ENSO-induced persistence of droughts and storms over the U.S. Affiliated Pacific Islands: development of process-oriented diagnostics to identify errors in climate models

Abstract
During the space-time evolution of El Niño–Southern Oscillation (ENSO), the insular
U.S. Affiliated Pacific Islands (USAPI) experience drought-like conditions that persist for
3-4 seasons. Furthermore, environmental conditions favor frequent formation of cyclones
with evidence during the recent 2015-16 El Niño. In CMIP6-era models, therefore, to
represent weather and climate extremes over the USAPI during ENSO, a prerequisite lies
in models’ fidelity in translating equatorial Pacific sea surface temperature (SST)
anomalies into diabatic processes. Our goals are to develop process-oriented diagnostics
(POD) and relevant metrics at a level close enough to model formulations (e.g., at the
parameterization levels) that will identify the origin of biases and inform model
improvement decisions.

With a particular focus on model formulations that determine vertical processes, we
will assess parameterization schemes’ fidelity over different convective regimes along the
equatorial Pacific, as well as during different environmental conditions that exist during the
life cycle of ENSO. This represents a rigorous, targeted test bed for objectively diagnosing
the response of parameterization schemes. To identify model formulations that correspond
to the biases, we target four objectives: (i) During ENSO, examine parameterizations’
response to time-varying large-scale forcing and identify processes that lead to drought
persistence over the USAPI; (ii) Identify processes that determine tropical cyclone statistics
during ENSO; (iii) Examine upper troposphere vorticity budgets and their dependence on
the vertical gradient of Q1; (iv) Diagnose model formulations that account for vertical
profiles of cloud properties, Q1, moisture and vertical velocity. Before applying the POD to
study extremes over the USAPI, it will be employed along the equatorial Pacific, the source
region for predictability of global climate variations during ENSO. Thus, outcomes are not
region-specific. The POD can be transitioned to ENSO teleconnection studies, in general.

Our proposed research targets the MAPP competition that focuses “Addressing Key
Issues in CMIP6-era Earth System Models”, and specifically, the competition identified
research topic – “representation of model processes relevant to Weather and climate
extremes, including drought”. The PODs will be applied to CMIP6-era models, and other
relevant solutions performed within CMIP6 framework. Deliverables include a set of
metrics that illustrate models’ fidelity in representing ENSO-teleconnection, and
identify sources of model biases that reveal model formulation deficiencies. Our
proposed research will enhance the POD framework that is being developed under the
auspices of MAPP sponsored Model Diagnostics Task Force. Importantly, our POD will
be user accessible, flexible and adaptable such that it can be transitioned to any group
of process-level evaluations during the model developments.

Climate Risk Area: Water Resources