Deep and abyssal ocean warming from 35 years of repeat hydrography

Abstract

Global and regional ocean warming deeper than 2000 m is investigated using 35 years of sustained repeat hydrographic survey data starting in 1981. The global long‐term temperature trend below 2000 m, representing the time period 1991–2010, is equivalent to a mean heat flux of 0.065 ± 0.040 W m−2 applied over the Earth's surface area. The strongest warming rates are found in the abyssal layer (4000–6000 m), which contributes to one third of the total heat uptake with the largest contribution from the Southern and Pacific Oceans. A similar regional pattern is found in the deep layer (2000–4000 m), which explains the remaining two thirds of the total heat uptake yet with larger uncertainties. The global average warming rate did not change within uncertainties pre‐2000 versus post‐2000, whereas ocean average warming rates decreased in the Pacific and Indian Oceans and increased in the Atlantic and Southern Oceans.

Deep temperature variability in Drake Passage

“…The Scotia Sea seems to be an exception to the pattern of significant deep warming, with Purkey and Johnson [] and Desbruyères et al . [] finding near‐zero trends below 2000 m in this area over the 1990s and 2000s, and Johnson et al . [] finding warming before 2005 and cooling after 2005.…”

“…The Argo profiling float program has enabled investigators to deduce, with increasing confidence, trends in ocean temperature in the top 2000 m contributing 0.4–0.6 W m −2 to the global energy budget [ Roemmich et al ., ; Wijffels et al ., ], with 67%–98% of this increase found in the Southern Ocean. Decadal repeat hydrographic sections have shown significant deep warming over most of the Southern Ocean, giving an average warming trend of 2.5 m°C yr −1 between 2000 and 4000 m south of the Subantarctic Front (SAF) [ Purkey and Johnson , ; Desbruyères et al ., ]. These decadal changes are associated with changes in the volume and properties of deep water masses, with Antarctic Bottom Water (AABW) warming, freshening, and decreasing in volume globally [ Purkey and Johnson , ].…”

State of the Climate in 2016

“…Deep-ocean properties show significant large-scale trends on decadal time scales in some deep basins, with the strongest anomalies at high latitudes near water mass formation regions Johnson 2010, 2013;Desbruyères et al 2016). The Argo Program's international partnership proposes to meet the technical challenge by deploying a new generation of Deep Argo floats globally.…”

“…From 2000 to 6000 m ( Fig. 3.6b), trends are estimated from differences between decadal surveys (Desbruyères et al 2016).…”

Observational Advances in Estimates of Oceanic Heating

“…Updating the hydrography dataset with section repeats up to 2015 has enabled a calculation and comparison of deep and abyssal warming rates during the 1990s and 2000s decades. The comparison of these decadal changes revealed no statistically significant difference in the magnitude and structure of the global decadal warming rate at deep and abyssal levels [14]. However, there are differences in the regional trends, specifically trend reversals in the deep Atlantic and deep Pacific consistent with the simulated redistribution of heat during hiatus periods [45].…”