoring + restoring melt flux Strong S restoring (V p = 50m/30 days) B5B4B3B2 BAB1 AMOC time evolution NYF Hindcast Large (10 Sv) initial increase in AMOC associated with loss of Labrador ice shelf in standard ocean-ice runs (B, B3) Strong salinity restoring (B1, B2) maintains ice shelf and damps AMOC spinup No salinity restoring (B4, B5) further enhances AMOC and induces spurious oscillation => AMOC strength and variability strongly dependent on salinity restoring NYF HINDCAST (IAF)

ATLANTIC TEMPERATURE TRENDATLANTIC SALINITY TREND YEAR oCoCpsu

TEMPERATURE TRENDS (ANOMALIES) WITH RESPECT TO YEAR 80

ROOT-MEAN-SQUARE INTERANNUAL VARIABILITY AND VARIANCE ATLANTIC PACIFIC INDIAN VR = IA variability / detrended IA variability (VR)

INITIAL CONDITION SENSITIVITY Sufficient historical data are not available to fully initialize an ocean – sea-ice simulation in, e.g., 1948, and the repeat cycle forcing necessarily introduces errors because of inappropriate initial conditions. Perform a 10-year branch integration for model years replacing the end of model year 120 fields with data from model year 113 corrected for linear drift (labeled as IC). CONTROL – IC RMS DIFFERENCE PROFILES (GLOBAL) 0.1 vs o C0.02 vs psu

The minimum number of forcing cycles is 3-4. Studies involving mid-depth and abyssal ocean may require significantly more cycles. Initial condition shock diminishes after the first years of each forcing cycle: Any analysis should focus on the remaining period. One can potentially use detrending to allow more of the low frequency variability to be distinguished. A guiding principle is that the main focus of a study should stay rather robust across cycles, e.g., when differenced between the last cycle and the previous one, the differences should be “small”. For variability studies the relevant question is not so much the absolute magnitude of the drift but rather the amplitude relative to the variability signal. There is also an ambiguity in separating numerical drift from long- term secular trend driven by the IAF data. SUMMARY:

What concerns and recommendations do you have for a comparison project based around CORE-II? Recommendations: Meaningful, focused, multiple papers with a few authors (Q13): Some focus areas may include (Q1-3): - forced vs. natural variability, - passive tracer studies, - bias attribution to individual model components, - natural variability differences between participating models, - forced variability applications, climate shift, trends, - AMOC variability (relevant for decadal prediction), - studies complementary to data assimilation and the 20 th Century coupled cases.

What concerns and recommendations do you have for a comparison project based around CORE-II? Concerns (Q11): Details of surface salinity forcing and under sea-ice treatment, including role of a sea-ice model, remain very important. It can turn into a beauty contest. Internal politics may come into play (may not be reconcilable). There may be institutional commitment / pressure to use other forcing data sets.