US National Oceanic and Atmospheric Administration
Climate Test Bed Joint Seminar Series (2009-2010)
IGES/COLA, Calverton, Maryland, 19 November 2009

The NCEP GODAS Ocean Analysis of the Tropical Pacific Mixed Layer Heat Budget on Seasonal to Interannual Time Scales

Yan Xue

Climate Prediction Center, NOAA/NWS/NCEP, Camp Springs, MD

Summary

The mixed layer heat budget in the tropical Pacific is diagnosed using pentad (5 day) averaged outputs from the Global Ocean Data Assimilation System (GODAS), which is operational at the National Centers for Environmental Prediction (NCEP). The GODAS is currently used by NCEP’s Climate Prediction Center (CPC) to monitor and to understand El Niño and La Niña in near real-time. The purpose of our study is to assess the feasibility of using an operational ocean data assimilation system to understand SST variability.

The climatological mean and seasonal cycle of mixed layer heat budgets derived from GODAS agree reasonably well with previous observational and model based estimates. However, significant differences and biases were noticed. Large biases were found in GODAS zonal and meridional currents, which contributed to biases in the annual cycle of zonal and meridional advective heat fluxes. The warming due to tropical instability waves in boreal fall is severely underestimated due to use of 4-week data assimilation window. On interannual time scales, the GODAS heat budget closure is good for weak-to-moderate El Niños. A composite for weak-to-moderate El Niños suggests that zonal and meridional temperature advection and vertical entrainment/diffusion all contributed to the onset of the event, and that zonal advection played the dominant role during the decay of the event and the transition to La Niña. The net surface heat flux acts as a damping during the development stage, but plays a critical role in the decay of El Niño and the transition to the following La Niña.

The GODAS heat budget closure is generally poor for strong La Niñas. Despite the biases, the GODAS heat budget analysis tool is useful in monitoring and understanding the physical processes controlling the SST variability associated with ENSO. Therefore it has been implemented operationally at CPC in support of NOAA’s operational ENSO forecasting.
 

Paper published

Huang, B., Y. Xue, D. Zhang, A.Kumar and M. J. McPhaden 2010: The NCEP GODAS Ocean Analysis of the Tropical Pacific Mixed Layer Heat Budget on Seasonal to Iinterannual Time Scales. J. Climate, Early Online Release.

Contact  Yan Xue