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South Pacific Subtropical Decade-Long Warming Detected

It is indicated that the climate system of the Earth continues to accumulate excess heat by the persistent energy imbalance at the top of the atmosphere, as inferred from satellite measurements. One of the most challenging questions in studies of global climate change is whether the excess heat has already penetrated to the deep ocean, as a result of only sparse and irregular measurements of heat in the ocean below a depth of 2,000 m being obtained. In this study a comprehensive analysis of satellite and in situ measurements were performed to report that as significant warming has occurred in the deep ocean in the subtropical South Pacific Ocean over the decade 2005-2104. The local accumulation of heat accounted for up to a quarter of global heat increase in the deep ocean, with inferred directly and indirectly deep ocean heat, below 2,000 m contribution of 2.4 ± 1.4 and 6.1-10.1 ± 4.4%, respectively. It was also demonstrated by this study that this accumulation of heat is consistent with a decade-long intensification of the subtropical convergence, which is possibly linked to a persistent La Niña-like state.

71% of the Earth’s surface is covered by the ocean which serves as a major energy reservoir (Levitus et al, 2005), and there has been a well-documented rise of global mean sea level that is one of the most certain indicators of global warming (IPCC, 2013; Church & White, 2011). Sea level rise that results from the thermal expansion of the entire water column as the ocean warms, and the input of freshwater from the melting glaciers and ice sheets, mainly Antarctica and Greenland. The volume of the ocean is expanding as the ocean is warming, as is evident from both satellite and in situ measurements, and the question of heat sequestration at depth has attracted a great amount of attention (Trenberth & Fasullo, 2010; Hansen et al., 2011; Loeb et al., 2012; Abraham et al., 2013; Balmaseda et al., 2013; Johnson & Lyman, 2014; Cazenave et al., 2014; Llovel et al., 2014; Chen & Tung, 2014; Purkey et al., 2014; Desbruyères et al., 2016).

There is the potential to provide a more complete view of the horizontal and vertical distributions of heat in the ocean by a combination of satellite data and in situ observing systems of the present. Since 1992 the sea level has been continuously measured by altimetry satellites with a near global coverage and high accuracy. Since the early 2000s the Argo floats, that have a near global coverage that was achieved in 2005, and Gravity Recovery and Climate Experiment  (GRACE) satellites that were launched in 2002, have provided measurements of the temperature and salinity of the ocean in the upper 2,000 m and estimates of the mass changes, respectively. The steric (results from density in seawater) sea level variability, which is mostly a function of the heat content of the of the full depth (Jayne et al., 2003), is given by the difference between the total sea level, as observed by altimetry, and the mass-related sea level, as observed by GRACE. The contribution from the deep ocean below 2,000 m can be inferred indirectly as the difference between the satellite-based altimetry minus GRACE, and Argo-based steric sea level (Willis et al., 2008; Llovel et al., 2014). No significant deep ocean warming was shown by the most recent satellite altimetry, GRACE, and Argo measurements to have occurred on the global scale during the period 2005-2013 (Llovel et al., 2014; Dieng et al., 2015). There are several studies, on the other hand, that were based on repeat hydrography sections, most of which were in the pre-Argo era, that have reported a deep warming that was statistically significant from the 1990s to the 2000s, which were concentrated mainly in the Southern Ocean (Abraham et al., 2013; Purkey & Johnson, 2010, 2012; Desbruyères et al., 2016). As a result of the existing observing systems being subject to uncertainties and temperature measurements below 2,000 m are rather sparse and irregular, the topic of warming of the deep ocean is still controversial and requiring further research. Warming of the deep ocean can initiate and advance in regions where the interactions between the atmosphere and the ocean internal dynamics favour transfer of heat from the surface to the deeper waters. It is important that such regions be identified, preferably by the use of as many independent observing systems as possible, and to understand the associated dynamics. This study, which involved a comprehensive analysis of satellite and in situ measurements, and atmospheric reanalyses, reports warming signatures of the deep ocean that were observed in the subtropical South Pacific over the decade of 2005-2014, showing that this warming is consistent with an intensification a decade-long of the Southern Hemispheric westerly and trade winds and associated subtropical convergence, that was possibly linked to the persistent La Niña–like conditions.

Discussion and conclusions

Theoretically, an ideal method to derive deep steric changes below a depth of 2,000 m is the combined use of altimetry, GRACE and Argo. The indirect method has already been found to be useful for budget studies of sea level (Llovel et al., 2014; Purkey et al., 2014; Dieng et al., 2015), though the uncertainties that are associated with the observing systems are decreasing (von Schuckmann et al., 2016). It was shown by the results of the study of Volkov et al. with respect to local trends in box B, should be used with appropriate care, as they have the largest uncertainty.

The local deep ocean warming that is inferred indirectly is likely to be robust, given that the SSL_2,000 accounts for about half of the SSH rise in box B and the residual 3.8-3.9 ± 1.3 mm/yr is about twice as great as the global mean SSH_Mass change of 2.0 ± 0.2 mm/yr. The local SSH_Mass change that is the result of the melting of glaciers in Antarctica and Greenland is not expected to exceed the global mean SSH_Mass rise (Tamisiea & Mitrovica, 2011). The residual SSL_Deep rise in box B would still be significant at 1.8 ± 1.3 mm/yr if the SSH_Mass rise was uniform around the globe.

The indirect estimates of this study of the contribution of the deep ocean to sea level rise in box B range from 1.8 ± 1.4 mm/yr to 3.0 ± 1.3 mm/yr. The direct estimate of the change in temperature in the deep ocean along P16 transect in 2005=2014 provides more confidence in the residual calculation. The moderate warming of 0.012 ± 0.007^oC that is directly measured is equivalent of a rise in sea level of 0.7 ± 0.4 mm/yr, which agrees, within the error bars, with most indirect estimates. In box B the deep ocean contributed from 2.4 ± 1.4% to 6.1 – 10.1 ± 4.4% of the global ocean mean SSL and heat increase, respectively (Text S2), according to the direct and indirect estimates and assuming that the halosteric (wholly solid) contribution is negligible.

According to Volkov et al. by realising the pitfalls of the residual calculation they noted that this study is the first report on the consistency between the direct and indirect estimates of the warming in the deep ocean in the subtropical South Pacific. They reconciled the studies that were recently published with contrasting reports of temperature changes in the deep ocean based on repeat hydrography (Purkey & Johnson, 2010; Desbruyères et al., 2016) and the residual calculation for the global ocean (Llovel et al., 2014, Dieng et al., 2015), by focusing on the regional instead of the global accumulation of heat, which can be independent of each other. They showed that the warming of the deep ocean that was inferred directly and indirectly was consistent with the warming of the upper ocean in 2005-2014, and was likely to be driven, or at least favoured, by wind-driven convergence that was persistent that is intrinsic to La Niña conditions. It is suggested by this that the warming that was observed reflects, at least partly, an interannual or Interdecadal variation.

However, Volkov et al. noted that given the scarcity of observations at depth, the mechanism that is presented is hypothetical and there are other mechanisms that could also be relevant. The abyssal warming, e.g., can be triggered by changes in buoyancy forcing in regions of the formation of Antarctic Bottom Water (Purkey & Johnson, 2012, 2013; Masuda et al., 2010). Their study highlights, overall, the importance of the implementation of the Deep Argo array in the region as well as globally, which would help to reduce uncertainties in sea level and planetary energy-imbalance budgets (Johnson et al., 2015).

It is indicated by the findings that are reported in this paper that the South Pacific is a region where heat can be sequestered effectively by a convergence-favourable wind forcing. The decade-long regional accumulation of heat at depth could, therefore, potentially contribute to the “hiatus” in SSTs that has been documented (IPCC, 2013). The subtropical South Pacific began losing heat in 2014, however, and this tendency continued throughout 2015. The Pacific Ocean and the atmosphere began exhibiting features of the El Niño event that was pending at the beginning of 2014, which had already expressed itself by early 2016 as one of the most powerful on record. A weakening of westerly winds above the Antarctic Circumpolar Current (ACC) and in the tropics the trade winds has been observed that was triggered by the El Niño event of 2014-2016; also decreased, were the wind stress curl and associated Ekman pumping in the subtropical South Pacific. It is reasonable to hypothesise as a motivation for continued research is that the tendency to cooling that began in 2014 in the South Pacific continues, and then the heat that accumulated at depth could be released into the atmosphere and the surrounding ocean that would have far-reaching effects on weather and climate.

Volkov, D. L., et al. (2017). "Decade-long deep-ocean warming detected in the subtropical South Pacific." Geophysical Research Letters 44(2): 2016GL071661.