The next figure illustrates how the oceans have warmed during the period that the floats have been in operation, up to August 2020. The vertical scale is the global ocean temperature change in degrees Celsius averaged from 65oS to 65oN (excluding the polar regions), while the horizontal scale gives the depth up to 1,900 meters (6,200 feet).
The fluctuations in each Argo depth profile arise from seasonal variations in temperature from summer to winter, which are more pronounced at the surface than at greater depths. If you focus your attention on any yearly summer peak at zero depth, you will notice that it moves to the right – that is, to later times – as the depth increases. In other words, there is a time delay of any temperature change with depth.
From a correlation analysis of the Argo data, Humlum finds that the time delay at a depth of 200 meters (650 feet) is a substantial 20 months, so that it takes 20 months for a temperature increase or decrease at the tropical surface to propagate down to that depth. A similar, though smaller, delay exists between any change in sea surface temperature (SST) and corresponding temperature changes in the atmosphere and on land, as shown in the figure below.