A three box THC model, both nonlinear and linearized versions, used for:

Tziperman and P. J. Ioannou. Transient growth and optimal excitation of thermohaline variability. J. Phys. Oceanogr., 32(12):3427–3435, December 2002. http://www.seas.harvard.edu/climate/eli/reprints/Tziperman-Ioannou-2002.pdf