... EL NINO...WILL IT LIVE UP TO EXPECTATIONS? ...

By: William R. Deedler, Weather Historian, WFO Detroit/Pontiac Mi

 

With all the coverage the climate event "El Nino" has had recently, one would have to
be from Mars to have missed at least one news item about it. The "event" is nothing new
and has been going on for centuries. However, reliable data to measure El Nino and its
effects has only recently been available to scientists for interpretation. El Ninos are detected
by a host of meteorological equipment and methods including weather satellites, moored
and drifting buoys, and sea level temperature analysis. This data is used in computer models
of the oceans and atmosphere to help predict the evolution and dissipation of the El Nino
life cycle.

The term "El Nino" is spanish for the "boy child" and refers to the Christ child because
the effect generally is associated with the month of December and Christmas. The term
was coined by the Peruvian fisherman as far back as the 1600's to describe the change in
the Pacific Ocean currents off the coast of South America. The phenomenon originates in
the western tropical Pacific Ocean and generally occurs every 2 to 7 years. The normal
easterly trade winds relax and a westerly wind begins to dominate. As a result, the warmer
surface sea water of the tropics spreads east and northward toward the South and North
American continents. This causes a stronger atmospheric link to develop between the
Pacific tropics and the higher latitudes and thus, invokes a shift in mid-latitude weather
patterns including those of North America.

Along with the shift in the mid-latitude weather patterns, abnormally low pressure develops
over the tropics in the eastern Pacific, while unusually high pressure takes shape over
Indonesia and Australia. Normal global atmospheric circulation patterns are disrupted and
since the U.S. is generally downwind of El Nino, the effects can be quite noteworthy. As a
result of El Nino, the tropical connection or "pineapple express" increases across mainly
the southern U.S. in the winter bringing additional storminess and rainfall. At the same
time, the northern U.S. tends to see a more pleasant winter with relatively milder and
drier conditions. While this is the general accepted scenario, other factors along with the
"El Nino effect" must be examined and considered.

One important factor is the timing of El Nino and its relation to the winter months in the
Northern Hemisphere. Evidently, when El Nino peaks during the winter months, its effects on
the climate also peaks. Thus, the storm track should be quite active from the eastern Pacific,
across California and southern Rockies and eastward through the remaining southern half of
the U.S. While the northern half of the country has more tranquil, milder and subsequently,
drier winter weather. Another equally important consideration of El Nino is its amplitude
which is mainly judged by how much above normal the eastern Pacific water temperatures
rise. The more above normal the eastern Pacific water temperatures are, the more amplified
or stronger El Nino determined to be.

A good example of when El Nino peaked strongly and coincided with a winter season was
during the months of December through February, 1982-83. Temperatures during this major
El Nino averaged about 5 degrees above normal across the Midwest and Great Lakes states.
Snowfall across the region was well below normal and in Southeast Lower Michigan, only
20 inches fell in Detroit and 33 in Flint. The average or "normal" seasonal snowfall for
Detroit is around 40 inches, while Flint has close to 45. This El Nino event has been the one
most commonly referred and equated to when speaking of the upcoming winter (1997-98)
expectations across the country.

Other recent major El Nino events occurred in the winter of 1957-58 and 1972-73. Looking
at the climate statistics from Detroit during all three winters showed interesting similarities
and as well as differences (see Table-1). Before getting into the winter data, a brief look at
Detroit's climate records for each of the fall seasons preceding these three winters is of
interest and can be compared to this fall (all normals used are current).

Temperatures during the fall of '57 averaged about normal in Detroit. Even though
September had normal rainfall, a very wet October (4.83"/+2.73 departure) and wet
November (3.29"/+.62), brought the total fall precipitation to well over three inches above
normal. So, basically the fall of '57 was near normal in temperature but on the wet side.
The fall of '72 trended more toward a cool but again, wet scenario. Temperatures averaged
nearly 2 1/2 degrees below normal with October having the biggest departure of around 4
degrees. Each fall month had higher than normal precipitation making the fall season total
about 1 1/4" above normal with September and November each having a half-inch surplus.
It also should be noted that November's snowfall was more than double the norm with
about 7 inches. The fall of '82 was the only one of the three with temperatures that averaged
above normal at about a degree and a half. And, even though the fall started out dry (both
September and October below normal), a very wet November (5.68/+3.01) more than made
up for the earlier deficit. So, the fall of '82 ended up mild and wet and thus, the one common
denominator in all three fall periods was they tended to be wet.

Looking at this fall, September fit into the slightly cooler and wetter mold, meanwhile
October was just about normal for temperatures (warm the first half/cold the second) and
a little drier than usual. November 1997 came in below normal in both temperature and
precipitation. Therefore, autumn of '97 will average cooler than normal, similar to that of
1972 but with less rainfall but close to the same snowfall (7.0"). Next, will briefly scanning
the upper wind patterns of all three fall seasons (Monthly Weather Review) further enhance
the above speculation?

The upper wind pattern (700 MB mean) of 1957 and 1972 showed a more persistent low
pressure trough axis over northern and eastern Canada and the eastern U.S. than in 1982.
In fact, when a trough did develop over eastern Canada and U.S. during the fall of '82, it
was more transitory in nature rather than a semi-permanent feature such as in 1957, 1972
and most of this fall. In place of the low pressure trough, a high pressure ridge, occasionally
unusually strong and persistent, was one of the more dominant features during the fall of '82
along with the strong westerlies from the Pacific.

The strong and more southerly placed Pacific jet stream associated with El Nino did develop
each fall but not until later in November during the fall of '72, which is similar to this fall.
While the general upper wind patterns of the fall of 1957 and 1972 were more alike than
1982, there were also differences. The most notable difference was the strength and southern
penetration of the polar jet stream across Canada and the eastern half of the U.S. A broader
and more pronounced cold area of low pressure held tough over this region through the fall
of '72, breaking down only occasionally, another similarity to this past fall. While a low
pressure trough was also over this region during the fall of '57, it was weaker and routinely
shunted east of the Great Lakes by the overpowering Pacific jet stream, especially from fall
to mid-winter. But in the fall of '72, a split flow in the upper wind pattern took shape over
the country which lasted at least into early winter. Speaking of winter, lets move on and see
how the winters of 1957, 1972 and 1982 unfolded...

                                      TABLE - 1
_______________________________________________________________________
YEAR              1957-58      1972-73          1982-83          NORMAL
------------------------------------------------------------------------------
MONTH:        TMP/SNW/PRE    TMP/SNW/PRE      TMP/SNW/PRE      TMP/SNW/PRE
------------------------------------------------------------------------------
DECEMBER    34.1/5.1/4.60    29.4/12.5/3.11   37.3/1.4 /3.29   28.3/10.4/2.82
(DEPART)   +5.8/-5.3/+1.78  +1.1/+2.1/+.29   +9.0/-9.0 /+.47
------------------------------------------------------------------------------
JANUARY     26.6/ 3.4/0.80   28.8/ 2.4/1.65   28.7/ 1.5 / .84  22.9/10.2/1.76
(DEPART)   +3.7/-6.8/-.96   +5.9/-7.8/-.11   +5.8/-8.7/-.92
------------------------------------------------------------------------------
FEBRUARY    22.8/4.1/0.67    25.3/12.8/1.08   31.6/4.3/.89    25.4/ 8.9/1.74 
(DEPART)   -2.6/-4.8/-1.07  - 0.1/+3.9/-.66  +6.2/-4.6/-.85
______________________________________________________________________________
WINTER         1957-58          1972-73           1982-83            NORMAL
TMP/SNW/PRE  27.8/18.0/6.07    27.8/45.0/5.84   32.5/20.0/5.02     25.5/40.6/6.32
(DEPART)     +2.3/-22.6/-.25   +2.3/+4.4/-.48   +7.0/-20.6/-1.30
______________________________________________________________________________

LEGEND: TMP: TEMPERATURES (f)        - NORMALS USED ARE CURRENT
                  SNW: SNOW(inch)            AS OF 1997         
                  PRE: PRECIPITATION (inch)

NOTE: Winter snowfall refers to snowfall measured the entire season, whereas winter 
precipitation refers to rain and snow (melted) during the December-February period.

One of the first things one notices when looking at the data is all three winter seasons
started out milder than normal with two seasons, 1957 and 1982, well above normal.
In addition, as one might expect, snowfall was well below normal during those two
Decembers. In fact, December of 1982 turned out to be the fifth warmest December on
record with the sixth least amount of snowfall. While temperatures in the winter of '72-73
also started out above normal, it was much less so than the winter of '57-58 and '82-83.
The other notable difference was the above normal, not below, snowfall that began in
November of '72 (7.0"/+4.1) and continued into December. December of '72 was also
wet with above normal precipitation (3.11") but not as wet as December of '57, which
placed eighth wettest December on record with 4.60". So, even with a strong El Nino
in place, the winter of 1972-73 began on a snowy and soggy note, somewhat contrary
to what might be expected.

The most striking and consistent trend during the three winters occurred in January. All
three Januarys averaged significantly above normal in temperature and well below normal
in snowfall. Looking further, all three January's precipitation (rain/snow) totals were also below
normal. By February, temperature results were a little more mixed with 1958 averaging below
normal, 1973 near normal and 1983 above normal. Still, two out of the three winters recorded less
snowfall.

The final tally of the three winter statistics show some interesting and somewhat surprising
results. While the winters of 1957-58 and 1982-83 were nearly identically deficient in
snowfall, the winters of 1957-58 and 1972-73 were amazingly identical in average winter
temperature. Although the winters of 1957-58 and 1972-73 both had a mean temperature
of 27.8 degrees, why did they differ so much in snowfall even though precipitation totals
were close? The answer, to a large extent, must lie in the upper wind pattern data referred
to earlier, when discussing the fall pattern.

While the general upper wind pattern of the falls of 1957 and l972 were more similar to
each other than 1982, there were also some differences. The most notable difference was
the stronger polar jet stream and its penetration further south during the fall and early winter
of '72-73. By January of '73, there was a shift in this polar jet northeastward to northeast
Canada and New England.The strong Pacific jet stream associated with El Nino could have
been the catalyst here that caused this breakdown and shift of the polar jet. This, in return,
lessened the frequency and intensity of cold air driven into the Great Lakes by mid-winter.
The change in the upper wind pattern is reflected nicely in Detroit's climate statistics. What
had started out as a somewhat snowy winter through December abruptly changed to mild
and nearly snowless one in January. January started out colder than normal but a strong
"January Thaw", caused or enhanced by El Nino, surged into the region mid-month sending
the temperature close to 60 on the 18th. The remainder of the month into early February
stayed on the mild side, but a shift back to the earlier upper wind pattern in mid February
brought back the snow and near normal temperatures. This resilient polar jet stream was
one of the main reasons the winter of '72-73 in Southeast Lower Michigan, even though
milder than normal, still recorded above normal snowfall. The interaction, between the
strong Pacific jet from the west and polar jet from the north, or "split flow", created a
fairly active storm track during much of the winter and, subsequently, more snow, but with
a notable break in the action during January. It was a different story late in the winter of
'57-58.The El Nino pattern weakened somewhat and allowed the polar jet to become the
more dominant wind flow across the Great Lakes with little phasing or merging with the
Pacific jet, resulting in colder weather but less snow.

As stated earlier, when looking at all three El Nino fall seasons, to a large extent this years
autumn has best resembled that of the fall of '72. This is not to say the pattern will not
change in December; in fact it should, if the past strong El Ninos are any indication. But,
the upper wind pattern of the fall into winter of 82-83 stands alone with its alternating
ridge of high pressure in the east with a mild zonal flow across the country.

All in all, several El Ninos of various intensities have occurred in recent history (since 1950)
during the years of 1951-52, 1957-58, 1963-64, 1965-66, 1969-70, 1972-73, 1976-77,
1982-83, 1986-87, 1987-88, 1991-92, 1997-98. When checking out all El Ninos and their
effect on our weather, even more variable results were found yet there still remained a few
common threads. While the results for February showed six out of the eleven to be milder
and drier, the remaining five were colder with variable snowfalls.So out of the three winter
months, February's weather, especially temperatures, seemed least affected by El Nino.
A possible theory could be that by late winter, the Pacific water temperature would have
normally cooled somewhat, creating less of an influence. During the El Nino winters,
December and January appear to be impacted more by the phenomena. Out of the eleven
El Nino winters, seven (nearly 65%) averaged above normal in temperature during December.
And, out of those seven that were milder than normal, five (about 70%) were also drier. In
January, the results were even more compelling with EIGHT out the eleven (nearly 75%)
milder than what is typically seen in January. Out of those eight, five (over 60%) were also
drier. This certainly adds credence to the "January Thaw" phenomena occasionally seen in
these parts but is El Nino responsible? At the very least, El Nino could be considered a
contributing factor to the phenomena This is not to say there were no Januarys in which
temperatures averaged significantly below normal, as there were two, in 1970 and.1977.

 

Most long time residents of Michigan will likely remember January 1977 for its brutal cold.
The monthly average temperature in Detroit was a pitiful 12.8, a good ten degrees below
normal and top ranking for the coldest January ever recorded in Detroit. If ever there were
contrary evidence against the MILD El Nino effect in Michigan, the winter of 1976-77
certainly fits the bill. During that El Nino winter, the December through January temperature
averaged a mere 17 degrees in Detroit, eight and a half degrees below normal. Temperatures
in February did recover to near normal levels, but the winter on the whole averaged 19.8,
or nearly six degrees below normal. Thus, the winter of 1976-77 (during an El Nino) ended
up the third coldest winter on record in Detroit. Meanwhile the winter of 1982-83, the
previously highlighted warm El Nino winter, came in the fourth warmest with an average
temperature of 32.5! Quite a contrast in temperature for the same basic weather phenomena.
It was also bit surprising to learn that out of the eleven El Ninos since 1950, only the winter
of 1982-83, placed in the top ten warmest winters in Detroit. Breaking it down further by
month, only 1982 and 1965 placed in December's top ten warmest, there were NONE in
January, which is quite surprising since January typically had the largest departures above
normal during El Ninos, and again, NONE made the list in February. On the flip side, there
was one December (1976) listed in the top ten coldest December, two in January (1970, 1977)
and none in February.

Regarding snowfall, while the average snowfall drops significantly to 27.4" when the
three major El Nino winters are used, if all eleven El Ninos snowfalls are averaged,
which gives a better sample, the average snowfall rises to 38.2", just a few inches shy
of the 40.6" normal. While no El Nino winters placed in the top ten snowiest winters,
three of the winters (1965-66,1957-58 and 1982-83) did place in the top ten snowless,
adding support to at least the drier El Nino winter scenario. By monthly though, the
snow results were more mixed. For December, one (1951) placed in the top ten snowiest
while two (1965, 1982), ranked in the top ten snowless. January's top ten results were
one and one (1987/snowiest, 1983/snowless) and fnally February had one (1988) place
in the snowiest and two (1966, 1992) in the snowless category. Incidently, since 1870
(when records began in Detroit) only one winter month had no snow and that was way
back in December 1889. Who knows, maybe it was during an El Nino winter?

In conclusion, even though this winter's El Nino is very strong and has been equated to
that of 1982-83, this does not necessarily mean the winter of 1997-98 in Southeast Lower
Michigan will be as mild and snowless. Since there are so many factors which influence
the weather in Southeast Lower Michigan and the Great Lakes it is difficult, if not
impossible to rely on only one indicator to predict our weather.


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