Freshwater transports in HadCM3

Abstract

The hydrological cycle can influence climate through a great variety of processes. A good representation of the hydrological cycle in climate models is therefore crucial. Attempts to analyse the global hydrological cycle are hampered by a deficiency of suitable observations, particularly over the oceans. Fully coupled general circulation models are potentially powerful tools in interpreting the limited observational data in the context of large-scale freshwater exchanges. We have looked at large-scale aspects of the global freshwater budget in a simulation, of over 1000 years, by the Hadley Centre coupled climate model (HadCM3). Many aspects of the global hydrological cycle are well represented, but the model hydrological cycle appears to be too strong, with overly large precipitation and evaporation components in comparison with the observational datasets we have used. We show that the ocean basin-scale meridional transports of freshwater come into near balance with the surface freshwater fluxes on a time scale of about 400 years, with the major change being a relative increase of freshwater transport from the Southern Ocean into the Atlantic Ocean. Comparison with observations, supported by sensitivity tests, suggests that the major cause of a drift to more saline condition in the model Atlantic is an overestimate of evaporation, although other freshwater budget components may also play a role. The increase in ocean freshwater transport into the Atlantic during the simulation, primarily coming from the overturning circulation component, which changes from divergent to convergent, acts to balance this freshwater budget deficit. The stability of the thermohaline circulation in HadCM3 may be affected by these freshwater transport changes and this question is examined in the context of an existing conceptual model.

Appendix 1

1.1 Implications of the rigid-lid formulation for salinity

Because of its unrealistic requirement of zero volume divergence, the ocean model rigid-lid approximation obviously introduces errors in the velocities. However, the simulation of large-scale ocean volume transports appears to be acceptable (e.g. Wood et al. 1999; Vellinga et al. 2002). This is possible because typical large-scale volume transports are of order (1–100) Sv, while freshwater divergences from large regions (the cause of volume divergence in the real world) are typically a few tenths of a Sv (Wijffels et al. 1992; Wijffels 2001). Hence the distortion of transports implied by the rigid lid is fractionally small.

Even if F _s has zero global average, Eq. 3 allows a drift in global volume-integrated salinity, because the geographical covariance of S _* and ρ_* with F _s can lead to a non-zero global average for –(S _*/ρ_*)F _s . Therefore a constant reference salinity S _* = 35 psu and density ρ_* = 1026 kg m^–3 are used instead. The use of the reference salinity means that freshwater fluxes will have a larger effect on the surface salinity in the model than in reality at locations where the reference salinity S _* is larger than the local S, and a smaller effect where S _* < S. Areas where the net freshwater flux is into the ocean (F _s > 0) tend to be colocated with areas where its effect on salinity in the model is enhanced, because the sea surface salinity is fresher than average (S ₀ > S). Areas where F _s < 0 tend to coincide with areas where the effect on salinity is suppressed, because S _* < S. Hence the use of a reference salinity will generally tend to freshen most areas of the sea surface. However, calculations suggest that the errors in model F _s relative to observations generally have a much larger influence.