Emergence of El Niño as an Autonomous Component in the Climate Network

We construct and analyze a climate network which represents the interdependent structure of the climate in different geographical zones and find that the network responds in a unique way to El Niño events. Analyzing the dynamics of the climate network shows that when El Niño events begin, the El Niño basin partially loses its influence on its surroundings. After typically three months, this influence is restored while the basin loses almost all dependence on its surroundings and becomes autonomous. The formation of an autonomous basin is the missing link to understand the seemingly contradicting phenomena of the afore-noticed weakening of the interdependencies in the climate network during El Niño and the known impact of the anomalies inside the El Niño basin on the global climate system.

Figure 1

Spatial distribution of the in-weighted degrees of nodes (in the SAT network) averaged over (a) non-El Niño times ($⟨I_l^y{⟩}_{y\mathrm{\in ̸}\mathrm{El}\mathrm{Ni}\tilde{\mathrm{n}}\mathrm{o}}$) (b) El Niño times ($⟨I_l^y{⟩}_{y\in \mathrm{El}\mathrm{Ni}\tilde{\mathrm{n}}\mathrm{o}}$). The nodes of the network are shown by small dots. The nodes belonging to $\mathbf{C}$ (ENB) are shown as larger blue circles.

Figure 2

Weighted degrees as a function of time ($y$ axis) and the node index ($x$ axis), for the SAT network. (a) Total weighted degree $D_l^y$. (b) The microscopic contributions to the weighted incoming degree of $\mathbf{C}$, $I_{\mathbf{C}}^y$. (c) The microscopic contributions to the weighted outgoing degrees, $O_{\mathbf{C}}^y$. One should bare in mind that each point is compiled from records of 565 d: $365\mathrm{d}+200\mathrm{d}$ of shifts. The representative point in all figures for each 565 period is the beginning date of the period.

Figure 3

Plot of the weighted in and out degrees of $\mathbf{C}$, $I_{\mathbf{C}}^y$ and $O_{\mathbf{C}}^y$, respectively. The weighted degrees are shown for the networks based on the (a) SAT and (b) 850 hPa records. The blue curve shows the NINO3.4 index (see, e.g., 25) which represents an average sea surface temperature (scaled to fit into the image). The two dashed lines represent a threshold of $\pm 0.4°\mathrm{C}$, which is an accepted criterion indicating El Niño and La Niña events. The dotted curve shifts the NINO3.4 in 1 yr. This is shown since the correlation evaluations depend on integrated data from 1 yr.

Figure 4

Number of nodes (in the SAT network) outside $\mathbf{C}$ having sum of weights below 2.5 directing towards $\mathbf{C}$ (in), $I^{*}$, or from $\mathbf{C}$ (out), $O^{*}$. The blue and orange markers correspond to periods in which NINO3.4 is above the threshold. See the corresponding blue curves in the previous figure.

Figure 5

Cross covariance of the number of links below various thresholds going into and out from cluster $\mathbf{C}$ (in and out, respectively, Fig. 4). In the positive direction of the $x$ axis, the number of in links lags in time after the out links. The figure is based on the SAT network.