Ocean Assimilation Product Research Articles

On the Significance of Ageostrophic Meridional Eddy-Induced Heat Flux in the Surface Ocean of the Antarctic Circumpolar Current

Abstract Eddy-induced heat flux (EHF) convergence plays an important role in balancing the cooling of mean flows in the heat budget of the Southern Ocean. This study investigates the EHF in the Southern Ocean and the surface ocean heat budget over the Antarctic Circumpolar Current (ACC) estimated through a high-resolution ocean assimilation product. In contrast to previous studies in which the estimation of the EHF in the Southern Ocean was based on the assumption that mesoscale eddies are quasigeostrophic turbulence, we find that more than one-third of the total meridional EHF in the surface layer is attributed to ageostrophic currents of eddies and that the ageostrophic component of the EHF convergence is as important as its geostrophic component for the surface ocean heat budget over the ACC. In particular, the ageostrophic meridional EHF convergence accounts for 22% of the warming needed to balance the cooling from the mean flows during winter, equivalent to warming the surface ocean of the ACC by 0.14°C. The ageostrophic meridional EHF is likely caused by the stirring effect of ageostrophic secondary circulations in mesoscale eddies, which are induced by the turbulent thermal wind balance to restore the vertical shear of the upper layer in mesoscale eddies destructed by intense winter winds.

Contributions of Surface Heat Fluxes and Oceanic Processes to Tropical SST Changes: Seasonal and Regional Dependence

Abstract The present study employs six surface heat flux datasets and three ocean assimilation products to assess the relative contributions of surface heat fluxes and oceanic processes to the sea surface temperature (SST) change in the tropical oceans. Large differences are identified in the major terms of the heat budget equation. The largest discrepancies among different datasets appear in the contribution of vertical advection. The heat budget is nearly balanced in the shortwave-radiation- and horizontal-advection-dominant cases but not balanced in some of the latent-heat-flux- and vertical-advection-dominant cases. The contributions of surface heat fluxes and ocean advections to the SST tendency display remarkable seasonal and regional dependence. The contribution of surface heat fluxes covers a large geographical area. The oceanic processes dominate the SST tendency in the near-equatorial regions with large values but small spatial scales. In the Pacific and Atlantic Oceans, the SST tendency is governed by the dynamic and thermodynamic processes, respectively, while a wide variety of processes contribute to the SST tendency in the Indian Ocean. Several regions have been selected to illustrate the dominant contributions of individual terms to the SST tendency in different seasons. The seasonality and regionality of the interannual air–sea relationship indicate a physical connection with the mean state.

Distinctive precursory air–sea signals between regular and super El Niños

Statistically different precursory air–sea signals between a super and a regular El Nino group are investigated, using observed SST and rainfall data, and oceanic and atmospheric reanalysis data. The El Nino events during 1958–2008 are first separated into two groups: a super El Nino group (S-group) and a regular El Nino group (R-group). Composite analysis shows that a significantly larger SST anomaly (SSTA) tendency appears in S-group than in R-group during the onset phase [April–May(0)], when the positive SSTA is very small. A mixed-layer heat budget analysis indicates that the tendency difference arises primarily from the difference in zonal advective feedback and the associated zonal current anomaly (u′). This is attributed to the difference in the thermocline depth anomaly (D′) over the off-equatorial western Pacific prior to the onset phase, as revealed by three ocean assimilation products. Such a difference in D′ is caused by the difference in the wind stress curl anomaly in situ, which is mainly regulated by the anomalous SST and precipitation over the Maritime Continent and equatorial Pacific.

Heat advection processes leading to El Niño events as depicted by an ensemble of ocean assimilation products

AbstractThe oscillatory nature of El Niño‐Southern Oscillation results from an intricate superposition of near‐equilibrium balances and out‐of‐phase disequilibrium processes between the ocean and the atmosphere. The main objective of the present work is to perform an exhaustive spatiotemporal analysis of the upper ocean heat budget in an ensemble of state‐of‐the‐art ocean assimilation products. We put specific emphasis on the ocean heat advection mechanisms, and their representation in individual ensemble members and in the different stages of the ENSO oscillation leading to EN events. Our analyses consistently show that the initial subsurface warming in the western equatorial Pacific is advected to the central Pacific by the equatorial undercurrent, which, together with the equatorward advection associated with anomalies in both the meridional temperature gradient and circulation at the level of the thermocline, explains the heat buildup in the central Pacific during the recharge phase. We also find that the recharge phase is characterized by an increase of meridional tilting of the thermocline, as well as a southward upper‐ocean cross‐equatorial mass transport resulting from Ekman‐induced anomalous vertical motion in the off‐equatorial regions. Although differences between data sets are generally small, and anomalies tend to have the same sign, the differences in the magnitude of the meridional term are seen to be key for explaining the different propagation speed of the subsurface warming tendency along the thermocline. The only exception is GECCO, which does not produce the patterns of meridional surface Ekman divergence (subsurface Sverdrup convergence) in the western and central equatorial Pacific.

An optimal XBT‐based monitoring system for the South Atlantic meridional overturning circulation at 34°S

AbstractThe South Atlantic is an important pathway for the interbasin exchanges of heat and freshwater with strong influence on the global meridional overturning stability and variability. Along the 34°S parallel, a quarterly, high‐resolution XBT transect (AX18) samples the temperature structure in the upper ocean. The AX18 transect has been shown to be a useful component of a meridional overturning monitoring system of the region. However, a feasible, cost‐effective design for an XBT‐based system has not yet been developed. Here we use a high‐resolution ocean assimilation product to simulate an XBT‐based observational system across the South Atlantic. The sensitivity of the meridional heat transport, meridional overturning circulation, and geostrophic velocities to key observational and methodological assumptions is studied. Key assumptions taken into account are horizontal and temporal sampling of the transect, salinity, and deep temperature inference, as well as the level of reference for geostrophic velocities. With the current sampling strategy, the largest errors in the meridional overturning and heat transport estimations are the reference (barotropic) velocity and the western boundary resolution. We show how altimetry can be used along with hydrography to resolve the barotropic component of the flow. We use the results obtained by the state estimation under observational assumptions to make recommendations for potential improvements in the AX18 transect implementation.

Heat Budget of the Upper Ocean in the South-Central Equatorial Pacific

Abstract The double intertropical convergence zone (ITCZ) over the tropical Pacific, with a spurious band of maximum annual sea surface temperature (SST) south of the equator between 5°S and 10°S, is a chronic bias in coupled ocean–atmosphere models. This study focuses on a region of the double ITCZ in the central Pacific from 5°S to 10°S and 170°E to 150°W, where coupled models display the largest biases in precipitation, by deriving a best estimate of the mixed layer heat budget for the region. Seven global datasets of objectively analyzed surface energy fluxes and four ocean assimilation products are first compared and then evaluated against field measurements in adjacent regions. It was shown that the global datasets differ greatly in their net downward surface energy flux in this region, but they fall broadly into two categories: one with net downward heat flux of about 30 W m−2 and the other around 10 W m−2. Measurements from the adjacent Manus and Nauru sites of the Atmospheric Radiation Measurement Program (ARM), the Tropical Atmosphere Ocean (TAO) buoys, and the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) are then used to show that the smaller value is more realistic. An energy balance of the mixed layer is finally presented for the region as primarily between warming from surface heat flux of 7 W m−2 and horizontal advective cooling in the zonal direction of about 5 W m−2, with secondary contributions from meridional and vertical advections, heat storage, and subgrid-scale mixing. The 7 W m−2 net surface heat flux consists of warming of 210 W m−2 from solar radiation and cooling of 53, 141, and 8 W m−2, respectively, from longwave radiation, latent heat flux, and sensible heat flux. These values provide an observational basis to further study the initial development of excessive precipitation in coupled climate models in the central Pacific.

Causes of the El Niño and La Niña Amplitude Asymmetry in the Equatorial Eastern Pacific

Abstract The amplitude asymmetry between El Niño and La Niña is investigated by diagnosing the mixed-layer heat budget during the ENSO developing phase by using the three ocean assimilation products: Simple Ocean Data Assimilation (SODA) 2.0.2, SODA 1.4.2, and the Global Ocean Data Assimilation System (GODAS). It is found that the nonlinear zonal and meridional ocean temperature advections are essential to cause the asymmetry in the far eastern Pacific, whereas the vertical nonlinear advection has the opposite effect. The zonal current anomaly is dominated by the geostrophic current in association with the thermocline depth variation. The meridional current anomaly is primarily attributed to the Ekman current driven by wind stress forcing. The resulting induced anomalous horizontal currents lead to warm nonlinear advection during both El Niño and La Niña episodes and thus strengthen (weaken) the El Niño (La Niña) amplitude. The convergence (divergence) of the anomalous geostrophic mixed-layer currents during El Niño (La Niña) results in anomalous downwelling (upwelling) in the far eastern equatorial Pacific, which leads to a cold nonlinear vertical advection in both warm and cold episodes.

Mechanisms controlling seasonal‐to‐interannual mixed layer temperature variability in the southeastern tropical Indian Ocean

We use an Estimating the Circulation and Climate of the Ocean assimilation product to investigate seasonal‐interannual mixed layer temperature (MLT) budgets in the southeastern tropical Indian Ocean (SETIO) during 1993–2006. We examine spatial inhomogeneity of the SETIO MLT budget, contrasting three subregions with different forcing/circulation characteristics to better understand the area mean budget over the full SETIO. The subregions are the equatorial zone (box 1), the Sumatra‐Java upwelling zone (box 2), and east of the thermocline ridge (box 3). Seasonally, surface heat flux dominates MLT in all regions; advection and subsurface processes generally play secondary roles. On interannual scales, surface heat flux makes major contributions in all three boxes to termination of SETIO cooling associated with the Indian Ocean Zonal/Dipole Mode. Ocean dynamics show vital differences between regions: Subsurface processes cool box 1 and 2 but warm box 3. Horizontal advection warms box 1 but cools box 2 and 3. Averaging the MLT budget over the SETIO obscures regional physics. We explain spatial variations of the SETIO MLT budget in terms of differences in forcing, circulation, MLT distribution, and mixed layer and barrier layer thicknesses. We also examine SETIO MLT budget differences during 1994, 1997, and 2006, years exhibiting notable SETIO cooling events. In box 1, horizontal advection dominates warming after the 1994 and 2006 coolings, while in 1997, surface heat flux dominates warming. In box 2, cooling peaks earlier in 1994 than in 1997 and 2006 because of subsurface processes. Last, we show that the MLT budget is very different from heat budgets for fixed depth layers (e.g., the top 50–60 m).