# World Ocean Thermocline Weakening and Isothermal Layer Warming
## 4 Results and Discussions  
### Decadal Variability of Global (\(H_{ttl}\), \(G\))  
In analyzing global spatial and temporal variability, we divided the 1,111,647 synoptic data pairs of [\(H_{ttl}(t,\textbf{r})\), \(G(t,\textbf{r})\)] into six temporal periods: 1971-1980, 1981-1990, 1991-2000, 2001-2010, 2011-2017. For each time-period, we average the (\(H_{ttl}\), \(G\)) data in 4\({}^{\circ}\) (longitude) \(\times\) 2\({}^{\circ}\) (latitude) cell if the number of data pairs is larger than 5, and identify the cell with no data if the number of data pairs is less than 5. Figure 3 shows decadal variations of the global distributions of (\(H_{ttl}\), \(G\)). The number of data points are on the right panels. The white grid cells denote no data or a number of data pairs less than  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Ocean**} & \multirow{2}{*}{**Number of Data**} & \(\partial H_{ttl}/\partial\tau\) & \(\partial G/\partial\tau\) \\  & & **(W/m\({}^{2}\))** & **[10\({}^{-3}\) °C/(m \(\times\) yr)]** \\ \hline Global Oceans & 1,111,647 & \(0.142\pm 0.014\) & \(-2.11\pm 0.31\) \\ \hline North Atlantic (10\({}^{\circ}\) N–60\({}^{\circ}\) N) & 335,747 & \(0.128\pm 0.022\) & \(-1.49\pm 0.046\) \\ \hline Equatorial Atlantic (10\({}^{\circ}\) S–10\({}^{\circ}\) N) & 42,599 & \(0.154\pm 0.036\) & \(-2.28\pm 0.58\) \\ \hline South Atlantic (10\({}^{\circ}\) S–60\({}^{\circ}\) S) & 57,540 & \(0.0824\pm 0.04\) & \(-2.43\pm 0.73\) \\ \hline Indian Ocean & 150,789 & \(0.160\pm 0.028\) & \(-2.77\pm 0.49\) \\ \hline Equatorial Pacific (10\({}^{\circ}\) S–10\({}^{\circ}\) N) & 89,924 & \(0.156\pm 0.066\) & \(-3.51\pm 0.63\) \\ \hline Southern Ocean (South of 60\({}^{\circ}\) S) & 7942 & \(-0.0272\pm 0.027\) & \(-1.19\pm 1.6\) \\ \hline Western North Pacific & & & \\ (10\({}^{\circ}\) N–60\({}^{\circ}\) N, 120\({}^{\circ}\) E–160\({}^{\circ}\) E) & 101,447 & \(0.139\pm 0.04\) & \(-2.07\pm 0.55\) \\ \hline Central North Pacific & & & \\ (10\({}^{\circ}\) N–60\({}^{\circ}\) N, 160\({}^{\circ}\) E–140\({}^{\circ}\) W) & 88,967 & \(0.135\pm 0.028\) & \(-1.65\pm 0.44\) \\ \hline Eastern North Pacific & & & \\ (10\({}^{\circ}\) N–60\({}^{\circ}\) N, 140\({}^{\circ}\) W–85\({}^{\circ}\) W) & 91,572 & \(0.104\pm 0.062\) & \(-2.05\pm 0.43\) \\ \hline Western South Pacific & & & \\ (10\({}^{\circ}\) S–60\({}^{\circ}\) S, 145\({}^{\circ}\) E–170\({}^{\circ}\) W) & 79,472 & \(0.140\pm 0.037\) & \(-0.821\pm 0.19\) \\ \hline Central South Pacific & & & \\ (10\({}^{\circ}\) S–60\({}^{\circ}\) S, 170\({}^{\circ}\) W–120\({}^{\circ}\) W) & 23,632 & \(0.125\pm 0.045\) & \(-0.378\pm 0.3\) \\ \hline Eastern South Pacific & & & \\ (10\({}^{\circ}\) S–60\({}^{\circ}\) S, 120\({}^{\circ}\) W–70\({}^{\circ}\) W) & 16,965 & \(0.005\pm 0.084\) & \(-1.61\pm 1.10\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Trends of \(H_{ttl}\) and \(G\) for different parts of world oceans.  
5. The most striking features are thermocline weakening and ITL warming. A strong thermocline gradient (\(G>0.3\)\({}^{\circ}\)C/m) occupied a vast area of the middle to high latitudes (north of 30\({}^{\circ}\) N) of the Northern Hemisphere as well as the eastern equatorial Pacific and Atlantic Oceans during 1971-1980, and 1981-1990. Such strong thermocline areas shrank during 1991-2000, and almost disappeared in 2011-2017. However, \(H_{\mathit{ITL}}\) increased steadily from 1971-1980 to 2011-2017. There was no area with \(H_{\mathit{ITL}}\) of more than \(2.5\times 10^{8}\) J/m\({}^{2}\) in the middle to high latitudes of the Northern Hemisphere during 1971-1980. Warm areas (\(H_{\mathit{ITL}}>2.5\times 10^{8}\) J/m\({}^{2}\)) showed up in these latitudes especially the western North Atlantic Ocean and equatorial Pacific and Atlantic Oceans in 1981-1990. The warm areas increased in 1991-2000, 2001-2010, and 2011-2017.  
Figure 3. Decadal distributions of synoptic relative ITL heat content (\(H_{\mathit{ITL}}\), left panels) and thermocline gradient (\(G\), right panels) of the world oceans. The white grid cells denote no data.