South Pacific convergence zone

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The South Pacific Convergence Zone (SPCZ), a reverse-oriented

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.[1] 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

The SPCZ occurs where the southeast trades from transitory

ENSO
.

Measuring SPCZ position

The climatological position can be estimated by computing its mean position over 30 or more years.[4] There are several metrics to measure the position of the SPCZ. The location of maximum rainfall, maximum of low level convergence, maxima of the 500 hPa vertical motion, and the minimum in outgoing longwave radiation (OLR) are four indicators of the SPCZ axis.[4] 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.

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.[4]

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.[5] 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.[5] 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.[4]

Climate modelling

Besides observations of the SPCZ and movement in its position, there have been modelling studies as well.

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.[6]

Related oceanography

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.[5]

See also

References

World Wide Web

  1. ^ E. Linacre and B. Geerts. Movement of the South Pacific Convergence Zone. Retrieved on 2006-11-26.
  2. ^ Glossary of Meteorology. South Pacific convergence zone. Archived 2007-09-30 at the Wayback Machine Retrieved on 2006-11-26.
  3. .
  4. ^
    doi:10.1029/2001GL014201.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
  5. ^ a b c Braddock K. Linsley, Alexey Kaplan, Yves Gouriou, Jim Salinger, Peter B. deMenocal, Gerard M. Wellington, and Stephen S. Howe. Tracking the extent of the South Pacific Convergence Zone since the early 1600s. Archived 2011-06-06 at the Wayback Machine Retrieved on 2006-11-26.
  6. ^ a b Matthew J. Widlansky, Axel Timmermann, Karl Stein, Shayne McGregor, Niklas Schneider, Matthew H. England, Matthieu Lengaigne, and Wenju Cai. Changes in South Pacific rainfall bands in a warming climate
  • (in French)

Tropical textbook : from trade winds to cyclone (2 vol) Archived 2012-12-16 at

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