Tropical El Nino-Southern Oscillation Research Articles

Does increased atmospheric resolution improve seasonal climate predictions?

AbstractWe assess the impact of atmospheric horizontal resolution on the prediction skill and fidelity of seasonal forecasts. We show the response to an increase of atmospheric resolution from 0.8 to 0.3° horizontal grid spacing in parallel ensembles of forecasts. Changes in the prediction skill of major modes of tropical El Nino Southern Oscillation (ENSO) and extratropical North Atlantic Oscillation (NAO) variability are small and not detected and there is no discernible impact on the weak signal‐to‐noise ratio in seasonal predictions of the winter NAO at this range of resolutions. Although studies have shown improvements in the simulation of tropical cyclones as model resolution is increased, we find little impact on seasonal prediction skill of either their numbers or intensity. Over this range of resolutions it appears that the benefit of increasing atmospheric resolution to seasonal climate predictions is minimal. However, at yet finer scales there appears to be increased eddy feedback which could strengthen weak signals in predictions of the NAO. Until prediction systems can be run operationally at these scales, it may be better to use additional computing resources for other enhancements such as increased ensemble size, for which there is a clear benefit in extratropical seasonal prediction skill.

Decadal modulation of the relationship between intraseasonal tropical variability and ENSO

The El Nino Southern Oscillation (ENSO) amplitude is modulated at decadal timescales, which, over the last decades, has been related to the low-frequency changes in the frequency of occurrence of the two types of El Nino events, that is the Eastern Pacific (EP) and Central Pacific (CP) El Ninos. Meanwhile ENSO is tightly linked to the intraseasonal tropical variability (ITV) that is generally enhanced prior to El Nino development and can act as a trigger of the event. Here we revisit the ITV/ENSO relationship taking into account changes in ENSO properties over the last six decades. The focus is on two main components of ITV, the Madden–Julian Oscillation (MJO) and convectively coupled equatorial Rossby waves (ER). We show that the ITV/ENSO relationship exhibits a decadal modulation that is not related in a straight-forward manner to the change in occurrence of El Nino types and Pacific decadal modes. While enhanced MJO activity associated to EP El Nino development mostly took place over the period 1985–2000, the ER activity is enhanced prior to El Nino development over the whole period with a tendency to relate more to CP El Nino than to EP El Nino. In particular the relationship between ER activity and ENSO was particularly strong for the period 2000–2015, which results in a significant positive long-term trend of the predictive value of ER activity. The statistics of the MJO and ER activity is consistent with the hypothesis that they can be considered a state-dependent noise for ENSO linked to distinct lower frequency climate modes.

An early onset of ENSO influence in the extra-tropics of the southwest Pacific inferred from a 14, 600 year high resolution multi-proxy record from Paddy's Lake, northwest Tasmania

Abstract Tropical El Nino Southern Oscillation (ENSO) is an important influence on natural systems and cultural change across the Pacific Ocean basin. El Nino events result in negative moisture anomalies in the southwest Pacific and are implicated in droughts and catastrophic wildfires across eastern Australia. An amplification of tropical El Nino activity is reported in the east Pacific after ca. 6.7 ka; however, proxy data for ENSO-driven environmental change in Australia suggest an initial influence only after ca. 5 ka. Here, we reconstruct changes in vegetation, fire activity and catchment dynamics (e.g. erosion) over the last 14.6 ka from part of the southwest Pacific in which ENSO is the main control of interannual hydroclimatic variability: Paddy's Lake, in northwest Tasmania (1065 masl), Australia. Our multi-proxy approach includes analyses of charcoal, pollen, geochemistry and radioactive isotopes. Our results reveal a high sensitivity of the local and regional vegetation to climatic change, with an increase of non-arboreal pollen between ca. 14.6–13.3 ka synchronous with the Antarctic Cold Reversal, and a sensitivity of the local vegetation and fire activity to ENSO variability recorded in the tropical east Pacific through the Holocene. We detect local-scale shifts in vegetation, fire and sediment geochemistry at ca. 6.3, 4.8 and 3.4 ka, simultaneous with increases in El Nino activity in the tropical Pacific. Finally, we observe a fire-driven shift in vegetation from a pyrophobic association dominated by rainforest elements to a pyrogenic association dominated by sclerophyllous taxa following a prolonged (>1 ka) phase of tropical ENSO-amplification and a major local fire event at ca. 3.4 ka. Our results reveal the following key insights: (1) that ENSO has been a persistent modulator of southwest Pacific climate and fire activity through the Holocene; (2) that the climate of northwest Tasmania is sensitive to long-term shifts in tropical ENSO variability; and (3) that there has been possible stationarity in the spatial influence of ENSO over this region through the Holocene.

Co-evolution of monsoonal precipitation in East Asia and the tropical Pacific ENSO system since 2.36 Ma: New insights from high-resolution clay mineral records in the West Philippine Sea

Clay mineralogical analysis and scanning electron microscope (SEM) analysis were performed on deep-sea sediments cored on the Benham Rise (core MD06-3050) in order to reconstruct long-term evolution of East Asian Summer Monsoon (EASM) rainfall in the period since 2.36 Ma. Clay mineralogical variations are due to changes in the ratios of smectite, which derive from weathering of volcanic rocks in Luzon Island during intervals of intensive monsoon rainfall, and illite- and chlorite-rich dusts, which are transported from East Asia by winds associated with the East Asian Winter Monsoon (EAWM). Since Luzon is the main source of smectite to the Benham Rise, long-term consistent variations in the smectite/(illite + chlorite) ratio in core MD06-3050 as well as ODP site 1146 in the Northern South China Sea suggest that minor contributions of eolian dust played a role in the variability of this mineralogical ratio and indicate strengthening EASM precipitation in SE Asia during time intervals from 2360 to 1900 kyr, 1200 to 600 kyr, and after 200 kyr. The EASM rainfall record displays a 30 kyr periodicity suggesting the influence of El Nino–Southern Oscillation (ENSO). These intervals of rainfall intensification on Luzon Island are coeval with a reduction in precipitation over central China and an increase in zonal SST gradient in the equatorial Pacific Ocean, implying a reinforcement of La Nina-like conditions. In contrast, periods of reduced rainfall on Luzon Island are associated with higher precipitation in central China and a weakening zonal SST gradient in the equatorial Pacific Ocean, thereby suggesting the development of dominant El Nino-like conditions. Our study, therefore, highlights for the first time a long-term temporal and spatial co-evolution of monsoonal precipitation in East Asia and of the tropical Pacific ENSO system over the past 2.36 Ma.

Why the twenty-first century tropical Pacific trend pattern cannot significantly influence ENSO amplitude?

Although the climate is highly expected to change due to global warming, it is unclear whether the El Nino-Southern Oscillation (ENSO) will be more or less active in the future. One may argue that this uncertainty is due to the intrinsic uncertainties in current climate mod- els or the strong natural long-term modulation of ENSO. Here, we propose that the global warming trend cannot significantly modify ENSO amplitude due to weak feed- back between the global warming induced tropical climate change and ENSO. By analyzing Coupled Model Intercom- parison Project Phase 5 and observation data, we found that the zonal dipole pattern of sea surface temperature (SST; warming in the eastern Pacific and cooling in the western Pacific or vice versa; 'Pacific zonal mode' (PZM)) is highly correlated to change in ENSO amplitude. Additionally, this PZM is commonly identified in control experiments (pre- industrial conditions), twentieth century observations, and twenty-first century scenario experiments (representative concentration pathways 4.5 and 8.5 W m −2 (RCP 4.5, 8.5)). PZM provides favorable conditions for the intensification of ENSO by strengthening air-sea coupling and modi- fying ENSO pattern. On the other hand, the twenty-first century SST trend pattern, which is different from PZM, is not favorable towards changing ENSO amplitude. Fur- thermore, we performed an intermediate ocean-atmosphere coupled model simulations, in which the SST trend pattern and PZM are imposed as an external anomalous heat flux

Mid-Holocene tropical Pacific climate state, annual cycle, and ENSO in PMIP2 and PMIP3

Using the Paleoclimate Modeling Inter-comparison Project Phase 2 and 3 (PMIP2 and PMIP3), we investigated the tropical Pacific climate state, annual cycle, and El Nino-Southern Oscillation (ENSO) during the mid-Holocene period (6,000 years before present; 6 ka run). When the 6 ka run was compared to the control run (0 ka run), the reduced sea surface temperature (SST) and the reduced precipitation due to the basin-wide cooling, and the intensified cross-equatorial surface winds due to the hemispheric discrepancy of the surface cooling over the tropical Pacific were commonly observed in both the PMIP2 and PMIP3, but changes were more dominant in the PMIP3. The annual cycle of SST was weaker over the equatorial eastern Pacific, because of the orbital forcing change and the deepening mixed layer, while it was stronger over the equatorial western pacific in both the PMIP2 and PMIP3. The stronger annual cycle of the equatorial western Pacific SST was accompanied by the intensified annual cycle of the zonal surface wind, which dominated in the PMIP3 in particular. The ENSO activity in the 6 ka run was significantly suppressed in the PMIP2, but marginally reduced in the PMIP3. In general, the weakened air-sea coupling associated with basin-wide cooling, reduced precipitation, and a hemispheric contrast in the climate state led to the suppression of ENSO activity, and the weakening of the annual cycle over the tropical eastern Pacific might lead to the intensification of ENSO through the frequency entrainment. Therefore, the two opposite effects are slightly compensated for by each other, which results in a small reduction in the ENSO activity during the 6 ka in the PMIP3. On the whole, in PMIP2/PMIP3, the variability of canonical (or conventional) El Nino tends to be reduced during 6 ka, while that of CP/Modoki El Nino tends to be intensified.

Present-day and future Amazonian precipitation in global climate models: CMIP5 versus CMIP3

The present study aims at evaluating and comparing precipitation over the Amazon in two sets of historical and future climate simulations based on phase 3 (CMIP3) and 5 (CMIP5) of the Coupled Model Intercomparison Project. Thirteen models have been selected in order to discuss (1) potential improvements in the simulation of present-day climate and (2) the potential reduction in the uncertainties of the model response to increasing concentrations of greenhouse gases. While several features of present-day precipitation—including annual cycle, spatial distribution and co variability with tropical sea surface temperature (SST)—have been improved, strong uncertainties remain in the climate projections. A closer comparison between CMIP5 and CMIP3 highlights a weaker consensus on increased precipitation during the wet season, but a stronger consensus on a drying and lengthening of the dry season. The latter response is related to a northward shift of the boreal summer intertropical convergence zone in CMIP5, in line with a more asymmetric warming between the northern and southern hemispheres. The large uncertainties that persist in the rainfall response arise from contrasted anomalies in both moisture convergence and evapotranspiration. They might be related to the diverse response of tropical SST and ENSO (El Nino Southern Oscillation) variability, as well as to spurious behaviours among the models that show the most extreme response. Model improvements of present-day climate do not necessarily translate into more reliable projections and further efforts are needed for constraining the pattern of the SST response and the soil moisture feedback in global climate scenarios.

Influence of Tropical Indian Ocean Warming and ENSO on Tropical Cyclone Activity over the Western North Pacific

In this study, we have investigated contributions of tropical Indian Ocean (IO) sea surface temperature (SST) warming and El Nino—Southern Oscillation (ENSO) to the interannual variability of tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) between 1948 and 2010 and the involved physical mechanisms. Both ENSO and tropical IO Basin Mode (IOBM) warming are found to play important roles in modulating the WNP TC genesis frequency, but their effects are significantly different. The time series of seasonal empirical orthogonal function of tropical IO and tropical Pacific SST with trend and multi-decadal variability removed are defined as the IOBM index and ENSO index, respectively. The results show that the IO warming year is usually the El Nino decaying year. The number of total TCs, especially weak TCs, decreases during the tropical IO warming year when an anomalous anticyclonic circulation is observed over the tropical Northwest Pacific off the equator. On the other hand, the number of intense TCs increases in the El Nino developing year because of the eastward shifts of both the western Pacific monsoon trough and the cyclonic shear of the equatorial westerlies, and thus the eastward shift of the main TC genesis region. It is also found that the relationship between ENSO and the frequency of intense TCs has a decadal variation. During 1968–1987, the number of intense TCs was not related to ENSO with a correlation coefficient of only 0.19, while the correlation coefficient is 0.63 and 0.73 during 1948–1967 and 1988–2007, respectively. The extent to which the WNP anomalous anticyclone is forced by IO warming is investigated by using the global climate model (European Centre Hamburg Model, ECHAM) with imposed SST anomalies over the tropical IO. The results suggest that the WNP anomalous anticyclone that develops from June to September results mainly from the northward shift of the tropical IO warming from boreal spring to boreal summer.

El Niño-Southern Oscillation sensitivity to cumulus entrainment in a coupled general circulation model

Received 6 July 2011; revised 26 August 2011; accepted 14 September 2011; published 22 November 2011. [1] A series of 200 year long integrations is performed using the Geophysical Fluid Dynamics Laboratory CM2.1 by varying the Tokioka parameter, a minimum entrainment rate threshold in the cumulus parameterization. Changing the threshold alters both the tropical Pacific mean state and the El Nino–Southern Oscillation (ENSO) variability. Increasing the Tokioka parameter causes a basin‐wide cooling in the tropical Pacific with the reduction of high clouds. The degree of cooling in the western part of the basin is bigger than that in the east. As a result, the east–west asymmetry in the tropical Pacific sea surface temperature (SST) decreases with increasing the Tokioka parameter. Accompanied with the reduced east–west SST asymmetry are the increase of mean precipitation over the eastern Pacific and the eastward shift of the atmospheric responses to the ENSO‐related SST forcing. The eastward shifted wind stress anomaly associated with ENSO leads to the stronger ENSO variability. In this way the magnitude of ENSO simulated in this model increases with the Tokioka parameter. Implication of our results on the relationship between the tropical Pacific mean state and ENSO is discussed.

Sensitivity of the tropical Pacific seasonal cycle and ENSO to changes in mean state induced by a surface heat flux adjustment in CCSM3

The influence of mean climate on the seasonal cycle and the El Nino-Southern Oscillation (ENSO) in the tropical Pacific climate is investigated using the Climate Community System Model Version 3 (CCSM3). An empirical time-independent surface heat flux adjustment over the tropical ocean is applied to the oceanic component of CCSM3. In comparison with the control run, the heat flux-adjusted run simulates a more realistic mean climate not only for the sea surface temperature (SST) but also for wind stress and precipitation. Even though the heat flux adjustment is time-independent, the seasonal cycles of SST, wind stress and precipitation over the equatorial eastern Pacific are more realistic in the flux-adjusted simulation. Improvements in the representation of the ENSO variability in the heat flux-adjusted simulation include that the Nino3.4 SST index is less regular than a strong biennial oscillation in the control run. But some deficiencies also arise. For example, the amplitude of the ENSO variability is reduced in the flux-adjusted run. The impact of the mean climate on ENSO prediction is further examined by performing a series of monthly hindcasts from 1982 to 1998 using CCSM3 with and without the heat flux adjustment. The flux-adjusted hindcasts show slightly higher predictive skill than the unadjusted hindcasts with January initial conditions at lead times of 7–9 months and July initial conditions at lead times of 9–11 months. However, their differences during these months are not statistically significant.

Biophysical coupling in remotely-sensed wind stress, sea surface temperature, sea ice and chlorophyll concentrations in the South Indian Ocean

Satellite records of chlorophyll, sea-surface temperature, sea-ice concentration and wind-stress curl, together with reanalysis wind fields, have been analysed to identify connections between the physical environment and phytoplankton growth. The study focusses on the BROKE-West survey region in the Indian Ocean sector of the Southern Ocean: 20° to 90°E, 70° to 50°S. Correlation and regression analyses showed that, of the parameters that can be routinely monitored from space, wind stress and sea surface temperature were most strongly correlated with chlorophyll across the BROKE-West survey area during the period of the survey. Each of these factors, as well as sea ice concentration, was found to be strongly correlated with chlorophyll at different locations. Three distinct regions were identified: The continental shelf and slope (max. depth=3000 m), where satellite data were most obstructed by ice and clouds, supported high concentrations of chlorophyll throughout the growth season (October to April), and although the most important factors determining chlorophyll were likely to be those not observable using remote sensing, close to 100% of variability in surface chlorophyll could be predicted by sea-surface temperature and sea ice concentration. Offshore to the west of 45°E, the eastern Weddell Gyre was found to retain sea-ice later into the growth season and support chlorophyll concentrations on the order of 0.1 mg m -3. Wind-driven advection of sea ice to the north and south of the Antarctic Divergence, located at 64°S, produced a strong positive correlation between chlorophyll concentrations along the landward and off-shore edges of the seasonal ice zone and the Southern Annular Mode. Offshore to the east of 45°E, chlorophyll concentrations encountered during the BROKE-West cruise were found to be unusually high. This was attributed to a southerly excursion of the Antarctic Circumpolar Current corresponding to negative Southern Annular Mode indices during the cruise. Significant, regionally varying correlations were found between the physical and biological parameters examined and climatological indices, including the Southern Annular Mode and the tropical El Nino-Southern Oscillation signal. © 2010 Elsevier Ltd.

ENSO amplitude change in observation and coupled models

Observations show that the tropical El Nino-Southern Oscillation (ENSO) variability, after removing both the long term trend and decadal change of the background climate, has been enhanced by as much as 60% during the past 50 years. This shift in ENSO amplitude can be related to mean state changes in global climate. Past global warming has caused a weakening of the Walker circulation over the equatorial Indo-Pacific oceans, as well as a weakening of the trade winds and a reduction in the equatorial upwelling. These changes in tropical climatology play as stabilizing factors of the tropical coupling system. However, the shallower and strengthening thermocline in the equatorial Pacific increases the SST sensitivity to thermocline and wind stress variabilities and tend to destabilize the tropical coupling system. Observations suggest that the destabilizing factors, such as the strengthening thermocline, may have overwhelmed the stabilizing effects of the atmosphere, and played a deterministic role in the enhanced ENSO variability, at least during the past half century. This is different from the recent assessment of IPCC-AR4 coupled models.

Modelling mid-Holocene tropical climate and ENSO variability: towards constraining predictions of future change with palaeo-data

Palaeoclimate simulations provide an opportunity for climate model evaluation as well as having a potential role in assigning relative likelihood to different ensemble members in probabilistic climate change prediction, supplementing constraints provided by the instrumental record. Here we take some initial steps towards such an approach by performing ensemble experiments with the Hadley Centre HadCM3 model under pre-industrial and mid-Holocene (6,000 years before present) forcing conditions. We examine the changes in both mean tropical climate and El Nino-Southern Oscillation (ENSO) variability, as palaeoclimate records suggest that ENSO amplitude was reduced in the mid-Holocene. Experiments are performed with perturbations to physical parameters in the atmosphere–surface component of the model, and with different implementations of heat and freshwater flux adjustments. Heat flux adjustments are required to stabilise model versions in which perturbations cause a net radiative imbalance. While we find broad agreement between different model versions in terms of changes in mean climate in the mid-Holocene, a detailed and quantitative comparison with the geographically-sparse palaeo-record is limited by systematic model biases. In the simulations without seasonally-varying flux adjustments there are modest reductions in ENSO amplitude of the order of 10–15%, lower than the range of reductions inferred from coral proxy records. We examine the mechanisms for these changes, and discuss the implications for the design of future ensemble experiments to formally quantify uncertainty in climate change predictions using palaeoclimate simulations.

An idealized study of the impact of extratropical climate change on El Niño–Southern Oscillation

Extratropical impacts on the tropical El Nino-Southern Oscillation (ENSO) are studied in a coupled climate model. Idealized experiments show that the remote impact of the extratropics on the equatorial thermocline through oceanic tunnel can substantially modulate the ENSO in both magnitude and frequency. First of all, an extratropical warming can be conveyed to the equator by the mean subduction current, resulting in a warming of the equatorial thermocline. Second, the extratropical warming can weaken the Hadley cells, which in turn slow down the mean shallow meridional overturning circulations in the upper Pacific, reducing the equatorward cold water supply and the equatorial upwelling. These oceanic dynamic processes would weaken the stratification of the equatorial thermocline and retard a buildup (purge) of excess heat content along the equator, and finally result in a weaker and longer ENSO cycle. This study highlights a nonlocal mechanism in which ENSO behavior is related to the extratropical climate conditions.

Tropical influence on Euro-Asian autumn rainfall variability

The connection between autumn rainfall variability in the Euro-Asian domain and tropical climate is documented using state-of-the-art global observational datasets and re-analyses. Results suggest a robust statistical relationship between the El Nino Southern Oscillation (ENSO) and autumn rainfall in parts of southwest Europe, northern Africa and southwest Asia. The correlation between area-mean anomalies over this region (Pea) and the NINO3.4 index is 0.68, stationary over the last 50 years. Global ENSO-like tropical climate anomalies are observed in conjunction with Pea anomalies confirming the relationship found with the NINO3.4 index. Overall, the connection with Indo-Pacific variability is stronger than that with the eastern Pacific.While rainfall anomalies in southwest Europe and southwest Asia appear to largely co-vary as one pattern under the influence of ENSO, our results suggest that different mechanisms may be contributing to the observed anomalies. In the North Atlantic/European region, it is speculated that while a PNA-like mode maybe the prevailing teleconnection mechanism for high Pea, for low Pea tropical Atlantic ENSO related SST anomalies may be playing a more relevant role forcing northeastward propagating Rossby waves. Over southwest Asia, a more direct connection to the Indo-Pacific region is suggested by the upper air anomaly observed over southern Asia, possibly the Rossby wave response to enhanced heating in the Indian Ocean.

Significant Change of Extratropical Natural Variability Associated with Tropical ENSO Anomaly

Abstract The natural variability over the North Pacific, where the influence of tropical El Nino–Southern Oscillation (ENSO) events is substantial, is examined to determine whether there is a large change owing to a difference in the ENSO forcing anomaly. The hindcast ensemble runs of the Seasonal Forecast Model of the National Centers for Environmental Prediction are analyzed for this assessment. Four sets of 10-member ensemble hindcasts out to 7 months with T42 horizontal resolution and another four sets with T62 resolution are examined in detail. The results consistently indicate that the natural variability, on both seasonal and monthly time scales, is significantly smaller during El Nino boreal winters than during La Nina boreal winters. The implication is that the predictability on both seasonal and monthly time scales over the North Pacific is potentially higher during El Nino winters than during La Nina winters.

Climatology and variability of historical Soviet snow depth data: some new perspectives in snow - Indian monsoon teleconnections

This study presents the monthly climatology and variability of the historical soviet snow depth data. This data set was developed under the bilateral data exchange agreement between United States of America and the former Union of Soviet Socialist Republics. The original data is for 284 stations for periods varying from 1881 upto 1985. The seasonal cycle of the mean snow depth has been presented both as spatial maps and as averages over key locations. The deepest snow (=80 cms/day) areas are found over Siberia (in Particular over 80′–100 ′E, 55′–70 ′N) during March. Over the course of the annual cycle average snow depth over this region changes dramatically from about 10 cms in October to about 80 cms in March. The variability is presented in the form of spatial maps of standard deviation. To investigate the interaction of snow depth with Indian monsoon rainfall (IMR), lag and lead correlation coefficients are computed. Results reveal that the winter-time snow depth over western Eurasia surrounding Moscow (eastern Eurasia in central Siberia) shows significant negative (positive) relationship with subsequent IMR. Following the monsoon the signs of relationship reverse over both the regions. This correlation structure is indicative of a midlatitude longwave pattern with an anomalous ridge (trough) over Asia during the winter prior to a strong (weak) monsoon. As the time progresses from winter to spring, the coherent areas of significant relationship show southeastward propagation. Empirical orthogonal function analysis of the snow depth reveal that the first mode describes a dipole-type structure with one centre around Moscow and the other over central Siberia, depicting similar pattern as the spatial correlation structure. The decadal-scale IMR variations seem to be more associated with the Northern Hemisphere midlatitude snow depth variations rather than with the tropical ENSO (El Nino Southern Oscillation) variability.