The Important Role Of The Tropical Cyclones In The Energy Balance And Water Mass Balance Of The Ocean

Although oceans receive a huge amount of thermal energy, such energy cannot be efficiently converted into mechanical energy because the ocean is heated and cooled from the same geopotential level, the upper ocean. Therefore, in order to maintain the quasi-steady oceanic circulation external sources of mechanical energy are required to balance the loss of mechanical energy due to friction and dissipation. Wind stress and tidal dissipation are the primary sources of mechanical energy. However, some important sources of mechanical energy in the world oceans remain unaccounted, such as tropical cyclones. Tropical cyclone activity is a vitally important component of the atmospheric circulation system at low- and mid- latitudes. In the commonly used low spatial resolution wind stress data, such as NCEP reanalysis, these strong nonlinear events are smoothed out. As a result, their contributions to the global mechanical energy input are excluded.Using a hurricane-ocean coupled model, the important role of the tropical cyclones in the energy balance and water mass balance of the ocean is examined. The coupled model was constructed from two independently developed and tested models: a three-layer reduced gravity ocean model and an axisymmetric hurricane model. The energy input to the ocean, including the energy input to surface waves, to surface current, especially to the near-inertial motions, induced by about 1600 tropical cyclones from 1984 to 2005 were examined. Meanwhile, a simple slab model was used to estimate the increase of the gravitational potential energy in the oceanic surface mixed layer. In addition, the mixed layer deepening and vertical diffusivity induced by tropical cyclones in the world oceans are estimated.When the tropical cyclones pass through, much mechanical energy was input to the oceans. The energy input averaged over the period from 1984 to 2005 is estimated as follows: 1.70TW to the surface waves and 0.10TW to the surface currents (including 0.03TW to the near inertial motions). The rate of gravitational potential energy increase due to tropical cyclones is 0.05TW. Although the energy induced by tropical cyclones is much smaller than that induced by smoothed wind stress, most of this energy is distributed in the low- and mid- latitudes, especially in the northwest Pacific and north Atlantic, which may be very important for the oceanic general circulation.Tropical cyclones are one of the strongest time varying components in the atmospheric circulation. The activity of tropical cyclones has enhanced during the past decades, apparently due to global warming. Accordingly, both the energy input from tropical cyclones and the increase of gravitational potential energy of the ocean show strong interannual variability with an increasing rate of 19% over the past 22 years. The variability of the energy input to the ocean induced by tropical cyclones is similar with the intensity of tropical cyclones, which is denoted by PDI (Power Dissipation Index); the correlation coefficient is 0.93. The energy input is also associated with the number of tropical cyclones in each year, the correlation coefficient is 0.33.The mechanical energy input to the ocean induced by tropical cyclones can efficiently deepen the mixed layer at the low- and mid- latitudes. The deepening of mixed layer at low latitudes can enhance the meridional pressure difference and thus the overturning circulation and poleward heat flux, and decrease the energy for subsurface diapycnal mixing at the same time. The total volume flux due to mixed layer deepening in the world ocean is from 1984 to 2005, with in the North Pacific. Qiu and Huang (1995) discussed subduction and obduction in the oceans, they estimated that basin-integrated subduction rate is and obduction rate is in the North Pacific. Therefore, the total volume flux due to mixed layer deepening induced by tropical cyclones is approximately 50% of the subtropical water mass formulation rate through subduction and much larger than obduction. The obduction rate was accumulated from to ; the mixed layer deepening was distributed from to . The cyclone induced mixed layer deepening play a capitally important role in water mass balance in the world oceans, and water mass balance without taking into consideration of the contribution due to tropical cyclones may not be acceptable. 10 0N 30 0NThe energy input to the ocean induced by tropical cyclones can be transported into the ocean interior; accordingly, diapycnal mixing in the upper ocean (below the mixed layer) is greatly enhanced. Our results indicate that the vertical diffusivity induced by tropical cyclones is on the order of , with the maximum of in the western Pacific. The mixing induced by tropical cyclones varied greatly with time and took place at low- and mid- latitudes with strong stratification; the strong ocean mixing induced by tropical cyclones may play vitally important roles in the oceanic general circulation.