An Oceanic Mixing Model: Application to Global Climate and to the New Zealand West Coast

Abstract

A new vertical mixing model is described. It combines a box-advection-diffusion model with a bulk mixed layer model, which simulates wind mixing and penetrative convection. It is shown that mixed layer models can have steady periodic solutions if either the mixing decays to zero at depth, or vertical advection is included. The latter approach is adopted. This mixing model is applied to a series of latitude bands between 50°N and 50°S, using average climatological data. These latitude zones are coupled in a simple two-dimensional thermohaline circulation scheme which allows for poleward heat transport. The model successfully simulates present day seasonal cycles of temperature and mixed layer depth. The model is then subjected to an additional heat flux resulting from an increase in greenhouse gases. For an equilibrium warming of 3°C for CO2 doubling, the model predicts the following transient response: a 0.5°C to 0.8°C temperature rise from 1850 to 1990, and a 1.5°C to 2.0°C rise from 1850 to 2050. The ocean acts as a thermal buffer, so that the actual surface warming lags the equilibrium warming by 25 to 50 years. Heat storage in the mixed layer and the deep ocean contribute about equally to this lag. The seasonal cycle of mixed layer depth pumps more heat down to deeper waters, compared to a fixed mixed layer depth model. The heat uptake depends strongly on possible changes in the global thermohaline circulation, which could therefore affect sea level predictions. The climatic warming also leads to a reduction in winter mixing depth in the higher latitudes, whereas the mixing depth in other seasons and latitudes would be mainly affected by wind changes. A scenario for reduced CO2 emissions shows that the surface warming can be slowed dramatically, but that a longterm sea level rise from thermal expansion may be inevitable. The model is also applied to a region off the West Coast of the South Island, New Zealand. Meteorological data from a coastal weather station are used to drive simulations of ocean temperatures for the years 1973-1989. Measurements were performed at sea to correlate the winds at the coastal station with those offshore. The model results show general agreement with satellite derived sea surface temperatures and some shipboard observations. However the poor quality of the available forcing data precludes a detailed analysis of temperature variations off Westland.