Air‐sea interaction regimes in the sub‐Antarctic Southern Ocean and Antarctic marginal ice zone revealed by icebreaker measurements

Abstract

This study analyzed shipboard air‐sea measurements acquired by the icebreaker Aurora Australis during its off‐winter operation in December 2010 to May 2012. Mean conditions over 7 months (October–April) were compiled from a total of 22 ship tracks. The icebreaker traversed the water between Hobart, Tasmania, and the Antarctic continent, providing valuable in situ insight into two dynamically important, yet poorly sampled, regimes: the sub‐Antarctic Southern Ocean and the Antarctic marginal ice zone (MIZ) in the Indian Ocean sector. The transition from the open water to the ice‐covered surface creates sharp changes in albedo, surface roughness, and air temperature, leading to consequential effects on air‐sea variables and fluxes. Major effort was made to estimate the air‐sea fluxes in the MIZ using the bulk flux algorithms that are tuned specifically for the sea‐ice effects, while computing the fluxes over the sub‐Antarctic section using the COARE3.0 algorithm. The study evidenced strong sea‐ice modulations on winds, with the southerly airflow showing deceleration (convergence) in the MIZ and acceleration (divergence) when moving away from the MIZ. Marked seasonal variations in heat exchanges between the atmosphere and the ice margin were noted. The monotonic increase in turbulent latent and sensible heat fluxes after summer turned the MIZ quickly into a heat loss regime, while at the same time the sub‐Antarctic surface water continued to receive heat from the atmosphere. The drastic increase in turbulent heat loss in the MIZ contrasted sharply to the nonsignificant and seasonally invariant turbulent heat loss over the sub‐Antarctic open water.

Surface heat budget in the Southern Ocean from 42°S to the Antarctic marginal ice zone: four atmospheric reanalyses versus icebreaker Aurora Australis measurements

“…Reprocessing the shipboard measurements for research purposes was conducted by Yu et al (2017). The reprocessing had three main objectives, as follows.…”

“…The energy budget balance at high latitudes has been less studied observationally. One reason is that most field measurements are made during off-winter seasons (e.g., Wendler et al 2005;Walden et al 2017;Yu et al 2017), which is insufficient to describe one annual cycle of the surface energy budget. The other reason is that the surface energy budget is strongly modulated by ice motion and ocean heat transport (e.g., Maykut & McPhee 1995).…”

“…CFSR SD LH and SH have the best agreement with the observed variances, while the SD LH and SH in the other three reanalyses are slightly weaker. LH and SH are usually weak at high latitudes (Yu et al 2017) because air temperature is usually low. The atmospheric water-holding capacity decreases exponentially with decreasing temperature according to the Clausius-Clapeyron equation, which results in low humidity at low temperatures and reduced sea-air thermal heat transfer.…”

“…Automated meteorological measurement systems are deployed onboard the vessel so that measurements of surface flux-related variables, including wind, air and sea temperature, humidity, pressure, precipitation, LW↓ and SW↓, are recorded routinely. Yu et al (2017) processed the icebreaker's measurements between December 2010 and April 2012, and showed that the shipboard measurements provide valuable in situ insights into the air-sea heat exchange processes in the ACC and MIZ. The most marked feature is the significant seasonal changes of turbulent LH and SH fluxes in the Antarctic ice margin when compared to those in the Subantarctic open ocean.…”

“…The surface meteorological measurements from the icebreaker Aurora Australis provide a rare opportunity to evaluate the flux products and to understand how the heat budget changes in the remote Southern Ocean. This study aims to take advantage of the analysis of Yu et al (2017) and use the derived ship-based fluxes as a reference to examine the performance of atmospheric reanalysis flux products in the ship-tracked region. Satellite products are not included because the lack of satellite retrievals makes it unfeasible to use these products to estimate the heat exchanges in the MIZ.…”