Weakening Of Trade Winds Research Articles

Dynamic shifts in the southern Benguela upwelling system since the latest Miocene

This study reconstructs the upwelling history of the Southern Benguela Upwelling System (SBUS), examining its sensitivity to both regional and global climatic influences. We analyze planktic foraminiferal relative abundance and stable isotope data from ODP Site 1087 to investigate the history of SBUS since ∼ 6.1 Ma. During the latest Miocene-earliest Pliocene, the SBUS experienced increased productivity, fueled by wind-driven upwelling and an enriched global nutrient reservoir associated with the late Miocene biogenic bloom. The early Pliocene witnessed an apparent weakening of the SBUS due to the deepening of the global thermocline and weak trade winds. Significantly, our proxy records indicate strengthening of the SBUS beginning before the onset of intense northern hemisphere glaciation (iNHG) between 3.5 and 3 Ma. Despite contraction of upwelling cells after ∼ 3 Ma, our study challenges the notion of water column stability during the Matuyama Diatom Maximum event (3 to 2 Ma); we alternatively propose the existence of water column instability and probable presence of upwelling during this time. Post 2 Ma, the main upwelling cells shifted northward in response to the strengthening and equatorward migration of the Hadley cell, marked by long-term marginal upwelling. The SBUS achieved its modern state by ∼ 1 Ma with complete isolation from the coastal upwelling zone. Our observations indicate a strong correlation between the history of SBUS and global climatic events, with a strong correlation between them prior to the initiation of iNHG. Subsequently, local climatic influences have become more predominant, overshadowing the signals from global climatic events. The data also suggest linkages between shifts in the SBUS strengths and southwest African climate.

Tropical Atlantic variability in EC-EARTH: impact of the radiative forcing

Understanding the impact of radiative forcing on climate variability and change in the Tropical Atlantic is crucial for different socio-economic sectors, given their substantial impacts in both local and remote regions. To properly evaluate the effect of a changing climate on the variability, the use of standard transient historical and scenario simulations requires very large ensembles. A computationally cheaper alternative implemented in this study consists of performing two 250-year-long atmosphere-ocean coupled simulations with EC-EARTH 3.3 (CMIP6 version) with fixed radiative forcing at the years 2000 and 2050, representative of present and future climate conditions, respectively. The changes in the leading modes of Tropical Atlantic variability (TAV), including the Atlantic Niño/Niña and the Subtropical North Atlantic pattern, have been assessed in three target seasons: spring (MAM), summer (JJ) and early winter (ND). While the change in sea surface temperature (SST) climatology shows homogeneous warming, the difference between future and present SST variability exhibits a distinct behaviour consistent along the seasonal cycle, with a decrease in the equatorial region and an increase at subtropical latitudes. This study explores the processes associated with the suppressed/enhanced TAV, with a particular focus on the less-explored early winter season. In agreement with previous studies, the Atlantic Meridional Overturning Circulation (AMOC) shows a weakening in strength, but the results also show an increase in variability. The AMOC-related deepening of the equatorial thermocline and the flattening linked to weakened trade winds are consistent with the suppressed SST variability of the Atlantic Niño/Niña. On the other hand, the enhanced SST variability at subtropical latitudes is mainly associated with an increase in turbulent heat flux variability, with a minor contribution of the mixed layer depth variability. Variability in turbulent heat flux is influenced primarily by latent heat flux, connected to changes in precipitation variability.

Balancing Volume, Temperature, and Salinity Budgets During 2014–2018 in the Tropical Pacific Ocean State Estimate

AbstractA state estimate of the tropical Pacific Ocean is used to analyze regional volume, temperature, and salinity budgets during 2014–2018. The simulated ocean state is constrained by both model dynamics and assimilated observations. Comparisons with independent mooring data show that the state estimate is consistent with the observed variability in temperature and velocity. Budgets are analyzed between 5°S and 5°N in the upper 300 m, inside a box defined to represent the central and eastern equatorial Pacific. Transports through the faces of this box are quantified to understand the processes responsible for variability in box‐mean properties. Vertical mixing across 300 m is negligible; temperature and salinity tendencies are balanced by surface fluxes and advective divergence, which is decomposed into geostrophic and ageostrophic components. The onset and recovery of the 2015/2016 El Niño event is found to be dominated by anomalous surface fluxes and horizontal advection. During the onset phase, weaker trade winds cause the shallow meridional overturning circulation to slow down, which reduces the poleward transport of heat and leads to upper ocean warming. Anomalous precipitation and advection of fresh water from the western Pacific drive the net freshening of the region. Relaxation from El Niño conditions is dominated by wind‐driven meridional advection at 5°N. As the meridional advection regains strength, Ekman advection efficiently exports the warm, fresh surface water out of the equatorial region. Quantifying the heat and salt transport changes in response to wind variability strengthens our understanding of global ocean heat transport.

Interannual Variability of Barrier Layer in the Tropical Atlantic and Its Relationship with the Tropical Atlantic Modes

Abstract This study reveals the role of the tropical Atlantic variability in modulating barrier layer thickness (BLT) in peak seasons. Based on reanalysis data during 1980–2016, statistical and dynamical analyses are performed to investigate the mechanism of BLT variability associated with the tropical Atlantic modes. The regions with significant correlation between BLT and tropical Atlantic modes are located on the northwest and southeast coasts of the tropical Atlantic, which are consistent with BLT maximum variability regions. In boreal spring, BLT decreases in the northwest because less latent heat release affected by weak trade wind related to the Atlantic meridional mode (AMM) shoals the isothermal layer depth (ITLD). In the south equatorial Atlantic, deepened mixed layer depth (MLD) is controlled by the decreasing freshwater input brought by a northward shift of the intertropical convergence zone (ITCZ) and further leads to a thinner barrier layer (BL). However, a shoaling MLD appears in the north equatorial Atlantic, which results from excessive freshwater input, causing a thick BL there. In boreal summer, positive runoff anomaly caused by the Atlantic equatorial mode (AEM) leads to upper warming of the tropical northwest Atlantic and a shallowing ITLD, favoring a thinner BL there. However, a southward shift of ITCZ brings more freshwater into the south equatorial Atlantic, inducing a shallowing MLD as well as a thicker BL. AEM-driven horizontal heat advection of the south equatorial current contributes to a thick ITLD in the central southern tropical Atlantic and thus increases BLT. Significance Statement This research aims to reveal how the tropical Atlantic meridional and equatorial interannual climatic modes affect barrier layer thickness (BLT). These two climate modes can affect the wind field, ocean current, and precipitation through air–sea interaction processes, and further affect mixing, heat–salt transport, and stratification in the upper ocean and thus BLT. This finding is important because the barrier layer restricts the exchange of heat, momentum, mass, and nutrients between the mixed layer and the thermocline, thereby impacting local and remote weather events, the ecological environment, and the climate. Our results provide guidance for interpreting the interannual variability of BLT in the tropical Atlantic.

Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System

The holopelagic macroalgae sargassum has proliferated across the tropical Atlantic since 2011, of consequence for coastal populations from West Africa to the Caribbean with limited early warning of major beaching events. As part of an interdisciplinary project, ‘Teleconnected SARgassum risks across the Atlantic: building capacity for TRansformational Adaptation in the Caribbean and West Africa’ (SARTRAC), an ensemble forecast system, SARTRAC-EFS, is providing seasonal predictions of sargassum drift. An eddy-resolving ocean model hindcast provides the winds and currents necessary to generate ensemble members. Ensemble forecasts are then obtained for different combinations of ‘windage’, the fractional influence of winds on sargassum mats, and in situ rates of growth, mortality, and sinking. Forecasts for north and south of Jamaica are evaluated with satellite-observed distributions, associated with beaching events in specific years of heavy inundation, 2015 and 2018-20. These seasonal forecasts are evaluated, on lead times of up to 180 days. Forecasts are subject to leading modes of tropical climate variability, in particular the Atlantic Meridional Mode (AMM). More accurate forecasts for a given year are obtained with ensemble members from hindcast years with a similar spring AMM-index. This is most clearly evident during negative AMM phases in spring of 2015 and 2018, when positive sea surface temperature anomalies and anomalously weak trade winds were established across the northern tropics. On this evidence, SARTRAC-EFS is potentially useful in providing early warning of high sargassum prevalence. Extended to sargassum drift off West Africa, extensive cloud cover limits availability of the satellite data needed for full application and evaluation of SARTRAC-EFS in this region, although experimental forecasts off the coast of Ghana are found highly sensitive to the windage that is associated with strong onshore winds during boreal summer. Alongside other forecast systems, SARTRAC-EFS is providing useful early warnings of sargassum inundation at seasonal timescale.

Intercomparison of FLEXPART and CALPUFF dispersion models. An application over a small tropical island

A typical practice in air quality modeling assessment is the intercomparison between different dispersion models results and air quality measurements at different atmospheric conditions. In this study, a comparison between the results of two Lagrangian dispersion models, the Lagrangian Particle Dispersion Model FLEXPART and the Lagrangian Puff Model CALPUFF (regulatory model), coupled to the same meteorological fields produced by the Weather Research and Forecasting (WRF) model, was done. As a case study, atmospheric dispersion of anthropogenic nitrogen oxides (NOx) emissions (considered as a passive tracer) was considered, during a typical case of severe pollution over the densely populated area of the Guadeloupe archipelago (West French Indies), including complex terrain and, of course, coastal influence. Even though Lagrangian models usually provide better results of plume dispersion under strong winds, in this case study weak trade winds are dominant, in order to check both models under non-ideal atmospheric conditions. As a result, compared to NOx ground level concentration (glc) observations, FLEXPART shows better agreement than CALPUFF. However, as a regulatory model, CALPUFF overestimates both glc observations and FLEXPART maximum NOx glc results, with higher values when a higher horizontal resolution is applied. Also, differences between models results arise in the spatial distribution of NOx over a 1 × 1 km2 horizontal resolution grid domain, showing quite homogenous isopleths with smooth contours for CALPUFF vs. fragmented isopleths with irregular contours for FLEXPART.

Investigating the Role of the Tibetan Plateau in ENSO Variability

ABSTRACTThe role of the Tibetan Plateau (TP) in El Niño–Southern Oscillation (ENSO) variability is investigated using coupled model experiments with different topography setups. Removing the TP results in weakened trade winds in the tropical Pacific, an eastward shift of atmospheric convection center, a shallower mixed layer in the equatorial Pacific, and a flattened equatorial thermocline, which leads to an El Niño–like sea surface temperature (SST) response. In association with these mean climate changes in the tropical atmosphere–ocean system, the ENSO variability exhibits a much stronger amplitude in the world without the TP. Detailed diagnoses reveal that in the absence of the TP, both thermocline feedback in the eastern equatorial Pacific and Ekman pumping feedback in the central-eastern equatorial Pacific are enhanced substantially, leading to stronger ENSO variability. The changes of these two feedbacks are caused by the eastward shift of the atmospheric convection center and enhanced ocean sensitivity; the latter is due to the shallower mixed layer and flattened thermocline. This study suggests that the presence of the TP may be of fundamental importance for modern-day tropical climate variability; namely, the TP may have played a role in suppressing ENSO variability.

Investigating the Role of the Tibetan Plateau in the Formation of Atlantic Meridional Overturning Circulation

AbstractThe Tibetan Plateau (TP) over the Eurasian continent has significant effects on both regional and global climate. It can even affect the remote Atlantic meridional overturning circulation (AMOC), as shown in this study. Through coupled modeling experiments, we demonstrate that removing the TP immediately weakens the meridional wind over East Asia, resulting in stronger westerlies in the midlatitudes. The stronger westerlies enhance the southward Ekman flow and surface latent and sensible heat losses in the subpolar North Atlantic, cooling the surface ocean and leading to stronger North Atlantic deep-water formation and stronger AMOC during the first few decades after the TP removal. At the same time, accompanying the weakened trade winds in the tropical Pacific, more moisture is transported from the tropical Pacific to the North Atlantic, freshening the surface ocean and triggering a weakening of the AMOC. The AMOC weakening in turn results in southward expansion and melting of sea ice, providing more freshwater to the North Atlantic, which furthers the weakening of the AMOC. The positive feedback between the AMOC and sea ice eventually leads to AMOC shutdown. We illustrate that there would be no AMOC without the TP. These results call for a revisiting of how ocean circulation and global climate may have responded to the TP uplift and other tectonic changes on the geological time scale.

Higher Sea Levels at Hawaii Caused by Strong El Niño and Weak Trade Winds

AbstractHawaii experienced record-high sea levels during 2017, which followed the 2015 strong El Niño and coincided with weak trade winds in the tropical northeastern Pacific. The record sea levels were associated with a combination of processes, an important contributing factor of which was the persistent high sea level (~10 cm above normal) over a large region stretching between Hawaii and Mexico. High sea levels at Mexico are known to occur during strong El Niño as the coastal thermocline deepens. Planetary wave theory predicts that these coastal anomalies propagate westward into the basin interior; however, high sea levels at Hawaii do not occur consistently following strong El Niño events. In particular, Hawaii sea levels remained near normal following the previous strong El Niño of 1997. The processes controlling whether Hawaii sea levels rise after El Niño have so far remained unknown. Atmosphere-forced ocean model experiments show that anomalous surface cooling, controlled by variable trade winds, impacts sea level via mixed layer density, explaining much of the difference in Hawaiian sea level response after the two recent strong El Niño events. In climate model projections with greenhouse warming, more frequent weak trade winds following El Niño events are expected, suggesting that the occurrence of high sea levels at Hawaii will increase as oceanic anomalies more often traverse the basin.

Spatial Patterns and Trends in Surface Air Temperatures and Implied Changes in Atmospheric Moisture Across the Hawaiian Islands, 1905–2017

AbstractWhile the Hawaiian Islands are experiencing long‐term warming, spatial and temporal patterns are poorly characterized. Drawing on daily temperature records from 309 stations (1905–2017), we explored relationships of surface air temperatures (Tmax, Tmin, Tavg, and diurnal temperature range) to atmospheric, oceanic, and land surface variables. Statistical modeling of spatial patterns (2006–2017) highlighted the strong negative influence of elevation and moisture on air temperature and the effects of distance inland, cloud frequency, wind speed, and the local trade wind inversion on the elevation dependence of surface air temperature. We developed time series of sea level air temperature and surface lapse rate by modeling surface air temperature as a simple function of elevation and found a strong long‐term (1905–2017) warming trend in sea level Tmin, twice that of Tmax (+0.17 vs +0.07°C/decade), suggesting regional warming, possibly enhanced by urbanization and cloud cover effects. Removing this trend, sea level Tmax and Tmin tracked SST and rainfall at decadal time scales, while Tmax increased with periods of weakened trade winds. Sea level air temperatures correlated with North Pacific climate indices, reflecting the influence of regional circulation via SST, rain, clouds, and trade winds that modulate environmental warming across the Hawaiian Islands. Increasing (steeper) Tmax surface lapse rates for the 0‐ to 1,600‐m elevation range (into the cloud zone) over 1978–2017 coincide with observations of marine boundary layer drying and rising cloud base heights, suggesting a need to better understand elevation‐dependent warming in this tropical/subtropical maritime environment and associated changes to cloud formation and persistence.

Climate diagnostics of the extreme floods in Peru during early 2017

From January through March 2017, a series of extreme precipitation events occurred in coastal Peru, causing severe floods with hundreds of human casualties and billions of dollars in economic losses. The extreme precipitation was a result of unusually strong recurrent patterns of atmospheric and oceanic conditions, including extremely warm coastal sea surface temperatures (SST) and weakened trade winds. These climatic features and their causal relationship with the Peruvian precipitation were examined. Diagnostic analysis and model experiments suggest that an atmospheric forcing in early 2017, which was moderately linked to the Trans-Niño Index (TNI), initiated the local SST warming along coastal Peru that later expanded to the equator. In January 2017, soil moisture was increased by an unusual expansion of Amazonian rainfall. By March, localized and robust SST warming provided positive feedback to the weakening of the trade winds, leading to increased onshore wind and a subsequent enhancement in rainfall. The analysis points to a tendency towards more frequent and stronger variations in the water vapor flux convergence along the equator, which is associated with the increased precipitation in coastal Peru.

Analysis of ENSO simulation biases in FIO-ESM version 1.0

As the most significant interannual variability in the climate system, El Niño-Southern Oscillation (ENSO) has critical effects on global weather and climate patterns. To simulate and predict ENSO, coupled general circulation models (CGCMs) have become a key tool. However, the accurate simulation of ENSO is still a challenge for CGCMs. The performance of El Niño simulations conducted through FIO-ESM v1.0 is examined based on the outputs of the Coupled Model Intercomparsion Project phase 5 (CMIP5) historical experiments. The results show that FIO-ESM v1.0 suffers from similar common problems to other CMIP5 models, including an eastward shift in the central locations of El Niño, adopting a regular period of roughly 3 years, addressing excessively high amplitude, spurious eastward propagation of El Niño events, and Aborted El Niño events. El Niño composite and mixed layer heat budget analyses indicate that these simulation biases are mainly associated with the mean state biases, including a warm Sea Surface Temperature (SST) bias for the central-eastern Pacific, a cold SST bias for the western and central Pacific, seasonal cycles of SST of the equatorial eastern Pacific, and weaker trade winds. Weaker SST-cloud-shortwave radiation feedback in La Niña events than in El Niño events is what creates spurious ENSO amplitude symmetry in the model. We suggest that the improvement of El Niño simulations may be realized by focusing on the mean state and SST-cloud-shortwave radiation feedback in the tropical region. Specifically, further incremental improvements in the mean state of the tropical Pacific should constitute the first step to realizing more accurate ENSO simulation.

Deciphering latitudinal shifts in coccolith accumulation in the eastern tropical Pacific Ocean through the Pleistocene

Abstract Middle and Late Pleistocene coccolithophore assemblages from Ocean Drilling Program (ODP) sites 1241 and 1242 in the northeastern tropical Pacific (north ETP) were analyzed to reconstruct coccolithophore-productivity and surface-water conditions over the last 925 kyr. Stratigraphic control was provided by δ18O data and geomagnetic time scales and the identification of synchronous calcareous nannofossil events, formerly described in mid and low latitudes. Changes in sea surface coccolithophore-productivity were inferred by estimates of the so-called N ratio – an independent proxy on conversion to accumulation rates – and the total coccolith-derived carbonate accumulation rate (CAR). CAR variations follow those of the N ratio but show larger amplitude changes. Fluctuations in the N ratio and total CAR values imply coccolithophore-productivity maxima during glacial times, most likely as a result of a shallower nutri-thermocline, defined as the boundary beneath the sea surface at which nutrient-rich/cool and nutrient-poor/warm waters are separated. Low interglacial values of the N ratio and CAR suggest lower coccolithophore-productivity and a deeper nutri-thermocline (deep stratification of the mixed layer). This result implies that coccolithophore-productivity-controlled CARs and dissolution were driven by the ratio between CaCO3 production and organic matter oxidation, and that effects of dissolution were enhanced when coccolith production fell. The data suggest high coccolithophore-productivity and a shallow nutri-thermocline during MIS 22 through MIS 6, likely due to the reinforcement of northeastern wind-driven advection of nutrient-rich waters, which is compatible with a La Nina-like state. The dominance of Gephyrocapsa caribbeanica during the Mid-Brunhes Event (MBE) is consistent with this eutrophic environment. Conversely, data from MIS 5 suggests a weakening of trade winds in the ETP, which would have reduced the normal upwelling of cool water and warmed SST, and which is compatible with the oligotrophic setting of an El Nino-like state. During MIS 4 through MIS 1, coccolithophore-productivity was not significantly different from MIS 5 and therefore not indicative of a prominent La Nina-like state, as previously reported for the Equatorial Cold Tongue (ECT). Comparison with coccolithophore data from Site 1240 supports a reduction of coccolithophore-productivity and nutri-thermocline deepening from the ECT to the Costa Rica margin and suggests that nutrient availability was the primary control on the distribution of the whole assemblage during the past 925 kyr.

Puerto Rico sea level trend in regional context

Sea level (SL) is rising in Puerto Rico due to regional influences from weaker trade winds, in addition to global influences of thermal expansion and polar ice melt. The linear increase of SL has steepened in recent decades from +0.175 to +0.725 cm/yr. The faster rate of SL rise is partly attributed to diminishing ice volume (r = −0.80), notwithstanding a recent decline in the Atlantic Multi-decadal Oscillation. A sophisticated point-to-field regression analysis (N = 5980) demonstrates that daily fluctuations of SL in Puerto Rico are significantly enhanced by locally warmer sea temperatures via reduced evaporation, and correspond with lower air pressure accompanying late summer storms. These features point to regional air-sea interactions that affect SL rise over and above the background global signal. Extrapolating these trends, a rise of more than 0.3 m is expected by 2050.

Decadal variability and trends of oceanic barrier layers in tropical Pacific

Decadal variability and trends of the isothermal layer depth (ILD), mixed layer depth (MLD), and barrier layer thickness (BLT) were analyzed for the tropical Pacific during 1979–2015. The decadal variability of ILD, MLD, and BLT shows a close connection with the Pacific Decadal Oscillation (PDO). At PDO positive phase, the eastward shift of precipitation and weakened trade winds result in thinner BLT in western Pacific and thicker BLT in central and eastern Pacific. The situation is reversed at PDO negative phase. The differences in BLT can be up to 9–15 m. The spatial distributions of decadal trends of ILD and MLD are complex, but a thickening of BLT in the western tropical Pacific is clearly present. The raw trends of ILD, MLD, and BLT averaged in the tropical Pacific (30° N–30° S, 120° E–75° W) from 1979 to 2015 are 1.62, 1.20, and 0.51 m per decade, respectively. PDO can explain about 25% of the increasing trends of BLT, while El Nino-Southern Oscillation (ENSO) only explains about 1.7%. Global warming and/or variability at longer time scales is responsible for the remaining increasing trends. The BLT change is related to the warming and freshening of the western Pacific warm pool in recent decades. The ocean-atmosphere interactions about trade winds, wind-driven ocean circulation, temperature, and precipitation/evaporation are discussed.

Variations in terrigenous input into the eastern Equatorial Atlantic over 120ka: Implications on Atlantic ITCZ migration

Variation in the location of the Intertropical Convergence Zone (ITCZ) in eastern equatorial Atlantic (EEA) drastically affects rainfall in equatorial Africa and hence sedimentation on the African margin. Sediment core DY26III-Nig-S60-GC2 taken from the slope of the EEA offshore Nigeria (4°28′56′′.388′E, 3°33′10′′.764′N; water depth: 2946 m) was studied to decode the climatological response to the ITCZ migration in the past 120 ka. Based on oxygen isotope stratigraphy and radiocarbon dating, the sedimentation rates of this core range from 0.64 to 6.90 cm/ka and average of 3.17 cm/ka. The periods MIS 2, MIS 4 and MIS 5b from our studied core are characterized by Minima of Fe/K and maxima of K/Al, Ti/Al and magnetic susceptibility along with more kaolinite. This reflects a period when the region supposedly dominated by less precipitation and strong trade winds which suggest southward position of the ITCZ. Conversely, the periods MIS 1, MIS 3 and MIS 5a exhibit an opposite trend; these wet periods are of weak trade winds and northerly ITCZ excursion.Fluctuations in Fe/K, K/Al and Ti/Al records reflect changes in the low-latitude insolation cycle (23 kyr) and thus indicate a close coupling between terrigenous input and climate changes. This work thus highlights the importance of the terrigenous input as a prerequisite for the interpretation of ITCZ position in the EEA.

Interannual Weakening of the Tropical Pacific Walker Circulation Due to Strong Tropical Volcanism

In order to examine the response of the tropical Pacific Walker circulation (PWC) to strong tropical volcanic eruptions (SVEs), we analyzed a three-member long-term simulation performed with HadCM3, and carried out four additional CAM4 experiments. We found that the PWC shows a significant interannual weakening after SVEs. The cooling effect from SVEs is able to cool the entire tropics. However, cooling over the Maritime Continent is stronger than that over the central-eastern tropical Pacific. Thus, non-uniform zonal temperature anomalies can be seen following SVEs. As a result, the sea level pressure gradient between the tropical Pacific and the Maritime Continent is reduced, which weakens trade winds over the tropical Pacific. Therefore, the PWC is weakened during this period. At the same time, due to the cooling subtropical and midlatitude Pacific, the Intertropical Convergence Zone (ITCZ) and South Pacific convergence zone (SPCZ) are weakened and shift to the equator. These changes also contribute to the weakened PWC. Meanwhile, through the positive Bjerknes feedback, weakened trade winds cause El Nino-like SST anomalies over the tropical Pacific, which in turn further influence the PWC. Therefore, the PWC significantly weakens after SVEs. The CAM4 experiments further confirm the influences from surface cooling over the Maritime Continent and subtropical/midlatitude Pacific on the PWC. Moreover, they indicate that the stronger cooling over the Maritime Continent plays a dominant role in weakening the PWC after SVEs. In the observations, a weakened PWC and a related El Nino-like SST pattern can be found following SVEs.

The Record-Breaking Hot Summer in 2015 over Hawaii and Its Physical Causes

Abstract Hawaiian surface air temperature (HST) during the summer of 2015 (from July to October) was about 1.5°C higher than the climatological mean, which was the hottest since records began in 1948. In the context of record-breaking seasonal-mean high temperature, 98 exceptional local heatwave days occurred during the summer of 2015. Based on diagnoses and simulations, this paper demonstrates that the record-high HST during the summer of 2015 arose mainly from the combined effects of the interannual and interdecadal variability of sea surface temperature anomalies (SSTAs). The interannual variability of SSTAs, with an El Niño–like pattern in the tropics and cold (warm) anomalies over the western (eastern) North Pacific, was the primary contributor to the abnormally high HST in the summer of 2015. This interannual tropical–extratropical SSTA pattern was accompanied by low-level southwesterly anomalies over the central North Pacific, which weakened the climatological northeasterly trade winds and reduced the ventilation effect, warming Hawaii. Numerical experiments further revealed that the SST warming in the subtropical eastern North Pacific was mostly responsible for the weakened trade winds and warming over Hawaii. Interdecadal SST warming in the tropics was a secondary factor. By superimposing the positive SSTAs over the Indo-Pacific warm pool and tropical North Atlantic Ocean upon the climatological-mean maximum SST regions, it was found that these anomalies led to enhanced convection over the Maritime Continent and the oceans around Mexico, causing anomalous subsidence and reduced cloud cover over the tropical central North Pacific. The reduced cloudiness increased the amount of downward solar radiation, thus warming Hawaii.

Multimodel Ensemble Sea Level Forecasts for Tropical Pacific Islands

AbstractSea level anomaly extremes impact tropical Pacific Ocean islands, often with too little warning to mitigate risks. With El Niño, such as the strong 2015/16 event, comes weaker trade winds and mean sea level drops exceeding 30 cm in the western Pacific that expose shallow-water ecosystems at low tides. Nearly opposite climate conditions accompany La Niña events, which cause sea level high stands (10–20 cm) and result in more frequent tide- and storm-related inundations that threaten coastlines. In the past, these effects have been exacerbated by decadal sea level variability, as well as continuing global sea level rise. Climate models, which are increasingly better able to simulate past and future evolutions of phenomena responsible for these extremes (i.e., El Niño–Southern Oscillation, Pacific decadal oscillation, and greenhouse warming), are also able to describe, or even directly simulate, associated sea level fluctuations. By compiling monthly sea level anomaly predictions from multiple statistical and dynamical (coupled ocean–atmosphere) models, which are typically skillful out to at least six months in the tropical Pacific, improved future outlooks are achieved. From this multimodel ensemble comes forecasts that are less prone to individual model errors and also uncertainty measurements achieved by comparing retrospective forecasts with the observed sea level. This framework delivers online a new real-time forecasting product of monthly mean sea level anomalies and will provide to the Pacific island community information that can be used to reduce impacts associated with sea level extremes.

A space–time statistical climate model for hurricane intensification in the North Atlantic basin

Abstract. Climate influences on hurricane intensification are investigated by averaging hourly intensification rates over the period 1975–2014 in 8° × 8° latitude–longitude grid cells. The statistical effects of hurricane intensity and sea-surface temperature (SST), along with the climatic effects of El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO) and the Madden–Julian Oscillation (MJO), are quantified using a Bayesian hierarchical model fit to the averaged data. As expected, stronger hurricanes tend to have higher intensification rates, especially over the warmest waters. Of the three climate variables considered, the NAO has the largest effect on intensification rates after controlling for intensity and SST. The model shows an average increase in intensification rates of 0.18 [0.06, 0.31] m s−1 h−1 (95 % credible interval) for every 1 standard deviation decrease in the NAO index. Weak trade winds associated with the negative phase of the NAO might result in less vertical wind shear and thus higher mean intensification rates.