Argo, Temperature, and OHC

By Willis Eschenbach | Whats Up With That? | March 2, 2014

I’ve been thinking about the Argo floats and the data they’ve collected. There are about 4,000 Argo floats in the ocean. Most of the time they are asleep, a thousand metres below the surface. Every 10 days they wake up and slowly rise to the surface, taking temperature measurements as they go. When they reach the surface, they radio their data back to headquarters, slip beneath the waves, sink down to a thousand metres and go back to sleep …

At this point, we have decent Argo data since about 2005. I’m using the Argo dataset 2005-2012, which has been gridded. Here, to open the bidding, are the ocean surface temperatures for the period.

Figure 1. Oceanic surface temperatures, 2005-2012. Argo data.

Dang, I like that … so what else can the Argo data show us?

Well, it can show us the changes in the average temperature down to 2000 metres. Figure 2 shows that result:

Figure 2. Average temperature, surface down to 2,000 metres depth. Temperatures are volume-weighted.

The average temperature of the top 2000 metres is six degrees C (43°F). Chilly.

We can also take a look at how much the ocean has warmed and cooled, and where. Here are the trends in the surface temperature:

Figure 3. Decadal change in ocean surface temperatures.

Once again we see the surprising stability of the system. Some areas of the ocean have warmed at 2° per decade, some have cooled at -1.5° per decade. But overall? The warming is trivially small, 0.03°C per decade.

Next, here is the corresponding map for the average temperatures down to 2,000 metres:

Figure 4. Decadal change in average temperatures 0—2000 metres. Temperatures are volume-averaged.

Note that although the amounts of the changes are smaller, the trends at the surface are geographically similar to the trends down to 2000 metres.

Figure 5 shows the global average trends in the top 2,000 metres of the ocean. I have expressed the changes in another unit, 10^22 joules, rather than in °C, to show it as variations in ocean heat content.

Figure 5. Global ocean heat content anomaly (10^22 joules). Same data as in Figure 4, expressed in different units.

The trend in this data (6.9 ± 0.6 e+22 joules per decade) agrees quite well with the trend in the Levitus OHC data, which is about 7.4 ± 0.8 e+22 joules per decade.

Anyhow, that’s the state of play so far. The top two kilometers of the ocean are warming at 0.02°C per decade … can’t say I’m worried by that.

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