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.
In the 1990s and early 2000s, many studies of Arctic sea ice export focused on the Arctic and North Atlantic oscillations as the primary drivers of export.However, other studies, such as those by Watanabe and Hasumi and Vinje, suggested that the Arctic and North Atlantic oscillations did not always explain the variability in sea ice export.
In 2006, the Arctic dipole anomaly was formally proposed by Bingyi Wu, Jia Wang, and John Walsh, using the NCEP/NCAR reanalysis datasets spanning 1960–2002. N, where the first leading mode corresponds to the Arctic oscillation. When defined for the winter season (October through March), the first leading mode (Arctic oscillation) accounts for 61% of the total variance, while the second leading mode (Arctic dipole anomaly) accounts for 13%.It is defined as the spatial distribution of the second leading empirical orthogonal functions mode of monthly mean sea level pressure north of 70°
While the Arctic oscillation has an annular structure centered over and covering the entire Arctic,the Arctic dipole anomaly has two poles of opposite sign: one over the Canadian Arctic Archipelago and northern Greenland, the other over the Kara and Laptev seas. This dipole structure leads to a pressure gradient with a zero isopleth oriented from the Bering Strait, across the Arctic to the Greenland and Barents seas. As a result, anomalous winds are generally directed parallel to the zero isopleth either towards the Greenland and Barents seas (positive Arctic dipole anomaly) or toward the Bering Strait (negative Arctic dipole anomaly).
Although the Arctic oscillation is responsible for more of the total variance in mean sea level pressure over the Arctic, the meridional winds anomalies that arise as a result of the spatial structure of the Arctic dipole anomaly make it the primary driver of the variability of Arctic sea ice export.During the positive phase of the Arctic dipole anomaly, anomalous winds drive sea ice from the central Arctic out through the Fram Strait and into the Greenland Sea via the Transpolar Drift Stream. In contrast, during the negative phase, anomalous winds reduce the removal of sea ice through the Fram Strait. This is supported by Watanabe et al., as well as Wang et al., which show that sea ice export is enhanced during the positive phase of the Arctic dipole anomaly, while export is reduced during the negative phase.
However, the Arctic oscillation cannot be ignored when considering sea ice export from the Arctic. By itself, circulation associated with a positive phase Arctic Oscillation results in an increase in sea ice export, while the negative phase of the Arctic oscillation is associated with reduced Arctic sea ice export.When considering sea ice export in connection with the Arctic dipole anomaly, the Arctic oscillation determines the sign of the dominant mean sea level pressure anomaly, while the Arctic dipole anomaly determines the location of the dominant mean sea level pressure anomaly (over the Canadian Arctic Archipelago and northern Greenland, or over the Kara and Laptev seas). Therefore, while the Arctic dipole anomaly determines whether the overall export of sea ice will be promoted or restricted, the Arctic oscillation will either enhance or diminish the influence of the Arctic dipole anomaly.
The Arctic dipole anomaly has also been suggested to play an important role in the occurrence of several extreme sea ice minima that have occurred since the mid-1990s, including the minimum in 2007.Wang et al. suggest that in addition to anomalous winds driving sea ice out of the Arctic through the Fram Strait, the positive phase of the Arctic dipole anomalies may also increase the flow of relatively warm waters from the North Pacific through the Bering Strait into the Arctic Ocean. Warmer waters, in addition to increased sea ice export, could result in reduced sea ice areal extent. Additionally, preconditioning of sea ice from the previous winter and summer seasons, as well as multidecadal trends, plays a role in determining the minimum sea ice extent for a given year.
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, 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.
The quasi-biennial oscillation (QBO), is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause. Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QBO domain, easterlies dominate, while at the bottom, westerlies are more likely to be found. At the 30mb level, with regards to monthly mean zonal winds, the strongest recorded easterly was 29.55 m/s in November 2005, while the strongest recorded westerly was only 15.62 m/s in June 1995.
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 North Atlantic Oscillation (NAO) is a weather phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. Through fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic. It is part of the Arctic oscillation, and varies over time with no particular periodicity.
The East Greenland Current (EGC) is a cold, low salinity current that extends from Fram Strait (~80N) to Cape Farewell (~60N). The current is located off the eastern coast of Greenland along the Greenland continental margin. The current cuts through the Nordic Seas and through the Denmark Strait. The current is of major importance because it directly connects the Arctic to the Northern Atlantic, it is a major contributor to sea ice export out of the Arctic, and it is a major freshwater sink for the Arctic.
The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic poles north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called the Antarctic oscillation or Southern Annular Mode (SAM). The index varies over time with no particular periodicity, and is characterized by non-seasonal sea-level pressure anomalies of one sign in the Arctic, balanced by anomalies of opposite sign centered at about 37–45° N.
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. There is evidence of reversals in the prevailing polarity 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 Siberian High is a massive collection of cold dry air that accumulates in the northeastern part of Eurasia from September until April. It is usually centered on Lake Baikal. It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than −40 °C (−40 °F). The atmospheric pressure is often above 1,040 millibars (31 inHg). The Siberian High is the strongest semi-permanent high in the northern hemisphere and is responsible for both the lowest temperature in the Northern Hemisphere, of −67.8 °C (−90.0 °F) on 15 January 1885 at Verkhoyansk, and the highest pressure, 1083.8 mbar at Agata, Krasnoyarsk Krai on 31 December 1968, ever recorded. The Siberian High is responsible both for severe winter cold and attendant dry conditions with little snow and few or no glaciers across Siberia, Mongolia, and China. During the summer, the Siberian High is largely replaced by the Asiatic low.
A polar vortex is an upper-level low-pressure area lying near one of the Earth's poles. There are two polar vortices in the Earth's atmosphere, overlying the North and South Poles. Each polar vortex is a persistent, large-scale, low-pressure zone less than 1,000 kilometers (620 miles) in diameter, that rotates counter-clockwise at the North Pole and clockwise at the South Pole, i.e., both polar vortices rotate eastward around the poles. As with other cyclones, their rotation is driven by the Coriolis effect. The bases of the two polar vortices are located in the middle and upper troposphere and extend into the stratosphere. Beneath that lies a large mass of cold, dense Arctic air.
The Atlantic Multidecadal Oscillation (AMO), also known as Atlantic Multidecadal Variability (AMV), 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 effects of global warming in the Arctic, or climate change in the Arctic include rising air and water temperatures, loss of sea ice, and melting of the Greenland ice sheet with a related cold temperature anomaly, observed since the 1970s. Related impacts include ocean circulation changes, increased input of freshwater, and ocean acidification. Indirect effects through potential climate teleconnections to mid latitudes may result in a greater frequency of extreme weather events, ecological, biological and phenology changes, biological migrations and extinctions, natural resource stresses and as well as human health, displacement and security issues. Potential methane releases from the region, especially through the thawing of permafrost and methane clathrates, may occur. Presently, the Arctic is warming twice as fast as the rest of the world. The pronounced warming signal, the amplified response of the Arctic to global warming, is often seen as a leading indicator of global warming. The melting of Greenland's ice sheet is linked to polar amplification. According to a study published in 2016, about 0.5 °C (0.90 °F) of the warming in the Arctic has been attributed to reductions in sulfate aerosols in Europe since 1980.
The Molloy Deep is a bathymetric feature in the Fram Strait, within the Greenland Sea east of Greenland and about 160 km west of Svalbard. It is the location of the deepest point in the Arctic Ocean.
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.
The West Spitsbergen Current (WSC) is a warm, salty current that runs poleward just west of Spitsbergen,, in the Arctic Ocean. The WSC branches off the Norwegian Atlantic Current in the Norwegian Sea. The WSC is of importance because it drives warm and salty Atlantic Water into the interior Arctic. The warm and salty WSC flows north through the eastern side of Fram Strait, while the East Greenland Current (EGC) flows south through the western side of Fram Strait. The EGC is characterized by being very cold and low in salinity, but above all else it is a major exporter of Arctic sea ice. Thus, the EGC combined with the warm WSC makes the Fram Strait the northernmost ocean area having ice-free conditions throughout the year in all of the global ocean.
Infragravity waves are surface gravity waves with frequencies lower than the wind waves – consisting of both wind sea and swell – thus corresponding with the part of the wave spectrum lower than the frequencies directly generated by forcing through the wind.
The Subtropical Indian Ocean Dipole (SIOD) is featured by the oscillation of sea surface temperatures (SST) in which the southwest Indian Ocean i.e. south of Madagascar is warmer and then colder than the eastern part i.e. off Australia. It was first identified in the studies of the relationship between the SST anomaly and the south-central Africa rainfall anomaly; the existence of such a dipole was identified from both observational studies and model simulations .
In recent decades, sea ice in the Arctic Ocean has been melting faster than it re-freezes in winter. The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that greenhouse gas forcing is predominantly responsible for the decline in Arctic sea ice extent. A 2007 study found the decline to be "faster than forecasted" by model simulations. A 2011 study suggested that this could be reconciled by internal variability enhancing the greenhouse gas-forced sea ice decline over the last few decades. A 2012 study with a newer set of simulations also projected rates of retreat which were somewhat less than that actually observed.
Caroline C. Ummenhofer is a physical oceanographer at the Woods Hole Oceanographic Institution where she studies extreme weather events with a particular focus on the Indian Ocean. Ummenhofer makes an effort to connect her discoveries about predicting extreme weather events and precipitation to helping the nations affected.