Warming Patterns in the CESM1 Large Ensemble and their Impacts on TC Projections

Recent studies have pointed out that current coupled climate models generally miss the increase in N Atlantic hurricane activity observed in the latter half of the 20th century, as well as an observed “La Niña-like” tropical east Pacific cooling during the same period. These studies suggest a connection between these two model biases in simulating historical climate.

Patterns of future warming projected by current generation coupled models are distinctly “El Niño-like”. Projections of tropical cyclone (TC) activity based on these systems also feature decreases in the N Atlantic. The concern is obvious – that the biases exhibited by historical simulations in the present day carry into future projections of warming and consequently TC activity.

In this presentation we will examine projections of TC activity for 2070-2100 using the Community Earth System Model version 1 (CESM1). We use a time-slice approach in which CESM1 at O(25km) resolution is forced with a sub-ensemble of future SSTs drawn from a large-ensemble (LENS) of lower resolution coupled simulations using CESM1. In the mean, the CESM1 LENS clearly exhibits an El Niño-like warming pattern such as that found in CMIP5 and CMIP6. However, significant internal variability exists. Examining the sub-ensemble results, we see a clear anti-correlation between projected east Pacific warming and projected N Atlantic and far NE Pacific (adjacent to Mexico and Central America) TC activity. We explore factors that contribute to the east Pacific warming signal in the LENS.