Earth Gauge » Multi-Annual Climate Cycles

Climate Fact: AMO and THC

administrator — Wed, 08 Oct 2008 21:31:36 +0000

The Atlantic Multidecadal Oscillation (AMO), or the North Atlantic’s periodic shift (65-year period) from predominately warm to predominately cool regimes, controls much of the climatic variability in the Northern Hemisphere. During warm (positive) AMO phases, the Northern Hemisphere is generally warmer, by as much as a few degrees Fahrenheit when compared to the cool phases. This cycle appears to be linked to periodic fluctuations in the strength of the thermohaline circulation (THC), or the “conveyor belt” that moves heat and salt around the world’s oceans. In the North Atlantic, a surface current transports warm water north and then “overturns” near Greenland and becomes a deep cold water current that moves south. The northward movement of warm water is strongest at about 30 degrees North (about the same latitude as New Orleans), where over 40 billion gallons of water pass each second! The strengthening of this current, which has been happening since the 1970’s, has corresponded to the rise in North Atlantic sea surface temperatures and Northern Hemisphere land temperatures.

Seasons: Winter, Spring, Summer, Fall

Sources: Knight, JR et al. “A signature of persistent natural thermohaline circulation cycles in observed climate.” Geophysical Research Letters 32 (2005): L20708 and Bryden, et al. “Slowing of the Atlantic meridional overturning circulation at 25 degrees N.” Nature 438 (2005): 655-657.

Climate Fact: Diatoms and Dinoflagellates

administrator — Wed, 08 Oct 2008 20:44:46 +0000

During the warm seasons (spring through fall), the water in the Baltic Sea is stable and stratified. This means that the warmest and least dense water is on the surface, and as you dive deeper and deeper, layers of progressively colder, saltier, and denser water are encountered. During the decades of the 1970’s and 1980’s, the North Atlantic Oscillation (NAO) was in a negative phase, and winters in the Baltic Sea were generally cold and dry. As a result, during the winter, the water on the surface would cool to the point where it was colder than the water below it, and mixing between the layers would occur until a new stable state (one where the cold water is on the surface) was reached. This annual mixing was important for the survival of the sea’s diatoms, which depended on the mixing that would take place during the spring as the sea reverted from the winter to summer water column formation. Diatoms are photosynthetic, single-celled organisms, which form a large part of the marine food pyramid’s base. A positive state of the NAO since the late 1980’s has corresponded to milder winters, and the deep mixing that would occur as the seasons changed no longer occurs. As a result, the diatoms that depend on the mixing have been largely replaced by another type of single-celled organism known as dinoflagellates, which may be best known for their whip-like tails that propel them through the water. This change has also corresponded to other changes in the sea’s species’ composition.

Seasons: Winter, Spring, Summer, Fall

Sources: Alheit, J. et al. “Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980’s.” ICES Journal of Marine Science 62 (2005): 1205-1215 and Hinrichsen, HH et al. “Correlation analyses of Baltic Sea winter water mass formation and its impact on secondary and tertiary production.” Oceanologia 49 (2007): 381-395.

Climate Fact: Seabird Shift

administrator — Wed, 08 Oct 2008 20:00:36 +0000

Climate variability in the mid- to high-latitudes of the Northern Hemisphere, or the area from about 35 degrees North to the poles, is largely controlled by two naturally occurring climate oscillations, the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO). In 1977, both oscillations shifted from negative to positive phases, which resulted in a warming of the ocean waters in the northeastern Pacific around Alaska, and a cooling of the waters in the northeastern Atlantic around Scandinavia. This shift in sea surface temperatures (SSTs) was one of the largest ever recorded. In 1989, the opposite trend happened. The magnitude of this shift, however, was much less pronounced. The population trends of two species of seabird, the Common Murre and the Thick-billed Murre, were studied in relation to these shifts. These species thrive under essentially the opposite environmental conditions. The large magnitude shift in 1977 caused populations of both species to decline throughout the entire hemisphere, while both populations grew after the smaller shift in 1989. Because these shifts produced opposite trends in local environmental conditions, it might be expected that one species would thrive and one species would decline in number during each of the regime shifts. Since this was not the case, this phenomenon illustrates how it can be difficult for top predatory species to adapt to any rapid climatic fluctuation.

Seasons: Spring, Summer, Fall

Source: Irons, D.B. et al. “Fluctuations in circumpolar seabird populations linked to climate oscillations.” Global Change Biology 14 (2008): 1455-1463.

Climate Fact: Rainfall Declines in Southeast Australia

administrator — Wed, 08 Oct 2008 19:43:43 +0000

Autumn (March to May) rainfall in southeast Australia is important for soil moisture and river recharge because the region is dependent on reliable water sources for cereal crop production. Since 1950, there has been a 40 percent decline in the region’s average autumn rainfall. This has been linked to fewer occurrences of La Niña events, more El Niño events, and a change in the “pressure wave-train” circulation pattern that brings rainfall from the sub-tropical Indian Ocean to southeast Australia. The increased frequency of El Niño events and the change in the wave-train circulation pattern, as well as a rise in atmospheric pressure over the region, have been linked to rising sea-surface temperatures in the Pacific and Indian Oceans. The last two autumns have seen record low inflows into the Murray River, an important source of water for the region’s farmers.

Seasons: Spring, Summer

Sources: Commonwealth Scientific and Industrial Research Organisation (CSIRO): Media Center. “Understanding autumn rain decline in SE Australia.” 23 May 2008. Accessed Online 9 June 2008 <http://www.csiro.au/news/UnderstandingDeclineAutumnRain.html> and Wahlquist, Asa. “Dry future well ahead of schedule.” The Australian, 7 June 2008. Accessed Online 9 June 2008 <http://www.theaustralian.news.com.au/story/0,25197,23822411-11949,00.html>

Climate Fact: Maritime Influences on Mountain Hemlock

administrator — Wed, 08 Oct 2008 18:25:55 +0000

In the Pacific Northwest, Mountain Hemlocks grow at elevations between 3,600 and 7,500 feet. These shade tolerant trees grow underneath the faster growing but shorter-lived Douglas Firs, and gradually make their way to the top of the canopy over their 700 year life spans. At the region’s high elevations, some of the world’s most extensive seasonal snowpack forms. The extent of this snowpack and its seasonal duration determine how much Mountain Hemlocks can grow each year. Warmer years (years when more precipitation falls as rain instead of snow, and the accumulated snow pack melts earlier in the spring) tend to be years of enhanced tree growth in the northern portion of the Hemlock’s range, and wider tree rings correspond to those years. In the southern extent of the Hemlock’s range (southern Oregon), however, the climate is different and warm temperatures and lack of snowpack limit hemlock growth. Here, the growth trends are essentially the opposite seen in the more northern areas of the species’ range. The Pacific Decadal Oscillation (PDO), a 50-year cycle of sea-surface temperature shifts in the northern Pacific, seems to largely control the depth and duration of Pacific Northwest snowpack. Positive (warm) phases of the PDO, which occurred between about 1920 until about 1945, and from about 1977 until just a few years ago, correspond to warmer temperatures in the Pacific Northwest and less snowpack.

Seasons: Winter, Spring, Summer, Fall

Source: Peterson, DW and Peterson, DL. “Mountain Hemlock Growth Responds to Climatic Variability at Annual and Decadal Time Scales.” Ecology 82 (2001): 3330-3345.

Climate Fact: El Niño and Tropical Pacific Cyclones

administrator — Wed, 08 Oct 2008 17:45:29 +0000

The tropical Pacific basin is one of the planet’s warmest ocean regions, with surface water temperatures rarely falling below 83 degrees Fahrenheit. These perennially warm temperatures provide the fuel for tropical cyclone formation, and the strongest cyclones on record have formed here. Because these waters are already above the threshold for tropical cyclone formation, slight increases in water temperature do not seem to have much of an effect on the average annual number and average intensity of cyclones forming here. Instead, the El Niño Southern Oscillation cycle appears to control how many cyclones form annually. In El Niño years, the region’s monsoon trough (a region of low pressure associated with the monsoon) extends farther east over the tropical waters than it does during La Niña years. This leads to a larger area where tropical cyclone formation is likely, and on average three times as many tropical cyclones form during El Niño than during La Niña years.

Seasons: Spring, Summer, Fall

Sources: Matsuura, T et al. “A mechanism for interdecadal variability of tropical cyclone activity over the western North Pacific.” Climate Dynamics 21 (2003): 105-117 and Chan, JCL. “Interannual variations of intense typhoon activity.” Tellus 59A (2007): 455-460.

Climate Fact: Copepod Range Change

administrator — Wed, 08 Oct 2008 17:28:24 +0000

The Labrador Sea Water (LWS) is a stream of cold, fresh, and oxygen rich water that travels down the western Atlantic coast from the Labrador Sea, which is located between Greenland and Newfoundland, towards the Equator. This stream forms in the late fall/early winter after the seasonal accumulation of glacial melt water, which is less dense than salt water, has pooled on the surface. As the strong northwesterly winter winds start to blow across the Labrador Sea, the fresh water pool cools, sinks to a depth of about 5,000 feet, and travels down the East Coast of Canada. The North Atlantic Oscillation (NAO), or the cyclical change in the pressure difference between the Azores High and Icelandic Low, controls the strength of these winds. During positive NAO phases (when the Azores High is particularly high and the Icelandic Low is particularly low), the winds are stronger, which leads to more cooling on the surface and a stronger flow of the Labrador Sea Water. The stronger the flow of this water, the cooler the waters along America’s east coast are. Over the last 30 years, the NAO was strongly and predominately positive, and copepod species (small crustaceans that collectively constitute the biggest source of protein in the oceans) that were previously found only in the waters off Canada, began showing up as far south as New Jersey, and increased in abundance all along the New England coast.

Seasons: Fall, Winter

Source: Johns, DG. “Arctic boreal plankton species in the Northwest Atlantic.” Canadian Journal of Fisheries and Aquatic Sciences 58 (2001): 2121-2124.

Climate Fact: Warming and Water Discharge

administrator — Wed, 08 Oct 2008 17:23:02 +0000

The Arctic Ocean is surrounded by a series of river drainage basins, which collectively occupy an area 1.5 times that of the ocean basin itself. No other ocean basin’s temperature and salinity levels are more dependent on what happens on the adjacent land surface. These temperature and salinity levels in turn influence the behavior of the world’s ocean conveyor system, which transports heat and nutrients to and from different ocean basins. The Eurasian Arctic has warmed by about 1.3 degrees Fahrenheit since the 1930’s. As this has happened, the total combined annual discharge of the six largest Eurasian rivers that flow into the Arctic Ocean has grown by 34 trillion gallons, or seven percent. These trends have corresponded to increases in winter precipitation and thawing permafrost. Total discharge also appears to be related to the behavior of the North Atlantic Oscillation (NAO), or the cyclical change in the pressure difference between the Azores High and Icelandic Low. Positive phases bring more precipitation to Scandinavia and Siberia, and the NAO has been predominately and strongly positive over the last 30 years.

One of the six rivers studied is Russia’s Lena River. To see a colorful LANDSAT image of the Lena River Delta, visit http://earthasart.gsfc.nasa.gov/lena.html.

Seasons: Spring, Summer, Fall

Sources: Peterson, BJ et al. “Increasing River Discharge to the Arctic Ocean.” Science 298 (2002): 2171-2173 and Serreze, MC et al. “Large-scale hydro-climatology of the terrestrial Arctic drainage system.” Journal of Geophysical Research 108 (2003): doi:10.1029/2001JD000919 and McGuire, AD et al. “Land cover disturbance and feedbacks to the climate system in Canada and Alaska.” Chapter 9 (pages 139 - 161) in Imzd Change Science: Ohsel-virzg, Moniloritig, and Ulzder*stnditzg Trojectol-ies of Challge OH the Earth’s SU+KZE. ditcd by Gutman, C., Janetos, A.C., Justice, C.0, Moran, E.F., Mustard, J.F., Rindf~lssR, .R., Skole, D., Turner 11, B.L., and Cochrane, M.A. Dordrecht, Ncthcrlands, Kluwer Adademic Publishers.

Climate Fact: Crater Lake Water Levels and PDO

administrator — Wed, 08 Oct 2008 17:13:23 +0000

About 7,700 years ago, a volcanic eruption 42 times more powerful than the 1980 Mt. St. Helens event happened at Mt. Mazama in the southern Oregon Cascade Mountains. The top 5,000 feet of the mountain collapsed soon afterwards, leaving behind a huge caldera, or crater, which has since filled with about 4.6 trillion gallons of water that make up America’s deepest lake - Crater Lake. Crater has been called “the world’s largest rain gauge,” as the lake’s surface comprises about 78.5 percent of its watershed and there are no streams flowing into or out of the lake. Thus, trends in Crater Lake’s water levels provide an effective proxy for local temperature and precipitation trends. The historical record shows that these lake levels are influenced by the behavior of the Pacific Decadal Oscillation (PDO), or the periodic shift in heat distribution in the Pacific Ocean. During negative (wet and cool) phases, the mid-latitude storm track that brings moisture to the Pacific Northwest tends to consistently travel over southern Oregon. During positive (warm and dry) phases, the storm track moves and southern Oregon does not receive as much precipitation. Lake levels reached 300 year lows during the late 1980’s and early 1990’s, after years of a strongly positive PDO.

Seasons: Winter, Spring, Summer, Fall

Sources: Peterson, DL et al. “Detecting Long-Term Hydrological Patterns at Crater Lake, Oregon.” Northwest Science 73 (1999): 121-130 and Uhler, John W. “Crater Lake National Park Information Page.” Online Posting, 2007. 4 September 2008 < http://www.crater.lake.national-park.com/info.htm> and United States Geological Survey: National Water Information Service. 4 September 2008 <http://waterdata.usgs.gov/nwis>

Climate Fact: ENSO and Tropical Cyclone Landfall Frequency

administrator — Wed, 08 Oct 2008 16:45:58 +0000

The El Niño Southern Oscillation (ENSO) cycle, or the cyclical movement of heat in the tropical Pacific Ocean, affects the upper atmosphere over the Atlantic Ocean. This affects both the frequency of Atlantic tropical cyclone formation as well as the positioning of the region’s high and low pressure centers that steer the tropical cyclones. La Niña conditions are 19 percent more common during years when tropical cyclones frequently make landfall along the Atlantic and Gulf Coasts of the United States. El Niño conditions are 10 percent more common during years when few tropical cyclones make landfall along the U.S. coast.

Seasons: Spring, Summer, Fall

Source: Lyons, Steven. “U.S. Tropical Cyclone Landfall Variability: 1950-2002.” Weather and Forecasting 19 (2004): 473-480.

Climate Fact: North Atlantic Basin Warming

administrator — Tue, 07 Oct 2008 20:38:39 +0000

The amount of energy that the North Atlantic Basin accumulated over the last 50 years is equivalent to almost four trillion tons of TNT (1.610 × 1022 joules). This energy has not been distributed uniformly, as the tropical and subtropical regions of the North Atlantic have warmed the most, and the subpolar region has actually cooled. More energy has been gained than lost, however, and if this gain was averaged out across the entire North Atlantic Basin, each square meter would have experienced an increased heat flux of 0.42 Watts. This heat gain and change in heat distribution is related to the behavior of the North Atlantic Oscillation (or the cyclical change in the pressure difference between the Azores High and Icelandic Low), which transitioned from a predominately negative phase during the 1950’s and 1960’s to a predominately positive phase during the 1980’s and 1990’s. While multi-decadal heat fluxes from one ocean basin to another have been part of Earth’s climate for centuries, each of the world’s ocean basins have warmed over the last 50 years and the average temperature of the upper 3000 meters (which is about 70 percent of the world’s ocean water) rose by 0.07 degrees Fahrenheit. While this number may seem small, the same amount of energy it would take to raise the world’s ocean heat content by just 0.18 degrees would be enough to raise the average global atmospheric temperature to the boiling point of water, or 212 degrees!

Seasons: Winter, Spring, Summer, Fall

Source: Lozier, MS, et al. “The Spatial Pattern and Mechanisms of Heat-Content Change in the North Atlantic.” Science 319 (2008): 800-803 and Sources: Levitus, S. et al. “Warming of the world ocean, 1955-2003.” Geophysical Research Letters 32 (2005): L02604, doi: 10.1029/2004GL01592 and Miles, Edward. “Multiple Stresses, Thresholds, and Ocean Acidification.” Cannon House Office Building, Washington, DC. 20 September 2007

Climate Fact: ENSO and Gulf Coast Lightning

administrator — Tue, 07 Oct 2008 20:27:42 +0000

The El Niño-Southern Oscillation (ENSO), or the cyclical movement of heat in the tropical Pacific Ocean, affects atmospheric phenomena throughout the world. The cycle affects the strength and position of the Pacific Jet Stream, an upper atmosphere wind current that flows from the Pacific over North America. During La Niña phases of the cycle, the Jet Stream is weaker than average and flows in an arcing pattern over the northern U.S. During El Niño phases, the Jet Stream strengthens and flows over the southern United States. In general, this leads to a stormier Gulf Coast, and a 100-200 percent regional increase in the frequency of warm season lightning strikes compared to neutral conditions.

Seasons: Spring, Summer, Fall

Sources: Cook, AR and Schaefer, JT et al. “The Relation of the El Niño-Southern Oscillation (ENSO) to Winter Tornado Outbreaks.” Monthly Weather Review 136 (2008): 3121-3137 and LaJoie, M and Laing, Arlene. “The Influence of the El Niño-Southern Oscillation on Cloud-to-Ground Lightning Activity along the Gulf Coast. Part I: Lightning Climatology.” Monthly Weather Review 136 (2008): 2523-2542.

Climate Fact: Sea Level Rise on the East Coast

administrator — Tue, 07 Oct 2008 20:13:09 +0000

Over the past century, measurements taken at geologically stable locations show that the global sea level rose by eight inches, with most of this rise happening over the second half of the 20th century. Measurements taken at most coastal locations, however, rarely correspond to this global value because near-shore environments are naturally dynamic. Storms, natural sedimentation, water course alterations, and land subsidence (often due to groundwater withdrawals) can all work to either raise or lower a locality’s relative sea level. There are also significant annual and multiannual variations in local sea levels, which on the East Coast of the United States appear to come in one- to three-year cycles (depending on the location), with a difference of about eight inches from peak to peak. These fluctuations appear to be linked to the behavior of the North Atlantic Oscillation, or the periodic change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands, located about 900 miles west of Portugal. Changes in the Icelandic Low have been linked to changes in the behavior of the North Wall of the Gulf Stream, or the stream of water that runs off the U.S. East Coast towards the middle of the Atlantic. Fluctuations in the strength and position of the North Wall appear to be largely responsible for causing the changes in the waves and winds that determine sea levels along the East Coast.

Seasons: Winter, Spring, Summer, Fall

Sources: Hong, BG et al. “Sea Level on the U.S. East Coast: Decadal Variability Caused by Open Ocean Wind-Curl Forcing.” Journal of Physical Oceanography 30 (2000): 2088-2098 and Kolker AS and Hameed, S. “Meteorologically driven trends in sea-level rise.” Geophysical Research Letters 34 (2007): L23616 and Hameed, S and Piontkovski, S. “The dominant influence of the Icelandic Low on the position of the Gulf Stream.” Geophysical Research Letters 31 (2004): L09303.

Climate Fact: North Atlantic Seabird Success

administrator — Tue, 07 Oct 2008 19:51:53 +0000

Seabirds, such as auks, gulls, petrels, terns, and gannets, have spent tens of millions of years adapting to life on the ocean. Some species, such as the Sooty Tern, can spend years at sea before returning to land. The success of these species is dependent on the success of their food sources (such as fish and plankton), and the success of these food sources is dependent on oceanic conditions such as sea-surface temperatures and the behavior of ocean currents. In the North Atlantic, changes in the North Atlantic Oscillation (NAO), or the cyclical change in the pressure difference between the Azores High and Icelandic Low, affect ocean currents. A 30-year study of summertime seabird breeding success indicates that seabird species on both sides of the Atlantic have more offspring in years when the waters are warmer, and in years when the previous year’s winter featured a predominately negative NAO (a negative NOA means that the difference between the aforementioned pressure centers is lower and the westerly winds blowing across the ocean are not as strong). The North Atlantic Basin has experienced an overall warming trend over the last 50 years, although the sub-polar waters, where most of the breeding activity takes place, have actually cooled. Also during this period, the NAO has transitioned from being predominately negative to predominately positive.

Seasons: Winter, Spring, Summer, Fall

Source: Sandvik, H et al. “A latitudinal gradient in climate effects on seabird demography: results from interspecific analyses.” Global Change Biology 14 (2008): 703-713.

Climate Fact: California’s Castle Lake and Climate

administrator — Tue, 05 Aug 2008 15:18:51 +0000

During the last ice age, an advancing glacier carved out a basin in the Siskiyou Mountains of what is northern California today. As the glacier melted, Castle Lake was formed. Every spring, as the ice on the lake melts and warm water on the bottom of the lake moves to the surface, stirring up nutrients in the process, blue-green algae and single celled organisms called diatoms bloom. These organisms ultimately feed the rest of the life in the lake, and are thus called the lake’s ”primary producers.” The earlier the ice melts, the warmer the lake’s water is during the summer, and the more primary production there is. Water temperature during the summer is regulated by the El Nino Southern Oscillation (ENSO) cycle. During summers that follow strong El NiÃ±o events (such as the summers of 1983 and 1998) water temperature and primary production are generally lower than average. Additionally, the area around Castle Lake has experienced a warming of 1.6 degrees Fahrenheit over the past 50 years, and during this same period in California and Nevada, the date when mountain snowpack melts has advanced an average of 19 days earlier in the year. All of these trends favor earlier ice-melt dates.

Season: Winter, Spring, Summer, Fall

Source: Park, S et al. “Climatic forcing and primary productivity in a subalpine lake: Interannual variability as a natural experiment.” Limnology and Oceanography 49 (2004): 614-619.

Climate Fact: Winter Weather and the North Atlantic Oscillation (Chicago, IL)

administrator — Wed, 05 Mar 2008 22:48:07 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar? air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States.? In Chicago, for example, there are on average three times as many days each year when the temperature drops below zero degrees Fahrenheit during negative phases, versus positive phases.? Over the last thirty years, the index has been predominately positive. The index is currently hovering around neutral, and it has been mostly positive this winter.

Season: Winter

Climate Fact: Winter Weather and the North Atlantic Oscillation (Boston, MA)

administrator — Wed, 05 Mar 2008 22:46:08 +0000

The North Atlantic Oscillation (NAO), which is part of a large system known as the Arctic Oscillation, is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the Northeast relatively mild, but it also keeps the cold polar air in Canada’s Maritime Provinces and Quebec, making winters in these regions more severe. Over the last thirty years, the index has been predominately positive, which has contributed to the increased winter temperatures in the northeastern U.S. during this period. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Sources: Thompson, David W.J. “Regional Climate Impacts of the Northern Hemisphere Annular Mode.” Science 293, 85 (2001) and National Oceanic and Atmospheric Administration: Climate Prediction Center. North Atlantic Oscillation. Accessed Online 19 February 2008 and Wettstein, JJ and Mearns, LO. “The Influence of the North Atlantic-Arctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada.” Journal of Climate 15, 3586 (2002)

Climate Fact: Winter Weather and the North Atlantic Oscillation (Atlanta, GA)

administrator — Wed, 05 Mar 2008 22:43:42 +0000

The North Atlantic Oscillation (NAO) is a cyclical change in the difference in atmospheric pressure between a low pressure center around Iceland and a high pressure center around the Azores Islands in the North Atlantic. When this difference in pressure is larger (i.e. the low pressure center is especially low and the high pressure center is especially high), the NAO is in a “positive” phase, whereas when the difference in pressure is smaller, the NAO is in a “negative” phase. This oscillation influences the subpolar westerly winds that flow between 35 and 55 degrees north. During positive phases, the westerlies are stronger and tend to “block” the polar air masses from invading the lower latitudes. This tends to keep winter weather in the mid- latitudes relatively mild and reduce the occurrence of below average winter temperatures in the United States. In Atlanta, for example, there are on average five times as many days each year when trace snow falls occur during negative phases, versus positive phases. Over the last thirty years, the index has been predominately positive. The index is currently positive, and has been mostly positive this winter.

Season: Winter

Climate Fact: New England’s Snowfall Trends

administrator — Wed, 05 Mar 2008 22:35:40 +0000

A rise in New England’s average temperature over the second half of the 20th Century was accompanied by a decrease in the ratio of precipitation that falls as snow. This trend was strongest for stations in Maine. Between 1949 and 2000, the percentage of annual precipitation? that fell as snow declined from 30 to 23 percent. Also during this period, the winter North Atlantic Oscillation (NAO) index became more positive. When the NAO is in a negative phase, high pressure builds over Greenland, which pushes cold air south into New England causing winters to be colder and more precipitation to fall as snow, especially during the months of December and March when temperatures are closer to the freezing point than they are during January and February. When the NAO is in a positive phase, the mid-latitude jet stream is farther north than it is during neutral and negative phases, which causes the United States to be warmer than normal. The trend toward a positive NAO helps to explain the decrease in snowfall that has occurred.

Season: Winter, Spring

Source: Huntington, TG et al, (2003): “Changes in the Proportion of Precipitation Occurring as Snow in New England (1949-2000).” Journal of Climate, vol 17, pp, 2626-2636

Climate Fact: ENSO and Carbon Concentrations

administrator — Wed, 05 Mar 2008 22:35:19 +0000

Atmospheric carbon dioxide (CO₂)levels have risen from 280 parts per million in pre-industrial times to around 385 parts per million today. The rate of this rise has varied from year to year. The two phases of the El Niño Southern Oscillation Cycle (ENSO), El Niño (positive) and La Niña (negative), appear to have a significant influence on this variation. The influence that the cycle has on temperature and rainfall in the tropics appears to in turn affect the rates of vegetation growth, which takes CO₂ out of the atmosphere, and plant respiration, which? takes place? when plants use the energy reserves that they stored during years that were more conducive to growth; respiration releases CO₂ into the atmosphere. ENSO’s influence seems to be especially strong in southeastern Asia and the western Amazon; during warm and dry El Niño years, plants in these regions grow at a slower rate, there are more fires, and plants consume more of their stored energy. During cool and wet La Niña phases, there is more plant growth, fires are rarer, and plants are storing more energy reserves than they are consuming. The influence that ENSO phases have on tropical forest conditions is thought to largely account for the variation in annual growth rates in atmospheric CO₂concentrations. In the same ENSO cycle, these rates can vary as much as 225 percent between the El Niño and La Niña phases.

Seasons: Winter, Spring, Summer, Fall

Source: Heimann, Martin and Reichstein, Markus. “Terrestrial ecosystem carbon dynamics and climate feedbacks.” Nature (2008), vol 451. pp. 289-292