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Kathryn A. Kelly

Abstract

Part of the heat transported poleward from the Tropics by the ocean is stored near the energetic western boundary currents. These storage reservoirs provide a source of interannual to decadal climate fluctuations through their impact on the ocean–atmosphere heat fluxes. Changes in ocean heat storage result from the difference between surface fluxes and the convergence of oceanic heat transport. To estimate the heat budget for 26°–40°N, 140°E–180°, sea surface temperature and subsurface temperatures are assimilated into a one-dimensional model of the upper ocean that is forced by heat fluxes from the NCEP–NCAR reanalysis. Heat transport convergences are inferred as the residual of the heat budget for the period 1970–2000 using the “unknown control” from a Kalman filter/smoother technique. The estimates of heat transport convergence compare qualitatively with direct estimates from a three-dimensional model that uses geostrophic currents from the TOPEX/Poseidon radar altimeter for 1993–99; this period contains the largest lateral fluxes and the largest heat loss from the ocean in the 31-yr record. The analysis of the heat budget demonstrates that, on interannual to decadal time scales, the heat storage rate in the upper ocean is better correlated with lateral heat transport convergence than with surface fluxes. In addition, heat content and surface flux are negatively correlated, demonstrating the dominance of oceanic feedback over atmospheric forcing. The close relationship between heat content and surface fluxes suggests the possibility of predicting surface flux anomalies: there is a small but significant skill in predicting surface flux anomalies up to one year in advance using heat content. SST has no prediction skill.

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Kathryn A. Kelly

Abstract

An inverse model to infer the near-surface velocity from the heat equation was applied to a series of six infrared satellite images from northern California. The inversion used a two-dimensional nondiffusive heat equation with a simple representation of surface heat fluxes and vertical entrainment. The along-isotherm component of the velocity was in the null space of this problem. An overdetermined problem was defined by adding weighted constraints on the energy, divergence and curl of the velocity and representative solutions were chosen from the family of solutions corresponding to a designated misfit level for the heat equation. A series of solutions with an average energy of 250 km2 d−2 and a divergence of 0.36 d−1 compared well with simultaneous Doppler acoustic log (DAL) measurements in regions with strong temperature gradients. The decorrelation time for the solutions was about one day. Horizontal advection accounted for about 40% of the variance of the temporal temperature derivative. There was little statistical skill in predicting temperature changes with the velocity fields inferred from previous images because of the short decorrelation time; however the average solutions provided a qualitative picture of movement of features seen in the series of images. The strongest region of divergence, the leading edge of a cold eddy near the coast, was consistent with average divergence calculations from DAL surveys for the same region. The velocity solutions from these inversions supported the theory that the apparent cold filaments in the images were actually meanders of the California Current.

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Shenfu Dong and Kathryn A. Kelly

Abstract

A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40°–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in the upper-ocean heat content are dominated by the advection– diffusion term. Within the advection term itself, the largest variations are from the geostrophic advection anomaly. In the western Gulf Stream region the largest component of anomalous advection is the advection of the anomalous temperature by the mean current; elsewhere, the advection of the mean temperature by the anomalous current is also important. Other studies have shown that upper-ocean heat content is a more robust indicator of the potential contribution of the ocean to interannual heat flux anomalies than is sea surface temperature. The analysis here shows that the dominant term in interannual variations in heat content in the Gulf Stream region is anomalous advection by geostrophic currents. In fact, these ocean-forced variations in heat content appear to force air–sea fluxes: the surface heat flux anomalies in the western Gulf Stream region are negatively correlated with the anomalous upper-ocean heat content, that is, a large heat loss to the atmosphere corresponding to a positive heat content anomaly.

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Kathryn A. Kelly and Bo Qiu

Abstract

Satellite-derived temperature and geostrophic velocities were assimilated into a mixed layer model to obtain estimates of the net surface heat flux as the residual of the upper ocean heat budget. The heat budget included eddy diffusion. advection, and vertical entrainment. Assimilation was done using a Kalman filter on both the temperature tendency and the temperature of the mixed layer. The error in temperature tendency was used to derive a new surface heat flux estimate. Experiments performed on the actual data suggested that better surface flux estimates could be obtained by allowing the model to predict the mixed layer depth than by adjusting the depth to a climatological value. A systematic error in the temperature tendency appeared to be due to errors in the estimate of the mean sea surface height from the altimeter; a partial correction for these errors was computed. The agreement between the time series of spatially averaged surface flux and that obtained from the ECMWF atmospheric model was surprisingly good. The temporally averaged surface flux estimates from the mixed layer model were in good agreement with the Bunker climatological values, except in February and March, when the model mixed layer shoaled more rapidly than expected from climatology.

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Kathryn A. Kelly and Bo Qiu

Abstract

The assimilation of temperature and altimetric velocity into a numerical model of the upper-ocean mixed layer in Part I allowed an analysis of the upper ocean heat budget for the western North Atlantic Ocean over the 2.5-year period of the Geosat Exact Repeat Mission (November 1986–April 1989). The balance of terms varied regionally: south of the Gulf Stream advection was relatively unimportant in the heat budget, and the ocean responded passively to changes in surface flux. Within the Gulf Stream and to the north of it, cooling of the upper ocean by advection was as large as 0.15°C/day for periods of several weeks. An analysis of the advection term showed that cooling by Ekman transport was opposed by warming from the geostrophic currents of the Gulf Stream, with cooling typically stronger by a factor of 2 because nonuniform Ekman transport disrupted the normal alignment between isotherms and sea surface height contours. There is a complex ocean-atmosphere coupling in this region: in addition to its increase during strong wind events, warming by geostrophic currents is a function of the strength of the Gulf Stream and its recirculation gyres. Over the 2.5-year period, the winds became progressively stronger, causing an increase in cooling by Ekman transport. Advective cooling was balanced by an increasingly positive surface flux (warming of the ocean by the atmosphere) at the rate of about 20% of the annually averaged surface flux per year. This positive trend in the surface flux was also observed in the estimates from the atmospheric general circulation model of the ECMWF.

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Shenfu Dong and Kathryn A. Kelly

Abstract

Formation and the subsequent evolution of the subtropical mode water (STMW) involve various dynamic and thermodynamic processes. Proper representation of mode water variability and contributions from various processes in climate models is important in order to predict future climate change under changing forcings. The North Atlantic STMW, often referred to as Eighteen Degree Water (EDW), in three coupled models, both with data assimilation [GFDL coupled data assimilation (GFDL CDA)] and without data assimilation [GFDL Climate Model, version 2.1 (GFDL CM2.1), and NCAR Community Climate System Model, version 3 (CCSM3)], is analyzed to evaluate how well EDW processes are simulated in those models and to examine whether data assimilation alters the model response to forcing. In comparison with estimates from observations, the data-assimilating model gives a better representation of the formation rate, the spatial distribution of EDW, and its thickness, with the largest EDW variability along the Gulf Stream (GS) path. The EDW formation rate in GFDL CM2.1 is very weak because of weak heat loss from the ocean in the model. Unlike the observed dominant southward movement of the EDW, the EDW in GFDL CM2.1 and CCSM3 moves eastward after formation in the excessively wide GS in the models. However, the GFDL CDA does not capture the observed thermal response of the overlying atmosphere to the ocean. Observations show a robust anticorrelation between the upper-ocean heat content and air–sea heat flux, with upper-ocean heat content leading air–sea heat flux by a few months. This anticorrelation is well captured by GFDL CM2.1 and CCSM3 but not by GFDL CDA. Only GFDL CM2.1 captures the observed anticorrelation between the upper-ocean heat content and EDW volume. This suggests that, although data assimilation corrects the readily observed variables, it degrades the model thermodynamic response to forcing.

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Bo Qiu and Kathryn A. Kelly

Abstract

A horizontally two-dimensional mixed-layer model is used to study the upper-ocean heat balance in the Kuroshio Extension region (30°–40°N, 141°–175°E). Horizontal dependency is emphasized because, in addition to vertical entrainment and surface thermal forcing, horizontal advection and eddy diffusion make substantial contributions to changes in the upper-ocean thermal structure in this region. By forcing the model using the wind and heat flux data from ECMWF and the absolute sea surface height data deduced from the Geosat ERM, the mixed-layer depth (hm) and temperature (Tm) changes in the Kuroshio Extension are hindcast for a 2.5-year period (November 1986–April 1989). Both phase and amplitude of the modeled Tm and hm variations agreed well with the climatology. The horizontal thermal patterns also agreed favorably with the available in situ SST observations, but this agreement depended crucially on the inclusion of horizontal advections.

Although the annually averaged net heat flux from the atmosphere to the ocean (Q net) is negative over the Kuroshio Extension region, the effect of the surface thermal forcing, when integrated annually, is to increase Tm because the large, negative Q net in winter is redistributed in a much deeper mixed layer than it is in summer when Q net > 0. This warming effect is counterbalanced by the vertical turbulent entrainment through the base of the mixed layer (35% when annually integrated), the Ekman divergence (16%), the geostrophic divergence (12%), and the horizontal eddy diffusion (35%). Though small when averaged in space and time, the temperature advection by the surface flows makes a substantial contribution to the local heat balances. While it warms the upstream region of the Kuroshio Extension (west of 150°E), the current advection tends to cool the upper ocean over the vast downstream region due to the presence of the recirculation gyre.

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Kathleen A. Edwards and Kathryn A. Kelly

Abstract

A seasonal heat budget is based on observations that span the broad California Current (CC) region. Budget terms are estimated from satellite data (oceanic heat advection), repeat ship transects (heat storage rate), and the Comprehensive Ocean–Atmosphere Data Set (COADS) (surface heat flux). The balance between terms differs with distance from shore. Offshore, a local balance between the heat storage rate and net heat flux (Q 0) holds; the latter is dominated by its shortwave component Q SW. Shoreward of ∼500 km, oceanic heat advection shifts the phase of the heat storage rate to earlier in the year and partially offsets an increase in Q 0 due to cloud clearing. During the summer maximum of Q 0, the ∼500-km-wide CC region loses heat to alongshore geostrophic transport, offshore Ekman transport, and, to a lesser degree, cross-shore geostrophic transport and eddy transport. The advective heat loss is neither uniform in space nor temporal phase; instead, the region of geostrophic and eddy heat loss expands cross shore with the annual widening of the California Current to ∼500 km. This expansion begins in spring with the onset of equatorward winds. A region of relatively positive wind stress curl widens at the same gradual rate as the CC, suggesting a coupling mechanism between the two.

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Kathryn A. Kelly and David C. Chapman

Abstract

The effect of steady, deep-ocean forcing on the flow over a continental slope and shelf region is examined using a linear and time-independent numerical model which includes continuous stratification, vertical and horizontal diffusion of momentum and density and linear bottom friction. The penetration of the pressure forcing is measured by the vertically averaged kinetic and potential energy as a function of cross-shore location. The most important factor governing the penetration of energy across the continental slope is the vertical structure of the imposed forcing: a surface-intensified pressure perturbation can penetrate easily onto the upper slope. Increasing the stratification also increases the energy penetration but not as effectively. Diffusion is relatively unimportant. The velocity field over the continental shelf is depth-independent regardless of the stratification or the location or vertical structure of the forcing function, and relatively little energy penetrates shoreward of the shelf break.

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Kathryn A. Kelly, LuAnne Thompson, and John Lyman

Abstract

Observations of thermosteric sea level (TSL) from hydrographic data, equivalent water thickness (EWT) from the Gravity Recovery and Climate Experiment (GRACE), and altimetric sea surface height (SSH) are used to infer meridional heat transport (MHT) anomalies for the Atlantic Ocean. An “unknown control” version of a Kalman filter in each of eight regions extracts smooth estimates of heat transport convergence (HTC) from discrepancies between the response to monthly surface heat and freshwater fluxes and observed mass and heat content. Two models are used: model A using only the heat budget for 1993–2010 and model B using both heat and mass budgets for 2003–10. Based on the small contributions of mass to SSH, model A is rerun using SSH in place of TSL to improve temporal resolution and data consistency. Estimates of MHT are derived by summing the HTC from north to south assuming either negligible anomalies at 67°N or setting MHT to observed values near 40°N. Both methods show that MHT is highly coherent between 35°S and 40°N. The former method gives a large drop in coherence north of 40°N while the latter method gives a less dramatic drop. Estimated anomalies in MHT comparable to or larger than that recently observed at the Rapid Climate Change and Meridional Overturning Circulation and Heatflux Array (RAPID/MOCHA) line at 26.5°N have occurred multiple times in this 18-yr period. Positive anomalies in coherent MHT correspond to increased heat loss in the North Atlantic subtropical gyre demonstrating the feedback of oceanic heat transport anomalies on air–sea fluxes. A correlation of MHT with the Antarctic Oscillation suggests a southern source for the coherent MHT anomalies.

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