On September 2017 there was an interesting article in the New York Times, entitled: “What could we lose if a NASA Climate mission goes dark?” I recommend reading this article (click here; please contact me if you would like to read it but cannot access it through this link). It refers to the satellite mission GRACE (Gravity Recovery and Climate Experiment), which has been the focus of the majority of my research since graduate school, and will also be part of my work during my short visits to Longyearbyen, here in Norway.
Here I briefly explain how GRACE works and I provide a couple of examples of the applicability of GRACE in climate research, including those that immediately affect our society. I also give one example of how I have most recently used GRACE to study Arctic Ocean circulation.
GRACE measures the changes in gravity over the Earth, so let’s begin by thinking about gravity: that invisible force that makes things fall to the ground. We learn about this early on in life – Evan is an avid gravity user during meal times while sitting on his high chair. Later on, we learn this force pulls two masses together, e.g., the mass of Earth and our body mass (hence our feet are kept on the ground). But this force is not the same everywhere on Earth at any given time: e.g., a mountain has more mass – and thereby more gravitational attraction – than a valley (see figure above). But also, and more importantly, this force is not the same all the time at any given location: it varies because the amount of mass in the surface of the planet changes constantly, primarily due to the water cycle, e.g., the same mountain will have more mass when it is snow-covered during the winter than when it is snow-free during the summer.
Generally, water is transported from land to the ocean, from the ocean to land, or from one region to another within the ocean or within the land. For example, glaciers lose mass to the ocean when they melt (e.g., Greenland; Antarctica; Alaskan glaciers…). And the ocean gains mass from melting glaciers, a process which is a major contributor to mean sea level rise (the other major contributor being expansion of the ocean’s volume due to warming). Groundwater pumping for human usage also decreases mass in the region where water is being pumped from, significantly contributing to water depletion over time. A great but sad example of this phenomenon is California’s drought over the last few years. According to the USGS, the drought is still ongoing despite recent accumulation of snowpack in the mountains and the filling up of the reservoirs, because groundwater takes much longer to replenish than the surface water and the snowpack. Since 2002, all of these incredibly important changes in mass – on land and in the ocean – have been measured from space thanks to GRACE.
GRACE consists of two identical satellites flying over the same orbit, one following the other (see image of satellites above). When the leading satellite senses an area of relatively larger mass on Earth, it gets attracted to it and speeds up, increasing the distance between the two satellites. This distance increases only up until the trailing satellite begins to sense (and thus be also more attracted gravitationally by) the same larger mass. Then, the second satellite also speeds up, reducing the distance between the two of them again. Similarly, over regions with less mass, the satellites slow down because they are less attracted by it. But the leading satellite slows down first, and the trailing satellite slows down soon after. The satellites are equipped with instruments that measure the distance between them constantly and very precisely. Knowing the exact position of the satellites at all times (using all available GPS in space), these data are converted to global maps of changes in mass over time, both in the ocean and on land.
Using ocean mass (or ocean bottom pressure, as discussed previously in my Post 3) measured by GRACE, my colleague Dr. Rebecca Woodgate and I recently discovered that about 2/3rds of the variability in summer ocean flow entering from the Pacific Ocean into the Arctic Ocean between Alaska and Russia through the Bering Strait (the exact opposite Arctic gateway from where we are in Norway now, see the map above) is controlled by changes in ocean bottom pressure in the East Siberian Sea (ESS) in the Russian Arctic. These results have been published in the scientific journal Geophysical Research Letters, and you can find it here, or send me a message if you would like me to send you a PDF version of it.
Compared to the salty Atlantic water (>34 parts of salt per thousand parts of water, typically denoted psu for practical salinity units), which enters the Arctic mainly through Fram Strait (recall my post 3), the Pacific water entering the Arctic through Bering Strait is considered fresh (<33psu). In fact, Pacific water is one of the three main sources of freshwater into the Arctic, the other two sources being sea ice melt and meteoric water (i.e., rivers and rain or snow). The Arctic freshwater – its changes and distribution in the Arctic, as well as its export from the Arctic – affects biology, atmospheric-ocean gas exchange, ocean acidification, sea ice cover and potentially global circulation and climate. Pacific waters are also comparatively rich in nutrients, which are very important for local and regional ecosystems. Furthermore, during the summer, warmer Pacific waters melt a significant portion of sea ice in the western side of the Arctic Ocean. These are only a few examples of why it is important to study the ocean flow through the Bering Strait.
The Bering Strait is much shallower (~50m deep) and narrower (~80km wide) than the Fram Strait (~3000m deep; ~350km wide), making it an obvious (and comparatively easier) place to measure ocean properties as water enters the Arctic. It is widely accepted through the oceanography community that there are two major factors that affect how much water flows through the Bering Strait: 1) the local winds, and 2) the sea level difference between the Pacific (higher sea level) and the Arctic (lower sea level) ocean, which in oceanography jargon is often referred to as the “pressure-head” forcing. During the winter, the local winds are strong and drive most of the flow: Northward winds through the strait tend to increase the flow into the Arctic, and southward winds slow down (or may even reverse) the flow through the strait. During the summer, the winds are much weaker, so we expect the second forcing, the sea level difference between Pacific and Arctic oceans, to be more important then. And, we find this is the case indeed. Through statistical analysis, we firstly find that most of the variability of the GRACE ocean bottom pressure (which here can be thought of as sea level) between a region of ~10 degrees of latitude north and south of the strait, occurs in the East Siberian Sea (ESS) during the summer, and both in the ESS and in the Bering Sea during the winter.
Secondly (and very excitingly), we find that this sea level variability in the ESS is in excellent agreement (more so during the summer, when the overall ocean flow is larger) with the velocity of the currents that has been directly measured in the strait since 1990s (using instruments moored year-round at the bottom of the ocean), as part of the University of Washington’s Bering Strait program led by Dr. Woodgate. Lastly, we find that these bottom pressure (or sea level) changes in the East Siberian Sea are, in turn, driven by Arctic winds (see figure above).
In summary, using GRACE we have finally discovered the characteristics of the “pressure-head” that drives most of the flow through the Bering Strait – details that had been previously unknown. We now can measure this forcing, track its changes, and then ideally model it and potentially forecast it. And none of this would have been possible without GRACE. Although GRACE is at the end of its lifetime, a follow-on mission (GRACE-FO) is planned to continue the legacy of GRACE, and is expected to be launched in 2018, thanks again to a joint collaboration between the US and Germany. As the New York times article says, “…we’re living at the start of a dark era of warming climates. But we’re also living in a golden age of environmental data, in which our technology in space can deliver surprising measurements with profound implications.”. Let’s hope that the “will to spend the money to continue these measurements” will remain.