Adrian Hines

A global perspective on Langmuir turbulence in the ocean surface boundary layer

The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface-wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of wave-forced turbulence is developed using re-analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under-estimated without forcing by surface waves. Wave-forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surface-wave-forced Langmuir turbulence is an important process in the OSBL that requires parameterization. Citation: Belcher, S. E., et al. (2012), A global perspective on Langmuir turbulence in the ocean surface boundary layer, Geophys. Res. Lett., 39, L18605, doi: 10.1029/2012GL052932.

Assessment of the FOAM global data assimilation system for real-time operational ocean forecasting

Abstract An operational system to forecast the state of the global ocean a few days ahead has been implemented at the UK Met. Office (UKMO). The system, known as the Forecasting Ocean Assimilation Model (FOAM), consists of a 1°×1° resolution numerical ocean model driven by surface fluxes from the UKMO numerical weather prediction (NWP) suite and a modified successive correction data assimilation scheme for thermal observations. The assimilation scheme is assessed here in a series of 1-year integrations by comparison with ‘independent’ thermal profile observations and climatology. Assimilating temperature observations significantly reduces model errors in the upper ocean and results in temperature analyses that on average are closer to independent observations than climatology. The specific results depend on location and depth. The extent to which the data assimilation scheme is able to compensate for uncertainties in the surface forcing fluxes is also assessed by comparing integrations forced with climatological and NWP fluxes. Assimilating data is able to compensate for uncertainties in the surface heat forcing fluxes and significantly reduces the impact from uncertainties in the surface wind stress.

Forecasting the ocean state using NEMO:The new FOAM system

The Forecasting Ocean Assimilation Model (FOAM) deep ocean analysis and forecasting system has been running operationally at the Met Office for over 10 years.The system has recently been transitioned to use the Nucleus for European Modelling of the Ocean (NEMO) community model as its core ocean component. This paper gives an end-to-end description of the FOAM-NEMO operational system and presents some preliminary assessment of operational and hindcast integrations including verification statistics against observations and forecast verification against model best guess fields.Validation of the sea surface height fields is presented, which suggests that the system captures and tracks the major mesoscale features of the ocean circulation reasonably well, with some evidence of improvement in higher-resolution configurations.

The Forecasting Ocean Assimilation Model (Foam) System

We present a detailed technical description of the present FOAM system and discuss some representative examples of the scientific investigations we undertake to track-down problems within the system and to understand the importance (“impact”) of the various inputs to it. We also provide an historical perspective on the development of the system and the changing demands for it, and describe the way in which we are adapting to meet these demands.

Solution of the linear thermocline equations driven by wind stress and thermohaline forcing

Abstract In this paper, new steady‐state solutions of the linearized thermocline equations satisfying prescribed fluxes of heat and salt at the base of the surface Ekman layer, are presented for a semi‐infinite ocean of constant depth. A decomposition into vertical modes is used to solve the problem. The solution is first determined in terms of a derivative of the unknown density at the surface and this derivative is then determined from an integral equation arising from applying the surface thermohaline boundary conditions. Solutions forced by wind stress alone, and by wind stress and thermohaline forcing are considered. The wind‐driven solution exhibits a temperature field with many realistic features, such as largest meridional gradients in the sub‐polar gyre, and the latitudinal spreading of isotherms towards the eastern boundary. The wind‐driven salinity field increases towards the poles, contrary to the observed annual mean salinity field. The stability of the sub‐tropical gyre is enhanced, whilst the sub‐polar gyre is de‐stabilized. With the addition of the thermohaline forcing the deficiencies of the salinity field associated with the wind‐driven solution are largely corrected, whilst the solution retains a reasonable representation of the climatological temperature field. Temperature and salinity anomaly fields relative to the Levitus climatology, calculated from the Met. Office Forecasting Ocean Assimilation Model, are shown to be qualitatively similar to the anomaly fields derviedfrom the model discussed in this paper. This result serves to underline the message that the combination of wind and surface buoyancy forcing are essential when modelling the large‐scale temperature and salinity fields using the thermocline equations.

Unsteady Abyssal Circulation Driven by a Discrete Buoyancy Source in a Continuously Stratified Ocean

Abstract Analytical and numerical models are presented for linear quasigeostrophic buoyancy-driven flow forced bya time periodic pulsating point mass source in a continuously stratified, incompressible β-plane ocean withconstant Brunt–Vaisala frequency. The source represents the seasonal introduction of dense water into the abyssalocean and is located on a linear sloping bottom of arbitrary orientation. The ocean domain is horizontallyunbounded and of infinite depth. Rayleigh friction is incorporated into the horizontal momentum equations andappears at order Rossby number in the quasigeostrophic expansions. In the density equation the influence ofRayleigh friction and Laplacian friction are each considered in turn. Analytical solutions are obtained in the case of 1) a midlatitude β plane with no bottom slope and 2) an fplane with a bottom slope. In both of these problems the fluid is initially at rest and the mass source is switchedon and maintained. A three-dimensional radiating field of baroclinic Rossb...