Deep ocean heat

Newly published research on deep ocean temperatures has found warming during 1990’s and 2000’s. The observed warming is generally stronger in the south and it appears that a large part of the heat in the deep ocean is transferred through the Southern Ocean. The finding is important for global energy budget and for global sea level estimates.

Cross-section from one measurement route. Different colors represent the temperature change.

Earth’s climate has warmed during recent decades. This is most likely due to top of atmosphere energy imbalance caused by the increases in the greenhouse gases in the atmosphere. About 80% of the energy from the imbalance has gone to heating the oceans due to their large heat capacity.

The slow reaction of the oceans to warming also slows the reaction of the surface temperature to the forcing. Even if we would keep greenhouse gases unchanged, the oceans would keep warming for centuries. The amount of heat flux to the deep ocean affects directly the climate sensitivity. This heat flux varies considerably between different models. The insufficient knowledge of the heat flux to the deep ocean might be biggest factor causing variation between different model projections. Because of this, it is important to gain better knowledge on the deep ocean heat flux.

The deep ocean water is ventilated in high latitudes when dense water sinks to the bottom. In North Atlantic it happens in Nordic and Labrador Seas and in Antarctic it happens in Weddell Sea, Ross Sea, and Adelie Coast. There is some previous research showing that deep waters near Antarctic have warmed. Also, nearer the surface the Southern Ocean apparently has warmed faster than world oceans in average.

Sarah Purkey and Gregory Johnson have studied the deep ocean temperatures globally, but concentrating on the role of the Southern Ocean in the matter. Globally deep waters below 4000m were analysed and in Southern Ocean the analysis concentrated to the depths of 1000-4000m.

The measurements used in this study cover the time span from 1990’s to february 2010. Measurements are not evenly distributed, but there are some measurement routes (28 of them were used in this study) that have been measured every now and then from ships. The measurements in these routes cover rather well the whole route both horizontally and vertically, so the measurements form a kind of temperature cross-section along that route (see the figure above for an example). The temporal span between the measurements is for some routes rather large. For example, one route was measured in 1981 and next time in 2010. On the other hand in one route the shortest time span between measurements was three years.

The warming was calculated from the measurements for 24 ocean basins. Additionally the warming was calculated for the Southern Ocean as a whole. For each basin a heat flux was calculated based on the warming in the basin in question. Also a global estimate was derived for the heat flux. Throughout the study the sea level rise from the calculated warming were also derived, but here we’ll concentrate on the warming and heat flux results.

Purkey and Johnson say this about their depth selections in this study:

In many of the basins north of the SAF, warming trends on pressure levels become significantly different from zero at 97.5% confidence below around 4000 m, making this pressure level a natural division for this study. In many of the repeat sections within the Southern Ocean consistently strong warming extends higher in the water column south of the SAF. Hence we also analyze contributions to SLR and heat gain from warming found from 1000–4000 m south of the SAF.

The effect of the heat transferred to deep ocean in the Southern Ocean shows well in the Atlantic, Indian, and Pacific oceans. General trend is that more warming is found towards the south and less warming towards the north. Of all basins few show cooling but only in one the cooling is statistically significant. Most basins show statistically significant warming.

In the global analysis, the effect of Southern Ocean is strong. Between the depths of 1000m and 4000m almost all the warming has happened in the Southern Ocean. Here are their global heat flux results:

The warming below 4000 m is found to contribute 0.027 (±0.009) W m^–2. The Southern Ocean between 1000–4000 m contributes an additional 0.068 (±0.062) W m^–2, for a total of 0.095 (±0.062) W m^–1 to the global heat budget (Table 1).

(I see that there is a typo in the units of the 0.095 number, it should also be W m^–2, not W m^–1.)

In order to compare their results to other studies, they also calculated heat flux values below 2000m and below 3000m. Deepest earlier global studies have extended to 3000m and currently used Argo reaches depths of 2000m. The total result in this new study for all three analysis (below 2000m, 3000m, and 4000m) is about 0.1 W m^–2. They say:

From 1993 to 2008 the warming of the upper 700 m of the global ocean has been reported as equivalent to a heat flux of 0.64 (±0.11) W m^–2 applied over the Earth’s surface area (Lyman et al. 2010). Here, we showed the heat uptake by AABW contributes about another 0.10 W m^–2 to the global heat budget. Thus, including the global abyssal ocean and deep Southern Ocean in the global ocean heat uptake budget could increase the estimated upper ocean heat uptake over the last decade or so by roughly 16%.

This study seems to suggest that in the warming of the deep world ocean the Southern Ocean plays a remarkably large role. The warming found in this study has been poorly known before, so this study seems to make the ocean heat budget, and even the whole Earth heat budget, more accurate.

Source: Sarah G. Purkey and Gregory C. Johnson, Warming of Global Abyssal and Deep Southern Ocean Waters Between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets, Journal of Climate 2010, doi: 10.1175/2010JCLI3682.1. [abstract, full text]