Composite 500 mb and Surface Fields: A Graphical Comparision of May-June Fields Relevant to Severe Thunderstorm Formation for Recent Type 1 El Niño and La Niña Years

John Monteverdi
Department of Geosciences
San Francisco State University

May-June Composites

Maps Created Using Climatic Diagnostics Center Composite Generation Page

This study was occasioned by the widespread media coverage of a study that suggested that Great Plains tornado events are more likely in El Niño years and less likely in La Niña years. Further, that study suggested that during La Niña years maximum tornado occurrence should be in the Tennessee River/Ohio River Valleys and upper Midwest.

These conclusions, based upon numbers of tornadoes and not tornado days, seem to contradict the observations of experienced storm chasers who found that Spring 1998 ( El Niño was still in force in Spring 1998) was one of the worst seasons in recent memory for tornadic supercells in the southern and central Great Plains, with significant activity shifted northeastward or eastward (e.g., Tennessee outbreak).

A logical inference from the study was that the synoptic patterns normally associated with severe thunderstorms (and, in certain cases, tornadic supercells) are more likely to occur (or, to have occurred) in El Niño years. Since this also flies in the face of the experience of storm chasers/researchers in the Plains, I have quickly assembled composite charts for recent Type 1 (large) El Niño and La Niña years to examine the contention that El Niño patterns would be more favorable for severe storm environments.

As you will see, this study does NOT substantiate the contention. In fact, composite patterns for La Niña events would suggest much more favorable environments for severe storm formation, whereas those for El Niño years appear to be very unfavorable.



1. Precipitation and Mid-tropospheric Vertical Motion Composites


Click To See Full Size

Composite May-June Precipitation Anomalies, La Niña

Precipitation fields composited for the eight La Niña May-June periods since 1950 suggest a very dry May-June for most of TX, but generally normal or wet conditions forTX Panhandle, OK Panhandle, Kansas, NE New Mexico, SE Colorado and Nebraska.


La Niña 500 mb Anomalous Omega

(Negative Numbers Indicate Upward Motion)

Suggests synoptic-scale vertical wind fields favoring destabilization from SW TX, TX Panhandle, SE and E Colorado, KS and Nebraska . Note that vertical motion field is somewhat consistent with the precipitation forecast in the southern Plains.


2. La Niña and El Niño Composite Comparisons

 Type 1 El Niño

(Year Shown is Last Year of Season, i.e., 1997-98 = 1998)

1973, 1978, 1983, 1992, 1998

 La Niña

(Year Shown is Last Year of Season, i.e., 1997-98 = 1998)

1965, 1971, 1974, 1976, 1989, 1996

La Niña - Type 1 El Niño

Differenced La Niña and El Niño Fields

(La Niña Means) - (El Niño Means)




El Niño 500 mb Heights

Stronger zonal flow in southern tier. Strong west winds in mid troposphere over southern Great Plains. Chart implies very strong 300 mb (anvil level and storm relative) flow.


La Niña 500 mb Heights

Deeper trough in west with more amplitude with mean ridge axis in Southern Great Plains. Suggests that mid tropospheric southwest winds in the west portions of south Great Plains.

Differenced 500 mb Heights

(Read Below)


El Niño 500 mb Height Anomalies

Suggests greatest upper divergence (east side of anomalous troughing inferred at 300 mb) shifted northeast towards Great Lakes through Ohio Valley.


La Niña 500 mb Height Anomalies

Suggests upper divergence (east side of anomalous troughing inferred at 300 mb) over region from Four Corners, SE Colorado-TX and OK Panhandles, KS.

This should be compared to the 500 mb height and height anomalies composites to the left in this row and the previous.

During El Niño years there is a tendency for troughing in the northern Plains eastward. But during La Niñas the composite suggests that troughing occurs in the Great Basin.

Hence, the difference between the two normalized composites indicates that during La Niñas 500 mb heights are much higher from the northern Plains eastward to the Atlantic than the heights for the El Niño composites, with lower heights in the Great Basin during La Niñas.



El Niño 500 mb Anomalous Omega

(Negative Numbers Indicate Upward Motion)

Suggests a tendency for subsidence and capping over southern and central Plains. The exception is extreme southern portion of west Texas.


La Niña 500 mb Anomalous Omega

(Negative Numbers Indicate Upward Motion)

Suggests synoptic-scale vertical wind fields from SW TX, TX Panhandle, SE and E Colorado, KS and Nebraska favoring destabilization.

Differenced 500 mb Omega

(Negative Numbers Indicate Upward Motion)

This verifies (also suggested by the500 mb patterns above) that the south-central Plains and southwest desert areas should experience synoptic-scale upward motion to a greater degree during La Niña.



El Niño Surface Pressure Field

Compare to La Niña pattern. This pattern actually suggests a weaker than normal low pressure region east of the Rockies and over the desert Southwest.


La Niña Surface Pressure Field

Suggests deeper return moisture through TX and strong low level shear (when taking into account the midtropospheric winds suggested by 500 mb chart above).

Differenced Surface Pressure Field

(Read Below)


El Niño Surface Pressure Anomaly

Suggests anomalous flow from west over southern Plains. Note the positive pressure anomaly over the West. This suggests focus of moist flow from Gulf would be the Ohio River Valley.


La Niña Surface Pressure Anomaly

Suggests much stronger than normal flow from the Gulf to the latitude of Nebraska. Note suggestion of forcing for surface flow towards the north from deep south portion of Gulf.

Recall that during spring in 1998, there was a tendency for surface storminess to be centered in the northern Ohio River Valley/Tennessee River Valleys northward.

This is reflected in the difference between the La Niña and El Niño composites: note that during La Niñas the surface pressures are much higher in those areas than during El Niños.

Also, there is a much higher frequency of surface cyclogenesis and cyclone passage over the great Basin and western Plains during La Niñas.



El Niño Anomalous Component, Surface Winds

Suggests what we saw last year--a lot of dry air in the southern Plains. This is consistent with previous two charts. Note anomalous northerly and westerly component ot the surface wind field in the southern Plains.


La Niña Anomalous Component, Surface Winds

Suggests great moisture flow and warm advection from the Gulf. Also,together with the 500 mb flow shown above, differential temperature and moisture advection and wind shear profiles favorable for severe storms/supercells.

Anomalous Component, Surface Winds

As one would expect from the surface charts above, the vector difference between composite La Niña and composite El Niño surface winds suggests much greater transport from the Gulf of Mexico in La Niña years.