The open-ocean sensible heat flux and its significance for Arctic boundary layer mixing during early fall

Abstract

Abstract. The increasing ice-free area during late summer has transformed the Arctic to a climate system with more dynamic boundary layer (BL) clouds and seasonal sea ice growth. The open-ocean sensible heat flux, a crucial mechanism of excessive ocean heat loss to the atmosphere during the fall freeze season, is speculated to play an important role in the recently observed cloud cover increase and BL instability. However, lack of observations and understanding of the resilience of the proposed mechanisms, especially in relation to meteorological and interannual variability, has left a poorly constrained BL parameterization scheme in Arctic climate models. In this study, we use multiyear Japanese cruise-ship observations from R/V Mirai over the open Arctic Ocean to characterize the surface sensible heat flux (SSHF) during early fall and investigate its contribution to BL turbulence. It is found that mixing by SSHF is favored during episodes of high surface wind speed and is also influenced by the prevailing cloud regime. The deepest BLs and maximum ocean-atmosphere temperature difference are observed during cold air advection (associated with the stratocumulus regime), yet, contrary to previous speculation, the efficiency of sensible heat exchange is low. On the other hand, the SSHF contributes significantly to BL mixing during the uplift (low pressure) followed by the highly stable (stratus) regime. Overall, it can explain ∼ 10 % of the openocean BL height variability, whereas cloud-driven (moisture and radiative) mechanisms appear to be the other dominant source of convective turbulence. Nevertheless, there is strong interannual variability in the relationship between the SSHF and the BL height which can be intensified by the changing occurrence of Arctic climate patterns, such as positive surface wind speed anomalies and more frequent conditions of uplift. This study highlights the need for comprehensive BL observations like the R/V Mirai for better understanding and predicting the dynamic nature of the Arctic climate.

Anthropocene climate bifurcation

“…Thus, the global average surface albedo is 24 185 = 0.13 = 13 %. The values for cloud albedo and atmospheric (Ganeshan and Wu, 2016). Finally, for the purposes of sensible and latent heat transport (see appendix), the wind speed U is 5 m s −1 (Nugent et al, 2014) and the drag coefficient is C D = 1.5 × 10 −3 .…”

“…Today, there is widespread agreement that the climate of the Earth is changing, but the precise trajectory of future climate change is still a matter of debate. Recently there has been much interest in the possibility of "tipping points" (or bifurcation points) at which abrupt changes in the Earth climate system occur (see Brovkin et al, 1998;Ghil, 2001;Alley et al, 2003;Seager and Battisti, 2007;Lenton et al, 2008;Ditlevsen and Johnsen, 2010;Lenton, 2012;Ashwin et al, 2012;Barnosky et al, 2012;Drijfhout et al, 2015;Bathiany et al, 2016;North and Kim, 2017;Steffen et al, 2018;Dijkstra, 2019;Wallace-Wells, 2019). Section 12.5.5 in IPCC (2013) gives an overview of such potential abrupt changes.…”

Evaluation of the Planetary Boundary Layer Height From ERA5 Reanalysis With MOSAiC Observations Over the Arctic Ocean

“…However, in transitional seasons (i.e., spring and autumn), the influencing factors on the PBLH are more complex. These include rapidly changing atmospheric circulation patterns and atmospheric conditions (Ganeshan & Wu, 2016). In addition, our subsequent research in Section 3.1.1 also found that high sea ice concentration ( SIC ) also has a significant impact on the differences in PBLH from ERA5 and observations especially in spring and autumn.…”

Sensitivity of low‐tropospheric Arctic temperatures to assimilation of AIRS cloud‐cleared radiances: Impact on midlatitude waves

“…The slightly lower, but spatially consistent, low-tropospheric temperatures in the CLD3 experiment result from the differences in the type of AIRS radiances assimilated and their associated coverage, as noted in Figure 1. The Arctic atmosphere has abundant low-level clouds associated with mid-tropospheric subsidence that are sensitive to the stability of the lower troposphere (Barton et al, 2012;Taylor et al, 2015;Ganeshan and Wu, 2016). It is possible that assimilation of CCRs in the CLD3 experiment is effective in capturing an important Arctic feature that would be missed by clear-sky data.…”