Winter 2023-24 Outlook

Local Winter Outlook:

Winter 2023-24 Temperature Outlook:

Locally, the odds have been tilted toward warmer-than-normal (not just a tenth of a degree warmer than normal, but among the warmest third of the winters from 1991-2020) in northeast Iowa, southeast Minnesota, and western Wisconsin. The probabilities were shifted to 40 to 50%.

Why warmer-than-normal?

• Favorable things for a warmer-than-normal winter.
  1. This winter is expected to be influenced by a moderate (92% chance during winter) to strong (62% chance during this winter) El Niño. There is even a 1 in 3 chance that this El Niño could rival the 3 strongest El Niño since 1949-50 (1982-83, 1997-98, and 2015-16).
     • La Crosse, WI:
       • Super El Niños (an average ONI of 2.0°C or warmer over the DJF 3-month period):  There have been 3 of these since 1949-50 and all 3 were among the warmest third of winters.
       • Strong El Niños (an average ONI of 1.5-1.9°C over the DJF 3-month period):  There have been 4 of these since 1949-50. 1 was among the warmest third, 2 was near normal, and 1 was among the coldest third of winters.
       • Moderate El Niños (an average ONI of 1.0-1.4°C over the DJF 3-month period): There have been 5 of these since 1949-50. 2 were among the warmest third and near normal, and 1 was among the coldest third of winters.
       • Moderate to super El Niños statistics: A total of 12 El Niños.  6 (50%) of these winters were among the warmest third, 4 (33.3%) of these winters were near normal, and just 2 of these winters (16.7%) were among the coldest winters.  
     • Rochester, MN:
       • Super El Niños (an average ONI of 2.0°C or warmer over the DJF 3-month period):  There have been 3 of these since 1949-50 and all 3 were among the warmest third of winters.
       • Strong El Niños (an average ONI of 1.5-1.9°C over the DJF 3-month period):  There have been 4 of these since 1949-50. 2 were among the warmest third, and there was 1 near normal, and another 1 among the coldest third of winters.
       • Moderate El Niños (an average ONI of 1.0-1.4°C over the DJF 3-month period): There have been 5 of these since 1949-50. 2 were among the warmest third and 3 were near normal.
       • Moderate to super El Niños statistics: A total of 12 El Niños.  7 (58.3%) of these winters were among the warmest third, 4 (33.3%) of these winters were near normal, and just 1 of these winters (8.3%) were among the coldest winters.
  2. Since 1982-83, there has even been a trend for all El Niños to be either warmer than normal (8 out of 13) or near normal (4 out of 13) during winter. Only the 2009-10  El Niño (moderate-strong) winter was among the coldest third.
  3. Very few winters have been in the coldest third over the past decade (optimal climate normal). Both La Crosse and Rochester have only had 1 winter (2013-14) in the coldest third. 
  4. The CFS version 2 climate model is also favoring warmer-than-normal temperatures for this winter.
  5. ECMWF is forecasting slightly warmer than normal temperatures this winter. Its temperature anomalies are currently around 1°C warmer than normal and even this would result in a near-normal winter.
  6. In addition to the ongoing El Niño, there is a notable atmospheric response to a strengthening positive Indian Ocean Dipole event. These are both likely to curtail the organization and associated impacts of MJO activity for the foreseeable future.
  7. Eurasian snow cover remains below average, so there is currently no signal to suggest that it will adversely impact the winter polar vortex.
• Unfavorable things for a warmer-than-normal winter. 
  1. We may see short periods of colder-than-normal conditions from Madden Julian Oscillation (MJO) and Sudden Stratospheric Warming events, but the brevity of these events would still likely result in near to above-normal temperatures.

Winter 2023-24 Precipitation Outlook:

Locally, there are equal chances that this winter will be drier, near, and wetter than normal. 

Why equal chances?  Mixed signals. Wetter and near-normal precipitation seems to have a better probability of occurrence over drier than normal, but due to uncertainty, the forecast was left as equal chances.  

• Things favoring a wetter-than-normal winter.
  1. During the past 15 years (optimal climate normal), La Crosse, WI has had 8 winters that were wetter than normal (4 in the top 10 for wettest), 4 winters were near-normal precipitation, and 3 winters were among the driest third of winters.  Rochester, MN has had 10 winters that were wetter than normal winters (5 among the ten wettest - this includes the wettest which was last winter), 4 winters with near-normal precipitation, and only 1 winter that was drier than normal (2019-20).
  2. 4 out of the last 5 El Niños have been wetter than normal. However, this is a small sample size, so whether this is an actual trend is uncertain.
  3. Strong El Niños have a tendency to be wetter than normal in the Mississippi River Valley. La Crosse and Rochester have seen 5 in the wettest third, 1 near normal, and 1 in the driest third. All three of the strongest El Niños (anomalies in the 3.4 Niñ​o Region of 2°C or greater than normal during meteorological winter) were among the wettest third.​
  4. The CFS version 2 has been trending wetter than normal in February, but other months look to be near normal.
• Things favoring a drier-than-normal winter and equal chances.
  1. Precipitation trends since the spring of 2023: Areas south of Interstate 90 have been very dry since March. Precipitation deficits range from 6 to 15". This has resulted in the development of moderate (D1) to extreme (D3) drought.
  2. Inconsistent signals in the dynamic models. Most favor near-normal precipitation.
  3. Statistical tools are trending toward wetter-than-normal just west of our area and near-normal for the Upper Mississippi River Valley

Seasonal snow has been generally near to below normal during the 13 moderate to strong El Niños since 1949-50.

• In La Crosse WI, 4 were in the lowest third, 7 were near-normal, and just 2 were among the snowiest third. 
• In Rochester MN, 2 were in the lowest third, 7 were near-normal, and 4 were among the snowiest third.  

Snowstorms will occur at times this winter. However, the frequency, number, and intensity of these events cannot be predicted on a seasonal timescale.

What is El Niño?:

What is El Niño?

El Niño literally means "the little boy." in Spanish.  South American fishermen first noticed periods of unusually warm water in the Pacific Ocean in the 1600s. The full name they used was El Niño de Navidad, because El Niño typically peaks around December. El Niño is sometimes referred to as simply "a warm event" or "a warm episode".  El Niño refers to abnormally warm water temperatures across the central and eastern equatorial waters (5°N-5°S, 120°-170°W)] of the Pacific Ocean.  El Niños typically occur every 2 to 7 years. They don't occur on a regular schedule.

During El Niño, trade winds weaken. Warm water is pushed back east, toward the west coast of the Americas.

El Niño can affect our weather significantly. The warmer waters cause the Pacific jet stream to move south of its neutral position. With this shift, areas in the northern U.S. and Canada are dryer and warmer than usual. But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding.

El Niño also has a strong effect on marine life off the Pacific coast. During normal conditions, upwelling brings water from the depths to the surface; this water is cold and nutrient-rich. During El Niño, upwelling weakens or stops altogether. Without the nutrients from the deep, there are fewer phytoplankton off the coast. This affects fish that eat phytoplankton and, in turn, affects everything that eats fish. The warmer waters can also bring tropical species, like yellowtail and albacore tuna, into areas that are normally too cold.

How does El Niño Develop?
 

During El Niño, the warm pool of water usually in the western Pacific Ocean begins to slosh to the east. As this warm pool moves east, the rising branch of the Pacific cell of the Walker Circulation follows, and the rest of the Walker Circulation gets shifted around. Instead of a strong rising motion over the Maritime Continent, upward flow weakens, and anomalous sinking motion occurs. The usual rising, moist air over South America is replaced by another anomalous sinking branch. The reversal of the circulation generally means that less rainfall than average dominates in these two areas during El Niño. In contrast, increased rising motion enhances rainfall over the central Pacific Ocean and even parts of equatorial eastern Africa.

Generalized Walker Circulation (December-February) anomaly during El Niño events, overlaid on a map of average sea surface temperature anomalies. Anomalous ocean warming in the central and eastern Pacific (orange) helps to shift a rising branch of the Walker Circulation to the east of 180° longitude, while sinking branches shift to over the Maritime continent and northern South America. NOAA Climate.gov drawing by Fiona Martin. 

How long does El Niño typically last?
 

El Niño episodes typically last 9-12 months. They both tend to develop during the spring (March-June), reach peak intensity during the late autumn or winter (November-February), and then weaken during the spring or early summer (March-June). The longest El Niño lasted 16 consecutive 3-month seasons from Autumn 2014 (September through November) into the Spring of 2016 (March through May). Generally, El Niño occurs more frequently than La Niña.

Global El Niño Impacts:

During El Niño winters (December through February), warmer-than-normal temperatures are typically found across western and central Canada, from India east into the Maritime continent, Japan, southeast Australia, and parts of eastern Brazil. Colder-than-normal temperatures are typically found across the southern U.S. and parts of northern Mexico. Wetter-than-normal conditions are typically seen across the southern U.S., northern Mexico, southern parts of South America, along the equator from east of the Maritime continent east to South America, southeast China, and east-central Africa. Drier-than-normal conditions are typically seen from Hawaii west to the Maritime continent and then southeast through northern Australia to Fiji and French Polynesia, and southern Africa.

U.S. El Niño Impacts:

There has been a fair amount of variability in the winter temperature and precipitation patterns during El Niño, but also that there are some clear tendencies for above or below-normal temperature or precipitation in some regions. 

Footnotes:

1. The terciles, technically, are the 33.33 and 66.67 percentile positions in the distribution. In other words, they are the boundaries between the lower and middle thirds of the distribution, and between the middle and upper thirds. These two boundaries define three categories: below-normal, near-normal, and above-normal. In the maps, the CPC forecasts show the probability of the favored category only when there is a favored category; otherwise, they show EC (“equal chances”). Often, the near-normal category remains at 33.33%, and the category opposite the favored one is below 33.33% by the same amount that the favored category is above 33.33%. When the probability of the favored category becomes very large, such as 70% (which is very rare), the above rule for assigning the probabilities for the two non-favored categories becomes different. 

El Niño Winter Temperatures

Author: Rebecca Lindsey (October 24, 2018)

Midwest El Niño Winter (DJF) Average Temperature Departures (25 Winters since 1949-50)

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1986-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
2018-2019

El Niño Winter Precipitation

Author: Rebecca Lindsey (October 24, 2018)

Midwest El Niño Winter Winter (DJF) Precipitation Departures (25 Winters since 1949-50)

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1986-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
2018-2019

El Niño Seasonal Snow

Authors: Michelle L'Heureux & Brian Brettsheider (October 26, 2023)
Collecting historical measurements of snow is a tricky endeavor, fraught with measurement errors, so creating a dataset of sufficient quality for climate studies is hard. But, Brian Brettschneider, who is the NWS Climate Service Program manager for the Alaska region and a clever, outside-the-box thinker, realized that the new ECMWF ERA5 reanalysis dataset may be the ticket (footnote #1). The jet stream tends to extend eastward and shift southward during El Niño winters. You can think of the jet stream as a river of air, which carries more moisture and precipitation along the southern tier of the United States during El Niño. As a result, it is not surprising to see a stripe of increased snowfall (blue shading) over the southern half of the country. Obviously, snowfall is limited in its southernmost reaches because it needs to be cold enough to snow, so the effects are strongest in the higher and colder elevations of the West. To the north, however, there is a reduction in snowfall (brown shading), especially around the Great Lakes, interior New England, the northern Rockies, and the Pacific Northwest, extending through far western Canada, and over most of Alaska. In fact, El Niño appears to be the great snowfall suppressor over most of North America.
	Figure 1: Snowfall during all El Niño winters (January-March) compared to the 1991-2020 average (after the long-term trend has been removed). Blue colors show more snow than average; brown shows less snow than average. NOAA Climate.gov map, based on ERA5 data from 1959-2023 analyzed by Michelle L'Heureux.

In the map below, over many regions, the anomalies become stronger (anomaly = difference from the long-term average), which makes sense because El Niño affects the climate. Stronger El Niño events tend to land a larger punch on our atmosphere, thus increasing the chance of seeing expected El Niño impacts.

While the maps above may excite or depress you depending on your situation and snow preferences, it is very important to recognize that the map is the showing the average of all winters with El Niño (footnote #2). Relying on the average is a bit dangerous because a few heavy snowfall winters can give the impression that most winters are above average. This is why it’s important to recognize there can be large variations from winter to winter.

Below is a map showing a count of El Niño winters: Here, we ask how many of the 13 moderate-to-strong El Niño winters had below-average snowfall. If it is in red shading, that means most winters had below-average snowfall. The deepest reds mean almost all past winters had below-average snowfall (black indicates no snowfall at all, which makes sense if you’re sitting on a beach in South Florida). If it is in grey shading, that means most moderate-to-strong winters had above-average snowfall.

Another major caveat related to these maps is they are just based on snowfall during El Niño. When the long-term trends are removed, there is also a trend in snowfall, and it looks like this over North America during January-March.

Unsurprisingly, because of climate change, over most of the contiguous United States, we have trended toward less snowy winters. This doesn’t mean that it never snows, or we cannot get big snowstorms (footnote #3), but that snowfall has gradually trended downward over time. In contrast, wintertime snowfall may have actually increased somewhat over time over the colder northern latitudes of Alaska and parts of Canada (this trend reverses in the spring; see footnote #4). Why would that be the case? Well, if you think about it, the warming of our planet allows the air to hold more moisture. If the atmospheric circulation allows for it, then that moisture can be wrung out of the air and precipitate. Snowfall also depends on the air temperature remaining below the freezing point. At more northern latitudes, despite warming air temperatures, it still remains cold enough in the winter to fall as snow. But there is no such luck in more southern locations which are often closer to the freezing point. There, the tendency toward warmer winters is a snow killer. 

The predictability of seasonal snowfall may be somewhat similar to precipitation in that one or two big events can dramatically affect the seasonal average. Thus, in general, the expected prediction skill is likely to be lower than for temperature. However, because temperature also plays an important role in snowfall, some predictability is likely nonetheless. And like for seasonal temperature and precipitation, knowing the state of ENSO is a pretty reasonable place to start.

Midwest El Niño Seasonal Snow Departures (25 Winters since 1949-50)

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1986-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
2018-2019

Footnotes:

1. We have to be careful to not take any one dataset literally, but this ECMWF ERA5 data seems to pass a few sniff tests. Sniff test #1 was “Does ERA5 snowfall reproduce the winter pattern of snowfall made with other datasets?” The answer, at least when compared with winter 2022-23, is yes. Sniff test #2 was “Does ERA5 snowfall reproduce the historical ENSO pattern that is found within other datasets?” Here again, the answer is yes, we were able to reproduce ENSO composite maps that were made with the Rutgers gridded snow data in this older ENSO blog post. Sniff test #3 was compared with our old ENSO snowfall composites made from an even older (not quality-controlled) station-based dataset that has been discontinued. With that said, ERA5 is a newer dataset, it is “reanalysis,” which means that a very short-range weather model is used to produce snowfall from in situ observations (from the ECMWF website, it outputs the “mass of snow that has fallen to the earth’s surface”). Essentially a reanalysis is predicting what observed snowfall would have looked like based on past observational inputs from satellites, stations, buoys, and other observing systems. Therefore, we recommend you treat some of the finer details with a healthy degree of suspicion and try to corroborate them in other datasets. Hopefully, this blog post will motivate the creation of additional snowfall datasets and scientists will explore how well ERA5 compares with these other snowfall measurements.
    
2. Brian Brettsheider emphasizes that composites (average historical maps during El Niño) are retrospectives and they are not a forecast. A forecast takes into account conditions beyond just El Niño, such as long-term climate trends, soil moisture, sea ice, and other global boundary forcings.
    
3. In fact, because a warmer atmosphere carries more moisture, there is published evidence that extreme snowfall events can intensify as a response to global warming (e.g. O’Gorman, P., 2014: Contrasting responses of mean and extreme snowfall to climate change. Nature, 512, 416–418).
    
4. This pattern of snow trends drastically changes if you look at the shoulder seasons, say April-June, which is warmer even in those northern latitudes. Rebecca took a look at this in this climate.gov article on spring snow cover in the Northern Hemisphere. In this season, trends are toward less snow cover over Alaska and western Canada. The ERA5 snowfall trends in April-June also reproduce the same features.

El Niño AWSSI

Author: Midwestern Regional Climate Center

CONUS AWSSI during El Niño (25 Winters since 1951-52)

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1986-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
2018-2019

La Crosse, WI AWSSI during El Niño (25 Winters since 1951-1952)
 

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1987-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
	The AWSSI Chart for 2023-2024 can be found at https://mrcc.purdue.edu/ research/AWSSI/ chart?stn=LSE
2018-2019	2023-2024

Rochester, MN AWSSI during El Niño (25 Winters since 1951-1952)
 

1951-1952	1953-1954	1957-1958	1958-1959

1963-1964	1965-1966	1968-1969	1969-1970

1972-1973	1976-1977	1977-1978	1979-1980

1982-1983	1986-1987	1987-1988	1991-1992

1994-1995	1997-1998	2002-2003	2004-2005

2006-2007	2009-2010	2014-2015	2015-2016
	The AWSSI Chart for 2023-2024 can be found at https://mrcc.purdue.edu/ research/AWSSI/ chart?stn=RST
2018-2019	2023-2024

Additional Information

• Information Sheet (2-page) - pdf
• The Accumulated Winter Season Severity Index (AWSSI)
• Barbara E. Mayes Boustead, Steven D. Hilberg, Martha D. Shulski, and Kenneth G. Hubbard. Journal of Applied Meteorology and Climatology, Vol. 54, No. 8, August 2015: 1693-1712. 
• Abstract | Full Text | PDF (2545 KB)
• An Accumulated Winter Season Severity Index. Barbara Mayes Boustead, NOAA/NWS, Valley, NE; and S. Hilberg, M. D. Shulski, and K. G. Hubbard. Presented at 93rd Annual Meeting of the American Meteorological Society, January 2013. 
• An Accumulated Winter Season Severity Index (AWSSI). Webinar for the NWS Central Region, February 2017. 
• For more information, please contact Barb Mayes Boustead (barbara.mayes@noaa.gov) at the National Weather Service or Steve Hilberg (hberg@illinois.edu).

El Niño Severe Weather:

El Niño Severe Weather:

Author: Michael K. Tippett and Chiara Lepore
April 27, 2017

ENSO shifts the atmospheric circulation (notably, the jet stream) in ways that affect winter temperature and precipitation over the U.S. The changes in spring (March-May) are similar to those during winter but somewhat weaker.

These shifts would also be expected to impact thunderstorm activity: El Niño tends to shift the jet stream farther south over the U.S., which blocks moisture from the Gulf of Mexico, reducing the fuel for thunderstorms. On the other hand, La Niña is associated with a more wavy and northward-shifted jet stream, which might be expected to enhance severe weather activity in the south and southeast. Indeed, historic tornado outbreaks in 1974, 2008, and 2011 started during La Niña conditions.

However, tornado and severe weather activity are more variable (“noisier” and harder to predict) than ordinary weather (think temperature and precipitation), and any ENSO signal is harder to see. Until recently, the only solid evidence showing that more tornadoes occur during La Niña conditions was for winter (January-March), when the ENSO signal is strongest, but average tornado activity is relatively low (Cook & Schaefer, 2008).

A recipe for tornadoes

A clearer picture of the impact of ENSO emerges when we look at the ingredients that are conducive to tornado and thunderstorm occurrence (Allen et al., 2015a).  Instead of only looking at individual weather events, it’s important to consider the environmental cues for the outbreak of severe weather.   

Two important ingredients for tornadoes are atmospheric instability (e.g., warm, moist air near the surface and cool dry air aloft) and vertical wind shear (winds at different altitudes blowing in different directions or speeds). Measures of these tornado-friendly ingredients can be combined into indexes that are less noisy than actual tornado reports and let us see how the phases of ENSO make the environment more or less favorable for severe weather (footnote 1).

In much of the U.S., La Niña conditions are associated with increases in these environmental factors and in tornado and hail reports. The largest signal is present in the south and southeast (including parts of Texas, Oklahoma, Kansas, Louisiana, Arkansas, and Missouri), except in Florida where the opposite relation is observed. Positive values indicate increased activity and negative values indicate decreased activity compared to the long-term average (1979-2015).

					Figure 1:  Sea surface temperature pattern showing the warm phase of the Pacific Decadal Oscillation (top). The status of the PDO between 1950 and this year, shown at the bottom, indicates a predominantly positive phase from about 1978 to 1998 and a negative phase since 1999.   Image Credit: Climate Impacts Group, University of Washington.

In the South and Southeast, where the signal is strongest, we see a clear shift in activity with the ENSO phase, but with a tremendous range of variability, meaning some El Niño years still have high severe weather activity, and some La Niña years are relatively inactive. While increased tornado activity is generally associated with La Nina conditions, blaming a year’s high activity on the weak La Nina conditions would exaggerate the strength of the historical relationship (footnote 2).

					Figure 2: Regional (100-90W, 31-36N) totals of March-May tornado reports, hail events, a tornado environment index (TEI), and a hail environment index (HEI) expressed as a percentage of their 1979-2015 average and conditioned on the ONI. Box edges mark the 25th and 75th percentiles, and whiskers extend 1 and a half times the interquartile range. Figure by climate.gov; data from the authors.

Footnotes

1. The tornado environment index (TEI) and hail environment index (HEI) are functions of monthly averages of convective precipitation, convective available potential energy, and storm-relative helicity. TEI and HEI are calibrated to match the recent climatology of tornado numbers and hail events. See Tippett et al. (2012) and Allen et al. (2015b) for more details.
2. Inside baseball: Further details of the ENSO relation

Overall, La Nina conditions are associated with enhanced U.S. tornado activity, but more detailed aspects of ENSO may also be relevant (Lee et al., 2012). The Gulf of Mexico sea surface temperature is close to the part of the U.S. most strongly impacted by severe weather: warm Gulf of Mexico surface water in spring enhances low-level moisture transport and southerly flow and is associated with enhanced US tornado and hail activity (Molina et al., 2016). The Gulf of Mexico sea surface temperature is negatively correlated with the tropical Pacific sea surface temperature, meaning when the tropical Pacific is cooler than average (La Niña), the Gulf of Mexico is usually warmer than average.

Seasonal (May-July) averages of Gulf of Mexico SST can be predicted with some skill (Jung and Kirtman, 2016). Atmospheric angular momentum is related to ENSO and also shows the impact of tropical forcing on tornado activity (Gensini and Marinaro, 2016).

Local El Niño Statistics:

Past El Niño Winters Statistics for the Local Area:

In the tables below, red represents a value in the upper third of winters, blue represents a value in the lower third of winters, and black represents a near-normal.  For La Crosse, the temperature and precipitation data extend back to the 1872-73 winter and snowfall back to 1895-96. For Rochester, the temperature and precipitation data extend back to the 1886-87 winter and snowfall back to 1908-09.

La Crosse, WI:
 

Rochester, MN

Arctic Oscillation:

Climate Variability: Arctic Oscillation (AO)

Author: LuAnn Dahlman
August 30, 2009

The Arctic Oscillation (AO) refers to an atmospheric circulation pattern over the mid-to-high latitudes of the Northern Hemisphere. The most obvious reflection of the phase of this oscillation is the north-to-south location of the storm-steering, mid-latitude jet stream. Thus, the AO can have a strong influence on weather and climate in major population centers in North America, Europe, and Asia, especially during winter.

The AO's positive phase is characterized by lower-than-average air pressure over the Arctic paired with higher-than-average pressure over the northern Pacific and Atlantic Oceans. The jet stream is farther north than average under these conditions, and storms can be shifted northward of their usual paths. Thus, the mid-latitudes of North America, Europe, Siberia, and East Asia generally see fewer cold air outbreaks than usual during the positive phase of the AO.

Conversely, AO's negative phase has higher-than-average air pressure over the Arctic region and lower-than-average pressure over the northern Pacific and Atlantic Oceans. The jet stream shifts toward the equator under these conditions, so the globe-encircling river of air is south of its average position. Consequently, locations in the mid-latitudes are more likely to experience outbreaks of frigid, polar air during winters when the AO is negative. In New England, for example, higher frequencies of coastal storms known as "Nor'easters" are linked to AO's negative phase.

This graph shows monthly values for the Arctic Oscillation index.

AO phases are analogous to the Southern Hemisphere's Antarctic Oscillation (AAO), a similar pattern of air pressure and jet stream anomalies in the Southern Hemisphere. Viewed from above either pole, these patterns show a characteristic ring shape or "annular" pattern; thus, AO and AAO are also referred to as the Northern Annular Mode (NAM) and Southern Annular Mode (SAM), respectively.

Monthly and Daily values for the Arctic Oscillation Index are available from NOAA's Climate Prediction Center.

References:

• Thompson, D.W.J., S. Lee, and M.P. Baldwin, 2002: Atmospheric Processes Governing the Northern Hemisphere Annular Mode/North Atlantic Oscillation. From the AGU monograph on the North Atlantic Oscillation, 293, 85-89.
• Thompson, D.W.J., and J.M. Wallace, 2001: Regional Climate Impacts of the Northern Hemisphere Annular Mode. Science, 293, 85-89.
• Thompson, D.W.J., and J.M. Wallace, 2000: Annular modes in the extratropical circulation. Part I: Month-to-month variability. J. Climate, 13, 1000-1016.
• Thompson, D.W.J., and J.M. Wallace 1998: The Arctic Oscillation signature in wintertime geopotential height and temperature fields. Geophys. Res. Lett. 25, 1297-1300.