Tropical Atlantic SST in coupled models; sensitivity to vertical mixing Wilco Hazeleger Rein Haarsma KNMI Oceanographic Research The Netherlands

SST error along equator in coupled models Davey et al 2001

SPEEDO (SPEEDy-Ocean) SPEEDY Land Bucket Model Fix SST LAND OCEAN ATMOSPHERE ICE Slab Mixed layer Linear Ocean MICOM OGCM ICE

SpeedO Model Atmosphere (Speedy): T30 (3.5 degree) 7 layers Simplified parameterizations Ocean (MICOM): Isopycnic coordinate primitive equation model (MICOM, Atlantic basin) 22 layers 1 degree horizontal resolution SST prescribed outside the Atlantic Land: Land Bucket model

Seasonal cycle Tropical Atlantic SST and thermocline in coupled SPEEDO Month (1=Jan) Seasonal cycle SST along equator (colors SPEEDO, contours Da Silva) Annual mean temperature at the equator

Experiments Mixed layer depth with Kraus Turner mixing parameterization: Ocean-only and fully coupled : 1) Control 2) Enhanced wind mixing 3) Enhanced buoyancy mixing

SST and SST error MICOM ocean-only Annual mean SST control run SST error control run

SST and SST error MICOM ocean-only Mean SST with enhanced wind mixing SST error with enhanced wind mixing

Divergence in mixed layer in MICOM ocean-only Control experimentEnhanced wind mixing experiment Divergence in 1/s (*1e7)

Upwelling derived from surface drifter data 7e7

SST budget in MICOM ocean-only (1S,10W) Entrainment Radiation Latent heat flux Sensible heat flux Horizontal advection Diffusion (all in W/m^2) ControlEnhanced wind mixing

Heat budget from PIRATA

SST error along equator in MICOM ocean-only Control (alfa=0.3, beta=0.15) Wind mixing efficiency enhanced (alfa=0.8, beta=.15) Buoyancy mixing efficiency enhanced (alfa=0.3, beta=0.4)

Summary ocean-only experiments Enhanced efficiency wind mixing -> deeper mixed layers Divergence is set by the winds (Ekman transport) But… The actual entrainment velocity reduces when divergence takes place over a (slightly) thicker mixed layer More mixing- > warmer SST in the cold tongue Realistic divergence and temperature budgets at slightly higher wind mixing efficiency What about the coupled model??

SST error along equator in coupled SPEEDO Control (alfa=0.3, beta=0.15) Wind mixing efficiency enhanced (alfa=0.8, beta=.15) Buoyancy mixing efficiency enhanced (alfa=0.3, beta=0.4)

Tropical Atlantic SST in coupled SPEEDO

Seasonal cycle Tropical Atlantic SST and thermocline in coupled SPEEDO with enhanced wind mixing Colors SPEEDO Contours Da Silva Seasonal cycle SST along equator (colors SPEEDO, contours Da Silva) Annual mean temperature at the equator

Atmospheric vertical motion SPEEDO (between 50W and 20E) ControlEnhanced ocean vertical mixing

Zonal velocity averaged between 50E and 20W in SPEEDO Enhanced ocean wind mixing efficiency minus Control

Summary and conclusions The reversed zonal gradient of Tropical Atlantic SST in coupled models is generated by a too strong entrainment at the base of the mixed layer If the depth over which the air/sea momentum flux is distributed increase the entrainment velocity reduces (and increase SST) -> it’s all in the wdT/dz term (here corrected via mixing parameterization, could be done in other ways as well) Coupled model reacts favourably to enhanced wind mixing efficiency-> semi annual harmonic disappears zonal SST gradient correct (SST error within 1 degree K) meridional mean circulation atmosphere improves