TRIPOLE INDEX FOR THE INTERDECADAL PACIFIC OSCILLATION (TPI(IPO)) |
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Tripole Index for the Interdecadal Pacific Oscillation (TPI(IPO))The Tripole Index (TPI) for the Interdecadal Pacific Oscillation (IPO) is based on the difference between the mean SST in the central equatorial Pacific and the mean SST in the northwestern and southwestern Pacific, representing a measure of interdecadal variability in the Pacific. The Interdecadal Pacific Oscillation is associated with a distinct ‘tripole’ pattern of sea surface temperature (SST) anomalies, with three major centers of action and variations on decadal timescales, evident in the second principal component (PC) of the global low-pass-filtered SST field. This index is based on the difference between the mean SST over the central equatorial Pacific and the mean SST in the northwestern and southwestern Pacific. The TPI is an easily calculated, non-PC-based index that tracks decadal SST variability associated with the IPO. The TPI also provides a simple metric in physical units (°C) to assess decadal and interdecadal variability in SST fields. Henley et al. (2015). Several studies have shown that the IPO influences large-scale climate processes, including the modulation of ENSO-related variability and impacts on precipitation in the Southern Hemisphere, particularly in Australia and the South Pacific. The relationship between the IPO and the modulation of ENSO effects on Australian precipitation was classically described by Power et al. (1999), and further expanded in subsequent analyses on regional atmospheric teleconnections. Paleoclimate reconstructions indicate that the interdecadal variability associated with the IPO existed long before the instrumental period. In particular, reconstructions based on Pacific ice cores show a signal consistent with the interdecadal mode over several centuries, reinforcing the physical persistence represented by the TPI. Porter et al. (2021). Composites of SST and sea level pressure associated with the positive and negative phases of the IPO show that the tripole pattern exhibits spatial stability across different epochs, confirming its usefulness in long-term climate studies. The spatial structure of the tripole and its relationship with atmospheric anomalies were detailed and directly compared with the phases defined by the TPI in Henley et al. (2015). 
The map above shows the low-pass correlations (composite of 10 realizations). *For monthly maps of SST anomalies, refer to the Maps tab. |
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The TPI (Tripole Index) is a metric developed to represent the tripolar pattern of Sea Surface Temperature (SST) anomalies associated with the Interdecadal Pacific Oscillation (IPO). It is calculated from the difference between the mean SST anomaly in the central equatorial Pacific and the mean anomalies in the northern and southern subtropical Pacific regions, as defined in Henley et al. (2015).
The regions used to compute the index are:
Region 1: 25°N–45°N, 140°E–145°W
Region 2: 10°S–10°N, 170°E–90°W
Region 3: 50°S–15°S, 150°E–160°W
This spatial configuration was formally introduced in Henley et al. (2015), which established the TPI as the standard representation of the IPO in the scientific literature.
Climatology from Jan/1971 to Jan/2000, following the interval used in SST datasets such as ERSST and HadISST (source of the TPI data provided by NOAA/PSL).
The calculation begins with determining the monthly SST anomalies obtained by subtracting the climatology (1971–2000) from each point of the ocean grid. This procedure removes the long-term mean and isolates the actual variability of the system, as described in Henley et al. (2015) and other studies that use the index.
After computing the anomalies, the spatial average for each of the three regions is obtained. Region 2 represents the tropical component of the IPO, while Regions 1 and 3 represent its subtropical components, as defined in Henley et al. (2015).
The monthly TPI index is then defined by the expression:
TPI = SSTRegion 2 − ( SSTRegion 1 + SSTRegion 3 ) / 2
This formula captures the characteristic tripolar pattern of the IPO, in which the tropical Pacific exhibits anomalies opposite to those found in the subtropical bands, as detailed in Henley et al. (2015).
Thus, positive TPI values indicate that the tropical Pacific is warmer relative to the subtropical regions, characterizing the positive (warm) phase of the IPO; negative values indicate the negative (cool) phase, as interpreted in climate variability studies that use the TPI to monitor the IPO.
| SOUZA, C. A. de; REBOITA, M. S. Ferramenta para o Monitoramento dos Padrões de Teleconexão na América do Sul. Terrae Didatica, Campinas, SP, v. 17, n. 00, p. e02109, 2021. DOI: 10.20396/td.v17i00.8663474 |
