From South to North

I am a South African whose passion for climate dynamics has taken me across hemispheres. My research aims to improve our understanding of the key processes determining Earth's climate and climate variability on a variety of timescales ranging from seasonal to decadal to much longer geological scales. I focus on the climatic role of ocean general circulation, ocean-atmospheric interactions, and cloud dynamics.

Knowledge of the processes giving rise to present-day climate variability and past climate change is required in order to anticipate the influence of human activity on climate. My research efforts acknowledge that, to fully understand, model, and predict changes in climate characteristics that have a large impact on society (especially temperature and precipitation patterns), a fully coupled ocean-atmosphere perspective is needed - one that accounts for changes in important variables such as the thermal structure of the slowly-adjusting ocean. Complimenting observations with theory, I endeavor to accompany complex simulations of climate phenomena with simple models capturing the essential dynamics required to explain unanswered questions within climate science.

Research

DYNAMICAL CONTROLS ON TROPICAL CLIMATE IN A WARMER WORLD

The warmest open ocean surface temperatures on Earth are found within the western equatorial Pacific warm pool, supporting deep atmospheric convection that drives the large-scale meridional Hadley and zonal Walker circulation cells. The nature of these cells determines large-scale precipitation patterns within the tropics and subtropics. How will large-scale atmospheric circulation change in response to the current anthropogenically-forced rise in atmospheric CO2 concentrations? By how much will western equatorial Pacific warm pool temperatures rise? Will the mean east-west Sea Surface Temperature (SST) gradient along the equatorial Pacific and the associated Walker circulation increase or decrease? How will the meridional temperature gradient and Hadley circulation respond? The answers to these question have important societal implications.

Adapted from Burls and Fedorov (2014) this figure illustrates the robust relationship seen between the meridional albedo gradient and the east-west SST gradient along the equatorial Pacific across a range of climate simulations.

SIMULATING PAST WARM CLIMATES

Sensitivity studies and paleoclimate simulations provide valuable tests for current theories, and the state-of-the-art models used to simulate the response of Earth's climate to elevated CO2 levels. Uncertainties in the forecasts provided by climate models are largely associated with the parameterization of sub-grid scale phenomena such as atmospheric convection in the tropics or processes in the deep ocean. One way to constrain these uncertainties is to reproduce and study climatic conditions of the past as suggested by paleodata.

The subsiding flow of the atmospheric Hadley circulation controls dry conditions over vast subtropical bands where the main arid regions of the globe reside. In the context of future changes in the atmospheric hydrological cycle, understanding precipitation changes in the subtropics is of particular importance given the scarcity of water resources in these locations. A major puzzle arises when contrasting the drier conditions in the subtropics predicted by climate models under global warming scenarios against the wetter conditions seen in reconstructions of past warm climates, including the warm, ∼400 ppm CO2, Pliocene.

Modeling results from Burls and Fedorov (2017) address this puzzle and highlight the importance of correctly predicting how the meridional temperature gradient between the tropics and the subtropics will change with global warming. The figure on the left from Burls and Fedorov (2017) illustrates our finding that the weaker meridional sea surface temperature gradients during the Pliocene supported weaker Hadley circulation to the point that the dynamic decrease in moisture divergence from the subtropics more than compensates for the thermodynamic increase to the point that the subtropics become wetter.

The hydrological cycle responce to Pliocene large-scale sea surface temperature forcing has interesting implications for the density driven meridional overturning circulation in the Ocean. Today, relatively fresh surface waters in the subpolar North Pacific prevent deep water formation, however under Pliocene conditions the hydrological cycle changes support much saltier surface water in the region allowing cool winter conditions to give rise to deep convection. The schematic on the right illustrates this result as presented in Burls et al. (2017)

INTERANNUAL TROPICAL CLIMATE VARIABILITY: AN OCEAN ENERGETICS PERSPECTIVE

During my time in the Department of Oceanography at the University of Cape Town, I investigated the role that oceanic processes play in the generation and evolution of coupled ocean-atmosphere variability in the tropical Atlantic Ocean - a region of great importance for African and South American climate. I focused on variability at the seasonal and inter-annual scales, as these are the time scales that have the largest impact on flood and drought events that often devastate large parts of Africa. Viewing upper-ocean variability within the tropical Atlantic from an energetics perspective, I assessed the role of ocean dynamics, in particular the role of ocean memory.

Based on ocean reanalysis data, and a simulation of the tropical Atlantic region that I configured and integrated using the Regional Ocean Modeling System (ROMS-TAtl), I investigated the feedbacks between the equatorial ocean thermocline, the work done by tropical wind stress and surface heat fluxes. The results that I obtained highlighted the impact that small basin size has on the behavior and predictability of inter-annual, tropical Atlantic climate variability. Unlike in the Pacific where seasonal and inter-annual variability involve distinctly different physical processes, my results showed that the latter is a modulation of the former in the Atlantic, whose seasonal cycle has similarities with El Niño and La Niña in the Pacific (Burls et al., 2011).

The ocean memory mechanism associated with inter-annual fluctuations in equatorial Atlantic Sea Surface Temperature (SST) appears to operate on much shorter time scales than that associated with the ENSO. Ocean memory in the Atlantic is largely associated with inter-annual modulations of a seasonally active delayed negative feedback response (e.g. see figure on the left from Burls et al., 2012). Observed differences between the ENSO in the Pacific and inter-annual variability in the Atlantic (referred to in the literature as the Atlantic zonal mode) can then be accounted for in terms of these distinctions. We showed anomalous wind power over the tropical Atlantic to be a potential predictor for anomalous events within the equatorial Atlantic. However, because these events are due to a modulation of seasonally active coupled processes, and not independent processes operating on inter-annual time scales as seen in the Pacific, the lead time of this potential predictability is limited.

Publications

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Burls, N. J., and Fedorov, A. V. 2017: Wetter subtropics in a warmer world: contrasting past and future hydrological cycles, PNAS, 114 (49) 12888-12893, https://doi.org/10.1073/pnas.1703421114.

Burls, N.J., Fedorov, A.V., Sigman, D.M., Jaccard, S.L., Tiedemann, R. and Haug, G.H., 2017: Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene, Sicence Advances, 3, e1700156.

Burls N.J., Muir L., Vincent E.M., and Fedorov A.V., 2016: Extra-tropical origin of equatorial Pacific cold bias in climate models with links to cloud albedo, Climate Dynamics, doi:10.1007/s00382-016-3435-6.

Fedorov, A.V., Burls N.J., Lawrence K.T., and Peterson L.C., 2015: Tightly linked ocean zonal and meridional temperature gradients over the past 5 million years, Nature Geoscience, 8, 975–980, doi:10.1038/ngeo2577.

Burls, N. J., and A. V. Fedorov, 2014b: Simulating Pliocene warmth and a permanent El Niño-like state: the role of cloud albedo, Paleoceanography, 29(10), 893-910, doi:10.1002/2014PA002644.

Luebbecke, J.F., N.J. Burls, C.J.C. Reason, M.J. McPhaden, 2014: Variability in the South Atlantic Anticyclone and the Atlantic Nino mode, Journal of Climate, 27, 8135-8150. doi:http://dx.doi.org/10.1175/JCLID-14-00202.1.

Burls, N. J., and A. V. Fedorov, 2014a: What controls the mean east-west sea surface temperature gradient in the equatorial Pacific: the role of cloud albedo, Journal of Climate, 27 (7), 2757-2778, http://dx.doi.org/10.1175/JCLI-D-13-00255.1.

Burls, N. J., C. J. C. Reason, P. Penven, and S. G. Philander, 2012: Energetics of the Tropical Atlantic Zonal Mode. Journal of Climate, 25, 7442-7466, doi:10.1175/JCLI-D-11-00602.1.

Burls, N. J., C. J. C. Reason, P. Penven, and S. G. Philander, 2011: Similarities between the tropical Atlantic seasonal cycle and ENSO: an energetics perspective. Journal of Geophysical Research, 116, C11010, doi:10.1029/2011JC007164.

The Team

Natalie Burls

Assistant Professor

Ehsan Erfani

Postdoctoral Associate

Keri Kodama

Graduate Student

Abdullah Al Fahad

Graduate Student

Funding

News Items

Science News article featuring Pliocene research

Coverage of 2017 Science Advances paper on Pacific Meridional Overturning (PMOC) during the Pliocene by Max Planck Institute and Mason

Invited to speak at the 33rd Annual South Africa Society for Atmospheric Sciences Conference hosted by the University of Venda

Nature Geoscience paper on tightly linked sea surface temperatures gradients over the last 5 million years in SciTechDaily

Contact

Phone

+01 703-993-5756

Mailing Address

Dept. of Atmospheric, Oceanic, & Earth Sciences
George Mason University
Research Hall, Mail Stop 6C5
4400 University Drive
Fairfax, VA 22030 USA