Revisiting the Thermocline Depth in the Equatorial Pacific

The thermocline depth is defined as the depth of the maximum vertical temperature gradient. In the equatorial Pacific, the depth of 20 degrees C isotherm is widely used to represent the thermocline depth. This work proposes that under the circumstance of a significant mean climate shift, it is better to use the original definition of the thermocline depth in studying the long-term changes in mean climate and tropical coupled climate variabilities. For instance, during the transient period of global warming, the tropical thermocline is usually enhanced because the surface layer warms more and faster than the lower layers. The depth of maximum vertical temperature gradient shoals, which is consistent with the enhanced thermocline. However, the 20 degrees C isotherm depth deepens, which suggests a weakened thermocline. This discrepancy exists in both the observations and the future climate simulations of coupled models.