Warm water formation and escape in the upper tropical Atlantic Ocean: 2. A numerical model study

Abstract

We seek the simplest mass and heat balance scenario within the upper tropical Atlantic Ocean using a 2.5‐layer numerical model, with the help of a newly suggested entrainment formula. The model ocean is driven by the seasonal wind and heat flux associated with the movement of the Intertropical Convergence Zone (ITCZ). The entrainment rate is scaled by the product of wind stress and shear at the bottom of the mixed layer. On an annual average, the northward transport of the tropical warm water is about 11 Sv, with roughly 10 Sv associated with entrainment of upper thermocline water. Out of the total 10 Sv of the needed upper thermocline water, 9 Sv enters the equatorial cell from the South Atlantic. The seasonal response to the ITCZ movement is most striking in the entrainment rate and the northward warm water escape rate. The entrainment is significant during May/December and ceases between January and March. The locally forced equilibrium response between the interface shear and the zonal wind stress east of 30°W appears to be responsible for this cycle. The warm water escape toward the North Atlantic takes place mainly from October to June and nearly stops during July/September. As a result of seasonal variations of the two key processes, namely, the entrainment and the northward warm water escape, the tropical warm water pool stores heat during May/October and lets heat escape in November/April.