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The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place from 7 January to 11 July 2020 in the tropical North Atlantic between the eastern edge of Barbados and 51∘ W, the longitude of the Northwest Tropical Atlantic Station (NTAS) mooring. Measurements were made to gather information on shallow atmospheric convection, the effects of aerosols and clouds on the ocean surface energy budget, and mesoscale oceanic processes. Multiple platforms were deployed during ATOMIC including the NOAA RV Ronald H. Brown (RHB) (7 January to 13 February) and WP-3D Orion (P-3) aircraft (17 January to 10 February), the University of Colorado's Robust Autonomous Aerial Vehicle-Endurant Nimble (RAAVEN) uncrewed aerial system (UAS) (24 January to 15 February), NOAA- and NASA-sponsored Saildrones (12 January to 11 July), and Surface Velocity Program Salinity (SVPS) surface ocean drifters (23 January to 29 April). The RV Ronald H. Brown conducted in situ and remote sensing measurements of oceanic and atmospheric properties with an emphasis on mesoscale oceanic–atmospheric coupling and aerosol–cloud interactions. In addition, the ship served as a launching pad for Wave Gliders, Surface Wave Instrument Floats with Tracking (SWIFTs), and radiosondes. Details of measurements made from the RV Ronald H. Brown, ship-deployed assets, and other platforms closely coordinated with the ship during ATOMIC are provided here. These platforms include Saildrone 1064 and the RAAVEN UAS as well as the Barbados Cloud Observatory (BCO) and Barbados Atmospheric Chemistry Observatory (BACO). Inter-platform comparisons are presented to assess consistency in the data sets. Data sets from the RV Ronald H. Brown and deployed assets have been quality controlled and are publicly available at NOAA's National Centers for Environmental Information (NCEI) data archive (https://www.ncei.noaa.gov/archive/accession/ATOMIC-2020, last access: 2 April 2021). Point-of-contact information and links to individual data sets with digital object identifiers (DOIs) are provided herein.

Recent studies have demonstrated that the difference in sea surface temperature anomalies (SSTAs) between the tropical Atlantic main development region (MDR) and the tropical Pacific (Nino 3...

The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m−2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic. How the tropical mixed layer of the ocean reacts to near-inertial waves has rarely been observed directly. Here, the authors present new data that shows strongly elevated vertical diffusive heat flux in the presence of near-inertial waves, causing a cooling of the mixed layer that is particularly strong in summer.

The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.

International audience; Decade-long satellite sea surface slinity (SSS) observations show that rain dilution prevails in wakes of tropical depressions (∼-0.1 pss) and tropical storms (∼-0.05 pss) on the left (right) side of Northern (Southern) Hemisphere storms. For stronger storms, the rain-induced dilution is dominated by the saltier water entrainment, leading to surface median salinification of 0.3 pss for the most intense storms, peaking on the right-hand side at around twice the maximum wind radius. The magnitude of the salty wake increases for stronger slowly moving storms. The vertical salinity gradient in the upper ocean is a key factor explaining the geographic distribution of the SSS response. A striking example is the systematic mixing of fresh near-surface river plume waters with saltier subsurface waters. It is also found that barrier layers lead to saltier and warmer storm wakes compared to wakes produced over barrier layer free areas.

This study finds that the winter barrier layer (BL) in the Bay of Bengal (BoB) can modulate the subsequent Indian summer monsoon (ISM) onset and associated rainfall variability. In the years when the prior winter BL is anomalously thick, there is anomalous sea surface cooling caused by intensified latent heat flux loss, and this anomalous cooling persists into the following year due to positive cloud–SST feedback. During December–February, it is shown that the vertical entrainment of warmer subsurface water due to anomalously thick BL acts to limit excessive cooling of the sea surface and maintain deep atmospheric convection over the BoB. Then, during March–May, the thinner mixed layer linked to anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer. This tends to reinforce the cold SST anomalies, and advance the onset of ISM and enhance June ISM precipitation through an increase in the tropospheric temperature gradient between land and sea. We also find that most CMIP5 models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL. This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation. The present results indicate that it is important to realistically capture the winter BL of the BoB in air–sea coupled models for improving the simulation and prediction of ISM.

We show the importance of salinity for rapidly intensifying Atlantic tropical cyclones and demonstrate the potential for improved prediction of rapid intensification through the inclusion of salinity. Tropical Cyclone (TC) rapid intensification (RI) is difficult to predict and poses a formidable threat to coastal populations. A warm upper ocean is well-known to favor RI, but the role of ocean salinity is less clear. This study shows a strong inverse relationship between salinity and TC RI in the eastern Caribbean and western tropical Atlantic due to near-surface freshening from the Amazon-Orinoco River system. In this region, rapidly intensifying TCs induce a much stronger surface enthalpy flux compared to more weakly intensifying storms, in part due to a reduction in SST cooling caused by salinity stratification. This reduction has a noticeable positive impact on TCs undergoing RI, but the impact of salinity on more weakly intensifying storms is insignificant. These statistical results are confirmed through experiments with an ocean mixed layer model, which show that the salinity-induced reduction in SST cold wakes increases significantly as the storm’s intensification rate increases. Currently, operational statistical-dynamical RI models do not use salinity as a predictor. Through experiments with a statistical RI prediction scheme, it is found that the inclusion of surface salinity significantly improves the RI detection skill, offering promise for improved operational RI prediction. Satellite surface salinity may be valuable for this purpose, given its global coverage and availability in near real-time.

Rapid Intensification (RI) of hurricanes is difficult to predict and poses a formidable threat to coastal populations. While a warm upper-ocean is well-known to favor RI, the role of salinity is less clear. In this study, using a suite of observations, we demonstrate that the subsurface oceans' influence on Atlantic hurricane RI exhibits two regimes. In the western region, which includes the Gulf of Mexico and the western Caribbean Sea, temperature stratification plays an important role in hurricane RI with little impact from salinity. On the other hand, in the eastern region dominated by the Amazon-Orinoco plume, salinity stratification prominently impacts RI. While a weak temperature stratification aids cold wake reduction for hurricanes in the western region, a strong salinity stratification causes less hurricane-induced mixing and surface cooling in the eastern region. Finally, in both regions, the relevance of the cold wake, and consequently the ocean sub-surface, is enhanced during RI compared to weaker intensification.

The causes of the seasonal cycle of vertical turbulent cooling at the base of the mixed layer are assessed using observations from moored buoys in the tropical Atlantic Intertropical Convergence Zone (ITCZ) (4°N, 23°W) and trade wind (15°N, 38°W) regions together with mixing parameterizations and a one-dimensional model. At 4°N the parameterized turbulent cooling rates during 2017–2018 and 2019 agree with indirect estimates from the climatological mooring heat budget residual: both show mean cooling of 25–30 W m (Formula presented.) during November–July, when winds are weakest and the mixed layer is thinnest, and 0–10 W m (Formula presented.) during August–October. Mixing during November–July is driven by variability on multiple time scales, including subdiurnal, near-inertial, and intraseasonal. Shear associated with tropical instability waves (TIWs) is found to generate mixing and monthly mean cooling of 15–30 W m (Formula presented.) during May–July in 2017 and 2019. At 15°N the seasonal cycle of turbulent cooling is out of phase compared to 4°N, with largest cooling of up to 60 W m (Formula presented.) during boreal fall. However, the relationships between wind speed, mixed layer depth, and turbulent mixing are similar: weaker mean winds and a thinner mixed layer in the fall are associated with stronger mixing and turbulent cooling of SST. These results emphasize the importance of seasonal modulations of mixed layer depth at both locations and shear from TIWs at 4°N.

This review describes the climate change–induced responses of the tropical atmospheric circulation and their impacts on the hydrological cycle. We depict the theoretically predicted changes and diagnose physical mechanisms for observational and model-projected trends in large-scale and regional climate. The tropical circulation slows down with moisture and stratification changes, connecting to a poleward expansion of the Hadley cells and a shift of the intertropical convergence zone. Redistributions of regional precipitation consist of thermodynamic and dynamical components, including a strong offset between moisture increase and circulation weakening throughout the tropics. This allows other dynamical processes to dominate local circulation changes, such as a surface warming pattern effect over oceans and multiple mechanisms over land. To improve reliability in climate projections, more fundamental understandings of pattern formation, circulation change, and the balance of various processes redistributing land rainfall are suggested to be important.

Sea surface temperature (SST) is one of the most important parameters for tropical cyclone (TC) intensification. Here, it is shown that the relationship between SST and TC intensification varies considerably from basin to basin, with SST explaining less than 4% of the variance in TC intensification rates in the Atlantic, 12% in the western North Pacific, and 23% in the eastern Pacific. Several factors are shown to be responsible for these interbasin differences. First, variability of SST along TCs’ tracks is lower in the Atlantic. This is due to smaller horizontal SST gradients in the Atlantic, compared to the Pacific, and stronger damping of prestorm SST’s contribution to TC intensification by the storm-induced cold SST wake in the Atlantic. The damping occurs because SST tends to vary in phase with TC-induced SST cooling: in the Gulf of Mexico and northwestern Atlantic, where SSTs are highest, TCs tend to be strongest and their translations slowest, resulting in the strongest storm-induced cooling. The tendency for TCs to be more intense over the warmest SST in the Atlantic also limits the usefulness of SST as a predictor since stronger storms are less likely to experience intensification. Finally, SST tends to vary out of phase with vertical wind shear and outflow temperature in the western Pacific. This strengthens the relationship between SST and TC intensification more in the western Pacific than in the eastern Pacific or Atlantic. Combined, these factors explain why prestorm SST is such a poor predictor of TC intensification in the Atlantic, compared to the eastern and western North Pacific.

Sea surface temperature (SST) and tropical cyclone heat potential (TCHP) are metrics used to incorporate the ocean’s influence on hurricane intensification into the National Hurricane Center’s Statistical Hurricane Intensity Prediction Scheme (SHIPS). While both SST and TCHP serve as useful measures of the upper-ocean heat content, they do not accurately represent ocean stratification effects. Here, it is shown that replacing SST within the SHIPS framework with a dynamic temperature Tdy, which accounts for the oceanic negative feedback to the hurricane’s intensity arising from storm-induced vertical mixing and sea surface cooling, improves the model performance. While the model with SST and TCHP explains about 41% of the variance in 36-h intensity changes, replacing SST with Tdy increases the variance explained to nearly 44%. These results suggest that representation of the oceanic feedback, even through relatively simple formulations such as Tdy, may improve the performance of statistical hurricane intensity prediction models such as SHIPS.

The Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) provides measurements of the upper ocean and near-surface atmosphere at 18 locations. Time series from many moorings are nearly 20 years in length. However, instrumental biases, data dropouts, and the coarse vertical resolutions of the oceanic measurements complicate their use for research. Here an enhanced PIRATA dataset (ePIRATA) is presented for the 17 PIRATA moorings with record lengths of at least seven years. Data in ePIRATA are corrected for instrumental biases, temporal gaps are filled using supplementary datasets, and the subsurface temperature and salinity time series are mapped to a uniform 5-m vertical grid. All original PIRATA data that pass quality control and that do not require bias correction are retained without modification, and detailed error estimates are provided. The terms in the mixed-layer heat and temperature budgets are calculated and included, with error bars. As an example of ePIRATA’s application, the vertical exchange of heat at the base of the mixed layer (Q−h) is calculated at each PIRATA location as the difference between the heat storage rate and the sum of the net surface heat flux and horizontal advection. Off-equatorial locations are found to have annual mean cooling rates of 20–60 W m−2, while cooling at equatorial locations reaches 85–110 W m−2 between 10° and 35°W and decreases to 40 W m−2 at 0°. At most off-equatorial locations, the strongest seasonal cooling from Q−h occurs when winds are weak. Possible explanations are discussed, including the importance of seasonal modulations of mixed-layer depth and the diurnal cycle.

This study explores the seasonally lagged impact of wintertime sea surface temperature (SST) in the Atlantic main development region (MDR) on the subsequent Atlantic hurricane season. It is found that wintertime SST anomalies in the MDR can persist into the summer, explaining 42% of the variance in the subsequent hurricane season's SST during 1951-2010. An anomalously warm wintertime in the MDR is usually followed by an anomalously active hurricane season. Analysis shows an important constraint on the seasonal evolution of the MDR SST by the water vapor feedback process, in addition to the well-known wind-evaporation-SST and cloud-SST feedback mechanisms over the tropical North Atlantic. The water vapor feedback influences the seasonal evolution of MDR SST by modulating seasonal variations of downward longwave radiation. This wintertime thermal control of hurricane activity has significant implications for seasonal predictions and long-term projections of hurricane activity over the North Atlantic.

Three surface drifters equipped with temperature and salinity sensors at 0.2 m and 5 m depths were deployed in April/May 2015 in the subtropical South Pacific with the objective of measuring near-surface salinity differences seen by satellite and in situ sensors and examining the causes of these differences. Measurements from these drifters indicate that water at a depth of 0.2 m is about 0.013 psu fresher than at 5 m and about 0.024°C warmer. Events with large temperature and salinity differences between the two depths are caused by anomalies in surface freshwater and heat fluxes, modulated by wind. While surface freshening and cooling occurs during rainfall events, surface salinification is generally observed under weak wind conditions (≤4 m/s). Further examination of the drifter measurements demonstrates that (i) the amount of surface freshening and strength of the vertical salinity gradient heavily depend on wind speed during rain events, (ii) salinity differences between 0.2 m and 5 m are positively correlated with the corresponding temperature differences for cases with surface salinification, and (iii) temperature exhibits a diurnal cycle at both depths, whereas the diurnal cycle of salinity is observed only at 0.2 m when the wind speed is less than 6 m/s. The amplitudes of the diurnal cycles of temperature at both depths decrease with increasing wind speed. The mean diurnal cycle of surface salinity is dominated by events with winds less than 2 m/s.

Despite a globally uniform increase in the concentrations of emitted greenhouse gases, radiatively forced surface warming can have significant spatial variations. These define warming patterns that depend on preexisting climate states and through atmospheric and oceanic dynamics can drive changes of the hydrological cycle with global-scale feedbacks. Our study reviews research progress on the hydrological cycle changes and their effects on multiscale climate variability. Overall, interannual variability is expected to become stronger in the Pacific and Indian Oceans and weaker in the Atlantic. Global monsoon rainfall is projected to increase and the wet season to lengthen despite a slowdown of atmospheric circulation. Strong variations among monsoon regions are likely to emerge, depending on surface conditions such as orography and land-sea contrast. Interdecadal climate variability is expected to modulate the globally averaged surface temperature change with pronounced anomalies in the polar and equatorial regions, leading to prolonged periods of enhanced or reduced warming. It is emphasized that advanced global observations, regional simulations, and process-level investigations are essential for improvements in understanding, predicting, and projecting the modes of climate variability, monsoon sensitivity, and energetic fluctuations in a warming climate.

Over the past decade, measurements from the climate-oriented ocean observing system have been key to advancing the understanding of extreme weather events that originate and intensify over the ocean, such as tropical cyclones (TCs) and extratropical bomb cyclones (ECs). In order to foster further advancements to predict and better understand these extreme weather events, a need for a dedicated observing system component specifically to support studies and forecasts of TCs and ECs has been identified, but such a system has not yet been implemented. New technologies, pilot networks, targeted deployments of instruments, and state-of-the art coupled numerical forecast models have enabled advances in research and forecast capabilities and illustrate a potential framework for future development. Here, applications and key results made possible by the different ocean observing efforts in support of studies and forecasts of TCs and ECs as well as recent advances in observing technologies and strategies are reviewed. Then a vision and specific recommendations for the next decade are discussed.

Analysis of Bay of Bengal tropical cyclone (TC) track data for the months of May–June during 1979–2014 reveals a meridional dipole in TC intensification: TC intensification rates increased significantly in the northern region and decreased in the southern region. The dipole is consistent with changes in the large-scale TC environment estimated using the Genesis Potential Index (GPI) for the same period. While an increase in lower troposphere cyclonic vorticity and midtroposphere humidity in the northern Bay of Bengal made the environment more favorable for TC intensification, enhanced vertical wind shear in the southern Bay of Bengal tended to reduce TC development. These environmental changes were associated with a strengthening of the monsoon circulation for the months of May–June, driven by a La Nina-like shift in tropical Pacific SSTs and associated tropical wave dynamics. Finally, analysis of a suite of climate models from the Coupled Model Intercomparison Project Phase 5 archive shows that most models correctly reproduce the link between ENSO and premonsoon Bay of Bengal TC activity at interannual timescales, demonstrating the robustness of our main conclusions.

This study examines the benefit of using Ocean Mean Temperature (OMT) to aid in the prediction of the sign of Indian Summer Monsoon Rainfall (ISMR) anomalies. This is a statistical examination, rather than a process study. The thermal energy needed for maintaining and intensifying hurricanes and monsoons comes from the upper ocean, not just from the thin layer represented by sea surface temperature (SST) alone. Here, we show that the southwestern Indian OMT down to the depth of the 26 °C isotherm during January–March is a better qualitative predictor of the ISMR than SST. The success rate in predicting above- or below-average ISMR is 80% for OMT compared to 60% for SST. Other January–March mean climate indices (e.g., NINO3.4, Indian Ocean Dipole Mode Index, El Niño Southern Oscillation Modoki Index) have less predictability (52%, 48%, and 56%, respectively) than OMT percentage deviation (PD) (80%). Thus, OMT PD in the southwestern Indian Ocean provides a better qualitative prediction of ISMR by the end of March and indicates whether the ISMR will be above or below the climatological mean value.

To incorporate the effects of tropical cyclone (TC)-induced upper ocean mixing and sea surface temperature (SST) cooling on TC intensification, a vertical average of temperature down to a fixed depth was proposed as a replacement for SST within the framework of air-sea coupled Potential Intensity (PI). However, the depth to which TC-induced mixing penetrates may vary substantially with ocean stratification and storm state. To account for these effects, here we develop a “Dynamic Potential Intensity” (DPI) based on considerations of stratified fluid turbulence. For the Argo period 2004–2013 and the three major TC basins of the Northern Hemisphere, we show that the DPI explains 11–32% of the variance in TC intensification, compared to 0–16% using previous methods. The improvement obtained using the DPI is particularly large in the eastern Pacific where the thermocline is shallow and ocean stratification effects are strong.

The intense sea surface temperature cooling caused by tropical cyclone-induced mixing lasts several weeks and may thus influence a later cyclone passing over it. Using a 28 year analysis spanning the North Atlantic, eastern Pacific, and Northwest Pacific, we systematically demonstrate that, on average, when tropical cyclones encounter lingering wakes, they experience sea surface temperatures that are ∼0.25-0.5 ◦ C colder. Consequently, the intensification rates are ∼0.4-0.7 ms −1 36 h lower for cyclones when they interact with wakes, consistent with the maximum potential intensity theory. The probability for cyclones to encounter lingering wakes varies positively with cyclone frequency, is ∼10% on average, and has been as high as 27%-37% in the past. These large interaction probabilities reduce the mean intensification rates for cyclones by 3%-6% on average and by ∼12%-15% during the most active years. "Cyclone-cyclone interactions" may therefore represent a mechanism through which tropical cyclones self-regulate their activity to an extent on intraseasonal time scales.

The postmonsoon (October-November) tropical cyclone (TC) season in the Bay of Bengal (BoB) has spawned many of the deadliest storms in recorded history. Here it is shown that the intensity of major TCs (wind speed > 49 m s −1 ) in the postmonsoon BoB increased during 1981-2010. It is found that changes in environmental parameters are responsible for the observed increases in TC intensity. Increases in sea surface temperature and upper ocean heat content made the ocean more conducive to TC intensification, while enhanced convective instability made the atmosphere more favorable for the growth of TCs. The largest changes in the atmosphere and ocean occurred in the eastern BoB, where nearly all major TCs form. These changes are part of positive linear trends, suggesting that the intensity of postmonsoon BoB TCs may continue to increase in the future.

The impacts of canonical and Modoki El Nino on tropical Atlantic sea surface temperature (SST) are quantified using composite analysis. Results show that El Nino Modoki fails to produce significant warming in the tropical Atlantic, in contrast to the well known warming following canonical El Nino events. El Nino Modoki instead induces significant cooling in the northeastern tropical Atlantic and near-neutral conditions elsewhere in the tropical Atlantic. It is shown that the difference in SST response stems primarily from a much stronger Pacific/North American (PNA) teleconnection pattern and stronger atmospheric Kelvin wave response during canonical events compared to Modoki. The stronger PNA pattern and Kelvin waves during canonical events generate anomalously weak surface winds in the tropical North Atlantic, driving anomalously weak evaporative cooling and warmer SSTs. Past research has shown significant decadal variability in the frequency of noncanonical El Ninos relative to canonical events. If such variability continues, it is likely that the impact of El Nino on tropical Atlantic SST will also fluctuate from one decade to the next.

The seasonal cycle of the mixed layer heat budget in the northeastern tropical Atlantic (0°–25°N, 18°–28°W) is quantified using in situ and satellite measurements together with atmospheric reanalysis products. This region is characterized by pronounced latitudinal movements of the intertropical convergence zone (ITCZ) and strong meridional variations of the terms in the heat budget. Three distinct regimes within the northeastern tropical Atlantic are identified. The trade wind region (15°–25°N) experiences a strong annual cycle of mixed layer heat content that is driven by approximately out-of-phase annual cycles of surface shortwave radiation (SWR), which peaks in boreal summer, and evaporative cooling, which reaches a minimum in boreal summer. The surface heat-flux-induced changes in the mixed layer heat content are damped by a strong annual cycle of cooling from vertical turbulent mixing, estimated from the residual in the heat balance. In the ITCZ core region (3°–8°N) a weak seasonal cycle of mixed layer heat content is driven by a semiannual cycle of SWR and damped by evaporative cooling and vertical turbulent mixing. On the equator the seasonal cycle of mixed layer heat content is balanced by an annual cycle of SWR that reaches a maximum in October and a semiannual cycle of turbulent mixing that cools the mixed layer most strongly during May–July and November. These results emphasize the importance of the surface heat flux and vertical turbulent mixing for the seasonal cycle of mixed layer heat content in the northeastern tropical Atlantic.

Long-term and direct measurements of surface shortwave radiation (SWR) have been recorded by the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) since 1997. Previous studies have shown that African dust, transported westward from the Sahara and Sahel regions, can accumulate on mooring SWR sensors in the high-dust region of the North Atlantic (8°–25°N, 20°–50°W), potentially leading to significant negative SWR biases. Here dust-accumulation biases are quantified for each PIRATA mooring using direct measurements from the moorings, combined with satellite and reanalysis datasets and statistical models. The SWR records from five locations in the high-dust region (8°, 12°, and 15°N along 38°W; 12° and 21°N along 23°W) are found to contain monthly-mean accumulation biases as large as −200 W m−2 and record-length mean biases on the order of −10 W m−2. The other 12 moorings, located mainly between 10°S and 4°N, are in regions of lower atmospheric dust concentration and do not show statistically significant biases. Seasonal-to-interannual variability of the accumulation bias is found at all locations in the high-dust region. The moorings along 38°W also show decreasing trends in the bias magnitude since 1998 that are possibly related to a corresponding negative trend in atmospheric dust concentration. The dust-accumulation biases described here will be useful for interpreting SWR data from PIRATA moorings in the high-dust region. The biases are also potentially useful for quantifying dust deposition rates in the tropical North Atlantic, which at present are poorly constrained by satellite data and numerical models.

The seasonal variability of surface layer salinity (SLS), evaporation (E), precipitation (P), E-P, advection and vertical entrainment over the global ocean is examined using in situ salinity data, the National Centers for Environmental Prediction's Climate System Forecast Reanalysis and a number of other ancillary data. Seasonal amplitudes and phases are calculated using harmonic analysis and presented in all areas of the open ocean between 60° S and 60° N. Areas with large amplitude SLS seasonal variations include: the intertropical convergence zone (ITCZ) in the Atlantic, Pacific and Indian Oceans; western marginal seas of the Pacific; and the Arabian Sea. The median amplitude in areas that have statistically significant seasonal cycles of SLS is 0.19. Between about 60° S and 60° N, 37% of the ocean surface has a statistically significant seasonal cycle of SLS and 75% has a seasonal cycle of E-P. Phases of SLS have a bimodal distribution, with most areas in the Northern Hemisphere peaking in SLS in March/April and in the Southern Hemisphere in September/October. The seasonal cycle is also estimated for surface freshwater forcing using a mixed-layer depth climatology. With the exception of areas near the western boundaries of the North Atlantic and North Pacific, seasonal variability is dominated by precipitation. Surface freshwater forcing also has a bimodal distribution, with peaks in January and July, 1–2 months before the peaks of SLS. Seasonal amplitudes and phases calculated for horizontal advection show it to be important in the tropical oceans. Vertical entrainment, estimated from mixed-layer heaving, is largest in mid and high latitudes, with a seasonal cycle that peaks in late winter. The amplitudes and phases of SLS and surface fluxes compare well in a qualitative sense, suggesting that much of the variability in SLS is due to E-P. However, the amplitudes of SLS are somewhat different than would be expected and the peak of SLS comes typically about one month earlier than expected. The differences of the amplitudes of the two quantities is largest in such areas as the Amazon River plume, the Arabian Sea, the ITCZ and the eastern equatorial Pacific and Atlantic.

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.

Dust storms are a persistent feature of the tropical North Atlantic and vary over a wide range of temporal scales. While it is well known that mineral aerosols alter the local radiative fluxes, far less is understood about the oceanic response to such forced changes to the radiative budget, particularly on long time scales. This study uses an observation-based climatology of dust surface forcing and an ocean general circulation model to examine the influence of anomalous atmospheric dust cover over the tropical North Atlantic on upper ocean temperature and circulation during 1955–2008. It is found that surface temperature anomalies from the model experiments are forced primarily by local radiation-induced changes to the surface heat budget. The subsurface temperature anomalies are additionally influenced by upper ocean circulation anomalies, which are the response to dust-forced steric changes in dynamic height. The results herein suggest that on decadal time scales dust-forced variability of ocean surface and subsurface temperatures are of a magnitude comparable to observed variability. On longer time scales dust-forced sea surface temperature anomalies vary in phase with the Atlantic multidecadal oscillation, implying that tropical North Atlantic multidecadal variability is related to changes in dust emissions from West Africa.

Tropical Atlantic sea surface temperatures and surface winds vary together in a pattern termed the Atlantic Meridional Mode. Simulations with an ocean general circulation model that is driven by a record of dust radiative forcing suggest that dust emissions over West Africa substantially influence the state of the tropical Atlantic Ocean on interannual to decadal scales.

A combination of satellite and in situ datasets is used to investigate the impact of interannual changes in atmospheric dust content on the sea surface temperature (SST) of the tropical North Atlantic Ocean. Throughout most of the region the authors find, in agreement with previous studies, that positive anomalies of dust are associated with a significant reduction in surface shortwave radiation (SWR), while negative anomalies of dust are associated with an enhancement of SWR. Statistical analysis for 1984–2000 suggests that changes in dustiness in the tropical North Atlantic (10°–25°N, 20°–60°W) explained approximately 35% of the observed interannual SST variability during boreal summer, when climatological dust concentrations are highest. Measurements from a long-term moored buoy in the central tropical North Atlantic are used to investigate the causes of anomalously cool SST that occurred in conjunction with a period of enhanced dustiness at the start of the unexpectedly quiet 2006 hurricane season. It is found that surface SWR varied out of phase with dustiness, consistent with historical analyses. However, most of the anomalous cooling occurred prior to the period of enhanced dustiness and was driven primarily by wind-induced latent heat loss, with horizontal oceanic heat advection and SWR playing secondary roles. These results indicate that dust-induced changes in SWR did not play a major direct role in the cooling that led up to the 2006 Atlantic hurricane season.

Sea-surface temperature interannual anomalies (SSTAs) in the thermocline ridge of the southwestern tropical Indian Ocean (TRIO) have several well-documented climate impacts. In this paper, we explore the physical processes responsible for SSTA evolution in the TRIO region using a combination of observational estimates and model-derived surface layer heat budget analyses. Vertical oceanic processes contribute most to SSTA variance from December to June, while lateral advection dominates from July to November. Atmospheric fluxes generally damp SSTA generation in the TRIO region. As a result of the phase opposition between the seasonal cycle of vertical processes and lateral advection, there is no obvious peak in SSTA amplitude in boreal winter, as previously noted for heat content anomalies. Positive Indian Ocean Dipole (IOD) events and the remote influence of El Nino induce comparable warming over the TRIO region, though IOD signals peak earlier (November-December) than those associated with El Nino (around March-May). Mechanisms controlling the SSTA growth in the TRIO region induced by these two climate modes differ strongly. While SSTA growth for the IOD mostly results from southward advection of warmer water, increased surface shortwave flux dominates the El Nino SSTA growth. In both cases, vertical oceanic processes do not contribute strongly to the initial SSTA growth, but rather maintain the SSTA by opposing the effect of atmospheric negative feedbacks during the decaying phase.

Recent observations have shown evidence of intraseasonal oscillations (with periods of approximately 1–2 months) in the northern and southern tropical Atlantic trade winds. In this paper, the oceanic response to the observed intraseasonal wind variability is addressed through an analysis of the surface mixed layer heat balance, focusing on three locations in the northwestern tropical Atlantic where in situ measurements from moored buoys are available (14.5°N, 51°W; 15°N, 38°W; and 18°N, 34°W). It is found that local heat storage at all three locations is balanced primarily by wind-induced latent heat loss, which is the same mechanism that is believed to play a dominant role on interannual and decadal time scales in the region. It is also found that the intraseasonal wind speed oscillations are linked to changes in surface wind convergence and convection over the western equatorial Atlantic warm pool. These atmospheric circulation anomalies and wind-induced SST anomalies potentially feed back on one another to affect longer time-scale variability in the region.

The surface Walker and tropical tropospheric circulations have been inferred to slow down from historical observations and model projections, yet analysis of large-scale surface wind predictions is lacking. Satellite measurements of surface wind speed indicate strengthening trends averaged over the global and tropical oceans that are supported by precipitation and evaporation changes. Here we use corrected anemometer-based observations to show that the surface wind speed has not decreased in the averaged tropical oceans, despite its reduction in the region of the Walker circulation. Historical simulations and future projections for climate change also suggest a near-zero wind speed trend averaged in space, regardless of the Walker cell change. In the tropics, the sea surface temperature pattern effect acts against the large-scale circulation slow-down. For higher latitudes, the surface winds shift poleward along with the eddy-driven mid-latitude westerlies, resulting in a very small contribution to the global change in surface wind speed. Despite its importance for surface wind speed change, the influence of the SST pattern change on global-mean rainfall is insignificant since it cannot substantially alter the global energy balance. As a result, the precipitation response to global warming remains 'muted' relative to atmospheric moisture increase. Our results therefore show consistency between projections and observations of surface winds and precipitation.

Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 20°C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of −0.1 to 0.6 m day−1 and 26°–29.5°C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (

[1] Trends in tropical Atlantic sea surface temperature (SST), Sahel rainfall, and Saharan dust are investigated during 1980–2006. This period is characterized by a significant increase in tropical North Atlantic SST and the transition from a negative to a positive phase of the Atlantic multidecadal oscillation (AMO). It is found that dust concentrations over western Africa and the tropical North Atlantic Ocean decreased significantly between 1980 and 2006 in association with an increase in Sahel rainfall. The decrease in dust in the tropical North Atlantic tended to increase the surface radiative heat flux by 0.7 W m−2 which, if unbalanced, would lead to an increase in SST of 3°C. Coupled models significantly underestimate the amplitude of the AMO in the tropical North Atlantic possibly because they do not account for changes in Saharan dust concentration.

[1] In this study the causes of the seasonal cycle of mixed layer salinity in the tropical North Atlantic Ocean are investigated from a combination of satellite, atmospheric reanalysis, and in situ data sets. Results indicate that the salinity balance varies regionally, leading to a seasonal cycle in sea surface salinity (SSS) with significant spatial inhomogeneity. For example, horizontal salinity advection plays a key role in the salinity balance of the western tropical North Atlantic (10–25°N, 50–65°W), where seasonal variations in SSS are relatively large. In contrast, in the north-central basin (15–25°N, 20–50°W), freshening from meridional advection balances an excess of evaporation over precipitation, resulting in a very weak seasonal cycle of mixed layer salinity. Farther south (5–15°N, 20–45°W), seasonal variations of precipitation are more significant and drive a pronounced seasonal cycle of mixed layer salinity. Throughout most of the tropical North Atlantic the sum of the surface moisture flux and horizontal advection underestimates the mixed layer salinity tendency during boreal fall and winter. This is the time of year with highest wind speeds and highest negative buoyancy flux, suggesting that vertical turbulent entrainment of high-salinity thermocline water may be important. Daily measurements from a moored buoy in the central tropical North Atlantic are consistent with this interpretation. The results of this study highlight the complexity of the seasonal salinity balance in the tropical North Atlantic Ocean and the need for continued in situ monitoring of upper ocean salinity and currents to complement future space-based surface salinity measurements.

[1] The 2005 Atlantic hurricane season was the most active and destructive on record. One of the factors that likely contributed to this record-breaking season was the presence of exceptionally warm sea surface temperatures (SST) in the tropical North Atlantic. Two long term moored buoys were well positioned to record the anomalous atmospheric and oceanic conditions associated with this warm event. Here we present results from a mixed layer heat budget analysis based on measurements from these buoys. We find that the primary cause of the anomalous warming was a weakening of the northeasterly trade winds and an associated decrease in latent heat loss from the ocean. Important secondary factors include changes in shortwave radiation and horizontal oceanic heat advection.

[1] This paper addresses the atmospheric and oceanic causes of the seasonal cycle of sea surface temperature (SST) in the tropical Atlantic on the basis of direct observations. Data sets include up to 4 years (September 1997 to February 2002) of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near-surface drifting buoys, and a blended satellite in situ SST product. We analyze the mixed layer heat balance at eight PIRATA mooring locations and find that the seasonal cycles of latent heat loss and absorbed shortwave radiation are responsible for seasonal SST variability in the northwest basin (8°N–15°N along 38°W). Along the equator (10°W–35°W), contributions from latent heat loss are diminished, while horizontal temperature advection and vertical entrainment contribute significantly. Zonal temperature advection is especially important during boreal summer near the western edge of the cold tongue, while horizontal eddy temperature advection, which most likely results from tropical instability waves, opposes temperature advection by the mean flow. The dominant balance in the southeast (6°S–10°S along 10°W) is similar to that in the northwest, with both latent heat loss and absorbed solar radiation playing important roles.

International audience; Cyclone Nargis (Figure 1a) made landfall in Myanmar (formerly Burma) on 2 May 2008 with sustained winds of approximately 210 kilometers per hour, equivalent to a category 3-4 hurricane. In addition, Nargis brought approximately 600 millimeters of rain and a storm surge of 3-4 meters to the low-lying and densely populated Irrawaddy River delta. In its wake, the storm left an estimated 130,000 dead or missing and more than $10 billion in economic losses. It was the worst natural disaster to strike the Indian Ocean region since the 26 December 2004 tsunami and the worst recorded natural disaster ever to affect Myanmar.