Julie L. McClean

Frontal Scale Air–Sea Interaction in High-Resolution Coupled Climate Models

Abstract The emerging picture of frontal scale air–sea interaction derived from high-resolution satellite observations of surface winds and sea surface temperature (SST) provides a unique opportunity to test the fidelity of high-resolution coupled climate simulations. Initial analysis of the output of a suite of Community Climate System Model (CCSM) experiments indicates that characteristics of frontal scale ocean–atmosphere interaction, such as the positive correlation between SST and surface wind stress, are realistically captured only when the ocean component is eddy resolving. The strength of the coupling between SST and surface stress is weaker than observed, however, as has been found previously for numerical weather prediction models and other coupled climate models. The results are similar when the atmospheric component model grid resolution is doubled from 0.5° to 0.25°, an indication that shortcomings in the representation of subgrid scale atmospheric planetary boundary layer processes, rather t...

Eulerian and Lagrangian Statistics from Surface Drifters and a High-Resolution POP Simulation in the North Atlantic

Abstract Eulerian and Lagrangian statistics were calculated from the North Atlantic surface drifter dataset for the years 1993–97 and a high-resolution eddy-resolving configuration of the Los Alamos National Laboratory (LANL) Parallel Ocean Program (POP) model. The main purpose of the study was to statistically quantify the state of the surface circulation in the North Atlantic Ocean for this period and compare it with the equivalent modeled state. Diffusivities and time and length scales are anisotropic over most of the ocean basin, except in most of the subpolar regions. Typical time and length scales are 2–4 days and 20–50 km. Longest timescales are found in the energetically quiescent regions in the south and southeast sectors of the basin. The longest length scales are found in the energetic western boundary current system, the most dispersive region of the domain. In many respects the eddy-resolving model reproduced a surface circulation in good statistical agreement with that depicted by the drifte...

Observations and modeling of the 1991–1992 El Niño signal off central California

Five research cruises were conducted over the continental shelf and slope near the Farallon Islands, California, in February, May, August, and October/November 1991 and February 1992. The observations consisted of shipboard hydrographic and acoustic Doppler current profiler data and moored current meter measurements. Water mass anomalies were calculated for each cruise by subtracting seasonal means based on historical data. In general, the maximum anomalies were observed subsurface in the 100-to 150-m range. In May 1991, equatorward, upwelling favorable winds elevated the thermocline resulting in cold, salty anomalies nearshore, with cold, fresh anomalies offshore associated with the advection of Pacific Subarctic Water into the region from the north. Warm, fresh anomalies and a strongly depressed thermocline were observed during the February 1992 cruise. A combination of coastal sea level and wind stress data and output from the Los Alamos National Laboratory parallel ocean program model was used to explain the cause of these anomalies. The February 1992 anomalies were shown to be due to both the deepening of the Aleutian low in the North Pacific associated with the 1991–1993 El Nino/Southern Oscillation event in the equatorial Pacific and poleward propagating intraseasonal coastal trapped Kelvin waves also arising from this event. The anomalous poleward wind forcing produced onshore flow, deepening of the thermocline, and downwelling at progressively southward locations. The “downwelling” Kelvin waves propagated northward with the two signals meeting somewhere near the cruise region. Both the model and the coastal sea level data showed the phase speed of the waves to slow by about 50% after passing the Gulf of California. This may be due to the scattering of energy from the fastest baroclinic mode into a slower mode. The strongest wave signal in the equatorial Pacific did not necessarily produce the strongest anomalies off central California.

Thermohaline Stratification of the Indonesian Seas: Model and Observations*

The Indonesian Throughflow, weaving through complex topography, drawing water from near the division of the North Pacific and South Pacific water mass fields, represents a severe challenge to modeling efforts. Thermohaline observations within the Indonesian seas in August 1993 (southeast monsoon) and February 1994 (northwest monsoon) offer an opportunity to compare observations to model output for these periods. The simulation used in these comparisons is the Los Alamos Parallel Ocean Program (POP) 1/6 deg (on average) global model, forced by ECMWF wind stresses for the period 1985 through 1995. The model temperature structure shows discrepancies from the observed profiles, such as between 200 and 1200 dbar where the model temperature is as much as 38C warmer than the observed temperature. Within the 58‐288C temperature interval, the model salinity is excessive, often by more than 0.2. The model density, dominated by the temperature profile, is lower than the observed density between 200 and 1200 dbar, and is denser at other depths. In the model Makassar Strait, North Pacific waters are found down to about 250 dbar, in agreement with observations. The model sill depth in the Makassar Strait of 200 m, rather than the observed 550-m sill depth, shields the model Flores Sea from Makassar Strait lower thermocline water, causing the Flores lower thermocline to be dominated by salty water from the Banda Sea. In the Maluku, Seram, and Banda Seas the model thermocline is far too salty, due to excessive amounts of South Pacific water. Observations show that the bulk of the Makassar throughflow turns eastward into the Flores and Banda Seas, before exiting the Indonesian seas near Timor. In the model, South Pacific thermocline water spreads uninhibited into the Banda, Flores, and Timor Seas and ultimately into the Indian Ocean. The model throughflow transport is about 7.0 Sv (Sv [ 106 m3 s21) in August 1993 and 0.6 Sv in February 1994, which is low compared to observationally based estimates. However, during

Comparisons of mesoscale variability in the Semtner-Chervin 1/4° model, the Los Alamos Parallel Ocean Program 1/6° model, and TOPEX/POSEIDON data

Measures of mesoscale variability in the Semtner-Chervin 1/4° and the Los Alamos Parallel Ocean Program (POP) 1/6° models were compared with those obtained from TOPEX/POSEIDON (T/P) data. The objectives of these comparisons were two-fold: the first was to validate the models using altimetry as a measure of the variability of the real ocean, and the second was to evaluate the effect of increased model resolution/decreased horizontal friction. Mesoscale root-mean-square (rms) sea surface height (SSH), eddy kinetic energies, and length scales were used to quantify the mesoscale variability. Results showed that the models reproduced the distribution and much of the magnitude of this variability associated with the major current systems; however, in the oceans interiors the magnitude was underrepresented. The 1/6° and 1/4° models were found to explain about 60% and 50% of the global T/P variability, respectively. Estimates of eddy kinetic energy (and rms velocities) from T/P and the models were compared, demonstrating that the models were less energetic than the T/P fields. Independent comparisons were made with lagrangian drifters in the Pacific basin. Excellent agreement was found between the total POP velocity fields and the drifter data in the tropics, where the T/P geostrophic values were too high due to error amplification by the 1/ƒ factor. In the midlatitudes, the drifter values exceeded those derived from the total model velocities; the T/P results lay between the two. Differences are attributed to the drifter analysis choices and possible residual noise in the altimetry data. The effect of increased resolution/decreased friction was best seen in the length scales where the POP scales agreed more closely with the T/P values than with the 1/4° model The distribution and magnitude of the POP length scales were generally in agreement with the T/P values between 10° and 40°. Near the equator, discrepancies were due to the long equatorial and instability waves, whose long wavelengths were too great to be resolved by the the combination of noise in the altimeter slopes, and the particular definition of length scale chosen.

Reconstructing the Ocean's Interior from Surface Data

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numericalsimulation. The decomposition yields strikinglyrealistic subsurfacefields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example,seasurfacetemperature(SST)andSSH,andsparse(orclimatological)estimatesoftheregionaldepthresolveddensity.Themethodcouldbefurtherelaboratedtointegrateadditionalsubsurfaceinformation,such as mooring measurements.

Estimates of bottom flows and bottom boundary layer dissipation of the oceanic general circulation from global high-resolution models

[1]xa0This paper (1) compares the bottom flows of three existing high-resolution global simulations of the oceanic general circulation to near-bottom flows in a current meter database and (2) estimates, from the simulations, the global energy dissipation rate of the general circulation by quadratic bottom boundary layer drag. The study utilizes a data-assimilative run of the Naval Research Laboratory Layered Ocean Model (NLOM), a nonassimilative run of NLOM, and a nonassimilative run of the Parallel Ocean Program z-level ocean model. Generally speaking, the simulations have some difficulty matching the flows in individual current meter records. However, averages of model values of (the time average of the cube of bottom velocity, which is proportional to the dissipation rate) computed over all the current meter sites agree to within a factor of 2.7 or better with averages computed from the current meters, at least in certain depth ranges. The models therefore likely provide reasonable order-of-magnitude estimates of areally integrated dissipation by bottom drag. Global dissipation rates range from 0.14 to 0.65 TW, suggesting that bottom drag represents a substantial sink of the ∼1 TW wind-power transformed into geostrophic motions.

Vertical heat transport in eddying ocean models

[1]xa0The effect of mesoscale eddies on the vertical heat transport of the ocean is examined using two eddy-resolving numerical models. The global heat transport by the mean flow and diffusion are both downwards and are balanced by an upward eddy heat flux. Mean and eddy advective heat fluxes dominate the subpolar regions, while diffusive flux is important primarily in the subtropics. In the subtropical abyss, the mean advective heat flux is balanced by a combination of eddy and diffusive fluxes and the classical Munk-type advective-diffusive heat balance must be modified. The Munk and Wunsch (1998) expression for the vertical turbulent diffusivity over-estimates the diffusivity by as much as a factor of four near the base of the main thermocline. This implies that the mixing required to close the meridional overturning circulation determined by Munk and Wunsch (1998) may be an over-estimate due to the neglect of mesoscale eddies.

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). n nThe northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

Formation of subtropical mode water in a high-resolution ocean simulation of the Kuroshio Extension region

Abstract A high-resolution numerical model is used to examine the formation and variability of the North Pacific Subtropical Mode Water (STMW) over a 3-year period. The STMW distribution is found to be highly variable in both space and time, a characteristic often unexplored because of sparse observations or the use of coarse resolution simulations. Its distribution is highly dependent on eddies, and where it was renewed during the previous winter. Although the potential vorticity fluxes associated with down-front winds can be of the same order of magnitude or even greater than the diabatic ones due to air–sea temperature differences, the latter dominate the potential vorticity budget on regional and larger scales. Air–sea fluxes, however, are dominated by a few strong wind events, emphasizing the importance of short time scales in the formation of mode waters. In the Kuroshio Extension region, both advection and mixing play important roles to remove the STMW from the formation region.