Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

Abstract

This thesis examines the effect of mean large-scale currents on the vertical structure of the upper ocean during two recent observational programs: the Long Term Upper Ocean Study (LOTUS) and the TROPIC HEAT experiments. The LOTUS experiment took place in the northwest Atlantic Ocean, a mid-latitude region away from strong mean currents, and extended over one entire seasonal cycle. The TROPIC HEAT experiments took place in the central equatorial Pacific Ocean during two 12-day periods in 1984 and 1987, at opposite extremes of the seasonal cycle. We use observations from these field experiments as well as one-dimensional numerical models of the upper ocean to analyze the dynamics of the vertical structure of the upper ocean at the equator and in mid-latitudes. Due to the different nature of the observations, we focus on the long term mean structure of the upper ocean in the LOTUS observations (Chapters 2 and 3), and on the diurnal cycle in the equatorial upper ocean in our analysis of the TROPIC HEAT observations (Chapters 4 and 5). In the LOTUS observations, we find that the observed current is coherent with the wind over low frequencies (greater than an inertial period). Using a wind-relative averaging method we find good agreement with Ekman transport throughout the first summer and winter of the LOTUS experiment, with the exception of a downwind component in the wintertime. The mean current spiral is flat compared to the classic Ekman spiral, in that it rotates less with depth than does the Ekman spiral. The mean current has an e-folding depth scale of 12m in the summer and 25 min the winter. Diurnal cycling is the dominant variability in the summer and determines the vertical structure of the spiral. In the winter, diurnal cycling is almost non-existent due to greatly reduced solar insolation. There is a persistent downwind shear in the upper 15 m during the winter which may be partially due to a bias induced by surface wave motion but which is also consistent with a logarithmic boundary layer. The Price et al. (1986) model is reasonably successful in simulating the current structure during the summer, capturing both the mean and the diurnal variation. The model is less successful in the winter, though it does capture the overall depth scale of the current spiral. In our analysis of the TROPIC HEAT observations, we extend the Price et al. (1986) model to the equatorial upper ocean. The model is initialized with the stratification and shear of the Equatorial Undercurrent (EUC), and is driven with heating and wind stress. A surface mixed layer is determined by bulk stability requirements, and a transition layer below the mixed layer is simulated by requiring that the gradient Richardson number be no less than 1/4. A principal result is that the nighttime phase of the diurnal cycle is strongly affected by the EUC, resulting in deep mixing and large dissipation at night consistent with observations of the equatorial upper ocean during TROPIC HEAT. Other features of the equatorial circulation (upwelling and the zonal pressure gradient) are of little direct importance to the diurnal cycle. The daytime (heating) phase of the simulated diurnal cycle is unaffected by equatorial circulation and is very similar to its mid-latitude counterpart. Solar heating produces a stably stratified surface layer roughly 10 m thick within which there is little, 0(3 x 10-8 W kg-1), turbulent dissipation. The diurnal stratification, though small compared to the EUC, is sufficient to insulate the EUC from wind stress during the day. For the typical range of conditions at the equator, diurnal warming of the sea surface is 0.2-0.5°C, and the diurnal variation of surface current (diurnal jet) is 0.1-0.2 m s-1, consistent with observations. The nighttime (cooling) phase of the simulated diurnal cycle is quite different from that seen at mid-latitudes. As cooling removes the warm, stable surface layer, the wind stress can work directly against the shear of the EUC. This produces a transition layer that can reach to 80 m depth, or nearly to the core of the EUC. Within this layer the turbulent dissipation is quite large, 0(2 x 10-7 W kg-1). Thus, the simulated dissipation has a diurnal range of more than a factor of five, as observed in the 1984 TROPIC HEAT experiment, though the diurnal cycle of stratification and current are fairly modest. Dissipation estimated from the model is due to wind working directly against EUC, and is similar to observed values of dissipation in both magnitude and depth range. Overall dissipation values in the model are set by the strength of the wind stress rather than the structure of the EUC, and rise approximately like u*3 for a given Undercurrent. This suggests that the lower values of dissipation observed in the 1987 TROPIC HEAT experiment were due to the lower wind stress values rather than the relatively weak Undercurrent. The main findings of this thesis are: 1) When the diurnal cycle in solar heating is strong, it determines the local vertical structure of the upper ocean (in both the LOTUS and TROPIC HEAT observations). The Price et al. (1986) model and its extension to the equator simulate the upper ocean fairly well when the diurnal cycle is strong. Under these conditions it is necessary to make measurements very near the surface ( < 10 m depth) to fully resolve the wind-driven flow. 2) When surface waves are strong, surface-moored measurements of current may have a significant wave bias. To accurately estimate this bias, simultaneous measurements of current, current meter motion, and surface waves are needed. 3) Mean currents strongly amplify the nighttime phase of the diurnal cycle in the equatorial upper ocean, and therefore alter the mean structure of the equatorial upper ocean.