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 . 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 .
31-597: The SPCZ occurs where the southeast trades from transitory anticyclones 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 zone of convergence . The location of the SPCZ is affected by ENSO and Interdecadal Pacific oscillation conditions. It generally stretches from
62-479: A positive feedback loop develops between the convective tropical cyclone and the upper level high, the two systems are strengthened. This loop stops once ocean temperatures cool to below 26.5 °C (79.7 °F), reducing the thunderstorm activity, which then weakens the upper-level high-pressure system. When the subtropical ridge in the Northwest Pacific is stronger than in other areas, it leads to
93-467: A subtropical ridge . The evolution of an anticyclone depends upon variables such as its size, intensity, and extent of moist convection , as well as the Coriolis force . Sir Francis Galton first discovered anticyclones in the 1860s. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as cum sole . Subtropical high-pressure zones form under
124-621: A buildup of particulates in urban areas under the high pressure, leading to widespread haze . If the surface level relative humidity rises towards 100 percent overnight, fog can form. The movement of continental arctic air masses to lower latitudes produces strong but vertically shallow high-pressure systems. These systems affect their pressure. The surface level, sharp temperature inversion can lead to areas of persistent stratocumulus or stratus cloud , colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of
155-439: 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
186-531: A wet monsoon season for Asia . The subtropical ridge position is linked to how far northward monsoon moisture and thunderstorms extend into the United States . Typically, the subtropical ridge across North America migrates far enough northward to begin monsoon conditions across the Desert Southwest from July to September. When the subtropical ridge is farther north than normal towards
217-605: Is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure , clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above (opposite to a cyclone ). Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as
248-416: Is a significant problem in large urban centers during summer months such as Los Angeles, California and Mexico City . The existence of upper-level (altitude) high pressure allows upper level divergence which leads to surface convergence . If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because
279-706: The Four Corners , thunderstorms of the New Mexican Monsoon can spread northward into Arizona and New Mexico . When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime. On weather maps, high-pressure centers are associated with the letter H in English, within the isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within
310-616: The Solomon Islands through Vanuatu , Fiji , Samoa , and Tonga . Low-level convergence along this band forms cloudiness as well as showers and thunderstorms . 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. The convergence zone shifts east or west depending on
341-411: The subtropical ridge , deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm-core lows such as tropical cyclones , due to descending cool air from the backside of upper troughs such as polar highs , or from large-scale sinking such as
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#1732783106023372-465: The Azores high pressure may bring about anticyclonic gloom during the winter because they pick up moisture as they move over the warmer oceans. High pressures that build to the north and move southwards often bring clear weather because they are cooled at the base (as opposed to warmed) which helps prevent clouds from forming. Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over
403-575: The SPCZ axis. Figure 1 shows qualitative agreement between all of these SPCZ indicators. The position of the SPCZ can change on seasonal, interannual, and possibly longer timescales. Research into SPCZ movements of the 20th century are linked to changes in the IPO and ENSO. 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
434-472: 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
465-411: 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. Determining the position of the SPCZ over longer timescales in the past (pre-20th century) has been studied using coral records of
496-436: The air subsidence at their center, act to steer tropical cyclones around and out their periphery. Due to the subsidence within this type of system, a cap can develop which inhibits free convection and hence mixing of the lower with the middle level troposphere. This limits thunderstorms and other low-pressure weather activity near their centers and traps low-level pollutants such as ozone as haze under their base, which
527-431: The day, there is more incoming solar radiation and heating so temperatures rise rapidly near the surface. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, allowing the escape of heat and giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to
558-577: The descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature. Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence , or converging height lines near or above
589-485: The ensemble of coupled models. 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. Tropical textbook : from trade winds to cyclone (2 vol) Archived 2012-12-16 at archive.today , 897 pp., Florent Beucher, 25 mai 2010, Météo-France, ISBN 978-2-11-099391-5 Anticyclone An anticyclone
620-535: The equator and to the poles aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres. This circulation known as the Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas . Because these anticyclones strengthen with height, they are known as warm core ridges. The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by
651-400: The existence of El Niño, or the phase of ENSO . The climatological position can be estimated by computing its mean position over 30 or more years. 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
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#1732783106023682-413: The formation of clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow. In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure . The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system,
713-462: The highest height line contour. On Jupiter , there are two examples of an extraterrestrial anticyclonic storm; the Great Red Spot and the recently formed Oval BA on Jupiter. They are powered by smaller storms merging unlike any typical anticyclonic storm that happens on Earth where water powers them. Another theory is that warmer gases rise in a column of cold air, creating a vortex as
744-406: The level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold. Heating of the earth near the equator forces upward motion and convection along the monsoon trough or Intertropical Convergence Zone . The divergence over the near-equatorial trough leads to air rising and moving away from
775-607: 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 . 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
806-419: 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. Besides observations of the SPCZ and movement in its position, there have been modelling studies as well. Widlansky et al. (2012) used
837-438: 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
868-487: The southwest Pacific. 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
899-424: The stronger the wind. The coriolis force caused by Earth 's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down
930-416: The warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans, polar lows can develop. However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses. The circulation around mid-level (altitude) ridges, and
961-470: The wind flowing out of high-pressure systems and causes wind to flow more outward (more ageostrophically ) from the center. High-pressure systems are frequently associated with light winds at the surface and subsidence of air from higher portions of the troposphere . Subsidence will generally warm an air mass by adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during