92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 11:00 AM
Linear Trends in Sea Surface Temperature of the Tropical Pacific Ocean and Implications for the El Nino-Southern Oscillation
Room 355 (New Orleans Convention Center )
Michelle L'Heureux, NOAA/CPC, Camp Springs, MD; and D. Collins and Z. Z. Hu

A simple principal component decomposition of monthly sea surface temperature (SST) variability in the tropical Pacific Ocean demonstrates that nearly all of the linear trends during 1950-2010 are found in two leading patterns. The first pattern is strongly related to SST variations associated with the canonical El Niņo-Southern Oscillation (ENSO) pattern. The second pattern also shares characteristics with the canonical ENSO pattern and its existence solely depends on the presence of linear trends across the tropical Pacific Ocean. The decomposition also uncovers a third pattern, often referred to as ENSO Modoki or Central Pacific ENSO, but the linear trend is small and dataset dependent over the full 61-year record and is insignificant within each season. ENSO Modoki is also reflected in the equatorial zonal SST gradient between the Niņo-4 region, located in the western Pacific, and the Niņo-3 region in the eastern Pacific. It is only in this zonal SST gradient that a marginally significant trend arises early in the Northern Hemisphere spring (March-May) during both El Niņo and La Niņa and also in the late summer /early fall (July-September) during El Niņo. Yet these SST trends in the zonal gradient do not resemble an ENSO Modoki-like dipole because they are exclusively associated with significant positive SST trends in either the eastern or western Pacific, with no corresponding significant negative trends. Insignificant trends in the zonal SST gradient are evident during the wintertime months (November-February). Given the presence of positive SST trends across much of the equatorial Pacific Ocean, using fixed SST thresholds to define ENSO events likely needs to be reconsidered.

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