Peter A. Rochford

An optimal definition for ocean mixed layer depth

A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the worlds oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data, respectively: (1) from 11 ocean weather stations in the northeast Pacific and (2) from the World Ocean Atlas 1994. Using these two data sets, a detailed statistical error analysis is presented for the ILD and MLD estimation by season. MLD variations with location due to temperature and salinity are properly accounted for in the defining density (Δσt) criterion. Overall, the optimal estimate of turbulent mixing penetration is obtained using a MLD definition of ΔT =0.8°0, although in the northeast Pacific region the optimal MLD criterion is found to vary seasonally. The method is shown to produce layer depths that are accurate to within 20 m or better in 85% or more of the cases. The MLD definition presented in this investigation accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one-dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed layer.

Efficient and Accurate Bulk Parameterizations of Air–Sea Fluxes for Use in General Circulation Models

Abstract Efficient and computationally inexpensive simple bulk formulas that include the effects of dynamic stability are developed to provide wind stress, and latent and sensible heat fluxes at the air–sea interface in general circulation models (GCMs). In these formulas the exchange coefficients for momentum and heat (i.e., wind stress drag coefficient, and latent and sensible heat flux coefficients, respectively) have a simple polynomial dependence on wind speed and a linear dependence on the air–sea temperature difference that are derived from a statistical analysis of global monthly climatologies according to wind speed and air–sea temperature difference intervals. Using surface meteorological observations from a central Arabian Sea mooring, these formulas are shown to yield air–sea fluxes on daily timescales that are highly accurate relative to those obtained with the standard algorithm used by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), where the ...

Mixed layer depth variability and barrier layer formation over the North Pacific Ocean

Seasonal variability in the isothermal and isopycnal surface mixed layers of the North Pacific Ocean is examined using the Naval Research Laboratory Ocean Mixed Layer Depth (NMLD) Climatology. A comparison with observations from 11 ocean weather stations in the northeast Pacific Ocean is performed that validates the NMLD climatology in this region. The general features of the isothermal layer depth (ILD) and mixed layer depth (MLD) obtained from these mixed layers are explained with wind stress, surface net heat flux, and freshwater flux climatologies, given guidance from a mixed layer model. Departures from a surface-forced interpretation of turbulent mixing are found near the Kuroshio, where horizontal heat transport is important. The much deeper ILD in the northeast Pacific in winter and spring relative to the MLD reveals a 50 m “barrier layer” between the bottom of the MLD and the top of the thermocline. A detailed analysis shows this barrier layer extends over most of the North Pacific subpolar gyre. It forms when the seasonal thermocline is deepened in winter by surface cooling, such that salinity stratification due to evaporation minus precipitation less than zero (E - P <0) becomes important in the formation of the MLD. A shallower halocline forms over the subpolar gyre than in other regions of the North Pacific because of precipitation dominating over evaporation in the annual mean. A mechanism for maintaining the shallow halocline is provided by upward vertical motion driven by positive wind stress curl in the presence of diapycnal mixing. Numerical models show this as part of a shallow meridional overturning cell.

Importance of solar subsurface heating in ocean general circulation models

The importance of subsurface heating on surface mixed layer properties in an ocean general circulation model (OGCM) is examined using attenuation of solar irradiance with depth below the ocean surface. The depth-dependent attenuation of subsurface heating is given by global monthly mean fields for the attenuation of photosynthetically available radiation (PAR), kPAR. These global fields of kPAR are derived from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) data on the spectral diffuse attenuation coefficient at 490 nm (k490), and have been processed to have the smoothly varying and continuous coverage necessary for use in OGCM applications. These monthly fields provide the first complete global data sets of subsurface optical fields that can be used for OGCM applications of subsurface heating and bio-optical processes. The effect on global OGCM prediction of sea surface temperature (SST) and surface mixed layer depth (MLD) is examined when solar heating, as given by monthly mean kPAR and PAR fields, is included in the model. It is found that subsurface heating yields a marked increase in the SST predictive skill of the OGCM at low latitudes. No significant improvement in MLD predictive skill is obtained when including subsurface heating. Use of the monthly mean kPAR produces an SST decrease of up to 0.8°C and a MLD increase of up to only 4–5 m for climatological surface forcing, with this primarily confined to the equatorial regions. Remarkably, a constant kPAR value of 0.06 m−1, which is indicative of optically clear open ocean conditions, is found to serve very well for OGCM prediction of SST and MLD over most of the global ocean.

The NRL Layered Global Ocean Model (NLOM) with an Embedded Mixed Layer Submodel: Formulation and Tuning*

Abstract A bulk-type (modified Kraus–Turner) mixed layer model that is embedded within the Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) is introduced. It is an independent submodel loosely coupled to NLOMs dynamical core, requiring only near-surface currents, the temperature just below the mixed layer, and an estimate of the stable mixed layer depth. Coupling is achieved by explicitly distributing atmospheric forcing across the mixed layer (which can span multiple dynamic layers), and by making the heat flux and thermal expansion of seawater dependent upon the mixed layer models sea surface temperature (SST). An advantage of this approach is that the relative independence of the dynamical solution from the mixed layer allows the initial state for simulations with the mixed layer to be defined from existing near-global model simulations spun up from rest without a mixed layer (requiring many hundreds of model years). The goal is to use the mixed layer model in near-global multidecadal simul...

Air–Sea Flux Estimates And The 1997–1998 Enso Event

Bulk formulae for wind stress, sensible and latent heat flux are presented that are suitable for strong mesoscale events such as westerly wind bursts that contribute to the El Niño-Southern Oscillation (ENSO). Their exchange coefficients for heat and momentum have a simple polynomial dependence on wind speed and a linear dependence on air–sea temperature difference. The accuracy of these formulae are validated with respect to air–sea fluxes estimated using the standard algorithm adopted by the Tropical Ocean-Global AtmosphereCoupled-Ocean Atmosphere Response Experiment (TOGA COARE). The comparison ismade for observations from 96 Tropical Atmosphere Ocean (TAO) array and National Oceanographic Data Center (NODC) moorings in the equatorial and North Pacific Ocean spanning years 1990–1999. The bulk formulae are shown to have very small median root–mean-square differences with respect to the TOGA COARE estimates: ≈ 0.003 N m-2, 1.0 W m-2, and 10.0 W m-2 for the wind stress, sensible heat flux, and latent heat flux, respectively.The variability of air–sea fluxes during the 1997–1998 ENSO is also examined, along with a possible relationship between air–sea fluxes and surface ocean mixed layer depth (MLD). The wind stress and latent heat flux during the 1997 El Niño are found to be greater in the warm pool of the western Pacific than in the central Pacific where the ENSO is most clearly seen. These differences disappear upon the start of La Niña. The MLD in the equatorial Pacific is found to be moderately correlated to air–sea fluxes just before the start of the 1998 La Niña and poorly correlated otherwise.

The Impact of Water Turbidity on Interannual Sea Surface Temperature Simulations in a Layered Global Ocean Model

Abstract The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) with an embedded bulk-type mixed layer model is used to examine the effects of ocean turbidity on sea surface temperature (SST) and ocean mixed layer depth (MLD) simulations over the global ocean. The model accounts for ocean turbidity through depth-dependent attenuation of solar radiation in the mixed layer formulation as determined from the diffusive attenuation coefficient at 490 nm (k490) obtained by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS). Interannual model simulations are used to assess the first-order effects of ocean turbidity on SST and MLD simulation. Results are reported from three model experiments performed using different values for the attenuation of photosynthetically available radiation (kPAR). It is shown that, although allowing incoming solar radiation to vary in time and space is desirable for predicting SST, in an OGCM use of a constant kPAR with a value of 0.06 m−1 is generally sufficient in the deep ...

Sensitivity of the Arabian Sea mixed layer to 1994–1995 operational wind products

The evolution of the upper ocean in the strong seasonally forced Arabian Sea, as observed by a mooring deployed in 1994–1995, is investigated using the Naval Research Laboratory Layered Ocean Model (NLOM). Model simulations were sensitive to the choice of surface wind products used for forcing, and results are reported for simulations forced by monthly mean climatologies and 12 hourly 1994–1995 wind products from two operational atmospheric forecast models, the European Centre for Medium-Range Weather Forecast model and the Navy Operational Global Atmospheric Prediction System model of Fleet Numerical Meteorology and Oceanography Center (FNMOC). The NLOM yields the best prediction of sea surface temperature (SST) and mixed layer depth when using FNMOC forcing. Surface cooling is found to be responsible for the seasonal SST minimum during the NE monsoon. Heat advection is found to be important for supporting the surface cooling during the second half of the NE monsoon. Strong entrainment and appreciable advective cooling are responsible for the SST minimum of the SW monsoon. The NLOM wind experiments strongly suggest that thermal convection may be important in the central Arabian Sea during the winter months.