Intermittent stability and frontogenesis in an area influenced by land runoff

Abstract

Abstract. A frontal structure in Liverpool Bay was studied using conductivitytemperature-depth (CTD) and current meter data. At times of weak vertical mixing by wind and tides, a density-driven gravity flow, induced by river discharge, distorts the thermohaline field, so that an expanding stratified region develops, bounded offshore by a surface front. Expansion of this frontal structure is increasingly curbed by rotation as a near-geostrophic balance is attained. Owing to friction, the space scale and time scale of frontogenesis are somewhat greater than two internal Rossby radii and an inertial period, respectively. At times of increased mixing, the frontal structure is destroyed and the enhanced friction between layers inhibits gravity flow, hampering the frontogenesis mechanism. Large variability in mixing conditions results in an intermittent frontal structure.

Scales and structure of frontal adjustment and freshwater export in a region of freshwater influence

“…Where the volume of fresh water input is sufficient to offset the negative buoyancy contribution from the temperature a surface-advected plume may develop in the winter. Further evidence for winter stratification, in spite of destabilizing temperature gradients, is found in Sharples and Simpson (1995) and Czitrom and Simpson (1998). Yankovsky and Chapman (1997), and references therein, help explain some of the features of results presented here.…”

“…The limitations of using SST as an indicator of the fronts' position should therefore be considered. Between May and August the salinity and temperature structures contribute equally to the vertical density distribution (Czitrom and Simpson, 1998), and CEFAS SmartBuoy data shows temperature and salinity to be significantly negatively correlated during the summer months. For example, during July 2007 the correlation coefficient for the raw 30 minute time series is -0.8, significant at the 99% level.…”

“…If Vext Vtot < 0.3 the two watermasses are considered to be weakly defined. 92 image classifications were rejected on this basis, the majority during equinox periods (March-April and SeptemberOctober), when estuarine and oceanic water temperatures are most similar and there is an absence of vertical temperature variations (Czitrom and Simpson, 1998). Additionally, 6 classifications were rejected on account of the water mass temperature differences (|µ 1 − µ 2 |) being 1.5…”

“…This allows the density driven estuarine circulation to dominate and more permanent stratification to develop (Linden and Simpson, 1988b;Simpson, 1993, 1995;Simpson et al, 1990). A stratified region, delimited by a strengthening surface front propagates westward, compensated at depth by onshore flows (Czitrom and Simpson, 1998;Sharples and Simpson, 1993). Isopycnals on the eastern side of the expanding stratified zone slope downwards forming a bottom-attached front.…”