PROJECT GOALS:
Our continuing research – an outgrowth of our previous PACS research – is aimed at (1) understanding how the ENSO variability interacts with interdecadal variability, and (2) understanding the WHWP: its seasonal and nonseasonal variability, how it is forced (heat balance), and how it affects the IAS regional climate; and (3) physically understanding how the Atlantic and Pacific sectors interact to alter the rain-producing tropospheric structures overlying the IAS and surrounding land regions. The research is organized into three tasks around these objectives, and is being done in roughly the same order.

METHODOLOGY:
We approach these tasks using statistical analyses of modern data sets (Kaplan et al.; Da Silva et al.; the NCAR/NCEP atmospheric reanalysis; and Xie and Arkin rainfall) to identify and diagnose the physical relationships. The work will build on some of the published results from our previous PACS research. For example, in the first task (already completed) our previous calculation of a global ENSO mode of SST anomaly (Figure 1) (Enfield and Mestas-Nuñez, 1999) is used to separate the ENSO variability from the interdecadal variability in the NINO3 index region, and the separate SSTA indices are used to construct composite means of the tropospheric direct circulation around the globe, with special attention given to the inter-American sector. Our WHWP analysis has begun by describing the annual and nonseasonal variability in the size and intensity of the warm pool, and relating it to the surface fluxes to understand the heat balance. In the latter stages of our work we plan to relate both the Atlantic and Pacific variability and the IAS/WHWP variability to (a) the tropospheric structures affecting static stability and rainfall, and (b) the rainfall itself.

RESULTS AND ACCOMPLISHMENTS:
Our early work (Mestas-Nuñez and Enfield, 2000) looks at the tropospheric direct circulation associated with two components of the NINO3 time series: the canonical ENSO and the residual (decadal) components. The total, the ENSO and the residual NINO3 series are shown in Figure 2. Notice how the canonical ENSO of the middle plot (the global ENSO contribution) reproduces the warm and cold events in the data (upper plot) but with a very different ranking for the intensities (e.g., 1982-83 and 1997-98 are no longer dominant). The difference is explained by the residual (lower plot), which is mainly interdecadal in nature. In the paper we construct composites of the 3-D tropospheric direct circulation associated the canonical ENSO and interdecadal components, using the irrotational velocity and velocity potential at 850 hPa (lower troposphere) and 200 hPa, (upper troposphere) the vertical velocity at 500 hPa (middle troposphere). In Figure 3 we simply use the 200 hPa plots as a proxy and compare each of the two anomalous composites to the long term boreal winter mean (DJF). We see that the decadal circulation (warm phase) is quite different from the ENSO circulation, especially over northern South America and the Caribbean. This confirms that the phase of Pacific decadal variability will have a large impact on the tropospheric signal for ENSO, and suggests that El Niño (La Niña) effects over that region will be more predictable when the decadal mode is in its cool (warm) phase.

Our initial analysis of the WHWP (Figure 4) shows that the warm pool, as defined by the 28.5 °C isotherm, is minimal in January-February. It begins to strengthen and expand over the eastern North Pacific in the boreal spring. The warming and expansion continues into the Gulf of Mexico in the early summer. Finally, the warm pool in the IAS region moves south into the Caribbean and expands eastward into the tropical North Atlantic, in the late summer and early fall. The WHWP area index of water warmer than 28.5 °C undergoes a strong annual cycle from nearly zero in winter to a maximum in September(Figure 5). It is during the May-October period of warmest SST and greatest areal extent that large scale convective systems develop overhead and the region experiences its greatest rainfall and incidence of tropical storms. When the annual cycle of the index is compared to the variability of individual months (lower plot) we see that the interquartile range (dashed curves) of the index are as large as the annual mean area.

In an analysis of the heat balance (Wang and Enfield, 2000) it is clear that shortwave radiative influx is largely responsible for the annual mean growth and decay of the WHWP. However, the nonseasonal fluctuations seem to be controlled more by the feedbacks between the cloudiness and back-radiation. Because the WHWP indices are correlated with indices in both the Pacific (NINO3) and Atlantic (NATL), it is unclear at this point what degree of independence the WHWP anomalies have with respect to the neighboring oceans, or what the sequence of cause and effect linking them may be.

FUTURE WORK:
We believe that the large nonseasonal variability in the warm-water area (and intensity) of the WHWP will be reflected in the conditioning of the lower troposphere and in the associated rainfall and tropical storm development. Our future work will be aimed at exploring these aspects through analysis of the NCEP reanalysis to see how tropospheric thermodynamic structure and convective processes vary between extremes of WHWP extent. We will similarly analyze the effects of the Atlantic and Pacific, recently identified by Enfield and Alfaro (1999). Later in 2000 our group hopes to be joined by an OGP/UCAR post-doc (Alessandra Giannini), who plans to analyze these same relationships through experiments with a high-resolution regional model (MM5) of the atmosphere over the IAS region.

NEW RESULTS
Mestas-Nunez, A.M and D.B. Enfield, 2000: Equatorial Pacific SST Variability: ENSO and Non-ENSO Components and their Climatic Associations., resubmitted to Journal of Climate
(two page summary postscript file for download).

Wang, C. and D. B. Enfield, 2000: The tropical western hemisphere warm pool. Submitted to Science.

Enfield, D.B. and E.J. Alfaro, 1999: The dependence of Caribbean rainfall on the interaction of the tropical Atlantic and Pacific Oceans, Journal of Climate, 12:2093-2103.

Enfield, D.B. and A.M. Mestas-Nunez, 1999: Multiscale variabilities in global sea surface temperatures and their relationships with tropospheric climate patterns, Journal of Climate, 12:2719-2733.

Landsea, C.W., R.A. Pielke, Jr., and A.M. Mestas-Nunez, 1999: Atlantic basin hurricanes: Indices of climatic changes. Climatic Change, 42: 89-129.

Mestas-Nunez, A.M and D.B. Enfield, 1999: Rotated global modes of non-ENSO sea surface temperature variability, Journal of Climate, 12:2734-2746.

David B. Enfield
enfield@aoml.noaa.gov
phone: (305) 361-4351
fax: (305) 361-4392

Alberto Mestas-Nuñez
mestas@aoml.noaa.gov
phone: (305) 361-4372
fax: (305) 361-4392

Chunzai Wang
wang@aoml.noaa.gov
phone: (305) 361-4325
fax: (305) 361-4392

Christopher W. Landsea
landsea@aoml.noaa.gov
phone: (305) 361-4357
fax: (305) 361-4392

NOAA Atlantic Oceanographic and Meteorological Laboratory
4301 Rickenbacker Cswy
Miami Florida, 33149