El Nino-Southern Oscillation Timescale Research Articles

Assessment of rainfall variability and future change in Brazil across multiple timescales

AbstractRainfall variability change under global warming is a crucial issue that may have a substantial impact on society and the environment, as it can directly impact biodiversity, agriculture, and water resources. Observed precipitation trends and climate change projections over Brazil indicate that many sectors of society are potentially highly vulnerable to the impacts of climate change. The purpose of this study is to assess model projections of the change in rainfall variability at various temporal scales over sub‐regions of Brazil. For this, daily data from 30 CMIP5 models for historical (1900–2005) and future (2050–2100) experiments under a high‐emission scenario are used. We assess the change in precipitation variability, applying a band‐pass filter to isolate variability on daily, weekly, monthly, intra‐seasonal, and El Nino Southern Oscillation (ENSO) time scales. For historical climate, simulated precipitation is evaluated against observations to establish model reliability. The results show that models largely agree on increases in variability on all timescales in all sub‐regions, except on ENSO timescales where models do not agree on the sign of future change. Brazil will experience more rainfall variability in the future that is, drier or more frequent dry periods and wetter wet periods on daily, weekly, monthly, and intra‐seasonal timescales, even in sub‐regions where future changes in mean rainfall are currently uncertain. This may provide useful information for climate change adaptation across, for example, the agriculture and water resource sectors in Brazil.

Statistical link between external climate forcings and modes of ocean variability

In this study we investigate statistical link between external climate forcings and modes of ocean variability on inter-annual (3-year) to centennial (100-year) timescales using de-trended semi-partial-cross-correlation analysis technique. To investigate this link we employ observations (AD 1854–1999), climate proxies (AD 1600–1999), and coupled Atmosphere-Ocean-Chemistry Climate Model simulations with SOCOL-MPIOM (AD 1600–1999). We find robust statistical evidence that Atlantic multi-decadal oscillation (AMO) has intrinsic positive correlation with solar activity in all datasets employed. The strength of the relationship between AMO and solar activity is modulated by volcanic eruptions and complex interaction among modes of ocean variability. The observational dataset reveals that El Nino southern oscillation (ENSO) has statistically significant negative intrinsic correlation with solar activity on decadal to multi-decadal timescales (16–27-year) whereas there is no evidence of a link on a typical ENSO timescale (2–7-year). In the observational dataset, the volcanic eruptions do not have a link with AMO on a typical AMO timescale (55–80-year) however the long-term datasets (proxies and SOCOL-MPIOM output) show that volcanic eruptions have intrinsic negative correlation with AMO on inter-annual to multi-decadal timescales. The Pacific decadal oscillation has no link with solar activity, however, it has positive intrinsic correlation with volcanic eruptions on multi-decadal timescales (47–54-year) in reconstruction and decadal to multi-decadal timescales (16–32-year) in climate model simulations. We also find evidence of a link between volcanic eruptions and ENSO, however, the sign of relationship is not consistent between observations/proxies and climate model simulations.

Spatial distribution and the interdecadal change of leading modes of heat budget of the mixed-layer in the tropical Pacific and the association with ENSO

Using heat budget diagnosis of ocean mixed layer from the Global Ocean Data Assimilation System, the spatial distribution of the leading modes of the heat budget was examined. The analysis was for the tropical Pacific in 1979–2013 and was based on combined empirical orthogonal function (CEOF) analysis. The interdecadal changes of the leading modes and their associations with El Nino-Southern Oscillation (ENSO) were also analyzed. The first leading CEOF mode (CEOF1) corresponds to the ENSO mature phase. The contribution from the zonal advection was relatively small along the equator, except the region near the Pacific coast of Central America. The vertical entrainment and diffusion (surface heat flux) had pronounced maxima with positive (negative) values along the equatorial central and eastern Pacific. The meridional advection displayed a different spatial pattern with large positive values on both sides of the equator and smaller values along the equator. The total meridional advection anomaly was mainly determined by advection of anomalous temperature by climatological current responsible for broadening of the ENSO SSTA pattern meridionally. The zonal advection varied almost simultaneously with the tendency of ocean temperature anomaly in the mixed layer. The second leading CEOF mode (CEOF2) included contribution to SSTA tendency during the ENSO developing phase. The distribution pattern and amplitude of the zonal advection in the eastern Pacific in CEOF2 was similar to but with opposite sign to that in CEOF1. The amplitudes of the other dynamical and thermodynamical terms were smaller than that in CEOF1 and spatial distributions displayed an opposite variation between the Pacific coast of Central America and central and eastern tropical Pacific in CEOF2. A comparison of two periods (1979–1999 and 2000–2013) suggested that coupling in the tropical Pacific weakened at ENSO time scales and shifted to a relatively higher frequency regime (from 2 to 4 years averaged in 1979–1999 to 1.5–3 years) after 2000.

On the observed variability of monsoon droughts over India

In the present study, the observed variability of monsoon droughts over India has been examined using a drought monitoring index, namely the Standardized Precipitation Evapo-transpiration Index (SPEI). For calculating the SPEI over different time periods, long term (1901–2010), high resolution, monthly gridded temperature and rainfall data sets have been used. The drought time series shows significant interannual, decadal and long term trends. The analysis suggests a general increase in the intensity and percent area affected by moderate droughts during the recent decades. In particular, the frequency of multi-year (24 months) droughts has shown a statistically significant increase, which is attributed to increase in surface air temperatures and thus drying of the atmosphere. The wavelet analysis of SPEI suggests significant spectral peaks at quasi-biennial (2–3 years), ENSO (5–7 years) and decadal (10–16 years) time scales, with significant multi-decadal variations. The variability of monsoon droughts over India is significantly influenced by the tropical sea surface temperature anomalies. The Canonical correlation analysis (CCA) suggests that the major portion of the drought variability is influenced by the El Nino/Southern Oscillation (ENSO). Global warming, especially the warming of the equatorial Indian Ocean represents the second coupled mode and is responsible for the observed increase in intensity of droughts during the recent decades.

Solomon Sea circulation and water mass modifications: response at ENSO timescales

The South Pacific low latitude western boundary currents (LLWBCs) carry waters of subtropical origin through the Solomon Sea before joining the equatorial Pacific. Changes in their properties or transport are assumed to impact El Nino Southern Oscillation (ENSO) dynamics. At ENSO timescales, the LLWBCs transport tends to counterbalance the interior geostrophic one. When transiting through the complex geography of the Solomon Sea, the main LLWBC, the New Guinea Coastal Undercurrent, cannot follow a unique simple route to the equator. Instead, its routes and water mass properties are influenced by the circulation occurring in the Solomon Sea. In this study, the response of the Solomon Sea circulation to ENSO is investigated based on a numerical simulation. The transport anomalies entering the Solomon Sea from the south are confined to the top 250 m of the water column, where they represent 7.5 Sv (based on ENSO composites) for a mean transport of 10 Sv. The induced circulation anomalies in the Solomon Sea are not symmetric between the two ENSO states because of (1) a bathymetric control at Vitiaz Strait, which plays a stronger role during El Nino, and (2) an additional inflow through Solomon Strait during La Nina events. In terms of temperature and salinity, modifications are particularly notable for the thermocline water during El Nino conditions, with cooler and fresher waters compared to the climatological mean. The surface water at Vitiaz Strait and the upper thermocline water at Solomon Strait, feeding respectively the equatorial Pacific warm pool and the Equatorial Undercurrent, particularly affect the heat and salt fluxes. These fluxes can change by up to a factor of 2 between extreme El Nino and La Nina conditions.

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Nino/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Nina and warm Atlantic SSTs prevailed, and the 1980–1990s when El Nino and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Nino and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.

Response of the terrestrial carbon cycle to the El Ni˜no-Southern Oscillation

Land plays a dominant role in the interannual variability of the global carbon cycle. The canonical warming and drying of the terrestrial tropics observed during El Ni˜no events calls for the study of the role of precipitation and temperature on carbon cycle variability. Here we use a dynamic vegetation and terrestrial carbon model vegetationglobal- atmosphere-soil (VEGAS) to investigate the response of terrestrial carbon cycle to El Ni˜no-Southern Oscillation (ENSO) for the period 1980–2004. The simulated global total land–atmosphere flux (Fta) by VEGAS agrees well with the atmospheric CO2 inversion modelling results on ENSO timescales and is dominated by the tropics. Analysis of composites of terrestrial responses and climate factors during ENSO events and lead–lag correlations have identified that in the tropics, anomalous precipitation lags ENSO by 1 month and temperature by 5–6 months, while simulated soil moisture lags by 5 months.Warmer and drier conditions there cause suppression of Net Primary Production (NPP) and enhancement of Heteotrophic Respiration (Rh) simultaneously, resulting in the lagging of tropical Fta by 6 months. Sensitivity simulations reveal that 2/3 of Fta change comes from NPP and 1/3 from Rh. In VEGAS, fire burning accounts for about 25% of total Fta anomalies. Precipitation during ENSO events contributes 56% of variation of Fta; temperature accounts for 44%, which includes 25% from the enhancement of Rh and 7% from the increase of the vegetation respiration.We identify the remaining 12% variation of Fta to be from an indirect effect of temperature through its effect on soil wetness, which in turn affects NPP. Such insight into the direct and indirect effects of climatic factors highlights the critical role of soil moisture in ecosystem and carbon cycle–a poorly constrained factor.

Protracted’ ENSO episodes and their impacts in the Indian Ocean region

Abstract Evidence for a quasi-decadal (9–13 yr) signal interacting with the quasi-biennial (QB) (2–2.5 yr) and low-frequency (LF) (2.5–8 yr) components of the El Nino Southern Oscillation (ENSO) phenomenon to produce both ‘protracted’ El Nino and La Nina episodes is examined using historical patterns of climatic variables over the Indian Ocean basin in the global context. Across the Indian Ocean basin, seasonal rainfall correlations on LF ENSO and quasi-decadal timescales show that the lower frequency signal displays ‘ENSO-like’ characteristics. Thus, there is evidence that the quasi-decadal signal alone can provide an important modulation of Indian Ocean climatic patterns. However, examinations of the Indian Ocean situation in the wider global context provide strong evidence that ‘protracted’ El Nino and La Nina episodes result from interactions between QB, LF and quasi-decadal signals in the climate system. The seasonal tropospheric velocity potential correlations show that QB, LF and quasi-decadal changes in mass overturning and convective regimes are of global extent. These mass overturnings are most coherent on LF ENSO timescales, but are also clearly evident in seasonal QB and quasi-decadal signal sequences. The situation in October–December (OND) also suggests that a propensity exists in the climate system for drought or flooding extremes to occur over central-eastern Africa to central Asia on QB to decadal timescales, and that this may be particularly manifest during ‘protracted’ El Nino and La Nina episodes. An investigation of individual ‘protracted’ El Nino and La Nina episodes reveals that the magnitude and phasing of the QB, LF and quasi-decadal signals can lead to periods during ‘protracted’ El Nino (La Nina) episodes, which start to take on ‘La Nina-like’ (‘El Nino-like’) characteristics, before reverting back again. This phasing nature involves opposing combinations of the above signals, especially on QB and LF ENSO timescales. Ultimately, it can lead to modulations of climatic variables that are different from those usually observed over ‘classical’ ENSO-sensitive regions. Such characteristics may have major consequences for global teleconnection patterns, and thus have the potential to impact on both regional and remote climatic signatures.

Transport of mass, heat, salt, and nutrients in the southern California Current System: Annual cycle and interannual variability

Net fluxes of mass, heat, salt, nutrients, oxygen, and chlorophyll into a control volume within the southern California Current System (CCS) were computed from data collected on 55 cruises over a 14 year period (1984–1997). This analysis builds on an earlier work [Roemmich, 1989] by using an additional 39 cruises over 10 years, allowing for reliable estimates of the temporal variability in the fluxes on seasonal and interannual timescales and a reduction in the corresponding error budgets. A close balance was found between geostrophic convergence and Ekman divergence for the 14 year, seasonal, and interannual cruise subsets using three different wind products. Wind data taken concomitantly with the hydrographic sampling provided the best balance and hence the best flux estimates. The southern CCS was found to be a region with higher evaporation over precipitation and net heat gain by the ocean from the atmosphere (86 W m−2 in the 14 year mean) in all seasons. Significant variability in both the Ekman and geostrophic transports and the net property fluxes was found to be related to low‐frequency (interpentadal and El Niño‐Southern Oscillation timescale) changes in the dominant wind and circulation patterns in the CCS. Variability in primary productivity, estimated from the derived nutrient fluxes, accompanied the environmental changes. Application of this model to the ongoing data collection will further reduce the error bars on the fluxes and will allow for continued monitoring of changes in the physical and biological structure of the southern CCS.

A geochemical model of the Peru Basin deep-sea floor—and the response of the system to technical impacts

A geochemical model of the Peru Basin deep-sea floor, based on an extensive set of field data as well as on numerical simulations, is presented. The model takes into account the vertical oscillations of the redox zonation that occur in response to both long-term (glacial/interglacial) and short-term (El Nino Southern Oscillation (ENSO) time scale) variations in the depositional flux of organic matter. Field evidence of reaction between the pore water NO3− and an oxidizable fraction of the structural Fe(II) in the clay mineral content of the deep-sea sediments is provided. The conditions of formation and destruction of reactive clay Fe(II) layers in the sea floor are defined, whereby a new paleo-redox proxy is established. Transitional NO3− profile shapes are explained by periodic contractions and expansions of the oxic zone (ocean bottom respiration) on the ENSO time scale. The near-surface oscillations of the oxic–suboxic boundary constitute a redox pump mechanism of major importance with respect to diagenetic trace metal enrichments and manganese nodule formation, which may account for the particularly high nodule growth rates in this ocean basin. These conditions are due to the similar depth ranges of both the O2 penetration in the sea floor and the bioturbated high reactivity surface layer (HRSL), all against the background of ENSO-related large variations in depositional Corg flux. Removal of the HRSL in the course of deep-sea mining would result in a massive expansion of the oxic surface layer and, thus, the shut down of the near-surface redox pump for centuries, which is demonstrated by numerical modeling.

ENSO-Related Precipitation Changes in New Caledonia, Southwestern Tropical Pacific: 1969–98

Abstract An analysis of El Nino–Southern Oscillation (ENSO) related precipitation changes in New Caledonia, southwestern tropical Pacific, based on 21 selected stations covering the 1969–98 period is performed. The analysis at the ENSO timescale is complemented by an investigation of basin-scale precipitation changes in order to set the context, by an examination of changes in the flow rates of two major rivers of the island, and by a comparison between actual precipitation (P) records and the output of a simple linear regression model. The results indicate that a 20%–50% decrease in precipitation generally occurs during El Nino events, and vice versa during La Nina events. In accord with these P changes, the flow rates of the two rivers can almost double during La Nina years, and decrease by more that 50% during El Nino years. The magnitudes of the precipitation anomalies are however not strictly proportional to the strength of ENSO. Hence, it is found that the simple linear regression model based on the...

High- and Low-Frequency Intraseasonal Variance of OLR on Annual and ENSO Timescales

Abstract Using 20 yr of outgoing longwave radiation observations, the complex behavior of the higher- (6–25-day) and lower- (25–70-day) frequency bands of tropical intraseasonal convective oscillations is investigated. Emphasis is given to the mean annual cycle and interannual variability of both bands and to the interaction between the two bands. The focus with regard to the interannual variability within each band is on the warm and cold events associated with the El Nino–Southern Oscillation (ENSO) cycle. The study encompasses the tropical and subtropical Indian and Pacific Oceans (including Australasia). The strongest intraseasonal signals are, for the most part, aligned with the intertropical convergence zone (ITCZ) and South Pacific convergence zone. In some cases, the 6–25-day signal is not collocated with the Madden–Julian oscillation (MJO) signal and/or occurs remotely from the ITCZ. In these cases, the higher-frequency intraseasonal convective perturbations are associated with phenomena independ...

Dynamical Response of the Tropical Total Ozone to Sea Surface Temperature Changes

Interannual variations of total ozone in the equatorial region are discussed in terms of the lower-stratospheric dynamical processes associated with sea surface temperature (SST) changes on an El Nino-Southern Oscillation time scale. Tropospheric diabatic heating by the latent heat release in convective systems drives the interannual variations of tropopause height and atmospheric vertical motions. The adiabatic cooling associated with the dynamically forced ascending motion is subject to radiative heating, which further extends the upwelling motion into the stratosphere. SST variations can thus influence total ozone amounts by changing the tropopause height and vertical advection through modulation of tropospheric diabatic heating. Mechanistic relationships that describe total ozone fluctuations through the advection effect and the tropopause effect of SST variations are derived using the equivalent potential temperature. Total ozone fluctuations are estimated using a quantitative formulation of these mechanistic equations together with the observed SST data. These results indicate that the interannual variations of the zonal wave component of equatorial total ozone are governed by the tropopause effect of SST variations. Preliminary investigation suggests that the annual cycle of tropical total ozone is coupled to the tropopause height changes under the influence of the planetary wave-induced lower-stratospheric temperature rather than local SST.