Evaluation of Subtropical North Atlantic Ocean Circulation in CMIP5 Models against the Observational Array at 26.5°N and Its Changes under Continued Warming

Observationally based metrics derived from the Rapid Climate Change (RAPID) array are used to assess the large-scale ocean circulation in the subtropical North Atlantic simulated in a suite of fully coupled climate models that contributed to phase 5 of the Coupled Model Intercomparison Project (CMIP5). The modeled circulation at 26.5°N is decomposed into four components similar to those RAPID observes to estimate the Atlantic meridional overturning circulation (AMOC): the northward-flowing western boundary current (WBC), the southward transport in the upper midocean, the near-surface Ekman transport, and the southward deep ocean transport. The decadal-mean AMOC and the transports associated with its flow are captured well by CMIP5 models at the start of the twenty-first century. By the end of the century, under representative concentration pathway 8.5 (RCP8.5), averaged across models, the northward transport of waters in the upper WBC is projected to weaken by 7.6 Sv (1 Sv ≡ 106 m3 s−1; −21%). This reduced northward flow is a combined result of a reduction in the subtropical gyre return flow in the upper ocean (−2.9 Sv; −12%) and a weakened net southward transport in the deep ocean (−4.4 Sv; −28%) corresponding to the weakened AMOC. No consistent long-term changes of the Ekman transport are found across models. The reduced southward transport in the upper ocean is associated with a reduction in wind stress curl (WSC) across the North Atlantic subtropical gyre, largely through Sverdrup balance. This reduced WSC and the resulting decrease in the horizontal gyre transport is a robust feature found across the CMIP5 models under increased CO2 forcing.

Interannual variability in the transport of fish eggs in the Kattegat and Öresund

The dispersal and retention of early life stages are essential factors for the recruitment of fish stocks, and often depend on local meteorological and hydrographical conditions. A BAltic sea Long-Term large Scale Eutrophication Model (BALTSEM) was used to examine interannual variation in the retention and dispersal of fish eggs, Atlantic cod (Gadus morhua) at spawning areas in the Kattegat and Öresund and to investigate the mechanisms responsible for this variation. Model simulations over period 1971–2006 showed that years with high retention were related to variable weak winds and a strong outflow of water from the Baltic Sea, while years with low retention were related to strong westerly winds. Low egg retention was associated with enhanced southward transport from the Kattegat towards the Great Belt and from the Öresund towards the Baltic Sea. This southward transport of eggs was highly correlated with the local westerly wind component. The southbound egg transport and the westerly wind component both increased after 1989, and were connected with a change in the large-scale atmospheric circulation pattern, as represented by the NAO index. Results of this study provide information about the connectivity between neighboring areas and suggest that sedimentation is an important mortality factor.

On Challenges in Predicting Bottom Water Transport in the Southern Ocean

Changes in the Southern Ocean lower-limb overturning circulation are analyzed using a set of climate models. In agreement with some recently developed theoretical models, it is found that the overturning can be strongly affected by winds. In particular, the simulated strengthening of large-scale southward transport in the abyss is explicitly driven by zonal wind stress. However, there is a considerable range among the climate models in their projected changes of Southern Ocean wind stress. Furthermore, the strengthening of large-scale southward transport tends to be compensated by eddy-induced northward flows in the abyss, particularly at eddy-permitting resolution. As a result, the net Antarctic Bottom Water (AABW) export may only be weakly affected. However, none of the models considered accounts for the possibility that a fraction of the eddy kinetic energy may be converted to diapycnal mixing. If this were the case, the presented energetic arguments suggest that stronger Southern Ocean winds would result in a stronger AABW transport.

Volume, Freshwater, and Heat Fluxes through Davis Strait, 2004–05*

Davis Strait volume [−2.3 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1); negative sign indicates southward transport], freshwater (−116 ± 41 mSv), and heat (20 ± 9 TW) fluxes estimated from objectively mapped 2004–05 moored array data do not differ significantly from values based on a 1987–90 array but are distributed differently across the strait. The 2004–05 array provided the first year-long measurements in the upper 100 m and over the shelves. The upper 100 m accounts for 39% (−0.9 Sv) of the net volume and 59% (−69 mSv) of the net freshwater fluxes. Shelf contributions are small: 0.4 Sv (volume), 15 mSv (freshwater), and 3 TW (heat) from the West Greenland shelf and −0.1 Sv, −7 mSv, and 1 TW from the Baffin Island shelf. Contemporaneous measurements of the Baffin Bay inflows and outflows indicate that volume and freshwater budgets balance to within 26% and 4%, respectively, of the net Davis Strait outflow. Davis Strait volume and freshwater fluxes nearly equal those from Fram Strait, indicating that both are significant Arctic freshwater pathways.

Early Life History of Pomatomus saltatrix off the East Coast of South Africa

Several authors have stated that southward transport of the early life-history stages of Pomatomus saltatrix occurs by passive drift in the Agulhas Current, a strong western boundary current which flows southwards following the edge of the continental shelf of eastern South Africa. However, an extensive study of the ichthyoplankton occurring in shelf and Agulhas Current waters from 29�S to 34�S located P. saltatrix larvae only in the shelf waters off KwaZulu-Natal. Sampling of fish eggs in the inshore shelf waters 60 km south of Durban on a regular basis has confirmed spawning of P. saltatrix to occur in this region. The data thus suggest that larval dispersal might occur in shelf waters where, although strongly influenced by the Agulhas Current, southward transport is not as rapid.

El Niño Events and Variability in a Pacific Mackerel (Scomber japonicus) Survival Index: Support for Hjort's Second Hypothesis

From 1928 to 1965 the higher survival rates to age 1 for Pacific mackerel (Scomber japonicus) in the California Current occurred during years of decreased southward transport (as indicated by sea level), which in turn is related to El Niño events. During such years, plankton biomass was relatively low. Conversely, Pacific mackerel survival rates to age 1 were low during periods of increased southward transport, when plankton biomass was high. We conclude that survival rates during the early life history may be more influenced by hydrographic processes in a direct manner than by biological interactions. This interpretation is consistent with Hjort's (1914. Rapp. P.-V. Réun. Cons. Int. Explor. Mer 20: 1–228) second hypothesis to explain interannual variability in recruitment to fish populations.