Earth Gauge » Interannual Climate Variability

Climate Trivia: El Niño Frequency

kraus — Mon, 08 Mar 2010 15:10:17 +0000

Much of our weather in the United States depends on what is happening in the tropical Pacific Ocean. During an El Niño event, which is happening now, the eastern tropical Pacific is warmer than average. During La Niña events, the eastern tropical Pacific is cooler than average. While South America’s west coast may seem far away, what happens there has been shown to affect weather throughout the United States. El Niño events mean more winter Nor’easters on America’s East Coast. El Niño events also result in a more southerly winter storm track, which means more rain and snow for the Southwest but less for the Pacific Northwest. Hurricane season in the Atlantic is less active during El Niño phases and more active during La Niña phases. An intermediate stage, known as the neutral phase, means more snowfall throughout the Mississippi River basin.

Trivia Question: What phase has been more common over the last 25 years?

a) El Niño
b) La Niña

The correct answer is a. El Niño events have become more common since the mid-1970’s. Duing the 1950’s and 1960’s, La Niña events were more common. See below for a graph of the last 60 years of El Niño (red) and La Niña (blue) event frequency.

Seasons: Winter, Spring, Summer, Fall

Sources: Kim, HM et al. “Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones.” Science 325 (2009): 77-80 and Twine, TE et al. “Effects of El Niño-Southern Oscillation on the Climate, Water Balance, and Streamflow of the Mississippi River Basin.” Journal of Climate 18 (2005): 4840-4861 and Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf and Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf.

Climate Trivia: El Niño Events and Frost Days – Great Basin

kraus — Mon, 08 Mar 2010 14:54:12 +0000

Winter is ending and the growing or “frost free” season is almost here! The frost free season is defined as the continuous period of the year when the temperature does not drop below freezing. When this season starts and how long it lasts have important implications for the plants and animals that live around us, especially plants we grow for food. In the United States over the second half of the 20th century, the average length of the frost free season increased at a rate of two days per decade. How many frost days there are each year is influenced – as is much of our weather – by the El Niño-Southern Oscillation (ENSO), or the periodic warming and cooling of the eastern tropical Pacific. This warming and cooling changes how air in the upper-atmosphere moves, which in turn affects weather across the United States.

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the Great Basin region have on average…

a. fewer frost days?
b. more frost days?

The correct answer is a. All other things being equal, the Great Basin region experiences fewer frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

(Sources: Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf)

Climate Trivia: El Niño and Frost Events – Pacific Northwest

kraus — Mon, 08 Mar 2010 14:50:05 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the Pacific Northwest have on average….

a. fewer frost days?
b. more frost days?

The correct answer is a. All other things being equal, the Pacific Northwest experiences fewer frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Sources: Meehl, GA et al. “Current and Future U.S. Weather Extremes and El Niño.” Geophysical Research Letter 34 (2007) L20704 and Easterling, David. “Observed Climate Variability and Change.” NOAA/National Climatic Data Center. Ashville, NC: 31 January 2007 http://www.ametsoc.org/atmospolicy/documents/Easterling-Observed-Change-Jan-07.pdf.

Climate Trivia: El Niño and Frost Events – Eastern U.S.

kraus — Mon, 08 Mar 2010 14:44:28 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the eastern U.S. experience on average…

a. fewer frost days?
b. more frost days?

The correct answer is b. All other things being equal, the eastern U.S. experiences more frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Climate Trivia: El Niño and Frost Events – Southern U.S.

kraus — Mon, 08 Mar 2010 14:34:45 +0000

Trivia Question: During El Niño years such as this year, when the eastern tropical Pacific is relatively warm, does the southern U.S. have on average…

a. fewer frost days?
b. more frost days?

The correct answer is b. All other things being equal, the southern U.S. experiences more frost days during El Niño years compared to La Niña years.

Seasons: Late Winter, Early Spring, Fall

Climate Number: 510 Years

kraus — Mon, 01 Mar 2010 14:49:22 +0000

Better understanding how fluctuations in climate have affected regional wildfire frequency over the past few centuries may help to improve our ability to predict severe wildfire seasons. Some of the West’s older groves have experienced dozens of wildfires over the past few centuries. The trees that survived these fires recorded black scars in their annual tree rings, providing us with the ability to know what year a given area burned. Core samples (profiles of the annual rings taken from a part of the tree trunk) are used to figure out when and where of fires occurred in the past. Enough of these fire scars from a long enough period of time make it possible to investigate the possible links between large scale climate variability and fire in given regions of the West. A recent investigation using core samples from hundreds of trees across the West found that fire years are more common during drought years and the occurrence of drought is controlled by large-scale movements of water in the oceans. Specifically, different phases of the El Nino-Southern Oscillation, the periodic warming and cooling of the eastern tropical Pacific, and the Atlantic Multidecadal Oscillation, the periodic warming and cooling of the North Atlantic, influence the frequency of fire in the West. Fire in all regions of the West is more common when the North Atlantic is warmer. The Pacific Northwest has more fires when the eastern tropical Pacific is warmer and the Southwest has more fires when the eastern tropical Pacific is cooler than average. The oldest trees used in this study had fire records going back to 1400 CE, 510 years ago.

For Comparison: The Mongol Empire was at its peak 510 years ago, with its soldiers sacking cities as far west as Damascus. In Florence, Italy, the Medici family was building the banking empire that would revolutionize western finance.

Seasons: Winter, Spring, Summer

Sources: Kitzberger, T et al. “Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America.” Proceedings of the National Academy of Sciences 104 (2007): 543-548 and Trouet, V et al. “Fire-climate interactions in the American West since 1400 CE.” Geophysical Research Letters 37 (2010): L04702.

Climate Fact: North American and Eurasian Snow

kraus — Mon, 22 Feb 2010 15:13:48 +0000

Snow is both a product of the weather and a weather maker. It has long been recognized that snow exhibits a cooling effect on local and regional scales. Snow reflects more sunlight than bare ground, meaning that it absorbs less energy. More snow cover also means soils stay moist for longer following the spring melting than they otherwise would. More soil moisture means that more of the sun’s energy that would have been spent heating the ground is instead spent evaporating water. In North America, lots of snow over the continent affects the storm track, or the latitudinal band where travelling cyclonic high and low pressure systems are most common. More specifically, the cold temperatures the snow cover induces eventually force the storm track over North America to veer south. As it does this, the storm track downstream in Eurasia veers north, allowing warmer air masses to penetrate further into the continent than they otherwise would. The presence of these warmer air masses generally means less snow there. Thus, years of above average snow cover in North America tend to be years of below average snow cover in Eurasia.

Seasons: Winter, Spring

Source: Sobolowski, S et al. “Modeled Climate State and Dynamic Responses to Anomalous North American Snow Cover.” Journal of Climate 23 (2010): 785-799.

Climate Number: Two Watts per Square Meter

kraus — Mon, 01 Feb 2010 15:32:18 +0000

The amount of solar energy Earth receives varies according to the “11-year solar cycle,” which corresponds to a cycle in the frequency and distribution of sunspots on the sun’s surface. The difference in solar energy between high and low points of the solar cycle is about two watts per square meter. About 35 percent of the variation in Earth’s climate since 1600 can be explained directly by shifts in solar forcing. Indirect effects of solar variation are also important and can have pronounced regional effects on climate. For example, the behavior of the El Niño-Southern Oscillation (ENSO), the system of winds and ocean currents that affects temperature distributions in the tropical Pacific Ocean, is believed to be influenced by solar activity. Temperature distributions in the tropical Pacific in turn have noticeable impacts on weather throughout the world, including the strength of the Indian Monsoon; the position of the Northern Hemisphere storm tracks that bring winter and spring precipitation to the United States; and the intensity of the dry season in the Amazon Basin.

For Comparison: The total amount of solar irradiance that reaches Earth varies by two watts per square meter, from 1365 watts per square meter at a low points in the solar cycle to around 1367 watts per square meter at high points – about a 0.1 percent variation. The noticeable effects of the solar cycle, despite the relatively nominal absolute change in solar radiation, illustrate the non-linear nature of Earth’s climate: small changes in certain variables can have large impacts, particularly at regional scales.

Seasons: Winter, Spring, Summer, Fall

Source: Wasco, C and Sharma, A. “Effect of solar variability on atmospheric moisture storage.” Geophysical Research Letters 36 (2009): L03703.

Climate Fact: Lake Warming in California and Nevada

kraus — Mon, 25 Jan 2010 14:29:50 +0000

Air temperatures are fickle – they fluctuate significantly from day to day, from season to season and from year to year. The temperature of a water body fluctuates as well, but is much more constant than the surrounding air temperature. Water has a higher heat capacity than air, which means it takes far more energy to raise the temperature of a given volume of water than a given volume of air. It also means that once warmed, the water must lose lots of energy to fall in temperature. This higher heat capacity causes seasonal changes in water temperature to “lag” behind the ambient air temperature. This phenomenon is easily observable. The air temperature can remain below freezing for weeks before ice cover begins to form on lakes. The higher heat capacity of water means that temperature fluctuations on multi-annual time scales in lake water as a whole are much smoother than temperature fluctuations in the atmosphere, making lakes effective indicators of longer term warming or cooling trends. Records from satellite imaging systems that collect nighttime infrared emissions show that several large lakes in the Sierra Nevada region (Tahoe, Mono, Pyramid, Almanor and Clear Lake) have collectively been warming at a rate of 0.2 degrees Fahrenheit per year since 1992. This trend may have implications for the life in these lakes that has adapted to the traditionally cold high-altitude temperatures.

Seasons: Winter, Spring, Summer and Fall

Source: Schneider, P et al. “Satellite observations indicate rapid warming trend for lakes in California and Nevada.” Geophysical Research Letters 36 (2009): L22402.

Climate Fact: Arctic Temperature Trend Amplification and the AMO

kraus — Mon, 25 Jan 2010 14:26:27 +0000

Temperature records suggest that the Earth’s surface temperatures warmed during the early part of the 20th century, cooled from the period 1940-1970 and have since been warming. While Arctic temperature trends have corresponded to this general warming and cooling pattern, it has followed these trends more severely. During the warming period from 1910-1940, the Arctic warmed about 5.4 times the rate of the rest of the planet and from 1970-2008 it warmed at about twice the rate. During the cooling period of 1940-1970 it cooled at about nine times the rate of Earth’s average trend. What is behind this temperature trend amplifications in the Arctic? The best explanation is the behavior of the Atlantic Multidecadal Oscillation (AMO), or the periodic 65-year warming and cooling of North Atlantic sea-surface temperatures due to a strengthening and weakening of the ocean currents that bring warm waters from the tropics into the far north. If the Atlantic had a cold anomaly in the far northern regions that was balanced by a warm anomaly in the subtropics, the Arctic would noticeably cool while the rest of the planet would show little or no change in temperature. The coincidence of warm AMO periods (warm waters in the Arctic) and cool AMO periods (cool waters in the Arctic) during periods of global warming and cooling respectively helps to explain why the Arctic has shown such severe warming or cooling trends compared to globally averaged rates.

Seasons: Winter, Spring, Summer, Fall

Source: Chylek, P et al. “Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation.” Geophysical Research Letters 36 (2009): L14801.

Climate Fact: The Ozone Hole and Climate

kraus — Mon, 11 Jan 2010 15:28:14 +0000

Near the center of Antarctica in the polar vortex, strong westerly winds that blow in a circle around the continent during winter trap an envelope of air near the South Pole, prohibiting this air from mixing with warmer air masses closer to the equator. The extreme cold in the vortex causes clouds to form in the lower part of the stratosphere. Conditions in these stratospheric clouds are just right for a complex series of chemical reactions to take place, resulting in the destruction of ozone molecules and the formation of the ozone hole. This hole is at its maximum during the austral (Southern Hemisphere) spring months of September through December; stratospheric ozone concentrations during these months can fall by 33 percent. Once temperatures warm sufficiently, the strong westerly winds slow and the polar vortex breaks up, allowing ozone rich air to blow in and ozone poor air to blow out. This movement of ozone poor air is noticeable in parts of New Zealand and South America, where ozone concentrations can temporarily drop by 10 percent.

While the vortex is a local phenomenon, the strength and annual duration of the westerly winds that create the polar vortex are influenced by a larger phenomenon called the Southern Annular Mode (SAM), the difference in atmospheric pressure between 40 and 65 degrees South. When this difference is relatively large, the westerly winds around Antarctica are particularly strong, leading to a stronger vortex and more ozone destruction. High concentrations of ozone, however, can affect the movements of air between the stratosphere and troposphere, ultimately affecting the SAM itself. Better understanding this “coupling” between the SAM and the ozone hole will be needed for better weather and climate prediction, as well as for predicting future ozone concentrations.

Seasons: Winter, Spring, Summer, Fall

Sources: Sparling, B. “The Antarctic Ozone Hole.” NAS Educational Resources. 2001. Accessed Online 10 January 2009 and Fogt, RL et al. “Intra-annual relationships between polar ozone and the SAM.” Geophysical Research Letters 36 (2009): L04707 and Son, SW et al. “Ozone hole and Southern Hemisphere climate change.” Geophysical Research Letters 36 (2009): L15705.

Climate Trivia: East Coast Winter Storm Frequency and ENSO

kraus — Mon, 07 Dec 2009 15:59:51 +0000

December is East Coast Winter Storm (ECWS) season. These storms are powered by warm water that flows from the Gulf Stream. The Gulf Stream current flows along the Eastern Seaboard past Florida and the Carolinas before reaching Cape Hatteras, where the warm water heads out into the Atlantic. ECWS’s travel northward along the coast causing high winds and coastal property damage comparable to hurricanes. They also bring heavy snowfall, causing further weather complications. On average, there are 12 ECWS’s during the December to February season, with January being the most active month. One of the best predictors of how intense an ECWS season will be is the ocean temperature along the coast of the southeastern U.S. during the previous summer (Gulf of Mexico temperatures were above average this past summer). The warmer these waters are, the stronger the Gulf Stream generally is and the more active the winter storm season will be. Interestingly, conditions in the eastern tropical Pacific affect ECWS activity as well. What eastern tropical Pacific conditions are most conducive to an active ECWS season?

a)   El Niño conditions (warmer eastern tropical Pacific SSTs)
b)   La Niña conditions (cooler eastern tropical Pacific SSTs)
c)   Neutral Conditions (average eastern tropical Pacific SSTs)

The correct answer is a. More active ECWS seasons tend to coincide with El Niño years. This is in contrast to Atlantic hurricane season trends, as El Niño conditions tend to suppress Atlantic Hurricane formation. Over the second half of the 20th century, the frequency of ECWS events showed little trend, but the storms did become slightly more intense.

Season: Winter

Sources: DeGaetana, AT et al. “Statistical Prediction of Seasonal East Coast Winter Storm Frequency.” Journal of Climate 15 (2002): 1101-1117 and Hirsch, ME et al. “An East Coast Winter Storm Climatology.” Journal of Climate 14 (2001): 882-899 and Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093 and National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif

Climate Trivia: ENSO and Regional Rainfall (Northwest)

kraus — Mon, 07 Dec 2009 15:58:02 +0000

Winter storm season is here. Storms will be blowing in from the Pacific, bringing rainfall to lower elevations and snow to the mountains. This year, the eastern tropical Pacific is in an El Niño phase, meaning that its waters are warmer than average. When the eastern Pacific is in an El Niño phase, the northwestern U.S. can expect:

a)   more than normal rainfall
b)   less than normal rainfall
c)   about average rainfall

The correct answer is b. The Northwest gets less than normal rainfall during El Niño winters and greater than normal rainfall during La Niña winters, when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and hit us. The storm track will spend most of its time south of us this winter, thus giving us below average rainfall through about April.

View a schematic diagram of how El Niño and La Niña events affect wintertime rainfall and temperature: http://www.earthgauge.net/climate-facts-image-library#5. For more information on El Niño, including seasonal forecasts by region, visit: http://www.elnino.noaa.gov/. Learn more about what Northern Hemisphere storm tracks are and how they work: http://www.earthgauge.net/wp-content/CF_Storm%20Tracks.pdf.

Season: Winter

Sources: Eichler, T and Higgins W. “Climatology and ENSO-Related Variability of North American Extratropical Cyclone Activity.” Journal of Climate 19 (2006): 2076-2093 and National Oceanic and Atmospheric Administration: Climate Prediction Center. Accessed Online 7 December 2009 (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/stormtracks/eisdiffobs.meta.gif

Climate Fact: ENSO and Regional Rainfall (South)

kraus — Mon, 07 Dec 2009 15:54:44 +0000

a)   more than normal rainfall and cooler temperatures
b)   less than normal rainfall and warmer temperatures
c)   about normal rainfall and about normal temperatures

The correct answer is a. The South gets more than normal rainfall during El Niño years and less than normal rainfall during La Niña years when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and miss us. During El Niño, not only does the storm track head right for us, it is even stronger than it is during the La Niña phase. Another effect of El Niño is cooler than normal winters in the South from about Texas eastward.

View a schematic diagram of how El Niño and La Niña events affect wintertime rainfall and temperature: http://www.earthgauge.net/climate-facts-image-library#5. For more information on El Niño, including seasonal forecasts by region, visit: http://www.elnino.noaa.gov/. Learn more about what Northern Hemisphere storm tracks are and how they work: http://www.earthgauge.net/wp-content/CF_Storm%20Tracks.pdf.

Seasons: Winter

Climate Trivia: ENSO and Regional Rainfall (Southwest)

kraus — Mon, 07 Dec 2009 15:50:46 +0000

Winter storm season is here. Storms will be blowing in from the Pacific, bringing rainfall to lower elevations and snow to the mountains. This year, the eastern tropical Pacific is in an El Niño phase, meaning that its waters are warmer than normal. When the eastern Pacific is in an El Niño phase, the southwest U.S. can expect:

a)   more than normal rainfall
b)   less than normal rainfall
c)   about normal rainfall

The correct answer is a. The Southwest gets more than normal rainfall during El Niño years and less than normal rainfall during La Niña years when the colder waters in the eastern Pacific cause the Pacific storm track to shift north and miss us. During El Niño, not only does the storm track head right for us, it is even stronger than it is during the La Niña phase. Hopefully, this El Niño winter will give the Southwest some relief from the current dry and drought conditions.

Season: Winter

Climate Fact: Atlantic and Pacific Niños

kraus — Mon, 02 Nov 2009 16:17:17 +0000

It has long been known that what happens in the tropical Pacific doesn’t just stay in the tropical Pacific. Much of the year-to-year variability in America’s weather, particularly winter weather, can be explained by conditions there. All other things being equal, warm El Niño conditions off the coast of equatorial South America mean a wetter southern U.S., while cool La Niña conditions mean a wetter northern U.S. This switch between warm and cool conditions is influenced by the strength of the winds that descend from the upper atmosphere and blow over the surface of the tropical Pacific. These winds help to “pull” cool waters from the ocean depths to the surface where they upwell off the coast of South America. The strength of these descending winds is in turn influenced by temperatures in the eastern tropical Atlantic Ocean, which has its own “Niños” and “Niñas”. In contrast to the Pacific, where El Niño and La Niña are most pronounced during the winter, the Atlantic events are most pronounced during the summer months. During warm Atlantic phases, the convection in the eastern tropical Atlantic Ocean is stronger, which affects the upper atmospheric winds all the way over to the Pacific. This strengthening in turn strengthens the descending winds over the tropical Pacific, which makes the formation of La Niña conditions more likely there during the winter months. Thus, summertime ocean temperatures in the tropical Atlantic Ocean can help predict the wintertime ocean temperatures in the tropical Pacific.

Seasons: Summer, Fall, Winter

Source: Rodr?guez-Fonseca, B et al. ”Are Atlantic Niños enhancing Pacific ENSO events in recent decades?” Geophysical Research Letters 36 (2009): L20705.

Climate Fact: Southern U.S. Drought Occurrence Linked to SST Variability: Causes of Northern Droughts Less Clear

kraus — Thu, 29 Oct 2009 19:23:51 +0000

Better drought prediction systems could potentially save billions of dollars. Current prediction systems largely rely on observations of the circumstances surrounding past droughts to understand the factors that led to drying. New research reveals that:
• Drought in the southern Great Plains states, such as the 1946-1956 drought, can be largely explained by persistently cool conditions in the eastern tropical Pacific.
• Drought in the northern Great Plains states, such as the 1930’s “Dust Bowl,” was likely caused by serendipitous atmospheric variability (i.e. a variety of atmospheric factors came together with the result being the Dust Bowl) and agricultural practices. The initial atmospherically forced drying resulted in dust storms that reinforced these dry conditions.

Predicting drought may be more difficult in the northern Great Plains than southern Great Plains, as the causes of drought in the former region are much more complex.

Seasons: Winter, Spring, Summer, Fall

Source: Hoerling, M et al. “Distinct causes for two principal U.S. droughts of the 20th century.” Geophysical Research Letters 36 (2009): L19708.

Climate Fact: North Atlantic Sea Level Cycles

kraus — Fri, 02 Oct 2009 15:34:30 +0000

Global sea level is rising at a rate of about 1.2 inches per decade due to an influx of glacial melt water and thermal expansion of the oceans. The relative sea level rise that each coastal location experiences, however, differs from this global average due to local factors such as land subsidence and land uplift (which typically happens as glaciers melt), as well as cyclical variability in ocean currents. Sea levels off America’s east coast fluctuate by around two inches over a period of 20-30 years. This fluctuation is linked to variability in the strength of the Atlantic Meridional Overturning Circulation (AMOC), which occurs in the North Atlantic around Greenland and is considered to be the “engine” behind Earth’s thermohaline circulation. This thermohaline circulation moves heat from the ocean surface to the depths and nutrients from the depths to the surface. Strengthening of the AMOC leads to strengthening of the currents that move heat from the tropical Atlantic to the northern portions of the Atlantic basin. When North Atlantic temperatures are at their warmest point in the cycle, sea levels off Europe’s coast peak. This crest in sea level then propagates westward, and about eight to ten years after sea levels off Europe’s coast reach their peak, sea levels off America’s east coast peak. This cycle is currently causing a relative decline in east coast sea levels.

Seasons: Winter, Spring, Summer, Fall

Source: Frankcombe, LM and Dijkstra, HA. “Coherent multidecadal variability in North Atlantic sea level.” Geophysical Research Letters 36 (2009): L15604.

Climate Fact: Holocene ENSO Variability

kraus — Mon, 28 Sep 2009 15:39:17 +0000

Variations in the temperature distribution gradient in the tropical Pacific (characterized by the state of the El Niño-Southern Oscillation (ENSO), the oscillation in the eastern tropical Pacific between cool (La Niña) and warm (El Niño) states) exerts a strong influence on upper-atmospheric circulation and affects weather throughout the world. Surface temperature distributions in the tropical Pacific are sensitive to variations in solar intensity and even slight changes in solar or volcanic activity can have marked effects on the tropical Pacific and Earth’s climate. ENSO’s properties are also strongly influenced by the behavior of the Peru (Humboldt) Current, which moves from the north to south along the South American coast and is in turn influenced by variations in the Antarctic Circumpolar Current. During the early Holocene (12,000 years ago to about 5,400 years ago), the tropical Pacific had a strong temperature gradient between the east and west (i.e. it was in a semi-permanent La Niña state) and El Niño events were effectively suppressed. Between about 5,400 and 4,600 years ago, the variability between the two phases became more pronounced. Because short-term solar influences cannot explain this shift, it is likely that changes originating from the poles as a result of longer-term changes linked to the deglaciation are needed to explain the transition.

Seasons: Winter, Spring, Summer, Fall

Source: Chazen, CR et al. “Abrupt mid-Holocene onset of centennial-scale climate variability on the Peru-Chile Margin.” Geophysical Research Letters 35 (2009): L18704.

Climate Fact: Climate and North Atlantic Hurricanes

kraus — Fri, 04 Sep 2009 17:50:51 +0000

The torrential rainfall and storm surges associated with hurricane landfall events can cause what are known as “overwash deposits” that leave definitive marks in the sediment layers that accumulate in coastal areas. Analyses of sediment cores from various locations along the Eastern Seaboard and Puerto Rico show that for the last 1500 years, Atlantic Hurricanes are more frequent when a) the El Niño-Southern Oscillation is in a La Niña phase, which reduces the amount of vertical wind shear over the North Atlantic and b) the North Atlantic is relatively warm, which usually coincides with positive phases of the Atlantic Multidecadal Oscillation. Over the 20th century, ocean temperatures in the North Atlantic main development region warmed during peak hurricane season, with the most pronounced warming occurring over the last four decades.

To view the chart of 20th century North Atlantic main development region peak hurricane season SST’s visit: http://www.earthgauge.net/climate-facts-image-library#6. This image is featured in the “Global Climate Change Impacts in the United States” report recently published by the U.S. Global Change Research Program. The image is in the public domain.

Seasons: Summer, Fall

Sources: Global Climate Change Impacts in the United States, Thomas R. Karl, Jerry M. Melillo, and Thomas C. Peterson, (eds.). Cambridge University Press, 2009 and Mann, ME et al. “Atlantic hurricanes and climate over the past 1,500 years.” Nature 460 (2009): 880-883.