ENSO Effects on Severe Climatology, U.S
With warming effects in the Pacific lasting from June 1997 through April 1998, the
winter El Nino of the ’97-’98 season was the most devastating to hit the United States
marking the seventh warmest and second wettest winter since 1895. Through the
combined heating of equatorial waters and intense convection off the South American
coast, a strong sub-tropical jet developed in association with a blocking ridge of highpressure from a stalled polar front to the north. With arctic air being confined into
Canada, the northern plains averaged from 5 to 150 F above normal for the month.
Elsewhere, the southern states received record-breaking levels of precipitation and
slightly cooler temperatures as winter storm tracks from the Pacific were being diverted
farther south in association with a stronger southerly jet. In many places, months of
almost continuous rainfall unleashed deadly floods, mudslides, and agricultural
deprivation on parts of southern California, Florida, and the Appalachian Mtns. One
specific storm in January, aided by increased southerly flow and a pulse of arctic air from
Canada, produced flooding rains from the lower Mississippi valley into the northeast in
addition to tornadoes and one of the worst ice storms of the decade.
The effect of ESNO conditions on upper-level systems of circulation is crucial in
defining weather patterns within the United States. It is also perhaps the least understood
facet of an El Nino event that plays a major role in severe weather activity from heavy
rains, to thunderstorms, and tornado activity. In 1998, 1424 tornadoes occurred during
the severe weather season, which, before the 2004 season of 1722 - mostly due to an
overly active hurricane season - was the highest annual number of tornadoes ever
reported. Possible connections between the ESNO occurrence and tornado activity are
still tenuous at best but can be indirectly linked with Pacific convection and affects on
mid-latitude jet streams.
One aspect of research is to examine a possible increase in tornadic activity in
association with ENSO fluctuations. It is a hypothesis that with unusually warm waters in
place in the Western Hemisphere Pacific, the relative strength of the sub-tropical jet
stream increases in response to steeper height gradients at the 500 mb level. A
strengthened feature such as this jet can provide areas of the southern United States with
un-seasonable amounts of moisture ahead of developing low-pressure cores (steered into
the unstable air by the same upper-air feature). It is possible that this unusual storm
activity can lead to conditions of instability with vertical/horizontal shear favorable for
tornadic development. It is believed, however, that these shear conditions are not the
primary effects of ENSO events, just the amount of available moisture for storm
initiation. The data will be supported by a variety of past studies mostly in response to the
recent and costly tornadic outbreaks coincidently (or not) associated with the El Nino of
1998. The importance of collecting and correlating past data on tornadic occurrences is to
understand the greater trend in severe weather history and its association with strong
ENSO conditions.
In studying large-scale impacts of ENSO events (with imbalances in precipitation
around the world) the effects of these shifts can be used to illustrate specific instances of
climate change within the United States. One of the questions are research should answer
is how a 6-month change in sea surface temperatures is able to dramatically increase
severe weather activity on land and whether or not this affects the overall number and
distribution of tornadoes. In researching this distribution, the primarily focus was on two
specific locations across the country where tornado frequency could be carefully
monitored and compared to an average or base-year of frequency and intensity. Through
data available on statewide tornado occurrences from the National Climatic Data Center,
a comprehensive listing on significant (≥ F2) tornadoes and violent (F4/F5) tornadoes
was compiled on a case-by-case basis with an emphasis on not only correlation between
ENSO events, but also correlations between tornado frequency/intensity within each year
(month to month). This allowed us to take a more analytical approach in viewing the
original data set (divided by Fujita intensity but lacking any clear distinction between
either a single tornado or multiple events). Once analyzed, the data should help to
illustrate a clear picture as to the changing severe weather climatology in the past 50
years for both Oklahoma and Ohio on a local and national level.
In order to understand the logic behind our selection of these two states for a
frequency/distribution analyses on tornadoes, one must have some appreciation for the
historical perspective on severe weather in addition to the bigger (synoptic) picture of
weather affecting the United States. Tornadoes themselves form in association with a
complex interaction of frontal zones and wind shear providing both a level of vertical and
horizontal stability within storm updrafts. The key to understanding severe weather
begins at the larger or synoptic scale of upper-level and surface dynamics providing
varying degrees of instability essential in the formation of widespread convection. In
choosing Ohio and Oklahoma as two states to represent a specific mix of convective
ingredients, our approach to analyzing the data was first focused on the synoptic patterns
generally affecting both regions and how they have changed in response to global
influences such as ENSO.
Oklahoma, incidentally, suffers from the highest density of tornado strikes per area
in the United States due to its local and synoptic climatology, in place year after year
across the plains each spring. We felt compelled to see how the state with the highest
tornado rankings faired throughout the years in terms of frequency and intensity of
accounts as a possible indicator of climate change. A general pattern of convection
across the southern plains (OK) region, for example, focuses on local degrees of
instability such as daytime heating and the dryline in association with broader surface and
upper-level features. Typically, March through May in the Great Plains brings a special
blend of instability through cold, upper-level vortices along the polar front reacting with
buoyant masses of gulf moisture. With dew points in the 60s and 70s advecting farther
north in response to steeper ridge gradients (a product of the subtropical jet), the 500 mb
scheme becomes instrumental in decoding the varying risks for convectively available
potential energy and lifted indexes.
The Oklahoma and Texas panhandle regions provide an interesting dynamic found
only in the great plains turning a situation of absolute instability into one of
conditionality. Under this scenario, the severity of single cell convection becomes a
critical component to tornado-genesis as dry slots of descending air from the Rockies
cause temporary temperature inversions at the 10,000 ft layer in the atmosphere
(Bluestein, 26, 27). Lifted indexes values soar here in the spring and early summer
months as daytime heating forces all the buoyant gulf moisture to press against the ‘cap’
in an effort to form cells of convection. Potential energy rises throughout the afternoon
with the daytime heating maximized around 3 or 4 in the afternoon until subsiding in the
evening and overnight hours. The window of opportunity that allows the region to
proliferate with cyclonic activity occurs when a local feature, either warm sector or
dryline, moves across the state in association with a surface low and upper-level winds.
Here the cyclonic activity of the low brings unusually warm and buoyant air in
concurrence with much colder air aloft as the polar jet, in extreme cases, reverse tilts the
upper-air trough from northwest to southeast, overlapping the two airmasses in perfect
unison (Bluestein, 37-40). With such violent instability in place over a large area (and a
small degree of stability (dry slot) between the mixtures) it would seem only a matter of
time before the two layers were brought in contact. Local lifting mechanisms, allow this
to happen by temporarily increasing the buoyancy of the heated air-parcel above critical
levels (as a measure of the lifted indexes value) causing one updraft to break free to
dominant the surrounding atmosphere. The resulting detonation, of sorts, accelerates the
saturated airmass upward at speeds reaching 100 mph in the condensation of over a
billion gallons of air-born water. The adiabatic cooling of the water vapor relative to the
surrounding environmental conditions determines the height of the updraft and the
severity of weather that will ensue. Under this scenario, a conditionally unstable mixture
allows for limited convection in the form of single-cell breaches of the upper-level ‘cap’.
One updraft, instead of a multi-cell amalgamation, has the chance to become a highly
organized and long-lived severe storm with the increased threat for tornadoes. This
supercell, depending on the environmental conditions, can then initiate multi-cell
outbreaks of severe weather by dividing itself along right-turns in connection with upperlevel winds (Bluestein, 68).
In Ohio, we were interested in seeing how our own state, which represents a
crucial demographic of severe climatology in the upper-Midwest, compares to the annual
frequency and intensity of Great Plains tornadoes. Single-cell convection in the spring
and early summer shift from the Great Plains into the Ohio valley one day out from
inception with the original ingredients of ‘cyclogenesis’ still mostly intact. States like
Ohio, Indiana, and Illinois are directly influenced by the large-scale lifting mechanisms
associated with a mature midlatitude cyclone. The cold front is the primary focus for
severe weather in the upper-Midwest and defines an altogether different frequency and
evolution of the severe storm than the isolated supercell of the Great Plains. Here, storm
motion is generally faster, moving along at the same pace as its parent frontal boundary.
Also, the dynamics of the cellular interactions within storms change from a highly
ordered supercell with distinct updraft, downdrafts to a more disordered/chaotic mix of
storms in either a convective mesoscale complex of cells or a more ordered squall
complex aligned with the front. These multi-cell clusters yield a wider spread of severe
weather coverage with the potential for effects across multiple states rather than one or
two as exhibited with isolated cells across the Great Plains. And as the data suggests,
while an increase in the disorder of the storm complex prohibits most tornado formation,
the shear coverage of the event allows for an increased probability that one or two cells
will break off of the squall forming their own single-cell event.
With Ohio, at least in the past, this has typically been the case where on average the
tornado count is nowhere near the number of reported events in the Great Plains until
upper-air dynamics (usually occurring in April) inject the right amount of energy into the
mix. Powerful frontal boundaries in association with deep centers of low pressure,
saturated gulf stream moisture, and fierce winds aloft with arctic temperatures of 30 to
400 F below zero initiate the tornado outbreak as a unique consequence to the large-scale
forcing of the cold front. As the critical boundary displaces warm, saturated air with into
the upper-levels of the atmosphere, the upper-air dynamics take hold into shaping the
resulting complex. In Ohio, some of the most famous tornadoes to set down in the
United States were associated with rapidly developing storm systems along cold fronts
with fast, long tracts of unpredictable levels of destruction due to intense upper-level
dynamics (+160 knts) (Hoxit, Chappell, 1975).
A principal reason for studying such fast-moving and transient systems is their
association with the eastern United States based on the typical model for cyclonic
development. Famous tornadoes to develop under such scenarios of deeper surface lows,
faster upper-level winds, and a powerful lifting mechanism at the surface were events like
the tri-state tornado of 1925 and the super outbreak of 148 tornadoes (6 F5s) including
Xenia, Ohio. The ferocity of such storms can hardly be comprehended without
witnessing them first-hand but one statistic seems quite fitting: in one forty minute
segment of the record 3.5 hour tri-state tornado, 540 people were killed and 1,423 were
seriously injured in southern Illinois (toll of 695 across MO, Il, IN) (Hoxit, Chappell,
1975). Perhaps the biggest concern of ESNO effects on changing severe weather
climatology would be the displacement of such tornadoes into more populated,
underdeveloped areas such as the southeast. Here living conditions mimic those of areas
wiped off the face of the earth 75 years ago like Murphysboro, Il (234 dead in 1925) and
would likely suffer the most direct effects of such incredible tornadoes. It was for this
reason that we studied the interesting dynamics of the upper and lower atmosphere in
association with the greater ENSO picture. In areas like the Great Pains we feel that with
changing upper-air features such as a re-oriented and intensified sub tropical jet,
influences on patterns and intensities of sever weather events and outbreaks in the United
States can be affected and perhaps completely changed.
It is crucial to understand how these patterns might change especially in light of
recent, stronger ENSO events in ‘97/’98 that displaced severe weather and tornadoes
farther east in the winter into more populated regions with little or no experience in reoccurring tornado outbreaks with the stronger influx of jet moisture. States in these
regions, while responding positively in their severe weather counts for El Nino years (like
F5s in both Alabama and Tennessee in 1998 along with a deadly outbreak of F3s in
Florida) were not included in our study because we were more interested in seeing how
normal patters of non-ENSO events have changed in recent years. For our purposes, the
states of Oklahoma and Ohio represent more normal regions of tornado occurrences for
their specific regions.
In visually analyzing the data for both states along with a listing of El Nino/La
Nino years on their SOI values (see correlating graphs),
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“The southward diversion of
the frontal systems by the Pacific/North American (PNA) pressure pattern could
potentially affect numbers, locations, and strengths of tornadoes in the United States”
(Knowles, Pielke, 1993). A consequence of this pressure pattern is higher than normal
rainfall in the western and southern United States caused through an influx of gulf stream
moisture from the subtropical jet. In recent ENSO trends, oscillation in the strength and
location of this current is the determining factor in tornado frequency with the ’98
tornado season being a major indicator.
In both Ohio and Oklahoma, frequency of significant tornadoes (considered as F2
or greater) have seen a significant decline since the mid-eighties with only brief periods
of recovery in the past few years especially the moderate La Nina of ’99. Since La Nina
episodes are marked by stronger temperature contrasts between warmer air in the
southeast and cooler air in the upper west, there would be an increase in the number of
days favorable for tornadic development (Knowles, Pielke, 1993). La Nina, in a sense,
represents a brief return to normality for historically tornadic areas where stronger
tornadoes are aided in the increased temperature clash between warm gulf stream
moisture and cold/dry air to the west. The polar jet, also, increases in depth under Las
Nina condition brining colder air deeper into the west to strengthen temperature contrast
between areas to the east. Both Oklahoma and Ohio saw a greater number of significant
tornadoes in 1999 with a significant outbreak in Oklahoma (including one F5) and an F4
in Cincinnati, yet numbers of significant tornadoes still were only considered “normal”
compared to past years. Within the first three decades of observation, tornado numbers in
Oklahoma averaged nearly 20 F2 or greater accounts a year (5: Ohio) but just under 10
(3: Ohio) for years since 1980. Even for reported El Nino years such as 1973, and 1968
saw far greater numbers than Nino events in the 80’s and 90’s, and the strongest La Nino
ever recorded, incidentally, occurred may have been responsible for the superoutbreak of
tornadoes in April of 1974 as an exacerbation of climate dynamics already at peak levels
during the year. Either way, the data leads us to believe that it is possible the southern
oscillation isn’t the only component effecting the range and frequency of severe weather
in the United States.
Recent warming trends from greenhouse gas emissions might play a more critical
role in severe climatology than previously thought with areas of more moderately warm
and saturated air stabilizing the atmosphere against violent levels of convection. While
thunderstorm activity no doubt increases as a result of ESNO activity, conditional
instability needed to initiate violent convection (i.e. severe single cell) relies more on
extreme temperature contrasts than available moisture. Without the strength of upper-
level winds in the polar jet needed to initiate surface vorticity, more moderate
temperatures recently associated with the southeastern states (especially under increasing
numbers of ENSO occurrences) may put an end to the traditional tornado alley. Recently
we are seeing more tornadoes in southern states reaching the level of intensity usually
reserved for the Great Plains. In addition, many late fall outbreaks in October and
November are becoming the dominant source of significant tornado counts such as a 70
tornado outbreak on November 10th, 2002 in the Ohio and Tennessee valleys comprising most of the significant tornado counts for the year with the strongest (an F4)
in Ohio.
Such anomalies in the data illustrate a disturbing pattern of climate shift even at
the impossibly small scale of the tornado. With a significant decrease in severe weather
climatology over an entire geographic area in addition to time of year, concerns exist
over the relative adaptability of new areas to an increased tornado threat such as the
southeast. It appears that the Great Plains no longer provide the ‘ideal’ conditions for
tornadoes unless supported by La Nina conditions of increased temperature contrast.
With ENSO conditions persisting longer and more frequently due to excessive heat in the
tropical Pacific, we can expect lulls in tornado activity in the traditional tornado season
and more intense periods such as late winter and late fall where the upper-level
conditions are finally favorable for temperature contrasts to take place in association with
large amounts of saturated air. Under this scenario, tornado outbreaks that have typically
been a ‘positive’ sign of transition from winter to summer and occurring in relatively
remote regions may become more of an urban spectacle with disastrous consequences.
With larger populations at risk from tornadoes occurring under more unseasonable
circumstances, the risk for casualties drastically increases especially in the southeast
where tornadoes form rapidly in multi-cell clusters with little advanced warning.
The data ultimately suggests, however, that such short-term decreases in tornado
frequency and intensity under historic trends may only be the result of cyclical levels in
upper-air activity. Especially with only an amateur understanding of tornado-genesis and
the Fujita damage scale when data became available in 1950, we really have been taken
detailed reports as recently as the past ten years. With such a limited understanding of
tornadoes and sparse data at our disposal, it becomes impossible to make such hypothesis
of any permanence especially on numbers alone. Damage estimates, incidentally, have
increased almost exponentially in the past ten years mostly due to urban growth
throughout the country. The real consequence of global climate change may not actually
be a change in weather coverage but our own coverage across the earth that puts larger
populations at risk for ‘unseasonable’ weather. Thus, ENSO cannot be directly proven to
effect local shifts in climatology especially on the scale of the severe storm.