Dr. Jean O’Brien Dickey, AGU Fellow and the first woman to give AGU’s Bowie Lecture and to be elected president of the organization’s Geodesy section, passed away peacefully on 9 May 2018 at the age of 72 in Pasadena, Calif. A loving mother and sister, Jean is survived by two sons, two grandchildren, three sisters, and two brothers.
Jean was a pioneering geodesist with deep expertise in Earth rotation. Her leadership paved the way for NASA’s Gravity Recovery and Climate Experiment (GRACE) mission. In honor of her trailblazing career, we commemorate her many accomplishments.
From Engineering to Atmospheric Physics
Jean started her career studying engineering and later switched to physics at a small liberal arts college, now Saint Francis University in Pennsylvania. In her senior year, she entered an honors program at the U.S. Department of Energy’s Argonne National Laboratory. Jean did her Ph.D. work at Rutgers University in high-energy physics, finishing her degree in 1976. As a postdoctoral scholar at the California Institute of Technology (Caltech), she studied data from particle experiments performed at Fermilab. Following her time at Caltech, she joined the Jet Propulsion Laboratory (JPL) in 1980 and switched her attention to planet Earth.
Throughout her early career, she was often the only woman physicist in the room, but that did not deter Jean’s interest or enthusiasm. At the beginning of her long and successful career at JPL, Jean worked on lunar laser ranging but quickly changed her focus to the exchange of angular momentum between the solid Earth, atmosphere, and oceans. This momentum exchange was emerging as a problem of great scientific interest at the intersection of many areas of Earth science.
Although previous research had shown that atmospheric effects dominate fluctuations in the Earth’s rotation over timescales of 2 years or less, Jean and her colleagues’ spectral analyses of length-of-day (LOD) and atmospheric angular momentum data showed significantly increased variability centered at 42 days. . Upon detailed analysis, they discovered that two effects were at play: the approximately 50-day tropical Madden-Julian Oscillation and a 40-day oscillation associated with the interaction of nonzonal air flow with the topography below.
How Long Is a Day?
As Earth rotation and other geophysical observations continued to improve in accuracy and length of record, Jean’s focus shifted to other Earth properties. In 1991 Jean and Raymond Hide used a combination of Earth rotation results from optical astrometry and the space geodetic techniques of very long baseline interferometry and lunar laser ranging to make connections between fluctuations in LOD occurring on decadal timescales and the topography of the core-mantle boundary and electromagnetic torques imposed by the core and lower mantle.
In the early 1990s, Jean studied the effects of the El Niño–Southern Oscillation (ENSO) on Earth’s rotation. The very large ENSO event of 1982–1983 was associated with the largest interannual slowdown in LOD then recorded. Jean and her colleagues showed a very high correlation between interannual LOD and a modified Southern Oscillation index. Atmospheric and stratospheric components accounted for most of that signal, but their analysis left approximately 40 microseconds unaccounted for. Jean and her colleagues postulated that oceanic effects could account for this time, a topic Jean would take up in separate work.
Jean also studied the impact of the 1997–1998 ENSO event on the long-term LOD, Southern Oscillation index, and atmospheric angular momentum record and showed a slow, global, coherent poleward propagation of atmospheric angular momentum, originating near the equator, with approximately 4.7- and 2.4-year periodicities.
Ocean Currents and Earth’s Rotation
Jean later shifted her attention to the very small effects of oceanic currents on Earth rotation (on the order of 10 microseconds out of a total variability on the order of 1,000 microseconds). In 1993 Jean and coauthors noted that the strong Antarctic Circumpolar Current’s effect on LOD was insufficient to account for the observed variations in LOD at annual and semiannual periods that remained after the atmospheric component was accounted for, and thus, it was necessary to consider the full set of oceanic currents to properly account for the data.
Jean also studied the not-so-steady decrease in the Earth’s oblateness. This difference in its diameter measured at the equator and from pole to pole is measured by J2, a scaled version of the spherical harmonic of the gravitational field with degree 2 and order 0. It was known that glacial isostatic adjustment (GIA), a rebounding of polar land surfaces as glacial melting decreases their load, was the cause of an observed steady decrease in J2. However, another study had shown that J2 had begun to increase in 1997, indicating a pronounced global-scale mass redistribution in the Earth system. Jean and her colleagues determined that the observed increase in J2 was caused primarily by a surge in subpolar glacial melting and by mass shifts in the Southern, Pacific, and Indian oceans. When these effects were removed through data-constrained numerical modeling of the ocean and data on glacier melt, the steady GIA trend emerged undisturbed.
Moving the Field Forward
Jean’s publications on the variety of topics mentioned here are but a sample of her many well-cited peer-reviewed papers in high-impact AGU and other journals. Throughout all this work, she also played key roles in the leadership and governance of science.
Jean chaired the National Academy of Sciences/National Research Council Committee on Earth Gravity from Space in 1996–1997. Her leadership and that committee’s work provided a critical impetus for NASA to select the GRACE mission in open competition. After its 2002 launch, GRACE measured tiny monthly variations in Earth’s gravity field over 15 years. These variations reflected melting in Greenland and Antarctica, depletion of aquifers, and even changes in transport of large ocean currents. GRACE’s successor, GRACE Follow-On, was launched on 23 May 2018.
Jean’s community leadership and many scientific discoveries brought her accolades and awards at NASA and in the community. She was elected a fellow of the International Association of Geodesy (1991) and of AGU (1994). Jean was the first woman to give AGU’s Bowie Lecture (1993), speaking on Earth’s rotation as an interdisciplinary approach to Earth system science.
She was also the first woman to serve as president of AGU’s Geodesy section, from 1994 to 1995. She was awarded the NASA Exceptional Service Medal (1998) and the NASA Exceptional Scientific Achievement Medal (2003), and she was appointed senior research scientist at JPL (2007). Jean retired from JPL in 2017.
—Victor Zlotnicki and Susan Owen (email: [email protected]), Jet Propulsion Laboratory, California Institute of Technology, Pasadena