Air-sea Feedback Research Articles

Influence of winter Arctic sea ice concentration change on the El Niño–Southern Oscillation in the following winter

The present study reveals a close connection between the winter Arctic sea ice concentration (ASIC) change over the Greenland–Barents Seas (GBS) and the El Nino–Southern Oscillation (ENSO) in the following winter. When there is more winter ASIC over the GBS, an El Nino-like sea surface temperature (SST) warming tends to occur in the tropical central-eastern Pacific (TCEP) during the following winter. It is found that the winter ASIC increase over the GBS triggers an atmospheric wave train propagating southeastward from the high latitude Eurasia towards the subtropical North Pacific, with cyclonic wind anomalies over the subtropical North Pacific. A barotropic model experiment with anomalous convergence prescribed around the GBS reproduces reasonably well the atmospheric wave train. The induced spring SST warming and associated anomalous atmospheric heating over the subtropical North Pacific play an essential role in the formation and maintenance of lower-level westerly wind anomalies over the western tropical Pacific. These westerly wind anomalies induce SST warming in the TCEP during the following summer via triggering an eastward propagating equatorial warm Kelvin wave. The summer TCEP SST warming further develops into an El Nino event in the following winter via a Bjerknes-like positive air–sea feedback process. This result suggests that the winter ASIC change around the GBS is a potential predictor of the ENSO events with a lead time of 1 year.

The effect of atmosphere–ocean coupling on the prediction of 2016 western North Pacific tropical cyclones

AbstractWe examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence.

The Influence of Oceanic Barrier Layers on Tropical Cyclone Intensity as Determined through Idealized, Coupled Numerical Simulations

AbstractThe connection relating upper-ocean salinity stratification in the form of oceanic barrier layers to tropical cyclone (TC) intensification is investigated in this study. Previous works disagree on whether ocean salinity is a negligible factor on TC intensification. Relationships derived in many of these studies are based on observations, which can be sparse or incomplete, or uncoupled models, which neglect air–sea feedbacks. Here, idealized ensemble simulations of TCs performed using the Weather Research and Forecasting (WRF) Model coupled to the 3D Price–Weller–Pinkel (PWP) ocean model facilitate examination of the TC–upper-ocean system in a controlled, high-resolution, mesoscale environment. Idealized vertical ocean profiles are modeled after barrier layer profiles of the Amazon–Orinoco river plume region, where barrier layers are defined as vertical salinity gradients between the mixed and isothermal layer depths. Our results reveal that for TCs of category 1 hurricane strength or greater, thick (24–30 m) barrier layers may favor further intensification by 6%–15% when averaging across ensemble members. Conversely, weaker cyclones are hindered by thick barrier layers. Reduced sea surface temperature cooling below the TC inner core is the primary reason for additional intensification. Sensitivity tests of the results to storm translation speed, initial oceanic mixed layer temperature, and atmospheric vertical wind shear provide a more comprehensive analysis. Last, it is shown that the ensemble mean intensity results are similar when using a 3D or 1D version of PWP.

Estimating the Role of SST in Atmospheric Surface Wind Variability over the Tropical Atlantic and Pacific

Abstract The influence of sea surface temperature (SST) on interannual surface wind variability in the tropical Atlantic and Pacific is estimated using sensitivity experiments with the SINTEX-F GCM and the ensemble spread in a nine-member control simulation. Two additional estimates are derived for both SINTEX-F and the ERA-Interim reanalysis using regression analysis and singular value decomposition. All methods yield quite consistent estimates of the fraction of surface wind variability that is determined by SST and therefore potentially predictable. In the equatorial Atlantic, analysis suggests that for the period 1982–2014 approximately 2/3 of surface zonal wind variability in boreal spring and early summer is potentially predictable, while 1/3 is due to noise. Of the predictable component, up to about 35% may be driven from outside the tropical Atlantic, suggesting an important role for remote forcing and a diminished one for local feedbacks. In the northern tropical Atlantic, only 30% of boreal winter variability is predictable, most of which is forced from the Pacific. This suggests a minor role for local coupled air–sea feedbacks. For the equatorial Pacific, the results suggest high predictability throughout the year, most of which is due to local SST, with the tropical Atlantic only playing a minor role in boreal summer. In the tropical Atlantic, atmospheric internal variability is strongly dependent on the presence of deep convection, which, in turn, is related to mean SST. A similar, but weaker, state dependence of internal variability is evident in the tropical Pacific.

Millennial- and centennial-scale variations in the upper-water temperature of Western Pacific Warm Pool during the last deglaciation

During the last deglaciation period, climate changes in the northern and southern high-latitude regions show a pattern of so-called “bipolar seesaw” on millennial scale. Previous research interpreted that the weakening of the Atlantic Meridional Overturning Circulation (AMOC) and the associated north-south migration of the Intertropical Convergence Zone (ITCZ) influenced the interhemispheric heat redistribution and thus climate state. However, such a hypothesis lacks the support from the western tropical Pacific, in which the deglacial millennial-scale changes is unclear as the published reconstructions usually conflict with one another. As the major oceanic heat source, the Western Pacific Warm Pool (WPWP) affects the global climate system, with its sea surface temperature (SST) change forcing the tropical atmospheric circulation and its thermocline dynamic modulating the upper-ocean heat content and thermal structure. This study explores the upper-water temperature changes during the transition from the Last Glacial Maximum (LGM) to the Holocene, i.e., the last deglaciation (~23−8 ka). Foraminifera are inferred to calcify in equilibrium with the seawater they live in, make it possible to reconstruct the environmental properties of corresponding water masses where certain foraminifera dwell. We analyzed the δ 18O and Mg/Ca of the mixed-layer dwelling planktonic foraminifera Globigerinoides ruber and the upper-thermocline dweller Pulleniatina obliquiloculata from sediment core MD01-2386 locating at the core of WPWP, and reconstructed decadal-resolved records of SST and thermocline water temperature (TWT) for the last deglaciation. The reconstructed results show that: (1) SST and TWT started initial warming onsets around 20−21 ka, following the initial decrease of precession parameter; (2) SST and TWT stopped warming and plateaued during the interval of Heinrich stadial 1 (HS-1) and Younger Dryas (YD; (3) TWT varied with larger amplitudes than SST. Further, we compared the SST and TWT of core MD01-2386 with the proxy-records of temperature and precipitation of the WPWP and the model-output of El Nino variability. The records of SST, TWT and the planktonic foraminifera δ 18O from the western equatorial Pacific are coherent with the Borneo stalagmite δ 18O, thus suggesting the coupled feedbacks among the thermocline dynamic, sea-surface heating and atmospheric convective activity over the heart of WPWP during the last deglaciation. While, contrasts exist between the records of near-equator region and off-equator area and thus suggests opposite changes in the atmospheric convective activity near the equator and the hydrological cycle change outside the equator. This finding may indicate a relative migration or intensity change of the atmospheric convection center, rather than a north-south shift of the rain belt. In addition, the planktonic foraminiferal proxy records demonstrate significant centennial-scale variabilities, mainly on the periodicity of 220−260 a. This cycle is coherent with the 10Be record of ice core and roughly coincident with the Suess cycle of solar activity. To sum up, the results of this study allow us to propose three main conclusions. (1) The upper water temperature records of core MD01-2386 show obvious millennial-scale changes, which are closely coupled with the upper water temperature and atmospheric convection activity in the western equatorial Pacific, indicating consistent changes of the ocean-atmosphere coupled system in the western equatorial Pacific on the millennial-scale of the last deglaciation. (2) On the millennium scale in the last deglaciation, opposite changes in the air-sea feedbacks exist between the near-equator and off-equator areas of the warm pool, which may indicate relative changes of the atmospheric convevtion center but not a north-south shift of rain belt. (3) The planktonic foraminifera proxy records present significant cycling of 200−400 a centered at ~260 a and may reflect the linkage between centennial-scale solar activity and the upper water dynamics of the western equatorial Pacific.

AMOC sensitivity to surface buoyancy fluxes: the role of air-sea feedback mechanisms

We interrogate the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to surface heat and freshwater fluxes over the Subpolar Gyre in an ocean general circulation model and its adjoint. Surface heat loss out of the Subpolar Gyre in the winter strengthens the AMOC at a lead time of approximately 6 months. However, the same surface heat flux anomaly in the summer leads to a delayed AMOC weakening that emerges at a lag of 8 months. Under a summer surface cooling perturbation, the AMOC progressively weakens up to a lag of approximately 80 months, and then the negative overturning anomaly persists for years. Compared with the sensitivity to surface heat fluxes, seasonality in the AMOC sensitivity to surface freshwater fluxes is less pronounced, and there is no sign reversal between the response to summer and winter perturbations. We explain the mechanisms behind the large seasonal differences in the AMOC sensitivity to surface heat fluxes and highlight the role of evaporation. Heat flux anomalies over the Subpolar Gyre trigger changes in the rate of evaporation and hence affect the salinity of the mixed layer. Surface cooling gives rise to freshening in the following months, whereas warming leads to salinification. Persistent buoyancy changes due to salinity responses counteract the impact of heat fluxes to a varying extent depending on the seasonal mixed layer depth. On the other hand, air-sea feedback mechanisms exert a positive feedback on the AMOC response to surface freshwater flux perturbations both in the summer and in the winter months.

The Zonal Oscillation and the Driving Mechanisms of the Extreme Western North Pacific Subtropical High and Its Impacts on East Asian Summer Precipitation

AbstractThis paper scrutinizes the zonal oscillation of the western North Pacific subtropical high (WNPSH) via diagnosing its two extreme phases, which are defined by the top 10% strongest (positive phase) and the weakest (negative phase) WNPSH index (WNPSHI) days during summers in 1979–2016. Key findings include the following: a tripole pattern consisting of intensified (weakened) precipitation over the Maritime Continent and the East Asian summer monsoon regions, and suppressed (strengthened) precipitation over the western North Pacific summer monsoon region during positive (negative) WNPSH phases; a westward movement of WNPSH-induced precipitation anomalies that subsequently affects eastern China, Japan, and the Korean Peninsula at different time lags; an OLR–vorticity pattern explained by atmospheric responses to thermal sources is suggested to drive the oscillation; and the competitive interaction of local air–sea feedbacks, especially during the positive phase. In addition, moderate-to-strong positive correlations between the WNPSHI and the Niño-3.4 index are found on 1–2-, 2–3-, and 3–6-yr time scales; both exhibit decadal shifts to a higher-frequency mode, suggesting the intensification of both the zonal WNPSH oscillation and the ENSO under the changing climate and their close interdecadal association. A nonlinear quasi-biennial WNPSH–ENSO relationship is identified: the positive (negative) WNPSH phase sometimes occurs during 1) a decaying El Niño (La Niña) in the preceding summer/autumn, and/or 2) a developing La Niña (El Niño) in the current summer/autumn. A full ENSO transition from moderate-to-strong El Niño to La Niña is often seen during the positive phase, offering potential in predicting ENSO events and extreme WNPSH phases and thereby the summer monsoon rainfall in East Asia.

Towards understanding the suppressed ENSO activity during mid-Holocene in PMIP2 and PMIP3 simulations

The mechanisms of El Niño–Southern Oscillation (ENSO) variability change during the mid-Holocene (MH) were investigated through analyzing the model simulations from the Paleoclimate Modelling Intercomparison Project Phases (PMIP) phase-2 and phase-3. The majority of PMIP2 and PMIP3 model simulations show a lower level of ENSO activity in the MH simulation compared to the pre-industrial (PI) simulation, which is qualitatively consistent with that inferred from the paleoclimate proxies. Through employing the Bjerknes stability index, we quantified the dynamic and thermodynamic air-sea feedbacks in the PI and MH simulations. The quantitative analyses showed that the reduced ENSO variability in MH arose from the weakening in the thermocline (TH), zonal-advection (ZA) and Ekman (EK) feedback terms. We found that all the weakened TH, ZA, and EK terms are associated with the reduction in the response of anomalous thermocline depth (Dʹ) to the zonal wind stress anomaly ({tau ^{prime}_x}^{{}}) in MH compared to PI. The reduced Dʹ response is attributed to the flattened meridional structure of ENSO-related {tau ^{prime}_x}^{{}} field in MH, which is linked to the enhanced surface poleward mean meridional current in MH. Among many aspects of the mean state changes under the MH orbital forcing, this study identified that the surface mean meridional current change might be a key factor behind the suppressed ENSO variability in MH. Lastly, through comparing our findings with the ENSO future projection studies, we found that the wind-thermocline feedback is susceptible in a changing climate, which implies that minimizing the uncertainty in the wind-thermocline feedback change may help constrain future ENSO response.

An Assessment of ENSO Stability in CAMS Climate System Model Simulations

We present an overview of the El Nino–Southern Oscillation (ENSO) stability simulation using the Chinese Academy of Meteorological Sciences climate system model (CAMS-CSM). The ENSO stability was quantified based on the Bjerknes (BJ) stability index. Generally speaking, CAMS-CSM has the capacity of reasonably representing the BJ index and ENSO-related air–sea feedback processes. The major simulation biases exist in the underestimated thermodynamic damping and thermocline feedbacks. Further diagnostic analysis reveals that the underestimated thermodynamic feedback is due to the underestimation of the shortwave radiation feedback, which arises from the cold bias in mean sea surface temperature (SST) over central–eastern equatorial Pacific (CEEP). The underestimated thermocline feedback is attributed to the weakened mean upwelling and weakened wind–SST feedback (μa) in the model simulation compared to observation. We found that the weakened μa is also due to the cold mean SST over the CEEP. The study highlights the essential role of reasonably representing the climatological mean state in ENSO simulations.

Indian Summer Monsoon as simulated by the regional earth system model RegCM-ES: the role of local air–sea interaction

In this study the regional earth system model RegCM-ES is applied over the South Asia CORDEX domain to improve the simulation of the variability of the Indian Summer Monsoon. RegCM-ES has been integrated for 30 years, from 1979 to 2008, with a spatial resolution of 50 km in the atmospheric component, 18 km in the ocean and 50 or 12 km in the two hydrological models used for this study. A new hydrological component, the Cetemps Hydrological Model (CHyM), is included in the coupled system composed by the regional climate model RegCM4 and the ocean model MITgcm. In our analysis we show how the two way air–sea feedback simulated by the coupled system largely improves the representation of intraseasonal (ISV) and interannual (IAV) modes of variability of the Indian Summer Monsoon Rainfall (ISMR). The comparison with the Hydrological Discharge (HD) model shows large improvements in simulating the correct amount of freshwater river discharge over the Bay of Bengal (BoB), one of the fundamental factor responsible for the formation of the shallow barrier layer (BL) over the BoB. However, the improvements in river discharge do not have a clear effect on the BL due to excessive mixing in the ocean model. The improvements in the simulation of IAV are primarily due to the ability of RegCM-ES to reproduce the Intraseasonal Oscillation modes and to the complex mechanism related to the delayed response of the ISMR to the ENSO forcing. Our results highlight the very important role of fine scale air–sea feedbacks in modulating the ISMR and the internal dynamics of this phenomenon.

Consequences of different air-sea feedbacks on ocean using MITgcm and MERRA-2 forcing: Implications for coupled data assimilation systems

Ocean surface flux estimates from atmospheric and oceanic reanalyses contain errors that compensate for inaccuracies in the respective atmosphere and ocean models used to generate these reanalyses. A conundrum for climate studies is the discrepancy between surface fluxes that minimize model-data differences for an atmosphere-only model vs surface fluxes that minimize model-data differences for an ocean model. As a first step towards a consistent coupled ocean-atmosphere data-assimilation (DA) system, we compare surface net heat flux from a state-of-the-art atmospheric reanalysis, the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), to net heat flux from a state-of-the-art ocean state estimate, the Estimating the Circulation and Climate of the Ocean Version 4 (ECCO-v4).The possible impacts of the MERRA-2 and ECCO-v4 air-sea net heat flux difference in a coupled DA system were assessed using a set of experiments designed to imitate different “flavors” of a coupled DA system in an ocean-only setup. This was done by forcing the ECCO-v4 underlying ocean model - the Massachusetts Institute of Technology general circulation model (MITgcm) - with different sets of MERRA-2 fields and utilizing different forcing methods. By doing so we were able to turn off different air-sea feedbacks which, in a coupled DA setup, are partially muted by the constraining observations. The set of experiments, therefore, represents a range of active feedbacks in different “flavors” of coupled data-assimilation systems.For the period 1992–2011, MERRA-2 net heat flux has a global mean difference of −4.9 Wm−2 relative to ECCO-v4. When MERRA-2 surface fields are used to force MITgcm, imbalances in the energy and the hydrological cycles of MERRA-2, which are directly related to the fact that MERRA-2 was created without an interactive ocean, propagate to the ocean. The experiment in which MITgcm is forced with MERRA-2 fluxes (MERRA-2-flux experiment) results in a 2.5°C global mean Sea Surface Temperature (SST) cooling, a 1m reduction in global mean sea level, and other drastic changes in the large scale ocean circulation relative to those resulting when the MITgcm is forced with the optimized ECCO-v4 net heat flux (the ECCO-v4 experiment itself). When MITgcm is forced with MERRA-2 state variables (MERRA-2-state experiment), the SST is somewhat restored to the observed SST, but the errors are shifted to the water cycle, resulting in a global mean sea level increase of 2.7 m. To further explore the pros and cons of these two approaches, we introduce a new intermediate forcing method in which the ocean is forced with turbulent fluxes but has a long wave feedback. This method, unlike MERRA-2 state, preserves the MERRA-2 water and salinity cycles, and it reduces the SST error compared to the MERRA-2-flux experiment, but the SST is not as good as that in the MERRA-2-state experiment. Our results have implications for ocean-model forcing recipes and clearly reveal the undesirable consequences of limiting the feedbacks in either these types of experiments or in coupled DA.

Diversity of the Pacific–Japan Pattern among CMIP5 Models: Role of SST Anomalies and Atmospheric Mean Flow

This study investigates the reproducibility of the spatial structure and amplitude of the observed Pacific–Japan (PJ) pattern in the phase 5 of the Coupled Model Intercomparison Project (CMIP5) models. In particular, the role of sea surface temperature anomalies (SSTAs) and atmospheric mean flow in the diverse reproducibility of the PJ pattern among models is investigated. Based on the pattern correlation between simulated and observed PJ patterns, models are categorized into high and low correlation groups, referred to as HCG and LCG, respectively. The observed cold SSTAs in the western North Pacific (WNP) and equatorial central Pacific, organized convection and precipitation anomalies, and Rossby wave response are reproduced well in HCG models, whereas these features are not present in LCG models. The summer SSTAs are closely tied to the preceding El Niño–Southern Oscillation and its temporal evolution in the tropical Indo-Pacific Ocean in both observations and models, but the SSTAs in the Indian Ocean are weak in both HCG and LCG, implying a weak Indian Ocean capacitor effect. As a result, the reproducibility of the amplitude of the WNP center of the PJ pattern is mainly modulated by the SSTAs and local air–sea feedback over the WNP in the models. On the other hand, a model with stronger climatological southerly along the coast of East Asia tends to produce more realistic amplitude of the midlatitude center of the PJ pattern with clearer poleward wave-activity fluxes due to more efficient local barotropic energy conversion from the mean flow.

Improved seasonal predictive skill and enhanced predictability of the Asian summer monsoon rainfall following ENSO events in NCEP CFSv2 hindcasts

The dominant modes of the Asian summer monsoon (ASM) rainfall variability, as well as their seasonal predictive skill and predictability, are investigated using two sets of seasonal hindcasts made with the NCEP Climate Forecast System (CFSv2): one from the NCEP CFS Reanalysis and Reforecast Project (CFS_RR) and the other using a Multi-ocean Analyses Ensemble initialization scheme (CFS_MAE). The 1st and 2nd empirical orthogonal function (EOF) modes of the observed ASM rainfall anomalies correspond respectively to the contemporaneous and delayed responses to El Nino and the Southern Oscillation (ENSO) in its developing and decaying years. In general, CFSv2 is capable of skillfully predicting these two dominant ASM modes on the seasonal time scale up to 5 months in advance. Moreover, the predictive skill of the ASM rainfall in CFS_MAE is much higher with respect to the delayed ENSO mode than the contemporaneous one. The predicted principal component of the former maintains high correlation skill and small ensemble spread about two seasons ahead while the latter is significantly degraded in both measures after one season. A maximized signal-to-noise EOF analysis further shows that the delayed ASM response to ENSO is also the most predictable pattern at long leads in CFS_RR. The improved predictive skill of the ASM rainfall following ENSO events originates from the enhanced predictability associated with the active air-sea feedback in the Indo-northwestern Pacific domain from the ENSO peak to the ENSO demise phase, which are well captured in the CFSv2 hindcasts.

Modulation of the relationship between spring AO and the subsequent winter ENSO by the preceding November AO

Previous studies indicated that the spring Arctic Oscillation (AO) exerts significant influences on the subsequent winter El Niño-Southern Oscillation (ENSO). This analysis suggests that the spring AO-ENSO linkage is highly modulated by its preceding November AO. When November and the subsequent spring AO indices are in phase, the spring AO has a pronounced influence on ENSO. However, when the November and spring AO indices are out of phase, the spring AO-ENSO connection disappears. Modulation of the November AO on the spring AO-ENSO connection is mainly through the constructive and destructive superposition of the November and spring AO associated sea surface temperature (SST) anomalies in the tropical central-eastern Pacific in spring and summer, as well as the SST anomalies developed further in the tropical Pacific via the positive air-sea feedback.

Impact of atmospheric model resolution on simulation of ENSO feedback processes: a coupled model study

This study examines El Nino-Southern Oscillation (ENSO)-related air-sea feedback processes in a coupled general circulation model (CGCM) to gauge model errors and pin down their sources in ENSO simulation. Three horizontal resolutions of the atmospheric component (T42, T63 and T106) of the CGCM are used to investigate how the simulated ENSO behaviors are affected by the resolution. We find that air-sea feedback processes in the three experiments mainly differ in terms of both thermodynamic and dynamic feedbacks. We also find that these processes are simulated more reasonably in the highest resolution version than in the other two lower resolution versions. The difference in the thermodynamic feedback arises from the difference in the shortwave-radiation (SW) feedback. Due to the severely (mildly) excessive cold tongue in the lower (higher) resolution version, the SW feedback is severely (mildly) underestimated. The main difference in the dynamic feedback processes lies in the thermocline feedback and the zonal-advection feedback, both of which are caused by the difference in the anomalous thermocline response to anomalous zonal wind stress. The difference in representing the anomalous thermocline response is attributed to the difference in meridional structure of zonal wind stress anomaly in the three simulations, which is linked to meridional resolution.

Ocean eddies and climate predictability.

A suite of coupled climate model simulations and experiments are used to examine how resolved mesoscale ocean features affect aspects of climate variability, air-sea interactions, and predictability. In combination with control simulations, experiments with the interactive ensemble coupling strategy are used to further amplify the role of the oceanic mesoscale field and the associated air-sea feedbacks and predictability. The basic intent of the interactive ensemble coupling strategy is to reduce the atmospheric noise at the air-sea interface, allowing an assessment of how noise affects the variability, and in this case, it is also used to diagnose predictability from the perspective of signal-to-noise ratios. The climate variability is assessed from the perspective of sea surface temperature (SST) variance ratios, and it is shown that, unsurprisingly, mesoscale variability significantly increases SST variance. Perhaps surprising is the fact that the presence of mesoscale ocean features even further enhances the SST variance in the interactive ensemble simulation beyond what would be expected from simple linear arguments. Changes in the air-sea coupling between simulations are assessed using pointwise convective rainfall-SST and convective rainfall-SST tendency correlations and again emphasize how the oceanic mesoscale alters the local association between convective rainfall and SST. Understanding the possible relationships between the SST-forced signal and the weather noise is critically important in climate predictability. We use the interactive ensemble simulations to diagnose this relationship, and we find that the presence of mesoscale ocean features significantly enhances this link particularly in ocean eddy rich regions. Finally, we use signal-to-noise ratios to show that the ocean mesoscale activity increases model estimated predictability in terms of convective precipitation and atmospheric upper tropospheric circulation.

Causes of the large warm bias in the Angola\u2013Benguela Frontal Zone in the Norwegian Earth System Model

We have investigated the causes of the sea surface temperature (SST) bias in the Angola–Benguela Frontal Zone (ABFZ) of the southeastern Atlantic Ocean simulated by the Norwegian Earth System Model (NorESM). Similar to other coupled-models, NorESM has a warm SST bias in the ABFZ of up to 8 °C in the annual mean. Our analysis of NorESM reveals that a cyclonic surface wind bias over the ABFZ drives a locally excessively strong southward (0.05 m/s (relative to observation)) Angola Current displacing the ABFZ southward. A series of uncoupled stand-alone atmosphere and ocean model simulations are performed to investigate the cause of the coupled model bias. The stand-alone atmosphere model driven with observed SST exhibits a similar cyclonic surface circulation bias; while the stand-alone ocean model forced with the reanalysis data produces a warm SST in the ABFZ with a magnitude approximately half of that in the coupled NorESM simulation. An additional uncoupled sensitivity experiment shows that the atmospheric model’s local negative surface wind curl generates anomalously strong Angola Current at the ocean surface. Consequently, this contributes to the warm SST bias in the ABFZ by 2 °C (compared to the reanalysis forced simulation). There is no evidence that local air-sea feedbacks among wind stress curl, SST, and sea level pressure (SLP) affect the ABFZ SST bias. Turbulent surface heat flux differences between coupled and uncoupled experiments explain the remaining 2 °C warm SST bias in NorESM. Ocean circulation, upwelling and turbulent heat flux errors all modulate the intensity and the seasonality of the ABFZ errors.

Causes of ENSO Weakening during the Mid-Holocene

The causes of the change in amplitude of El Niño–Southern Oscillation (ENSO) during the mid-Holocene were investigated by diagnosing the model simulations that participated in the Paleoclimate Modelling Intercomparison Project phases 2 and 3. Consistent with paleoclimate records, 20 out of the 28 models reproduced weaker-than-preindustrial ENSO amplitude during the mid-Holocene. Two representative models were then selected to explore the underlying mechanisms of air–sea feedback processes. A mixed layer heat budget diagnosis indicated that the weakened ENSO amplitude was primarily attributed to the decrease in the Bjerknes thermocline feedback, while the meridional advective feedback also played a role. During the mid-Holocene, the thermocline response to a unit anomalous zonal wind stress forcing in the equatorial Pacific weakened in both models because of the increased ENSO meridional scale. A further investigation revealed that the greater ENSO meridional width was caused by the strengthening of the Pacific subtropical cell, which was attributed to the enhanced mean trade wind that resulted from the intensified Asian and African monsoon rainfall and associated large-scale east–west circulation in response to the mid-Holocene orbital forcing.

Reforecasting the ENSO Events in the Past 57 Years (1958–2014)

Abstract A set of ensemble seasonal reforecasts for 1958–2014 is conducted using the National Centers for Environmental Prediction (NCEP) Climate Forecast System, version 2. In comparison with other current reforecasts, this dataset extends the seasonal reforecasts to the 1960s–70s. Direct comparison of the predictability of the ENSO events occurring during the 1960s–70s with the more widely studied ENSO events since then demonstrates the seasonal forecast system’s capability in different phases of multidecadal variability and degrees of global climate change. A major concern for a long reforecast is whether the seasonal reforecasts before 1979 provide useful skill when observations, particularly of the ocean, were sparser. This study demonstrates that, although the reforecasts have lower skill in predicting SST anomalies in the North Pacific and North Atlantic before 1979, the prediction skill of the onset and development of ENSO events in 1958–78 is comparable to that for 1979–2014. In particular, the ENSO predictions initialized in April during 1958–78 show higher skill in the summer. However, the skill of the earlier predictions declines faster in the ENSO decaying phase, because the reforecasts initialized after boreal summer persistently predict lingering wind and SST anomalies over the eastern equatorial Pacific during such events. Reforecasts initialized in boreal fall overestimate the peak SST anomalies of strong El Niño events since the 1980s. Both phenomena imply that the model’s air–sea feedback is overly active in the eastern Pacific before ENSO event termination. Whether these differences are due to changes in the observing system or are associated with flow-dependent predictability remains an open question.

Why 1986 El Ni\xf1o and 2005 La Ni\xf1a evolved different from a typical El Ni\xf1o and La Ni\xf1a

1986 El Niño (EN) and 2005 La Niña (LN) experienced distinctive evolution features compared to the typical EN and LN events. The 1986 EN persisted for more than 2 years, whereas the 2005 LN transitioned into a warm episode in the following winter. The physical mechanisms that caused the distinctive evolution features are investigated through an oceanic mixed-layer heat budget (MLHB) analysis. For both cases, major differences with typical EN and LN lie in wind induced anomalous zonal advection and thermocline feedback terms. An anomalous cyclone appeared over the western North Pacific (WNPC) in late 1986, in contract to an anomalous anticyclone (WNPAC) during the mature winter of the typical EN. The eastward propagation of anomalous convective heating from tropical Indian Ocean in late 1986 holds a key for generating the anomalous WNPC in the mature winter of 1986/87 EN. The equatorial westerly anomaly south of the WNPC triggered downwelling Kelvin waves, prolonging the positive SSTA throughout 1987. The negative SSTA center of the 2005 LN shifted eastward by at least 20° in longitude compared to the typical LN. As a result, an anomalous WNPC, with pronounced westerly anomalies at the equator, developed in the 2005 LN mature phase. This led to a rapid decay of the 2005 LN, and by the following boreal summer a positive SSTA formed. The season-dependent air-sea feedback further strengthens the warming in northern fall, leading to a transition from LN to EN in the succeed winter.