Frequently Asked Questions

• The Long-Range Tropical Cyclone Outlook for the Southwest Pacific (TCO-SP) generates freely-available deterministic and probabilistic island-scale tropical cyclone (TC) count outlooks for 12 locations across the Southwest Pacific region.
• Updates are provided monthly (July-October) so that TCO-SP can consider the latest changes in ocean temperatures and atmospheric variability. Pre-season outlooks are generated every month between July and October and provide guidance for the November-April tropical cyclone season. Pre-season outlooks offer up to four months lead time before the official start of the Southwest Pacific TC season. In-season outlooks may be generated if required.  

TCO-SP provides guidance for 12 regional, sub-regional and island scale locations (see figure below):

• Regional: Southwest Pacific (SWP) basin (135°E-120°W).
• Sub-regional: Northern SWP (N SWP; Tuvalu, Wallis & Futuna, Tokelau), Central SWP (C SWP; Samoa, American Samoa, Niue), Northeast SWP (NE SWP; Northern Cook Islands, E. Kiribati: Line Islands, Marquesas, Tuamotu Archipelago, Gambier Islands, Pitcairn Islands) and Southeast SWP (SE SWP; Southern Cook Islands, Society Islands, Austral Islands).
• Island-scale: Fiji, Tonga, Vanuatu, Solomon Islands, New Caledonia, Papua New Guinea, Northern New Zealand.
  
  

TCO-SP outlook regions. Black labels indicate individual island-scale outlooks. White labels indicate sub-regional outlooks.

• The statistical method used to derive the outlooks is sensitive to the seasonal number of TCs. Where there are fewer than 1.5 TCs per season (on average) within a given EEZ, geographically neighbouring EEZs are grouped together. This has resulted in four sub-regional EEZs, including N SWP, C SWP, SE SWP and NE SWP.
• Figure below compares the average TC frequency per EEZ between 1970-2019 (top panel) and the island-scale and sub-regional groupings used for TCO-SP (bottom panel).
  
  

Top panel: Exclusive Economic Zones (EEZ) considered in this study with the shading seasonal TC climatology (Nov-Apr) between 1970 and 2019. Contours represent seasonal (Nov-Apr) TC track density between 1970 and 2019 (0.5 TCs/season intervals). Bottom panel: Location of 12 subregional and island scale models (including entire SWP region: 0°-35°S, 135°E-120W). Where individual EEZ climatology was <1.5 TCs per season, surrounding EEZs were merged to form the following subregions: Northern SWP (N SWP; Tuvalu, Wallis & Futuna and Tokelau), Central SWP (C SWP; Samoa, American Samoa and Niue), Northeast SWP (NE SWP; Northern Cook Islands, E.Kiribati: Line Islands, Marquesas, Tuamotu Archipelago, Gambier Islands and Pitcairn Islands), and Southeast SWP (Southern Cook Islands, Society Islands, Austral Islands. Island-scale models were derived for the following: Papua New Guinea, Solomon Islands, Vanuatu, New Caledonia, Fiji, Tonga and Northern New Zealand. Models for W.Kiribati: Gilbert Islands and C.Kiribati: Phoenix Islands have not been derived or included as part of a larger subregion as these locations have a low seasonal TC climatology (≤0.06 TCs) and are at minimal risk of TC activity.

• TCO-SP uses multivariate Poisson regression to derive its TC count predictions. Models are trained on historical relationships between seasonal TC frequency and ocean-atmosphere variability from 1970-present.
• Models consider a total of 14 indices representing coupled ocean-atmosphere variability (figure below) including El Niño-Southern Oscillation, the Southern Annular Mode, and Indian Ocean sea surface temperature variability.
• Before generating an outlook, 10 unique predictor models are tested per location. The model with the highest skill based on the training period is chosen to derive the next outlook.
• See Magee et al. (2020) for more information.

Indo-Pacific climate indices considered in TCO-SP. Indices representing El Niño-Southern Oscillation (ENSO) include: NINO1+2, NINO3, NINO4, NINO3.4, Southern Oscillation Index (SOI), ENSO Modoki Index (EMI), Coupled ENSO Index (CEI), Oceanic Nino Index (ONI), Trans NINO Index (TNI) and ENSO Longitudinal Index (ELI). Other indices considered include the Southern Annular Mode (SAM), Indian Ocean Dipole West pole (IOD W), Indian Ocean Dipole East pole (IOD E), and the Dipole Mode Index (DMI).

Pre-season TCO-SP outlooks will be generated every month between July and October, while in-season outlooks will be generated between November and January if required. 

• In conjunction with ENSO, the Southern Annular Mode (SAM) has found to influence the seasonal characteristics of TCs (Diamond and Renwick, 2014).
• Indian Ocean sea surface temperature variability has been shown to drive changes on where TCs form in the Southwest Pacific region. For example, during positive IOD E events (warm SSTs in the IOD E region), TCs are more likely to form towards the northeast of the basin, even during ENSO neutral conditions. The relationships between Indian Ocean SST variability and ENSO result in substantially different risk profiles for Pacific Island Nations compared to those when considering ENSO alone (Magee and Verdon-Kidd, 2018).

• Models perform well in capturing the variability of TC frequency for monthly pre-season (July-October) and in-season (November-January) outlooks.
• Magee et al. (2020) comprehensively evaluate model performance for each location.

TCO-SP is a statistically-driven, island and regional scale TC outlook for the Southwest Pacific region. Rolling monthly updates will be provided between July and January so predictive models used in TCO-SP can consider the latest changes in ocean temperature and atmospheric variability. The following details should be considered when using TCO-SP:

• Guidance from TCO-SP does not and should not replace the advice provided by your local Pacific Island National Meteorological Service.
• Guidance from TCO-SP will be considered in the consensus-based outlook led by NIWA for their Island Climate Update TC outlook.
• TCO-SP provides guidance for the tropical cyclone season only (November-April), and does not consider tropical cyclones outside of these months.
  
• Timescales associated with outlooks (months) are different to shorter-term weather forecasts (hours to days). In the case of outlooks, particularly long-range outlooks such as those presented here, TCO-SP provides guidance up to four months before the start of the Southwest Pacific TC season. As such, it is possible for daily or weekly changes in predictors (i.e. ocean temperatures and atmospheric variability) to influence TC numbers and result in discrepancies with the long-range TC outlooks.
• Monthly TCO-SP guidance will track any changes in ocean temperature/atmosphere variability, which may result in changes in guidance from one outlook to another. Subscribing to TCO-SP is the best way for end-users (e.g. Pacific Island Meteorological Services) to stay up to date with the latest TCO-SP updates.
• Users should evaluate model skill and model consensus contained within the guidance document to inform decision-making.
• TCO-SP is an experimental platform and should ideally be used in combination with other guidance for decision-making. TCO-SP does not accept any liability associated with decisions that are made using this guidance.
• It does not take a landfalling TC to cause significant and life-threatening impacts. TCs that pass neighbouring Pacific Island nations can also cause significant damage. Always be alert. Listen to the advice of your local meteorological office and/or disaster management office.