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INTELLiCast: SUPER EL NINO



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 Lawrence F. London, Jr. - Dragonfly Market Gardens
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Title: INTELLiCast: SUPER EL NINO
Dr. Dewpoint

SUPER EL NINO THREATENS ONE-TWO PUNCH FOR THE US THIS UPCOMING WINTER

8/26/97
by Joseph D'Aleo
Chief WSI/INTELLiCast Meteorologist

The current El Nino event in the eastern Pacific is on its way to becoming one of the strongest ever recorded. Ocean temperatures are running better than 9 degrees F above normal in parts of the eastern tropical Pacific. This rivals the Great El Nino of 1982/83 which was blamed for major worldwide weather disasters including floods, droughts and powerful wind storms which that winter cumulatively did over $8 billion dollars damage and caused many deaths.

The 1982/83 disasters sparked a flurry of sponsored research into this phenomena that has documented that the El Nino and the opposite phase of the Southern Oscillation, the La Nina have definite correlations with weather features and both temperatures and precipitation patterns worldwide. The climate researchers, buoyed by their success with the Southern Oscillation then turned their attention to other phenomena in the atmosphere, the oceans and on the sun and to various combinations of these phenomena with some more successes. Today, forecasters have a bevy of new statistical tools that can be used to project likely weather regimes months or seasons in advance and to provide warnings in advance of potential weather trouble.

Those tools tell us the upcoming winter could be a very stormy one with the potential for more heavy snows in the east and powerful storms with strong winds and flooding rains throughout the south.

THE FACTORS IN PLACE THIS WINTER

THE TROPICAL CONNECTION

There are at least two phenomena that are important here - the El Nino - Southern Oscillation and the QBO or Quasi-Biennial Oscillation.

SOUTHERN OSCILLATION

This is an oscillation of the relative strength of pressure features in the tropical Pacific. The variations in the pressure patterns affects the strength of the trade winds which in turn influences the ocean temperatures. We normally find cool water off the west coast of South America due to an ocean current called the Humboldt. The winds normally blow from the south along the west coast of South America which causes the surface ocean water to move away from the land. Cool deep water rises to the surface, a phenomenon known as upwelling. This oxygen and plankton rich water makes this an ideal place for fish to feed and for fishing boats to catch fish. When the winds weaken, the current weakens and the upwelling is reduced. Ocean surface temperatures warm. This affects the fisheries locally and as the research has shown also the weather both locally (producing unusually warm and wet weather in the normally dry coastal regions of Peru) and globally.

In the U.S., El Nino years produce wet and relatively cool weather in the southeast and in most years in the southwest including California. In El Nino years, warmer and drier than normal weather is likely in the northwest and north central states.

In the opposite phase of the Southern Oscillation, which we call the La Nina, the normal tropical pressure features and the resulting winds grow stronger than normal. This causes enhanced upwelling which produces cooler than normal surface water in the eastern tropical Pacific. This too influences the weather globally. In the United states, La Nina brings warmer and drier than normal conditions to the southeast and colder, wetter weather to the northwest and north central states.

THE QBO (QUASI-BIENNIAL OSCILLATION)

Another flip-flop pattern in the tropical region has been shown to relate to weather patterns in the winter season worldwide. This is the QBO or Quasi-Biennial Oscillation. The QBO reflects the fact that the winds in the stratosphere (at around 45 mb) over the equator in the Pacific alternately blow west and east in a cycle that averages about 28 months. The direction of the wind in combination with both the phases of the Southern Oscillation and the solar cycle has been shown to be associated with definite tendencies for specific regimes of pressure, temperatures and precipitation worldwide.

The stratospheric wind blew easterly in the snowblitz winter of 1995/96. They turned westerly just before last winter and will likely at least start out westerly this winter. Sometime during the upcoming winter or spring they will transition back to easterly. The exact timing may determine which pattern dominates and what part of the nation gets the worst of it.

In El Nino years with a westerly QBO and an active sun, very cold weather can affect the south and east. In El Nino years with an easterly QBO during the quiet sun years, temperatures tend to be above normal across the northern states and below normal in the southern states coast to coast. A very active southern jet stream brings very wet and stormy conditions to much of the southern half of the nation while it tends to stay drier than normal in the north. This is the situation expected late this upcoming winter.

In La Nina years with an easterly QBO and a quiet sun, the storm track is usually from the southwest to the mid-Atlantic which threatens heavy snows from the Midwest to the northeast. This was the pattern in the hard winter of 1995/96. When a westerly QBO blows in a La Nina year with a quiet sun, the storm track is further north with most of the heavy snows confined to the upper plains, the northern Great Lakes and southeastern Canada. This was the pattern much of last winter.

THE SOLAR CYCLE

Since before 1700, astronomers worldwide have observed and documented sun spots on the solar surface. The number of sunspots varies over time in a number of cycles, most notably an 11 year cycle. An active sun with a high number of sunspots has been shown to be a slightly warmer sun (a few tenths of a percent more energy emitted). Increased solar activity also leads to increased geomagnetic storms and ionospheric disturbances which influence communications and electrical transmissions on the earth and have also been shown also to have some influence of the position and strength of certain weather features.

At the solar maximum, the pressure over North America tends to be higher than normal in the west phase of the QBO and lower than normal over the oceans. This results in northerly winds and colder than normal weather in the eastern states but southerly and warmer than normal in the west. In the solar minimum, with a westerly QBO, the winds tend to blow from the south in the east resulting in warmer than normal conditions. With an easterly QBO near the minimum, a more zonal flow is observed with an active southern storm track and a mix of continental polar and maritime polar Pacific air across the north tending to keep that area above normal. As stated above, that is the likely pattern for late this upcoming winter.

The sunspot cycle reached the minimum in May of 1996 and is now slowly rising. Forecasts from the various panels of solar scientists are for numbers to rise slowly from the 30s (number of sunspots observed per month) in the fall to the 60s by spring. We have since May of 1996, seen the numbers slowly rise but it should be noted the forecasts have been recently verifying too high. In fact, interestingly there is a debate among astronomers as to whether this next cycle will be a large one with peak activity at or above the last two maxima or a much quieter one perhaps like cycle 20 in the 1960s. A pair of Yale University and NASA astronomers have taken the latter position based on their study of magnetic fields just below the sunís surface from telescopes. Should they be correct, a period of colder temperatures could result through the next decade.

Regardless of what happens over the next cycle, the numbers this winter will be in what is considered the low range.

THE NORTH ATLANTIC OSCILLATION

This is another flip-flop pattern, but here, the flip-flop is north-south instead of east-west. Normally in winter, low pressure is found in the north Atlantic with a center near Iceland (called the Icelandic low) and high pressure is found to south (near the Azores).

In some years, the low and/or high is stronger than normal which causes a stronger than normal westerly jet stream over the Atlantic which tends to drain the cold air from the North American cold air factory. This often results in milder than normal conditions in the U.S, especially across the southeastern states.

In other years, the features weaken and sometimes even reverse position with a high pressure surface and aloft developing or moving over the north Atlantic. This high often makes its way to Greenland and then eventually backs up to northern Canada. It is referred to as a blocking high because it tends to prevent or slow the normal west to east movement of weather features. It tends to cause cold air to pool over North America and often this leads to a deep cold winter trough in the eastern half of the nation. Often east coast storms develop in these situations causing problems for the eastern cities.

Blocking was very common in the 1960s but much less so in the past two decades during which the opposite, non-blocking, fast westerly phase has predominated. Significant persistent winter season blocking tends to be more common during the quiet phase of the solar cycle. During the last two decades, persistent blocking has occurred in only 4 years all at or near the solar minimum. Over the last 40 years, the sunspot number during the years with strong blocking has averaged 35 per month, while the average number during years when the index was strongly non-blocking was 102 per month.

In the hard winter of 1995/96, the North Atlantic Oscillation was at its strongest blocking level since 1966/69. Last winter, the this high latitude blocking was evident early in the winter and again in the spring. The blocking is favored in both the westerly and easterly QBO phases during quiet sun years and during the easterly phase of the El Nino. And so this winter with the solar activity still low (although increasing) and with a strong El Nino with QBO winds turning easterly, blocking should again be evident.

FALL PRECIPITATION

Research at the University or Virginia a few years back showed storm tracks tend to persist well season-to-season and indeed along the east coast, some studies we have done confirm this at least for the northeast. In Boston, for example, in the last four decades, when rainfall has averaged significantly above normal in the fall, 80% of the winters that followed have had above normal snows. We will thus be watching the storm tracks and precipitation totals in the upcoming fall.

PUTTING IT ALL TOGETHER FOR THE WINTER OF 1997/98

This winter we are experiencing a strong El Nino. The QBO is likely to start the winter at least with a westerly phase. By the spring we could be in an easterly phase. The solar activity will be increasing but still relatively low. There are indications we should see a repeat of North Atlantic blocking.

PART I - THE EARLY WINTER

These factors suggest that we start the winter much in the same way we ended last winter with a strong trough near the east coast and a strong ridge in the west. This would mean cold air and, if the east coast trough is positioned far enough west, potentially some major snow threats for eastern and possibly Midwestern cities. If the western ridge - eastern trough pattern is amplified enough, very cold air masses could plunge into the eastern half of the nation and threaten freezes into Florida.

A stronger than normal trough should be spinning in the central Pacific. Whether California and Texas get heavy rains will depend on whether the troughs pinwheeling around the massive Pacific low can break through underneath the western North American ridge. If they do, powerful storms could rake the California coast with damaging winds and waves, heavy rains and flooding. If they donít, the winter could start out like last winter ended in California, dry.

PART II - THE LATE WINTER

If the QBO changes phase in mid or late winter as seems likely, a split flow would be favored with strong southern branch storms lashing California and redeveloping near the Gulf coast and dumping very heavy rains and triggering severe weather in the southeast states. If enough cold air is in position to the north, snow or ice could affect the areas along the northern edge of the precipitation (like Atlanta). If the northern branch features phase with these southern lows, the storms could turn north and bring the threat of heavy rains and snows and strong winds. Because North Atlantic blocking is favored in the quiet sun years and with an easterly QBO in an El Nino, blocking may really show itself late and potentially keep the east colder and possibly snowier than otherwise might be expected with this pattern.

ANALOG YEARS

1982/83 is being referred to as a possible analog. Surely with regards to the strength of the El Nino, that is a good analog. That year also had a westerly QBO. However, the solar activity was high and decreasing instead of low and increasing. The North Atlantic Oscillation was in a strong non-blocking phase whereas this year we are coming off two years with blocking and expecting a third.

Examples of years where all the conditions were met that we think could be alternative analog years include 1963/64 and 1977/78 for the early winter and 1965/66, 1986/87 for the late winter

These were all hard winters. Boston averaged 54 inches in those 5 winters, New York 33 inches, Philadelphia 36 inches and Washington DC 29 inches, all above normal. Chicago averaged 34 inches, Cleveland 55 inches, Cincinnati 26 inches, Minneapolis 43 inches. Snowfall data for the upper plains was missing before 1967 but for the other three years Fargo (which had over 100 inches last winter) averaged just 32 inches in those 5 analog years).

California numbers show the situation is not totally clear yet there. In the 5 analog years, there was a large variation in rainfall. San Francisco was very wet in 1982/83 (44 inches), wet in 1965/66 (24 inches) and 1977/78 (22 inches) but dry in 1963/64 (12 inches) and 1986/87 (13 inches). Los Angeles was very wet in 1977/78 (27 inches) and 1982/83 (26 inches), moderately wet in 1963/64 (18 inches), dry in 1965/66 (12 inches) and very dry in 1986/87 (just 5 inches).

We will continue to monitor and review the situation and will update this report and forecast during the fall season.


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