Someone asked me the other day if heat dome is a real meteorological term. This was the first time that I’d heard it, but apparently I live under a rock (check the twitter hashtag #HeatDome). It seems to be another buzz term, the summer equivalent of polar vortex, which the popular media has been misusing for the past few years. However, unlike polar vortex, heat dome is not a word that I’ve ever heard meteorologists use. Google trends tells us that the term has existed for a few years now and spiked previously in 2011, which I presume was during another heat wave. My guess is that heat dome is a fancy way of saying high pressure system.
It’s clear from any surface analysis that high pressure has dominated the eastern half of the country for the past few days. Although there’s not much of a northerly flow over the East Coast (at least not from what I’ve looked at), subsidence and clear skies within the broad area of high pressure allow the temperature to rise as it pleases.
Below is a map showing temperature anomalies for Sunday, July 24, 2016 (also known as today). You’ll notice that most of the CONUS (continental United States) is painted red, indicative of warmer than average temperatures. You can see evidence of the ridging in the contours over the eastern CONUS. (This would be more apparent if I’d chosen a better contour interval.)
To put this into perspective, I downloaded temperature data for Reagan National Airport (DCA), just south of DC, from the National Centers for Environmental Information. The data plotted in the histogram are maximum temperatures for all 31 days in every July from 1981 through 2015, which is 5 years longer than the time period that the National Weather Service uses for climate statistics. The maximum temperature reported at DCA for July 23, 2014 was 98 degrees. There are only 79 days out of 1085 (7.3%) that have temperatures of at least 98 degrees. Another way to put this is that about 93% of all July days in DC have high temperatures cooler than 98.
Clearly, this isn’t a common occurrence. But, what if we rephrased the question: how many July days in DC are warmer than 95 degrees? Can you really tell the difference between 95 and 98? It turns out that roughly 17% of all July days have high temperatures of at least 95 degrees in DC and about half of all July days have maximum temperatures of at least 89 degrees. 24 of the 79 days (30%) with high temperatures greater than 98 degrees have happened since 2010, but only 20% of the days with high temperatures greater than 90 degrees have occurred since 2010. The average temperature in July from 2010-2015 is 2.33 degrees higher than the average temperature in July from 1981-2009.
So what do all these numbers actually mean? Well, in a nutshell, DC has been warmer in the last half-decade than it was before. This is probably partially related to anthropogenic global warming. The numbers also tell us that while 98 degrees is an abnormally high temperature for DC, temperatures of at least 90 degrees are as about as common as getting heads on a coin flip.
I suppose after a post about all this heat, it would be nice to take a look to the future; when will we see abnormally cold temperatures? As shown in the map below, the CFS model has a hint of abnormally cold temperatures for the eastern CONUS about 2.5 weeks from now. I don’t want to get anyone’s hopes up though; you don’t need me to know to be cautious in believing 3 week forecasts.
The El Niño Southern Oscillation, typically just referred to as ENSO, has important implications on global weather, including that of California’s North Coast.
What is ENSO?
ENSO refers to the oscillation of the temperature of the surface of the water located between Indonesia and the west coast of South America, typically right along the equator.
The water just off the Maritime Continent is usually the warmest in the Pacific, so it’s called the Pacific Warm Pool (see the map to the left). Occasionally, something called a westerly wind burst (WWB) can push the warm water out of the Warm Pool and towards South America. If enough of these WWBs occur, an El Niño can form. Eventually, the warm water retracts back to the Warm Pool, often in earnest. This leads to an overall cooling of the equatorial Pacific and is called La Niña. The complete cycle, from average conditions to El Niño to La Niña and back to average, is called ENSO. El Niño is often called the warm phase of ENSO and La Niña is often called the cold phase of ENSO.
How does warm water affect the weather?
You’re forgiven if you’re wondering how some warm water in the middle of a giant ocean affects weather around the world. The easiest way to understand this rather complicated concept is to consider a convection cell, which is based on the simple fact that warm air rises.
There’s an important relationship between temperature and density that almost always holds true: as one goes up the other goes down. When objects are less dense than their surroundings, they tend to float, and vice-versa. This is why a person can float on water; the human body is less dense than water. This is also why a balloon filled with helium floats away, but a balloon filled with regular air doesn’t. The density of helium is nearly 7x less than the density of air, so a helium balloon is bound to float up until either the air pressure becomes so low that the two are the same density, or it pops.
Let’s consider a hot air balloon, for example. When the temperature inside the balloon is the same as the temperature outside the balloon, it sits on the ground. Heating the air inside the balloon will cause the density of the air inside the balloon to decrease, which leads to a floating balloon. At some point, the density inside the balloon will be the same as the air outside the balloon (density in the atmosphere decreases as you go up). If the source of hot air is turned off then, the air inside the balloon will cool off and its density will increase. This will cause the balloon to sink.
It turns out that this balloon analogy is a very useful
way to illustrate how the atmosphere behaves. During an El Niño, the equatorial Pacific ocean becomes fairly warm. Warm water heats the air, which leads to rising air. This process is called convection. As the air along the equator rises, air along the edges sinks to replace it (you can’t just move air from somewhere and not expect anything to happen). This creates an entire 3-dimensional circulation called a convection cell.
The image to the right shows a typical El Niño convection cell. Convection cells set off other cells and, often, a chain forms, as is shown. The image makes it appear as though the chain of convection cells is linearly confined to the Equator, but that is not always true. These cells often form along northeastward paths (in the Northern Hemisphere) following great circles. These chains of convection cells are the reason that El Niño can have global impact, even though the warm water is localized to a relatively small part of the Pacific.
These convection cells create a link from the warm water in the Pacific to weather around the world. This linkage is critical to the weather/climate system and is known as a teleconnection. Teleconnections can be caused by anything in the atmosphere or the ocean including hurricanes and Nor’easters. We’ll discuss these in much more detail in a future post because they are especially important to understanding how regional climates work.
All El Niños are not created equally.
The eastern extent of the warm water is very important to defining an El Niño. Sometimes, the warm water only makes it to the central Pacific. These events are called Central Pacific El Niños, or Modoki El Niños. We call El Niño events where the warm water makes it to the South American coast Canonical El Niños.
The convection cell diagram, above, demonstrates the importance of the warm water location. Imagine moving the primary cell (the one just past the dateline at 180° longitude) 30° to the west. This would cause the entire chain of convection cells to move accordingly, leading to changes in how they affect the globe.
How El Niño and La Niña events affect the world
I’ll begin by emphasizing that there’s no such thing as a typical El Niño or La Niña. Every event is different and needs to be treated as such if you’re after accurate forecasts. So, we’ll take a look at how some recent strong El Niños have affected the globe.
I’ve placed a black oval over the El Niño region in the first map, to help you get your bearings. These maps show the average temperature anomalies over each season in each year. If you look down each column, you’ll see how temperatures evolved throughout the year. If you look across each row, you can compare seasons between years.
These temperature anomalies, which are simply the deviation from the average temperature between 1981–2010, are in degrees Celsius. (You can figure out the Fahrenheit equivalent by multiplying by 1.8 or simply double the value for an estimate.) ‘Warm’ colors indicate that temperatures were warmer than average, and vice-versa.
Let’s take a look at things that stand out most to me:
As you look down each column, you’ll notice that the El Niño develops throughout the year. The warm waters in the central Pacific get warmest during the summer and autumn.
The distribution of the warm water is different during each El Niño:
In 1997 the warmest waters stretch west from the South American coast.
In 2002, the warmest waters are confined to the central Pacific and don’t touch the South American coast (this is a central Pacific El Niño).
In 2015, the warmest waters touch the South American coast, similar to 1997. However, the area of warm water is significantly larger and much of the Northern Pacific is also warm. Also, this El Niño event started earlier than the other two; you can see evidence of it in the spring.
I’ve placed red circles around the western half of the United States in summer and autumn to draw your attention to this region.
California is much warmer during thesummers of the 2002 and 2015 El Niño events, despite the fact that the 1997 El Niño is stronger at the time.
During the autumn, there’s warmth in California during the 1997 and 2015 El Niño events (remember, these two are canonical), whereas there’s very little signal during the Central Pacific based 2002 El Niño.
For those who are curious, the term signal refers to the amplitude of the El Niño event. A very strong El Niño would have a strong signal, for instance. So saying that there’s very little signal during the Central Pacific based 2002 El Niño means that the El Niño is weak. You can verify this by looking at the corresponding sea surface temperatures, they’re all green, there’s no red in sight.
For some added fun, take a look around the maps and see if you can notice any interesting differences. Australia, Argentina, and Europe are particularly interesting to me. We’ll explore those in more detail when we move to wine regions over there.
By the way, you can flip the colors to get an approximate idea of how La Niña events affect the globe. I’ll write up a post specifically on La Niña when it becomes relevant to us (I suspect that will be pretty soon).
Hopefully this image reinforces the fact that no two El Niño events are the same. The atmosphere/ocean system is dynamic and complex and there’s a lot about it that we don’t understand and cannot model. Even the simple question, Why does ENSO exist?, has no universally accepted answer.
I’m starting off this series of wine region climatologies with a look at Sonoma County and Napa Valley in California. For those who aren’t familiar with the term, meteorologists use climatology to refer to a region’s long-term weather. Typically, a climatology means an average. Climatologies are usually computed over a standard period of 30 years, from 1981 – 2010. The interval increases every 10 years to adjust for the changing climate.
The map on the right puts these two wine regions into geographical context, showing Santa Rosa to approximate Sonoma County and Napa to approximate the Napa Valley. These regions are located about 50 miles north of San Francisco, near some mountainous terrain.
Sonoma County, like most of the West Coast, has very little weather variability. The graph below shows a climatology of average daily temperature and rainfall from the official weather station in Santa Rosa, CA. I obtained the data from the National Centers for Environmental Information (NCEI, formally known as NCDC).
In the top graph, the red and blue lines show average maximum and minimum temperatures, respectively. The black line shows the average daily temperature, which is just the average of the red and blue lines. The bars in the bottom graph show average monthly rainfall.
The annual cycle is very clear in both plots, showing warm dry summers and cool wet winters. The box and whisker diagrams on the graph show the variability of these data and the red plus signs show outliers (< 1st quartile and > 3rd quartile). I won’t discuss box and whisker plots in detail here, but, if you’re interested, Wikipedia has a nice article on them.
The height of the black box component of the box and whisker diagrams is a measure of the spread of the data. You’ll notice that the boxes in the temperature graphs are fairly short, there’s just not much variation in the temperature of the region. The biggest outlier is found in June, roughly 10°F above average and it’s only happened twice in 30 years. Anyone that has lived on the East Coast, or in the Midwest, knows that 10°F temperature swings happen all the time there. Of course, these regions don’t have the moderating effect of the ocean that the West Coast enjoys.
Interestingly, there’s about a 14°F variation in the daily minimum temperature throughout the year and a 23°F variation in the daily maximum temperature. This really drives home Pacific’s moderating effect; the only main driver in Sonoma County’s temperatures is the sun.
Sonoma County’s rainfall is much more variable than its temperature, with 3 – 7 inches of rain each month throughout the winter. Many of the box plots are bottom heavy (skewed left), which suggests there are a handful of years with very little precipitation, probably related to California’s drought.
Napa Valley’s climate, shown to the right, has some small but important differences compared to Sonoma County’s. First off, notice that the variations in maximum and minimum temperatures throughout the year, as well as the temperatures themselves, are very similar to those found in Sonoma County.
Napa gets slightly less precipitation than Sonoma, by about 0.8 inches on average.
What does all this mean for wines?
The North Coast benefits from stable, predictable temperatures and rainfall patterns that form a long growing season that is beneficial to grapes. Since there’s not much temperature variability, it’s rare to get temperatures below freezing, so frost damage isn’t common.
The stable climate of the North Coast isn’t infallible, and there are certain weather and climate phenomena that affect the region. El Niño, which is currently quite strong, can have significant impacts on California’s weather. We’ll explore this topical impact later this week.