Journal of Oceanography, Vol. 61 (No. 4), pp. 655-662, 2005

Application of Nonlinear Constraints in a Three-Dimensional Variational Ocean Analysis

Yosuke Fujii*, Shiro Ishizaki and Masafumi Kamachi

Oceanographical Research Department, Meteorological Research Institute, Nagamine, Tsukuba 305-0052, Japan

(Received 17 March 2004; in revised form 2 September 2004; accepted 8 September 2004)

Abstract: Two kinds of nonlinear constraints, not previously studied in oceanography, have been adopted with the Preconditioned Optimizing Utility for Large-dimensional analyses (POpULar) in a three-dimensional oceanic variational analysis in the equatorial Pacific. One is the constraint for the variational Quality Control (QC) procedure and the other is used to avoid density and temperature inversions. Estimation of the large heat content anomaly in the upper ocean related to El Niño and La Niña phenomena is improved with the variational QC. For example, it prevents unusual but correct observation data on the thermocline deepening in the 1997/98 El Niño from being ignored. As a result, it improves the temperature field estimation in the eastern equatorial Pacific. The constraint for avoiding inversions prevents the low salinity layer at the surface and the barrier layer in the eastern equatorial Pacific in the El Niño period from being destroyed by the convective adjustment procedure performed after minimizing the cost function. Incorporating nonlinear constraints in variational analyses is thus a strong candidate for increasing the accuracy of analysis.