Warming in the Agulhas Region during the Global Surface Warming Acceleration and Slowdown

The Agulhas Region gains more heat during the global surface warming slowdown than acceleration period. Yet, mechanisms that cause excessive heat accumulation in this region remain largely unknown. We investigate the underlying physical processes and examine their influence on ocean heat changes in the last three decades. Heave is found to drive the increasing ocean heat content in the last three decades whereas elevated heat accumulation rate in slowdown compared than acceleration period is mainly attributed to spice. During the acceleration period, pure heaving of Subtropical Mode Water induced by wind stress change and pure warming caused by heat flux, leading to a strong heave component and relatively weak spice, drive the increases in ocean heat content. During the slowdown period, increasing salinity strengthens the spice, resulting in a higher heat accumulation rate compared to the acceleration period.


Results
Agulhas Region (AR) covers most of the Greater Agulhas System, located in the domain delimited by 10°E to 35°E and 36°S to 45°S 11 , including the south end of the Agulhas Current, the retroflection, the Return Current and its adjacent subtropical front. Sea surface temperature data and two reanalysis data sets (ORAP5 and ORAS4), were analyzed to study the heat distribution and warming mechanism from 1984 to 2013. Results from the two reanalysis datasets are highly consistent (Fig. S1); therefore, only results from ORAP5 are presented due to its higher spatial resolution (ORAP5: 0.25 o × 0.25 o × 75 vertical levels vs. ORAS4: 1 o × 1 o × 42 vertical levels). To compare the heat distribution pattern between the global surface warming acceleration and slowdown period, data from 1984-1997 is used to represent global warming acceleration period and 1998-2013 for slowdown.
The AR shows a strong warming trend at the surface in the last three decades 11 . The strongest surface warming signal is detected at the retroflection, Return current, and its adjacent subtropical frontal region (Fig. 1a). As sea surface temperature only captures surface signal, ocean heat content (OHC) is further used to investigate interior processes in the AR. In the last three decades, the increase of OHC anomalies is mainly detected at 0-700 m (Figs S1, S2). The rate of heat accumulation is shown to be much higher during the slowdown than the acceleration period (Fig. 1b,c), consistent with previous findings that more heat sinks into the interior ocean during the slowdown 3,12-14 .
In the upper 200 m layer, diabatic processes such as seasonality and mixing are dominant in the AR, resulting in substantial residue from heat decomposition 15 . Thus, in this study, we focus on the 200-700 m layer to investigate the heat evolution and distribution in the AR. To obtain a general trend of heat content changes induced by heave and spice, Ensemble Empirical Mode Decomposition (EEMD) is applied to all the three signals (total OHC, heave, and spice) (Fig. S3), decomposed into intrinsic mode functions (IMF) with different time scales 16 . In this study, each EEMD mode passes the significant test determined by calculating the spread function for the 99% and 95% confidence limit levels 17 . After removing the seasonal and annual viability, decadal variability plus the long-term nonlinear trend show an increasing pattern for heat content changes induced by heave during the three decades, while the OHC and spice component are observed to have an increase only in the slowdown period with high variability during the acceleration period. Moreover, OHC change induced by heaving process is much higher than that induced by spice (Fig. 1d). The mean value of the heave component in the AR is 4.7 × 10 8 J/m 2 , while that of spice component is −2.6 × 10 7 J/m 2 , suggestive that heave is a major contributor to the elevated heat accumulation in interior ocean, which is also consistent with previous findings that isopycnals in the AR have one of the largest deepening rates globally 5 . Noticeably, IMFs5-7 for spice display a similar pattern with that for the total OHC, which indicates that the decadal variabilities of the total OHC can be mainly attributed to the spice processes (Fig. 1d).
Heat content changes in ocean interior caused by heave are shown to vary at different density surface (Fig. 2). Strong isopycnal deepening, over 15 m/decade, is observed in Antarctic Intermediate Water (AAIM) and Central Water (CW) (σ = 26. 8-27.8), at the deep layer of the Return Current and upper layer of its adjacent subtropical frontal region (Figs 2, S4). Noticeably, Subtropical Mode Water (STMW: δ ≤ 26.5) in Agulhas Current displays a shoaling trend (Fig. 3), which indicates that heave's impact changes in both horizontal and vertical dimensions. To further identify the water mass that contributes the most to heat content changes induced by heave, isopycnals with σ at 26.5, 27, 27.5 are chosen to examine their various influence on ocean heat content. Compared to the previous acceleration period, density surface at 26.5 deepens around 40 meters in the Return Current and shoals in the Agulhas Current during the slowdown period. In contrast to the strong spatial variability detected in water mass at 26.5, isopycnals at 27 and 27.5 present a more uniform deepening signal with twice the deepening rate on average. Häkkinen et al. 5 claimed that the strongest deepening happens at isopycnal at 27 in the southern hemisphere. However, our study implies that the isopycnal of 27.5's deepening is stronger than that of 27 in the AR.
To obtain a higher resolution of heat content change across the water column, vertical profiles of water temperature change is examined to assess the relative contribution of heave and spice at different depth in the AR. The total temperature difference between the slowdown and acceleration period decreases with depth (Fig. 4a). Vertical mixing, inferred by the residue of the decomposition, can be important at the surface while temperature changes below 200 m are mainly attributed to heave and spice. Heave has a strong impact on temperature variations across the whole water column whereas spicing component decreases with depth, which is similar to the vertical profile of total temperature difference. The high temperature changes caused by spice indicates the elevated heat content accumulation in slowdown compared to acceleration period is mainly caused by elevated spicing.

Conclusion and Discussion
The Agulhas Region (AR) is one of the fastest warming regions in the world ocean in the last few decades 4,5 . Heat accumulates at a faster rate during the slowdown than the acceleration period. Therefore, it is considered as one of the possible oceanic heat sinks. Despite its essential role in global heat redistribution, a basic understanding of the mechanisms leading to its excessive heat accumulation is still lacking. This research decomposes temperature changes into heave and spice, which are two major adiabatic physical processes affecting ocean heat content, to assess their impacts on excessive heat accumulation in the AR during the slowdown period. Increasing heat content is mainly caused by heave at the deep layer (200-700 m) whereas the elevated heat accumulation rate observed in slowdown can be mainly attributed to stronger spice. Heave is found to increase in a monotonic fashion while more decadal variabilities are observed for the spice component. The total OHC also shows a strong decadal variability, indicating that decadal variability in OHC can be largely attributed to spicing processes. Noticeably, heave displays spatial variations both horizontally and vertically, further highlighting the complex nature of adiabatic physical processes in the AR.
In ventilated thermocline theory, the surface conditions can be advected into the interior along isopycnals 18 . Following previous methodology 15 , we investigate three possible surface processes contributing to the ocean interior warming, pure warming due to the heat flux from the atmosphere to the ocean, pure freshening caused by the changes of evaporation and precipitation, and pure heaving stimulated by wind stress change. Water mass change (pure warming or pure freshening) induced heaving can counteract or reinforce wind stress change induced heaving (pure heaving). Pairs of temperature or salinity change trend at pressure surfaces (θ z , S z ), its heave (N′θ z , N′S z ) and spice (θ n , S n ) components are compared during the acceleration and slowdown periods (Fig. 5). In the pure warming scenario, S z = 0, which requires the salinity changes on a neutral surface to be balanced by the vertical movement of the neutral surface (S n = N′S z ). Likewise, in the pure freshening scenario, the temperature changes at certain depth should equal the changes on the neutral surface (θ z = θ n ). And pure heaving scenario involves no temperature or salinity change along the neutral surface (θ n = 0, S n = 0). During the acceleration period, the correlation coefficient between S n and N′S z is 0.5358 (Fig. 5a), which indicates that increase of heat flux is one of the dominant warming mechanisms. Moreover, no temperature change of the STMW along the neutral surfaces (Fig. 5c) suggests that STMW experienced wind driven heaving during the acceleration period. However, during the slowdown period, STMW is saltier than that during the acceleration period (Fig. 4b), and the warming is governed by salinity change and pure heaving of most of the water column. A correlation coefficient of 0.7962   was found between θ z and θ n (Fig. 5d), and S n falls to near zero (Fig. 5b) with a correlation coefficient of 0.6406 between S z and N′S z . The salinity change trend is positive at where θ z and θ n are best correlated with each other, suggestive that it is more of a salting scenario during the slowdown period rather than freshening. Pure warming and pure freshening can lead to negative spice and positive heave 5 . During the acceleration period, pure heaving of STMW induced by wind stress change combines heat flux into the ocean, leading to the fast increase of the heave component and relatively weak spice. Whereas, during the slowdown period, strong pure heaving is offset by the increasing salinity, which can also lead to a larger spice comparing to the former period.