Background information for meteorological services
In the Southwest Pacific, tropical cyclones (TCs) usually develop in the wet season, from November through to April, but occasionally occur in October and May, and have even occurred in June.
Peak cyclone occurrence is usually from January to March (See figure below). In seasons with similar background climate conditions to present, several tropical cyclones occur in the Coral Sea region between the Solomon Islands and New Caledonia, and near Vanuatu, Fiji and Tonga, while a few affect other areas. On average, half of the tropical cyclones that developed since the 1969-70 season have reached hurricane force with mean wind speeds of at least 64 knots (118 km/h).
Finding past analogues to describe the current climate state (a well-coupled El Niño) that was preceded by poor coupling (strengthened oceanic anomalies in the NINO3.4 region (warmer than normal) and near neutral Southern Oscillation Index (SOI) values during the lead-up to the coming TC season) was difficult. The change in the ENSO state that occurred since the first TC projection was made has meant the guidance for the second half of the season needed to be updated. This is especially pertinent because there may be increased risk to the east of the Dateline, and also because of recent activity near the Southern Cook Islands (TC Pat).
Use of a joint ENSO index that combines the atmospheric index (SOI) with the most widely-used oceanic index (NINO3.4) has been described in Gergis and Fowler (2005), who call this the “Coupled ENSO Index” (CEI). Using the CEI to highlight years when the equatorial SSTs were above normal and atmospheric signals were also above normal (i.e. what Gergis & Fowler term a well coupled or “NINO-style” El Niño) for January through April, and preceded by months when equatorial SSTs were above normal and atmospheric signals were near normal (i.e. what Gergis & Fowler term a well coupled or “NINO3.4-style” El Niño) identified four analogues (1957/58; 1976/77; 1986/87; 2006/07)..
Global climate models indicate El Niño is likely through the Austral summer into autumn. It is possible that during this time we will see an elevated risk of storm tracks or TC origins east of the Dateline (especially for the Southern Cook Islands) and also near the Solomon Islands. A strong E-SE slant to TC tracks and ex-tropical transition in the region are expected, consistent with the subtropical jet and the South Pacific Convergence Zone (SPCZ) being northeast of normal during typical El Niño years. The recent northeast shift in the SPCZ relative to its normal position recently highlight the changes that have occurred in the atmosphere during late spring and the early part of summer.
TC intensity is related to how long developing systems reside in the tropics and draw on warm waters for their strength, and how the subtropical jet and SPCZ mutually guide and also contribute to shear during ex-tropical transition.
References
Gergis, J. and A.M. Fowler (2005), Classification of synchronous oceanic and atmospheric El Niño-southern Oscillation (ENSO) events for palaeoclimate reconstruction. International Journal of Climatology, 25, 1541-1565.
