ENSO Studies
PROJECT GOALS
The objective of this research is to better understand Pacific ENSO and ENSO-related climate variability in the Atlantic. We mainly focus on the following research topics: (1) analyzing the ENSO western Pacific patterns for improving our understanding of ENSO; (2) investigating roles of the western Pacific patterns in ENSO relative to other ENSO mechanisms; (3) studying atmospheric circulation cells associated with ENSO; (4) understanding how the Pacific ENSO affects the tropical North Atlantic and the Western Hemisphere warm pool (WHWP); and (5) understanding why the El Niño events originate and develop differently in the last five decades.
METHODOLOGY
We approach these tasks by using both analyses of modern data sets and numerical models. The data sets to be analyzed include the NCAR-NCEP reanalysis field, the NCEP SST, COADS data, and SODA ocean data assimilation product. The models to be used include the coupled ocean-atmosphere intermediate model of Zebiak and Cane, and a new version of the HYbrid-Coordinate Ocean Model (HYCOM), a primitive equation OGCM that evolved from the Miami-Isopycnic-Coordinate Ocean Model (MICOM).
RESULTS AND ACCOMPLISHMENTS
Our NOAA/OGP-sponsored research has produced many papers. We herein only summarize some of research results. The detailed papers are available as PDF files at NOAA/AOML website: http://www.aoml.noaa.gov/phod/docs.html.
We showed that the western Pacific anomaly patterns are robust features of ENSO and play a role in the evolution of ENSO. Based on observations and numerical modeling results, a unified ENSO oscillator is developed and formulated (Wang 2001a and b). This unified oscillator model includes the physics of the delayed oscillator, the recharge oscillator, the western Pacific oscillator, and the advective-reflective oscillator. The delayed oscillator, the recharge oscillator, the western Pacific oscillator, and the advective-reflective oscillator can be extracted as special cases of the unified oscillator. As suggested by this unified oscillator and other studies, ENSO may be a multi-mechanism phenomenon and the relative importance of different mechanisms may be time-dependent (Wang and Picaut 2003).
By analyzing the NCEP-NCAR reanalysis field, we found that the western Pacific ENSO patterns (identified previously at the sea surface and subsurface) are also manifested in the atmospheric troposphere (Wang 2002a; Wang 2003). During the mature and decay phases of ENSO, the tropical off-equatorial western Pacific shows maximum middle tropospheric anomalous descending motion, whereas the equatorial eastern Pacific displays anomalous ascending motion. We showed how the Walker cell, the Hadley cell, and the Ferrel cell evolve during different phases of ENSO (Fig. 3). We found a zonal cell over the North Pacific that is named as a mid-latitude zonal cell (MZC), similar to the equatorial zonal Walker cell.
Our data analyses reveal how the Pacific El Niño SST anomalies are transferred to the Atlantic sector via the tropospheric bridge (Wang 2002b and c; Wang and Enfield 2003; Wang 2003) and how the induced North Atlantic SST anomalies precede the subsequent anomalous growth of the Western Hemisphere warm pool (WHWP) (Wang and Enfield 2001, 2003). As the Pacific El Niño warming culminates near the end of the calendar year, an alteration of the low-latitude direct circulation occurs, featuring (1) an anomalous weakening of the convection over northern South America, (2) Walker circulation anomalies along the equatorial strip to the east and west, and (3) a weakened northward Hadley flow aloft. The Hadley weakening results in less subsidence over the subtropical North Atlantic, an associated breakdown of the anticyclone and a weakening of the northeast (NE) trades in the tropical North Atlantic (TNA). The wind weakening leads to less evaporative surface cooling and entrainment of colder water from below the shallow mixed layer, resulting in a spring warming of the TNA, which in turn induces the development of an unusually large summer WHWP and a wetter Caribbean rainy season. During the summers following Pacific El Niño, when the warm pool is larger than normal, the increased Hadley flow into the subtropical South Pacific reinforces the South Pacific anticyclone and trade winds, probably playing a role in the transition back to the cool phase of ENSO.
By analyzing the NCEP SST data, the TAO temperature data, and the ADCP velocity mooring data, we investigate ocean circulation influence on SST in the equatorial central Pacific (Wang and Weisberg 2001). The vertical velocity calculated from ADCP moorings shows that oceanic upwelling and downwelling are associated with cooling and warming, respectively, suggesting that a vertical velocity of component of either sign affects SST. The ocean circulation in the equatorial central/eastern Pacific on average provides a cooling effect requiring the net surface heat flux to be positive on average to maintain the mean background state.
PUBLICATIONS
Wang, C., 2001a: A unified oscillator model for the El Niño-Southern Oscillation. J. Climate, 14, 98-115.
Wang, C., 2001b: On the ENSO mechanisms. Adv. Atmos. Sci., 18, 674-691.
Wang, C., and R. H. Weisberg, 2001: Ocean circulation influences on SST in the equatorial central Pacific. J. Geophys. Res., 106, 19515-19526.
Wang, C., and D. B. Enfield, 2001: The tropical Western Hemisphere warm pool. Geophys. Res. Lett., 28, 1635-1638.
Wang, C., 2002a: Atmospheric circulation cells associated with the El Niño-Southern Oscillation. J. Climate, 15, 399-419.
Wang, C., 2002b: Atlantic climate variability and its associated atmospheric circulation cells. J. Climate, 15, 1516-1536.
Wang, C., 2002c: ENSO and atmospheric circulation cells. CLIVAR Exchanges, 7, 9-11.
Wang, C., and D. B. Enfield, 2003: A further study of the tropical Western Hemisphere warm pool. J. Climate, 16, 1476-1493.
Wang, C., 2003: ENSO, Atlantic climate variability, and the Walker and Hadley circulations.
The Hadley Circulation: Present, Past, and Future. H. F. Diaz and R. S. Bradley, Eds., Cambridge University Press, in press.
Wang, C., and J. Picaut, 2003: An overview of ENSO understanding. Ocean-Atmosphere Interaction and Climate Variability. C. Wang, S.-P. Xie, and J. Carton, Eds., AGU Geophysical Monograph Series, submitted.
