Cyclonic Niño

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Typhoon Chan-Hom in 2003 Typhoon Chan-Hom 2003.jpg
Typhoon Chan-Hom in 2003

Cyclonic Niño is a climatological phenomenon that has been observed in climate models where tropical cyclone activity is increased. Increased tropical cyclone activity mixes ocean waters, introducing cooling in the upper layer of the ocean that quickly dissipates and warming in deeper layers that lasts considerably more, resulting in a net warming of the ocean.

Contents

In climate simulations of the Pliocene, this net warming is then transported by ocean currents and part of it ends up in the Eastern Pacific, warming it relative to the Western Pacific and thus creating El Niño [lower-alpha 1] -like conditions. Reconstructed temperatures in the Pliocene have shown an El Niño-like pattern of ocean temperatures that may be explained by increased tropical cyclone activity and thus increased temperatures in the Eastern Pacific. Some of the heat is transported away from the tropics and may be responsible for past episodes of warmer-than-usual climate, such as in the Eocene and Cretaceous, although there is no agreement on the predominant effects of tropical cyclones on heat transport away from the tropics. There is evidence that under present-day climate when conditions are right, typhoons might start El Niño events.

Background

Tropical cyclones and ocean mixing

Depression of sea surface temperatures caused by Hurricane Katrina and Hurricane Rita in 2005 GulfMexTemps 2005Hurricanes.gif
Depression of sea surface temperatures caused by Hurricane Katrina and Hurricane Rita in 2005

Tropical cyclones are dangerous and destructive weather phenomena that are responsible for nearly $10,000,000,000 damage every year in the United States alone. [3] They also have diverse effects on the atmosphere and ocean, [lower-alpha 2] [5] their winds mix the upper ocean waters [6] and draw up cold deep water; in addition heat is extracted from the ocean although this effect is small. [7] The effects have usually been described as a temporary cooling of the water surface [8] by up to 6 °C (11 °F) [9] that tends to weaken the storm [7] but is dissipated by the sea and the atmosphere in one-two months. [10] This is accompanied by a much longer lasting warming of subsurface waters, although there is a certain complexity in response patterns; [11] [3] [12] part [lower-alpha 3] of [14] the subsurface warming tends to dissipate into the atmosphere through seasonal variations in the thermocline if it is not sufficiently deep. [15] Moreover, other effects of tropical cyclones on the ocean such as the precipitation can alter or counteract the wind-driven effects. [16] This potentially has effects on global heat transport; the effects on global climate is modest under current climate but could be stronger in warmer climates. [17]

The net result of the mixing would thus be a warming of the ocean [8] and a heat flux of between 0.26–0.4 petawatts (3.5×1011–5.4×1011 hp), [15] as well as – for a realistic distribution of tropical cyclones – a decreased heat transport out of the tropics [18] with about 1/3 of the heat accumulating in the equatorial regions. [lower-alpha 4] [22] Estimates of ocean heat content through satellite imaging support that tropical cyclone activity increases the heat content of the oceans, although there are some caveats [23] and the effect on global heat fluxes is not particularly large under present-day tropical cyclone activity; [2] however, according to one study the effect might be large enough to explain discrepancies between the steady state ocean mixing observed in the tropics and the amount required by planetary energetics, as the former is insufficient otherwise. [18]

Pliocene

Sea surface temperature anomalies during the Pliocene Pliocene sst anomaly.png
Sea surface temperature anomalies during the Pliocene

The concept has been formulated in discussions of Pliocene climates; during the Pliocene temperatures were 2–4 K (3.6–7.2 °F) higher than today and temperature gradients in the Pacific Ocean substantially smaller, [24] [25] meaning that the Eastern Pacific had similar temperatures to the Western Pacific, [26] equivalent to strong El Niño conditions. [25] Among the reconstructed effects are significantly moister conditions in the Southwestern United States than today. [27] As greenhouse gas concentrations were not higher than today, other explanations have been sought for these temperature anomalies. [28]

The existence of a permanent El Niño-like state however is not uncontested, and in some research results a more La Niña-like state of the Pacific Ocean. Climate models, sea surface temperatures reconstructed with alkenones [lower-alpha 5] and sometimes even reconstructions from foraminifera in the same drill core have yielded conflicting results. [1] Coral-based reconstructions have been used in a 2011 study to infer that the El Niño Southern Oscillation already existed during the Pliocene, including discrete El Niño events. [30]

Computer simulations concerning the Pliocene

Modelling with the CAM3 general circulation model has indicated that the number of tropical cyclones was much larger than today and their occurrence more extensive owing to higher sea surface temperatures and a weaker atmospheric circulation (the Hadley cell and Walker circulation) which results in less wind shear. Also, tropical cyclones last longer and occur throughout the year rather than being tied to specific reasons. [28]

This expansion of tropical cyclone activity would bring tropical cyclones within reach of zones of the ocean where sea currents below the surface transport water towards the Eastern Pacific. [31] Tropical cyclones induce mixing of the sea surface waters; [28] with a tenfold increase in ocean mixing within two bands 8–40° north and south of the equator – especially mixing occurring in the Central Pacific where tropical cyclone activity is low under present-day climate – heat would be introduced into these sea currents and eventually lead to a warming of the central and eastern Pacific Ocean similar to El Niño and a warming of the upwelling regions, [31] with a warming of about 2–3 °C (3.6–5.4 °F) in the zone of the East Pacific cold tongue. [22] This effect can take up to a century to set in and its strength is dependent on the exact pattern of ocean mixing. It is also subject to positive feedback, as the warming of the eastern Pacific in turn increases tropical cyclone activity; eventually a climate state featuring a permanent El Niño and a weaker El Niño Southern Oscillation can arise. [32]

During the mid-Piacenzian where carbon dioxide concentrations were close to present-day levels, Earth was about 2–4 °C (3.6–7.2 °F) warmer than present [33] and simulations indicate that tropical cyclones were more intense; [34] the modelled distribution of tropical cyclones however was different from the one reconstructed for other stages of the Pliocene. Simulations using the CESM climate model conducted in 2018 [35] showed a reduced temperature gradient between the East and West Pacific and a deeper thermocline in response to tropical cyclone driven mixing and anomalous eastward sea currents in the Pacific; this is accompanied by a cooling of the areas where mixing is strongest and a warming of the Eastern Pacific. [36] There are also effects on the East Asian monsoon such as a stronger winter monsoon [37] but in the simulations the background climate of the Piacenzian was more significant than the tropical cyclone effects. [38]

Subsequent findings

Later researchers have suggested that the increased winds may actually strengthen the El Niño Southern Oscillation [39] and that Eocene and Pliocene warm climates still featured an ENSO cycle. This does not necessarily imply that there still was an east-west temperature gradient in the Pacific Ocean, [40] which instead might have featured an eastward expanded Pacific warm pool. [24] Temperature reconstructions based on corals and reconstructed precipitation data from Chinese loess indicate that there was no permanent El Niño like state. [41] Another 2013 study with a different climate model indicated that tropical cyclones in the western Pacific may actually induce cooling of eastern Pacific sea surface temperatures. [42] A 2015 simulation of tropical cyclogenesis did not show increased tropical cyclone genesis in the Pliocene, although the simulation did not obtain a decreased East-West Pacific temperature gradient and it did obtain increased tropical cyclone activity in the parts of the Central Pacific most critical for the occurrence of Cyclonic Niño effects. [43] A 2018 simulation implied that adding tropical cyclone mixing induced climate phenomena to simulations of mid-Piacenzian climate can in some aspects improve and in others reduce the match between the modelled climate and the climate reconstructed from paleoclimate data. [37] A 2019 study concluded that tropical cyclone activity in the Western Pacific is correlated to El Niño-associated temperature anomalies months later. [44]

A 2010 climate simulation indicated that increasing the average winds of tropical cyclones induced warming in the Eastern Pacific and cooling in the Western Pacific, [45] consistent with an El Niño like response; there is also strengthening of the Hadley cell of the atmospheric circulation [46] and some heat is transported out of the tropics by the western boundary currents. [47] Similar East-West temperature changes were obtained in other 2010 [48] and 2011 studies; [49] in the latter high latitude temperatures warmed by about 0.5–1 °C (0.90–1.80 °F) and a global warming by 0.2 °C (0.36 °F) [50] and the former indicated that the heat is transported at depths of about 200 metres (660 ft) towards the Equatorial Undercurrent which then brings it into the Eastern Pacific. Similar effects but of much smaller magnitude are seen in the North Atlantic and other oceans [48] and there are also changes to the Indonesian Throughflow. [51] A 2013 study using tropical cyclones from the 2003 Pacific typhoon season including Typhoon Chan-hom showed that the tropical cyclone winds could induce eastward moving equatorial waves [52] and suggested that such typhoon induced waves can start El Niño events [53] when background conditions are favourable. [54] A 2014 study showed a total increase in ocean heat content caused by the typhoons and hurricanes active between 2004 and late 2005. [55] Another 2018 simulation shows that warm subsurface anomalies are transported eastward into the Eastern Pacific. [56]

Non-oceanic mechanisms for tropical cyclone-induced El Niños may exist as well. [57] Tropical cyclones in the Pacific induce westerly winds, so called westerly wind bursts that play a major role in the onset of El Niño events such as the 2014–16 El Niño event, and there is evidence that increased tropical cyclone activity precedes the onset of El Niño. [58] Such processes also influence the intensity of the El Niño. [59]

Concurrent effects on worldwide climate

Increased tropical cyclone activity during warmer climates might increase ocean heat transport, which could explain why climate records of warmer past climates often do not show much warming in the tropics compared to high latitude temperatures; the increased heat transport would remove heat more effectively from the tropics [60] and thus keep temperatures stable even with changing rates of ocean heat transport. [61]

Such alteration of ocean heat transport by tropical cyclones has been used to explain other past climate states where Earth was warmer than today and the temperature gradient between the poles and the tropics smaller. This was the case for example during the late Cretaceous, during the Paleocene-Eocene thermal maximum during which temperatures in the Arctic exceeded 20 °C (68 °F) at times, [62] during the Eocene [5] and during the Pliocene between 3 and 5 million years ago. [19] [63]

Effects

El Nino induced changes in atmospheric circulation ENSO - El Nino.svg
El Niño induced changes in atmospheric circulation

The "Cyclonic Niño" effect could partially explain temperature distributions in the Pliocene [32] and a flattening of the oceanic thermocline during the Pliocene. [40] The permanent El Niño conditions may have had effects similar to that of present-day El Niño, although this is not undisputed. [26] A permanent El Niño would suppress hurricane activity in the North Atlantic less effectively than a present-day El Niño, owing to different thermodynamic effects of transitory warming. [64]

Stronger tropical cyclones are expected to cause more mixing of the ocean and thus a stronger effect on heat transport. Anthropogenic global warming is expected to increase the frequency of intense tropical cyclones and thus may induce a Cyclonic Niño effect. [65] Increased hurricane activity in the Central Pacific could be a consequence. [66]

Notes

  1. Present-day El Niño is a climate phenomenon that occurs every three to seven years, during which warm water masses appear in the Eastern Pacific, suppressing upwelling there. In the Western Pacific conversely precipitation and temperature both decrease. [1] Tropical Kelvin waves associated with the Madden-Julian oscillation and Yanai waves can favour the onset of El Niño events. [2]
  2. As an example, during the little ice age when tropical cyclone activity in the area was depressed, the ocean was more strongly stratified close to Great Bahama Bank, probably reflecting a lowered tropical cyclone mediated mixing. [4]
  3. According to one study 3/4 of the warming [13]
  4. The distribution of tropical cyclones implies that the transport of heat towards the poles is inhibited by the cyclone-induced mixing. [19] Tropical cyclones usually occur within the area of the subtropical overturning circulation which retains the mixed-down heat. [20] One 2015 computer simulation observed a strong heat accumulation in the tropics as a consequence. [21]
  5. Alkenones are organic compounds that can be used to reconstruct past temperatures. [29]

Related Research Articles

El Niño Warm phase of a cyclic climatic phenomenon in the Pacific Ocean

El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific, including the area off the Pacific coast of South America. The ENSO is the cycle of warm and cold sea surface temperature (SST) of the tropical central and eastern Pacific Ocean. El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific. El Niño phases are known to occur close to four years, however, records demonstrate that the cycles have lasted between two and seven years. During the development of El Niño, rainfall develops between September–November. The cool phase of ENSO is La Niña, with SSTs in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific. The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.

Cloud feedback is the coupling between cloudiness and surface air temperature where a surface air temperature change leads to a change in clouds, which could then amplify or diminish the initial temperature perturbation. Cloud feedbacks can affect the magnitude of internally generated climate variability or they can affect the magnitude of climate change resulting from external radiative forcings.

La Niña A coupled ocean-atmosphere phenomenon that is the counterpart of El Niño

La Niña is a coupled ocean-atmosphere phenomenon that is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation climate pattern. The name La Niña originates from Spanish, meaning "the little girl", analogous to El Niño meaning "the little boy". It has also in the past been called anti-El Niño, and El Viejo. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3 to 5 °C. An appearance of La Niña persists for at least five months. It has extensive effects on the weather across the globe, particularly in North America, even affecting the Atlantic and Pacific hurricane seasons, in which more tropical cyclones in the Atlantic basin due to low wind shear and warmer sea surface temperatures, while reducing tropical cyclogenesis in the Pacific Ocean during a La Niña.

El Niño–Southern Oscillation Irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean

El Niño–Southern Oscillation (ENSO) is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and typically occur every few years with varying intensity per period.

Sea surface temperature Water temperature close to the oceans surface

Sea surface temperature (SST) is the water temperature close to the ocean's surface. The exact meaning of surface varies according to the measurement method used, but it is between 1 millimetre (0.04 in) and 20 metres (70 ft) below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a short distance of the shore. Localized areas of heavy snow can form in bands downwind of warm water bodies within an otherwise cold air mass. Warm sea surface temperatures are known to be a cause of tropical cyclogenesis over the Earth's oceans. Tropical cyclones can also cause a cool wake, due to turbulent mixing of the upper 30 metres (100 ft) of the ocean. SST changes diurnally, like the air above it, but to a lesser degree. There is less SST variation on breezy days than on calm days. In addition, ocean currents such as the Atlantic Multidecadal Oscillation (AMO), can effect SST's on multi-decadal time scales, a major impact results from the global thermohaline circulation, which affects average SST significantly throughout most of the world's oceans.

Pacific decadal oscillation robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin

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Climate variability is the term to describe variations in the mean state and other characteristics of climate "on all spatial and temporal scales beyond that of individual weather events." Some of the variability does not appear to be caused systematically and occurs at random times. Such variability is called random variability or noise. On the other hand, periodic variability occurs relatively regularly and in distinct modes of variability or climate patterns.

Madden–Julian oscillation

The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.

Tropical cyclone rotating storm system with a closed, low-level circulation

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Indian Ocean Dipole irregular oscillation of sea-surface temperatures in the Indian Ocean

The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.

Polar amplification

Polar amplification is the phenomenon that any change in the net radiation balance tends to produce a larger change in temperature near the poles than the planetary average. On a planet with an atmosphere that can restrict emission of longwave radiation to space, surface temperatures will be warmer than a simple planetary equilibrium temperature calculation would predict. Where the atmosphere or an extensive ocean is able to transport heat polewards, the poles will be warmer and equatorial regions cooler than their local net radiation balances would predict.  

Pliocene climate

During the Pliocene epoch climate became cooler and drier, and seasonal, similar to modern climates.

Tropical instability waves A phenomenon in which the interface between areas of warm and cold sea surface temperatures near the equator form a regular pattern of westward-propagating waves

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Tropical cyclones and climate change confluent article on tropical cyclones and climate change

Tropical cyclones and climate change concerns how tropical cyclones have changed, and are expected to further change due to climate change. The topic receives considerable attention from climate scientists who study the connections between storms and climate, and notably since 2005 makes news during active storm seasons. The 2018 U.S. National Climate Change Assessment reported that "increases in greenhouse gases and decreases in air pollution have contributed to increases in Atlantic hurricane activity since 1970."

The Atlantic Equatorial Mode or Atlantic Niño is a quasiperiodic interannual climate pattern of the equatorial Atlantic Ocean. It is the dominant mode of year-to-year variability that results in alternating warming and cooling episodes of sea surface temperatures accompanied by changes in atmospheric circulation. The term Atlantic Niño comes from its close similarity with the El Niño-Southern Oscillation (ENSO) that dominates the tropical Pacific basin. The Atlantic Niño is not the same as the Atlantic Meridional (Interhemispheric) Mode that consists of a north-south dipole and operates more on decadal timescales. The equatorial warming and cooling events associated with the Atlantic Niño are known to be strongly related to atmospheric climate anomalies, especially in African countries bordering the Gulf of Guinea. Therefore, understanding of the Atlantic Niño has important implications for climate prediction in those regions. Although the Atlantic Niño is an intrinsic mode to the equatorial Atlantic, there may be a tenuous causal relationship between ENSO and the Atlantic Niño in some circumstances.

The Tropical Atlantic Variability (TAV) is influenced by internal interaction and external effects. TAV can be discussed in different time scales: seasonal and interannual.

1997–98 El Niño event

The 1997–98 El Niño was regarded as one of the most powerful El Niño–Southern Oscillation events in recorded history, resulting in widespread droughts, flooding and other natural disasters across the globe. It caused an estimated 16% of the world's reef systems to die, and temporarily warmed air temperature by 1.5 °C, compared to the usual increase of 0.25 °C associated with El Niño events.

2014–16 El Niño event

The 2014–16 El Niño was a warming of the eastern equatorial Pacific Ocean that resulted in unusually warm waters developing between the coast of South America and the International Date Line. These unusually warm waters influenced the world's weather in a number of ways, which in turn significantly affected various parts of the world. These included drought conditions in Venezuela, Australia and a number of Pacific islands while significant flooding was also recorded. During the event, more tropical cyclones than normal occurred within the Pacific Ocean, while fewer than normal occurred in the Atlantic Ocean.

Pacific Centennial Oscillation is a climate oscillation predicted by some climate models.

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