Annular Modes

First, a mode of climate variability is just a long-term fluctuation of the climate, like El Nino, that isn’t just caused by the seasons. El Nino is caused from ocean-atmosphere interactions, but it is not annular though. What makes a mode annular is its shape. Annular modes are zonally symmetric: they follow lines of latitude. If you are above a pole and look down, an annular mode will just appear as a circle around you.

Annular mode response of the eddy kinetic energy is a circle around the South Pole (enclosed in blue) (Thompson and Barnes, 2014)

Earth has (at least) two types of modes (and so do Mars and Titan): Mode 1 is in the jet stream (zonal-wind, called the Northern/Southern annular mode [N/SAM]). The NAM describes changes in the position of the jet stream north to south (which is related to cold outbreaks).

The jet stream shifts north and south (blue arrows), signified by the positive and negative centers of action of the annular mode.

Mode two is in the eddies (deviations from the zonal-mean winds, called the Baroclinic annular modes [BAMs]). The BAMs describe the intensity of storms in the middle and high-latitudes (like Nor’easters or atmospheric rivers). Why care about annular modes? They explain the largest percentages of variance in the middle to high-latitude weather than any other climate mode. On Earth, the NAM is associated with anomalies in surface temp (the dreaded “polar vortex”), sea ice, or ozone concentration. The BAM impacts precipitation and clouds. So if modes can be predicted, then observable things that impact us can be predicted. I.e., if you can predict this single climate metric, you get a long way towards actually predicting the weather people care about. Now about modes on Mars. They share many similarities to Earth’s. Mars’s NAM describes shifts in the jet stream north and south (indicated by the red and blue blobs), and Mars’s BAMs describe the intensity of eddies. But, they each explain a lot more variance. A lot more. (Pic 3).

Mars’s annular mode in the northern hemisphere also indicates shifts of the jet stream north and south (Battalio and Lora, 2021)

Earth’s modes explain perhaps 25% of the variance in their described fields. Mars’s modes can explain up to 50% (or more). This is huge, because, again, if we can predict the modes, we might predict a lot of weather. But Mars’s modes have a super-cool bonus. First a one-sentence primer on Mars dust storms: Mars’s dust storms generally initiate in the north (from the same waves that are described by the BAMs) and stay there. But sometimes, something makes them travel south. (Pic 4)

Dust storm tracks on Mars as identified from the MDAD (Battalio and Wang, 2021).

We don’t exactly know what yet, but when they get to the south, they can grow to be very large, globally large. See my paper describing the Mars Dust Activity Database (Battalio and Wang, 2021) for everything you want to know about Mars’s dust storms. (Other twitter thread). This is important because just like precipitation links to Earth’s modes, dust relates to Mars’s modes. If you link Mars’s BAM to dust storms, dust storms peak before the BAM in the regions where they initiate. (This is like Earth’s BAM and precip, so not too surprising). (Pic 5)

Mars’s annular mode in the northern hemisphere also indicates shifts of the jet stream north and south (Battalio and Lora, 2021)

But… If you link them the other way, with the BAM leading the dust storms, the regions where the dust storms travel to and grow stands out. Something described by the mode at a given moment contains information about what the dust is going to do in the future. (Pic 6).

Meaning you don’t need to predict the modes to get a prediction of weather like you do on Earth. On Mars, you just need to know what the modes and dust storms are doing right now to make a prediction.. With good enough observations of the Martian atmosphere, we might predict when the largest dust storms might happen days in the future without having to run a weather model at all. Aside: There’s other weirdness about Mars’s modes climate dynamicists will find interesting, but it’s not worth boring everyone else with here. Go read the paper and email me with questions: joseph.battalio[at]yale[dot]edu. Or look for my presentation at @theAGU this year. We’re not done yet. Now Titan’s modes: Titan has two types of annular modes too. The BAMs (again the ones describing eddy intensity) explain more variance, up to 50%. Titan’s NAM/SAM (the modes describing the jet stream) explain even more, up to 70%. But the NAM/SAM is weird. Instead of north-south shifts of Titan’s winds in the middle latitudes, the mode explains shifts up-down. The reasons for this are related to Titan’s jet being connected to its equatorial superrotation, which will have to be another tweet thread. (Pic 7)

Titan’s “barotropic” annular mode shows vertical displacements of the jet (yellow arrows) rather than horizontal shifts like Earth and Mars. Adapted from Battalio and Lora (2021).

What makes Titan’s modes particularly curious is that the BAM and NAM are related to each other because they are both related to methane storms, at least in our model. This is not how Earth’s modes are; they’re uncorrelated (because of other dynamics not worth going into here). Regardless, because Titan’s modes are correlated and each explain huge amounts of the changes in the atmosphere, if we can predict Titan’s modes, we may predict methane storms. This will be immensely helpful for the safety of Dragonfly in the 2030s. I’m working on making these predictions. I won a Mars Data Analysis Program grant this year to figure it out.

Dust storm sequence D06_01 from the Mars Dust Activity Database. The sequence is shown in green. Other storms are shown in black (Battalio and Wang, 2021).