El Ñino is a climate pattern that occurs periodically in the Pacific Ocean that is is characterized by abnormally-warm sea surface temperatures (SST) in the Eastern Pacific Ocean, defined as when the three-month average of sea-surface temperatures in a strip between latitudes five degrees north and south and longitudes 170 degrees west and 120 degrees west exceeds the long-term mean by 0.5 degrees Celsius.
Typically building in strength from March through June, El Ñino has the greatest effect on weather in North America in the December – February time frame. The abnormally-warm SSTs cause a disruption in the normal Walker circulation, bringing more convection to the eastern Pacific and, through teleconnection, affecting weather throughout North America. Although there are several theories as to the cause of El Ñino, there is no consensus yet. Regardless of cause, its effect is clear.
I will be examining the El Ñino that occurred in 1982-1983 and the impacts it had on the subtropical jet stream (STJ) and on seasonal temperature and precipitation in the Gulf Coast States in the United States. The 1982-1983 El Ñino was the strongest since records on these patterns have been kept (1958). This strong alteration in the SST had a significant impact on the STJ and on weather in the US Gulf Coast.
El Ñino and the Subtropical Jet
The subtropical jet stream is a narrow ribbon of high speed winds, found on the equatorward side of the subtropical front. It is characterized by high speed winds increasing with altitude from about 400mb to 200mb or so, where they reach their maximum.
Note that the typical STJ tends to fade over the central and eastern Pacific before regaining some strength over Mexico and the southern US. The STJ’s root can be traced to temperature gradients at about 400mb. The tropics are generally devoid of significant horizontal temperature gradients at the surface, but at 400mb a relatively strong north-south gradient forms as a result of Hadley circulation. It is this gradient that is the root of the STJ, which itself is strongest at 200mb.
Now let’s examine the STJ in the winter of the 1982-83 El Ñino event.
In the winter of 1982-83, the band of strong winds extending off of Japan extended much further into the Pacific, across the date line and almost to California. Additionally, the “gap” in the STJ that normally exists over the eastern Pacific has nearly disappeared, with a larger-than-normal pocket of higher-speed STJ from south of Baja California extending to off the coast of the Carolinas. What caused this enhanced STJ during the winter of 1982-83?
During the 1982-83 El Ñino, there was a pool of significantly-warmer-than-normal air over the eastern equatorial Pacific, off of South America, and a commensurately-cool pocket of air over the North Pacific. This sets up an anomalously-strong horizontal temperature gradient along the path of the STJ. The temperature anomaly contributed to a significant height anomaly. Rembering that the root of the STJ is at about 400mb, we can see why this resulted in increased STJ at 200mb extending further east than normal climatology would suggest.
What caused these anomalies? The abnormally-warm SST during an El Ñino causes the overlying atmosphere to also become unusually warm in the central and eastern Pacific. As this warm air rises, forming the Hadley circulation, the north-south temperature gradient at 400 mb becomes stronger than in normal years. The 1982-1983 El Ñino was particularly strong, and thus SSTs were quite high. The effect of this was enhanced baroclinicity, starting at the surface and extending all the way up to 200mb, resulting in a strengthening of the STJ.
Now that we understand how El Ñino serves to enhance the STJ, we turn to the regional effects of it on weather in the US.
Seasonal Impacts of El Ñino
The extended El Ñino-influenced STJ over North America affects weather patterns over the Gulf states of the US, including precipitation and temperature.
Precipitation in the Gulf states is significantly higher in our El Ñino event than normal. The STJ plays an important role in cyclogenesis and transporting moisture. The STJ and jet streaks enhance upper-air divergence, which helps intensify surface lows.
The divergence in the left-exit regions of these jet streaks can be seen to contribute to the precipitation in the Pacific Northwest of the US as well as the area off coastal Carolina. These divergence-rich area are rich grounds for cyclogenesis and accompanying precipitation. Furthermore, the fast and strong STJ serves to transport moisture from the maritime environment in the eastern Pacific and the Gulf into the adjoining Gulf states of the US.
We can see from this that the 1982-1983 El Ñino did not have significant effects on temperature anomalies in the southeast. However, a pattern of warmer-than-normal surface temperatures did exist over the North Central, Great Lakes and Northeastern states. This is typical of an El Ñino year.
Why was it that the Gulf Coast states were, if anything, warmer than typical in an El Ñino? I suggest that this is because the strong El Ñino of 1982-83 had a strong STJ, which transported warmer air from the Pacific across Mexico into the region. This would have the effect of moderating temperatures in the Gulf area, and perhaps moving those states back towards climatological norms. Let’s compare this to the map showing strong El Ñino’s.
We can look to a specific day as an example, with the caveat that ENSO cannot predict specific weather on specific days. Looking at 0600Z on December 12, 1982, we can see that a significant trough existed in the central part of the country.
This jet streak is part of the anomalous STJ that existed during the 1982-83 El Ñino. Going back to our earlier conversation, the normal STJ during non-ENSO years doesn’t pick up steam until further east, towards the eastern seaboard. In El Ñino years, the STJ’s strength over Mexico and Texas is stronger, and the setup on December 12, 1982, is consistent with this. This positioned the divergence-rich left exit region of the jet streak over Mississippi, Alabama and Georgia.
The result of this was precipitation ‘downstream’ towards the Carolinas.
Additionally, we should note that the STJ is further north than the average for the 1982-83 DJF time period. As was noted in lesson 4, when the STJ is carried north, particularly when there is a strong jet streak over the eastern half of the US, there is a favorable breeding ground for storms in the coastal Atlantic. That is what seems to have happened on December 12, 1982. Indeed, on the map below, we can see the polar jet stream and STJ interacting:
Although the precise causes of El Ñino are not known, it is fairly clear that the seasonal changes to the STJ in the 1982-83 cold season are an effect of El Ñino, not the cause.
El Ñino is a warming of the SSTs of the central and eastern equatorial Pacific. As we demonstrated above, the warming of the SSTs causes a warming of the atmosphere. The enhanced baroclinicity related to this warming is the primary cause of the enhancement to the STJ during El Ñino episodes. There is anomalously-large north-south temperature gradient, which in turn contributes to similar height anomalies. This is the root cause of the extension of the STJ eastward from it’s normal location as shown my climatology.
Of course, general patterns such as the enhanced STJ during an El Ñino cannot be said to cause specific storms or specific localized weather conditions. Instead, the large-scale patterns established can help us establish conditional probabilities on specific weather patterns such as local or regional temperatures or precipitation. The teleconnections established by El Ñino in strong episodes, such as 1982-83, are fairly strong themselves and thus the probabilities can be forecast with greater definition. However, even the effects of strong ENSO episodes interact with many other atmospheric conditions and forces, and thus the exact impact cannot be predicted far in advance.