Warm Phase Of El Niño–Southern Oscillation Research Articles

Changes in Sea Salt Emissions Enhance ENSO Variability

AbstractTwo 150-yr preindustrial simulations with and without interactive sea salt emissions from the Community Earth System Model (CESM) are performed to quantify the interactions between sea salt emissions and El Niño–Southern Oscillation (ENSO). Variations in sea salt emissions over the tropical Pacific Ocean are affected by changing wind speed associated with ENSO variability. ENSO-induced interannual variations in sea salt emissions result in decreasing (increasing) aerosol optical depth (AOD) by 0.03 over the equatorial central-eastern (western) Pacific Ocean during El Niño events compared to those during La Niña events. These changes in AOD further increase (decrease) radiative fluxes into the atmosphere by +0.2 (−0.4) W m−2 over the tropical eastern (western) Pacific. Thereby, sea surface temperature increases (decreases) by 0.2–0.4 K over the tropical eastern (western) Pacific Ocean during El Niño compared to La Niña events and enhances ENSO variability by 10%. The increase in ENSO amplitude is a result of systematic heating (cooling) during the warm (cold) phase of ENSO in the eastern Pacific. Interannual variations in sea salt emissions then produce the anomalous ascent (subsidence) over the equatorial eastern (western) Pacific between El Niño and La Niña events, which is a result of heating anomalies. Owing to variations in sea salt emissions, the convective precipitation is enhanced by 0.6–1.2 mm day−1 over the tropical central-eastern Pacific Ocean and weakened by 0.9–1.5 mm day−1 over the Maritime Continent during El Niño compared to La Niña events, enhancing the precipitation variability over the tropical Pacific.

Interannual Variation of Summer Rainfall in the Taipei Basin

AbstractThe Taipei basin, located in northern Taiwan, is formed at the intersection of the Tanshui River valley (~30 km) and the Keelung River valley (~60 km). Summer is the dry season in northern Taiwan, but the maximum rainfall in the Taipei basin occurs during 15 June–31 August. The majority of summer rainfall in this basin is produced by afternoon thunderstorms. Thus, the water supply, air/land traffic, and pollution for this basin can be profoundly affected by interannual variations of thunderstorm days and rainfall. Because the mechanism for these interannual variations is still unknown, a systematic analysis is made of thunderstorm days and rainfall for the past two decades (1993–2013). These two variables are found to correlate opposite interannual variations of sea surface temperature anomalies over the National Oceanic and Atmospheric Administration Niño-3.4 region. Occurrence days for afternoon thunderstorms and rainfall amounts in the Taipei basin double during the cold El Niño–Southern Oscillation (ENSO) phase relative to the warm phase. During the latter phase, a stronger cold/drier monsoon southwesterly flow caused by the Pacific–Japan Oscillation weakens the thunderstorm activity in the Taipei basin through the land–sea breeze. In contrast, the opposite condition occurs during the cold ENSO phase. The water vapor flux over the East/Southeast Asian monsoon region converges more toward Taiwan to maintain rainfall over the Taipei basin during the cold ENSO phase than during the warm ENSO phase.

Interannual variability of temperature in the UTLS region over Ganges–Brahmaputra–Meghna river basin based on COSMIC GNSS RO data

Abstract. Poor reliability of radiosonde records across South Asia imposes serious challenges in understanding the structure of upper-tropospheric and lower-stratospheric (UTLS) region. The Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission launched in April 2006 has overcome many observational limitations inherent in conventional atmospheric sounding instruments. This study examines the interannual variability of UTLS temperature over the Ganges–Brahmaputra–Meghna (GBM) river basin in South Asia using monthly averaged COSMIC radio occultation (RO) data, together with two global reanalyses. Comparisons between August 2006 and December 2013 indicate that MERRA (Modern-Era Retrospective Analysis for Research Application) and ERA-Interim (European Centre for Medium-Range Weather Forecasts reanalysis) are warmer than COSMIC RO data by 2 °C between 200 and 50 hPa levels. These warm biases with respect to COSMIC RO data are found to be consistent over time. The UTLS temperature show considerable interannual variability from 2006 to 2013 in addition to warming (cooling) trends in the troposphere (stratosphere). The cold (warm) anomalies in the upper troposphere (tropopause region) are found to be associated with warm ENSO (El Niño–Southern Oscillation) phase, while quasi-biennial oscillation (QBO) is negatively (positively) correlated with temperature anomalies at 70 hPa (50 hPa) level. PCA (principal component analysis) decomposition of tropopause temperatures and heights over the basin indicate that ENSO accounts for 73 % of the interannual (non-seasonal) variability with a correlation of 0.77 with Niño3.4 index whereas the QBO explains about 10 % of the variability. The largest tropopause anomaly associated with ENSO occurs during the winter, when ENSO reaches its peak. The tropopause temperature (height) increased (decreased) by about 1.5 °C (300 m) during the last major El Niño event of 2009/2010. In general, we find decreasing (increasing) trend in tropopause temperature (height) between 2006 and 2013.

Investigating the seasonal predictability of significant wave height in the West Pacific and Indian Oceans

AbstractThis study investigates seasonal prediction skill of significant wave height (SWH) in the West Pacific and Indian Oceans. We forced the WAVEWATCH III model with 10 m winds from the National Centers for Environmental Prediction Reanalysis‐2 and from the Community Climate System Model version 4 North American Multi‐Model Ensemble retrospective forecasts for the period of January 1979 to December 2013. Results indicate potential for predicting SWH with several months lead time during boreal summers after the warm phase of El Niño–Southern Oscillation (ENSO) measured by deterministic and probabilistic skill scores in the Northwest Pacific and Bay of Bengal. During these summers, SWH is smaller than normal due to reduced atmospheric synoptic activity associated with an anomalously anticyclone in the western Pacific, leading to larger signal‐to‐noise ratio in the 10 m winds, hence increasing SWH prediction skill. It is shown that ENSO has a nonlinear influence on the number of extremely large SWH events, with reduced number of extreme occurrences during boreal summers after the warm phase of ENSO.

The effects of the El Niño Southern Oscillation on skin and skin-related diseases: a message from the International Society of Dermatology Climate Change Task Force.

The El Niño Southern Oscillation (ENSO) is a complex climate phenomenon occurring in the Pacific Ocean at intervals of 2-7 years. The term refers to fluctuations in ocean temperatures in the tropical eastern Pacific Ocean (El Niño [the warm phase of ENSO] and La Niña [the cool phase of ENSO]) and in atmospheric pressure across the Pacific basin (Southern Oscillation). This weather pattern is attributed with causing climate change in certain parts of the world and is associated with disease outbreaks. The question of how ENSO affects skin and skin-related disease is relatively unanswered. We aimed to review the literature describing the effects of this complex weather pattern on skin. El Niño has been associated with increases in the occurrence of actinic keratosis, tinea, pityriasis versicolor, miliaria, folliculitis, rosacea, dermatitis by Paederus irritans and Paederus sabaeus, and certain vector-borne and waterborne diseases, such as dengue fever, leishmaniasis, Chagas' disease, Barmah Forest virus, and leptospirosis, and with decreases in the occurrence of dermatitis, scabies, psoriasis, and papular urticaria. La Niña has been associated with increases in the occurrence of varicella, hand, foot, and mouth disease, and Ross River virus (in certain areas), and decreases in viral warts and leishmaniasis. Reports on the effects of ENSO on skin and skin-related disease are limited, and more studies could be helpful in the future.

Seasonal reversal of the influence of El Niño–Southern Oscillation on very large wildfire occurrence in the interior northwestern United States

AbstractSatellite‐mapped fire perimeters and the multivariate El Niño–Southern Oscillation index were used to examine the impact of concurrent El Niño–Southern Oscillation (ENSO) phase on very large fire (VLF) occurrences over the intermountain northwestern United States (U.S.) from 1984 to 2012. While the warm phase of ENSO promotes drier and warmer than normal conditions across the region during winter and spring that favor widespread fire activity the following summer, a reduction in VLFs was found during the warm phase of ENSO during summer concurrent with the fire season. This paradox is primarily tied to an anomalous upper level trough over the western U.S. and positive anomalies in integrated water vapor that extend over the northwestern U.S. during summers when the warm phase of ENSO is present. Collectively, these features result in widespread increases in precipitation amount during the summer and a curtailment of periods of critically low‐fuel moistures that can carry wildfire.

Anomalous Weather Patterns in Relation to Heavy Precipitation Events in Japan during the Baiu Season

Abstract Anomalous weather patterns (WPs) in relation to heavy precipitation events during the baiu season in Japan are investigated using a nonlinear classification technique known as the self-organizing map (SOM). The analysis is performed on daily time scales using the Japanese 55-year Reanalysis Project (JRA-55) to determine the role of circulation and atmospheric moisture on extreme events and to investigate interannual and interdecadal variations for possible linkages with global-scale climate variability. SOM is simultaneously employed on four atmospheric variables over East Asia that are related to baiu front variability, whereby anomalous WPs that dominated during the 1958–2011 period are obtained. Our analysis extracts seven typical WPs, which are linked to frequent occurrences of heavy precipitation events. Each WP is associated with regional variations in the probability of extreme precipitation events. On interannual time scales, El Niño–Southern Oscillation (ENSO) affects the frequency of the WPs in relation to the heavy rainfall events. The warm phase of ENSO results in an increased frequency of a WP that provides a southwesterly intrusion of high equivalent potential temperature at low levels, while the cold phase provides southeastern intrusion. In addition, the results of this analysis suggest that interdecadal variability of frequency for heavy rainfall events corresponds to changes in frequency distributions of WPs and are not due to one particular WP.

The sunshine state: investigating external drivers of sky conditions

Following rapid population growth and urbanization in Florida, there is an increased demand for energy. The state currently gets more than 50% of its electricity from burning imported natural gas. As the future of fossil fuels is uncertain and their impact on climate has proven negative, one could expect great potential and interest in further developing the solar industry, which utilizes the most prominent of renewable energy sources in Florida. Solar energy production is dependent on the supply of clear skies and plentiful insolation. This paper seeks to explain variations in the number of clear sunny days by identifying the months with clearest (cloudiest) sky conditions during the years 1950–2009 in relation to some of the most dominant low frequency climate patterns of variability in the Northern Hemisphere. The hypergeometric distribution is used to test for significant association between the phases of El Niño Southern Oscillation (ENSO), North Atlantic Oscillation, Pacific-North American, and Atlantic Multi-decadal Oscillation (AMO) and cloud cover in Florida. We find that ENSO and AMO significantly impact the occurrence of clear/cloudy skies with the warm phase of ENSO associated with cloudier conditions across much of the state and the warm phase of AMO bringing clearer conditions to northern stations.

Soil moisture variability in Iowa

ABSTRACTThirty years (1954–1983) of soil moisture data from 60 sites in Iowa are used to identify the dominant modes of spatial and temporal variability. Both soil moisture and precipitation exhibit a northwest to southeast gradient across Iowa, with wetter conditions in the southeast and drier conditions in the northwest. Climate and soil characteristics are responsible for the spatial variations in soil moisture. There is a pronounced annual soil moisture cycle that has three distinct phases: a moist phase (April–June), a drying phase (June–September), and a recharge phase (September–November). Spring soil moisture is strongly controlled by the amount of recharge during the previous fall. There is no evidence of a long‐term trend in soil moisture; however there is significant interannual variability and about 20% of the variance is attributable to El Niño‐Southern Oscillation (ENSO). The warm (cold) phase of ENSO is associated with above (below) average soil moisture in Iowa. The multi‐decadal soil moisture record is used to analyse drought frequency and magnitude in Iowa, and to place the 2012 Midwest drought in historical context. In general, Iowa drought is more common during the latter part of the warm season (August through November), but tends to be more severe during the early part of the warm season (April–July). We find that the 2012 drought was unusual because it persisted and intensified throughout the growing season.

Precipitation variability over UAE and global SST teleconnections

AbstractThe present study investigates the role of equatorial Pacific sea surface temperatures (SSTs) on the precipitation variability over the United Arab Emirates (UAE) and adjoining Middle East regions. Monthly precipitation data (1981–2011) assembled from rain gauge stations located in the UAE along with other global reanalysis data sets are used to explore the teleconnections. It is observed that statistically significant correlations exist between precipitation over the UAE and the equatorial Pacific and North Atlantic SSTs. Canonical correlation analysis between the monthly winter precipitation and the global SSTs (60°S to 60°N) reveals that the major portion of the precipitation variability is influenced by equatorial Pacific SSTs associated with El Niño–Southern Oscillation (ENSO). The moisture budget analysis reveals the distinct change in the anomalous circulation (cyclonic and anticyclonic) associated with strong convergence and divergence of the moisture flux during the warm and cold phases of ENSO, respectively. Further, the composite analysis of upper troposheric zonal wind shows the equatorward shift (~2° latitude) of subtropical jet stream (STJ) over the Middle East during the warm phase of ENSO affecting the weather in the UAE. The findings suggest that the teleconnection linking ENSO and the precipitation over UAE and adjoining regions is mediated by the response of STJ to Rossby waves.

Relationship between fine-mode AOD and precipitation on seasonal and interannual time scales

On seasonal and interannual time scales, weather is highly influential in aerosol variability. In this study, we investigate the relationship between fine-mode AOD (fAOD) and precipitation on these scales, in order to unravel the effect of wet weather on aerosol amount. We find with integrated satellite and ground observations that biomass burning related fAOD has a relatively greater seasonal variation than fossil fuel combustion related fAOD. It is also found that wet weather reduces biomass burning fAOD and increases fossil fuel combustion fAOD. Aerosol simulation models forced by reanalyses consistently simulate the biomass burning fAOD reduced during wet weather but only in the tropics and furthermore do not consistently increase fossil fuel combustion fAOD during wet conditions. The identified relationship between fAOD and precipitation in observations allows for seasonal predictability of fAOD, since average precipitation can be predicted a few to several months in advance due to the well-established predictability of El Niño-Southern Oscillation (ENSO). We reveal ENSO-covariant fAOD using a rotated component principal analysis of combined interannual variation of sea surface temperature, precipitation and fAOD. During the warm phase of ENSO, we find that fAOD increases over Indonesia and the eastern coastal area of China, and decreases over South Asia, the Amazon and the continental parts of China.

Impact of Strong Tropical Volcanic Eruptions on ENSO Simulated in a Coupled GCM

Abstract The impact of strong tropical volcanic eruptions (SVEs) on the El Niño–Southern Oscillation (ENSO) and its phase dependency is investigated using a coupled general circulation model (CGCM). This paper investigates the response of ENSO to an idealized SVE forcing, producing a peak perturbation of global-mean surface shortwave radiation larger than −6.5 W m−2. Radiative forcing due to volcanic aerosols injected into the stratosphere induces tropical surface cooling around the volcanic forcing peak. Identical-twin forecast experiments of an ENSO-neutral year in response to an SVE forcing show an El Niño–like warming lagging one year behind the peak forcing. In addition to a reduced role of the mean subsurface water upwelling (known as the dynamical thermostat mechanism), the rapid land surface cooling around the Maritime Continent weakens the equatorial Walker circulation, contributing to the positive zonal gradient of sea surface temperature (SST) and precipitation anomalies over the equatorial Pacific. Since the warm and cold phases of ENSO exhibit significant asymmetry in their transition and duration, the impact of a SVE forcing on El Niño and La Niña is also investigated. In the warm phase of ENSO, the prediction skill of the SVE-forced experiments rapidly drops approximately six months after the volcanic peak. Since the SVE significantly facilitates the duration of El Niño, the following transition from warm to cold ENSO is disrupted. The impact of SVE forcing on La Niña is, however, relatively weak. These results imply that the intensity of a dynamical thermostat-like response to a SVE could be dependent on the phase of ENSO.

The Hadley Circulation in Reanalyses: Climatology, Variability, and Change

Abstract Analysis of the annual cycle of intensity, extent, and width of the Hadley circulation across a 31-yr period (1979–2009) from all existent reanalyses reveals a good agreement among the datasets. All datasets show that intensity is at a maximum in the winter hemisphere and at a minimum in the summer hemisphere. Maximum and minimum values of meridional extent are reached in the respective autumn and spring hemispheres. While considering the horizontal momentum balance, where a weakening of the Hadley cell (HC) is expected in association with a widening, it is shown here that there is no direct relationship between intensity and extent on a monthly time scale. All reanalyses show an expansion in both hemispheres, most pronounced and statistically significant during summer and autumn at an average rate of expansion of 0.55° decade−1 in each hemisphere. In contrast, intensity trends are inconsistent among the datasets, although there is a tendency toward intensification, particularly in winter and spring. Correlations between the HC and tropical and extratropical large-scale modes of variability suggest interactions where the extent of the HC is influenced by El Niño–Southern Oscillation (ENSO) and the annular modes. The cells tend to shrink (expand) during the warm (cold) phase of ENSO and during the low (high) phase of the annular modes. Intensity appears to be influenced only by ENSO and only during spring for the southern cell and during winter for the northern cell.

On the relationship between total ozone and atmospheric dynamics and chemistry at mid-latitudes – Part 2: The effects of the El Niño/Southern Oscillation, volcanic eruptions and contributions of atmospheric dynamics and chemistry to long-term total ozone changes

Abstract. We present the first spatial analysis of "fingerprints" of the El Niño/Southern Oscillation (ENSO) and atmospheric aerosol load after major volcanic eruptions (El Chichón and Mt. Pinatubo) in extreme low and high (termed ELOs and EHOs, respectively) and mean values of total ozone for the northern and southern mid-latitudes (defined as the region between 30° and 60° north and south, respectively). Significant influence on ozone extremes was found for the warm ENSO phase in both hemispheres during spring, especially towards low latitudes, indicating the enhanced ozone transport from the tropics to the extra-tropics. Further, the results confirm findings of recent work on the connection between the ENSO phase and the strength and extent of the southern ozone "collar". For the volcanic eruptions the analysis confirms findings of earlier studies for the northern mid-latitudes and gives new insights for the Southern Hemisphere. The results provide evidence that the negative effect of the eruption of El Chichón might be partly compensated by a strong warm ENSO phase in 1982–1983 at southern mid-latitudes. The strong west-east gradient in the coefficient estimates for the Mt. Pinatubo eruption and the analysis of the relationship between the AAO and ENSO phase, the extent and the position of the southern ozone "collar" and the polar vortex structure provide clear evidence for a dynamical "masking" of the volcanic signal at southern mid-latitudes. The paper also analyses the contribution of atmospheric dynamics and chemistry to long-term total ozone changes. Here, quite heterogeneous results have been found on spatial scales. In general the results show that EESC and the 11-yr solar cycle can be identified as major contributors to long-term ozone changes. However, a strong contribution of dynamical features (El Niño/Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Antarctic Oscillation (AAO), Quasi-Biennial Oscillation (QBO)) to ozone variability and trends is found at a regional level. For the QBO (at 30 and 50 hPa), strong influence on total ozone variability and trends is found over large parts of the northern and southern mid-latitudes, especially towards equatorial latitudes. Strong influence of ENSO is found over the Northern and Southern Pacific, Central Europe and central southern mid-latitudes. For the NAO, strong influence on column ozone is found over Labrador/Greenland, the Eastern United States, the Euro-Atlantic Sector, and Central Europe. For the NAO's southern counterpart, the AAO, strong influence on ozone variability and long-term changes is found at lower southern mid-latitudes, including the southern parts of South America and the Antarctic Peninsula, and central southern mid-latitudes.

The vertical and spatial structure of ENSO in the upper troposphere and lower stratosphere from GPS radio occultation measurements

The vertical and spatial structure of the atmospheric El Niño‐Southern Oscillation (ENSO) signal is investigated using radio occultation (RO) data from August 2006 to December 2010. Due to their high vertical resolution and global coverage, RO data are well suited to describe the full 3‐dimensional ENSO structure in the troposphere and lower stratosphere. We find that interannual temperature anomalies in the equatorial region show a natural decomposition into zonal‐mean and eddy (deviations from the zonal‐mean) components that are both related to ENSO. Consistent with previous studies, we find that during the warm phase of ENSO, zonal‐mean temperatures increase in the tropical troposphere and decrease in the tropical stratosphere. Maximum warming occurs above 8 km, and the transition between warming and cooling occurs near the tropopause. This zonal‐mean response lags sea surface temperature anomalies in the eastern equatorial Pacific by 3 months. The atmospheric eddy component, in contrast, responds rapidly (within 1 month) to ENSO forcing. This signal features a low‐latitude dipole between the Indian and Pacific Oceans, with off‐equatorial maxima centered around 20° to 30° latitude in both hemispheres. The eddy response pattern attains maximum amplitude in the upper troposphere near 11 km and (with opposite polarity) in a shallow layer near the tropopause at approximately 17 km. The eddy ENSO signal tends to be out‐of‐phase between low and middle latitudes in both the troposphere and lower stratosphere.

Relationship between ENSO phases with Northwest India winter precipitation

AbstractWinter (December, January, February and March—DJFM) precipitation variability over Northwestern India is examined in association with large scale global forcing of El Niño Southern Oscillation (ENSO). During winter, Northwestern India receives precipitation in the form of snow, by eastward moving synoptic weather systems—western disturbances (WDs). An attempt is made to define relationship between wintertime large scale atmospheric fields with different phases of ENSO. A composite set of El Niño (10 years) and La Niña (8 years) is prepared to analyse role of large scale fields followed by ENSO response with wintertime precipitation over Northwestern India. Significant correlation between the two indicates increased (decreased) precipitation over Northwestern India during warm (cold) phase of ENSO. Higher geopotential height, with stronger southwesterly to northwesterly wind flow suggests more moisture flux from the Caspian and Arabian Sea. Empirical orthogonal function analysis also suggests instance of ENSO impact on Northwest India winter precipitation. Copyright © 2012 Royal Meteorological Society

Zonal and meridional patterns of phytoplankton biomass and carbon fixation in the Equatorial Pacific Ocean, between 110°W and 140°W

Primary production (P prim) and calcification (C calc) were measured in the eastern and central Equatorial Pacific during December 2004 and September 2005, between 110°W and 140°W. The design of the field sampling allowed partitioning of P prim and total chlorophyll a (B) between large (>3 μm) and small (0.45-3 μm) phytoplankton cells. The station locations allowed discrimination of meridional and zonal patterns. The cruises coincided with a warm El Niño Southern Oscillation (ENSO) phase and ENSO-neutral phase, respectively, which proved to be the major factors relating to the patterns of productivity. Production and biomass of large phytoplankton generally covaried with that of small cells; large cells typically accounted for 20-30% of B and 20% of P prim. Elevated biomass and primary production of all size fractions were highest along the equator as well as at the convergence zone between the North Equatorial Counter Current and the South Equatorial Current. C calc by >0.4 μm cells was 2-3% of P prim by the same size fraction, for both cruises. Biomass-normalized P prim values were, on average, slightly higher during the warm-phase ENSO period, inconsistent with a “bottom-up” control mechanism (such as nutrient supply). Another source of variability along the equator was Tropical Instability Waves (TIWs). Zonal variance in integrated phytoplankton biomass (along the equator, between 110° and 140°) was almost the same as the meridional variance across it (between 4° N and 4° S). However, the zonal variance in integrated P prim was half the variance observed meridionally. The variance in integrated C calc along the equator was half that seen meridionally during the warm ENSO phase cruise whereas during the ENSO-neutral period, it was identical. No relation could be observed between the patterns of integrated carbon fixation (P prim or C calc) and integrated nutrients (nitrate, ammonium, silicate or dissolved iron). This suggests that the factors controlling integrated P prim or C calc are more complex than a simple bottom-up supply model and likely also will involve a top-down grazer-control component, as well. The carbon fixation within the Equatorial Pacific is well balanced with diatom and coccolithophore production contributing a relatively steady proportion of the total primary production. This maintains a steady balance between organic and inorganic production, relevant to the ballasting of organic matter and the export flux of carbon from this important upwelling region.

Recurrent, thermally-induced shifts in species distribution range in the Humboldt current upwelling system

El Niño-Southern Oscillation (ENSO) is a global climate variablility, which fundamentally influences environmental patterns of the Humboldt Current System (HCS) off Chile and Peru. The surf clams Donax obesulus and Mesodesma donacium are dominant and highly productive bivalves of exposed sandy beaches of the HCS. Existing knowledge indicates that El Niño (EN, warm phase of ENSO) and La Niña (LN, cold phase of ENSO) affect populations of both species in a different way, although understanding of the mechanisms underlying these effects is still lacking. The aim of this study was to test hypotheses attempting to explain field observations on the effect of strong EN or LN events by using controlled experimental conditions. Growth and mortality rates of both species were registered during a four-week experiment under EN temperature conditions, normal temperature conditions and LN temperature conditions. While D. obesulus exhibited reduced growth and higher mortality under LN conditions, M. donacium showed reduced growth and higher mortality under EN conditions. The results clearly indicate different temperature tolerance windows for each species, possibly reflecting the evolutionary origins of the Donacidae and Mesodesmatidae in regions with contrasting temperature regimes. These results provide experimental support for previous hypotheses suggesting that thermal tolerance is the driving factor behind observed changes in the species distributions of D. obesulus and M. donacium during the extreme phases of ENSO.

Why Is ENSO Influencing Northwest India Winter Precipitation in Recent Decades?

AbstractThis study examines decadal changes of the El Niño–Southern Oscillation (ENSO) influence on the interannual variability of northwest India winter precipitation (NWIWP). The analysis is based on correlations and regressions performed using India Meteorological Department (IMD) records based on station data and reanalysis fields from 1950 to 2008. The authors find that the interannual variability of NWIWP is influenced by the ENSO phenomenon in the recent decades. This conclusion is supported by a consistency across the different observational datasets employed in this study and confirmed by numerical modeling. A physical mechanism for such an influence is proposed, by which western disturbances (WDs) are intensified over northwest India because of a baroclinic response due to Sverdrup balance related to large-scale sinking motion over the western Pacific during the warm phase of ENSO. This response causes an upper-level cyclonic circulation anomaly north of India and a low-level anticyclonic anomaly over southern and central India. The cyclonic circulation anomaly intensifies the WDs passing over northwest India.

Increasing influence of ENSO and decreasing influence of AO/NAO in the recent decades over northwest India winter precipitation

The interannual variability of northwest India winter precipitation (NWIWP) is examined in association with the variability of surface temperature, mean sea level pressure, tropospheric geopotential height, and wind patterns over the globe to study the changes in the large‐scale circulation features associated with NWIWP. The data for the study have been considered from 1950 to 2008 (for a period of 59 years). The analysis is based on correlations and composites performed using India Meteorological Department records based on station data. We find that the interannual variability of NWIWP is influenced by Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and El Niño–Southern Oscillation (ENSO) phenomena. Above‐normal NWIWP is associated with the positive phase of AO/NAO and the warm phase of ENSO, but showing strong secular variations. During the recent decades, the influence of ENSO over NWIWP has increased while the influence of AO/NAO has decreased. This conclusion is supported by a consistency across the different observation data sets employed in the study. A physical mechanism for such effect is proposed, by which western disturbances are intensified over NW India by the intensification of Asian westerly jet stream over Middle East during positive phase of AO/NAO and shift and intensification of Asian jet to the lower latitudes during the warm phase of ENSO.