Sea ice, high‐latitude convection, and equable climates

D S Abbot,Eli Tziperman

doi:10.1029/2007gl032286

Abstract

It is argued that deep atmospheric convection might occur during winter in ice‐free high‐latitude oceans, and that the surface radiative warming effects of the clouds and water vapor associated with this winter convection could keep high‐latitude oceans ice‐free through polar night. In such an ice‐free high‐latitude ocean the annual‐mean SST would be much higher and the seasonal cycle would be dramatically reduced ‐ making potential implications for equable climates manifest. The constraints that atmospheric heat transport, ocean heat transport, and CO2 concentration place on this mechanism are established. These ideas are investigated using the NCAR column model, which has state‐of‐the‐art atmospheric physics parameterizations, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean.

Highlights

Introduction[2] Paleoclimatic data suggest that during the late Cretaceous period ($100 to $65.5 Ma) and the early Paleogene period ($65.5 Ma to $34 Ma) the global-mean temperature was higher than its modern value and the equator to pole temperature difference and the amplitude of the high-latitude seasonal cycle were both much smaller than they are today, in continental interiors, which has led to the characterization of climates during these periods as ‘‘equable’’ [e.g., Greenwood and Wing, 1995]
[1] It is argued that deep atmospheric convection might occur during winter in ice-free high-latitude oceans, and that the surface radiative warming effects of the clouds and water vapor associated with this winter convection could keep high-latitude oceans ice-free through polar night
SCAM user’s guide, 2004, available at http://www.ccsm.ucar.edu/models/atm-cam/ docs/scam/) which has state-of-the-art atmospheric physics parameterizations, high vertical resolution, a full seasonal cycle, a thermodynamic sea ice model, and a mixed layer ocean, to argue that winter convection may be the driver of positive winter cloud radiative forcing (CRF) over ice-free high-latitude oceans and that sea ice prevents such convection so that the removal of winter sea ice could be an essential prerequisite for the high-latitude convective cloud feedback

Summary

Introduction

[2] Paleoclimatic data suggest that during the late Cretaceous period ($100 to $65.5 Ma) and the early Paleogene period ($65.5 Ma to $34 Ma) the global-mean temperature was higher than its modern value and the equator to pole temperature difference and the amplitude of the high-latitude seasonal cycle were both much smaller than they are today, in continental interiors, which has led to the characterization of climates during these periods as ‘‘equable’’ [e.g., Greenwood and Wing, 1995]. [4] Observations of winter cloud radiative forcing (CRF) in the modern climate suggest that sea ice may play an important role in the high-latitude convective cloud feedback mechanism that Abbot and Tziperman [2007] proposed. [8] Figure 1 displays the seasonal cycle of some important physical quantities for the ice-free and the ice states For both states we specify AHT = 100 W mÀ2, OHT = 0 (roughly equal to modern values at 80°N [Trenberth and Stepaniak, 2003]) and CO2 = 1000 ppm. [15] Without the radiative forcing of clouds and water vapor during winter, the ice-free state would not be stable at any of the AHT, OHT, and CO2 combinations displayed in [10] The radiative effects of the convective clouds and moisture are essential for the maintenance of the ice-free state. We calculate that without winter CRF the ice-free state would be unstable and sea ice would develop after only two years and without the winter radiative forcing of water vapor the ice-free state would be destroyed in a single season (not shown)

Sensitivity to Heat Transport

Discussion

Conclusion