On the porosity of barrier layers

Barrier layers are defined as the layer between the pycnocline and the thermocline when the latter are different as a result of salinity stratification. We present a revisited 2-degree resolution global climatology of monthly mean oceanic Barrier Layer (BL) thickness first proposed by de Boyer Montégut et al. (2007). In addition to using an extended data set, we present a modified computation method that addresses the observed porosity of BLs. We name porosity the fact that barrier layers distribution can, in some areas, be very uneven regarding the space and time scales that are considered. This implies an intermittent alteration of air-sea exchanges by the BL. Therefore, it may have important consequences for the climatic impact of BLs. Differences between the two computation methods are small for robust BLs that are formed by large-scale processes. However, the former approach can significantly underestimate the thickness of short and/or localized barrier layers. This is especially the case for barrier layers formed by mesoscale mechanisms (under the intertropical convergence zone for example and along western boundary currents) and equatorward of the sea surface salinity subtropical maxima. Complete characterisation of regional BL dynamics therefore requires a description of the robustness of BL distribution to assess the overall impact of BLs on the process of heat exchange between the ocean interior and the atmosphere

Southern Ocean bottom water characteristics in CMIP5 models

Southern Ocean deep water properties and formation processes in climate models are indicative of their capability to simulate future climate, heat and carbon uptake, and sea level rise. Southern Ocean temperature and density averaged over 1986–2005 from 15 CMIP5 (Coupled Model Intercomparison Project Phase 5) climate models are compared with an observed climatology, focusing on bottom water. Bottom properties are reasonably accurate for half the models. Ten models create dense water on the Antarctic shelf, but it mixes with lighter water and is not exported as bottom water as in reality. Instead, most models create deep water by open ocean deep convection, a process occurring rarely in reality. Models with extensive deep convection are those with strong seasonality in sea ice. Optimum bottom properties occur in models with deep convection in the Weddell and Ross Gyres. Bottom Water formation processes are poorly represented in ocean models and are a key challenge for improving climate predictions

Observational evidence of European summer weather patterns predictable from spring

Forecasts of summer weather patterns months in advance would be of great value for a wide range of applications. However, seasonal dynamical model forecasts for European summers have very little skill, particularly for rainfall. It has not been clear whether this low skill reflects inherent unpredictability of summer weather or, alternatively, is a consequence of weaknesses in current forecast systems. Here we analyze atmosphere and ocean observations and identify evidence that a specific pattern of summertime atmospheric circulation--the summer East Atlantic (SEA) pattern--is predictable from the previous spring. An index of North Atlantic sea-surface temperatures in March-April can predict the SEA pattern in July-August with a cross-validated correlation skill above 0.6. Our analyses show that the sea-surface temperatures influence atmospheric circulation and the position of the jet stream over the North Atlantic. The SEA pattern has a particularly strong influence on rainfall in the British Isles, which we find can also be predicted months ahead with a significant skill of 0.56. Our results have immediate application to empirical forecasts of summer rainfall for the United Kingdom, Ireland, and northern France and also suggest that current dynamical model forecast systems have large potential for improvement

Barrier layer characteristics of the Indian Ocean sector of the Southern Ocean during austral summer and autumn

Barrier layer in the Enderby Basin (EB) and the Australian Antarctic Basin (AAB) during late summer (December & January) and early autumn (February & March) are studied using temperature-salinity profiles collected between 1975 and 2012. A distinct difference in mixed layer depth is observed over the eastern (i.e. the EB) compared to western (i.e. the AAB) side of the Kerguelen Plateau (KP), with shallower mixed layer depths on the eastern side. Mixed layers show an increase from less than 50 m–∼150 m from south to north in the EB. During autumn, the wind strengthens and the upwelling over the eastern side of the KP (i.e. in the AAB) weakens, resulting in deeper mixed layers (∼80 m–100 m) compared to summer. During summer, deep barrier layer (BL) values (∼50 m or more) with porosity less than 0.3 was seen over the Chun Spur region. The fresher melt water from the EB brought by the Fawn trough current (FTC) across the KP may be responsible for the occurrence of BL over the region. During autumn, BL is spread over a much larger area around the Chun Spur, which could be attributed to the increase in the strength of FTC due to the intensification of wind over the region. A thorough study of BL condition over this region is required to understand the processes behind the discrepancies in sea ice conditions

A global relationship between the ocean water cycle and near-surface salinity

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C10025, doi:10.1029/2010JC006937.Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E-P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E-P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E-P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed-layer salinity (MLS) in different regions is thus derived, using a lower-order MLS dynamics that allows key balance terms (i.e., E-P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite-derived data sets and a salinity climatology. Major E-P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E-P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E-P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available.The study was supported by the NASA Remote Sensing Science for Carbon and Climate program under grant NNX07AF97G and by the NSF Physical Oceanography program under grant OCE‐0647949

Changes in global ocean bottom properties and volume transports in CMIP5 models under climate change scenarios

Changes in bottom temperature, salinity and density in the global ocean by 2100 for CMIP5 climate models are investigated for the climate change scenarios RCP4.5 and RCP8.5. The mean of 24 models shows a decrease in density in all deep basins except the North Atlantic which becomes denser. The individual model responses to climate change forcing are more complex: regarding temperature, the 24 models predict a warming of the bottom layer of the global ocean; in salinity, there is less agreement regarding the sign of the change, especially in the Southern Ocean. The magnitude and equatorward extent of these changes also vary strongly among models. The changes in properties can be linked with changes in the mean transport of key water masses. The Atlantic Meridional Overturning Circulation weakens in most models and is directly linked to changes in bottom density in the North Atlantic. These changes are due to the intrusion of modified Antarctic Bottom Water, made possible by the decrease in North Atlantic Deep Water formation. In the Indian, Pacific and South Atlantic, changes in bottom density are congruent with the weakening in Antarctic Bottom Water transport through these basins. We argue that the greater the 1986-2005 meridional transports, the more changes have propagated equatorwards by 2100. However, strong decreases in density over 100 years of climate change cause a weakening of the transports. The speed at which these property changes reach the deep basins is critical for a correct assessment of the heat storage capacity of the oceans as well as for predictions of future sea level rise

Development, amplification, and decay of Atlantic/European summer weather patterns linked to spring North Atlantic sea surface temperatures

A recent study identified a relationship between North Atlantic Ocean sea surface temperature (SST) gradients in spring and a specific pattern of atmospheric circulation in the following summer: the summer east Atlantic (SEA) pattern. It was shown that the SEA pattern is closely associated with meridional shifts in the eddy-driven jet in response to anomalous SST gradients. In this study, the physical mechanisms underlying this relationship are investigated further. It is shown that the predictable SEA pattern anomalies appear in June–July and undergo substantial amplification between July and August before decaying in September. The associated SST anomalies also grow in magnitude and spatial extent from June to August. The question of why the predictable atmospheric anomalies should occur in summer is addressed, and three factors are identified. The first is the climatological position of the storm track, which migrates poleward from spring to summer. The second is that the magnitude of interannual SST variability underlying the storm track peaks in summer, both in absolute terms, and relative to the underlying mean SST gradient. The third factor is the most interesting. We identify a positive coupled ocean–atmosphere feedback, which operates in summer and leads to the amplification of both SST and atmospheric circulation anomalies. The extent to which the identified processes are captured in the HadGEM3-GC2 climate model is also assessed. The model is able to capture the relationship between spring North Atlantic SSTs and subsequent ocean–atmosphere conditions in early summer, but the relationship is too weak. The results suggest that the real world might be more predictable than is inferred from the models

Ocean mixedlayer depth: A subsurface proxy for ocean-atmosphere variability

A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix. Using historical global hydrographic profile data, including conductivity-temperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used Δ criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the Δ criterion. The uncertainty for h mix/c is ±5 m for high- (<5 m) and ±8 m for low- (<20 m) resolution profiles. A quality index, QImix, which compares the variance of a profile above h mix to the variance to a depth of 1.5 × h mix, shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the well-mixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20–70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed