Science Source

Multi-model attribution of upper-ocean temperature changes using an isothermal approach

States that both air-sea heat exchanges and changes in ocean advection have contributed to observed upper-ocean (i.e. ~200 m) warming most evident in the late-twentieth century
States, however, that it is predominantly via changes in air-sea heat fluxes that human-induced climate forcings, such as increasing greenhouse gases, and other natural factors such as volcanic aerosols, have influenced global ocean heat content
The present study builds on previous work using two different indicators of upper-ocean temperature changes for the detection of both anthropogenic and natural external climate forcings
Compares mean temperatures above a fixed isotherm with the more widely adopted approach of using a fixed depth
Presents the first multi-model ensemble detection and attribution analysis using the fixed isotherm approach to robustly detect both anthropogenic and natural external influences on upper-ocean temperatures
States that although contributions from multidecadal natural variability cannot be fully removed, both the large multi-model ensemble size and properties of the isotherm analysis reduce internal variability of the ocean, resulting in better observation-model comparison of temperature changes since the 1950s
Shows that the high temporal resolution afforded by the isotherm analysis is required to detect natural external influences such as volcanic cooling events in the upper-ocean because the radiative effect of volcanic forcings is short-lived