9 February 2012
Southwest Pacific Tropical Cyclone
Outlook
Average or below average activity for most
islands during the late season, but near normal number of
total named storms for the region
New Zealand’s
National Institute of Water Atmospheric Research (NIWA)
and New Zealand MetService have issued a tropical cyclone
outlook on behalf of collaborating organisations from the
southwest Pacific, including Australia, the USA, the Pacific
Island National Meteorological Services, French Polynesia,
and New Caledonia.
Climate and weather forecasting
organisations across the Pacific are predicting near normal
tropical cyclone numbers for the 2011 – 12 season as a
whole. On average, ten named tropical cyclones occur in the
southwest Pacific (between 135°E and 120°W) each season
(November – April). The TC forecast update indicates that
a total of 6 – 8 named cyclones are expected for the late
season (February – April). Two named storms have occurred
since the start of the season in November 2011 through the
end of January 2012 (TC Fina and TC Grant). As such, the
November–April seasonal total is likely to be in the range
of 8 – 10 named storms. A small area of higher than normal
activity is expected to be focused well south of the
Southern Cook Islands and Austral Islands. For most island
groups, TC activity is expected to be normal or below normal
for the remainder of the season.
Despite normal or below
normal activity, most countries west of the International
Date Line, including Papua New Guinea, the Solomon Islands,
Vanuatu, New Caledonia, and New Zealand (from ex–tropical
cyclones) are likely to experience an interaction or two
with a tropical cyclone because of La Niña that is
currently in place. It should be recognised that the
expectation of activity (and associated risk) ascribed for
each island group is subjective, but these expectations are
based on the style of La Niña that exists at present, along
with international climate forecast centres that indicate
the high likelihood of La Niña diminishing from mid-autumn.
Note that the forecast of normal or below normal activity
for islands like New Caledonia, Vanuatu, Fiji, and Tonga
indicates two or three tropical cyclones can still be
expected during the season as a whole. In addition, most of
the tropics and sub–tropics can be severely affected by at
least one tropical cyclone during the remainder of the
season, and as such all nations should remain
vigilant.
Outlook in more detail
La Niña
conditions exist at present, and sea surface temperature
anomalies across the central and eastern Equatorial Pacific
Ocean are negative relative to normal. More frequent high
pressure anomalies have occurred over French Polynesia
during late austral spring and through mid-summer, with
lower pressures over eastern Australia. The style of La
Niña that has developed is moderately coupled between the
ocean and atmosphere, and the South Pacific Convergence Zone
has been displaced south and west of normal during the past
four months. The oceanic component of ENSO is expected to
weaken over the coming months, and as such the style of the
current La Niña is anticipated to degrade into an
atmospheric-dominated event from mid-autumn. Past situations
that describe a similar scenario (analogues) indicate that
normal or below normal tropical cyclone (TC) activity is
likely for most islands in the southwest Pacific during the
remainder of the TC season, with slightly elevated activity
well to the south of the Southern Cook Islands and Austral
Islands from an extra-tropical storm. Although some reduced
activity is indicated to the west of the International Date
Line during this time, all communities should remain alert
and prepared.
TC forecast suggests average overall numbers
of 8 – 10 named TCs for the southwest Pacific basin can be
expected by the end of April 2012. On average, ten named
tropical cyclones occur each year for the southwest Pacific
region. Southwest Pacific TCs are grouped into classes
ranging from 1 to 5, with 5 being the most dangerous. For
the coming season, at least one cyclone is forecast to reach
at least Category 3, with mean wind speeds of at least 64
knots or 118 km/h and one system may reach at least Category
4 strength, with mean wind speeds of at least 86 knots or
159 km/h.
Each year, tropical cyclones have a significant
impact on the southwest Pacific. Places like Vanuatu and New
Caledonia typically experience the greatest activity in the
region, with an average of about 3 or 4 TCs passing close to
those countries each year. The past years selected as being
similar to this season are 2000/01, 07/08 and 08/09. New
Zealand usually experiences more frequent ex-tropical
cyclone interactions during La Niñas, and is also
vulnerable to a close TC passage during seasons where the
Southern Oscillation Index is low or neutral. However in
only one of these selected years did an ex-tropical cyclone
come close to the country. While TC activity is generally
reduced for islands to the east of the International Date
Line during La Niñas, historical cyclone tracks (see Figure
2) indicate that TCs can affect parts of southwest French
Polynesia, including the Society and Austral Islands, and
the southern Cook Islands during La Niñas, especially late
in the season. All islands should remain vigilant as the
current La Niña continues to evolve with progression into
Austral summer. During moderate La Niñas with a strong
atmospheric component (strong SOI) but slightly weaker
equatorial SST anomalies(see explanation of analogue
selection below), ex-tropical cyclones are known for
transitioning into the mid-latitudes within ±15° of the
Dateline and they typically have a strong southeasterly or
southerly slant to their trajectory.
In the Pacific
Islands, please contact your local national
meteorological service for information about how this
guidance should be interpreted.
Additional background
information
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. In seasons with
similar background climate conditions to present, several
tropical cyclones occur over the Gulf of Carpentaria as well
as 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, nearly 80% of the tropical
storms that have developed since the 1969-70 season have
reached Tropical Cyclone force with mean wind speeds of at
least 34 knots (63 km/h).
We used a joint ENSO index that
combines the atmospheric index (SOI) with the most
widely-used oceanic index (NINO3.4), described in Gergis and
Fowler (2005) as the “Coupled ENSO Index” (CEI), to
identify possible analogues for TC activity in the 2010-11
late season. Past analogues to describe the current climate
state (re-development of La Niña through spring and into
summer) ahead of the late TC season were assessed. Winter
(JJA), spring (SON), and early summer (December) conditions
were examined for the tropical Pacific for 1969 to Present.
For the past late winter, the atmosphere signalled the
existence of a La Niña, but the ocean was still in a
neutral state. This was followed by strengthening of SST
anomalies in NINO3.4, whereby the redevelopment of La Niña
showed coupling between the ocean and atmosphere. Using the
CEI, we then highlighted years when the equatorial SSTs were
neutral but the atmosphere was La Niña-like, followed by
springtime conditions when the atmospheric signals were in
synch with the ocean (i.e. what Gergis Fowler term a well
coupled or “NINA-style” La Niña). This highlighted
three analogue tropical cyclone seasons (2000/01; 2007/08;
2008/09) for February-April. Note that the small number of
analogue years relates to the brevity of the high-quality TC
data set (only 41 years) and the limited number of similar
analogues to this season.
Global climate models suggest La
Niña is likely to persist through Austral summer and into
autumn. Despite moderate coupling of La Niña at present,
the event has shown some signs of plateauing, while SST
guidance indicates some weakening in the coming three
months. The three analogues identified also suggest some
coupling extending through the TC season (one analogue), or
deterioration of La Niña into an SOI-NINA event (an
atmospheric dominated event) for February-April. An analysis
of past tracks using the South Pacific Enhanced Archive of
Tropical Cyclones (SPEArTC) data set (Diamond et al., 2011)
suggests it is possible that reduced activity for large
parts of the southwest Pacific is likely in this scenario
(see Figure 1). Tracks associated with the analogue seasons
are seen in Figure 2. The late season projection is still
consistent with the subtropical jet and the South Pacific
Convergence Zone (SPCZ) being displaced southwest of normal
during La Niña years (see Figure 3). TC genesis regions are
expected to be shifted north and west of normal within the
Coral Sea during this time, and while there is reduced
likelihood of a Category 5 system occurring, one could
happen as indicated by one of the three analogue years.
Based on averages for the analogue years, a total of 6
storms for the late season are expected, and the range of
variation between analogues suggests 8 – 10 for the total
TC count (November-April) is likely. Southeasterly track
orientations during extratropical transition (ETT) are
expected in the region with a mean ETT to the east of the
Dateline south of Niue during the late season. The ETT
position also corresponds with the higher than normal count
of ex-tropical cyclones that are expected far to the east of
New Zealand during the late season.
TC intensity is
related to how long developing systems reside in the deep
tropics and feed on warm waters for their growth, and how
the subtropical jet and SPCZ mutually interact and
contribute to shear during ex-tropical transition. It should
also be noted that the interplay of the hemispheric-scale
atmospheric circulation with the timing of a Madden-Julian
Oscillation (MJO) passage can also significantly contribute
to TC activity in the region. As discussed in the ICU 137
teleconference (Week 1 of February 2012), enhanced
MJO-related convective activity over the western Equatorial
Pacific was forecast for weeks 2 and 3 of February 2012. The
MeteoFrance tropical cyclone forecast guidance, which is
MJO-based, illustrated high probability of TC action in the
SW Pacific for week two of February. The MeteoFrance
forecast has proved to have good skill with a one week lead
this season, and it currently suggests some activity could
continue into week 3 of February 2012. MJO pulses are
usually six weeks apart, on average, and as such additional
activity may occur in mid-to-late March and again in late
April.
For images and other information go to:
http://www.niwa.co.nz/news/southwest-pacific-tropical-cyclone-outlook-feb-2012
ENDS
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