View source for South Pacific convergence zone

{{short description|Seasonal storm pattern}}
The '''South Pacific Convergence Zone''' ('''SPCZ'''), a reverse-oriented [[monsoon trough]], is a band of low-level convergence, cloudiness and precipitation extending from the [[Western Pacific Warm Pool]] at the maritime continent south-eastwards towards French Polynesia and as far as the [[Cook Islands]] (160W, 20S). The SPCZ is a portion of the [[Intertropical Convergence Zone]] (ITCZ) which lies in a band extending east–west near the Equator but can be more extratropical in nature, especially east of the [[International Date Line]].  It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby landmass during the summer than any other portion of the [[monsoon trough]].<ref>E. Linacre and B. Geerts. [http://www-das.uwyo.edu/~geerts/cwx/notes/chap12/spcz.html Movement of the South Pacific Convergence Zone.] Retrieved on 2006-11-26.</ref> The SPCZ can affect the precipitation on Polynesian islands in the southwest Pacific Ocean, so it is important to understand how the SPCZ behaves with large-scale, global climate phenomenon, such as the ITCZ, [[El Niño–Southern Oscillation]], and the [[Interdecadal Pacific oscillation]] (IPO), a portion of the [[Pacific decadal oscillation]].

==Position==
<!-- Deleted image removed: [[File:Folland2002 figure1.png|thumbnail|right|Figure 1 of Folland et al. (2002) shows the mean position of the SPCZ based on four different measurements: maximum rainfall, maximum of low level [[Cloud|convergence]], maxima of the 500 hPa vertical motion, and the minimum in [[outgoing longwave radiation]] (OLR)]] -->

The SPCZ occurs where the southeast trades from transitory [[anticyclone]]s to the south meet with the semipermanent easterly flow from the eastern South Pacific anticyclone. The SPCZ exists in summer and winter but can change its orientation and location. It is often distinct from the ITCZ over Australia, but at times they become one continuous [[Convergence zone|zone of convergence]]. The location of the SPCZ is affected by [[El Niño|ENSO]] and [[Interdecadal Pacific oscillation]] conditions.  It generally stretches from the [[Solomon Islands]] through [[Vanuatu]], [[Fiji]], [[Samoa]], and [[Tonga]].  Low-level convergence along this band forms cloudiness as well as showers and [[thunderstorm]]s.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=south-pacific-convergence-zone1 South Pacific convergence zone.] {{Webarchive|url=https://web.archive.org/web/20070930210848/http://amsglossary.allenpress.com/glossary/search?id=south-pacific-convergence-zone1 |date=2007-09-30 }} Retrieved on 2006-11-26.</ref>  Thunderstorm activity, or convection, within the band is dependent upon the season, as the more equatorward portion is most active in the Southern Hemisphere summer, and the more poleward portion is most active during transition seasons of fall and spring.<ref>{{cite thesis |last1=Cocks |first1=Stephen Brenton |title=An observational study of the South Pacific Convergence Zone using satellite and model re-analysis data |type=PhD dissertation |publisher=Texas A&M University |date=2003 |url=https://oaktrust.library.tamu.edu/handle/1969.1/374 |hdl=1969.1/374}}</ref>  The [[convergence zone]] shifts east or west depending on the existence of El Niño, or the phase of [[ENSO]].

===Measuring SPCZ position===
The climatological position can be estimated by computing its mean position over 30 or more years.<ref name="Folland">{{cite journal | author=C. K. Folland, J. A. Renwick, [[Jim Salinger|M. J. Salinger]], A. B. Mullan | title=Relative influences of the Interdecadal Pacific Oscillation and ENSO in the South Pacific Convergence Zone | journal=Geophysical Research Letters | volume=29 | issue=13 | year=2002 | pages=21–1–21–4|doi=10.1029/2001GL014201 | bibcode=2002GeoRL..29.1643F| doi-access=free }}</ref> There are several metrics to measure the position of the SPCZ. The location of maximum rainfall, maximum of low level [[Cloud|convergence]], maxima of the 500 hPa vertical motion, and the minimum in [[outgoing longwave radiation]] (OLR) are four indicators of the SPCZ axis.<ref name="Folland" /> Figure 1 shows qualitative agreement between all of these SPCZ indicators.

==Changes in SPCZ position==
The position of the SPCZ can change on seasonal, interannual, and possibly longer timescales.

===Observations===
Research into SPCZ movements of the 20th century are linked to changes in the IPO and ENSO.<ref name=Folland /> Folland et al., 2002 defined an index to describe the [[Interdecadal Pacific oscillation]] (IPO) with [[sea surface temperature]] and night marine air temperature to determine how the SPCZ varies with the IPO. When the IPO index has negative temperature anomalies, the SPCZ is displaced southwest and moves northeastward when the IPO index has positive temperature anomalies. The [[Southern Oscillation Index]] (SOI) is a metric for describing warm- and cold-phase conditions associated with the [[El Niño–Southern Oscillation]] (ENSO) and can also describe movements of the position of the SPCZ. Negative SOI index values are associated with warm-phase or El Niño-like conditions and a northeastward displacement of the SPCZ. Positive SOI index values, on the other hand, describe cold-phase or La Niña-like conditions and a southwestward displacement of the SPCZ.<ref name="Folland" />

Determining the position of the SPCZ over longer timescales in the past (pre-20th century) has been studied using coral records of the southwest Pacific.<ref name="Linsley">Braddock K. Linsley, Alexey Kaplan, Yves Gouriou, [[Jim Salinger]], Peter B. deMenocal, Gerard M. Wellington, and Stephen S. Howe. [http://www.agu.org/pubs/crossref/2006/2005GC001115.shtml Tracking the extent of the South Pacific Convergence Zone since the early 1600s.] {{Webarchive|url=https://web.archive.org/web/20110606025520/http://www.agu.org/pubs/crossref/2006/2005GC001115.shtml |date=2011-06-06 }} Retrieved on 2006-11-26.</ref> Linsley et al. (2006) reconstructed sea-surface temperature and sea surface salinity in the southwest Pacific starting circa 1600CE by measuring the oxygen isotopic composition of four [[Porites]] coral records from [[Rarotonga]] and two from [[Fiji]]. Coral isotope measurements provide information on both sea surface temperature and sea surface salinity, so they can indicate times of increased or decreased temperature and/or precipitation associated with changes in the position of the SPCZ. Their coral oxygen isotope index indicated an eastward shift of the decadal mean position of the SPCZ since the mid 1800s. A shift of the SPCZ in this direction suggests there were more La Niña-like or cold-phase conditions in the Pacific, during this period, often called the [[Little Ice Age]].<ref name="Linsley" /> Additional paleoclimate studies are still needed in order to test the reliability of these coral results.

The IPO and ENSO can interact together to produce changes in the position of the SPCZ. West of about 140 W, both ENSO (measured with [[Southern Oscillation Index]]) and IPO strongly influence the SPCZ latitude, but farther east only ENSO is a significant factor. Only near 170 W is there any indication of an interaction between the two factors.<ref name="Folland" />

===Climate modelling===
Besides observations of the SPCZ and movement in its position, there have been modelling studies as well.<ref name="Widlanksy">Matthew J. Widlansky, [[Axel Timmermann]], Karl Stein, Shayne McGregor, Niklas Schneider, Matthew H. England, Matthieu Lengaigne, and Wenju Cai. [http://www.nature.com/nclimate/journal/v3/n4/full/nclimate1726.html   Changes in South Pacific rainfall bands in a warming climate]</ref> Widlansky et al. (2012) used a number of [[climate models]] of differing complexity to simulate rainfall bands in the southwest Pacific and see how the magnitude and areal extent was affected by the SPCZ and [[ENSO]]. During El Niño or warm-phase conditions, the SPCZ typically shifted northeastward with dryer conditions on islands to the southwest, in agreement with observations. Conversely, a southwestward shift in rainfall accompanied La Niña or cold-phase events in the simulations. Widlanksy et al. (2012) argued the sea surface temperature biases in models created uncertainty in the rainfall projections and produce what has been named “the double ITCZ problem”. The impact of sea surface temperature bias was further investigated by using uncoupled atmospheric models with prescribed sea surface temperatures, and those 3 models each with differing complexity showed less severe double ITCZ bias than the ensemble of coupled models.<ref name="Widlanksy" />

==Related oceanography==
<!-- Deleted image removed: [[File:Linsley2006 figure1.png|thumbnail|right|Figure 1 of Linsley et al. (2006) shows the annual average precipitation in the top panel and the annual average salinity in the bottom panel. Warmer colors indicate increased precipitation or salinity while colder colors indicate decreased precipitation or salinity. The salinity front, a function of SPCZ position, is labeled in the bottom panel.]] -->

At its southeast edge, the circulation around the feature forces a salinity gradient in the ocean, with fresher and warmer waters of the western Pacific lying to its west.  Cooler and saltier waters lie to its east.<ref name="Linsley" />

==See also==
*[[El Niño–Southern Oscillation]]
*[[Monsoon trough]]
*[[Tropical cyclogenesis]]
*[[Tropical cyclone]]
*[[Coral bleaching]]

== References ==

===World Wide Web===
{{reflist}}
* {{in lang|fr}}
[http://entreprise.meteofrance.com/jsp/site/Portal.jsp?&page_id=14199&document_id=23901&portlet_id=64720 Tropical textbook : from trade winds to cyclone (2 vol)] {{Webarchive|url=https://archive.today/20121216085839/http://entreprise.meteofrance.com/jsp/site/Portal.jsp?&page_id=14199&document_id=23901&portlet_id=64720 |date=2012-12-16 }}, 897 pp., Florent Beucher,  25 mai 2010, Météo-France, {{ISBN|978-2-11-099391-5}}

===Print===
* {{cite journal | author=C. K. Folland, J. A. Renwick, [[Jim Salinger|M. J. Salinger]], A. B. Mullan | title=Relative influences of the Interdecadal Pacific Oscillation and ENSO in the South Pacific Convergence Zone | journal=Geophysical Research Letters | volume=29 | issue=13 | year=2002 | pages=21–1–21–4|doi=10.1029/2001GL014201 | bibcode=2002GeoRL..29.1643F| doi-access=free }}

[[Category:Tropical meteorology]]
[[Category:Atmospheric dynamics]]
[[Category:Regional climate effects]]
[[Category:Physical oceanography]]