A. Evaporation from the warm tropical Pacific Ocean moistens the lower atmosphere.

1. Walker Circulation Trades driven by Walker Cell also promote eastern Pacific upwelling and nutrient-rich water A. Evaporation from the warm tropical Pacific Ocean moistens the lower atmosphere. B. East to west surface trade winds transport the warm moisture laden air to the western Pacific C. Moist air rises and feeds rain in the western Pacific D. Dried-out upper air moves eastward, and sinks in the eastern tropical Pacific http://www.gfdl.noaa.gov/research/climate/highlights/GFDL_V1N3_gallery.html

2. El Niño (warm ENSO, warm phase) events • Trade winds weaken due to weakening pressure gradient across the equatorial Pacific. • The weakened trade winds allow warmer water from the western Pacific to surge eastward, raising sea level in the eastern Pacific • The rise in sea level in the eastern Pacific and a lack of upwelling lead more warm surface water and a sinking of the thermocline (boundary between warmer surface and colder deep water). http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/def.rxml

3. With warmer surface waters over the central and eastern Pacific, convective clouds and thunderstorms shift eastward. • This results in dry conditions in Indonesia and Australia with wetter conditions in Peru and Ecuador. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/def.rxml

4. La Niña events are essentially the opposite of El Niño events - La Niña - El Niño http://eesc.columbia.edu/courses/ees/climate/labs/enso/index.html

5. Current La Niña event El Niño events Sea level and SST animation

6. ENSO conditions are measured by many methods, including satellites, moored buoys, drifting buoys, and sea level analysis. • sea surface temperatures (Nino 3, Nino 4, Nino 3.4) • sea level pressure at Tahiti and Darwin, Australia (Southern Oscillation Index, SOI) • Multivariate ENSO index (MEI) MEI included sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky. http://www.cdc.noaa.gov/people/klaus.wolter/MEI/mei.html

7. Pacific Decadal Oscillation • A long-lived El Niño-like pattern of Pacific climate variability. • Similar climatic pattern, but different behavior in time (greater persistence) • The climatic pattern is most notable in the North Pacific/North American sector, with a weaker center in the tropics (opposite for ENSO). Typical wintertime Sea Surface Temperature (colors),  Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns during warm and cool phases of PDO, ENSO http://jisao.washington.edu/pdo/

8. PDO index: the leading PC of monthly SST anomalies in the North Pacific Ocean, poleward of 20N http://jisao.washington.edu/pdo/

9. PDO and ENSO • PDO and ENSO are not independent, and PDO may actually be a manifestation of slowly varying ENSO – the relationship between the two is not yet completely understood Precipitation patterns related to PDO Precipitation patterns related to ENSO

10. Hydroclimatology and ENSO Gershunov, A. and T.P. Barnett. 1998. Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meterological Society, 79, 2715-2725. PDFRajagopalan, B., E. Cook, U. Lall, and B.K. Ray. 2000. Spatiotemporal variability of ENSO and SST teleconnections to summer drought over the United States during the 20th century. Journal of Climate 13, 4244-4255. PDF Climate and Water, 01/30/08

11. Gershunov and Barnett: • Possible discussion questions:-This paper indicates that during high (low) NPO phases we should expect to see a stronger climate response to El Nino (La Nina) . So what does this mean for hydroclimatology in the Southwest? Would this mean more rain than "normal" El Nino years and even less rain than "normal" La Nina years? -The authors use SLP data and HPF data to show their results. Are there other forms of data that may have been more useful, or are these two forms good enough for their claim? • I know we are in a weak La Nina right now, but this paper made me wonder what the current NPO status is. I did some looking around the NOAA website, but couldn’t find it. It looks like the southwest is impacted in a few of the combinations, so it would be interesting to see what influence the NPO is having right now. • In a world of secular warming trends (both ocean and surface) do any of the old "rules" or associations apply? The pattern of SSTs and SLPs over the oceans will obviously still exert strong atmospheric forcing, but will the storm track and/or extremes change? • A paper on the phasing of El Nino anomalies w/ respect to the annual cycle being modulated by multidecadal variations in either the Atlantic or the Pacific?

12. Rajagopalan et al. • As we accelerate climate change and sea temperatures rise (consequently SSTs), can we also that the effects of ENSO on regional summer drought also be amplified? • Also, the article mentions other low-frequency climate indicators that lead to drought. Can these known climate patterns also be used as a predictor of how the climate will react to global warming? • I am curious as to how the study of ENSO-PDSI (or any other drought index) has evolved since 2000. • It’s interesting that there are summer drought teleconnections with El Nino in the SW. I’d thought that ENSO largely influenced frontal, winter precipitation in the SW. Is this just a carry-over/memory effect of the lack of winter precipitation, or is it possible that ENSO may actually moderate the monsoon? • “In terms of linear analyses,the key story appears to be the role of ENSO in all three epochs as a modulator of drought, with the North Atlantic playing a role in the eastern United States in the third epoch.” My question is, then, what is modulating ENSO differently in the different epochs? Has there been much progress on this since 2000? I know the “Perfect Ocean for Drought” suggests CO2 forcing as one modulator, but what are others?

13. D.P. Brown and A. C. Comrie. 2004. A winter precipitation ‘dipole’ in the western United States associated with multidecadal ENSO variability. GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L09203, doi:10.1029/2003GL018726 Winter (DJF) precipitation anomalies during SON El Nino or La Nina years, for three phases of PDO (warm, cool, warm)

14. http://www.climas.arizona.edu/forecasts/archive/nov2007/nov2007figs/9_prec_outlook.htmlhttp://www.climas.arizona.edu/forecasts/archive/nov2007/nov2007figs/9_prec_outlook.html

15. ftp://ftp.wcc.nrcs.usda.gov/support/water/westwide/snowpack/wy2008/snow0801.gifftp://ftp.wcc.nrcs.usda.gov/support/water/westwide/snowpack/wy2008/snow0801.gif http://waterdata.usgs.gov/nwis/rt

17. Next week: Climate forcing of moisture variabilityMonday (back to hydroclimatology) Guest speaker, Dr. Katie Hirschboeck.  Hirschboeck, K.K., 1988. Flood hydroclimatology, in Baker, V.R., Kochel, R.C. and Patton, P.C., eds., Flood Geomorphology, John Wiley & Sons, 27 49. WednesdayMcCabe, G. J., M. A. Palecki, and J. L. Betancourt. (2004). "Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States." Proceedings of the National Academy of Sciences 101(12): 4136-4141. PDFSeager, 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(19): 4065-4088. PDF