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The Pacific Decadal Oscillation (PDO) is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales (meaning time periods of a few years to as much as time periods of multiple decades). There is evidence of reversals in the prevailing polarity (meaning changes in cool surface waters versus warm surface waters within the region) of the oscillation occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures from Alaska to California.
The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east.
The amplitude of a periodic variable is a measure of its change over a single period. There are various definitions of amplitude, which are all functions of the magnitude of the difference between the variable's extreme values. In older texts the phase is sometimes called the amplitude.
Alaska is a U.S. state in the northwest extremity of the United States West Coast, just across the Bering Strait from Asia. The Canadian province of British Columbia and territory of Yukon border the state to the east and southeast. Its most extreme western part is Attu Island, and it has a maritime border with Russia to the west across the Bering Strait. To the north are the Chukchi and Beaufort seas—southern parts of the Arctic Ocean. The Pacific Ocean lies to the south and southwest. It is the largest U.S. state by area and the seventh largest subnational division in the world. In addition, it is the 3rd least populous and the most sparsely populated of the 50 United States; nevertheless, it is by far the most populous territory located mostly north of the 60th parallel in North America: its population—estimated at 738,432 by the United States Census Bureau in 2015— is more than quadruple the combined populations of Northern Canada and Greenland. Approximately half of Alaska's residents live within the Anchorage metropolitan area. Alaska's economy is dominated by the fishing, natural gas, and oil industries, resources which it has in abundance. Military bases and tourism are also a significant part of the economy.
During a "warm", or "positive", phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. The Pacific Decadal Oscillation was named by Steven R. Hare, who noticed it while studying salmon production pattern results in 1997.
The improper expression Oceanic heat content (OHC) refers to the heat absorbed by the ocean, which is then stored as a form of internal energy or enthalpy. Oceanography and climatology are the science branches which study ocean heat content. Changes in the ocean heat content play an important role in the sea level rise, because of thermal expansion. It is with high confidence that ocean warming accounts for 90% of the energy accumulation from global warming between 1971 and 2010. About one third of that extra heat has been estimated to propagate to depth below 700 meters. Beyond the direct impact of thermal expansion, ocean warming contributes to increased rates of ice melt of glaciers in fjords of Greenland and ice sheets in Antarctica
Salmon is the common name for several species of ray-finned fish in the family Salmonidae. Other fish in the same family include trout, char, grayling and whitefish. Salmon are native to tributaries of the North Atlantic and Pacific Ocean. Many species of salmon have been introduced into non-native environments such as the Great Lakes of North America and Patagonia in South America. Salmon are intensively farmed in many parts of the world.
The Pacific Decadal Oscillation index is the leading empirical orthogonal function (EOF) of monthly sea surface temperature anomalies (SST-A) over the North Pacific (poleward of 20°N) after the global average sea surface temperature has been removed. This PDO index is the standardized principal component time series.A PDO 'signal' has been reconstructed as far back as 1661 through tree-ring chronologies in the Baja California area.
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.
Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northernmost and westernmost of the 32 Federal Entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of Baja California. It has an area of 70,113 km2 (27,071 sq mi), or 3.57% of the land mass of Mexico and comprises the northern half of the Baja California Peninsula, north of the 28th parallel, plus oceanic Guadalupe Island. The mainland portion of the state is bordered on the west by the Pacific Ocean, on the east by Sonora, the U.S. state of Arizona, and the Gulf of California, and on the south by Baja California Sur. Its northern limit is the U.S. state of California.
Several studies have indicated that the PDO index can be reconstructed as the superimposition of tropical forcing and extra-tropical processes.Thus, unlike ENSO (El Niño Southern Oscillation), the PDO is not a single physical mode of ocean variability, but rather the sum of several processes with different dynamic origins.
At inter-annual time scales the PDO index is reconstructed as the sum of random and ENSO induced variability in the Aleutian low, whereas on decadal timescales ENSO teleconnections, stochastic atmospheric forcing and changes in the North Pacific oceanic gyre circulation contribute approximately equally. Additionally sea surface temperature anomalies have some winter to winter persistence due to the reemergence mechanism.
ENSO can influence the global circulation pattern thousands of kilometers away from the equatorial Pacific through the "atmospheric bridge". During El Nino events, deep convection and heat transfer to the troposphere is enhanced over the anomalously warm sea surface temperature, this ENSO-related tropical forcing generates Rossby waves that propagate poleward and eastward and are subsequently refracted back from the pole to the tropics. The planetary waves form at preferred locations both in the North and South Pacific Ocean, and the teleconnection pattern is established within 2–6 weeks.ENSO driven patterns modify surface temperature, humidity, wind, and the distribution of clouds over the North Pacific that alter surface heat, momentum, and freshwater fluxes and thus induce sea surface temperature, salinity, and mixed layer depth (MLD) anomalies.
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporation or sea ice formation which result in an increase in salinity. The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height. The depth of the atmospheric mixed layer is known as the mixing height. Turbulence typically plays a role in the formation of fluid mixed layers.
The atmospheric bridge is more effective during boreal winter when the deepened Aleutian low results in stronger and cold northwesterly winds over the central Pacific and warm/humid southerly winds along the North American west coast, the associated changes in the surface heat fluxes and to a lesser extent Ekman transport creates negative sea surface temperature anomalies and a deepened MLD in the central pacific and warm the ocean from the Hawaii to the Bering Sea.
Midlatitude SST anomaly patterns tend to recur from one winter to the next but not during the intervening summer, this process occurs because of the strong mixed layer seasonal cycle. The mixed layer depth over the North Pacific is deeper, typically 100-200m, in winter than it is in summer and thus SST anomalies that form during winter and extend to the base of the mixed layer are sequestered beneath the shallow summer mixed layer when it reforms in late spring and are effectively insulated from the air-sea heat flux. When the mixed layer deepens again in the following autumn/early winter the anomalies may again influence the surface. This process has been named "reemergence mechanism" by Alexander and Deserand is observed over much of the North Pacific Ocean although it is more effective in the west where the winter mixed layer is deeper and the seasonal cycle greater.
Long term sea surface temperature variation may be induced by random atmospheric forcings that are integrated and reddened into the ocean mixed layer. The stochastic climate model paradigm was proposed by Frankignoul and Hasselmann,in this model a stochastic forcing represented by the passage of storms alter the ocean mixed layer temperature via surface energy fluxes and Ekman currents and the system is damped due to the enhanced (reduced) heat loss to the atmosphere over the anomalously warm (cold) SST via turbulent energy and longwave radiative fluxes, in the simple case of a linear negative feedback the model can be written as the separable ordinary differential equation:
where v is the random atmospheric forcing, λ is the damping rate (positive and constant) and y is the response.
The variance spectrum of y is:
where F is the variance of the white noise forcing and w is the frequency, an implication of this equation is that at short time scales (w>>λ) the variance of the ocean temperature increase with the square of the period while at longer timescales(w<<λ, ~150 months) the damping process dominates and limits sea surface temperature anomalies so that the spectra became white.
Thus an atmospheric white noise generates SST anomalies at much longer timescales but without spectral peaks. Modeling studies suggest that this process contribute to as much as 1/3 of the PDO variability at decadal timescales.
Several dynamic oceanic mechanisms and SST-air feedback may contribute to the observed decadal variability in the North Pacific Ocean. SST variability is stronger in the Kuroshio Oyashio extension (KOE) region and is associated with changes in the KOE axis and strength,that generates decadal and longer time scales SST variance but without the observed magnitude of the spectral peak at ~10 years, and SST-air feedback. Remote reemergence occurs in regions of strong current such as the Kuroshio extension and the anomalies created near the Japan may reemerge the next winter in the central pacific.
Saravanan and McWilliamshave demonstrated that the interaction between spatially coherent atmospheric forcing patterns and an advective ocean shows periodicities at preferred time scales when non-local advective effects dominate over the local sea surface temperature damping. This "advective resonance" mechanism may generate decadal SST variability in the Eastern North Pacific associated with the anomalous Ekman advection and surface heat flux.
Dynamic gyre adjustments are essential to generate decadal SST peaks in the North Pacific, the process occurs via westward propagating oceanic Rossby waves that are forced by wind anomalies in the central and eastern Pacific Ocean. The quasi-geostrophic equation for long non-dispersive Rossby Waves forced by large scale wind stress can be written as the linear partial differential equation:
where h is the upper-layer thickness anomaly, τ is the wind stress, c is the Rossby wave speed that depends on latitude, ρ0 is the density of sea water and f0 is the Coriolis parameter at a reference latitude. The response time scale is set by the Rossby waves speed, the location of the wind forcing and the basin width, at the latitude of the Kuroshio Extension c is 2.5 cm s−1 and the dynamic gyre adjustment timescale is ~(5)10 years if the Rossby wave was initiated in the (central)eastern Pacific Ocean.
If the wind white forcing is zonally uniform it should generate a red spectrum in which h variance increases with the period and reaches a constant amplitude at lower frequencies without decadal and interdecadal peaks, however low frequencies atmospheric circulation tends to be dominated by fixed spatial patterns so that wind forcing is not zonally uniform, if the wind forcing is zonally sinusoidal then decadal peaks occurs due to resonance of the forced basin-scale Rossby waves.
The propagation of h anomalies in the western pacific changes the KOE axis and strengthand impact SST due to the anomalous geostrophic heat transport. Recent studies suggest that Rossby waves excited by the Aleutian low propagate the PDO signal from the North Pacific to the KOE through changes in the KOE axis while Rossby waves associated with the NPO propagate the North Pacific Gyre oscillation signal through changes in the KOE strength.
Temperature and precipitation
The PDO spatial pattern and impacts are similar to those associated with ENSO events. During the positive phase the wintertime Aleutian low is deepened and shifted southward, warm/humid air is advected along the North American west coast and temperatures are higher than usual from the Pacific Northwest to Alaska but below normal in Mexico and the Southeastern United States.
Winter precipitation is higher than usual in the Alaska Coast Range, Mexico and the Southwestern United States but reduced over Canada, Eastern Siberia and Australia
McCabe et al. showed that the PDO along with the AMO strongly influence multidecadal droughts pattern in the United States, drought frequency is enhanced over much of the Northern United States during the positive PDO phase and over the Southwest United States during the negative PDO phase in both cases if the PDO is associated with a positive AMO.
The Asian Monsoon is also affected, increased rainfall and decreased summer temperature is observed over the Indian subcontinent during the negative phase.
|PDO Indicators||PDO positive phase||PDO negative phase|
|Pacific Northwest, British Columbia, and Alaska||Above average||Below average|
|Mexico to South-East US||Below average||Above average|
|Alaska coastal range||Above average||Below average|
|Mexico to South-Western US||Above average||Below average|
|Canada, Eastern Siberia and Australia||Below average||Above average|
|India summer monsoon||Below average||Above average|
The PDO index has been reconstructed using tree rings and other hydrologically sensitive proxies from west North America and Asia.
MacDonald and Casereconstructed the PDO back to 993 using tree rings from California and Alberta. The index shows a 50–70 year periodicity but is a strong mode of variability only after 1800, a persistent negative phase occurring during medieval times (993–1300) which is consistent with La Niña conditions reconstructed in the tropical Pacific and multi-century droughts in the South-West United States.
Several regime shifts are apparent both in the reconstructions and instrumental data, during the 20th century regime shifts associated with concurrent changes in SST, SLP, land precipitation and ocean cloud cover occurred in 1924/1925, 1945/1946, and 1976/1977:
The NOAA Earth System Research Laboratory produces official ENSO forecasts, and Experimental statistical forecasts using a linear inverse modeling (LIM) methodto predict the PDO, LIM assumes that the PDO can be separated into a linear deterministic component and a non-linear component represented by random fluctuations.
Much of the LIM PDO predictability arises from ENSO and the global trend rather than extra-tropical processes and is thus limited to ~4 seasons. The prediction is consistent with the seasonal footprinting mechanismin which an optimal SST structure evolves into the ENSO mature phase 6–10 months later that subsequently impacts the North Pacific Ocean SST via the atmospheric bridge.
Skills in predicting decadal PDO variability could arise from taking into account the impact of the externally forcedand internally generated Pacific variability.
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.
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.
Climatology or climate science is the scientific study of climate, scientifically defined as weather conditions averaged over a period of time. This modern field of study is regarded as a branch of the atmospheric sciences and a subfield of physical geography, which is one of the Earth sciences. Climatology now includes aspects of oceanography and biogeochemistry. Basic knowledge of climate can be used within shorter term weather forecasting using analog techniques such as the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), the North Atlantic oscillation (NAO), the Northern Annular Mode (NAM) which is also known as the Arctic oscillation (AO), the Northern Pacific (NP) Index, the Pacific decadal oscillation (PDO), and the Interdecadal Pacific Oscillation (IPO). Climate models are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate. Weather is known as the condition of the atmosphere over a period of time, while climate has to do with the atmospheric condition over an extended to indefinite period of time.
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.
The Tropical Ocean Global Atmosphere program (TOGA) was a ten-year study (1985-1994) of the World Climate Research Programme (WCRP) aimed specifically at the prediction of climate phenomena on time scales of months to years.
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. 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.
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 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.
The Azores High also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda-Azores High, is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean, at the Horse latitudes. It forms one pole of the North Atlantic oscillation, the other being the Icelandic Low. The system influences the weather and climatic patterns of vast areas of North Africa and southern Europe, and to a lesser extent, eastern North America. The aridity of the Sahara Desert and the summer drought of the Mediterranean Basin is due to the large-scale subsidence and sinking motion of air in the system. In its summer position, the high is centered near Bermuda, and creates a southwest flow of warm tropical air toward the East Coast of the United States. In summer, the Azores-Bermuda High is strongest. The central pressure hovers around 1024 mbar (hPa).
The Atlantic Multidecadal Oscillation (AMO) is a climate cycle that affects the sea surface temperature (SST) of the North Atlantic Ocean based on different modes on multidecadal timescales. While there is some support for this mode in models and in historical observations, controversy exists with regard to its amplitude, and in particular, the attribution of sea surface temperature change to natural or anthropogenic causes, especially in tropical Atlantic areas important for hurricane development. The Atlantic multidecadal oscillation is also connected with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations.
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.
The Aleutian Low is a semi-permanent low-pressure system located near the Aleutian Islands in the Bering Sea during the Northern Hemisphere winter. It is a climatic feature centered near the Aleutian Islands measured based on mean sea-level pressure. It is one of the largest atmospheric circulation patterns in Northern Hemisphere and represents one of the "main centers of action in atmospheric circulation."
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 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.
Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances. The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Australia, which defines the Southern Oscillation.
Tropical instability waves, often abbreviated TIW, are 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. These waves are often present in the Atlantic Ocean, extending westward from the African coast, but are more easily recognizable in the Pacific, extending westward from South America. They have an average period of about 30 days and wavelength of about 1100 kilometers, and are largest in amplitude between June and November. They are also largest during La Niña conditions, and may disappear when strong El Niño conditions are present.
The Arctic dipole anomaly is a pressure pattern characterized by high pressure on the arctic regions of North America and low pressure on those of Eurasia. This pattern sometimes replaces the Arctic oscillation and the North Atlantic oscillation. It was observed for the first time in the first decade of 2000s and is perhaps linked to recent climate change. The Arctic dipole lets more southern winds into the Arctic Ocean resulting in more ice melting. The summer 2007 event played an important role in the record low sea ice extent which was recorded in September. The Arctic dipole has also been linked to changes in arctic circulation patterns that cause drier winters in Northern Europe, but much wetter winters in Southern Europe and colder winters in East Asia, Europe and the eastern half of North America.
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.
A climate oscillation or climate cycle is any recurring cyclical oscillation within global or regional climate, and is a type of climate pattern. These fluctuations in atmospheric temperature, sea surface temperature, precipitation or other parameters can be quasi-periodic, often occurring on inter-annual, multi-annual, decadal, multidecadal, century-wide, millennial or longer timescales. They are not perfectly periodic and a Fourier analysis of the data does not give a sharp spectrum.
The Tropical Atlantic SST Dipole refers to a cross-equatorial sea surface temperature (SST) pattern that appears dominant on decadal timescales. It has a period of about 12 years, with the SST anomalies manifesting their most pronounced features around 10–15 degrees of latitude off of the Equator. The term Tropical Atlantic SST dipole is only one of the characteristic names used to refer to this mode of variability; other definitions include the interhemispheric SST gradient or the Meridional Atlantic mode. This decadal-scale SST pattern constitutes one of the key features of SST variability in the Tropical Atlantic Ocean, with another one being the Atlantic Equatorial Mode or Atlantic Niño, which occurs in the zonal (east-west) direction at interannual timescales, with sea surface temperature and heat content anomalies being observed in the eastern equatorial basin. Its importance in climate dynamics and decadal-scale climate prediction is evident when investigating its impact on adjacent continental regions such as the Northeast Brazil, the Sahel as well as its influence on North Atlantic cyclogenesis.
The Tropical Atlantic Variability (TAV) is influenced by internal interaction and external effects. TAV can be discussed in different time scales: seasonal and interannual.
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