The migration of humans into the eastern Pacific was relatively rapid and focused around 900-1250 CE. Although the causes for this migration are varied, we put forward evidence to suggest that a change in the mean state of the tropical South Pacific from La Nina like to El NinAo like conditions accompanied by an increase in climate "shocks" around the period of migration, could have created conditions to promote migration east into the Pacific. We use a range of sediment archives and hydroclimate proxies located in sites within the region of migration, to reconstruct climate conditions in the 'sending' islands, and 'receiving' archipelagos. Climate in the period immediately prior to the eastward migration was characterised by a drier southwest Pacific. During the period following settlement of the central region of eastern Polynesia, the mean climate state in receiving islands became wetter, with fewer climate "shocks".Results from socio-hydrological models highlight the sensitivity of growing populations to droughts within a drying climate. Using these lines of evidence, we suggest that as populations grew, particularly in drought-sensitive islands, people with some knowledge of eastern "Gateway islands", chose to move east, fortuitously at a time when wetter conditions supported their long-term settlement.

Convectively coupled equatorial Rossby waves (CCERWs) are an intrinsic part of the spectrum of tropical weather systems, and can bring extreme precipitation to tropical locations. They are usually interpreted as modified versions of the theoretical dry equatorial Rossby wave solutions of the shallow water equations. However, the structure and dynamics of CCERWs are rather different to their theoretical cousins. Here, a vorticity budget is presented for both theoretical equatorial Rossby waves and for CCERWs (based on reanalysis data). The different strengths of the vorticity budget terms between the theoretical waves and CCERWs gives insights into CCERW propagation and growth mechanisms, and provides a focus and testbed for future model and forecast improvements.

Previous work has explained the physical mechanisms behind nocturnal offshore propagation of convection southwest of Sumatra. Low‐level moisture flux convergence due to the land breeze front controls the progression of convection, typically a squall line, away from the coast overnight. However, the diurnal convection over the mountains occurs on only 57% of days in December–February (DJF) and propagates offshore on only 49% of those days. We investigate day‐to‐day variability in dynamical and thermodynamical conditions to explain the variability in diurnal convection and offshore propagation, using a convection‐permitting simulation run for 900 DJF days. A convolutional neural network is used to identify regimes of the diurnal cycle and offshore propagation behavior. The diurnal cycle and offshore propagation are most likely to occur ahead of an active Madden‐Julian oscillation, or during El Niño or positive Indian Ocean Dipole; however, any regime can occur in any phase of these large‐scale drivers, because the major control arises from the local scale. When the diurnal cycle of convection occurs over land, low‐level wind is generally onshore, providing convergence over the mountains, and low‐level humidity over the mountains is high enough to make the air column unstable for moist convection. When this convection propagates offshore, midlevel offshore winds provide a steering flow, combined with stronger convergence offshore due to more onshore environmental winds. Low‐level moisture around the coast also means that as the convection propagates, the storm‐relative inflow of air into the system adds greater instability than would be the case on other days.

The South Pacific Convergence Zone is a band of intense austral summer rainfall in the tropical Pacific. Changes in the South Pacific Convergence Zone are linked to Pacific sea surface temperatures on decadal timescales, but its behaviour and impacts over longer timescales remain poorly understood due to limited proxy records and model uncertainties. We combine new plant wax hydroclimate records with existing proxy evidence and climate model simulations to investigate South Pacific Convergence Zone changes over the past 1500 years. Our findings indicate that between 1000 and 200 years ago, the eastern South Pacific Convergence Zone became wetter while the western part became drier. Model simulations suggest that these centennial-scale changes were driven by Pacific sea surface temperature gradients. This eastward shift coincides with Polynesian colonisation, implying hydroclimate shifts both 'pushed' migration eastward and 'pulled' successful eastern settlement.

Mesoscale ocean eddies contribute to the mixing and transport of water properties throughout the global ocean. Sea surface temperature anomalies associated with these eddies can influence atmospheric boundary layer stability, and thus the formation of clouds. The Maritime Continent experiences the modulation of convection and precipitation by processes operating over multiple spatial and temporal scales. However, mesoscale air-sea interactions, such as those associated with the eddies the region generates, remain understudied. Applying a sea surface height-based eddy detection and tracking algorithm, we show that lower latitude eddies, such as those in the Maritime Continent, are generally fewer in number, weaker, and shorter-lived, but larger and faster-propagating, compared to those at higher latitudes. Crucially, we highlight that eddies in the Maritime Continent can significantly modify air-sea heat exchange and the near-surface wind field. However, changes to column water vapor, cloud, and rainfall are less distinct. Compared to the Kuroshio Extension, a representative case study for the extratropics, atmospheric anomalies associated with eddies in the Maritime Continent are weaker, and decreasing in magnitude toward the lower latitudes. We hypothesize that weaker sea surface temperature anomalies associated with eddies in the Maritime Continent, coupled with their faster propagation and intraseasonal variability in convection over the region, reduce the likelihood and intensity of the instantaneous atmospheric imprint. This study therefore emphasizes the importance of the spatial and temporal scales with regard to air-sea interactions and their influence on cloud and rainfall across the Maritime Continent.

The extratropical response to the Madden-Julian Oscillation (MJO) is modulated by two prominent modes of low-frequency sea surface temperature (SST) variability: the Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO). Utilizing the UK Earth System Model (UKESM) 1100 year pre-industrial control simulation from CMIP6, this study offers a unique opportunity to explore decadal variability with an extensive dataset, surpassing the limitations of previous studies which focussed on reanalysis products. The results underscore a statistically significant influence of both AMV and PDO on the extratropical response across all MJO phases. Non-linear interactions between the MJO teleconnection and SST forcing are observed prominently in the modification of the response to MJO phase 6 (enhanced convection over the western Pacific), with AMV+ and PDO+ background states amplifying distinct teleconnection patterns, notably the negative North Atlantic Oscillation (NAO-) and the deepened Aleutian Low responses, respectively. These changes are greater in magnitude than would be expected from the linear superposition of the individual atmospheric responses to the SST mode and the MJO. The amplification of the MJO phase 6 teleconnection to the North Atlantic aligns with prior research based on ERA5 reanalysis data. While modulation of the response to MJO phase 3 (enhanced convection over the eastern Indian Ocean) is evident, it is less pronounced compared to phase 6, and the mechanisms via which it acts are less clear. Intriguingly, alterations in the teleconnection, such as a weaker Aleutian Low during PDO+, contradict the anticipated modulation. Since MJO phase 3 and PDO+ tend to weaken and strengthen the Aleutian Low, respectively, it would be reasonable to expect that these effects would cancel. Instead, the weakening of the Low after MJO phase 3 is increased during PDO+. A possible mechanism for the modulation of the teleconnections is a linear superposition of Rossby wave modes excited by the MJO, contingent upon the SST state. In the case of MJO phase 6, this corresponds to an amplification of the existing modes, and hence of the expected response. For MJO phase 3, however, there is an indication that other Rossby wave modes may also be excited in certain SST states, leading to interference which is out of phase with the primary response. Acknowledging the limitations of observational and reanalysis datasets, this study underscores the pivotal role of climate models in the effective study of decadal and multi-decadal variability. Importantly, the study has significant implications for extratropical forecasting over the coming decades. The modulation of the MJO teleconnection by AMV and PDO suggests modifications in predictability, crucial for refining forecasting techniques. Furthermore, these results provide a contextual foundation for studies examining MJO teleconnections in future climates, enabling a more accurate dissection of responses influenced by internal and anthropogenically forced variability.

Convectively coupled Kelvin waves (CCKWs) are eastward‐propagating weather systems that organise convection locally and are linked to precipitation extremes across the Maritime Continent (MC). They are often embedded in active Madden–Julian Oscillation (MJO) phases. The MJO also propagates eastwards, but influences convection in the MC over longer time‐scales and larger areas. This article examines a case study during July 2021 of multiple CCKWs and westward‐propagating inertio‐gravity waves (WIGs) embedded within an active MJO. The final CCKW traversed the western MC, causing precipitation extremes across equatorial Indonesia and East Malaysia that led to numerous reports of flooding and landslides, with western Borneo the worst‐affected region. The MJO event was terminated abruptly following the passage of this CCKW. Analysis of the total column water budget reveals that the precipitation rate exceeded the vertically integrated moisture‐flux convergence provided by the CCKW, drying out the atmosphere and suppressing further convection. The performance of the UK Met Office prediction model was evaluated for this case study; parameterised convection configurations generally performed as well as or better than explicit convection models. This is possibly because they represented better the location and timing of the convective systems that developed because of interactions between CCKWs and WIGs. This research highlights how CCKWs should be viewed, not simply as convective systems that affect weather locally but, as having the potential to deliver larger‐scale impacts over the entire equatorial MC, as part of a complex multiscale interaction. Such interactions can involve the MJO influencing CCKWs downscale by providing enhanced convection. Conversely, the suppressed phase of CCKWs can dampen the MJO convective signal and terminate MJO propagation. Whilst weather prediction models may forecast rainfall associated with individual equatorial modes accurately, capturing their combined effect remains a challenge.

The South Pacific Convergence Zone (SPCZ) dominates the climate dynamics of the tropical South Pacific, significantly influencing global climate dynamics. The magnitude, slope, and spatial extent of the SPCZ's precipitation pattern, are responsive to changes in both trade winds and the Southern Hemisphere subtropical jet. These changes are, in turn, driven by modes such as the MJO and ENSO. Whilst the drivers of SPCZ variability on subseasonal to interannual timescales are well-documented, understanding of its variability over multi-decadal to millennial timescales remains limited. Quantitative reconstructions of South Pacific hydroclimate reveal rapid, and as yet unexplained, changes in the SPCZ over recent millennia that do not align with large-scale climate forcing. This study proposes that large-scale multi-decadal modes of variability may play a crucial role in driving SPCZ variability. Using an Intermediate General Circulation Model (IGCM4), we investigate the SPCZ's response to sea surface temperature (SST) forcing corresponding to the Atlantic Multi-decadal Variability (AMV) and Interdecadal Pacific Oscillation (IPO). Applying monthly varying SST anomalies at a range of magnitudes and in different combinations, we assess the linearity of the SPCZ response to these multi-decadal modes. Additionally, we explore the pathways associated with each response by applying SST anomalies both globally and within the respective basin. This research aims to enhance our understanding of the interaction between the SPCZ and multi-decadal modes of variability, providing insights into past changes in the proxy record and contributing to the development of future SPCZ projections.

Convectively coupled equatorial Kelvin waves (CCKWs) are eastward propagating weather systems that locally organise convection and have been linked to precipitation extremes across the Maritime Continent (MC). They are often embedded in convectively active phases of the Madden-Julian Oscillation (MJO) which too propagates eastwards but influences convection in the MC over longer timescales and larger areas. Previous high impact weather case studies have linked CCKWs to local precipitation extremes. In this study, we examine a case study during July 2021 of multiple CCKWs embedded within an active MJO. The final CCKW traversed the western MC causing precipitation extremes across equatorial Indonesia that lead to numerous reports of flooding and landslides, with the West Kalimantan region the worst affected. The MJO event itself was abruptly terminated following the passage of this CCKW. Through analysis of the moisture budget we find that the rainfall exceeded the convergence of moisture to produce the pronounced drying. Prior to the local MJO termination, we find there was enhanced westward propagating diurnal activity across the equatorial MC coinciding with a steady increase of total column water. We also examine observations of the extreme rainfall event in the West Kalimantan province. Comparing different deterministic model configurations, we find that the convection permitting models generally perform better when there are not multiple CCKWs present within the initial conditions. This research highlights how CCKWs should not simply be viewed as convective systems that locally affect weather but have the potential to have devastating impacts over the entire equatorial MC especially when involved in multiscale interactions both with the diurnal cycle and with the MJO.

The diurnal cycle of SST (dSST) is influenced by the development of diurnal warm layers in the upper ocean. Observations show that the dSST rectifies intraseasonal SSTs, potentially leading to changes in intraseasonal weather patterns such as the Madden-Julian Oscillation (MJO). Here we analyze 15-day forecast composites of the coupled ocean-atmosphere and the atmosphere-only configurations of the Numerical Weather Prediction (NWP) models of the UK Met Office to show that a strong dSST in the coupled model leads to a faster MJO propagation compared with the atmosphere-only version of the model. A set of experiments using the coupled model was designed to reduce the strength of the dSST by imposing instant vertical mixing in the top 5 and 10 m of the ocean model. On a 15 lead-day time scale, weakening the dSST slows the MJO phase speed in the coupled model. On a 7 lead-day time scale, all coupled model runs display an underlying 5% increase in the MJO phase speed compared to the atmosphere-only model due to the presence of thermodynamic coupling unrelated to the dSST. The MJO phase speed increase due to the dSST is linearly related to the mean tropical dSST at lead day 1 in the coupled model. An additional 4% of the MJO phase speed increase between the control coupled model and the atmosphere-only model on a 7 lead-day timescale can be attributed to the presence of the dSST in the coupled model. Over 15 lead days, the coupled model produces a two-way feedback between the MJO and the dSST. The MJO conditions set the strength of the dSST in the coupled model. Consistent with observations, the dSST in the coupled model rectifies intraseasonal anomalies of SSTs such that stronger dSST leads to positive intraseasonal SST anomalies. The MJO convection response to these SST anomalies peaks 7 days later, and subsequently feeds back onto SST anomalies. The phase relationship between MJO convection, dSST and intraseasonal SST anomalies is consistent with the relationship between dSST and MJO propagation speed. Overall, our experiments demonstrate the importance of high vertical resolution of the upper ocean in predicting the eastward propagation of the MJO in an NWP setting, potentially creating repercussions for seasonal predictions and climate projections should this feedback be unrepresented in the models.

The Maritime Continent (MC) is the rainiest region on Earth, where extreme precipitation constitutes a major hazard. Convectively coupled Kelvin waves (CCKWs) are weather systems that travel eastwards across the equatorial waveguide and can trigger convection in their convergent phase. CCKWs are linked to up to a fourfold increase in precipitation rates across the equatorial Maritime Continent (Ferrett et al., 2020). However, not all CCKWs produce precipitation extremes. Recent studies reveal that CCKWs arriving in phase with the local diurnal cycle of convection may be more likely to cause high impact weather events or when part of a multiscale interaction with the MJO, organising the large-scale precipitation on a more localised scale (Baranowski et al., 2016; Baranowski et al., 2020; Latos et al., 2021; Senior et al., 2023).Current methods for studying these mechanisms have some limitations. For example, composite studies of CCKWs are useful for revealing statistical links but smooth out key interactions. Case studies are useful for identifying mechanisms in particular high-impact weather events but are difficult to generalise. Modelling such high-impact weather events provides additional insights; however, lacks the capability for fine-tuning.Hence, we have developed a methodology for introducing synthetic CCKWs into convection-permitting Met Office Unified Model (MetUM) forecasts. This involves generating 3D CCKW structures on key dynamical fields using ERA5 data and adding these to the model’s initial conditions. The methodology will be presented, and a comparison of diagnostics from control and perturbation experiments will be provided. We will then discuss how the methodology will be applied to studying the mechanisms through which CCKWs cause precipitation extremes across various locations in the MC. Since CCKWs are an important dynamical predictor of extreme precipitation, understanding these mechanisms is crucial for providing accurate forecasts of hazardous weather in the MC.Baranowski, D.B. et al., (2016) Phase locking between atmospheric convectively coupled equatorial Kelvin waves and the diurnal cycle of precipitation over the Maritime Continent. Geophysical Research Letters, 43(15), 8269–8276. https://doi.org/10.1002/2016GL069602.Baranowski, D.B. et al., (2020) Social-media and newspaper reports reveal large-scale meteorological drivers of floods on Sumatra. Nature Communications, 11, 2503. https://doi.org/10.1038/s41467-020-16171-2.Ferrett, S. et al., (2020) Linking extreme precipitation in Southeast Asia to equatorial waves. Quarterly Journal of the Royal Meteorological Society, 146(727), 665–684. https://doi.org/10.1002/qj.3699.Latos, B. et al., (2021) Equatorial waves triggering extreme rainfall and floods in Southwest Sulawesi, Indonesia. Monthly Weather Review, 149(5), 1381–1401. https://doi.org/10.1175/MWR-D-20-0262.1.Senior, N.V. et al., (2023) Extreme precipitation at Padang, Sumatra triggered by convectively coupled Kelvin waves. Quarterly Journal of the Royal Meteorological Society, 149(755), 2281–2300. https://doi.org/10.1002/qj.45

The Madden‐Julian oscillation (MJO) exerts a downscale influence on the diurnal cycle (DC) of precipitation over the Maritime Continent (MC). We assess the characteristics of this downscale influence in GPM‐IMERG data across the western MC, comparing the MJO cycles of daily mean precipitation, DC amplitude, DC timing, and additional diurnal characteristics. During a typical MJO event, islands and surrounding waters experience their greatest DC amplitude 2–4 days ahead of their greatest daily mean precipitation. The MJO has a greater influence on daily mean precipitation over water and on DC amplitude over land. Greatest DC amplitude over land leads greatest DC amplitude over surrounding waters by 3–6 days. Diurnal cycle timing varies systematically by MJO phase in most locations, particularly eastern Sumatra, eastern Borneo and the eastern Makassar Strait where the diurnal maximum may systematically vary in timing by over 4 hours. Over these regions, the diurnal maximum propagates westward before, and eastward after, the active MJO crosses the western MC. As the active MJO crosses, the diurnal maximum gets earlier across western land on large islands, and later across eastern land, creating a west‐east regime divide in DC timing variability. Additional diurnal characteristics quantify further changes in the nature of the diurnal oscillation. MJO‐induced changes in the diurnal timing of convective cloud cover may influence local radiation budgets. These results provide a detailed benchmark for the modulation of the DC by the MJO against which this scale interaction in models may be assessed.

The South Pacific Convergence Zone (SPCZ) is a critical region of intense precipitation, particularly during the austral summer (November—March), situated in the tropical Pacific Ocean. This region is known to be highly sensitive to variations in sea surface temperatures (SSTs) across the Pacific. Previous research has demonstrated that decadal-scale shifts in the SPCZ are influenced by SST variability, but our understanding of longer-term changes—spanning multi-decadal to millennial timescales—remains limited. These constraints arise from the scarcity of high-resolution hydroclimate proxy records and systemic biases in even state-of-the-art coupled climate models.To address this knowledge gap, we combine newly developed hydroclimate proxies (from Nuku Hiva and Tahiti, French Polynesia) with previously established proxies and novel climate model simulations to explore SPCZ dynamics over the last 1500 years. Our findings provide new insights into centennial- and millennial-scale precipitation variability and its potential drivers.Our proxy data indicate a spatial shift in SPCZ precipitation patterns from 1000 yrs BP to 200 yrs BP. During this interval, the eastern SPCZ experienced a significant increase in precipitation, whilst the western SPCZ underwent notable drying. This north-eastward shift in precipitation was likely driven by changes in Pacific SST gradients, as shown by our climate model simulations and proxy SST reconstructions. Our modelling results show that a previously hypothesised weakening of the tropical Pacific zonal SST gradient is consistent with our new proxy reconstructions and offer a plausible mechanism for the observed hydroclimatic shifts.An intriguing corollary of our study is the coincidence of this millennial-scale hydroclimatic changes with key events in human history. The eastward shift in SPCZ precipitation overlaps temporally with the Polynesian colonization of the eastern SPCZ region, including islands such as the Marquesas, and eventually Hawaii. This suggests a potential linkage between climate-driven changes in the SPCZ and patterns of human migration and settlement. Enhanced precipitation in the eastern SPCZ would have likely improved freshwater availability, agricultural potential, and overall habitability of these islands, potentially facilitating successful colonization. Conversely, drier conditions in the western SPCZ may have influenced resource pressures, encouraging exploration and eastward movement.Ultimately, these findings emphasize the need for continued development of high-resolution proxies and improvements in coupled climate models to deepen our understanding of long-term Pacific region climate dynamics and their societal impacts.

Hydroclimate in the tropical South Pacific is dominated by the South Pacific Convergence Zone (SPCZ), a region of low-level atmospheric convergence responsible for providing fresh water to 11 million people. The SPCZ is known to change in orientation and intensity in response to interannual climate phenomena, including El Niño Southern Oscillation (ENSO) and the interdecadal Pacific Oscillation (IPO), principally through modulation of trade wind strength (i.e., Walker circulation strength), and the resultant moisture inflow. Understanding how the orientation and intensity of the SPCZ changed under past climate states is important to predict future SPCZ changes, currently poorly represented in existing GCM’s. However, our knowledge of the dynamics of the SPCZ beyond the last 1000 years is limited by a lack of proxy archives and a large spread in climate model ensembles. We present a 60 ka plant wax record of paleoprecipitation collected from a peat sediment core from the island of Nuku Hiva, French Polynesia, located in the northeastern margin of the SPCZ. We demonstrate that Nuku Hiva was drier during the last glacial maximum (LGM) and wetter during the early Holocene compared to modern conditions. This indicates that the SPCZ was located further to the south during the LGM and further to the north during the early Holocene. We find a strong correlation between our SPCZ precipitation record and foraminifera based reconstructions of western Pacific warm pool thermocline depth. Given that both modern western Pacific thermocline depth and Nuku Hiva precipitation are influenced by easterly trade wind speed, we deduce that trade wind speeds were likely lower during the LGM and higher during the early Holocene, highlighting the long term dependence of SPCZ orientation on Walker circulation strength. This study, will help constrain future predictions of SPCZ precipitation change.

The diurnal warm layer in the upper ocean develops during low surface winds and high incoming solar radiation conditions, often increasing sea‐surface temperatures (SSTs) by up to 1°C. The suppressed phase of the Madden–Julian Oscillation (MJO) favours the formation of such a layer. Here, we analyse the coupled ocean–atmosphere and atmosphere‐only numerical weather prediction systems of the UK Met Office to reveal that important differences arise from the representation of the diurnal warm layer in the coupled model. Though both models are skilful in predicting the MJO to at least a 7‐day lead time, the coupled model predicts approximately 12% faster MJO RMM phase speed propagation than the atmosphere‐only model due to the ability to resolve diurnal warming in the upper ocean that rectifies onto MJO‐associated SST anomalies. The diurnal warming of SST (dSST) in the coupled model leads to an increase in daily mean SST compared with the atmosphere‐only model persisted foundation SST. The strength of the dSST in the coupled model is modulated by MJO conditions. During suppressed MJO conditions on lead day 1, the dSST is enhanced, leading to 0.2°C warmer daily mean MJO‐associated SST anomalies and increased convection in the coupled model by lead day 7. During active MJO convection, the dSST is suppressed, leading to 0.1°C colder MJO‐associated SST anomalies in the coupled model and reduced convection by lead day 7. This variability in dSST further amplifies the MJO propagation speed, underlining the importance of the two‐way feedback between the MJO and the diurnal cycle of SST and the need to accurately represent this process in coupled models.

A multi-season convection-permitting regional climate simulation of the Maritime Continent (MC) using the Met Office Unified Model (MetUM) with 2.2 km grid spacing is presented and evaluated. The simulations pioneer the use of atmosphere–ocean coupling with the multi-column K profile parametrisation (KPP) mixed-layer ocean model in atmospheric convection-permitting climate simulations. Comparisons are made against a convection-parametrised simulation in which it is nested and which in turn derives boundary conditions from the ERA5 reanalysis. This paper describes the configuration, performance of the mean state and variability in the two simulations compared against observational datasets. The models have both minor sea surface temperature (SST) and wet precipitation biases. The diurnal cycle, representation of equatorial waves, and relationship between SST and precipitation are all improved in the convection-permitting model compared to the convection-parametrised model. The Madden–Julian oscillation (MJO) is present in both models with a faster-than-observed propagation speed. However, it is unclear whether fidelity of the MJO simulation is inherent to the model or whether it predominantly arises from the forcing at the boundaries.

Tropical-Antarctic teleconnections are known to have large impacts on Antarctic climate variability at multiple timescales. Anomalous tropical convection triggers upper-level quasi-stationary Rossby waves, which propagate to high southern latitudes and impact the local environment. Here the teleconnection between the Indian Ocean Dipole (IOD) and Antarctica was examined using daily gridded reanalysis data and the linear response theory method (LRTM) during September-November of 1980-2015. The individual contribution of the IOD over the Antarctic climate is challenging to quantify, as positive IOD events often co-occur with El Ni & ntilde;o events. However, using the LRTM, the extratropical response due to a positive IOD was successfully extracted from the combined signal in the composite map of anomalous 250-hPa geopotential height. Applying the method to a set of models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6), significant differences were observed in the extratropical response to the IOD among the models, due to bias in the Rossby waveguide and IOD precipitation pattern. The LRTM was then applied to evaluate the extratropical response of the 850-hPa temperature, wind anomalies, and sea-ice concentration anomalies in observation data, as well as models that represented both the IOD precipitation and the extratropical waveguide adequately. The IOD induced cold southerly flow over the west of the Ross Sea, Weddell Sea, and Antarctic Peninsula, causing cold surface-temperature anomalies and the increase of sea ice, and warm northerly flow over the east of the Ross Sea and Amundsen Sea, causing warm surface-temperature anomalies and the decrease of sea ice. We recommend the LRTM as a complementary method to standard analysis of climate variability from observations and global climate models. Using a novel statistical technique based on linear response theory (LRTM), we have investigated the extratropical Rossby wave response at 250-hPa geopotential height anomaly to the positive phase of the Indian Ocean Dipole (IOD) in reanalysis data and a set of state-of-the-art general circulation models (GCMs). The biases and uncertainties in IOD-Antarctic teleconnection in the GCMs were further evaluated in the context of stationary Rossby wave theory. We recommend LRTM as a complementary technique for future tropical-Antarctic teleconnection studies. image

Theories of ocean-atmosphere interaction during a Madden-Julian Oscillation (MJO) are generally based on a thermodynamic model with surface fluxes dictating changes in sea surface temperature. Evidence from a two month ocean glider deployment in early 2019 in the southeast Indian Ocean suggests the impact of mesoscale dynamics on upper-ocean stratification likely affects ocean-atmosphere interaction at MJO scales. Until midFebruary, local surface fluxes consistent with a convectively suppressed MJO phase drove near-surface ocean evolution. With the advection of a fresh-core eddy to the glider location in late February, ocean dynamics then becomes an additional driver of this evolution by modulating local stratification and generating a barrier layer of approximate to 12 m thickness for 10 days. One-dimensional modelling experiments based on the ocean and atmospheric conditions experienced during our sampling period show that the ocean subsurface structure within the eddy induce changes in SST of physical significance for ocean-atmosphere interaction. Moreover, results also suggest that the presence of a thick eddy-induced barrier layer during the MJO suppressed phase modulates the magnitude of temperature anomalies forced by surface fluxes during the following enhanced MJO phase. As eddies are abundant in this area, their dynamics must be considered to correctly represent SST variability for MJO modelling.

In the South-East Pacific, understanding how precipitation will respond to changes in temperature is crucial in anticipating future climate changes. However, our understanding of the regional climate temperature sensitivity is limited by a lack of any long-term paleoclimate reconstructions which extend before the climatically stable Holocene. To address this knowledge gap,we reconstruct temperature and precipitation using biomarker and pollen proxies sampled from a 50\,ka year-long sediment core located in Nuku Hiva (French Polynesia). The hydrogen isotopic composition of plant waxes is more positive during the last glacial period indicating drier conditions. Pollen analysis indicates that allwetter cloud forest species decreased in abundance in the glacial period, whilst drier-adapted herbaceous taxa increased. Temperature-sensitive bacterial lipids indicate much cooler conditions in the glacial period than in the Holocene. Crucially, changes in reconstructed temperature and precipitation are highly correlated over this time period, which is at odds with state-of-the-art modelling studies. These results willaid models in predicting future changes in precipitation in a region already experiencing the devastating effects of anthropogenic climate change.

Diurnal warm layers develop in the upper ocean on sunny days with low surface wind speeds. They rectify intraseasonal sea-surface temperatures (SSTs), potentially impacting intraseasonal weather patterns such as the Madden-Julian Oscillation (MJO). Here we analyse 15-lead-day forecast composites of coupled ocean-atmosphere and atmosphere-only numerical weather prediction (NWP) models of the UK Met Office to reveal that the presence of diurnal warming of SST (dSST) leads to a faster MJO propagation in the coupled model compared with the atmosphere-only model. To test the feedback between the MJO and the dSST, we designed a set of experiments with instantaneous vertical mixing over the top 5 or 10 m$$ 10\;\mathrm{m} $$ of the ocean component of the coupled model. Weaker dSST in the mixing experiments leads to a slower MJO over 15 lead days. The dSST produces a 3%$$ 3\% $$ increase in the MJO phase speed between the coupled and the atmosphere-only model. An additional 5%$$ 5\% $$ increase is found for other coupling effects, unrelated to the dSST. A two-way feedback manifests in the coupled model over the 15 lead days of the forecast between the MJO and the dSST. The MJO regime dictates the strength of the dSST and the dSST rectifies the intraseasonal anomalies of SST in the coupled model. Stronger dSST in the coupled model leads to stronger intraseasonal anomalies of SST. The MJO convection responds to these SSTs on a seven-lead-day timescale, and feeds back into the SST anomalies within the next three lead days. Overall, this study demonstrates the importance of high vertical resolution in the upper ocean for predicting the eastward propagation of the MJO in an NWP setting, which is potentially impactful for seasonal predictions and climate projections, should this feedback be unrepresented in the models. Novel experiments suppressing the diurnal cycle of sea-surface temperatures (SSTs) in numerical weather prediction systems of the UK Met Office reveal that the presence of diurnal warm layers in the coupled ocean-atmosphere model speeds up the MJO propagation speed compared with the atmosphere-only version of the model. The experiments demonstrate a two-way feedback between the MJO and diurnal warm layers: the MJO sets the strength of diurnal warming, diurnal warming enhances daily mean intraseasonal SST anomalies, and these anomalies lead to earlier arrival of MJO convective anomalies on a seven-lead-day timescale. image