ENSO precursor signal: the Victoria mode (VM) index
The Victoria mode (VM) is defined as the second empirical orthogonal function mode (EOF2) of North Pacific SST anomalies poleward of 20°N. The EOF1 of monthly SST anomalies over the North Pacific poleward of 20°N is conventionally referred to as the Pacific decadal Oscillation (PDO). The EOF2 exhibits a tilted SST anomaly dipole pattern oriented in the northeast–southwest direction. This mode is referred to as the so-called VM. The VM index (VMI) is defined as the second principal component (PC2) time series associated with the EOF2. Recent studies have documented the wide-ranging impacts of the VM on the tropical Pacific climate (e.g., the ITCZ precipitation, the South China Sea summer monsoon, and ENSO).
Figure 1. Spatial patterns and corresponding PCs of the first two leading EOF modes of monthly SST anomalies over the North Pacific poleward of 20°N (after removing the global mean SST anomaly).
Ding, R. Q., J. P. Li, Y.-H. Tseng, C. Sun, and Y. P. Guo, 2014: The Victoria mode in the North Pacific linking extratropical SLP variations to ENSO. Journal of Geophysical Research, 120, 27-45, doi: 10.1002/2014JD022221.
ENSO precursor signal: the South Pacific Quadrapole (SPQ) index
The SPQ is defined as the second empirical orthogonal function (EOF2) mode of SSTA in the extratropical South Pacific. Figure 2a shows the SPQ obtained using the monthly normalized SSTA field in the extratropical South Pacific (poleward of 15°S) from 1979 to 2010 (after removing the monthly mean global average SSTAs). The SPQ accounts for 12.2% of the total SST variance and represents a zonal SSTA pattern with four centers between 30°S and 50°S. The SPQ index (SPQI) is correspondingly defined as the difference between the sum of normalized SSTA over the two positive centers (Boxes B and D in Figure 2b) and the sum of normalized SSTA over the two negative centers (Boxes A and C in Figure 2b). Figure 2b shows the simultaneous correlation coefficients between SSTA and SPQI during austral summer.
Figure 2. (a) Spatial pattern of the EOF2 mode of the South Pacific (140°E−70°W, 15°−60°S) monthly SSTA field for the period 1979–2010 (after removing the monthly mean global average SSTA). EOF2 accounts for 12.2% of the total variance. (b) Correlation map of the JFM-averaged SPQ index with the concurrent SSTA. Positive (red) and negative (blue) value, with correlation significant at the 90% level, are shaded. The JFM-averaged SPQ index (dotted line) defined by the four green boxes [positive regression boxes: B:(58°–36°S, 173°–145°W) and D:(37°–17°S, 103°–76°W); negative regression boxes: A:(47°–25°S, 142°E–179°W) and C:(59°–40°S, 113°–81°W)].
Ding, R. Q., P. J. Li, Y. H. Tseng, 2015: The impact of South Pacific extratropical forcing on ENSO and comparisons with the North Pacific. Climate Dynamics, 44, 2017–2034.