Tampa Bay Area, FL

Weather Forecast Office

ENSO's Effect on our Winter Weather

El Niño-Southern Oscillation (ENSO) effect on Winter Weather across West Central and Southwest Florida
El Niño and La Niña are opposite phases of a natural climate pattern across the tropical Pacific Ocean that swings back and forth every 3-7 years on average. Together, they are called ENSO (pronounced “en-so”), which is short for El Niño-Southern Oscillation. The ENSO pattern in the tropical Pacific can be in one of three states: El Niño, Neutral, or La Niña. El Niño (the warm phase) and La Niña (the cool phase) lead to significant differences from the average ocean temperatures, winds, surface pressure, and rainfall across parts of the tropical Pacific. Neutral indicates that conditions are near their long-term average.

During an El Niño Winter the southern tier of Alaska and the U.S. Pacific Northwest tend to be warmer than average, whereas the U.S. southern tier of states—from California to the Carolinas—tends to be cooler and wetter than average. Meanwhile La Niña typically brings above-average precipitation and colder-than-average temperatures along the northern tier of the U.S., along with below-average precipitation and above-average temperatures across the South. An analysis of the 300 millibar (≅30,000 feet) wind speeds, the appoximate location of the jetstream, during the different ENSO phases depicts rather well how the location of the jetstream shifts with the stronger winds (red areas in image below) further south during El Niño and further north during La Niña.
300 mb Wind Speeds during different ENSO phases
However, there are other climate patterns, such as the Madden-Julian Oscillation and Arctic Oscillation (AO), that can also play a role in determining winter weather. For example, the AO influences the number of arctic air masses that intrude into the U.S., but its predictability is limited to a couple of weeks. More local information about how the AO effects the weather across the Florida peninsula can be found at Arctic Oscillation: Impacts on West Central and Southwest Florida
Most images on this page can be magnified by clicking, and resized back to original size with a second click.

ENSO Details
ENSO History
Local Rainfall
Local Temperature
Local Severe
Local Freezes
Local Sky Cover
Links

ENSO State:
ENSO Phase Meter		Tropical Pacific SST Anomalies
CPC ENSO: Recent Evolution, Current Status and Predictions

What is ENSO? El Niño and La Niña are the warm and cool phases of a recurring climate pattern across the tropical Pacific—the El Niño-Southern Oscillation, or “ENSO” for short. The pattern shifts back and forth irregularly every two to seven years, bringing predictable shifts in ocean surface temperature and disrupting the wind and rainfall patterns across the tropics. These changes have a cascade of global side effects.		ENSO Cross Section over Pacific Ocean

Though ENSO is a single climate phenomenon, it has three states, or phases, it can be in. The two opposite phases, “El Niño” and “La Niña”, require certain changes in both the ocean and the atmosphere because ENSO is a coupled climate phenomenon. “Neutral” is in the middle of the continuum.
El Niño: A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean. Over Indonesia, cloudiness and rainfall tend to become reduced with a corresponding increase in the average surface pressure. In the central and eastern part of the tropical Pacific Ocean cloudiness and rainfall increases with a corresponding drop in the average surface pressure. The low-level surface winds, which normally blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start blowing the other direction (from west to east or “westerly winds”). La Niña: A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean. Over Indonesia, cloudiness and rainfall tend to increase with a corresponding drop in average surface pressure. Meanwhile cloudiness and rainfall decreases over the central and eastern tropical Pacific Ocean with a corresponding rise in average surface pressure. The normal easterly winds along the equator become even stronger. Neutral: Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average. However, there are some instances when the ocean can look like it is in an El Niño or La Niña state, but the atmosphere is not playing along (or vice versa).
ENSO Sea Surface Temperature Anomalies

So what is the criteria for an “El Niño” or “La Niña” to be declared officially?
El Niño criteria:
Average sea surface temperatures in the Niño 3.4 region (see image below) of the equatorial Pacific Ocean were at least 0.5°C (0.9°F) warmer than average (5°N-5°S, 120°W-170°W) in the preceding month, and the anomaly has persisted or is expected to persist for 5 consecutive, overlapping 3-month periods (e.g., DJF, JFM, FMA, etc), and the atmosphere over the tropical Pacific exhibits one or more of the changes commonly associated with El Niño:
El Niño Flow Chart
La Niña criteria:
Average sea surface temperatures in the Niño 3.4 region (see image below) of the equatorial Pacific Ocean (5°N-5°S, 120°W-170°W) were at least 0.5°C (0.9°F) cooler than average in the preceding month, and an average anomaly of at least -0.5°C has persisted or is expected to persist for 5 consecutive, overlapping 3-month periods (e.g., DJF, JFM, FMA, etc), and the atmosphere over the tropical Pacific exhibits changes commonly associated with La Niña, including one or more of the following:
ENSO Regions:
Location of the parts of the tropical Pacific used for monitoring sea surface temperature. As noted in the above criteria the Niño 3.4 region (red box in graphic below) is the area monitored to determine the ENSO phase.
ENSO Regions
Other climate patterns, such as the Madden-Julian Oscillation can also play a role in determining winter weather.

ENSO History:

We had a strong El Niño this past winter with temperatures averaging slightly below to slightly above normal, while rainfall was near to well above normal (more details can be found here.) Before this, La Niña conditions existed the previous three winters with temperatures generally averaging near to slightly above normal while rainfall ranged from well below normal to near normal. Following are U.S. maps of the Winter (December-February) departure from average precipitation and temperature for each category of El Niño and La Niña since 1950 (These graphics do not include this past winter at this time).

El Niño Precipitation		El Niño Temperature

La Niña Precipitation		La Niña Temperature

ENSO Local Rainfall:

During El Niño there tends to be more stormy conditions across the Florida peninsula during the winter and early spring. Cold fronts will move across the region and with the jet stream further south and stronger, rainfall is more widespread and therefore usually above average. This can be seen in the graphics below where rainfall totals during the dry season average much higher during El Niño winters then during La Niña winters.

Graph Legend

Another way to see this is by viewing the Box and Whisker Distribution Plots* for the three climate zones across the Florida peninsula shown below:


Northern Florida Peninsula	Central Florida Peninsula	Southern Florida Peninsula
*To better understand these plots visit the following CPC web page: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/box_whiskers/info.php

ENSO Local Temperature:

The link between ENSO and wintertime temperatures is a little bit weaker. With the average storm track farther south during El Niño temperatures tend to average slightly below normal thanks to the increased clouds and rain that keep daytime highs cooler. This is in contrast to La Niña where we usually see drier air over the state with more fair weather days. This drier air leads to warmer daytime and slightly cooler nighttime temperatures that overall result in slightly above average temperatures across the area.

What plays a larger role in controlling temperatures across Florida during the winter and early spring is the Arctic Oscillation (AO) as we saw back in early 2010. This oscillation shifts on a monthly and sometimes weekly basis and typically is not included in long term forecasts. For more details about the AO and its effect on West Central and Southwest Florida visit Arctic Oscillation: Impacts on West Central and Southwest Florida.

Graph Legend

This can also be seen in the Box and Whisker Distribution Plots* for the three climate zones across the Florida peninsula shown below:


Northern Florida Peninsula	Central Florida Peninsula	Southern Florida Peninsula
*To better understand these plots visit the following CPC web page: https://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/box_whiskers/info.php

ENSO Severe Weather:

During the El Niño phase of the ENSO, especially a Strong El Niño, there tends to be more stormy conditions across the Florida peninsula during the winter and early spring. Cold fronts will move across the region and with the jet stream further south and stronger we tend to see a higher number of severe weather events. When we look at the data since the winter of 1969-70, we see that Strong El Niño events have more than double the number of severe weather events (tornadoes, severe wind, and severe hail) than the other ENSO categories across our county warning area. On a side note, there is data available back in the 1950's and 1960's, but the number of reports is very low and unrepresentative.

Graph Legend

NWS Tampa Bay
County Warning Area WFO Tampa Bay County Warning Area

WFO Tampa Bay Average Number of Severe Weather Events per Cool Season by ENSO Category

Tornadoes:

Severe Wind:

Winds of 58 mph (50 knots) or stronger.
Damaging winds, often called “straight-line” winds to differentiate the damage they cause from tornado damage. Strong thunderstorm winds can come from a number of different processes. Most thunderstorm winds that cause damage at the ground are a result of outflow generated by a thunderstorm downdraft. Damaging winds are classified as those exceeding 50-60 mph.

Severe Hail:

Hail stones can grow as large as grapefruit in supercell thunderstorms, but it only takes a hail stone being one inch in diameter, or roughly the size of a U.S. quarter, to be considered severe. However, before January 5, 2010, the definition of severe hail was 3/4 inch in diameter, or roughly the size of a penny.

ENSO Local Freezes:

Freezing temperatures are possible into Central and South Florida during any winter no matter what the ENSO phase is that year. During El Niño conditions the likely cause of freezing temperatures is advection of cold air dragged southward behind low pressure systems that pass across the state. This is unlike La Niña conditions where the main cause is radiational cooling under clear skies and calm winds.

Long term averages indicate three to six days of freezing temperatures each winter across inland portions of Central Florida with only about one freeze over Southwest Florida. Farther north across the Nature Coast many more days of freezing temperatures occur with some locations such as Inverness and Bushnell having on average around 12 days, while farther north near Chiefland there are as many as 20 days with temperatures falling to or below 32° each winter. Overall, the difference in the number of days with freezing temperatures each winter is not that much between the ENSO phases as seen in the graphics below, but more dependent on the phase of the Arctic Oscillation (AO). For more details visit Arctic Oscillation: Impacts on West Central and Southwest Florida.

Graph Legend

ENSO Local Sky Cover:

Sky cover, or the amount of clouds, varies from day to day, but there are some trends depending on the phase of ENSO. During El Niño conditions we tend to have more cloudy days and less sunny (clear) days thanks to the jetstream being stronger and further south bringing storm systems across our area. During a La Niña winter the jetstream is usually further north and therefore the storm systems tend to track well to our north with less clouds and more sunshine. A breakdown of the average number of days in the different categories, clear (0/10 to 3/10), partly cloudy (4/10 to 7/10), and cloudy (8/10 to 10/10), shows how there are more cloudy days during El Niño and more clear (sunny days) during La Niña.

The following tables list the number of days in each sky cover category since the winter of 1970-71 at Tampa.
Link to 1970-2000 Table		Link to 2001-2024 Table