PROJECT GOALS:
The overall objective of this research is to investigate the underlying dynamics of TAV. The two major foci of the study are: 1) identification of key feedback loops in the tropical Atlantic sector and 2) investigation of the relative importance between local vs. remote influences on TAV.

METHODOLOGY:
This investigation is conducted using two types of numerical model simulations -- an atmospheric GCM (NCAR CCM3) simulation and a Hybrid Coupled Model simulation. The atmospheric GCM simulation contains three ensembles of CCM3 runs forced by the observed SST from 1950 to 1994 in different ocean domains. A comparison of these experiments allows the separation of atmospheric responses to local SST forcing in the tropical Atlantic from those due to the remote forcing. The Hybrid Coupled Model is used to gain further understanding of the role of local air-sea feedback in Tropical Atlantic Variability.

RESULTS AND ACCOMPLISHMENTS:
Several important findings have emerged as a result of this investigation:

1) A careful analysis of a suite of AMIP-type CCM3 integrations revealed a robust pattern of atmospheric response to local SST forcing in the tropical Atlantic. The dominant response appears to be associated with the variation in location and intensity of the Intertropical Convergence Zone (ITCZ) in response to changes in the SST gradient near the equator. Within the deep tropics, there is an indication of a positive feedback between surface heat flux and SST anomalies.

2) ENSO exerts a significant influence on Tropical Atlantic Variability during the boreal spring. The ENSO forcing contributes not only to the "dipole" correlation structure between tropical Atlantic SST and Nordeste rainfall, but also to positive correlations between SST anomalies and downward surface heat flux. The study further showed that the remote influence from the Pacific appears to be largely associated with an anomalous Walker Circulation that develops in the eastern Pacific-Atlantic sector during major ENSO events.

3) A preliminary analysis on the output of a 99-year integration of CCM3 coupled to a global ocean mixed layer model indicates that the model captures many salient features of TAV. In particular, the dominant tropical circulation and SST patterns in this integration bear a strong resemblance to those of the observations and of the dominant forced response found in the AMIP-type runs. The analysis also indicates that the mixed layer feedback plays an important role in enhancing the persistence of climate anomalies in the tropical Atlantic and the variable depth of the mixed layer is important for simulating realistic SST variability.

4) A hybrid coupled model study suggests that TAV may best fit into a weakly coupled scenario in which the air-sea feedback plays a role in enhancing the persistence of the cross-equatorial gradient of SST and the circulation anomalies, while the NAO provides an important source of external forcing to excite the coupled variability in the tropics.

FUTURE WORK:
Studies are needed to further understand the feedback loops in the tropical Atlantic sector and interactions between TAV and NAO. To facilitate a comprehensive understanding of Atlantic climate variability, we plan to develop a high-resolution, fully coupled Atantic sector climate model. This is a collaborative research effort between scientists at Texas A&M and NCAR. We hope that this effort will not only lead to an improved understanding of the underlying local physics of TAV, but also aid the ongoing global modeling activity at NCAR and other institutions. Ultimately, we envision this coupled model evolving into a research prediction system for Tropical Atlantic Variability.

Principal Investigators:
P. Chang
ping@ocean.tamu.edu
tel: (409)-845-8196
fax: (409)-845-8879