ALBERT

Description: The Eddy Kinetic Energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production and the associated EKE, we investigate the decadal changes of EKE. We show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the Interdecadal Pacific Oscillation (IPO) lead to up- and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30–40 % of the density anomalies in the investigated region.

Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multi-decadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the 20th Century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multi-decadal variations associated with the Pacific Decadal Oscillation and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multi-decadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean Dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.

Description: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.

Description: Highlights: • We compare the simulated Arctic Ocean in 15 global ocean–sea ice models. • There is a large spread in temperature bias in the Arctic Ocean between the models. • Warm bias models have a strong temperature anomaly of inflow of Atlantic Water. • Dense outflows formed on Arctic shelves are not captured accurately in the models. In this paper we compare the simulated Arctic Ocean in 15 global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves.

Description: On interannual to decadal times scales, model simulations suggest a strong relationship between anomalies in the deep water formation rate, the strength of the subpolar gyre, and the meridional overturning circulation in the North Atlantic. Whether this is valid, can only be confirmed by continuous, long observational time series. Several measurement components are already in place, but crucial arrays to obtain time series of the meridional volume and heat transport in the subpolar North Atlantic are still missing. Here we summarize the recent developments of the deep water formation rates and the subpolar gyre transports. We discuss how existing observational components in the subpolar North Atlantic could be supplemented to provide long-term monitoring of the meridional heat and volume transport. Through a combined analysis of observations and model results the temporal and spatial scales that had to be covered with instruments are discussed, together with the key regions with the highest variability in the velocity and temperature fields.

Description: A systematic intercomparison of three realistic eddy-permitting models of the North Atlantic circulation has been performed. The models use different concepts for the discretization of the vertical coordinate, namely geopotential levels, isopycnal layers, terrain-following (sigma) coordinates, respectively. Although these models were integrated under nearly identical conditions, the resulting large-scale model circulations show substantial differences. The results demonstrate that the large-scale thermohaline circulation is very sensitive to the model representation of certain localised processes, in particular to the amount and water mass properties of the overflow across the Greenland–Scotland region, to the amount of mixing within a few hundred kilometers south of the sills, and to several other processes at small or sub-grid scales. The different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes.

Description: Seasonal changes in eddy energy are used to investigate the role of high-frequency wind forcing in generating eddy kinetic energy in the oceans. To this end, we analyze two experiments of an eddy-permitting model of the North Atlantic driven by daily and monthly mean wind stress fields, and compare results with corresponding changes in the variance of the wind fields, and related results from previous studies using altimeter and current meter data. With daily wind-stress forcing the model is found to be in general agreement with altimetric observations and reveal a complex pattern of temporal changes in variability over the North Atlantic. Observations and the model indicate enhanced levels of eddy energy during winter months over several areas of the northern and, particularly northeastern North Atlantic. Since the wind-generated variability is primarily barotropic, its signal can be detected mostly in the low-energy regions of the northern and north-eastern North Atlantic, which are remote from baroclinically unstable currents. There the winter-to-summer difference in simulated eddy kinetic energy caused by the variable wind forcing is 〈0.5 cm2 s2 between 30° and 55°N, and is 1–3 cm2 s2 north of 55°N. Seasonal changes in kinetic energy are insignificant along the path of the North Atlantic current and south of about 30°N. The weak depth dependence of the seasonal changes in eddy energy implies that the relative importance of wind-generated eddy energy is maximum at depth where the general (baroclinic) variability level is low. Accordingly, a significant correlation is found between the seasonal cycle in the variance of wind stress and the seasonal cycle in eddy energy over a substantially wider area than near the surface, notably across the entire eastern North Atlantic between the North Atlantic Current and the North Equatorial Current.

Description: Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.

Description: We examine the diffusive behavior of the flow field in an eddy-resolving, primitive equation circulation model. Analysis of fluid particle trajectories illustrates the transport mechanisms, which are leading to uniform tracer and potential vorticity distributions in the interior of the subtropical thermocline. In contrast to the assumption of weak mixing in recent analytical theories, the numerical model indicates the alternative of tracer and potential vorticity homogenization on isopycnal surfaces taking place in a nonideal fluid with strong, along-isopycnal eddy mixing. The eastern, ventilated portion of the gyre appears to be sufficiently homogeneous to allow the concept of an eddy diffusivity to apply. A break in a random walk behavior of particle statistics occurs after about 100 days when along-flow dispersion sharply increases, indicative of mean shear effects. During the first months of particle spreading, eddy dispersal and mean advection are of similar magnitude. Eddy kinetic energy is of O(60–80 cm2 s−2) in the model thermocline, comparable to the pool of weak eddy intensity found in the eastern parts of the subtropical oceans. Eddy diffusivity in the model thermocline (Kxx = 8 × 107, Kyy = 3 × 107 cm2 s−1) seems to be higher by a factor of about 3 than oceanic values estimated for these area. Below the thermocline, model diffusivity decreases substantially and becomes much more anisotropic, with particle dispersal preferentially in the zonal direction. The strong nonisotropic behavior, prominent also in all other areas of water eddy intensity, appears as the major discrepancy when compared with the observed behavior of SOFAR floats and surface drifters in the ocean.

Description: This study focuses on an important aspect of air–sea interaction in models, namely, large-scale, spurious heat fluxes due to false pathways of the Gulf Stream and North Atlantic Current (NAC) in the “storm formation region” south and east of Newfoundland. Although high-resolution eddy-resolving models show some improvement in this respect, results are sensitive to poorly understood, subgrid-scale processes for which there is currently no complete, physically based parameterization. A simple method to correct an ocean general circulation model (OGCM), acting as a practical substitute for a physically based parameterization, is explored: the recently proposed “semiprognostic method,” a technique for adiabatically adjusting flow properties of a hydrostatic OGCM. The authors show that application of the method to an eddy-permitting model of the North Atlantic Ocean yields more realistic flow patterns and watermass characteristics in the Gulf Stream and NAC regions; in particular, spurious surface heat fluxes are reduced. Four simple modifications to the method are proposed, and their benefits are demonstrated. The modifications successfully account for three drawbacks of the original method: reduced geostrophic wave speeds, damped mesoscale eddy activity, and spurious interaction with topography. It is argued that use of a corrected (eddy permitting) OGCM in a coupled modeling system for simulating present climate (as now becomes possible because of increasing computer power) should lead to a more realistic simulation in regions of strong air–sea interaction as compared with that obtained with an uncorrected model. The method is also well suited for the simulation of the uptake and transport of passive tracers, such as anthropogenic carbon dioxide or components of ecosystem models.