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Gary Lagerloef

Abstract

Current measurements core the axis of a deep trough normal to the coast and from the adjacent shelf show that the mean flow is barotropic and follows depth contours, conserving potential vorticity, to form a cyclonic vortex or meander over the trough. The data are interpreted as an example of an inertial Taylor-Proudman column on the continental shelf. The scale of the topographic variations dominates the potential vorticity balance and a simple steady-state numerical model is in good agreement with the data when the vorticity balance approaches the limit U · BH = 0. Streamlines following isobaths converge over steeper topography and current speeds are nearly proportional to the local topographic gradient. Estimates from the data support this behavior and indicate that the ∼20 cm s−1 mean current around the trough is driven by a typical cross-shelf-averaged velocity scale of ∼5 cm s−1.

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Gary S. E. Lagerloef

Abstract

A technique is presented that applies modal decomposition to estimate dynamic height (0–450 db) from XBT temperature profiles. STD data are used to establish empirical relationships between vertically integrated temperature profiles and empirical dynamic height modes. These are then applied to XBT data to estimate dynamic height. A standard error of 0.028 dynamic meters is obtained for the waters of the Gulf of Alaska—an ocean region subject to substantial freshwater buoyancy forcing and with a TS relationship that has considerable scatter. The residual error is a substantial improvement relative to the conventional TS correlation technique when applied to this region. Systematic errors between estimated and true dynamic height were evaluated. The 20-year-long time series at Ocean Station P (50°N, 145°W) indicated weak variations in the error interannually, but not seasonally. There were no evident systematic alongshore variations in the error in the ocean boundary current regime near the perimeter of the Alaska gyre. The results prove satisfactory for the purpose of this work, which is to generate dynamic height from XBT data for coanalysis with satellite altimeter data, given that the altimeter height precision is likewise on the order of 2–3 cm. While the technique has not been applied to other ocean regions where the T–S relation has less scatter, it is suggested that it could provide some improvement over previously applied methods, as well.

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Gary S. E. Lagerloef

Abstract

Climatic dynamic topography variations in the Alaska gyre during the period 1968–1990 are described with an objective analysis of more than 12000 STD and XBT stations, and COADS wind stress data. Interannual dynamic height and SST variations were correlated and were consistent with recently described large-scale climatic shifts in the North Pacific. The gyre was cantered more to the east circulation appeared stronger, and SST was lower during the early to mid-1970s than during the 1980s. The Aleutian low (NP and PNA indices) intensified during the interim, but the response did not appear as a gyre spinup. Instead, the associated wind stress anomalies forced a slowly varying dynamic height anomaly across the eastern and northern part of the gyre through Ekman convergence, which had the effect of displacing the gyre's low somewhat to the WSW in the 1980s. The wind curl spectrum was white, and the slow oceanic response was modelled as stochastic-forced climate variability with a simple first-order Markov autoregression process. Forcing was assumed to be Ekman pumping of the pycnocline, and the damping coefficient was estimated from the data to be ∼1 yr−1. A hindcast with observed winds gave estimated dynamic height patterns similar to those observed, with a canonical correlation of 0.79 at 99% confidence. This response was weak in the western half of the gyre, where slow baroclinic variability may have been influenced by long Rossby wave propagation. A simple autoregression simulation using artificial white noise forcing shows the evolution of decadal Variations similar in nature to those observed. This result, along with the low frequency correlation between dynamic height and SST, suggests that the upper-ocean climatic variability in this region is primarily wind forced.

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Patrick F. Cummins and Gary S. E. Lagerloef

Abstract

Low-frequency variability of the depth of the main pycnocline at Ocean Weather Station P and over the northeast Pacific is examined in terms of the one-dimensional response to local Ekman pumping according to the Hasselmann stochastic climate model. The model is forced with monthly wind stress curl anomalies derived from the National Centers for Environmental Prediction reanalysis for the period 1948–2000. An empirical orthogonal function analysis shows that the leading mode of the response bears the signature of the Pacific (inter) Decadal Oscillation (PDO) and that the associated principal component captures the “regime shift” of 1976/77. The correlation is 0.77 between annually averaged pycnocline displacement anomalies hindcast from the model and anomalies in the depth of the main pycnocline at station P (50°N, 215°E) observed over a 43-yr period. The comparison indicates that variability in the depth of the upper layer on interannual to interdecadal timescales at station P occurs largely as an integrated response to local Ekman pumping. In addition, the results suggest that the PDO mode dominates the observed variability.

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Fabrice Bonjean and Gary S. E. Lagerloef

Abstract

A diagnostic model of the tropical circulation over the 0–30-m layer is derived by using quasi-linear and steady physics. The horizontal velocity is directly estimated from sea surface height (TOPEX/Poseidon), surface vector wind (SSM/I) and sea surface temperature (AVHRR + in situ measurements). The absolute velocity is completed using the mean dynamic height inferred from the World Ocean Atlas (WOA). The central issue investigated in this study is the more accurate estimate of equatorial surface currents relative to prior satellite-derived method. The model formulation combines geostrophic, Ekman, and Stommel shear dynamics, and a complementary term from surface buoyancy gradient. The field is compared with velocity observations from 15-m-depth buoy drifter and equatorial Tropical Ocean–Atmosphere (TAO) current meters. Correlations with TAO data on the equator are much higher in the eastern Pacific cold tongue than before. The mean field in the cold tongue is also much more accurate, now showing the equatorial minimum that splits the South Equatorial Current into northern and southern branches. The mean current strength is somewhat less than in drifter composites because the mean dynamic topography from WOA remains too smooth. However, the seasonal cycle and interannual variations are robust, especially anomalies on the order of 1 m s−1 during the 1997–98 ENSO. This direct method using satellite measurements provides surface current analyses for numerous research and operational applications.

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Gary S. E. Lagerloef, Robin D. Muench, and James D. Schumacher

Abstract

We present current observations from depths of 20, 50, 100 and 175 m obtained over a 3-year period near the northeast Gulf of Alaska shelf break, and emphasize a 2-year continuous segment from 50 m depth. These records indicate that a moderate (∼16 cm s−1 at 50 m) mean flow was directed, at all depths, along-shelf toward the northwest. Variance was high and had nearly normal distributions in along- and across-shelf components. The mean alongshelf speed varied from ∼12 cm s−1 in summer to ∼20 cm s−1 in winter and a more distinct seasonal cycle was evident in monthly kinetic energies of wind and current. Very low-frequency (VLF) (<0.1 cpd) current fluctuations were prominent flow features and were vertically coherent between 50 and 100 m depth (coherency squared >0.8 at 95% level). Fluctuating kinetic energy of the current contained in the VLF band was a significant fraction (34%) of the total kinetic energy, and displayed an increase over the two years with no seasonal trend. Trends in the wind kinetic energy in the VLF band were much different and most (∼66%) of the VLF current kinetic energy was not correlated with that of the winds. These current fluctuations were probably related to oceanic-scale features such as eddies or meanders in the Alaska Current.

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Oleg Melnichenko, Peter Hacker, Nikolai Maximenko, Gary Lagerloef, and James Potemra

Abstract

A method is presented for mapping sea surface salinity (SSS) from Aquarius level-2 along-track data in order to improve the utility of the SSS fields at short length [O(150 km)] and time [O(1 week)] scales. The method is based on optimal interpolation (OI) and derives an SSS estimate at a grid point as a weighted sum of nearby satellite observations. The weights are optimized to minimize the estimation error variance. As an initial demonstration, the method is applied to Aquarius data in the North Atlantic. The key element of the method is that it takes into account the so-called long-wavelength errors (by analogy with altimeter applications), referred to here as interbeam and ascending/descending biases, which appear to correlate over long distances along the satellite tracks. The developed technique also includes filtering of along-track SSS data prior to OI and the use of realistic correlation scales of mesoscale SSS anomalies. All these features are shown to result in more accurate SSS maps, free from spurious structures. A trial SSS analysis is produced in the North Atlantic on a uniform grid with 0.25° resolution and a temporal resolution of one week, encompassing the period from September 2011 through August 2013. A brief statistical description, based on the comparison between SSS maps and concurrent in situ data, is used to demonstrate the utility of the OI analysis and the potential of Aquarius SSS products to document salinity structure at ~150-km length and weekly time scales.

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Eric S. Johnson, Fabrice Bonjean, Gary S. E. Lagerloef, John T. Gunn, and Gary T. Mitchum

Abstract

Comparisons of OSCAR satellite-derived sea surface currents with in situ data from moored current meters, drifters, and shipboard current profilers indicate that OSCAR presently provides accurate time means of zonal and meridional currents, and in the near-equatorial region reasonably accurate time variability (correlation = 0.5–0.8) of zonal currents at periods as short as 40 days and meridional wavelengths as short as 8°. At latitudes higher than 10° the zonal current correlation remains respectable, but OSCAR amplitudes diminish unrealistically. Variability of meridional currents is poorly reproduced, with severely diminished amplitudes and reduced correlations relative to those for zonal velocity on the equator. OSCAR’s RMS differences from drifter velocities are very similar to those experienced by the ECCO (Estimating the Circulation and Climate of the Ocean) data-assimilating models, but OSCAR generally provides a larger ocean-correlated signal, which enhances its ratio of estimated signal over noise. Several opportunities exist for modest improvements in OSCAR fidelity even with presently available datasets.

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Gary S. E. Lagerloef, Roger Lukas, Robert A. Weller, and Steven P. Anderson

Abstract

The Hasselmann feedback model was applied to hindcast western Pacific warm pool sea surface temperatures (SST) with heat flux observations obtained near 2°S, 156°E from October 1992 to February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The model versus observed SST correlations were greater than 0.85. Two important feedback (or damping) timescales emerged, with e-folding times of λ −1 = 0.2 days and 8 days, fitting to the diurnal and subdiurnal variations, respectively. Distinct mixed layer depth scales were also found for the respective timescales. A time-varying depth parameter with a median of ∼5 m was derived for the shorter timescale and varied with the observed daily minimum mixed layer depth. A constant ∼16 m was optimal for the longer timescale, which is similar to the time-averaged observed mixed layer depth of 14.8 m and the Monin–Obukhov scale of ∼17 m. This bears on the choice of mixed layer parameters for climate model simulations of warm pool conditions observed in TOGA COARE. The low-frequency time- and depth-scale parameters give a negative feedback of about 95 W m−2 °C−1, which is significantly greater than previous studies have indicated. This restoring influence was treated separately from fluxes across the air–sea interface such as latent, radiative, and sensible heat loss or cloud shading, and is thus attributed to oceanic mixed layer processes. The frequency band where the damping or feedback becomes important is defined by ωλ, which is found to coincide with the diurnal cycle and the ∼50-day Madden–Julian oscillations for the respective λ −1 timescales. This indicates a possible dynamic connection between the surface heat forcing and mixed layer dissipation timescales, which the authors suggest might be accounted for if the dissipation is parameterized as being proportional to the amplitude of SST variations.

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Robert W. Helber, Robert H. Weisberg, Fabrice Bonjean, Eric S. Johnson, and Gary S. E. Lagerloef

Abstract

The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steady-state model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(±6) × 106 m3 s−1. Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.

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