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El Niño, the North Atlantic Oscillation and New England Climate; Winter Season Teleconnections and Climate Prediction AIRMAP Climate Change Research Center University of New Hampshire Earth Sciences
The following summary gives a brief description of the relations between New England regional climate and both the El Niño/ Southern Oscillation and North Atlantic Oscillation. For more detailed discussions of possible physical mechanisms responsible for the observed teleconnections please read the following sections of this report. Also, unless otherwise stated, this entire paper focuses almost entirely on winter climate and winter season teleconnection patterns. This is because atmospheric circulation is strongest during this season and, as a result, it is during this time of the year that teleconnection patterns are most easily identified and examined. There is an apparently weak correlation between the Southern Oscillation Index and New England temperature, such that during El Niño winters New England regional temperatures are generally above average. The opposite is true during La Niña events; therefore La Niña conditions are somewhat more favorable for New England regional snowfall. Also, there is some evidence to support the fact that El Niño winters generally produce a greater number of Atlantic coastal storms and fewer storms originating in the Lee of the Canadian Rocky Mountains. Unfortunately, since both of these storm tracks are equally likely to deliver moisture to New England the result is that the SOI has virtually no statistical relationship with precipitation variability in New England. However, another possible source for moisture in New England includes tropical storms and hurricanes and studies have shown that there are a greater number of Atlantic Hurricanes during La Niña events, while the opposite is true during El Niños. Compared with the El Niño/ Southern Oscillation there is a relative abundance of evidence supporting associations between the North Atlantic Oscillation and New England regional climate. One of the most well documented links between the NAO and the climate in the Northeastern United States is related to regional temperature. When the NAO is in its "negative mode" the northeastern United States is often colder than normal. As a result, negative NAO conditions are sometimes associated with greater proportions of snow as opposed to rainfall. The opposite is true during positive NAO winters. Another regional climate variable that is related with the phase of the NAO is regional sea-surface temperatures (SSTs). The association between the NAO and New England regional SSTs is very significant, since regional SSTs are in turn related with precipitation (and streamflows), snowfall and air temperatures across the entire region. Winters with below average SSTs typically have below average temperatures, less precipitation (particularly inland), lower streamflow levels, and more frequent snowfall (particularly in southern coastal regions). 2. Introduction Overview In New England, where the day-to-day weather can appear random and totally beyond prediction, one might guess that accurate seasonal climate forecasting is a near impossibility. However, recent research has made tremendous progress in developing accurate models for many important large-scale processes within the earth’s climate system. Furthermore, teleconnection studies help us understand where our region’s climate fits into the context of the global climate system. The following primer is an overview of our current understanding of teleconnections and New England climate. The main objective is to explain which large-scale climatic processes are the most influential to New England climate and how this knowledge may be used for the purpose of seasonal climate forecasting in the future. Some examples of important questions that the following primer addresses are:
Again, determining physical explanations for extreme climate anomalies of the past may provide insights into how such events could be predicted in the future. Climate Data and Analysis Detailed analysis, of weather observations collected from several locations over the course of many years, is the only way to develop a meaningful understanding of the climate in any region. This requires an extensive network of reliable weather stations continuously documenting daily observations of precipitation, temperature, snowfall, wind speed and direction, atmospheric pressure, and even local sea-surface temperatures (SST). Ongoing efforts to monitor and predict United States climate is lead by the National Oceanographic and Atmospheric Administration’s (NOAA) National Climate Data Center (NCDC) and Climate Prediction Center (CPC). Once quality data are acquired, hourly and daily weather observations can be condensed into monthly, seasonal or annual averages, which are often most useful for identifying significant trends, cycles and changes of climatologic regimes. Still, conclusions drawn from averages of higher resolution data sets must be interpreted cautiously because important information can be lost when data are averaged into arbitrary time steps (like months). It is also important to remember that each climate record itself can only represent one small piece of a much larger, more detailed and complex puzzle. The Coupled Ocean/Atmosphere System Our current knowledge of the Earth’s climate system indicates that there is in fact structure with in it. An atmosphere that once appeared to operate randomly we now know has controlling elements that operate on quasi-periodic cycles; some of which appear to have continued for hundreds and thousands of years. For example, some specific controlling elements include the geographic distribution of: continents, snow-covered regions, sea-ice, and average sea-surface temperatures, all of which play important rolls in governing patterns of regional climate variability around the world. During the first part of the 20th century Sir Gilbert Walker made great strides in atmospheric studies by identifying three major large-scale oscillations in sea-level pressure: the North Atlantic Oscillation the North Pacific Oscillation and the "Southern Oscillation" (Philander, 1990). Each of these "oscillations" is basically an atmospheric pressure seesaw pattern that alternates between two specific locations and varies over periods of weeks, months and years (Rasmusson, 1985). Although Walker may have imagined the influence that the underlying ocean has on atmospheric pressure patterns, it was later that the (now famed) El Niño phenomenon was linked to the Southern Oscillation (Philander, 1990). It is now accepted that these pressure seesaws, with centers of action generally located over the oceans, are closely linked to sea-surface temperature (SST) fluctuations. Furthermore, scientists now exploit the often-predictable lags between ocean and atmosphere dynamics in their efforts to predict climate in many regions around the world http://www.cpc.ncep.noaa.gov It is now accepted that …[atmospheric] pressure seesaws, with centers of action generally located over the oceans, are closely linked to sea-surface temperature (SST) fluctuations. Why Study Teleconnections? Establishing "teleconnections has continued to be the focus of much oceanic and atmospheric research in recent years as we edge closer to understanding how our climate system operates. This is because understanding the dynamics of significant teleconnection patterns has already proven to be very useful for predicting extreme climate conditions (such as flooding or drought) in some regions. For example, regions with significant teleconnections to the El Niño/Southern Oscillation benefit from reasonably accurate seasonal climate predictions, thanks to the predictability of the El Niño/ Southern Oscillation system (Piechota and Dracup, 1999; Cordery and McCall, 2000). Hence, once teleconnection patterns are established, the long-range predictability of regional climate comes to depend more on the predictability of the large-scale climate indices themselves. How are Teleconnections Identified? Teleconnections are identified by using statistical tools to compare indices for regional climate (as expressed in temperature, precipitation, streamflow… etc.) with indices for large-scale atmospheric circulation patterns, such as the ElNiño/ Southern Oscillation or the North Atlantic Oscillation indices. It is important to note that oceanic and atmospheric patterns are not part of a stationary or linear system. The climate system is dynamic, which means that the remote atmospheric and ocean responses to large-scale fluctuations in the climate system will never occur exactly the same way twice. For example, no two El Niño events are exactly alike and therefore the teleconnections related to each event also vary (Diaz and Kiladis, 1992). As a result of the climate system’s non-linearity, the global climatic teleconnections associated with extreme phases of any large-scale atmospheric circulation pattern may not be equal and opposite in all regions. Therefore, it is necessary to not only determine the linear response of a regional climate to the ENSO system, but to also consider each region’s climate during the opposite phases (La Niña or El Niño) independently when investigating ENSO teleconnections (e.g., Hoerling and Kumar, 2000). …no two El Niño events are exactly alike and therefore the teleconnections related to each event also vary. With this in mind, the following section introduces two important large-scale atmospheric circulation patterns that have been shown to be very significant contributors to monthly, seasonal, annual and decadal-scale changes in North American climate. Finally, the apparent role that each of these phenomena play in terms of New England climate, based on previously published work, will be introduced and discussed in some detail. 3. El Niño/Southern Oscillation (ENSO) What are El Niño and La Niña? The terms El Niño and La Niña refer to sea-surface temperature (SST) events in the eastern and central equatorial Pacific Ocean, off the coast of Peru (also called the El Niño region). El Niño represents the warm phase, and La Niña the cold, but these terms have come to represent much more than regional SSTs. These SST anomalies are associated with the " Southern Oscillation ", which is an atmospheric pressure seesaw between Tahiti and Darwin, Australia. Thus, when the sea-level pressure (SLP) in Darwin is below average, the SLP in Tahiti is almost always above average, and visa-versa. The SSTs in the El Niño region are intricately coupled with the atmospheric Southern Oscillation and together they form the El Niño/ Southern Oscillation (ENSO), which, through distinct teleconnection patterns, is held responsible for many significant variations in tropical and extratropical climate (Rasmusson, 1985, Ropelewski and Halpert, 1987). Under "normal" conditions the eastern equatorial Pacific has unusually cold SSTs for this latitude. This is caused by tropical Trade Winds (i.e., easterlies that blow off the coast of Peru) that force equatorial surface waters west and promote the upwelling of cold, nutrient-rich, deep water along the Peruvian coast. Under normal conditions the cold SSTs stabilize the lower atmosphere by inhibiting convection as far west as the central Pacific Ocean (Tahiti) forcing a regional atmospheric high-pressure system (Diaz and Markgraf, 1992) see Figure 1. La Niña conditions can be thought of as an extreme case of this "normal" pattern where the Eastern Equatorial Pacific Ocean surface temperatures are colder than normal, Trade Winds strengthen and the regional high-pressure system becomes more stable, more extensive, and generally more pronounced. In the West Pacific, La Niña events are characterized by above average SSTs, below-normal atmospheric pressure and heavy rains and flooding. Conversely, during El Niño events, the Trade Winds fail and the associated cold upwelling water slows, leading to unusually high SSTs, a persistent low-pressure system, and dramatically increased atmospheric convection in the central and eastern Pacific Ocean (Barry and Chorley, 1998) see Figure 2. In the Western Tropical Pacific, during an El Niño event, Indonesia, Papua New Guinea and Thailand experience unusually low SSTs, high sea-level pressures (SLP), and resulting moisture deficiencies see (Figure 2 for global map of teleconnections related to El Niño). How does an El Niño event have an impact on climate thousands of miles away? or: "Extra-tropical El Niño Teleconnections" To answer this question it is necessary to explain some fundamentals of the Earth’s climate system. Primarily, you need to understand that the difference between the amounts of solar energy that reaches the earth in the tropics verses that reaching the earth in the temperate zones is disproportionately large compared with the differences between surface temperatures in these two regions. This is because the Earth’s oceans and atmosphere constantly disperse heat and moisture away from the tropics to the higher latitudes, providing a more even distribution of the sun’s energy on the earth’s surface. Hurricanes are an example of one mechanism responsible for actively transporting heat and moisture away from the tropics and delivering it (albeit catastrophically) to the higher latitudes. The Hadley Cells are another very important mechanism, responsible for exporting heat and moisture away from the tropics, at work during every season of the year. …the [ENSO] signal that is received in the extra-tropics appears to be most commonly manifest in anomalous Jet Stream configurations and associated shifts in the location of major regional storm-tracks. As a result of extensive climate monitoring and modeling research our understanding of the physics responsible for causing fairly consistent ENSO teleconnections, to extratropical regions thousands of miles away from the equatorial Pacific, is improving. It is generally thought that significant changes in the location of tropical energy sources and sinks (i.e.: the location of cold and warm pools of ocean surface waters) force atmospheric responses that are large enough in-scale to have an impact on the spatial structure of the Hadley Cells. Therefore, a spatial redistribution of tropical convection sends a signal through the oceans and upper atmosphere forcing a global extra-tropical response to tropical forcing. In turn, the signal that is received in the extra-tropics appears to be most commonly manifest in anomalous Jet-Stream configurations and associated shifts in the location of major regional storm-tracks (Hoerling and Kumar, 2000). For a more in-depth description of the ENSO system including interesting educational material related to ENSO: How Does ENSO affect New England Climate? The atmospheric response over North America to ENSO warm and cold events is strongest in Northern Hemisphere winter and weakest in the summer (Hoerling and Kumar, 2000). Also, general atmospheric circulation is strongest during the winter and the highest levels of atmospheric pressure variability occur during this season; therefore most ENSO teleconnection studies examine winter climate variability. Based on storm track data from winters between 1951-1997, Kunkel and Angel (1999) found an increased frequency of Gulf-of-Mexico-generate cyclones and a relative decrease in frequency of Canadian Shield-generated cyclones during El Niño events see Figure 3. The opposite was true during La Niña winters. Rogers (1984) noted related sea-level pressure (SLP) patterns, with generally high pressures over the Canadian Shield region and relatively low pressures in the Gulf of Mexico/ Cape Hatteras region during El Niño winters. Also, Hirsch et al. (2001) note a marked increase in East coast winter storms during El Niño winters again, see Figure 3. Despite the relevance of the above mentioned storm tracks to winter weather in New England (Ludlum, 1976), results from a number of studies indicate that no clear link exists between ENSO and New England climate (e.g.: Ropelewski and Halpert, 1986, 1987; Kahya and Dracup, 1993; Dracup and Kahya, 1994; and Piechota and Dracup, 1996). Perhaps a decline in storm activity along one track is "replaced" by storms from another source, with a net result of no consistent relationship between in New England precipitation and ENSO. There is some evidence that New England experiences above normal temperatures during El Niño winters see Figure 2 however the statistical strength of this association is somewhat weak. …a decline in storm activity along one track is "replaced" by storms from another source, with a net result of no consistent relationship between in New England precipitation and ENSO. One study, however, has identified a significant (negative) linear relationship between the Southern Oscillation Index (SOI) with regional precipitation in southern New England, during the month of April (Richman et al., 1991). Richman et al’s (1991) study concluded that negative SOI (El Niño) events were concurrent with above average April precipitation in the Mid-Atlantic States, including (particularly southern) New England. These results (as well as the ENSO-temperature association) can be confirmed, with longer and more recent data sets, at the following web site: http://www.cdc.noaa.gov/USclimate/Correlation/ The evidence that ENSO has significant influences on extratropical atmospheric circulation is well established (Hoerling and Kumar, 2000) however the lack of any substantial and consistent ENSO "signal" in New England regional climate variability has been made evident through a great deal of past research and analysis. This does not mean that a strong El Niño event could not cause an exceptionally warm New England winter with a great many nor’easters and well above average precipitation. It does mean that any seasonal forecast, based on the phase of ENSO alone, is unlikely to be a very accurate predictor of conditions in New England specifically. For current ENSO-related climate predictions and forecasts: http://www.pmel.noaa.gov/tao/elNiño/forecasts.html http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ http://www.cpc.ncep.noaa.gov (follow link to seasonal "outlook") 4. North Atlantic Oscillation (NAO) What is the NAO? The NAO is the dominant mode of SLP variability in the North Atlantic region (Hurrell, 1995). The NAO index, defined as the SLP difference between the Azores High and the Icelandic Low (Rogers, 1984), describes the steepness of a north-south atmospheric pressure gradient across the North Atlantic Ocean see Figure 4. Winter weather patterns throughout the North Atlantic basin have historically been affected by changes in the NAO (Rogers and Van Loon, 1979; Hurrell and Van Loon, 1997). When the NAO changes between its modes of variability, the North Atlantic Ocean experiences changes in wind speed and direction, which affects heat and moisture transport to the surrounding continents and seas (Hurrell et al., 2001). The NAO index, defined as the SLP difference between the Azores high and the Icelandic low, describes the steepness of a north-south atmospheric pressure gradient across the North Atlantic Ocean. North Atlantic Oscillation Teleconnections How do changes in the atmospheric pressure gradient between Iceland and the Azores make a significant index for climate in regions remote to those islands? Like the "Southern Oscillation" there is an atmospheric pressure seesaw between Iceland and the Azores. Hence, if the atmospheric pressure in Iceland is low, then the atmospheric pressure in the Azores is usually high (and visa-versa). While the sub-polar (Icelandic) Low and sub-tropical (Azores) High characterize the average conditions, deviations from this mean can result in dramatic shifts in Northern Hemisphere climate. Figure 4 shows the North Atlantic region with its neighboring continents and NAOteleconnection related patterns. As you can see from the top panel Figure 4, when the NAO index is positive there is a strong north-south atmospheric pressure gradient across the North Atlantic region. With a strong sub-polar low (counter-clockwise atmospheric motion in the northern hemisphere) to the north and a strong subtropical high (clock-wise atmospheric motion in the northern hemisphere) to the south, the westerlies flow unrestrained across the North Atlantic. This drives warm and moist maritime air derived from westerlies blowing over the Gulf Stream into northern Europe, bringing them relatively mild winters. When the NAO index is negative bottom panel of Figure 4 the character and position of the North Atlantic Jet Stream is significantly different; this causes a drastic reorganization of regional temperatures and storm tracks. For example: during negative NAO winters Northeastern Europe and Scandinavia has been known to receive well-below-normal temperatures (2-4° C) and significantly less precipitation (50% to 75% less) than during positive NAO winters (Kushnir, 1999). So, a positive NAO is associated with strong westerlies and a negative NAO is linked with a reorganization of the Jet Stream and associated changes in regional temperatures and storm tracking. Since Europe is "downstream" of the centers of action for the NAO it is a somewhat simpler matter to explain the physics of how and why the NAO is significant to the climate of that region. Never the less, recent studies have shown that the NAO is strongly related to climate in many other regions of the Northern hemisphere including the (upwind) East Coast of the United States. …a positive NAO is associated with strong westerlies and a negative NAO is linked with a reorganization of the Jet Stream and associated changes in regional temperatures and storm tracking. How does the NAO affect New England climate? Due to the presence of prevailing westerlies, North Atlantic climate variability is often wrongly assumed to have little or no affect on the climate of North America. However, Figure 5 illustrates some of the more important known teleconnections between the NAO, related coastal SSTs, and New England regional climate. Research has found significant associations between the NAO and New England regional tropospheric airflow (Yarnal and Leathers, 1988; Hurrell, 1996; Hartley and Keebles, 1998; and Shabbar et al., 2001), storm track patterns (Rogers, 1990; Hartley and Keebles, 1998), snowfall variability (Hartley and Keebles, 1998), tree-ring chronologies (Cook et al., 1998), streamflow variability (Bradbury et al., 2001) and coastal SSTs (Rogers and van Loon, 1979). While most of these studies focus on winter season climate variability, Rogers and van Loon (1979) and Rogers (1990) have shown that some climate anomalies related to the phase of the winter NAO (SST and SLP) show persistence into the following seasons, suggesting that spring and summer conditions in NE may also be influenced by the phase of the winter NAO. A conceptual model describing our current understanding of how the NAO relates with NE climate involves the direct and indirect means by which the NAO influences regional atmospheric flow. Figure 6 and Figure 7 illustrate the general climatological relationships between mid-tropospheric flow and regional precipitation and temperature. For example, Figure 6 illustrates the general relationship between mean mid-tropospheric flow and precipitation. However, while the mean East Coast trough position, and associated precipitation regimes, is largely a function of geography, the character and location of the trough may vary depending on many factors. Recent research has shown that east-west variability in the location of the East Coast trough can depend on regional SSTs and the phase of the NAO (Bradbury et al., 2001b). Recent research has shown that east-west variability in the location of the East Coast trough can depend on regional SSTs and the phase of the NAO. When the NAO index is negative, the Icelandic Low is abnormally high and located farther to the southwest of Iceland see Figure 4, lower panel. This weakened low is associated with frequent North Atlantic blocking , and probably contributes to an enhanced pressure trough in the East coast region Figure 4 and Figure 5. Under these conditions, two primary weather patterns result see Figure 5, upper right panel. 1) A deeper trough in the Jet Stream allows polar air masses to make their way further south than normal, resulting in colder regional air temperatures. 2) Coastal storms luding nor’easters) tend to increase in frequency (Jones and Davis, 1995), which, depending on the average storm track orientation (and temperatures), can deliver above average snow to NE (Hartley and Keebles, 1998). However, a negative NAO is also associated with eastward shifting of the East Coast trough, and if the trough is shifted far enough east the bulk of the coastal storms may miss the coast and track over the ocean. Residents of the New England region know that this is not unusual; weather forecasters frequently have difficulty predicting the track and impact that nor’easters will have. This is partly because coastal storms generally follow two main tracks 1) along the Atlantic seaboard and 2) well away from the New England coast: along the northern edge of the Gulf Stream see Figure 8 ( Colucci, 1976). There is evidence that above normal sea-surface temperatures in the Atlantic coastal region, north of the Gulf Stream, contributes to an increased occurrence of coastal storms reaching the New England region (Colucci, 1976; see Figure 5 left panels. This may prove to be very useful for the purpose of predicting future wet and dry periods since SSTs tend to be very persistent. For example, studies have shown that the general sign (warm or cold) of New England regional SSTs for an entire winter season (December — March) is often evident by December (Hartley, 1996). …above normal sea-surface temperatures in the coastal region north of the Gulf Stream contributes to an increased occurrence of coastal storms reaching the New England region. This may well prove to be very useful for the purpose of predicting future wet and dry periods since SSTs tend to be very persistent. Also of interest, and import for New England water resources, is the relationship between the NAO and streamflow at many inland sites in New England. A correlation study between the NAO and New England streamflow revealed significant associations between them Figure 9. When the NAO is positive, more streamflow is generally observed. The opposite is true for negative NAO months. It is possible that the increase in snowfall associated with negative NAO winters is one cause for the teleconnection observed between the NAO and New England streamflow. For example, if there is an increase in snowfall, accompanied by persistent cold temperatures, a greater amount of precipitation is expected to remain in the snow pack longer, as compared to winters when large snowfall events are less frequent. Consequently, if a large portion of the snow pack remains through March then streamflows for the average winter (December through March) would be lower under these conditions. Negative NAO and Drought in New England There is also significant evidence supporting a relationship between persistent negative NAO "events" and drought in NE. Namias (1966) attributed the persistence of the 1960s drought event to persistent below-average New England regional air and sea-surface temperatures, as well as an eastward-displaced trough in annually averaged atmospheric pressure fields. As should be apparent from the preceding discussion, these conditions are associated with negative NAO conditions. It is likely that well-below-normal SSTs persisted into the springs and summers following a series of negative NAO winters, causing a general decrease in coastal storm tracking through New England (e.g., Figure 8, storm track #1) and the resulting moisture deficiencies that characterized the early 1960s. The significance of the association between regional SSTs and New England winter streamflow and precipitation is illustrated in Figure 10 and Figure 11, respectively. …well-below-normal SSTs persisted into the springs and summers following a series of negative NAO winters, causing a general decrease in coastal storm tracking through New England and the resulting moisture deficiencies that characterized the early 1960s. Multi-Annual to Decadal associations between the NAO and New England Climate Further studies of New England streamflow and the winter NAO reveals their strong decadal-scale association when both records are smoothed Figure 12. Also, Figure 13 illustrates the strong interrelationships between the NAO, regional SST, streamflow and precipitation, suggesting that regional SST and the NAO are inseparable when considering the principal controls on long-term climate variability in New England. A physical explanation for the multi-annual climatological association between the NAO and NE SSTs is as follows. During positive NAO winters there is a strong pressure gradient and vigorous westerly airflow in the North Atlantic region. The high winds lead to evaporation in the surface waters and cooler-than-average SSTs in the Labrador Sea. This triggers regional thermohaline processes between the Labrador Sea and the Labrador Shelf that can significantly weaken the density-driven long-shore southward current (the Labrador Current) the following summer. The opposite is true during negative NAO winters; weak westerlies over the Labrador Sea promote a stronger Labrador Current, which results in cooler SST in the New England region 6-12 months later Figure 5 (lower panels) (Rossby and Benway, 2000). Also, the phase of the winter NAO is also a good predictor of the latitude of the Gulf Stream 1-2 years in advance (Taylor and Stephens, 1998). Recent studies have proposed that the strength of the Labrador Current may play an important role in controlling the relationship between the NAO and the north-south position of the Gulf Stream (Rossby and Benway, 2000). The lag between winter NAO conditions and New England regional SSTs coupled with the ocean’s "memory" for previous seasons conditions (due to its high heat capacity) provides a reasonable explanation for the observed multi-annual association between the NAO and NE regional SST. Also, decadal-scale shifts in NAO variability promote significant year-to-year persistence in New England regional SSTs, which, as outlined above, is likely to play an important role governing seasonal to decadal-scale climate change in New England. Prediction models for the NAO have been best at forecasting multi-annual trends in North Atlantic climate system (Sutton and Allen, 1997; Griffies and Bryan, 1997; Rodwell et al., 1999). Hence, to the extent that the NAO proves to be a predictable climate index it may also become an important tool for forecasting annual and multi-annual climate change (particularly drought) in NE. …to the extent that the NAO proves to be a predictable climate index it may also become an important tool for forecasting annual and multi-annual climate change (particularly drought) in NE. 5. More teleconnection and climate prediction related web sites: Do your own teleconnection investigation: http://www.cdc.noaa.gov/USclimate/Correlation/ For expert weekly drought assessments and seasonal outlooks: http://www.cpc.ncep.noaa.gov/products/ expert_assessment/drought_assessment.html For an overview of the principal Northern Hemisphere teleconnection patterns: http://www.cpc.ncep.noaa.gov/data/teledoc/telecontents.html For a wide range of interesting weather essays and tutorials: http://www.intellicast.com/DrDewpoint To learn about expert hurricane monitoring and prediction: http://www.aoml.noaa.gov/hrd CLIVAR; an international research program investigating climate variability and predictability: http://www.clivar.org 6. Glossary: Also see: http://www.esig.ucar.edu/elnino/glossary.html Azores high: particularly during the winter, a somewhat stationary "sub-tropical High" pressure cell generally sits over the Azores and Portugal, making for a consistently stable atmosphere throughout the whole of this region. Blocking: in the mid-latitudes blocking is an anomaly in the westerlies where the standing waves that characterize normal atmospheric flow increase in amplitude to the point that regional flow can become reversed (easterly); associated with radical dislocation of the polar front and Jet Stream. El Niño: anomalously warm sea-surface temperatures in the eastern Equatorial Pacific Ocean; the "warm phase" (or "ENSO warm event") in the oceanic component of the El Niño/ Southern Oscillation; opposite of La Niña conditions (see text for details). El Niño region:generally referring to a geographic region in the equatorial Pacific Ocean just west of the coast of Peru Extratropical: generally referring to all earth systems north of the Tropic of Cancer (23° N) and south of the Tropic of Capricorn (23° S). Gulf Stream: a warm North Atlantic Ocean current that travels northward between Florida and the Bahamas then diverges west-northwest in the Cape Hatteras region, eventually becoming the North Atlantic Current. Hadley Cell: low-latitude surface air movement toward the equator that with heating rises vertically, with poleward movement in the upper atmosphere where, with cooling the air descends somewhere in the subtropics.This forms a convection cell that dominates the dynamics of tropical and sub-tropical climates. Heat Capacity: the proportionality constant between an amount of heat and the change in temperature that this heat produces in the object. For example, the heat capacity of the ocean is greater than the heat capacity of the atmosphere, which is to say that it would require a great deal more heat energy to raise the temperature of the entire ocean by 1° C than it would to raise the same volume of air by 1° C. High-pressure system: usually associated with fair weather; the opposite of a low-pressure system; in the northern hemisphere air descends at the center of highest pressure where wind at the surface diverging in a clockwise direction. Icelandic low: particularly in the winter, Iceland (and the surrounding ocean) generally experiences a great deal of cyclone activity.Consequently, daily atmospheric pressure values, averaged over a month or a season, typically reveal a "sub-polar Low" in the North Atlantic near Iceland. Jet Stream: rapid upper-air flow from west to east across the upper to mid latitudes over the polar front (the boundary between warm air from the subtropics and cold air from the polar regions). La Niña: anomalously cold sea-surface temperatures in the eastern Equatorial Pacific Ocean; the "cold phase" (or "ENSO cold event") oceanic component of the El Niño/ Southern Oscillation; opposite of El Niño Conditions (see text for details). Low-pressure system: usually associated with cloudiness and storm weather; the opposite of a high-pressure system; in the northern hemisphere, air ascends at the center of lowest pressure where surface wind converges in a counter clockwise direction. Non-linear: referring to processes or relationships that are multi-dimensional in nature. Nor’easter: coastal low-pressure systems that track northeastward along the Atlantic seaboard; characterized by strong northeasterly winds off the Atlantic that often produce heavy amounts of precipitation, high surf, and coastal erosion; coastal storms with origins generally in the tropics and subtropics. Sea-level pressure (SLP): a measure of the force exerted on the earth, at sea level, by air molecules in the overlying atmosphere.Rapid changes in SLP usually indicate a transition from one air mass to another (frontal passage) or the passage of a storm. Sea-surface temperature (SST): a measurement of ocean water temperature taken at the air-sea interface. Southern Oscillation: a sea-level pressure seesaw between Tahiti and Darwin, Australia; the atmospheric component of the El Niño/ Southern Oscillation (ENSO). Southern Oscillation Index (SOI): a monthly index for the sea-level pressure see-saw portion of the El Niño/ Southern Oscillation system.This is a widely used index developed to monitor the Southern Oscillation using the difference between sea-level pressures at Darwin, Australia, and Tahiti. Large negative values of the SOI indicate an El Niño event, and large positive values indicate a La Niña event. Teleconnection: Significant correlations between fluctuations in meteorological parameters at widely separated points on the earth.The atmospheric pressure seesaw between Tahiti and Darwin, Australia (a.k.a. the Southern Oscillation) is a classic example of a teleconnection. Thermohaline: related to temperature and salinity, often referring to oceanic processes. Trade Winds: tropical easterly winds; winds blowing toward the west throughout most of the tropical regions of the globe. Walker, Sir Gilbert : A British Meteorologist assigned to the Indian Meteorological Department (in 1908) to investigate the nature and possible causes of year-to-year fluctuations in the Indian Monsoon. Walker coined the term "Southern Oscillation" and made countless other contributions to the field of meteorology and climate research, largely through global-scale teleconnection analyses. 7. Literature Sited: Barlow, M., S. Nigam, and E.H. Berbery, 2001. ENSO, Pacific Decadal Variability, and U.S. Summertime Precipitation, Drought and Stream Flow. Journal of Climate 14: 2105-2128. Barry, R.G. and R.J. Chorley, 1998. Atmosphere, Weather, and Climate, 7th edition, Routledge, London. Bradbury, J.A., S.L. Dingman, L.D. Meeker, B. D. Keim, 2001a: Relations of Large-Scale Atmospheric Circulation Patterns to Streamflow Variability in New England, EOS. Trans. AGU, 82(20), Spring Meet. Suppl., Abstract H22D-05. Bradbury, J.A., B.D. Keim, and C.P. Wake, 2001b: U.S. East Coast Pressure Trough Indices and New England Winter Climate Variability. Eos Trans. AGU, 82(47), Fall Meet. Suppl., Abstract A12A-0038, 2001 Colucci, S.J., 1976. Winter Cyclone Frequencies over the eastern United States and adjacent western Atlantic, 1964-1973. Bulletin of the American Meteorological Society 57(5), 548-553. Cordery, I., and M. McCall, 2000. A model for forecasting drought from teleconnections. Water Resources Research 36(3): 763-768. Diaz, H.F. and V. 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Dracup, 1999. Long-Range Streamflow Forecasting Using El Niño-Southern Oscillation Indicators. Journal of Hydrologic Engineering 4(2): 144-151. Philander, G. S., 1990. El Niño, La Niña, and the Southern Oscillation. Academic Press, Inc., New York. Taylor, A. H. and Stephens, J.A. 1998. The North Atlantic Oscillation and the Gulf Stream. Tellus 50A, 134-142. |
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