Fig 7.

Schematics of mechanisms behind transition formation and seasonal migration. (a) The annual average gradient in total phosphorus (which can be thought of as approximately following nutrient supply) is set by a balance between the southward transport and the gradual export of material below the mixed layer. (b) Seasonal changes in the slope of the total phosphorus (P _T) are caused by seasonal export and entrainment due to oscillations in the mixed layer depth. The gradient in P _T changes but the location of the transition stays relatively fixed. The changing gradient leads the meridional location of some transitions to move. (c) Seasonal forcing leads some transitions like the TZCF to move considerably over the season. (d) The trajectory of the chlorophyll transition, TZCF, over latitude relative to total phosphorus. Solid black circles denote each month. White markers denote January (triangle), April (circle), July (diamond), and October (square).