ABSTRACT
The effect of climate change on the Eastern Mediterranean (EM) region, a region that reflects a transition between Mediterranean and semi‐arid climates, was examined. This transition region ...is affected by global changes such as the expansion of the Hadley cell, which leads to a poleward shift of the subtropical dry zone. The Hadley cell expansion forces the migration of jet streams and storm tracks poleward from their standard course, potentially increasing regional desertification. This article focuses on the northern coastline of Israel along the EM region where most wet synoptic systems (i.e. systems that may lead to precipitation) are generated. The current climate was compared to the predicted mid‐21st century climate based on Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway (RCP) RCP4.5 and RCP8.5 scenarios using four Coupled Model Intercomparison Project Phase 5 (CMIP5) models. A warming of 1.1–2.6 °C was predicted for this region. The models predicted that rain in the region will become less frequent, with a reduction of 1.2–3.4% in 6‐h intervals classified as wet synoptic systems and a 10–22% reduction in wet events. They further predicted that the maximum wet event duration in the mid‐21st century would become shorter relative to the current climate, implying that extremely long wet systems will become less frequent. Three of the models predicted shrinking of the wet season length by up to 15%. All models predicted an increasing occurrence frequency of Active Red Sea Troughs (ARSTs) for the RCP8.5 scenario by up to 11% by the mid‐21st century. For the RCP4.5 scenario, a similar increase of up to 6% was predicted by two of the models.
Interannual climate modes, especially the southern annular mode (SAM), significantly influence the wave climate modulation of the Indian Ocean (IO). The present study, aligned with previous research, ...identifies two crucial swell generation regions in the IO: the extratropical southern Indian Ocean (ETSI) and the tropical southern Indian Ocean (TSIO). The SAM, governing Southern Ocean surface winds, significantly shapes wave generation in these zones, thereby dominantly regulating IO wave conditions. Positive SAM phases shifts the westerlies poleward creating significant negative anomalies in the northern ETSI, reducing wave generation and swell propagation into the northern IO (NIO) basins, while the positive wind anomalies in the western TSIO creates a new swell generation area that directs swells towards the Arabian Sea, elevating wave heights there during monsoons. Conversely, negative SAM phases enhance TSIO easterlies, making it the primary IO swell source, increasing swell activity in the Bay of Bengal, notably during premonsoon and monsoon seasons. SAM impact expands beyond swells, influencing NIO wave climate by altering wind seas through Hadley cell (HC) circulation shifts. Positive SAM phases trigger NIO and midlatitude anomalous warming, intensifying HC and NIO surface winds in the next season, thereby affecting convection and subsequent sea surface temperature (SST) anomaly changes. The “SAM positive anomaly wind‐SST oscillations (SPAWSO)” pattern emerges, where warm SST anomalies in DJF and JJA precede increased surface winds in MAM and SON, thus increasing the wind‐sea conditions in the NIO. SPAWSO, hence, acts as a delayed mode, season‐dependent positive air–sea interaction cycle linked to positive SAM phases, significantly impacting NIO's wind‐sea dynamics. Thus the present study provides better insights into the long‐term wave prediction accuracy in the IO by considering the direct swell influence and SPAWSO‐driven wind‐sea changes, aiding preparations for changing wave dynamics and their impacts.
Southern annular mode (SAM) induces an anomalous cooling in the extratropics and warming in the midlatitudes. The temperature gradient strengthens during SAM positive phase and enhances the poleward expansion of the Hadley cell which weakens the wind over the Indian Ocean (IO). The weak winds over IO causes a decrease in wave height especially the north IO.
Abstract
Several atmospheric variables are directly influenced by oceanic conditions, mainly the sea surface temperature (SST). SST trends and variability over the southwestern Atlantic (SWA) have ...affected the South American climate, principally the monsoonal period (austral summer). This study analysed the SST trends over the SWA for all seasons. In addition, possible large‐scale triggers of the austral summer's warmest SST over the SWA and the straight impacts of these warmest SSTs on the South American climate were investigated. The results showed a significant positive SST trend of approximately 0.02°C·year
−1
over the SWA for all seasons. Since 2000, positive SST anomalies have increased in frequency in all seasons, with the seven warmest summers occurring during the 2010s. The highest SST summers over the SWA have been related to four leading causes: (i) planetary waves triggered by warm anomalies over the subtropical South Pacific; (ii) an omega‐type blocking configuration over the South Atlantic; (iii) a Southern Annular Mode positive phase; and (iv) a strengthening of the Hadley Cell, responding to warm SST over tropical North Atlantic. They all reflected on intensification of the South Atlantic Subtropical High and the western boundary current, heating the SWA. The summer's warmest SST led to positive air temperature anomalies extending in almost all of eastern South America, with significantly highest temperatures over southeastern Brazil, northern Argentina and western Paraguay. Clouds decrease strengthen the incident shortwave in southeastern Brazil, warming the region. Significant reduction of cloudiness and precipitation indicate a low performance of the South Atlantic Convergence Zone. The comparison of the warmest summers in 1980s–1990s in the SWA ratified the role of SST intensification. In conclusion, the positive SST trends over the SWA are related to the summer's warmest SST after 2000s, causing heating and dryness in eastern South America, mainly over southeastern Brazil.
Projected precipitation changes in a warming climate vary considerably, spatially, and between intensities. The changes can be greater or less than the ∼7% K−1 Clausius‐Clapeyron (CC) prediction, ...owing to dynamic effects. Using two global‐climate‐model large ensembles, we quantify the dynamically induced changes to precipitation extremes from the present (1996–2005) to late‐21st‐century (2071–2080) climates, as a function of recurrence interval, focusing on the subtropics. We separate non‐CC changes into a term proportional to the present‐day vertical‐velocity spatial pattern (i.e., an amplification or damping thereof by a constant factor) and a residual. The amplitude term varies with recurrence interval, approximately canceling (doubling) CC for moderate (large) extremes, increasing precipitation variability. Contrastingly, the residual is quasi‐uniform across recurrence intervals but spatially heterogeneous, weakening extremes over dry zones. This residual may be related to Hadley cell expansion, although this explanation is insufficient to explain many features, and other possible mechanisms are discussed.
Plain Language Summary
In a warming climate, the most extreme rainfall is projected to intensify, including in the subtropical dry regions, for example, Southern California and the Mediterranean. This will occur partly because a warmer atmosphere can hold more moisture, which implies more intense rainfall, but also because the vertical motion in the atmosphere that generates rainfall may undergo changes. We study the component relating to vertical motion in the subtropics and find that this results partly from a weakening/strengthening of vertical motion (weakening for moderate rainfall but strengthening for the most extreme rainfall). This effect will lead to a stronger contrast between the moderate rainfall events and the most extreme as the 21st century progresses. Thus, water resources in subtropical regions will become increasingly challenging to manage. In addition to this vertical‐motion effect, a poleward shift of the circulation will have a drying effect in certain dry regions on both moderate rainfall and the most extreme rainfall, which may offset the effect of strengthening vertical motion.
Key Points
The non‐CC scaling for subtropical extreme precipitation is broken down into the amplitude of the vertical‐velocity pattern, plus a residual
The amplitude term is spatially uniform, but transitions from negative (dampening) to positive (amplification) over the PDF of precipitation
The residual is quasi‐uniform over the PDF of precipitation, but not spatially uniform (weakens extremes just over the dry zones)
Super‐rotation affects—and is affected by—the distribution of dust in the martian atmosphere. We modeled this interaction during the 2018 global dust storm (GDS) of Mars Year 34 using data ...assimilation. Super‐rotation increased by a factor of two at the peak of the GDS, as compared to the same period in the previous year which did not feature a GDS. A strong westerly jet formed in the tropical lower atmosphere, with strong easterlies above 60 km, as a result of momentum transport by thermal tides. Enhanced super‐rotation is shown to have commenced 40 sols before the onset of the GDS, due to dust lifting in the southern mid‐latitudes and tropics. The uniform distribution of dust in the tropics resulted in a symmetric Hadley cell with a tropical upwelling branch that could efficiently transport dust vertically; this may have significantly contributed to the rapid expansion of the storm.
Plain Language Summary
Dust plays a major role in driving the behavior of the atmosphere of Mars. During a global dust storm (GDS), winds lift, and transport dust throughout the atmosphere; in turn, dust affects wind direction and strength by heating and cooling the surrounding air. Using a technique that combines satellite observations with simulations of the martian atmosphere, we demonstrate that winds at tropical latitudes were greatly strengthened during the 2018 GDS as a result of heating by dust. We show that tropical winds were strengthened even before the onset of the storm, as a result of a dust‐driven modification to the tropical circulation pattern. This change in the tropical circulation may have played a role in the formation of the GDS.
Key Points
The martian atmosphere was already in a state of enhanced super‐rotation prior to the onset of the Mars Year 34 global dust storm
Super‐rotation doubled during the peak of the storm and tropical easterlies were strongly enhanced above 60 km
Dust lifting in the southern hemisphere led to enhanced tropical heating and increased vertical dust transport in the lead‐up to the storm
We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. ...The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.
The teleconnection between the Asian monsoon system and North Atlantic forcing is an enduring prospect of the Earth’s climate. During the Holocene interstadial, the Indian summer monsoon showed ...asynchronous weakening links to ice rafting events documented in the North Atlantic region. However, the sensitivity of the Indian Winter Monsoon in response to North Atlantic cold spells is unclear due to a lack of compelling evidence. This study aims to extract the deglacial Indian Winter Monsoon signals using lithogenic tracers in coastal sediments and explore its association with the North Atlantic cooling episodes. A 5.1 m sediment core was retrieved from Pottuvil Lagoon in the southeastern coast of Sri Lanka, and the concentrations of K, Rb, Mg, Al, and Ti in 101 sub-sections were analysed using ICP-MS. The core- chronology was established by Bacon 2.2 age-depth modelling based on calibrated AMS 14C dates. The monsoon signal was reconstructed using element proxies and compared with the drift ice indices from the North Atlantic deep-sea sediments. Results revealed distinct phases of intense monsoon activity at 2553–2984 years BP, 3899–5021 years BP, and 5244–5507 years BP intervals with intermittent weak phases during 2253–2553, 2984–3899, and 5021–5244 years BP. The episodes of the intensified Indian Winter Monsoon coincided with Bond Events 2, 3, and 4, showing a strong coherence with the North Atlantic’s deglacial climate. Thus, on a millennial scale, North-Atlantic cooling has triggered intense winter monsoon conditions over the tropical Indian Ocean region from the mid to late Holocene. In comparison with regional monsoon archives, the Pottuvil winter monsoon record exhibits an anti-phase association with the Indian Summer Monsoon during Holocene ice-rafted debris events. The geochemical approach executed in this study could provide new insight into the millennial-scale pacing of the winter counterpart of the Indian monsoon links to climate extremes of high northern latitudes.
New climate simulations using the HadCM3L model with a paleogeography of the Late Jurassic (155.5Ma) and proxy-data corroborate that warm and wet tropical-like conditions reached as far north as the ...UK sector of the Jurassic Boreal Seaway (~35 degree N). This is associated with a northern hemisphere Jurassic Hadley cell and an intensified subtropical jet which both extend significantly poleward than in the modern (July-September). Deposition of the Kimmeridge Clay Formation (KCF) occurred in the shallow, storm-dominated, epeiric Boreal Seaway. High-resolution paleo-environmental proxy data from the Kimmeridge Clay Formation (KCF; ~155-150Ma), UK, are used to test for the role of tropical atmospheric circulation on meter-scale heterogeneities in black shale deposition. Proxy and model data show that the most organic-rich section (eudoxus to mid-hudlestoni zones) is characterized by a positive delta super(13)C sub(org) excursion and up to 37wt % total organic carbon (%TOC). Orbital modulation of organic carbon burial primarily in the long eccentricity power band combined with a clear positive correlation between %TOC carbonate-free and the kaolinite/illite ratio supports peak organic carbon burial under the influence of very humid climate conditions, similar to the modern tropics. This reinterpretation of large-scale climate relationships, supported by independent modeling and geological data, has profound implications for atmospheric circulation patterns and processes affecting marine productivity and organic carbon burial further north along the Boreal Seaway, including the Arctic. Key Points * Late Jurassic ITCZ at midtemperate latitudes * Kimmeridge Clay deposition (35 degree N to 54 degree N) was controlled by tropical climate * Organic carbon and clay patterns support strong orbital contrasts in humidity
It has been suggested that dust storms efficiently transport water vapor from the near‐surface to the middle atmosphere on Mars. Knowledge of the water vapor vertical profile during dust storms is ...important to understand water escape. During Martian Year 34, two dust storms occurred on Mars: a global dust storm (June to mid‐September 2018) and a regional storm (January 2019). Here we present water vapor vertical profiles in the periods of the two dust storms (Ls = 162–260° and Ls = 298–345°) from the solar occultation measurements by Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). We show a significant increase of water vapor abundance in the middle atmosphere (40–100 km) during the global dust storm. The water enhancement rapidly occurs following the onset of the storm (Ls~190°) and has a peak at the most active period (Ls~200°). Water vapor reaches very high altitudes (up to 100 km) with a volume mixing ratio of ~50 ppm. The water vapor abundance in the middle atmosphere shows high values consistently at 60°S‐60°N at the growth phase of the dust storm (Ls = 195°–220°), and peaks at latitudes greater than 60°S at the decay phase (Ls = 220°–260°). This is explained by the seasonal change of meridional circulation: from equinoctial Hadley circulation (two cells) to the solstitial one (a single pole‐to‐pole cell). We also find a conspicuous increase of water vapor density in the middle atmosphere at the period of the regional dust storm (Ls = 322–327°), in particular at latitudes greater than 60°S.
Plain Language Summary
The most striking phenomenon on Mars is a planet‐encircling storm, “global dust storm.” Once it starts, the floating dust covers the whole atmosphere for more than several weeks. Recent studies suggest that dust storms effectively transport water vapor from the near‐surface to the middle atmosphere. In June to September 2018 and January 2019, a strong global dust storm and a regional storm occurred on Mars, respectively. This study investigates altitude profiles of water vapor in the Mars atmosphere measured during the dust storms, by using brand‐new measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter. We confirm that the water vapor expanded into the middle atmosphere, and we find that the water vapor reached very high altitudes (up to 100 km) during the dust storms. The dust storms intensify the atmospheric dynamics and heat the atmosphere. As a result, water vapor is lifted to higher altitudes and distributes along the meridional circulation.
Key Points
We present vertical profiles of water vapor in the Martian atmosphere during global and regional dust storms in 2018‐2019
We show a rapid and significant increase of water vapor in the middle atmosphere (40‐100 km) during both global and regional dust storms
Water vapor reaches very high altitudes, at least around 100 km, during the global dust storm
One of the most robust responses of the climate system to future greenhouse gas emissions is the melting of Arctic sea ice. It is thus essential to elucidate its impacts on other components of the ...climate system. Here we focus on the response of the annual mean Hadley cell (HC) to Arctic sea ice loss using a hierarchy of model configurations: atmosphere only, atmosphere coupled to a slab ocean, and atmosphere coupled to a full‐physics ocean. In response to Arctic sea ice loss, as projected by the end of the 21st century, the HC shows negligible changes in the absence of ocean‐atmosphere coupling. In contrast, by warming the Northern Hemisphere thermodynamic coupling weakens the HC and expands it northward. However, dynamic coupling acts to cool the Northern Hemisphere which cancels most of this weakening and narrows the HC, thus opposing its projected expansion in response to increasing greenhouse gases.
Plain Language Summary
The climate's response to anthropogenic emissions comprises different feedbacks of the different components in the climate system. One of the robust responses to increased greenhouse gases is the melting of Arctic sea ice, which is found to have large effects on the hydrological circulation in the atmosphere. Here we examine the effect of Arctic sea ice loss on the tropical circulation. We find that under Arctic sea ice loss ocean heat transport acts to transfer the Arctic signal to the tropics and to contract the tropical circulation. This contraction opposes the projected widening of the tropical circulation and thus shows that Arctic sea ice loss acts as a negative internal feedback in the response of the tropical circulation to increased greenhouse gases.
Key Points
Arctic sea ice loss affects the Hadley cell only through ocean‐atmosphere coupling
Arctic sea ice loss acts as a negative internal feedback in the response of the Hadley cell width to anthropogenic emissions
Ocean heat transport opposes the thermodynamic effect of Arctic sea ice loss on the Hadley circulation
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