El Niño expected to produce severe tropical storms in the Southwest Pacific

Meteorological and climate analysis centres across the Southwest Pacific are indicating above average numbers of tropical cyclones (TC) for the 2015–16 season (November 2015 to April 2016). The 30-year average number of all Southwest Pacific tropical storms that formed between 1981-2010 is 12.4 for the November and April TC season. The average number of storms that developed into named TCs (Category 1 or stronger) during the same interval is 10.4 for the SW Pacific basin[1]. The outlook indicates that 11 to 13 named TCs are expected for the coming season.  TC activity is elevated for a majority of the Pacific Island countries, especially those situated close to or east of the International Date Line (see Figure 1).

It should be recognised that the six-month outlook reflects an expectation of overall elevated activity during both the early season (November to January) and the late season (February to April) particularly east of the Dateline. Note that the TC activity outlook for islands like New Caledonia and Tonga indicates two or more cyclones could interact with each of those countries during the season despite subtle projected differences from normal. At least six severe TCs (Category 3 or higher[2]) are expected to occur anywhere across the Southwest Pacific during the season. All communities should remain vigilant and follow forecast information provided by their national meteorological service.

On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. For the coming TC season, the risk for New Zealand is slightly higher than normal. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it has an equal probability of passing east or west of Auckland city. Significant rainfall, damaging winds and coastal impacts can occur leading up to and during these episodic events.

Outlook analysis

Conditions associated with a strong El Niño are indicated by sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean, and the atmospheric circulation patterns that exist over French Polynesia and northern Australia. There is an expectation amongst a number of international forecast centres that the present El Niño event will be one of the strongest in the last 60 years. The current event still has the potential of surpassing the 1997/98 event. Taking this climate scenario into account, elevated TC activity can be expected for many islands in the Southwest Pacific during the 2015–2016 season, with 11 to 13 named TCs forming across the region during  the November 2015–April 2016 period.

Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the coming TC season, at least six storms are anticipated to reach at least Category 3, with mean wind speeds of at least 64 knots or 118 km/h (so-called ‘hurricane force’ winds). Of those systems, four storms may reach at least Category 4 strength, with mean wind speeds of at least 86 knots or 159 km/h. In addition, Category 5 strength TCs (winds greater than 106 knots or 196 km/h) are known to occur during seasons like the current one.  Therefore, all communities should remain alert and well prepared for severe events.

Tropical cyclones have a significant impact across the Southwest Pacific from year to year. Vanuatu and New Caledonia typically experience the greatest activity, with an average of 2 or 3 TCs passing close to land each year and there are indications that activity may be above average this season for many islands, including Vanuatu, Fiji, and Wallis & Futuna.  The outlook indicates elevated TC activity for the 2015–16 season for many islands east of the International Date Line that is typically associated with El Niño with reduced risk for Papua New Guinea.  Elevated risk is forecast for countries east of the International Date Line include Tokelau, Samoa, Tuvalu and further east in the Northern Cook Islands, the Southern Cook Islands, and more broadly across French Polynesia.

On average, New Zealand usually experiences at least one interaction per season with an ex-tropical cyclone during El Niño conditions. Most of the analog seasons identified for this forecast (1972/73; 1982/83; 1987/88; 1991/92; 1997/98 ) show multiple ex-tropical cyclones coming close (within 550 km) to the country. Significant wind, waves and rainfall are possible from these systems. Their effects can be spread over a larger area when the ex-tropical cyclone interacts with separate weather systems. 

Even though TC activity is expected to be relatively low for some countries, historical cyclone tracks (see supporting information for this forecast, Figure 3) indicate that TCs can affect all parts of French Polynesia (including the Society Islands, the Austral Islands, the Tuamotu Archipelago, and the Marquesas).

All Pacific Islands should remain vigilant in case conditions in the equatorial Pacific change during the TC season. Past El Niño seasons have seen TC tracks with increased sinuosity (irregular or looping motions rather than a curvilinear trajectory), which means they have potential to impact a large area.

New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and Meteorological Service of New Zealand (MetService) along with meteorological forecasting organizations from the Southwest Pacific, including the Australian Bureau of Meteorology, MeteoFrance and the Pacific Island National Meteorological Services have prepared this tropical cyclone outlook. 

Contacts for comment 

In New Zealand:

Mr. Chris Brandolino
Principal Scientist - Forecasting, NIWA

Mr Chris Noble
Manager, Specialist Weather Services
MetService New Zealand

Mrs. Elke Louw
Wellington RSMC (Regional Specialized Meteorological Centre)
Manger, Marine Weather Services
MetService New Zealand

Dr. Andrew Lorrey
Climate Scientist, NIWA

Dr. Nicolas Fauchereau
Climate Scientist, NIWA

In the Pacific Islands, please contact your local national meteorological service for information about how this guidance should be interpreted. 

For Australia and associated offshore islands, please contact the Australian Bureau of Meteorology for information about how this guidance should be interpreted.

For French Polynesia, Wallis, Futuna and New Caledonia, please contact MeteoFrance regional offices for information about how this guidance should be interpreted.

Additional background information

TCs in the Southwest Pacific usually develop between November and April, but occasionally they can occur in October and May, very rarely in June, July and August and relatively unknown in September.

Peak TC season is usually from January to March. In seasons with similar background climate conditions to present, TC activity was elevated near and to the east of the International Dateline, while activity over the Coral Sea was reduced relative to normal over the whole season. In addition, the strongest TC anomalies were focused in the region between Fiji, Vanuatu and Tuvalu, and over French Polynesia, which is associated with a northward displacement of the South Pacific Convergence Zone (SPCZ) during El Niño events. On average, nearly half of the TCs that developed since the 1969-70 season have reached hurricane force with mean wind speeds of at least 64 knots (118 km/h).

To find past analogs that describe the climate state leading into the upcoming TC season, the past May-September conditions were examined for the tropical Pacific from 1969 to the present. For the majority of winter and early spring 2014, the ENSO system evolved into a well-coupled El Niño state. Available information from international forecasting centres that issue global climate forecast model outputs and ENSO diagnostics are discussed by the international climate research community that are involved in the Island Climate Update. The collective guidance suggests the current El Niño event is expected to be one of the strongest on record. As such, an additional element used to select the TC analog seasons included tracks that occurred when well-coupled El Niño conditions continued into austral summer.

We used a joint ENSO index that combines the Southern Oscillation Index (SOI) with the most widely-used oceanic index of sea surface temperature anomalies in the equatorial central-western Pacific (NINO3.4). This joint ENSO index is described in Gergis and Fowler (2005) as the “Coupled ENSO Index” (CEI). Using the CEI, we selected analog TC seasons for the 2015-16 forecast. We highlighted seasons when the equatorial SSTs and the SOI were strongly indicative of  El Niño (as defined by the CEI with a 5-month mean SSTa threshold greater than + 0.5°C and an 3-month SOI threshold below - 1.0) during the May-September pre-TC season interval.

Five analog TC seasons (1972/73; 1982/83; 1987/88; 1991/92; 1997/98) typified by well-coupled ENSO conditions identified using the approach above. Note that the small number of analog seasons relates to the high-quality TC data period in the satellite era beginning in 1969/70 (only 45 seasons), the availability of TC track data (current only to the end of the 2013/14 season), and the limited number of similar analogs to this season.  The historic analogs suggest the most likely outcome for this season is El Niño. As such, the tropical cyclone guidance for November 2015 to April 2016 is built on the five analog seasons identified above.

The Island Climate Update TC forecast spans four areas of responsibility overseen by international monitoring and forecast agencies (RMSC Nadi, TCWC Brisbane, TCWC Port Moresby and TCWC Wellington). We used a high quality set of past TC tracks  from the South Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) (Diamond et al., 2012) which covers 135°E to 120°W longitude to draw on past TC track patterns for this seasonal outlook. This region encompases a basin that is defined by climatology rather than geopolitical or meteorological boundaries (Diamond et al., 2012).  The analog tracks and anomalies for this region suggests TC activity is likely to be above normal for many island nations this season. In particular, southwest Pacific islands situated adjacent to and east of the International Date Line including Vanuatu, Fiji, Wallis & Futuna, Samoa, Tokelau, Tuvalu, the Northern Cook Islands and Southern Cook Islands, the Society Islands and the Austral Islands are expected to see elevated activity (Table 1; Figure 2 & 3). Moreover, island groups well to the east of the International Dateline that typically see low TC activity, including the Marquesas, the Tuamotu Archipelago, and Pitcairn Island also have a chance of a TC interaction. The main TC genesis region is expected to lie within a band between 10 – 12°S (north of Fiji) just west of the International Date Line. There is an increased likelihood of multiple Category 4 or 5 systems occurring this season based on the selected analogs. A total of 12 named storms on average are expected; the range of variation between analog seasons suggests 11 to 13 for the total TC count (all named storms) within the forcast area could occur, which is above normal.

Based on previous research (Diamond et al., 2013), there are a wide range of TC track orientations during extra-tropical transition (ETT) during well-coupled El Niño conditions. Recent research has also indicated TC track sinuosity is high during well-coupled El Niño events (Philip Malsale, Vanuatu Meteorological Service, personal communication). This means that weaker TCs, such as a Category 1 or 2 systems, might ‘wander’ or stall, causing prolonged or widespread effects. Well-coupled El Niño TC tracks also tend to begin closer to the Equator than normal and also more frequently than normal east of the Dateline. They have a tendency to cover a zone between the genesis region north of Fiji and ETT usually occurs near Tonga (mean longitude of ~172°W and a heading of ~135° at 25°S). For the historical TC tracks for the selected analog seasons, there is a very large spread for the location where each system underwent ETT that presents significant uncertainties for maritime navigation risks.

A split of the analog TC seasons into early (November – January) and late (February – April) periods suggests TC activity will be elevated during both parts of the TC season relative to normal (Figure 4). A change in the foci of TC activity is also apparent as the season progresses (particularly for islands identified adjacent and to the east of the Date Line). Activity is expected to increase near French Polynesia as the season progresses .

TC intensity is related to how long developing cyclonic systems reside in the deep tropics and feed on warm waters for their growth. In addition, the subtropical jet and South Pacific Convergence Zone (SPCZ) mutually interact and contribute to shear during extra-tropical transition. It should also be noted that the interplay of a hemispheric-scale atmospheric circulation with the timing of the short-term Madden-Julian Oscillation (MJO) passage (typically on a 30-50 day cycle) has significant bearing on TC activity in the region. Increased and and more intense TC activity can be expected during the MJO 6-7 paired phase (Diamond and Renwick, 2015).  Real-time monitoring of the MJO is available from the Australian Bureau of Meteorology at http://www.bom.gov.au/climate/mjo/.

Previous work (Lorrey et al., 2013) indicates New Zealand interacts with at least one ex-tropical cyclone passing within 550km of the country every year. For the coming TC season, the risk for New Zealand is slightly elevated. If an ex-tropical cyclone passes close to the country, it has equal probability of passing east or west of Auckland city.

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.

Diamond, H.J., and J.A. Renwick, 2015: The climatological relationship between tropical cyclones in the southwest Pacific and the Madden-Julian Oscillation. International Journal of Climatology, 35: 676-686. doi: 10.1002/joc.4012.

Diamond, H.J., A.M. Lorrey, K.R. Knapp, and D.H. Levinson. 2012. Development of an enhanced tropical cyclone tracks database for the southwest Pacific from 1840-2011. International Journal of Climatology, 32: 2240–2250. doi:10.1002/joc.2412.

Diamond, H.J., A.M. Lorrey, and J.A. Renwick. 2013. A Southwest Pacific Tropical Cyclone

Climatology and Linkages to the El Niño–Southern Oscillation. Journal of Climate, 26(1), 3-25. doi:10.1175/JCLI-D-12-00077.1.

Lorrey, A.M., G. Griffiths, N. Fauchereau, H.J. Diamond, P.R. Chappell, and J. Renwick, 2013.  An ex-tropical cyclone climatology for Auckland, New Zealand. International Journal of Climatology, doi: 10.1002/joc.3753.