Hunga Volcano
© judithcurry.comFigure 1. The Hunga Tonga eruption from space.
The unlikely volcano, the warmest year, and the collapse of the polar vortex.

The climate events of 2022-24 have been were truly extraordinary. From an unlikely undersea volcanic eruption to the warmest year on record to the collapse of the polar vortex after three sudden stratospheric warming events. This rare convergence presents a unique learning opportunity for climatologists and climate aficionados alike, offering insights into a climate event that may not be repeated for hundreds or even thousands of years.

1. January 2022, the unlikely volcano

Never before have we witnessed an undersea volcanic eruption with a plume capable of reaching the stratosphere and depositing a large amount of vaporized water. This extraordinary event occurred in January 2022 when the Hunga Tonga volcano erupted. The conditions for such an event are rare: the volcano must be deep enough to propel enough water with the plume, but not too deep to prevent it from reaching the stratosphere. Most undersea volcanoes do not produce plumes at all, which makes Hunga Tonga's eruption all the more remarkable.

The Hunga Tonga volcano occupied a unique "sweet spot" at a depth of 150 meters the day before the eruption. In addition, the eruption itself must be exceptionally powerful for water vapor to rise into the stratosphere. The January 2022 eruption of Hunga Tonga was the most powerful in 30 years, since the 1991 eruption of Mt. Pinatubo.

Active undersea volcanoes at the appropriate depth are rare, and the likelihood of one erupting with such intensity is relatively low. We may be looking at an event that occurs once every few centuries, or maybe even once every millennium. Undoubtedly, it was an exceptionally rare event.

While the most powerful eruptions, such as Tambora in 1815, can indeed strongly influence hemispheric weather for a few years, our observations of eruptions such as Agung (1963), El Chichón (1982), and Pinatubo (1991) suggest that their effects dissipate within 3-4 years.

The idea that the Little Ice Age (LIA) was caused by increased volcanic activity is popular. However, the data suggest otherwise. Volcanic activity during the LIA was not unusually high, but rather lower than the Holocene average (although volcanic activity was exceptionally high in the early 19th century, towards the end of the LIA). The primary unusual climate forcing factor during the LIA was exceptionally low solar activity.

Volcanic eruptions that penetrate the stratosphere trigger significant radiative, chemical, and dynamical changes, with sulfur playing a key role. Volcanic sulfur dioxide (SO2) oxidates, combines, and aggregates forming sulfate aerosols. These aerosols scatter incoming shortwave radiation, resulting in reduced surface insolation and consequent surface cooling. They also absorb both incoming and outgoing infrared radiation, contributing to stratospheric warming.

The effect of the Hunga Tonga eruption, however, is quite the opposite. While there was some sulfur dioxide associated with Hunga Tonga, the main impact was from water vapor. Water vapor is a potent greenhouse gas, so the sudden 10% increase in stratospheric water vapor in a single day increased stratospheric opacity to outgoing infrared radiation. Unlike the lower troposphere, where the greenhouse effect is relatively saturated, the stratosphere, well above the Earth's average emission altitude (about 6 km), experiences a much more pronounced effect from the addition of water vapor. Also, the increased stratospheric water vapor content enhances infrared emissions from the stratosphere, thereby cooling it significantly.

Stratospheric water vapor
© judithcurry.comFigure 2. Stratospheric water vapor in ppm by latitude over time at 31.6 hPa altitude. The evolution of the Hunga Tonga water vapor is clearly seen from its tropical injection toward the poles.
The unlikely inverse volcanic eruption of Hunga Tonga is currently cooling the stratosphere while warming the surface. However, this effect will gradually diminish over time as the excess water vapor exits the stratosphere over the next 2-4 years. Figure 2 illustrates the movement of volcanic water from the tropical regions, where the dehydrated air from the troposphere enters, to the mid and high latitudes, where it will gradually leave the stratosphere in the coming years.

The question arises: why did it take more than a year to detect the effects of stratospheric changes on surface temperature after the explosion? Typically, radiative effects are expected to be instantaneous once water vapor or sulfate aerosols are placed in the stratosphere. However, our understanding of how volcanoes affect weather remains incomplete, and climate models struggle to accurately reproduce these phenomena.

Transport within the stratosphere is rapid in the longitudinal direction, but very slow with respect to latitude and altitude, with significant seasonal variations. Depending on factors such as the latitude of the eruption and the time of year, the effects of a volcanic eruption on weather can vary widely. The eruption of Tambora provides a precedent: it occurred in April 1815, but its effects on weather, which led to the "year without a summer," were not detected until June 1816, a span of 15 months after the eruption. This historical example underscores the possibility that events occurring more than a year after an eruption could indeed be attributed to it.

2. 2023, the hottest year on record

Beginning in June 2023, the last seven months of the year marked the warmest period on record, significantly exceeding previous records. Such an event is quite remarkable, given the considerable temperature variability observed from month to month. But how unlikely is it?

Using the HadCRUT5 dataset, we find that there have been 17 record-breaking warmest years since 1870. Any year in HadCRUT5 that beats all previous years becomes a record year, and the record increase is measured as the temperature difference above the prior record year (highest mark until then). For example, 2009 was the warmest year, but it was only 0.005ºC warmer than 2007, the previous record year. 2023 was the warmest year and was 0.17ºC warmer than 2016. It is the biggest difference from one record year to the previous record year in the entire series.

Figure 3 shows that in 2023, the temperature increase from the previous record was the largest in 153 years, at +0.17°C. This level of increase from previous records is remarkable, even for a year that has been recorded as the warmest on record.

The warmest years
© judithcurry.comFigure 3. The warmest years in the HadCRUT5 dataset from 1870 with the temperature increase from the previous record. 2023 constitutes the biggest jump.
In the warmest years, several months often stand out as the warmest (Figure 4, blue bars). In 2023, there were seven such months, trailing only 2016 and tying 2015. Notably, these seven warmest months were consecutive, spanning from June to December. The red bars in Figure 4 illustrate the number of consecutive record months for each record year. It's clear from the figure that years in the data set with five or more consecutive warmest months coincide with very strong El Niño years: 1877-78, 1997-98, and 2015-2016.

number of record months
© judithcurry.comFigure 4. The number of record months in the record years is shown in blue. In red is the number of those record months that were consecutive.
In 2023, the temperature statistics reflect conditions similar to the strongest El Niño years in over a century. But was this really the case? Determining whether El Niño was the catalyst for the record warming in 2023 is challenging. Relying solely on the surface temperature of the Pacific Ocean as the criterion for El Niño would lead to circular reasoning. El Niño is a complex phenomenon involving both the atmosphere and the ocean. The Multivariate ENSO Index (MEI v2) uses five variables - sea level pressure, sea surface temperature, surface zonal winds, surface meridional winds, and outgoing longwave radiation - to create a time series of ENSO conditions from 1979 to the present.

This index, when averaged over the entire year, shows that of all the record years since 1980, only 1997-98 and 2015-16 were the result of a very strong El Niño. 2023 was actually a weak El Niño year, despite very high sea surface temperatures.

Yearly average Multivariate ENSO Index
© judithcurry.comFigure 5. Yearly average Multivariate ENSO Index values for the warmest record years.
We can conclude that 2023 stood out as an exceptionally unusual record-warm year. While it rivaled very strong El Niño years in terms of exceeding previous temperature records, it did not actually fall into that category. Remarkably, despite the lack of a strong El Niño, it managed to set the highest temperature record by the largest margin in the data set spanning a century and a half.

In an article entitled "State of the climate - summer 2023", Judith Curry showed how unusual 2023 was in terms of the global radiation balance at the top of the atmosphere, the components of the surface energy balance, and the internal modes of climate variability driven by atmospheric and oceanic circulation patterns.

The magnitude of the anomalies displayed in 2023 across a wide range of variables has never before been recorded. It is an unprecedented climate event in our records.

3. January-March 2024, the collapse of the polar vortex

The polar vortex is a circular wind pattern that develops on rotating planets with an atmosphere. It results from the conservation of potential vorticity, a property depending on the Coriolis force and the potential temperature gradient. The potential temperature refers to the portion of the temperature of an air parcel that is not affected by its potential energy, and is often defined as the temperature the parcel would have if it were brought to the surface (1,000 hPa).

In the Northern Hemisphere, toward the end of summer, the Arctic experiences a sharp drop in temperature as the days shorten. To maintain potential vorticity, the wind around the polar regions intensifies in a west-to-east direction (known as the westerlies). The formation of the polar vortex in the stratosphere occurs when the prevailing easterly winds shift to westerly winds. This shift is evident in the zonal wind speed, which changes from negative to positive around September (see Figure 6 below). Finally, the vortex dissipates around April.

Winds in the stratospheric polar vortex can reach 180 km/h (110 mph) and form a formidable barrier to heat transport from the tropics. As a result, the atmosphere and surface inside the vortex become very cold and dry, reducing the energy loss to the planet, as cold surfaces radiate less.

In the atmosphere, as in any fluid, waves occur, the largest of which are planetary waves. These planetary waves originate in the troposphere as a result of large mountain ranges and temperature differences between oceans and land. They are most prevalent and pronounced during winter in the Northern Hemisphere. Under favorable conditions, these waves travel rapidly, similar to tsunamis, colliding with the boundaries of the polar vortex and imparting an easterly momentum. As a result, the winds that form the polar vortex reduce their speed, weakening it and allowing warmer air to enter, pushing cold air outward. This exchange causes colder winter conditions in the mid-latitudes.

When the winds slow enough to reverse direction, the polar vortex breaks into two or three smaller, displaced vortices. Stratospheric air entering the area previously occupied by the vortex descends, warming significantly in the process. This phenomenon, known as a sudden stratospheric warming (SSW) event, can raise temperatures in the polar stratosphere by up to 40°C in a matter of days. SSWs are relatively common in the Northern Hemisphere, typically occurring about once every two years. They often lead to harsher winter conditions in certain regions, especially eastern North America and eastern Eurasia, in the following weeks.

El Niño years typically promote SSW events and polar vortex breakdowns. This could be due to the increased ocean temperature contrasts during El Niño, which generate larger-amplitude planetary waves. Occasionally, about once every 10-20 years, two SSW events occur in the same winter. However, this winter's extended period (November to March) marks the first time since records began in the 1950s that three SSW events have been observed. The breakdown of the polar vortex occurred in January, February, and March, as shown in Figure 6 from NOAA's SSW monitoring. Each time, the red line representing the westerly wind speed dropped to the zero line. At this time of year, it is possible that the stratospheric polar vortex may not reform.

Westerly (positive) stratospheric zonal winds
© judithcurry.comFigure 6. Westerly (positive) stratospheric zonal winds at 60°N (red line) reached the zero-speed line three times this year, indicating a sudden stratospheric warming event and polar vortex break down each time.
According to Adam Scaife of the UK Met Office, this event isn't just unprecedented - it could be a once-in-250-year event. This finding comes from a recent statistical study of SSW events conducted using a seasonal forecasting system within a climate model. However, it's important to note a caveat: climate models still struggle to accurately represent the stratosphere and fail to reproduce the observed phenomenon that La Niña years also increase the likelihood of SSW events.

The impact of three SSW events this winter isn't particularly dramatic. While normal weather patterns may shift, leading to unusual temperatures and precipitation in some areas, the effects are temporary. However, these events do affect Arctic temperatures and therefore the amount of energy leaving the planet. The weakening of the polar vortex, as shown in Figure 6, results in increased heat transport to the Arctic this winter, leading to higher temperatures in the region.

Figure 7 illustrates this trend, with an orange line representing Arctic temperatures in 2023 according to the Danish Meteorological Institute, and a green line representing temperatures this year. Since the greenhouse effect is relatively weak during the Arctic winter due to limited water vapor in the atmosphere, the result is that more energy escapes from the planet due to the weakened vortex. This serves to mitigate and reduce the unusual warming observed in the second half of 2023, which contributed to it being the warmest year on record.

Arctic surface temperature for the year 2023
© judithcurry.comFigure 7. Arctic surface temperature for the year 2023 (orange) and 2024 (green), compared to the 1958-2002 average (blue).
Despite the additional heat being transported to the Arctic, leading to increased temperatures, there hasn't been a corresponding decrease in Arctic sea ice extent. In fact, this winter's sea ice extent exceeds the 2010-2020 average. It appears that, contrary to widespread fears of its disappearance, Arctic ice remains resilient and stable.
Arctic sea ice extent in 2024
© judithcurry.comFigure 8. Arctic sea ice extent in 2024 compared to the 2001-10 and 2011-20 decadal averages from the National Snow and Ice Data Center.
4. What can we expect in the near future?

The unlikely volcanic eruption is the likely cause of the extraordinary warming, which in turn led to the occurrence of the unprecedented three SSW events. Our understanding of the effects of these events supports this interpretation.

Historical data on the warmest years suggests a high probability that 2024 will again break the temperature record, similar to what happened in 1877-78, 1980-81, 1997-98, and 2015-16. However, if we have correctly identified a major cause of the warming as the Hunga Tonga eruption, we can expect that as the excess water vapor exits the stratosphere, it will induce a cooling effect at the surface, potentially lowering temperatures for the next 3-4 years. Studies such as Solomon et al. (2010) have already shown the negative impact on global warming of stratospheric drying. We should see the reversing of all the warming caused by the Hunga Tonga volcano.

In addition, other factors affecting temperatures, such as the decline in solar activity after the maximum of Solar Cycle 25 and a future shift of the Atlantic Multidecadal Oscillation to its cold phase, could contribute to a large pause in global warming. Using the 2023-24 temperature as a reference point, we could even see some cooling in the coming years. These are indeed interesting times in terms of climate dynamics.