Ocean Atmosphere Feedback Research Articles

Influence of tropical cyclone Jawad on the surface and sub-surface circulation in the Bay of Bengal: ocean–atmosphere feedback

Influence of tropical cyclone Jawad on the surface and sub-surface circulation in the Bay of Bengal: ocean–atmosphere feedback

Sensitivity simulation of sea surface temperature variability in coastal waters off East Africa in relation to the Indian Ocean Dipole

East African coastal waters in the tropical western Indian Ocean experience strong seasonality which varies yearly, leading to the establishment of a prominent interannual Indian Ocean Dipole (IOD). This has a significant influence on regional and global socio-economic, climatic and human development. Sea surface temperature (SST) variability in these waters and its association with ocean–atmosphere feedbacks and internal subsurface ocean dynamics in relation to the IOD is the subject of this study. The research used reference simulation accompanied with sensitivity simulations with forcings from higher frequency variabilities against climatological signals of ocean–atmosphere or internal subsurface ocean dynamics from 1980 to 2007. Wind forcing with higher-frequency variabilities was applied in all simulations. Cooling and warming during pure and positive IOD events leading El Niño events in the region is caused by a combination of both surface heat fluxes associated with ocean–atmosphere feedbacks, and internal subsurface ocean dynamics, from July and peaking in August and October, respectively. Such processes also dominate the warming (cooling) in the region during pure El Niño (La Niña) events from July to December (July to October) where the SST patterns extend southwards. The warmest (coolest) SST anomalies during positive (negative) IOD events co-occurring with El Niño (La Niña) events stay longer than other events, being characterised by bimodal peaking in August and December. Such SST patterns are significantly forced with ocean–atmosphere feedbacks that might be associated with Walker circulation driving links between the Indian and Pacific oceans; however, the peaking in August might be enhanced by small ocean dynamics off the Somali coast, probably owing to the existing upwelling systems during these conditions.

Deglacial and Holocene sea-ice and climate dynamics in the Bransfield Strait, northern Antarctic Peninsula

Abstract. The reconstruction of past sea-ice distribution in the Southern Ocean is crucial for an improved understanding of ice–ocean–atmosphere feedbacks and the evaluation of Earth system and Antarctic ice sheet models. The Antarctic Peninsula (AP) has been experiencing a warming since the start of regular monitoring of the atmospheric temperature in the 1950s. The associated decrease in sea-ice cover contrasts the trend of growing sea-ice extent in East Antarctica. To reveal the long-term sea-ice history at the northern Antarctic Peninsula (NAP) under changing climate conditions, we examined a marine sediment core from the eastern basin of the Bransfield Strait covering the last Deglacial and the Holocene. For sea-ice reconstructions, we focused on the specific sea-ice biomarker lipid IPSO25, a highly branched isoprenoid (HBI), and sea-ice diatoms, whereas a phytoplankton-derived HBI triene (C25:3) and warmer open-ocean diatom assemblages reflect predominantly ice-free conditions. We further reconstruct ocean temperatures using glycerol dialkyl glycerol tetraethers (GDGTs) and diatom assemblages and compare our sea-ice and temperature records with published marine sediment and ice core data. A maximum ice cover is observed during the Antarctic Cold Reversal 13 800–13 000 years before present (13.8–13 ka), while seasonally ice-free conditions permitting (summer) phytoplankton productivity are reconstructed for the late Deglacial and the Early Holocene from 13 to 8.3 ka. An overall decreasing sea-ice trend throughout the Middle Holocene coincides with summer ocean warming and increasing phytoplankton productivity. The Late Holocene is characterized by highly variable winter sea-ice concentrations and a sustained decline in the duration and/or concentration of spring sea ice. Overall diverging trends in GDGT-based TEX86L and RI-OH' subsurface ocean temperatures (SOTs) are found to be linked to opposing spring and summer insolation trends, respectively.

On the ocean's response to enhanced Greenland runoff in model experiments: relevance of mesoscale dynamics and atmospheric coupling

Abstract. Increasing Greenland Ice Sheet melting is anticipated to impact water mass transformation in the subpolar North Atlantic and ultimately the meridional overturning circulation. Complex ocean and climate models are widely applied to estimate magnitude and timing of related impacts under global warming. We discuss the role of the ocean mean state, subpolar water mass transformation, mesoscale eddies, and atmospheric coupling in shaping the response of the subpolar North Atlantic Ocean to enhanced Greenland runoff. In a suite of eight dedicated 60- to 100-year-long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated and with regular and significantly enlarged Greenland runoff, we find (1) a major impact by the interactive atmosphere in enabling a compensating temperature feedback, (2) a non-negligible influence by the ocean mean state biased towards greater stability in the coupled simulations, both of which make the Atlantic meridional overturning circulation less susceptible to the freshwater perturbation applied, and (3) a more even spreading and deeper mixing of the runoff tracer in the subpolar North Atlantic and enhanced inter-gyre exchange with the subtropics in the strongly eddying simulations. Overall, our experiments demonstrate the important role of mesoscale ocean dynamics and atmosphere feedback in projections of the climate system response to enhanced Greenland Ice Sheet melting and hence underline the necessity to advance scale-aware eddy parameterizations for next-generation climate models.

Collective geographical ecoregions and precursor sources driving Arctic new particle formation

Abstract. The Arctic is a rapidly changing ecosystem, with complex ice–ocean–atmosphere feedbacks. An important process is new particle formation (NPF), from gas-phase precursors, which provides a climate forcing effect. NPF has been studied comprehensively at different sites in the Arctic, ranging from those in the High Arctic and those at Svalbard to those in the continental Arctic, but no harmonised analysis has been performed on all sites simultaneously, with no calculations of key NPF parameters available for some sites. Here, we analyse the formation and growth of new particles from six long-term ground-based stations in the Arctic (Alert, Villum, Tiksi, Zeppelin Mountain, Gruvebadet, and Utqiaġvik). Our analysis of particle formation and growth rates in addition to back-trajectory analysis shows a summertime maxima in the frequency of NPF and particle formation rate at all sites, although the mean frequency and particle formation rates themselves vary greatly between sites, with the highest at Svalbard and lowest in the High Arctic. The summertime growth rate, condensational sinks, and vapour source rates show a slight bias towards the southernmost sites, with vapour source rates varying by around an order of magnitude between the northernmost and southernmost sites. Air masses back-trajectories during NPF at these northernmost sites are associated with large areas of sea ice and snow, whereas events at Svalbard are associated with more sea ice and ocean regions. Events at the southernmost sites are associated with large areas of land and sea ice. These results emphasise how understanding the geographical variation in surface type across the Arctic is key to understanding secondary aerosol sources and providing a harmonised analysis of NPF across the Arctic.

ENSO–IOD Inter‐Basin Connection Is Controlled by the Atlantic Multidecadal Oscillation

AbstractThe interactions between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter‐basin connection is time‐varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections, including the La Niña‐like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO.

Coupled Oceanic and Atmospheric Controls of Deglacial Southeastern South America Precipitation and Western South Atlantic Productivity

Various mechanisms were proposed as substantial drivers of (sub)tropical South American hydroclimate changes during the last deglaciation. However, the interpretation of past precipitation records from the regions affected by the South American Summer Monsoon, the dominant hydroclimatic system in (sub)tropical South America, still insufficiently consider feedbacks between oceanic and atmospheric processes evident in modern observational data. Here, we evaluate ocean-atmosphere feedbacks active in the region from 19 to 4 ka based on a multi-proxy record comprising lipid biomarker, bulk sediment elemental composition and foraminiferal geochemistry from a sediment core retrieved from the tropical western South Atlantic offshore eastern Brazil at ~22°S. Our proxy data together with existing paleoclimate records show that the consideration of large scale synoptic climatic features across South America is crucial for understanding the past spatio-temporal rainfall variability, especially during the last deglaciation. While the paleohydrological data from our study site show relatively stable precipitation across the deglaciation in the core region of the South Atlantic Convergence Zone, distinct hydroclimatic gradients developed across the continent during Heinrich Stadial 1, which climaxed at ~16 ka. By then, the prevalent atmospheric and oceanic configuration caused more frequent extreme climatic events associated with positive rainfall in the northern portion of eastern South America and in the southeastern portion of the continent. These climatic extremes resulted from substantial warming of the sub(tropical) western South Atlantic sea surface that fostered oceanic moisture transport towards the continent and the reconfiguration of quasi-stationary atmospheric patterns. We further find that enhanced continental precipitation in combination with low glacial sea level strongly impacted marine ecosystems via enhanced terrigenous organic matter input in line with augmented nutrient release to the ocean. Extreme rainfall events similar to those that occurred during Heinrich Stadial 1 are likely to recur in South America as a consequence of global warming, because the projected reduction of the intra-hemispheric temperature gradient may lead to the development of atmospheric patterns similar to those in force during Heinrich Stadial 1.

Evolutionary History of DMSP Lyase-Like Genes in Animals and Their Possible Involvement in Evolution of the Scleractinian Coral Genus, Acropora

Dimethlysulfoniopropionate (DMSP) lyase is an enzyme that mediates cleavage of DMSP into dimethyl sulfide (DMS) and acrylate. DMS is an aerosol substance that may affect cloud formation, solar radiation and ocean temperatures. DMSP lyases in marine organisms, such as marine bacteria, release DMS, which might contribute to atmosphere-ocean feedback. Although DMSP lyases were first identified in marine bacteria, eukaryotic DMSP lyases or genes similar to DMSP lyase, DMSP lyase-like (DL-L) genes have been found not only in coccolithophores (Emiliania huxleyi) and symbiotic algae of the Family Symbiodiniaceae, but also in animals, including scleractinian corals (Cnidaria: Anthozoa: Hexacorallia). Comparative genomic analysis showed that gene expansion events of DL-L genes have occurred specifically in the scleractinian genus, Acropora. In the present study, we performed molecular identification of DL-L genes in Acropora digitifera. Thirteen full-length Open Reading Frames were isolated, confirming that these duplicated DL-L genes are likely expressed. A comprehensive survey of available transcriptomic databases revealed that DL-L genes have been identified not only in scleractinians (Hexacorallia), but also Octocorallia (Anthozoa) and even in a jellyfish (Cnidaria: Hydrozoa). Molecular phylogenetic analyses showed that although some sequences from cnidarian transcriptomic databases apparently originated with their symbiotic algae, cnidarian sequences from Anthozoa and Hydrozoa clustered together, indicating that these evolved from a gene in the last common ancestor of Cnidaria, dating to the Precambrian. Interestingly, cnidarian species possessing DL-L genes apparently occur only in coral reefs or shallow, warmer environments, suggesting that these genes may be essential for animals to survive in such environments. Acropora-specific duplicated DL-L genes, which originated during the past warm geological periods, may enable them to adapt to environmental changes.

The Equatorial Pacific Cold Tongue Bias in CESM1 and Its Influence on ENSO Forecasts

Abstract The mean-state bias and the associated forecast errors of the El Niño–Southern Oscillation (ENSO) are investigated in a suite of 2-yr-lead retrospective forecasts conducted with the Community Earth System Model, version 1, for 1954–2015. The equatorial Pacific cold tongue in the forecasts is too strong and extends excessively westward due to a combination of the model’s inherent climatological bias, initialization imbalance, and errors in initial ocean data. The forecasts show a stronger cold tongue bias in the first year than that inherent to the model due to the imbalance between initial subsurface oceanic states and model dynamics. The cold tongue bias affects not only the pattern and amplitude but also the duration of ENSO in the forecasts by altering ocean–atmosphere feedbacks. The predicted sea surface temperature anomalies related to ENSO extend to the far western equatorial Pacific during boreal summer when the cold tongue bias is strong, and the predicted ENSO anomalies are too weak in the central-eastern equatorial Pacific. The forecast errors of pattern and amplitude subsequently lead to errors in ENSO phase transition by affecting the amplitude of the negative thermocline feedback in the equatorial Pacific and tropical interbasin adjustments during the mature phase of ENSO. These ENSO forecast errors further degrade the predictions of wintertime atmospheric teleconnections, land surface air temperature, and rainfall anomalies over the Northern Hemisphere. These mean-state and ENSO forecast biases are more pronounced in forecasts initialized in boreal spring–summer than other seasons due to the seasonal intensification of the Bjerknes feedback.

Preceding winter Okhotsk Sea ice as a precursor to the following winter extreme precipitation in South China

The winter extreme precipitation over South China (SC) experiences a large year-to-year variability, causing uncertainty in its prediction. Here, we find that the boreal winter sea ice concentration (SIC) in the Okhotsk Sea can serve as a precursor to the following winter's extreme precipitation frequency (EPF) over SC, which has important implications for its prediction. Further analysis reveals that the Okhotsk Sea SIC anomalies help to reinforce North Pacific Oscillation-like atmospheric variability over the North Pacific, which induces the development of El Niño-Southern Oscillation (ENSO)-like SST anomalies in the equatorial eastern Pacific. The ENSO may act as a “power amplifier” to boost the impact of the Okhotsk Sea winter SIC anomalies on the following winter EPF over SC via a positive atmosphere–ocean feedback process. Our findings suggest that the Okhotsk Sea SIC may act as a potential precursor for the winter EPF over SC leading by about 1 year, and further improve our understanding of extratropical-tropical interactions and aid predictability of winter extreme precipitation over SC.

The 1600 CE Huaynaputina eruption as a possible trigger for persistent cooling in the North Atlantic region

Abstract. Paleoclimate reconstructions have identified a period of exceptional summer and winter cooling in the North Atlantic region following the eruption of the tropical volcano Huaynaputina (Peru) in 1600 CE. A previous study based on numerical climate simulations has indicated a potential mechanism for the persistent cooling in a slowdown of the North Atlantic subpolar gyre (SPG) and consequent ocean–atmosphere feedbacks. To examine whether this mechanism could have been triggered by the Huaynaputina eruption, this study compares the simulations used in the previous study both with and without volcanic forcing and this SPG shift to reconstructions from annual proxies in natural archives and historical written records as well as contemporary historical observations of relevant climate and environmental conditions. These reconstructions and observations demonstrate patterns of cooling and sea-ice expansion consistent with, but not indicative of, an eruption trigger for the proposed SPG slowdown mechanism. The results point to possible improvements in future model–data comparison studies utilizing historical written records. Moreover, we consider historical societal impacts and adaptations associated with the reconstructed climatic and environmental anomalies.

The role of the Pacific Decadal Oscillation and ocean-atmosphere interactions in driving US temperature predictability

Heatwaves can have devastating impact on society and reliable early warnings at several weeks lead time are needed. Previous studies showed that north-Pacific sea surface temperatures (SST) can provide long-lead predictability for eastern US temperature, mediated by an atmospheric Rossby wave. The exact mechanisms, however, are not well understood. Here we analyze two different Rossby waves associated with temperature variability in western and eastern US, respectively. Causal discovery analyses reveal that both waves are characterized by positive ocean-atmosphere feedbacks at daily timescales. Only for the eastern US, a long-lead causal link from SSTs to the Rossby wave exists, which generates summer temperature predictability. We show that this SST forcing mechanism originates from the evolution of the winter-to-spring Pacific Decadal Oscillation (PDO). During pronounced winter-to-spring PDO phases (either positive or negative) eastern US summer temperature forecast skill more than doubles, providing a temporary window of enhanced long-lead predictability.

Late Holocene Orbital Forcing and Solar Activity on the Kuroshio Current of Subtropical North Pacific at Different Timescales

The North Pacific subtropical gyre (NPSTG) redistributes heat and moisture between low and high latitudes and plays a key role in modulating the global climate change and ecosystem. Recent evidence suggests intensification and poleward shift of the subtropical gyres over the last decades due to global warming, but insufficient observations have hampered insight into the integrated effects of ocean-atmosphere interactions at longer timescales. Here we present the first high-resolution (∼12 years) grain-size record from Core CF1 in the Okinawa Trough, western subtropical North Pacific, to reconstruct the evolution of the western boundary Kuroshio Current (KC) of NPSTG during the Late Holocene. Our results indicate the KC slow-down during 4.6–2.0 ka, followed by quick enhancement after 2.0 ka, with centennial-scale variabilities (500–700 years) superimposed on the long-term trend. Over millennial timescales, gradually increased pole-to-equator thermal gradient, due to orbital forcing mechanisms, resulted in long-term enhanced KC, whereas solar activity triggered phase changes in the tropical Pacific mean state and controlled KC anomalies on centennial timescales. We suggest that both forcing mechanisms resulted in ocean-atmosphere feedback provoking concurrent changes in mid-latitude westerly and subtropical easterly winds over the North Pacific, alternating their dominance as source regions causing the dynamic changes of KC at different timescales. Our findings offer insight into the role of external forcing mechanisms in the NPSTG changes before the Anthropocene, which have profound implications for the deeper understanding of changes in ocean gyres under global warming scenarios.

Barotropic energy conversion during Indian summer monsoon: implication of Central Indian Ocean Mode Simulation in CMIP6

The simulation and prediction of the Indian summer monsoon (ISM) and its intraseasonal component in climate models remain a grand scientific challenge for numerical simulations. Recently, an intraseasonal mode was proposed over the tropical Indian Ocean, named central Indian Ocean (CIO) mode. The CIO mode index and the monsoon intraseasonal oscillations (MISO) have a high correlation. In this study, the simulations of the CIO mode in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) models are examined. Although the coupled ocean–atmosphere feedbacks associated with the CIO mode are not fully reproduced, the results show that a better depiction of the CIO mode in CMIP6 models is favorable for a better simulation of northward-propagating MISO and heavy rainfall during the ISM. Dynamic diagnostics unveil that the rendition of the CIO mode is dominated by kinetic energy conversion from the background to the intraseasonal variability. Furthermore, kinetic energy conversion is controlled by the meridional shear of background zonal winds (\(\frac{\partial \overline{u}}{\partial y }\)), which is underestimated in most CMIP6 models, leading to a weak barotropic instability. As a result, a better simulation of \(\frac{\partial \overline{u}}{\partial y }\) is required for improving the CIO mode simulation in climate models, which helps to improve the simulation and prediction skill of northward-propagating MISO and monsoonal precipitation.

Air-sea coupling shapes North American hydroclimate response to ice sheets during the Last Glacial Maximum

The Western U.S. is vulnerable to hydrological stress, and insights from past climate periods are helpful for providing historical benchmarks for future climate projections. Myriad evidence from coupled models and paleoclimatic proxies suggests a major reorganization of west coast hydroclimate during the Last Glacial Maximum (LGM, ∼17–25 ka), such that the Southwest U.S. was wetter than modern day and the Pacific Northwest was drier. Yet the fundamental mechanisms underlying these hydroclimatic shifts remain unclear. Here, we employ a suite of targeted model simulations to probe the influence of LGM Northern Hemisphere ice sheets on west coast atmospheric dynamics. Whereas previous modeling studies have suggested that the southward shift of LGM west coast precipitation was driven only by the mechanical steering of atmospheric circulation by elevated ice sheet topography, we find this to be an artifact of earlier simulations that neglected realistic air-sea interaction. Instead, our simulations indicate that ice sheet albedo induced a pattern of North Pacific sea surface temperatures, reinforced by ocean-atmosphere feedbacks, that shifted the large-scale atmospheric circulation as well as the latitudinal distribution of west coast precipitation southward during the LGM. Crucially, we find that atmosphere-ocean feedbacks that sustained this ice sheet albedo-induced temperature pattern in the LGM could drive similar hydroclimatic changes today.

Projections of changes in ecosystem productivity under 1.5 °C and 2 °C global warming

The ecosystem plays an important role in mitigating global warming through carbon assimilation. In addition, climate change has both positive and negative impacts on ecosystem productivity. To combat climate change, the Paris Agreement has set the goal of limiting global warming well below 2 °C with an ultimate target of 1.5 °C above preindustrial levels. It remains unclear how ecosystem productivity will respond to these warming targets. Here, we project global changes in gross (GPP) and net (NPP) primary productivity under 1.5 °C/2 °C warming using a dynamic vegetation model coupled with a state-of-art climate model. We use site-level observations to validate the present-day simulations and the multimodel ensemble to validate the future projections. We identify the main drivers of ecosystem changes by distinguishing between the impacts of CO 2 fertilization, fast climatic responses to CO 2 radiative perturbation, and slow climatic feedbacks to ocean warming due to the large heat capacity. Compared with the present day, global GPP increases by 11.5% (20.9%) and NPP increases by 14.8% (27.1%) under the 1.5 °C (2 °C) warming scenario with similar magnitudes of change in the tropics and subtropics. Changes in the leaf area index mainly follow those of GPP/NPP but with more significant enhancement in boreal regions. CO 2 fertilization causes the dominant contributions to GPP changes over 83% of land areas. The slow responses to ocean warming cause negative changes in GPP/NPP and partly offset regional CO 2 fertilization over the tropics, where the climate is projected to be drier due to atmosphere-ocean feedback. The fast climatic responses to CO 2 radiative perturbation further decrease GPP/NPP but with a moderate magnitude. The projection suggests that ecosystem productivity will increase in a warmer climate but with large spatial heterogeneity. • Global GPP increases by 11.5% under 1.5 °C and 20.9% under a 2 °C warming world. • Fast responses to CO 2 fertilization cause dominant and uniform GPP enhancement. • Slow responses to the warming of sea surface temperature reduce tropical GPP.

Impacts of UV Irradiance and Medium-Energy Electron Precipitation on the North Atlantic Oscillation during the 11-Year Solar Cycle

Observational studies suggest that part of the North Atlantic Oscillation (NAO) variability may be attributed to the spectral ultra-violet (UV) irradiance variations associated to the 11-year solar cycle. The observed maximum surface pressure response in the North Atlantic occurs 2–4 years after solar maximum, and some model studies have identified that atmosphere–ocean feedbacks explain the multi-year lag. Alternatively, medium-to-high energy electron (MEE) precipitation, which peaks in the declining phase of the solar cycle, has been suggested as a potential cause of this lag. We use a coupled (ocean–atmosphere) climate prediction model and a state-of-the-art MEE forcing to explore the respective roles of irradiance and MEE precipitation on the NAO variability. Three decadal ensemble experiments were conducted over solar cycle 23 in an idealized setting. We found a weak ensemble-mean positive NAO response to the irradiance. The simulated signal-to-noise ratio remained very small, indicating the predominance of internal NAO variability. The lack of multi-annual lag in the NAO response was likely due to lagged solar signals imprinted in temperatures below the oceanic mixed-layer re-emerging equatorward of the oceanic frontal zones, which anchor ocean–atmosphere feedbacks. While there is a clear, yet weak, signature from UV irradiance in the atmosphere and upper ocean over the North Atlantic, enhanced MEE precipitation on the other hand does not lead to any systematic changes in the stratospheric circulation, despite its marked chemical signatures.

Impact of surface roughness parameterizations on tropical cyclone simulations over the Bay of Bengal using WRF-OML model

The impact of different surface roughness schemes in air-sea coupling on simulating tropical cyclones (TCs) over the Bay of Bengal is analyzed using Weather Research and Forecasting-Ocean Mixed layer (WRF-OML) modeling system. The sensitivity of three surface roughness schemes is tested by conducting three experiments for seven TCs namely Phailin (2013), Lehar (2013), Hudhud (2014), Vardah (2016), Gaja (2018), Fani (2019) and Amphan (2020). The first experiment (Opt0) is configured with surface drag and moist enthalpy from Garratt formulations, the second (Opt1) uses surface drag from Donelan and constant moist enthalpy formulations and the third experiment (Opt2) employs the modified moist enthalpy from Garratt formulations along with Donelan drag. Results of predicted track, intensities, precipitation and structure of the cyclones are highly sensitive to the surface exchange coefficients formulated through the roughness parameterization. The Opt2 followed by Opt1 experiments captured the deepening and mature phases of the storm close to the observed estimates for both pre- and post-monsoon TCs, by improving the ocean-atmosphere feedback of surface energy fluxes. Comparison of simulated moisture convergence, transport and precipitation with observations for all cyclones suggest that the Opt2 provides an improved representation of surface enthalpy fluxes playing a crucial role in the moisture transport and cloud microphysical processes. The improved simulation of TCs with Opt1 and Opt2 over Opt0 is attributed to the realistic simulation of mixed layer deepening and sea surface temperature cooling effects in the model. An interesting result found from the WRF-OML simulations with three roughness schemes is the maximum moisture transport is mainly concentrated on the right sectors while the moisture convergence and precipitation are distributed on the left sectors of the cyclones.

Circulation adjustment in the Arctic and Atlantic in response to Greenland and Antarctic mass loss

Following a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv (= 10^6 m ^3s ^{-1}), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65^{circ }N and 70^{circ }N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response.

The North Pacific Blob acts to increase the predictability of the Atlantic warm pool

The Atlantic warm pool (AWP) has profound impacts on extreme weather events and climate variability. Factors influencing the AWP and its predictability are still not fully understood. Other than local ocean–atmosphere feedbacks and El Niño Southern Oscillation, we find an extratropical precursor from the Northeast Pacific (known as the Blob), which leads the AWP by 1 year with a robust correlation (r = 0.68). A suite of Northeast Pacific pacemaker experiments successfully reproduces the leading influence of the Blob on the AWP. The preceding summer Blob-related sea surface temperature (SST) warming signal can be transmitted towards the lower latitudes through the seasonal footprint mechanism, leading to the central Pacific warming in the winter and following spring. Such a strong tropical Pacific SST heating excites an anomalous atmospheric wave train that resembles the Pacific/North American (PNA) teleconnection pattern. At the downstream portion of the PNA, the low sea surface pressure anomalies can be found over the AWP region during the following spring. The anomalous low initiates the AWP SST warming, and the AWP warmer SST can persist into summer and is further amplified due to ocean–atmosphere feedbacks. Our results show that the North Pacific Blob may act as a useful predictor of the AWP 1 year in advance through trans-basin interactions. A Blob-based prediction model shows considerable hindcast skill for the observed AWP SST anomaly.