El Nino-Southern Oscillation Period Research Articles

Southern Indian Ocean Dipole as a trigger for Central Pacific El Niño since the 2000s

Despite decades of effort, predicting the El Niño-Southern Oscillation (ENSO) since the 2000s has become increasingly challenging. This is due to the weaker coupling between the ENSO and well-known precursors in tropical ocean basins, particularly in the Indian Ocean. Here we show that the Southern Indian Ocean Dipole (SIOD), which is characterized by an east-west-oriented sea surface temperature dipole pattern over the southern Indian Ocean, has become a key precursor of Central Pacific El Niño since the 2000s with a 14-month lead. The role of the SIOD in the subsequent year’s ENSO is distinctive from the equatorial Indian Ocean Dipole mode in that it prolongs the ENSO period. The westward-shifted ENSO has sustained simultaneous SIOD events for longer periods since the 2000s, which leads to weak but persistent westerly anomalies over the western Pacific. This eventually results in the development of the Central Pacific El Niño in the subsequent year.

The BEAP Teleconnection and Its Relationship With ENSO in CMIP6: Present and Future Projections

AbstractThis study investigates the historical simulation of the Bay of Bengal–East Asia–Pacific (BEAP) teleconnection and its relationship with the El Niño–Southern Oscillation (ENSO) using 16 models from the Coupled Model Intercomparison Project phase 6 (CMIP6). The CMIP6 models can reasonably well reproduce the BEAP‐related diabatic heating variables and the ENSO sea surface temperature anomaly (SSTA). The CMIP6 models also show good performance in simulating the BEAP pattern at subseasonal and interannual time scales. Further investigation reveals that the BEAP–ENSO relationship changes from statistically insignificant to significant on an interdecadal timescale. Significant BEAP–ENSO correlation is associated with more positive SSTA over the tropical Indian Ocean (TIO) than in the period of insignificant BEAP–ENSO correlation. The positive SSTA difference acts as a heat source that intensifies the in situ atmospheric convection to generate considerable diabatic heating around the BEAP source region. The covariation of the Indo‐Pacific SSTA exhibits a tripole pattern, namely positive SSTA over the TIO, negative SSTA over the tropical central‐western Pacific, and positive SSTA over the tropical eastern Pacific. The SSTA tripole pattern is ascribed to the Indo‐Pacific capacitor effect and zonal advective feedback. The surface wind stress is weaker during the period of significant BEAP–ENSO correlation than during the non‐significant BEAP–ENSO period. The surface wind stress is also identified as a crucial factor in the projected changes in the BEAP teleconnection. The results indicate that the enhanced zonal wind stress results in more positive BEAP cases in the 21st‐century projection.

The Different Relationships between the ENSO Spring Persistence Barrier and Predictability Barrier

Abstract In this paper, we investigate the relationship between the El Niño–Southern Oscillation (ENSO) spring persistence barrier (PB) and predictability barrier (PD) and apply it to explain the interdecadal modulation of ENSO prediction skill using the anomaly correlation coefficient (ACC). Previous studies showed that a longer persistence (i.e., autocorrelation) tends to produce a higher prediction skill. Using the recharge oscillator model of ENSO, both analytical and numerical solutions suggest that the predictability (i.e., ACC) is related to the persistence of sea surface temperature (SST) and cross correlation between SST and subsurface ocean heat content in the tropical Pacific. In particular, a larger damping rate in SST anomalies will lead to a lower persistence and ACC and a stronger PD. However, a shortened ENSO period, which controls the cross correlation, will lead to a lower persistence but a higher ACC associated with a weaker PD. Finally, we apply our solutions to observations and suggest that a higher ACC associated with a weaker PD after 1960 is caused by the shortened ENSO period.

The Signal‐to‐Noise Paradox in ENSO Prediction: Role of ENSO Growth Rate and Period

AbstractThis paper investigates the relationship between heterogeneous predictability and homogeneous predictability for El Niño‐Southern Oscillation (ENSO) prediction. Previous studies showed that a larger heterogeneous predictability than homogeneous predictability, referred as the signal‐to‐noise paradox, is caused by a higher persistence in the observations than in the model. By using the recharge oscillator model, it is found analytically and numerically that both ENSO growth rate and period contribute to this paradox. A larger ENSO growth rate in the observation can lead to a higher persistence and heterogeneous predictability. However, a larger ENSO growth rate associated with a shortened ENSO period can also lead to this paradox although the persistence of the observation is lower than forecast model. Finally, we apply it to explain the paradox after 1960 is caused by a larger ENSO growth rate and a shortened ENSO period.

Contribution of Ekman Transport to the ENSO Periodicity Estimated With an Extended Wyrtki Index

AbstractOff‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtki‐Jin index, to evaluate Ekman transport. The extended method applied to reanalysis data sets shows that the ENSO period lengthens by approximately 30%, reconciling the discrepancy between the observed ENSO frequency and estimates from the conventional method. This result demonstrates that the self‐inhibitory process by Ekman transport is an inherent feature of the observed ENSO periodicity. In addition, Ekman transport efficiently works with a wide meridional structure of ENSO, suggesting that the meridional width of individual ENSO events explains the difference in their duration.

The Effect of ENSO (El Nino Southern Oscillation) phenomenon on Fishing Season of Small Pelagic Fishes in Indonesia Waters

The El Nino Southern Oscillation (ENSO) phenomenon causes changes in environmental conditions such as water temperature, salinity, and rainfall. In fisheries sector, the changing environment has affected the fishing seasons and Catch per Unit Effort (CPUE) of some pelagic species. This research was conducted by calculating CPUE and fishing season index for several small pelagic fishes in Makassar Strait, Bali Strait, and Aceh waters, then comparing the index value with the fishing season pattern in two extreme periods that are 2010-2011 and 2016 - 2017. An ANOVA test was conducted to assess the significant difference between normal and extreme conditions. The results of the analysis showed that there was a significant different in CPUE between average normal condition and ENSO period. Every single species showed different response to ENSO event, mostly decreased in CPUE relate to El Nino event, except for sardine in Bali and Makassar Strait and scad in Makassar Strait. ENSO affects shift in the fishing season of big-eye scad, scad, sardine, and neritic tuna in Makassar Strait, Bali Strait, and around Aceh waters. Indian mackerel in Makassar Strait showed no change in fishing season but the CPUE showed lower than normal condition. This study shows that ENSO was significantly affected fisheries in Indonesia waters.

A Theory of the Spring Persistence Barrier on ENSO. Part III: The Role of Tropical Pacific Ocean Heat Content

AbstractIn this paper, we investigate the relationship between upper ocean heat content (OHC) and El Niño–Southern Oscillation (ENSO) sea surface temperature (SST) anomalies mainly using the neutral recharge oscillator (NRO) model both analytically and numerically. Previous studies showed that spring OHC, which leads SST by 6–12 months, represents a major source of predictability for ENSO. It is suggested that this seasonality is caused by the seasonally varying growth rate in SST anomalies. Moreover, a shortened ENSO period will lead to a reduced SST predictability from OHC, with the most significant decrease occurring in the latter half of the calendar year. The cross-correlation relationship between OHC and ENSO SST anomalies is further identified in the damped and self-excited version of the recharge oscillator model. Finally, we suggest that the seasonal growth rate of ENSO anomalies is the cause of the seasonality in the effectiveness of OHC as a predictor in ENSO forecasting, particularly as it relates to the boreal spring persistence barrier and associated spring predictability barrier. We also explain the shorter lead time between spring OHC and ENSO SST anomalies after the turn of the twenty-first century in terms of the apparent higher frequency of the ENSO period.

Thermal Front Variability during The El Nino Southern Oscillation (ENSO) in The Banda Sea Using Remotely Sensed Data

The Banda Sea is one of the routes of global ocean currents that move from the Pacific Ocean to the Indian Ocean. This flow is known as Indonesian Through Flow (ITF). The Banda Sea is an area where warm and cold water masses meet, so it has the potential for a thermal front. This study aims to understand the variability of thermal front in the Banda Sea during the El Nino Southern Oscillation period. Southern Oscillation Index (SOI) and sea surface temperature (SST) data in 2010, 2012 and 2015 were used in this study. SOI data was obtained from http://www.bom.gov.au and SST data was obtained from http://oceancolor.gsfc.nasa.gov. The data were processed using ArcGIS 10.4 software and Ms. Office 2013. The results showed the La Nina period occurs in July - December 2010, the Normal period occurs in July - December 2012, and the El Nino period occurs in May - October 2015. In general, during La Nina, the mean SST has higher values than the other periods. On the other hand, the highest thermal front occurs during the El Niño period (10584), followed by the Normal period (7544) and the lowest during the La Niña period (5961), respectively.

A Theory of the Spring Persistence Barrier on ENSO. Part I: The Role of ENSO Period

AbstractIn this paper, we investigate the role of the period of El Niño–Southern Oscillation (ENSO) in the spring persistence barrier (SPB), mainly using the neutral recharge oscillator (NRO) model both analytically and numerically. It is suggested that a shorter ENSO period strengthens the SPB. Moreover, in contrast to the strict phase locking of the SPB in the Langevin equation, the phase of the SPB is no longer locked exactly to a particular time of the calendar year in the NRO model. Instead, the phases of the SPB for different initial months shift earlier with lag months of maximum persistence decline. In particular, the phase of the SPB will be shifted from the early summer to early spring, corresponding to the initial months of the early half year and later half year. This feature demonstrates that for the later half year, ENSO predictability decreases as the presence of ENSO period. For realistic parameters, the range of the phase change is modest, smaller than 2–3 months. A similar phase shift is also identified for the SPB in the damped ENSO regime, unstable ENSO regime, and observations. Our theory provides a null hypothesis for the role of ENSO period with regard to the SPB.

Impact of climate variabilities on trans-oceanic flight times and emissions during strong NAO and ENSO phases

This study investigates the impact of the North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO) on trans-oceanic round-trip flight times and consequent CO2 emissions over the north Atlantic and eastern Pacific regions. For three strongest winter periods of both polarity during 1979–2016, daily mean wind data are used to compute the wind-optimal flight trajectories at cruising altitudes. Results show that intensified upper-level jet streams during the +NAO winters provide stronger headwinds for westbound flights between the eastern US and the western Europe. This causes 4.24 ∼ 9.35 min increase in an averaged total round-trip journey time during the +NAO compared to −NAO winters. In the eastern Pacific region, the jet stream is extended eastward towards the southwestern US during the +ENSO period, which lengthens the travel time for westbound flights between Hawaii and the west coast of the US. The increase in travel time of westbound flights is greater than the corresponding decrease in travel time for eastbound flights, resulting in a 5.92 ∼ 8.73 min increase of the averaged total round-trip time during the +ENSO compared to the −ENSO periods. Extrapolating these results to overall trans-oceanic air traffic suggests that aircraft will take a total of 1908 ∼ 4207 (888 ∼ 1309) extra hours during the +NAO (+ENSO) than the −NAO (–ENSO) winters over the North Atlantic (Eastern Pacific) regions, requiring 6.9 ∼ 15 (3.2 ∼ 4.7) million US gallons of extra fuel burned at a cost of 21 ∼ 45 (9.6 ∼ 14) million US dollars and 66 ∼ 144 (31 ∼ 45) million kg of extra CO2 emissions to all trans-oceanic traffic. In +ENSO and +NAO winters, the chances of a given flight having a slower round-trip flight time with more fuel burn and CO2 emissions are 2–10 times higher than in a −ENSO or −NAO winter. These results have significant implications for the planning of long-term flight routes with climate variability.

The Summer/Fall Variability of the Southern East/Japan Sea in the ENSO Period

The summer/fall variability of the upper ocean circulations and properties of the East/Japan Sea occurring during the El Nino-Southern Oscillation (ENSO) period is investigated by use of the high-resolution HYCOM reanalysis data. From August to November of the La Nina period, the inflow through the Korea/Tsushima Strait is intensified, and so are the East Korean Warm Current (EKWC) and the Nearshore Branch of the Tsushima Warm Current (NBTWC) in the southern East/Japan Sea. The intensified EKWC induces an offshore current anomaly near 36°N, weakening the anticyclonic gyre circulation in Ulleung basin, and the intensified NBTWC leads to weakening of the Offshore Branch of the Tsushima Warm Current (OBTWC). The sea surface temperature begins to increase from the northeastern coast of Japan in August, and its positive anomalies expand to the largest area of southern East/Japan Sea in October and last until November. The positive anomalies of sea surface salinity are mainly observed along the south and east coast of Korea. The cyclonic gyre circulation anomaly in Ulleung Basin is accompanied by the negative SSH anomaly and shallowing of the upper ocean isopycnal lines, which leads to substantial cooling of the subsurface water. The reverse is true in the El Nino period.

Spatial and temporal variability of sea surface temperature during El-Niño Southern Oscillation and Indian Ocean Dipole in the Strait of Malacca and Andaman Sea

This study examined the long-term trends and the impacts of climate variability caused by El-Niño Southern/Oscillation (ENSO) and Indian Ocean Dipole (IOD) by using 33-year Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST). The data were retrieved from year 1982 to 2014, hence enabled the observation of both ENSO and IOD events based on this long-term SST trends in the Strait of Malacca (SoM) and Andaman Sea. The 4-km resolution of satellite-derived SST data had been gathered from the National Oceanic and Atmospheric Administration (NOAA). Geographical location portrayed that the seasonal cycle of SST in the SoM and Andaman Sea was characterised by the Asian Monsoon. The correlations between monthly SST in regions with wind velocity, SST anomaly, and indices (Oceanic Niño Index (ONI) and Dipole Mode Index (DMI)) were analysed based on wavelet coherence (WTC) analysis. The seasonal SST in SoM revealed that the temperature was above normal trend during the southwest monsoon, wherein the region experienced warmer temperature especially during the ENSO period due to seasonal wind velocity. The results demonstrated that in certain years, such as in 1992, ENSO was dominating while in 1994, IOD was influential. During the strong El-Niño event in 1997, ONI did not correlate with SST trends in the SoM, whereas during positive IOD in 1994 revealed that DMI did not correlate with SST trends in both SoM and Andaman Sea. The warming trend seemed to have significantly increased over the past 33 years, as displayed by the continuous increment in temperature throughout the years in the study region.

FIO‐ESM Version 2.0: Model Description and Evaluation

AbstractThe First Institute of Oceanography Earth System Model (FIO‐ESM) version 2.0 was developed and participated in the Climate Model Intercomparison Project phase 6 (CMIP6). In comparison with FIO‐ESM v1.0, all component models of FIO‐ESM v2.0 are updated, and their resolutions are fined. In addition to the non‐breaking surface wave‐induced mixing (Bv), which has also been included in FIO‐ESM v1.0, there are three more distinctive physical processes in FIO‐ESM v2.0, including the effect of surface wave Stokes drifts on air–sea momentum and heat fluxes, the effect of wave‐induced sea spray on air–sea heat fluxes and the effect of sea surface temperature (SST) diurnal cycle on air–sea heat and gas fluxes. The FIO‐ESM v2.0 has conducted the CMIP6 Diagnostic, Evaluation, and Characterization of Klima (DECK) and historical experiments. The historical simulation of FIO‐ESM v2.0 for 1850–2014 is evaluated, including the surface air temperature (SAT), precipitation, SST, Atlantic Meridional Overturning Circulation (AMOC), El Niño‐Southern Oscillation (ENSO), and so forth. The climate changes with respect to SAT and SST global warming and decreasing AMOC are well reproduced by FIO‐ESM v2.0. The correlation coefficient of the global annual mean SAT anomaly can reach 0.92 with observations. In particular, the large warm SST bias at the east coast of tropical Pacific from FIO‐ESM v1.0, which is a common challenge for all climate models, is dramatically reduced in FIO‐ESM v2.0 and the ENSO period within the range of 2–7 years is well reproduced with the largest variation of SST anomalies occurring in boreal winter, which is consistent with observations.

Analisis Heat Stress saat Kondisi El – Niño, La – Niña, dan Netral di Wilayah Jakarta dan Sekitarnya Periode 1993 – 2018

Jakarta and its surroundings areas that have high surface temperatures due to high total CO2 emissions. High CO2 emissions will cause surface temperatures to increase due to the phenomenon of greenhouse gases. An increase in temperature will also have an impact on increasing heat strs which will be dangerous if no steps are taken for prevention. The El Niño – Southern Oscillation (ENSO) factor is also considered influential in terms of the value of heat stress. Calculating heat stress in Jakarta and surrounding areas can be done by the Wet-Bulb Globe Temperature Index (WBGT) method. Data was obtained from six station points in Jakarta and surrounding areas in 1993 – 2018 consisting of daily air temperature and humidity and using Oceanic Nino Index (ONI) data to determine the ENSO period. The results show that the highest WBGT were identified in four regions, Tanjung priok, Kemayoran, Cengkareng and South Tangerang with ISBB values ​​of 30 – 32.5 (the heat stress increases to dangerous levels), while for the other two, Curug and Dramaga Bogor have WBGT values at 28.5 – 30 (heat stress increases). The most dangerous WBGT category was in Tanjung priok in April during El – Niño with 71% (58) days were in the dangerous category, while the WBGT category had the least heat stress at Dramaga in August when it was neutral with 17.7% (66) days were in the no heat stress category. When El-Niño ISBB was higher than when it is neutral and La-Niña and has higher minimum and maximum extreme values than when it is neutral and La – Niña.

Variability of lake surface water temperature: a case study during El Nino and La Nina events over the Himalayan lake region

ABSTRACT Lake surface water temperature (LSWT) is sensitive to long-term changes in thermal structure of lakes. This paper investigates the association of LSWT with meteorological parameters and its impact during El Nino and La Nina events over the Himalayan lake region, using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data from 2001 to 2017. The observed daytime rates of change of LSWT was −0.05°C/year whereas it was +0.01°C/year for nighttime. The cooling of lake during daytime could be attributed to increase in rainfall and decrease in solar radiation, whereas increase in nighttime LSWT is due to increasing lowest temperature. The variability in LSWT with respect to meteorological variables during El Nino Southern Oscillation period was highest for cold phase than warm phase. There was only 1% increase in LSWT in the El Nino years (2002 and 2009), due to decreased rainfall by 95% and 56% respectively. On the other hand, the LSWT decreased in strong La Nina years of 2007 (2%) and 2010 (6%) due to the drastically increased rainfall, decreased maximum temperature and solar radiation. The correlation revealed that, in addition to the increase in temperature, the LSWT warming is consistent with the decrease in rainfall and increase in solar radiation.

Anomaly Variation of Vegetation and Its Influencing Factors in Mainland China During ENSO Period

Considering the noncore region influenced by El Nino-Southern Oscillation (ENSO) events, China is hardly investigated in terms of the vegetation variation during the ENSO period.Therefore, this study focused on increasing knowledge of vegetation growth and variation during the ENSO period.The novelty of this paper is introduced the moving window correlation analysis method to determine the thresholds of vegetation response to sea surface temperature anomaly (SSTa) and southern oscillation index (SOI) and analyze the main reasons for such variations.China was divided into seven areas based on different weather conditions, and the variations in vegetation growth in various areas during the ENSO period were analyzed. The response of vegetation to ENSO events in China was analyzed from the perspectives of precipitable water vapor (PWV), temperature, and precipitation, thereby revealing the interplay of multi-factors on vegetation growth.The main conclusions include (1) a positive vegetation response to El Nino exists all over China with thresholds of SSTa ≥ 1.87°C and SOI ≤ -3.27hPa, whereas a negative response of vegetation variation to La Nina exists with thresholds of SSTa ≤ -1.05°C and SOI ≥ 1.7hPa; (2) the correlations (p <; 0.05) of PWV to normalized difference vegetation index (NDVI) (PWV-NDVI), temperature-NDVI, and precipitation-NDVI reached 0.84, 0.86, and 0.68, respectively, and PWV, temperature, and precipitation were negatively/positively abnormal during El Nino/La Nina periods; (3) in coastal areas of Southeast China, correlations between NDVI and PWV/temperature/ precipitation are poor and the opposite anomalies of PWV/temperature/ precipitation existed when compared to other areas of China.

Seasonal linkage of the Southern Hemisphere extratropical climate variability to two types of ENSO

This study uses the Climate Forecast System Reanalysis (CFSR) to investigate the responses of the Southern Hemisphere (SH) extratropical climate to two types of El Nino-Southern Oscillation (ENSO)—the eastern Pacific (EP) type and the central Pacific (CP) type in different seasons. The responses are denoted by the anomalies of climate variables associated with one-standard-deviation increase in the Nino3 or Nino4 index. The results show that in austral spring the differences in the ENSO-related anomaly (ERA) patterns of atmospheric circulation between the EP ENSO period (1979–1998) and CP ENSO period (1999–2010) are mainly associated with the change in the ENSO-PSA2 relationship. Such differences affect the ERA fields of surface air temperature and mixed layer temperature, and finally result in significant differences in sea-ice concentration anomalies in the Atlantic sector. In austral summer, significant correlation exists between the variations of SAM and both of the variations of Nino3 and Nino4 in 1979–1998, while the correlation between SAM and Nino4 disappears in 1999–2010. For all seasons, the strength of the climate ERAs depend on if there are close relationship between ENSO and the major climate variation modes of the SH extratropics. For the climate variables, the ERA patterns of surface air temperature are generally controlled by surface wind anomalies and mirrored by the mixed layer temperature anomalies. The mixed layer depth anomalies are primarily modulated by surface heat flux anomalies and occasionally by anomalous wind. There are strikingly strong anomalies of surface heat flux in the autumn of 1979–1998 related to the Nino3 variation, the period when there is only significant correlation between ENSO and PSA2. There are no evidence that the SH extratropical climate variability induced by Nino3 variations are stronger in the EP-ENSO period, and that variability induced by Nino4 variations are stronger in the CP-ENSO period.

What happens to the ocean surface gravity waves when ENSO and MJO phases combine during the extended boreal winter?

The safety of vulnerable coastal and offshore infrastructures requires an in-depth understanding of wave variability and climate drivers. We investigate the association of significant wave height (Hs) and peak wave period (Tp) with the co-occurrence of El Nino–Southern Oscillation (ENSO) and the Madden–Julian Oscillation (MJO) at the global scale. We calculate composites of daily anomalies in modelled Hs, Tp, and surface wind for periods of ENSO–MJO phase combinations. Calculations spanned November–March seasons over the period 1979–2018. Wave anomalies are widespread across the world’s oceans, with remarkable strength during several ENSO–MJO phase combinations, demonstrating strong tropic–tropic and tropic-extratropic teleconnections. Positive Hs anomalies are strongest in the Pacific Ocean during El Nino–MJO phase 8, in the Atlantic Ocean during ENSO-neutral-MJO phase 3, and in the Indian Ocean during ENSO-neutral-MJO phase 4. Positive Tp anomalies are strongest in the Pacific Ocean during La Nina–MJO phase 8, in the Atlantic Ocean during El Nino–MJO phase 1, and in the Indian Ocean during El Nino–MJO phase 8. In the Southern Ocean, the strongest Hs anomalies occur during El Nino–MJO phase 8, whereas in the Maritime Continent, they appear during ENSO-neutral-MJO phases 5–6. Despite previous studies finding low correlations of ENSO indices with wave parameters in the North Atlantic, our results suggest that ENSO-related conditions play a significant role in the area when combined with certain MJO-related conditions. This study also reveals that the wave anomalies associated with ENSO–MJO phase combinations can be twice as strong as those found in previous work, related only to the MJO. Therefore, considering multiple concurrent climate patterns in the analysis of wave anomalies is essential to developing more reliable coastal management plans.

Bird population declines in the Chametla wetland (Southern Gulf of California): Evidence of stress at the assemblage level

The Chametla wetland is used by shorebirds as a stopover site during their autumn migration and it is also an important breeding area for several species of waterbirds. The objective of this work was to compare the bird assemblages in Chametla wetland during three sampling periods: 1) 1991–1992; 2) 1997–1998 which was subjected to El Nino Southern Oscillation (ENSO) climate conditions and 3) 2005–2006. Bird communities were characterized in terms of species composition and diversity, using similarity analysis. Bird assemblage composition differed across years and seasons. Seasonal variations in composition and diversity were related to the presence/absence of phenological-characterized species (migratory vs. wintering species). The highest species richness was recorded under the ENSO period (1997–1998). We observed a sharp decrease in shorebird numbers, with evident stress at the assemblage level throughout the entire study period. There seems to be a transition of the bird assemblages from shorebird dominance to a dominance by long-legged wading birds and waterfowl species, which could be related to water level variation and changes in the quality/availability of food in the intertidal zone. The joint pressures of regional climate variation combined with local anthropogenic perturbations may lead to changes in bird assemblage in the Chametla wetland.

Derivation of delay equation climate models using the Mori-Zwanzig formalism.

Models incorporating delay have been frequently used to understand climate variability phenomena, but often the delay is introduced through an ad hoc physical reasoning, such as the propagation time of waves. In this paper, the Mori-Zwanzig formalism is introduced as a way to systematically derive delay models from systems of partial differential equations and hence provides a better justification for using these delay-type models. The Mori-Zwanzig technique gives a formal rewriting of the system using a projection onto a set of resolved variables, where the rewritten system contains a memory term. The computation of this memory term requires solving the orthogonal dynamics equation, which represents the unresolved dynamics. For nonlinear systems, it is often not possible to obtain an analytical solution to the orthogonal dynamics and an approximate solution needs to be found. Here, we demonstrate the Mori-Zwanzig technique for a two-strip model of the El Niño Southern Oscillation (ENSO) and explore methods to solve the orthogonal dynamics. The resulting nonlinear delay model contains an additional term compared to previously proposed ad hoc conceptual models. This new term leads to a larger ENSO period, which is closer to that seen in observations.