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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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Shenfu Dong and Kathryn A. Kelly

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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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Shenfu Dong, Sarah T. Gille, and Janet Sprintall

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The mixed layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from 1 June 2002 to 31 May 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer-Earth Observing System (EOS) (AMSR-E) sea surface temperature measurements are available. Temperature/salinity profiles from Argo floats are used to derive the mixed layer depth. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion, and air–sea flux are largely consistent with the evolution of the mixed layer temperature. The mixed layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air–sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the seasonal variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributor to the surface heat budget error is thought to be the air–sea heat fluxes, because only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. In particular, the lack of in situ measurements during winter is of fundamental concern. Sensitivity tests suggest that a proper representation of the mixed layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed layer depth and, because of this, also an imperfect estimate of the temperature of water entrained into the mixed layer from below.

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Shenfu Dong, Janet Sprintall, and Sarah T. Gille

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The location of the Southern Ocean polar front (PF) is mapped from the first 3 yr of remotely sensed Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperature (SST) measurements. In agreement with previous studies, the mean path of the Antarctic PF and its standard deviation are strongly influenced by bottom topography. However, the mean PF path diverges slightly from previous studies in several regions where there is high mesoscale variability. Although the SST and SST gradient at the PF show spatially coherent seasonal variations, with the highest temperature and the lowest temperature gradient during summer, the seasonal variations in the location of the PF are not spatially coherent. The temporal mean SST at the PF corresponds well to the mean PF path: the temperature is high in the Atlantic and Indian Ocean sections and is low in the Pacific Ocean section where the PF has a more southerly position. The relationship of the wind field with the Antarctic PF location and proxies for the zonal and meridional PF transports are examined statistically. Coherence analysis suggests that the zonal wind stress accelerates the zonal transport of the PF. The analysis presented herein also suggests that the meridional shifts of the Antarctic PF path correspond to the meridional shifts of the wind field.

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Shenfu Dong, Susan L. Hautala, and Kathryn A. Kelly

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Subsurface temperature data in the western North Atlantic Ocean are analyzed to study the variations in the heat content above a fixed isotherm and contributions from surface heat fluxes and oceanic processes. The study region is chosen based on the data density; its northern boundary shifts with the Gulf Stream position and its southern boundary shifts to contain constant volume. The temperature profiles are objectively mapped to a uniform grid (0.5° latitude and longitude, 10 m in depth, and 3 months in time). The interannual variations in upper-ocean heat content show good agreement with the changes in the sea surface height from the Ocean Topography Experiment (TOPEX)/Poseidon altimeter; both indicate positive anomalies in 1994 and 1998–99 and negative anomalies in 1996–97. The interannual variations in surface heat fluxes cannot explain the changes in upper-ocean heat storage rate. On the contrary, a positive anomaly in heat released to the atmosphere corresponds to a positive upper-ocean heat content anomaly. The oceanic heat transport, mainly owing to the geostrophic advection, controls the interannual variations in heat storage rate, which suggests that geostrophic advection plays an important role in the air–sea heat exchange. The 18°C isotherm depth and layer thickness also show good correspondence to the upper-ocean heat content; a deep and thin 18°C layer corresponds to a positive heat content anomaly. The oceanic transport in each isotherm layer shows an annual cycle, converging heat in winter, and diverging in summer in a warm layer; it also shows interannual variations with the largest heat convergence occurring in even warmer layers during the period of large ocean-to-atmosphere flux.

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Hosmay Lopez, Shenfu Dong, Sang-Ki Lee, and Gustavo Goni

Abstract

This study presents a physical mechanism on how low-frequency variability of the South Atlantic meridional heat transport (SAMHT) may influence decadal variability of atmospheric circulation. A multicentury simulation of a coupled general circulation model is used as basis for the analysis. The highlight of the findings herein is that multidecadal variability of SAMHT plays a key role in modulating global atmospheric circulation via its influence on interhemispheric redistributions of momentum, heat, and moisture. Weaker SAMHT at 30°S produces anomalous ocean heat divergence over the South Atlantic, resulting in negative ocean heat content anomalies about 15–20 years later. This forces a thermally direct anomalous interhemispheric Hadley circulation, transporting anomalous atmospheric heat from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) and moisture from the SH to the NH, thereby modulating global monsoons. Further analysis shows that anomalous atmospheric eddies transport heat northward in both hemispheres, producing eddy heat flux convergence (divergence) in the NH (SH) around 15°–30°, reinforcing the anomalous Hadley circulation. The effect of eddies on the NH (SH) poleward of 30° depicts heat flux divergence (convergence), which must be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream. This study illustrates that decadal variations of SAMHT could modulate the strength of global monsoons with 15–20 years of lead time, suggesting that SAMHT is a potential predictor of global monsoon variability. A similar mechanistic link exists between the North Atlantic meridional heat transport (NAMHT) at 30°N and global monsoons.

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Marlos Goes, Gustavo Goni, Shenfu Dong, Timothy Boyer, and Molly Baringer

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This work assesses the value of expendable bathythermograph (XBT) and Argo profiling float observations to monitor the Atlantic Ocean boundary current systems (BCS), meridional overturning circulation (MOC), and meridional heat transport (MHT). Data from six XBT transects and available Argo floats in the Atlantic Ocean for the period from 2000 to 2018 are used to estimate the structure and variability of the BCS, MOC, and MHT, taking into account different temporal and spatial mapping strategies. The comparison of Argo data density along these six XBT transects shows that Argo observations outnumber XBT observations only above mapping scales of 30 days and 3° boxes. The comparison of Argo and XBT data for the Brazil Current and Gulf Stream shows that Argo cannot reproduce the structure and variability of these currents, as it lacks sufficient resolution to resolve the gradients across these narrow jets. For the MHT and MOC, Argo estimates are similar to those produced by XBTs at a coarse mapping resolution of 5° and 30 days. However, at such a coarse resolution the root-mean-square errors calculated for both XBT and Argo estimates relative to a high-resolution baseline are higher than 3 Sv (1 Sv ≡ 106 m3 s−1) and 0.25 PW for the MOC and MHT, respectively, accounting for about 25%–30% of their mean values due to the smoothing of eddy variability along the transects. A key result of this study is that using Argo and XBT data jointly, rather than separately, improves the estimates of MHT, MOC, and BCS.

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Claude Frankignoul, Gaelle de Coëtlogon, Terrence M. Joyce, and Shenfu Dong

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Time series of Gulf Stream position derived from the TOPEX/Poseidon altimeter from October 1992 to November 1998 are used to investigate the lead and lag relation between the Gulf Stream path as it leaves the continental shelf and the changes in sea level pressure, surface wind stress, and sea surface temperature (SST), as given by the NCEP reanalysis. The dominant signal is a northward (southward) displacement of Gulf Stream axis 11 to 18 months after the North Atlantic Oscillation (NAO) reaches positive (negative) extrema. A SST warming (cooling) peaking north of the Gulf Stream is also seen to precede the latitudinal shifts, but it is a part of the large-scale SST anomaly tripole that is generated by the NAO fluctuations. There is no evidence that the Gulf Stream shifts have a direct impact onto the large-scale atmospheric circulation. A fast, passive response of the Gulf Stream to NAO forcing is also suggested by a corresponding analysis of the yearly mean Gulf Stream position estimated from XBT data at 200 m during 1954–98, where the NAO primarily leads the latitudinal Gulf Stream shifts by 1 yr. The fast Gulf Stream response seems to reflect buoyancy forcing in the recirculation gyres but, as the covariability remains significant when the NAO leads by up to 9 yr, large-scale wind stress forcing may become important after a longer delay. Because of the high NAO index of the last decades, the TOPEX/Poseidon period is one of unprecedented northward excursion of the Gulf Stream in the 45-yr record, with the Gulf Stream 50–100 km north of its climatological mean position.

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Chunzai Wang, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, and Sang-Ki Lee

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Most studies of African dust and North Atlantic climate have been limited to the short time period since the satellite era (1980 onward), precluding the examination of their relationship on longer time scales. Here a new dust dataset with the record extending back to the 1950s is used to show a multidecadal covariability of North Atlantic SST and aerosol, Sahel rainfall, and Atlantic hurricanes. When the North Atlantic Ocean was cold from the late 1960s to the early 1990s, the Sahel received less rainfall and the tropical North Atlantic experienced a high concentration of dust. The opposite was true when the North Atlantic Ocean was warm before the late 1960s and after the early 1990s. This suggests a novel mechanism for North Atlantic SST variability—a positive feedback between North Atlantic SST, African dust, and Sahel rainfall on multidecadal time scales. That is, a warm (cold) North Atlantic Ocean produces a wet (dry) condition in the Sahel and thus leads to low (high) concentration of dust in the tropical North Atlantic, which in turn warms (cools) the North Atlantic Ocean. An implication of this study is that coupled climate models need to be able to simulate this aerosol-related feedback in order to correctly simulate climate variability in the North Atlantic. Additionally, it is found that dust in the tropical North Atlantic varies inversely with the number of Atlantic hurricanes on multidecadal time scales because of the multidecadal variability of both direct and indirect influences of dust on vertical wind shear in the hurricane main development region.

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Marlos Goes, Jonathan Christophersen, Shenfu Dong, Gustavo Goni, and Molly O. Baringer

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Simultaneous temperature and salinity profile measurements are of extreme importance for research; operational oceanography; research and applications that compute content and transport of mass, heat, and freshwater in the ocean; and for determining water mass stratification and mixing rates. Historically, temperature profiles are much more abundant than simultaneous temperature and salinity profiles. Given the importance of concurrent temperature and salinity profiles, several methods have been developed to derive salinity solely based on temperature profile observations, such as expendable bathythermograph (XBT) temperature measurements, for which concurrent salinity observations are typically not available. These empirical methods used to date contain uncertainties as a result of temporal changes in salinity and seasonality in the mixed layer, and are typically regionally based. In this study, a new methodology is proposed to infer salinity in the Atlantic Ocean from the water surface to 2000-m depth, which addresses the seasonality in the upper ocean and makes inferences about longer-term changes in salinity. Our results show that when seasonality is accounted for, the variance of the residuals is reduced in the upper 150 m of the ocean and the dynamic height errors are smaller than 4 cm in the whole study domain. The sensitivity of the meridional heat and freshwater transport to different empirical methods of salinity estimation is studied using the high-density XBT transect across 34.5°S in the South Atlantic Ocean. Results show that accurate salinity estimates are more important on the boundaries, suggesting that temperature–salinity compensation may be also important in those regions.

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