Impacts of hurricane Frances (2004) on vertical mixing and air-sea CO₂ exchange / Peisheng Huang

2010

[Truncated abstract] Recent research has suggested that global hurricanes release large amounts of CO2 from the ocean to the atmosphere during their passage, with CO2 effluxes comparable to the net local annual CO2 fluxes, but these estimates were based on arbitrary assumptions within the short “forced stage” of the hurricane passages. The factors controlling the air-sea CO2 exchange and the integrated impact of hurricanes on the annual carbon cycle are still unclear. Here we use in situ measurements, satellite observations and numerical models to investigate the mechanisms of the variation of partial pressure of CO2 in surface ocean water (pCO2 surf) and integrated air-sea CO2 exchange fluxes during and after the passage of Hurricane Frances (2004) over Caribbean Sea. The results suggest that the pCO2 surf variation is dominated by the changing sea surface temperature. The hurricane-induced vertical mixing brings cold, higher-CO2 water to the surface through water entrainment at the bottom of the surface mixed layer. The sea surface temperature cooling is the major reason for the decline of pCO2 surf, while the entrainment of higher-CO2 water partially offsets this decline. The spatial heat budgets reveal that during the hurricane passage not only the entrainment at the bottom of surface mixed layer but also the horizontal water advection are important factors determining the spatial pattern of sea surface temperature. At the free surface, the hurricane-induced precipitation contributes a negligible amount to the air-sea heat exchange, but the precipitation produces a negative buoyancy flux in the surface layer that overwhelms the instability induced by the loss of heat to the atmosphere.

Integrated over the domain within 400 km of the hurricane eye on day 245.71 of 2004, the rate of change of the heat content in the surface water was estimated to be about 0.45 PW (1PW= 15 10 W), with about 20% (0.09 PW in total) of this due to theair-sea interface, and almost all the remainder (0.36 PW) was downward-transported by oceanic vertical mixing. This downward heat could potentially spread out and significantly influence the local ocean heat transport. Extrapolated to the global tropical cyclone population, the global tropical cyclone activity could be an important factor influencing the global ocean heat transport. Through the turbulent kinetic energy budget of vertical mixing, the shear production was found to be the major source of turbulent kinetic energy, amounting to 88.5% of the source of TKE, while the rest (11.5%) was attributed to the wind stirring at sea surface. The increase in ocean IV potential energy due to vertical mixing represented 7.3% of the energy deposited by wind stress. The surface distribution of pCO2 behind the hurricane mimicked the physical deepening processes, with a “right bias” to the hurricane track. The impact of the hurricane on local air-sea CO2 exchange was within a distance of about 100 km to the hurricane track on both sides. Application of different gas transfer velocities resulted in similar oceanic pCO2 variations but notably different CO2 effluxes from the ocean to the atmosphere. Hurricane Frances was estimated to have caused a CO2 efflux of about 3.504 – 10.363 Tg (1 Tg = 1012 g) C from ocean to the atmosphere, and globally, hurricanes in 2004 were estimated to have released a total CO2 of 0.047 – 0.141 Pg (1 Pg = 1015 g) C when extrapolating from Hurricane Frances. This efflux is remarkable when compared to the CO2 uptake by global oceans. The observed increased hurricane activity and severity were shown

to have accelerated the CO2 efflux from oceans into the atmosphere over recent decades...

Thesis (Ph.D.)--University of Western Australia, 2011

http://repository.uwa.edu.au:80/R/-?func=dbin-jump-full&object_id=30660&silo_library=GEN01