Drag Coefficient and Its Sea State Dependence under Tropical Cyclones

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper-ocean current observations [using electromagnetic autonomous profiling explorer (EM-APEX) floats deployed under five tropical cyclones] and a coupled ocean–wave (Modular Ocean Model 6–WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2–3 3 1023 forwindspeedsof25–55 m s21.Whilethedragcoefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is signif-icantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 458, a feature that is underestimated by current models of sea state–dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left-front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state–dependent parameterizations of the drag coefficient in tropical cyclone conditions

Estimates of net community production and export using high-resolution, Lagrangian measurements of O2, NO3−, and POC through the evolution of a spring diatom bloom in the North Atlantic

Budgets of nitrate, dissolved oxygen, and particulate organic carbon (POC) were constructed from data collected on-board a Lagrangian, profiling float deployed between April 4 and May 25, 2008, as part of the North Atlantic Bloom Experiment. These measurements were used to estimate net community production (NCP) and apparent export of POC along the float trajectory. A storm resulting in deep mixing and temporary suspension of net production separated the bloom into early (April 23–27) and main (May 6–13) periods over which ~264 and ~805 mmol C m-2 were produced, respectively. Subtraction of the total POC production from the NCP yielded maximum estimates of apparent POC export amounting to ~92 and 574 mmol C m-2 during the early and main blooms, respectively. The bloom terminated the following day and ~282 mmol C m-2 were lost due to net respiration (70%) and apparent export (30%). Thus, the majority of the apparent export of POC occurred continuously during the main bloom and a large respiration event occurred during bloom Termination. A comparison of the POC flux during the main bloom period with independent estimates at greater depth suggest a rapid rate of remineralization between 60 and 100 m. We suggest the high rates of remineralization in the upper layers could explain the apparent lack of carbon overconsumption (C:N>6.6) in the North Atlantic during the spring bloom