El Niño is a global phenomenon. Sea surface temperature anomaly during the 1997-98 El Niño. Source: NASA.

Here in Boulder, the nearest ocean is far away–820 miles from campus to the Southern California Bight. So, why do the tropical oceans matter, even to citizens of Colorado (and the world)?

One could sail the Pacific continuously along the equator from Indonesia to the Galápagos and travel 38% of the way around the planet (9,562 miles). The tropical Pacific alone (between 23°S and 23°N) accounts for 17% of the planet's surface area (32.6 million sq. miles). Knowing the tropical oceans is fundamental to knowing our planet.

Answers based solely on such facts may be of little practical consequence, since the majority of those 32.6 million sq. miles is uninhabited. The more pragmatic answer is that what happens in the tropics does not stay in the tropics, due in large part to the El Niño–Southern Oscillation (ENSO). ENSO is a natural cycle between warm and cool ocean temperatures in the tropical Pacific that occurs every 2–7 years. Because the global atmosphere is very sensitive to even small changes in tropical ocean temperatures, most of the published scientific papers about ENSO (now well over 10,000) focus on the impacts of ENSO outside of the tropical Pacific–including the nation’s breadbasket.

The western U.S. including Colorado is faced with the challenge of a growing, changing population and limited water. The effective management of freshwater resources requires predictability of rainfall and other hydrologic processes, and much of our current seasonal predictive capacity is derived from ENSO (1, 2). However, the impacts go well beyond seasonal weather conditions. To name just a few examples, ENSO influences major U.S. droughts including the Dust Bowl of the 1930s (3–6), U.S. tornado outbreaks (7, 8), global human health including Cholera (9, 10), Atlantic hurricane activity (11, 12), global monsoon systems (13–15), even temperature and winds on the West Antarctic Ice Sheet (16, 17).

Note: The CU Department of Atmospheric & Oceanic Sciences offers a Graduate Certificate in Oceanography.


  1. United States El Niño Impacts (by Mike Halpert, Deputy Director of the NOAA Climate Prediction Center).
  2. Barco, J., T. S. Hogue, M. Girotto, D. R. Kendall, and M. Putti, 2010: Climate signal propagation in southern California aquifers. Water Resources Research, 46, W00F05.
  3. Hoerling, M. P., and A. Kumar, 2003: The perfect ocean for drought. Science, 299, 691–694.
  4. Schubert, S. D., 2004: On the cause of the 1930s Dust Bowl. Science, 303, 1855–1859.
  5. Seager, R., Y. Kushnir, C. Herweijer, N. Naik, and J. Velez, 2005: Modeling of tropical forcing of persistent droughts and pluvials over western North America: 1856–2000. Journal of Climate, 18, 4065–4088.
  6. Karnauskas, K. B., A. Ruiz–Barradas, S. Nigam, and A. J. Busalacchi, 2008: North American droughts in ERA–40 global and NCEP North American regional reanalyses: A Palmer Drought Severity Index perspective. Journal of Climate, 21(10), 2102–2123.
  7. Cook, A. R. and J. T. Schaefer, 2008: The Relation of El Niño–Southern Oscillation (ENSO) to Winter Tornado Outbreaks. Monthly Weather Review, 136, 3121–3137.
  8. Lee, S. K., R. Atlas, D. Enfield, C. Wang, and H. Liu, 2013: Is There an Optimal ENSO Pattern That Enhances Large-Scale Atmospheric Processes Conducive to Tornado Outbreaks in the United States? Journal of Climate, 26, 1626–1642.
  9. Colwell, R. R., 1996: Global climate and infectious disease: the cholera paradigm. Science, 274, 2025–2031.
  10. Kovats, R. S., 2000: El Niño and human health. Bulletin of the World Health Organization, 78(9), 1127-1135.
  11. Gray, W. M., 1984: Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences. Monthly Weather Review, 112, 1649–1668.
  12. Bove, M. C., J. B. Elsner, C. W. Landsea, X. Niu, and J. J. O’Brien, 1998: Effect of El Niño on U.S. landfalling hurricanes, revisited. Bulletin of the American Meteorological Society, 79, 2477–2482.
  13. Walker, G. T., 1918: Correlation in seasonal variation of weather, Quarterly Journal of the Royal Meteorological Society, 44, 223-234.
  14. Kumar, K. K., B. Rajagopalan, and M. A. Cane, 1999: On the weakening relationship between the Indian monsoon and ENSO. Science, 284, 2156–2159.
  15. Karnauskas, K. B., and A. J. Busalacchi, 2009: The role of SST in the east Pacific warm pool in the interannual variability of Central American rainfall. Journal of Climate, 22(10), 2605–2623.
  16. Ding, Q., E. J. Steig, D. S. Battisti, and M. Kuttel, 2011: Winter warming in West Antarctica caused by central tropical Pacific warming. Nature Geoscience, 4, 398-403.
  17. Criscitiello, A. S., S. B. Das, K. B. Karnauskas, M. J. Evans, K. E. Frey, I. Joughin, E. J. Steig, J. R. McConnell, and B. Medley, 2014: Tropical Pacific influence on source and transport of marine aerosols to West Antarctica. Journal of Climate, 27(3), 1343–1363.