The Boundary Layer Response to Recent Arctic Sea Ice Loss and Implications for High-Latitude Climate Feedbacks

Abstract

This study documents and evaluates the boundary layer and energy budget response to record low 2007 sea ice extents in the Community Atmosphere Model version 4 (CAM4) using 1-day observationally constrained forecasts and 10-yr runs with a freely evolving atmosphere. While near-surface temperature and humidity are minimally affected by sea ice loss in July 2007 forecasts, near-surface stability decreases and atmospheric humidity increases aloft over newly open water in September 2007 forecasts. Ubiquitous low cloud increases over the newly ice-free Arctic Ocean are found in both the July 2007 and the September 2007 forecasts. In response to the 2007 sea ice loss, net surface [top of the atmosphere (TOA)] energy budgets change by 119.4 W m 22 (121.0 W m 22 ) and 217.9 W m 22 (11.4 W m 22 ) in the July 2007 and September 2007 forecasts, respectively. While many aspects of the forecasted response to sea ice loss are consistent with physical expectations and available observations, CAM4's ubiquitous July 2007 cloud increases over newly open water are not. The unrealistic cloud response results from the global application of parameterization designed to diagnose stratus clouds based on lower-tropospheric stability (CLDST). In the Arctic, the well-mixed boundary layer assumption implicit in CLDST is violated. Requiring a well-mixed boundary layer to diagnose stratus clouds improves the CAM4 cloud response to sea ice loss and increases July 2007 surface (TOA) energy budgets over newly open water by 111 W m 22 (114.9 W m 22 ). Of importance to high-latitude climate feedbacks, unrealistic stratus cloud compensation for sea ice loss occurs only when stable and dry atmospheric conditions exist. Therefore, coupled climate projections that use CAM4 will underpredict Arctic sea ice loss only when dry and stable summer conditions occur.

Effects of Arctic Sea Ice Decline on Weather and Climate: A Review

“…Also, Kay et al (2011) modelled the atmospheric response to the sea ice minimum of 2007. They applied the Community Atmosphere Model Version 4 (CAM4), and carried out short-term observationally constrained forecasts and longer term experiments in which the atmosphere freely evolved.…”

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

“…Models also have difficulties in representing the correct amount and vertical distribution of cloud hydrometeor phase partitioning over polar regions, under a wide range of annual temperatures. These biases lead to direct consequences for the surface radiation budget, near-surface temperature, and the lower ABL thermal stability and turbulent structure Karlsson and Svensson, 2011;Kay et al, 2011;Cesana et al, 2012;Liu et al, 2012).…”

Characteristics of water-vapour inversions observed over the Arctic by Atmospheric Infrared Sounder (AIRS) and radiosondes

“…This decoupling is strongest during winter when solar radiation is absent and strong surface inversions often form in cloud-free conditions, and weakest during summer, when clouds are more frequent and solar radiation warms the surface (Kahl, 1990;Devasthale et al, 2010). The low-level atmospheric stability changes not only with the season, but also with the fraction of open water (Kay and Gettelman, 2009;Kay et al, 2011) and with the presence or absence of cloud cover Sedlar et al, 2010). The large annual variability in solar radiation causes a large annual cycle in the near-surface temperature.…”