A Global View of Tropical Pacific Biases and Their Effect on Connections Between the Southern Hemisphere and the Equatorial Pacific Climate

Previous studies have shown that extra-tropical influences from the North Pacific can impact the tropical Pacific climate, offering some additional degree of predictability of El Nino/Southern Oscillation (ENSO). More recent work has also shown that similar physical mechanisms operate in the Southern hemisphere, and further suggest that the connection between the southern extra-tropics and the equatorial climate is even more direct than the north. The reason is that southeasterly trade winds cross the equator and allow signals to propagate from the south deep into the tropics, while in the northern hemisphere signals are essentially ‘blocked’ by the convergence of winds in the northern ITCZ. Simulating this connection from the southern hemisphere is problematic in coupled GCMS in which an erroneous southern ITCZ can potentially block extra-tropical signals originating in the south. Thus, in this project we will test the hypothesis:

That Pacific ITCZ biases in climate models weaken the southern hemisphere influence on the equator and diminish a potential source of ENSO predictability.

We will test this hypothesis using a collection of climate model simulations that offer multiple realizations of the mean state biases in the Pacific ITCZ. We will perform diagnostic studies using CMIP5 model simulations to test the model-dependence of the mean state and its influence on variability propagating from the southern hemisphere to the equator. We will perform a large number of experiments with climate models in which the ITCZ position is altered in two ways: First by externally imposed perturbations to the energy budget of the model, and second by altering the strength of regional radiative feedbacks using a novel approach that we have developed in prior work (funded by NOAA). This dual method approach of altering ITCZ builds upon recent work that suggests that in addition to local processes in the Pacific that can lead to biases in the ITCZ, the mean ITCZ position is also influenced by processes outside the tropics that alter the radiative balance of the planet. This experimental approach has the advantage of being able to (1) test which regions of the globe are key for the simulation of the Pacific ITCZ, and (2) examine mean state interactions with variability in a consistent framework. Further we will use a hierarchy of models including aqua planet models, AGCM-slab ocean mixed layer models, and fully coupled models, which will allow us to identify the fundamental mechanisms which control the position of the ITCZ and impact Pacific climate variability. The ultimate goal of this work is to design an experimental framework in which we can test how potential sources of predictability, particularly from the southern hemisphere, are affected by Pacific ITCZ biases.

This work will contribute to the goals of the ‘ESS – Climate Variability and Predictability (CVP): Improving Understanding of Tropical Pacific’ by identifying processes and regions outside the tropical Pacific that exert a remote influence on the mean climate. This will help to provide a more complete and global context for understanding tropical Pacific biases, and suggest novel ways in which models can be developed to reducing this external influence on these biases. A unique aspect of this work is that we are focused on additional (an perhaps unexploited) sources of predictability in the Pacific climate system, namely signals or precursors from the southern hemisphere. By testing the central hypothesis or this project, our work will enhance NOAA’s core capabilities in both ‘Modeling and understanding’ and in ‘Predictions’ of the Pacific climate system.