NOAA GFDL - Geophysical Fluid Dynamics LaboratoryGFDL - Geophysical Fluid Dynamics Laboratory

Biases in the Atlantic ITCZ in seasonal-interannual variations for a coarse and a high resolution coupled climate model

July 9th, 2012

Key Findings

  • The SST and precipitation biases associated with the Atlantic ITCZ are improved in GFDL’s CM2.5, a new high-resolution climate model based on CM2.1.
  • This marked improvement in CM2.5 is due to not only high-resolved orography, but also a significant reduction of biases in the seasonal meridional migration of the ITCZ.
Atlantic sea surface temperature simulated by GFDL's new high-resolution coupled climate model, CM2.5. Click for MPEG or Flash animation. (Image by Keith Dixon (NOAA/GFDL))
Atlantic sea surface temperature simulated by GFDL’s new high-resolution coupled climate model, CM2.5.
Click for MPEG animation. (Image by Keith Dixon (NOAA/GFDL))

Summary

Using two fully coupled ocean-atmosphere GFDL models, CM2.1 and CM2.5 (a new high-resolution climate model based on CM2.1), the characteristics and sources of SST and precipitation biases associated with the Atlantic ITCZ were investigated and compared.

CM2.5 has an improved simulation of the annual mean and the annual cycle of the rainfall over the Sahel and the northern South America, while CM2.1 shows excessive Sahel rainfall and lack of northern South America rainfall in boreal summer.

This marked improvement in CM2.5 is due to high-resolved orography, as well as a significant reduction of biases in the seasonal meridional migration of the ITCZ. In particular, the seasonal northward migration of the ITCZ in boreal summer is coupled to the seasonal variation of the SST and a subsurface doming of the thermocline in the northeastern tropical Atlantic, known as the Guinea Dome. Improvements in the ITCZ allow for better representation of the coupled processes that are important for an abrupt seasonally phase-locked decay of the interannual SST anomaly in the northern tropical Atlantic.

Many coupled GCMs suffer from serious biases in the tropical Atlantic (Davey et al. 2002). In particular, almost all climate models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC-AR4) show a warm SST bias in the eastern equatorial Atlantic, and produce a zonal SST gradient along the Atlantic equator which conflicts with observations (Richter and Xie 2008).

This research shows that many aspects of the simulation in the tropical Atlantic are significantly improved in the new model CM2.5 – yet others persist. The differences between CM2.5 and CM2.1 were not sufficient to reduce the warm SST biases in the eastern equatorial region and Angola-Benguela Area. The weak bias of southerly winds along the southwestern African coast associated with the excessive southward migration bias of the ITCZ may be a key to improve the warm SST biases there.