Presented at the recent ECCWO PICES symposium in DC
Presented at the recent ECCWO PICES symposium in DC
Big whorls absorb little whorls
Thus building their kinetic energy,
And big whorls persist,
Abiding in their vorticity.
– J C McWilliams, 1984
(as summarized by C S Harrison)
One line summary: The eddies abide
As an undergraduate, I took Geophysical Fluid Dynamics with Geoff Vallis. I loved the merger of mathematics and the bit of magic that is scaling and appoximoatation. One of my texts was the much used Cushman-Rosin book, which included the following poem. In 1920 Richardson paraphrased Jonathan Swift in a summary of his paper The Supply of Energy from and to Atmospheric Eddies:
Big whorls have little whorls
That feed on their velocity,
And little whorls have lesser whorls
And so on to viscosity.
– Lewis F. Richardson, 1920
This poem describes the conceptual paradigm that large eddies feed smaller eddies, which feed even smaller eddies, which then lead to dissipation, basically molecular friction, at very small scales. However, on rotating planets such as earth, this “forward cascade” of energy to smaller scales (larger wavenumbers), only works on some scales. The surprising result is that in the ocean, eddies actually merge together, forming large mesoscale eddies (~100-200 km in diameter, i.e. the size of West Virginia). This was presented in the seminal 1984 Jim McWilliams paper, The Emergence of Isolated Coherent Vortices in Turbulent Flow, which describes simulations done on the first supercomputer, the Cray 1-A (pictured below), which had less computing power than a first generation smart phone.
Fig. 1 Seymour Cray and the Cray 1A “I was one of those nerds before the name was popular”
The 1984 McWilliams paper has a number of great results. In it, he describes how energy moves from small scales to large scales, mediated by eddy merger, resulting in “hard-core” persistent vortices seen in the header image above. This finding was supported at the same time by the recent satellite altimetry results, which showed the existence and persistence of these eddies globally.
Fig. 2 Kinematics of eddy merger for two unequal sized eddies (McWilliams 1984). The smaller eddy is filamented and partially merges with the larger, increasing its kinetic energy.
I had the opportunity to present the above poem, summarizing JMW’s 1984 paper, at his 70th birthday celebration. He looked at me quizzically, not knowing who I was, and said, “Give me some context.”
This fall I am moving to University of Texas Rio Grande Valley to be a professor in the School of Earth, Environmental, and Marine Sciences stationed at the new Port Isabel Marine lab.
I am very excited about this position and all of its potential, for a large number of reasons: being part of building up a new program, turtle conservation, regional ocean modeling, connectivity and dispersal in the Gulf of Mexico, access to UT’s outstanding computational resources, teaching about climate change and data analysis, and especially mentoring the majority Hispanic student population to be awesome researchers. Also tacos, low cost of living, and a beautiful, warm, turquoise ocean.
On ESWN someone posted this great short video from Ohio State U on how biases affect assessment. This includes when we write letters of recommendation. are hiring, and really anytime we do assessment. We all do this unconsciously, and studies show just being aware can help us overcome our biases’ effect on our behavior. A great resource.
See link to accepted manuscript here:
Most Earth System models (used to study the global climate including the carbon cycle) do not resolve the most energetic scales in the the ocean, the mesoscale (10-100 km), encompassing all of the eddies and jets in the ocean. One key question is does this ocean mesoscale circulation affect the global carbon cycle? In particular, does the export of carbon from the surface to the deep ocean, a process that is responsible for removing a significant fraction of CO2 from the atmosphere, change if the ocean circulation is better represented? We found that the while the global integral of carbon export is similar between a mesoscale-resolving (0.1º) and a standard resolution (1º) Earth System model (<2% difference), regional differences are up to +/-50%, and occur through a number of processes. One key result is that in high-latitude regions where biological production is driven by natural iron fertilization from coastal sediment sources, the resolution of both coastal jets and mesoscale turbulence reduce iron delivery from the coasts signifigantly, limiting biological production and the resulting carbon export. The figure below (left) shows the iron concentration leaking through at the confluence of the Brazil and Malvinas currents off the Patagonian shelf, into the iron-limited waters of the South Atlantic sector of the Southern Ocean. This results in one of the largest, shortest phytoplankton blooms in the Earth’s ocean, shown here in December, late Austral Spring, where the phytoplankton abundance is shown in iron (Fe) units and in log scale. Note that white areas in the left plot are where the phytoplankton concentration in the right plot is high; here the plankton have rapidly used all of the iron that is available, as the Southern Ocean is rich in nitrate and other nutrients. The large bloom to the south, off Antarctica, is due to deep winter vertical mixing, while the blooms off South America and South Africa are driven by horizontal transport and local upwelling. Additionally, one can see the iron fertilization effects of islands and seamounts throughout the region. You can see an animation of this bloom here: https://youtu.be/oXnGiTSyzx8
How does mesoscale transport of iron affect carbon export? Read the paper for the full story! In the below plot, the top panel shows carbon export production (EP) at high resolution (top) and low resolution (bottom), with an inset of the comparative statistics for the two boxed regions. The short answer is that restriction of the iron transport by the mesoscale flow reduces the production and export in this region signifigantly, while also increasing the spatial and temporal variability. This has implications for observational campaigns, highlighting the difficulty in validating modeled carbon export for short, seasonal and turbulent blooms, which represent a large portion of the global carbon export.
Tonight and tomorrow we are having a symposium at CU Boulder on the Little Ice Age entitled “The Coldest Centuries in 8000 years: The Little Ice Age Causes and Human Consequence.” The keynote speech is given by one of my collaborators in the nuclear war modeling project, Alan Robock. The Little Ice Age (LIA) was a period of global cooling lasting from roughly 1250 to 1850 CE (Fig. 1). In preparation for this event I have been doing some reading on what we know about the LIA in terms of the climate proxy and historical records, as well as our understanding of what started it and kept it going. In particular, I am interested in the idea that the cause was a series of large volcanic eruptions, and how those might compare with nuclear war in terms of climate and human impacts.
Fig 1. Temperature reconstructions over the last 2000 years, showing the cold anomaly termed the “Little Ice Age”. Source: wikipedia , see link for description of records plotted. Note that the temperature anomaly has a different magnitude in different reconstructions.
The beginning of the Little Ice Age coincided with a number of large volcanic eruptions, recorded in ice cores from Antarctica and Greenland (Fig. 2). One idea is that the cluster of eruptions around 1250 CE initiated a global cooling event, which lead to changes in the global ocean overturning circulation (i.e. AMOC), leading to a climate feedback. Honestly I have yet to wrap my head around how this feedback works.
Fig 2. Records of volcanic eruptions in Antarctic ice cores. From Sigl et al. 2014
One of the interesting things about this era is that we have historical records of the human impacts of this global cooling, and of the individual volcanic eruptions. One map I found particularly fascinating links the large eruptions throughout the LIA to various famines (Fig 3). The stories from these famines are quite disturbing, from legalization of slavery in Japan during the Kangi Famine, where families would sell off one of their members to get food for the rest, to cannibalism and infanticide. Often famines would make populations more susceptible to disease, and it is thought that the “Dark Ages” in Europe were largely an effect of the LIA cooling.
Fig. 3. Links between various volcanic eruptions in the LIA and human famines. From PAGES magazine
To put this in perspective to what we might expect from nuclear war, the temperature reconstructions suggest that cooling was at most -1ºC during the LIA (Fig 1), where the climate simulations of Mills et al. 2014 suggest that the global cooling effect of regional nuclear war could more than this (Fig 3). Global nuclear war would have an even larger cooling effect, as it is expected to deliver more light-blocking aerosols to the stratosphere.
Fig 4. Atmospheric aerosol mass burden (soot in atmosphere), anomalies of global mean solar flux, temperature, and precipitation in a number of nuclear war model simulations. From Mills et al. 2014. Note that the maximum temperature anomaly is greater than 1ºC/K, larger that what is recorded for the LIA, and lasts for a number of years.
Would nuclear war have the same climate-related impacts on humans as volcanic eruptions in the past? Our farming methods have much improved, with much higher yields per acre, as has our ability to store and distribute food surplus. However, there would likely be very large disruptions in weather and food supply, resulting from crop failures, which would be expected to have unpredictable regional and global political impacts. The losers would likely be poorer regions and nations, especially those relying on subsistence farming, as is the case for any sort of climate disturbance. The regional and large-scale impacts of recent refugee crises in Syria, Central America and Myanmar come to mind. Though these were driven by political unrest, we might expect similar levels of migration during climate disturbances.
In short, the project is applying a state-of-the-art earth system modeling framework to study the climate implications of nuclear war (aka nuclear winter), funded by the Open Philanthropy Project.
The science summary is this: Like volcanoes and asteroid impacts, regional nuclear war and the resulting firestorms are expected to inject aerosols like soot into the stratosphere, where the lack of precipitation leads to residence times from years to decades. The soot (aka black carbon) absorbs radiation, blocking light to the surface, causing global cooling and an associated decrease in precipitation. At the same time, heating of the stratosphere destroys ozone, potentially creating a global ozone hole, leading to high levels of UV exposure for plants and animals world-wide. In the historical record, volcanic eruptions, causing less cooling than we expect for even a regional nuclear war, have resulted in climate perturbations leading to massive famines, riots and disease outbreak, e.g. “The Year Without a Summer” in 1816. Regional nuclear war is expected to lead to much more extensive global cooling, and could result in a global nuclear famine. Global nuclear war would likely lead to global temperatures far below the last ice age, and the cooling is expected to last for decades.
The details: In this study we are using expert knowledge of likely nuclear conflict scenarios to drive regional urban fire models, the output of which will be plugged into state-of-the art global earth system models as a climate forcing perturbation. The resulting impacts on temperature, precipitation, crops, ocean biogeochemistry and fisheries will be studied. The urban fire modeling is building on recent advances in urban wind modeling, and a state of the art fire model, which will be combined for the first time. The earth systems modeling effort is building on recent studies of asteroid impact effects on global climate (Toon et al. 2016, Bardeen et al. 2017), previous nuclear war modeling work by Mills et al. 2014, the Last Millennium Ensemble, which includes the effects of volcanic eruptions on climate, and concurrent efforts by myself and Clay Tabor to look at ocean biogeochemical effects of the K-Pg asteroid impact that killed the dinosaurs, as well as work I am doing with Samantha Stevens to look at climate variability (such as ENSO) effects on fisheries. The effects of volcanic, asteroid, and nuclear forcing on ocean biogeochemistry have not yet been explored.
In July we had our first meeting with nuclear policy experts. One of the likely scenarios our experts proposed was that these weapons could be easily hacked. Bruce Blair said something to the effect of: “We could go tonight, and I could show you where we could pitch a tent, here in Colorado, and hack into the data lines leading to NORAD.” He also reported that it is confirmed that Russia has an unmanned sub with a mega, dirty nuclear bomb, so that even with a “first strike” effort by another power (i.e. the US) to take out all nuclear capabilities of an opposing country could still result in a severe retaliation. This is beside the point that a first strike could result in a global climate crisis, in effect self-assured destruction. One of our issues as a group is do we try to figure out at what level nuclear bombing is not a global catastrophe, or do we focus more on the potentially harmful effects. This is a bit of a moral conundrum I feel ill-prepared for.