Air‐sea interaction over a thermal marine front in the Denmark Strait

Abstract

Abstract. An investigation was conducted into air-sea interaction in the Denmark Strait, where a distinct thermal front separates warm North Atlantic water from the cold East Greenland Current. The field data consisted of ship weather station data and rawinsonde soundings from R/V Aranda's expedition in August -September 1993. The surface energy balance differed drastically between the warm and cold side of the front (net fluxes of 95 W m -2 upward and 82 W m -2 downward, respectively). The difference resulted mostly from the contradictory turbulent fluxes. The air temperature, humidity and wind speed showed more variation on the warm side of the front. Lower wind speeds were observed on the cold side. The cross-frontal differences in the air temperature and wind speed were largest during front-parallel flow, but those in the sensible and latent heat flux were largest during cross-frontal flow. During cases of air advection across the front, the modification in the air temperature was strongest with a low wind speed. Downwind of the front, the sensible heat flux strongly depended on the south-north wind component. The rawinsonde data revealed temperature inversions and low-level jets. The wind profile was affected by the combined effects of baroclinity, surface layer stability, and stratification through the atmospheric boundary layer. The surface heterogeneity caused by the sea surface temperature front resulted in the Schmidt paradox: the area-averaged sensible heat flux was upward, while the area-averaged air temperature exceeded the area-averaged surface temperature. A mosaic method, extended by estimates of the local wind speed over the warm and cold water side, was applicable to parameterizing the area-averaged sensible heat flux.

Low-level jet characteristics over the Arctic Ocean in spring and summer

“…Baroclinicity was responsible for generation of strong and warm LLJs, the former as in Vihma et al (1998). Baroclinicity was a more important forcing mechanism in July and August (11 cases) than in April-June (two cases).…”

“…Also ReVelle and Nilsson (2008) suggested a high occurrence of LLJs (60-80 %) over polar oceans, and Vihma et al (1998) observed that 91 % of rawinsonde soundings in the very baroclinic ice-edge zone in the Denmark Strait included a LLJ. According to our understanding, the most important reasons for the relatively low occurrence of LLJs at Tara were that (a) the observations were made far from strongly baroclinic zones, such as the sea ice margin, and (b) the typical conditions in AprilAugust were not as stably stratified as in the autumn-winter data set of Andreas et al (2000).…”

“…LLJs are also common over sea ice far from orographic influence: Langland et al (1989) observed LLJs related to an ice breeze -a sea-breeze-type mesoscale circulation. Vihma et al (1998) observed LLJs over the ice edge zone in the Denmark Strait; the strongest LLJs were generated by baroclinicity. Andreas et al (2000) observed a high frequency of occurrence (80 %) of LLJs over the Antarctic sea ice zone, and suggested that they were primarily due to inertial oscillations: a mechanism analogous to the classical nocturnal jet (Blackadar, 1957;Thorpe and Guymer, 1977), but in the Antarctic related to synoptic-scale changes in the atmospheric boundary layer (ABL) stratification.…”

A climatology of low‐level jets in the mid‐latitudes and polar regions of the Northern Hemisphere

“…Therefore, the LLJs we identify close to the southeast coast of Greenland are barrier jets. However, the LLJs further away from the coast may be generated by the combined effects of barrier winds (important close to the coast) and baroclinicity (important close to the sea-ice edge and SST front, Vihma et al, 1998). At the southern tip of Greenland (red area marked 4 in Figure 2(d)), there are two dominant directions for LLJs: from the northeast and northwest (Figure 5(c)).…”

Acknowledging contributions

“…Similar jets have been observed near oceanic SST fronts. Vihma et al (1998) noted the presence of LLJs in 90% of rawinsonde soundings in a four week cruise at the Denmark Strait, which were most strong either when background flow was along the front, so that the change in air temperature across the SST front enhanced the jet via thermal wind, or when warm air was advected over a cold surface (akin to the nocturnal jet case mentioned above), or when flow originated from the steep topography of Greenland (possibly due to the upstream orographic influence on baroclinicity also discussed above). Wayland and Raman (1989) detected a LLJ over the Gulf Stream at about 100 m height related to the baroclinic gradient.…”