Principal Investigators:
David B. Enfield
Chunzai Wang

Other Scientists:
Sang-ki Lee (CIMAS/UM)
Alberto Mestas-Nunez (CIMAS/UM)
Chidong Zhang (MPO/UM)
Bruce Albrecht (MPO/UM)

INTRODUCTION
The tropical Western Hemisphere warm pool (WHWP) of water warmer than 28.5°C extends over parts of the eastern North Pacific, the Gulf of Mexico, the Caribbean, and the tropical North Atlantic (See slides presented in the AGU 2002 Fall Meeting, Ref. 1). The WHWP, like the warm pool in the western Pacific, is a critical component of the boreal summer climate of the Caribbean and surrounding land areas. The WHWP is correlated with summer rainfall in the United States and the Caribbean, and is related to the occurrence of Atlantic hurricanes. Atmospheric GCM model studies show that warm pools, together with continental orography, are responsible for the stereotypical ridge-trough configurations over the extratropical continents, the subtropical jet acceleration downstream of the continents, and possibly their variations. The WHWP provides a key source of moisture that is transported into the North American interior, and anomalies in its size and intensity may alter either the moisture source, the frequency and configuration of synoptic patterns that import the moisture, such as the Caribbean Low-Level Jet (CLLJ), Great Plains Low-Level Jet (GPLLJ), or intraseasonal northward moisture surges, or some combination. Interannual anomalies in the size of the WHWP are large in relation to climatological means, with the largest warm pools being about twice the size of the smallest. The WHWP SST anomalies are highly correlated with the SST anomalies in the tropical North Atlantic.
Despite importance of the WHWP, little is known about how the WHWP develops annually or how its significant interannual anomalies occur. This project aims to better understand physical processes controlling the seasonal and interannual variations of the WHWP, and to help improve prediction of seasonal-to-interannual climate variability mediated by the warm pool.

PROJECT GOALS
The overarching goal of the research is to understand which ocean and/or atmospheric processes are of primary importance for the annual cycle and interannual anomalies of the WHWP. In the process, we may find that what plays an overriding role in the annual cycle may not be the critical factor in forming persistent anomalies during most of a single warm pool season. In particular, we wish to understand what happens in the spring-summer growth season that frequently (but not always) causes a larger and warmer WHWP in the summer following El Niño events, i.e., how the normal growth process is altered and why the anomaly persists. The specific objectives are:

1. Understand the physical processes that control the seasonal variations of the WHWP.
2. Understand the mechanisms for WHWP interannual variability, including local processes and influences of ENSO and the tropical North Atlantic.
3. Improve our understanding of how Pacific El Niño events affect the North Atlantic and warm pool through the troposphere.
4. Determine how moisture transports from the warm pool into the US and rainfall in the southern and central US are related to the warm pool variability.

METHODOLOGY
We approach these tasks by using both analyses of modern data sets and a state-of-the-art numerical ocean model. The data sets to be analyzed include the NCAR-NCEP reanalysis field, the NCEP, Da Silva COADS, , and Southampton surface flux data sets, and the SODA ocean data assimilation product. The ocean model to be used is 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

• Completed a descriptive study of the annual and interannual variability and heat balance of the WHWP (Wang and Enfield 2001).
• Showed that the Amazon and warm pool alternate seasonally as western hemisphere foci for the Walker and Hadley circulations (Wang 2002; Wang and Enfield 2003).
• Demonstrated that El Niño anomalies cause large warm pools to ensue through alteration of the normal Walker-Hadley configurations (Wang and Enfield 2003; Wang 2003).
• Discovered that that a positive feedback involving longwave radiation may be responsible for the persistence of boreal summer warm pool anomalies (Wang and Enfield 2001, 2003).
  
  

REFERENCES
1. An oral presentation in the AGU 2002 Fall Meeting (AGU Meeting).

2. Wang, C., and D. B. Enfield, 2001: The tropical Western Hemisphere warm pool. Geophys. Res. Lett., 28, 1635-1638.
3. Wang, C., 2002: Atlantic climate variability and its associated atmospheric circulation cells. J. Climate, 15, 1516-1536.
4. Wang, C., and D. B. Enfield, 2003: A further study of the tropical Western Hemisphere warm pool. J. Climate, 16, 1476-1493.
5. 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.

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