Abrupt transitions in an atmospheric single-column model with weak temperature gradient approximation

Benjamin A Stephens,Charles S Jackson

doi:10.5194/wcd-1-389-2020

Abstract

Abstract. We document a feature of the tropical atmosphere that could be relevant to episodes of abrupt transitions in global climate that regularly occurred during the last ice age. Using a single-column model (SCM) incorporating the weak temperature gradient (WTG) approximation, we find that abrupt transitions occur as the sea surface temperature is steadily increased. Because these transitions arise from the interplay between local deep convection and the large-scale adjustments that are required to maintain weak temperature and pressure gradients, they are only present with the WTG approximation relevant for the tropics but may be of interest as a trigger for abrupt transitions in global climate. These transitions are marked by an abrupt change in the partitioning of rainfall between convective and large-scale (microphysics) subroutines in addition to various other features of the column, including cloudiness, vertical velocity, temperature, and humidity. We conclude that the transitions are initiated by a failure of evaporative cooling in the lower free troposphere. This leads to lower-column heating and a burst of convection that heats the upper free troposphere, increasing the large-scale rainfall rate, which allows for sufficient evaporative cooling to restabilize the column.

Highlights

Explanations for abrupt climate change during the last ice age have largely focused on the role of the ocean, the Atlantic meridional overturning circulation sensitivity to a freshening of the North Atlantic (Clark et al, 2002; Jackson et al, 2010)
We document the hysteresis and multiple equilibria we have found within the column under the weak temperature gradient (WTG) approximation with sea surface temperature (SST) forcing, and in the third subsection we describe in greater detail the abrupt transitions that have been our primary focus
We begin by describing the typical features of our Weather Research and Forecasting (WRF) single-column model (SCM) radiative-convective equilibrium experiments and how these experiments are typically affected by activating the WTG approximation without anomalous SST forcing

Summary

Introduction

Explanations for abrupt climate change during the last ice age have largely focused on the role of the ocean, the Atlantic meridional overturning circulation sensitivity to a freshening of the North Atlantic (Clark et al, 2002; Jackson et al, 2010). Using a single-column model (SCM), Sobel et al (2007) find the existence of dry and rainy states to be sensitive to both SST and the horizontal moisture advection rate, while Sessions et al (2010) find similar behavior in a cloud-resolving model These multiple equilibria have been understood in the context of explaining the spatial structure of tropical rainfall, but the fact that different equilibria exist under the WTG approximation is

Methods

Results

Discussion

Conclusion