Ocean impact on decadal Atlantic climate variability revealed by sea-level observations. Nature, 521 (7553). 508-510. 10.1038/nature14491 Contact NOC NORA team at publications@noc.soton.ac.ukThe NERC and NOC trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. properties and offer timeseries of sufficient length (Ext. Data Fig. 1) to study decadal 46 ocean circulation variations. Investigating ocean circulation using tide gauges is not new: 47 the first attempt to estimate the Gulf Stream using tide gauges was made in 1938 14 . The 48 principle is based on geostrophic dynamics: on timescales longer than a few days, ocean 49 circulation is in geostrophic balance so, looking downstream, the sea level is seen to 50increase from left to right in the northern hemisphere. 51 52Estimates of the Gulf Stream using tide gauges have focused on the use of gauges on the 53American east coast with an offshore estimate of sea level from either an island gauge 15 54 or a reconstructed sea level 16 . A weakness of this method is that the offshore 55 measurement lies in the eddy-filled ocean where sea-level fluctuations at any one point 56 are influenced by the mesoscale 17 even on long timescales, increasing the difficulty of 57 making estimates of ocean circulation that is coherent on large spatial scales. This is the 58 case for sea level at Bermuda, whose decadal fluctuations can be reproduced by 59 considering a Rossby wave response to wind forcing 16 . To make estimates of ocean 60 circulation that capture the fluctuations in large-scale circulation and less eddy variability, 61 measurements close to or on the western boundary are necessary 18 . We account for this 62 by focusing on the gradient of sea level along the US east coast. The mean dynamic sea 63 level decreases to the north along the east coast of the US (Fig. 1a) (Fig. 1a), we can construct a single sea-level composite 82 representative of the subtropical (subpolar) circulation by averaging sea-level from 83 linearly detrended, deseasonalised tide gauges, with the inverse barometer effect removed, 84 south (north) of the Cape (Fig. 1b, c). The difference, south minus north (Fig. 1d), 85 represents our circulation index. This index projects onto observed surface velocities 86 during the satellite era in the intergyre region, with a positive index associated with more 87 northwards flow and a more northerly path of this circulation (Extended Data Fig. 4). captured by the accumulated sea-level index, observationally supporting the hypothesis 113 6 that circulation changes and not only air-sea fluxes were involved in these changes 28 . For 114 the purposes of statistical analyses, the timeseries have had a 7-year low-pass, Tukey 115 filter applied to them, which is referred to with the prefix '7-year' from here on. The 7-116 year sea-level index leads the 7-year rate of heat content change by 2 years with a 117 maximum correlation of ...

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This paper examines the mean annual cycle of rainfall and general circulation features over West Africa and central Africa for 1958-97. Rainfall is examined using a 1400-station archive compiled by the first author. Other circulation features are examined using the NCEP-NCAR reanalysis dataset. Important features of the reanalysis zonal wind field are shown to compare well with the seasonal evolution described by the radiosonde observations. In addition to the well-known African easterly jet (AEJ) of the Northern Hemisphere, the seasonal evolution of its Southern Hemisphere counterpart is also described. Thermal wind calculations show that although the southern jet is weaker, its existence is also due to a local reversal of the surface temperature gradient. In the upper troposphere, a strong semiannual cycle is shown in the 200-mb easterlies and a feature like the tropical easterly jet (TEJ) is evident south of the equator in January and February. The paper describes the movement of the rainbelt between central and West Africa. An asymmetry in the northward and southward migration of the rainbelt is evident. The paper discusses the influence that the jets may have on rainfall and possible feedback effects of rainfall on the jets. Evidence suggests that the midtropospheric jets influence the development of the rainy season, but also that the rainfall affects the surface temperature gradient and in turn the jets. In the Northern Hemisphere, east of 20ЊE, the axis of the TEJ is located so that it may promote convection by increasing upper-level divergence. However, west of 10ЊE and in the Southern Hemisphere, the location of the TEJ is consistent with the suggestion that it is the equatorward outflow of convection that produces the TEJ.

[1] The exchange between the Persian (Arabian) Gulf and the Indian Ocean is investigated using hydrographic and moored acoustic Doppler current profiler data from the Straits of Hormuz during the period December 1996 to March 1998. The moored time series records show a relatively steady deep outflow through the strait from 40 m to the bottom with a mean speed of approximately 20 cm/s. A variable flow is found in the upper layer with frequent reversals on timescales of several days to weeks. The annual mean flow in the near-surface layer is found to be northeastward (out of the Persian Gulf) in the southern part of the strait, suggesting a mean horizontal exchange with the Indian Ocean that is superimposed on the vertical overturning exchange driven by evaporation over the gulf. The salinity of the deep outflow varies from 39.3 to 40.8 psu with highest outflow salinities occurring in the winter months (December-March). The annual mean deep outflow through the strait is estimated to be 0.15 ± 0.03 Sv. Calculation of the associated heat and freshwater fluxes through the strait yields estimates for the annual heat loss over the surface of the gulf of À7 ± 4 W/m 2 and an annual water loss (E-P-R) of 1.68 ± 0.39 m/yr. These values are shown to be in relatively good agreement with climatological surface fluxes derived from the Southampton Oceanography Centre global flux climatology after known regional biases in the radiative budget are taken into account.

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events.

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Cold ocean temperature anomalies have been observed in the mid- to high-latitude North Atlantic on interannual to centennial timescales. Most notably, a large region of persistently low surface temperatures accompanied by a sharp reduction in ocean heat content was evident in the subpolar gyre from the winter of 2013-2014 to 2016, and the presence of this feature at a time of pervasive warming elsewhere has stimulated considerable debate. Here, we review the role of air-sea interaction and ocean processes in generating this cold anomaly and place it in a longer-term context. We also discuss the potential impacts of surface temperature anomalies for the atmosphere, including the North Atlantic Oscillation and European heat waves; contrast the behavior of the Atlantic with the extreme warm surface event that occurred in the North Pacific over a similar timescale; and consider the possibility that these events represent a response to a change in atmospheric planetary wave forcing.

This article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important 'local' factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a 'wet mode' and a 'dry mode' in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958-1967 (representing the wet mode) and 1968-1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter-tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so).The results of this study suggest that the key factor controlling the occurrence of the 'wet Sahel' mode versus the 'dry' mode is the presence of deep, well-developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well-known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the 'wet Sahel' and 'dry Sahel' modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well-known dipole and no-dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea-surface temperatures (SSTs) in the Gulf of Guinea.