Response of Vietnam coastal upwelling to the 1997-1998 ENSO event observed by multisensor data

In this paper, we examine the behavior of the Vietnam coastal upwelling during the 1997-1998 El Niño-Southern Oscillation (ENSO) event. The baseline is 4 years of National Oceanic and Atmospheric Administration (NOAA) satellite Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data taken from 1997 to 2000. Comparison of upwelling images to simultaneous ERS-2 (European Remote Sensing Satellite) wind fields indicates that the summer monsoon winds constitute a major generation forcing. During the 1997 El Niño, the monsoon winds enhanced the upwelling and induced the upwelling center to move southward. During the 1998 La Niña, the monsoon winds weakened the upwelling. In contrast with the tropical Pacific, in the study area, La Niña implies a warm event and El Niño a cold event. We use empirical orthogonal function (EOF) methods to analyze the spatial and temporal variance of the upwelling. The three principal modes account for 37%, 15%, and 8% of the total variance, respectively. The first EOF modes reveal that the SST variance in the north and south subregions underwent a positive-negative sign switch in summer 1997. The second EOF modes represent the monthly evolution in normal years. The third modes seem to be sensitive to the 1998 La Niña event. Simultaneous TOPEX/POSEIDON and ERS-2 altimeter data provide further evidence for our analysis. Comparison with California coastal upwelling and mid-Atlantic Bight (MAB) coastal upwelling indicates that the Vietnam coastal upwelling is the most intensive one.     

Strong 2015–2016 El Niño and implication to global ecosystems from space data

During 2015, sea surface temperature (SST) in the central tropical Pacific (TP) was warmer than normal, what indicated about the potential for the development of El Niño Southern Oscillation (ENSO). By December 2015, El Niño intensified when SST anomaly in the Niño-3.4 tropical Pacific area reached +2.9 °C, which indicated about the strongest event of the past 36 years. El Niño normally impacts weather, ecosystems, and socioeconomics (agriculture, fisheries, energy, human health, water resource etc.) on all continents. However, the current El Niño is much stronger than the recent strong 1997–1998 event. Therefore, this paper investigates how the strength of El Niño impacts world ecosystems and which areas are affected. The vegetation health (VH) method and 36-year of its data have been used as the criteria of the impact. Specifically, the paper investigates VH-ENSO teleconnection, focusing on estimation of vegetation response to El Niño intensity and transition of the impact from boreal winter to spring and summer. Two types of ecosystem response were identified. In boreal winter, ecosystems of northern South America, southern Africa, eastern Australia, and Southeast Asia experienced strong vegetation stress, which will negatively affect agriculture, energy, and water resources. In Argentina, southeastern USA and the Horn of Africa ecosystem response is opposite. One of the worst disasters associated with ENSO is drought. The advantages of this study are in derivation of vegetation response to moisture, thermal, and combined conditions including an early detection of drought-related stress. For the first time, ENSO impact was evaluated based on all events with |SSTa|> 0.5 ºC and >2.0 ºC. The current strong El Niño has already triggered drought in Brazil, southern Africa, southeastern Asia, and eastern Australia during December–February. Such conditions will be transitioned from boreal winter to spring but not to summer 2016, except for two regions: northern Brazil and southeastern Asia.     

Climatic oscillations in aerosol optical depth over tropical oceanic regions pertaining to genesis of the Indian Ocean Dipole

Data on aerosol optical depth (AOD) derived from the ocean colour sensor of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) from September 1997 to December 2010 over the western tropical Indian Ocean (WTIO) (10° S to 10° N; 50° E to 70° E) and southeastern tropical Indian Ocean (SETIO) (10° S to equator; 90° E to 110° E) were analysed with a view to understanding its response to climatic oscillations in regard to the El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). This study demonstrates the existence of a bimodal distribution pattern of AOD in the atmosphere over both WTIO and SETIO, with the highest values being around 1.1 during the period of primary maximum during August over the WTIO and during October over the SETIO. A secondary maximum (～0.9) appeared during March over both areas. In addition, the existence of a see-saw oscillation in the distribution of AOD between the atmospheric columns over the study regions was revealed, with higher values during August–December over the SETIO. AOD data over the SETIO captured very well the influence of these atmospheric modes, whereas the influence was not as significant over the WTIO. Stronger El Niño (Niño index > 0.80) events produced a significantly positive (more than +0.03) anomaly in AOD values over the SETIO during October, whereas the lone mode of IOD events and La Niña were not sufficient to induce any significant change in the aerosol distribution over the area. The mode of El Niño co-occurring with a positive IOD (PIOD) strengthens this anomalous behaviour. A significantly negative anomaly (≤0.03) in AOD was observed with concurrent La Niña (Niño index < ?1.1) and negative IOD (NIOD) (dipole mode index ≤ 1.1) events. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) winds were utilized to verify these observations.     

Altimeter observations of sea-level variability off the west coast of North America

A combination of careful harmonic analysis and high-resolution numerical modelling of tides in altimeter data from TOPEX/Poseidon and ERS-2 satellites allows detailed calculation of seasonal and interannual variability of sea level in coastal areas of the north-east Pacific Ocean. These calculations show large changes in sea level in marginal seas during the 1997–8 El Niño and La Niña events, as well as providing information on the origin and life cycle of large anticyclonic eddies, observed to be generated off the west coast of Canada.     

Two types of El Niño and extratropical sea-level pressure variations

This study applies the nonlinear canonical correlation analysis (NLCCA) to explore the nonlinear relationship between the sea-level pressure (SLP) anomalies over the extratropical North Pacific and sea surface temperature (SST) anomalies in the tropical Pacific during 1985–2009. Our results suggest that the asymmetry between the warm eastern Pacific (EP) El Niño–Aleutian Low mode and the cool EP La Niña–anti-phase of the Aleutian Low mode is exhibited in the first NLCCA mode. Nonlinearity of the first NLCCA SST field is enhanced after 1998, and vice versa for the SLP field. The second NLCCA SST mode reveals weak nonlinearity representing the nonlinear central tropical Pacific (CP) El Niño–CP La Niña modes, while the second SLP field depicts the North Pacific Oscillation and anti-phase with the Aleutian Low phases. The nonlinearity of the second SST and SLP NLCCA modes is found to decrease gradually with time. During 1985–1997, the SST field exhibits linearity, while the SLP field shows weak nonlinearity. During 1997–2009, the SST and SLP fields both display weak linearity. Nonlinearity between the extratropical SLP and SST fields is further weakened from the first period. The Aleutian Low pattern could be excited by both EP and CP El Niños. Moreover, the CP El Niños have more connections with the North Pacific Oscillation state rather than the EP El Niños. Conclusively, this study reveals the asymmetric modes between the SLP and SST by the nonlinear method, and contributes to the understanding of the extratropical SLP variability response to two types El Niño events.     

Spatio-temporal variability of surface chlorophyll-a in the Halmahera Sea and its relation to ENSO and the Indian Ocean Dipole

ABSTRACT

Long-term satellite data are used to investigate the variability of ocean surface chlorophyll-a (chl-a) concentration in the Halmahera Sea (HS) under influence of the Australian-Indonesian Monsoon (AIM), the El Niño-Southern Oscillation (ENSO), and the Indian Ocean Dipole (IOD). In this study, we first analysed the seasonal variability of chl-a, and then examine the relationship between surface chl-a, sea surface temperature (SST), and sea surface wind stress in the area. Our results suggest that prevailing southeasterly winds play a fundamental role in generating chl-a blooms in the HS. Particularly on a seasonal timescale, through the mechanism of Ekman mass transport, strengthening of southeasterly wind stress during the Southeast Monsoon season (June – August) produces enhanced chl-a concentrations associated with ocean surface cooling in the area of study. On the other hand, the chl-a bloom completely diminishes during the Northwest Monsoon season (December – February) due to weakening of wind stress and Ekman transport. On an interannual timescale, sea level pressure and wind stress are coherent with ENSO and IOD phases. During El Niño and positive IOD events (La Niña and negative IOD events), both sea level pressure and wind stress greatly increase (decrease) over the HS. These conditions cause an anomaly in southerly (northerly) wind stress, which is favourable to an enhancement (reduction) of the chl-a concentration in the region. This study demonstrates that sea level pressure and wind stress are the critical factors in determining the magnitude of chl-a bloom in the HS.     

Remote sensing of US cornbelt areas sensitive to the El Ni@o-Southern Oscillation

A 16-year time-series (1982-1997) of monthly maximum Normalized Difference Vegetation Index (NDVI) values derived from coarse-scale Advanced Very High Resolution Radiometer (AVHRR) satellite images was investigated for its application in identifying agricultural regions sensitive to El Ni@o-Southern Oscillation (ENSO) impacts. Cumulative NDVI values over the growing season were used to test the temporal and spatial sensitivity of rainfed agricultural regions in the US cornbelt to ENSO climate teleconnections. The correlation (R) between NDVI anomalies and yield anomalies aggregated to the Agricultural Statistics Division (ASD) level was 0.59 (significant at the 99% level). Sea surface temperatures (SSTs) from the NINO3 region (between +5.0° and m 5.0° latitude and 90.0° and 180.0° west longitude in the Pacific Ocean), an indicator of the ENSO phenomenon, were significantly but weakly correlated with growing season NDVI anomalies, precipitation anomalies, and yield anomalies. Two classification schemes for El Niño, La Niña and Neutral years related to the crop growing season were applied--one based on simultaneous Pacific NINO3 SSTs (spring-summer), and one based on following winter Pacific NINO3 SSTs. The strongest differences in Vegetation Condition Index (VCI) spatial patterns among the three ENSO categories were found using the following winter SST classification. Classification of ENSO years is a key issue in analysing ENSO impacts on agriculture as represented by the NDVI, because the regularity of annual agricultural seasons is not synchronous with the quasi-regular Pacific SST cycles.     

NDVI anomaly patterns over Africa during the 1997/98 ENSO warm event

Normalized Difference Vegetation Index (NDVI) departure patterns for Africa during the 1997/98 El Niño/Southern Oscillation (ENSO) warm event show two dominant patterns. Over equatorial Eastern Africa, above normal NDVI anomalies persisted from October 1997 through the normal dry season (December-February) and into the long rains season in March-May. Over Southern Africa the spatial NDVI anomaly shows a dry western half and a relatively greener than normal eastern half. Correlations between the temporal NDVI anomalies with ENSO indices shows that the anomalous conditions over Eastern Africa were a direct result of anomalous warming of sea surface temperatures (～+3°C) in the western equatorial Indian Ocean (WIO) and a lagged response to the warming in the eastern Pacific Ocean (+4°C). We suggest that this anomalous warming of the WIO and the equatorial eastern Atlantic Ocean basin dampened the normal severe drought response pattern over Southern Africa where mild drought conditions were experienced. The overall continental response pattern shows a meridional dipole pattern, with above normal NDVI straddling the equator between 10° N and 10° S and normal to slightly below normal NDVI south of 15° S, predominantly over south-western Africa.     

Interannual variations in global SST deviations through SMMR from 1978 to 1987

We study variability of global sea surface temperature (SST) utilizing the data of scanning multichannel microwave radiometer (SMMR) on board the NASA Nimbus 7 satellite from 1978 to 1987. First, we model, and then remove from the SMMR SST data, the seasonal cycle by using an intercept, a trend and first five harmonics of the annual cycle to fit the data at each grid point by the method of least squares. A general negative nine‐year trend was observed. In order to analyse the deviations in the global SSTs, we calculate and remove zonally averaged temperatures. We then show Hoffmueller diagrams for the deviations along paths in different oceanic regions over the globe. These paths include a quadrangle in the south Pacific and paths in the north Pacific, Atlantic and along the equatorial Pacific. Both 1983 and 1987 El Niño events as well as the 1984–85 La Niña event are clearly depicted. During these events, the SSTs in the equatorial Pacific and Atlantic are completely out of phase. We also demonstrate spatial propagation of SST waves over interannual scales. In particular, a wave of a period of about 3–4 years following the North Pacific Current will be shown.     

Seasonal evolution of the interannual variability of chlorophyll-a concentration and its forcing factors in the northwestern Pacific from 1998 to 2010

The northwestern Pacific Ocean is a complex region with significant biological spatial variations on a seasonal timescale. To investigate the joint variation patterns on both seasonal and interannual timescales, a season-reliant empirical orthogonal function (S-EOF) analysis was applied to seasonal mean chlorophyll-a concentration (chl-a) anomalies in the northwestern Pacific Ocean during the period 1998–2010. The first two dominant modes accounted for nearly 31% of the total interannual variance, with the second S-EOF mode (S-EOF2) lagging behind the first S-EOF mode (S-EOF1) by one year. S-EOF1 featured a strong variation pattern to the north of 30° N, with maximum chl-a in winter and minimum chl-a in summer. However, S-EOF2 indicated an opposite seasonally evolving pattern compared with S-EOF1, with chl-a increasing along the Kuroshio and extension current from boreal winter to autumn. Both these modes revealed significant relationships with climate-related indices. The two modes corresponded to the central Pacific (CP) La Niña developing episodes and the turnaround from eastern Pacific (EP) La Niña to CP El Niño, respectively. Both modes were associated with the cold phase of the Pacific Decadal Oscillation, which played an important role in prolonging the impact of the El Niño/Southern Oscillation on chl-a seasonal evolution from 1998 to 2010. In addition, we discuss the possible factors dominating chl-a seasonal variation, in terms of the subregions of the northwestern Pacific Ocean. In the subtropical northwestern Pacific Ocean (15° N – 30° N), the chl-a growth was primarily nutrient-limited, whereas in the mid-latitude northwestern Pacific Ocean (35° N – 50° N), the chl-a growth was mainly light-limited.     

The regional impacts of climate change on coastal environments and the aquaculture of Japanese scallops in northeast Asia: case studies from Dalian,China, and Funka Bay,Japan

Climate changes affect coastal environments and aquaculture, threatening food security and economic growth. Japanese scallop (Mizuhopecten yessoensis) culture is economically important for the coastal communities of Dalian, China, and Funka Bay, Japan. In this study, we combined satellite remote-sensing data, in situ observations, and a suitable aquaculture site selection model to explore the interactions between marine environments and climate variability over a recent 10-year period (2003–2012). Our selection of appropriate zones in these two Far Eastern regions and our analyses of climatic event (Arctic Oscillation (AO), winter East Asian monsoon (EAM), and El Niño/La Niña Southern Oscillation (ENSO)) and meteorological (precipitation, temperature, and wind) data allowed us to determine the impacts of climate change on regional coastal environments and prospects for scallop aquaculture. These analyses showed that AO and EAM strongly influenced the aquaculture areas on the Dalian coast through their effects on temperature during winter. We also determined that wind was the main driving force behind regional environmental changes during spring. Conversely, ocean conditions and suitable areas in Funka Bay changed rapidly relative to oceanic and atmospheric circulation. In Funka Bay, areas appropriate for scallop aquaculture and variations in chlorophyll-a concentration (which reflect the availability of algal food for scallops) were strongly correlated with ENSO, precipitation, and air temperature. These correlations demonstrate the influence of oceanic and atmospheric parameters on the productivity of scallop aquaculture in Funka Bay. Adaptation to oceanic and atmospheric changes should be considered when developing plans and management strategies for coastal scallop aquaculture in northeast Asia.     

Chlorophyll variability in the northeastern Gulf of Mexico

Changes in chlorophyll concentration distribution in surface waters of the northeastern Gulf of Mexico (NEGOM) were examined using satellite and in situ data collected between November 1997 and August 2000. The patterns of chlorophyll distribution derived from in situ data consistently matched the satellite observations, even though the satellite-derived concentrations in coastal and offshore waters influenced by rivers were overestimated by the standard satellite data processing algorithms. River discharge and wind-driven upwelling were the major factors influencing surface chlorophyll-a variability for inshore regions. High in situ chlorophyll-a concentrations (≥1 mg m^?3) occurred inshore and particularly near major river mouths during the summer seasons of 1998, 1999 and 2000. Plumes of Mississippi River water extended offshore to the southeast of the delta over distances >500 km from the river delta for maximum periods of 14 weeks between May and September every year and could reach the Florida Keys in certain years. The offshore transport of the plume was initiated by eastward or southeastward winds and then by separate anticyclonic eddies located southeast of the Mississippi delta and nearby shelf every year. Chlorophyll concentrations during the winter to spring transition in 1998 off Escambia, Choctawhatchee, Apalachicola and Suwannee Rivers and off Tampa Bay were up to 4 times higher than during the same periods in 1999 and 2000. This was related to higher freshwater discharge during the 1997–1998 winter–spring transition, coinciding with an El Niño–Southern Oscillation event, and to the unusually strong upwelling observed along the coast in spring 1998.     

Calculation of the sensible heat flux of the global ocean using satellite data

The sensible heat flux of the global ocean is derived using satellite data of Special Sensor Microwave/Imager (SSM/I) precipitable water, Advanced Very High Resolution Radiometer (AVHRR) sea-surface temperature (SST) and scatterometer wind speed. Prior to heat flux derivation, the air temperature over the sea surface in the global ocean is obtained with an iterative solving technique applied to a simplified equation that specifies the relationship among boundary-layer parameters. It is found that a bias exists between the calculated air temperature and the climatology data, which is corrected by a linear model based on the climatology of air temperature. Using the corrected air temperature and the bulk formula, we calculate the sensible heat flux from January 1992 to October 1998. The heat flux calculation is consistent with previous results. An error analysis suggests that based on the bulk formula, the uncertainty caused by the air temperature is comparable with the error components from the SST and wind. Empirical orthogonal function (EOF) analysis is used to extract the temporal and spatial characteristics of the calculated sensible heat flux. The first EOF is characterized by a winter and summer oscillation with an annual cycle, the second shows another annual cycle in spring and autumn oscillation, the third EOF is related to the El Niño and La Niña cycle, reflecting a certain relationship between El Niño Southern Oscillation (ENSO) events and the global ocean sensible heat flux, and the fourth EOF indicates an increasing trend with a quasi-biennial oscillation and atmospheric influences.     

A multi-sensor climatological view of double ITCZs over the Indian Ocean

We characterize the climatological features of the double inter-tropical convergence zones (DITCZs) over the western Indian Ocean during November–December by a synergistic analysis of the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite (HOAPS III) data (1988–2005) and the National Aeronautics and Space Administration's (NASA's) A-Train data (2002–2009). We investigate rainfall, freshwater flux and cloud liquid water, cloud fraction and relative humidity over the DITCZs. In addition, the daily rainfall data from the Global Precipitation Climatology Project (GPCP) are used to document the DITCZs during the El Niño southern oscillation (ENSO) events. An analysis of the GPCP data shows that the DITCZs are clearly discernible during strong ENSO events (1997, 2002 and 2006), in sharp contrast to the DITCZs in the eastern Pacific Ocean, where they are absent during ENSOs. Further, these convergence zones on either side of the equator are of short duration, approximately 3–6 pentads during November and December. All satellite sensor data sets consistently capture the major features of DITCZs. As an accurate simulation of DITCZs in coupled global climate models remains a challenge, the results from the present study would provide a platform for evaluating these models.     

Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

The characterisation of the geostrophic surface flow field around the Cape Verde Archipelago in the northeast Atlantic Ocean with satellite altimeter data is presented. The aim is to analyse the main current systems present in the region 3°-30°N, 40°-10°W and their seasonal and interannual variability. A merged data set of Topex/Poseidon (T/P) and ERS-2 altimeter data for an 8-year period, beginning in June 1995, has been used and corrected sea surface heights were computed by applying a homogeneous set of relevant geophysical corrections. ERS-2 data were crossover adjusted to T/P. Monthly maps of sea level anomalies were created for the whole period and were used in the computation of monthly maps of absolute dynamic topography, geostrophic currents and eddy kinetic energy (EKE). The seasonal signal of the northeast Tropical Atlantic large-scale surface circulation appears as the prevailing cause of the variability in the region, particularly in the southernmost portion of the region being studied. This signal is also present in the flow field along the African coast and in the Guinea Dome. Regions of highest EKE values are clearly associated with the North Equatorial Counter-Current and with the currents along the African coast. The significant interannual variability found for 1998 seems to be associated with the 1997-1998 ENSO Pacific event, but other anomalous periods (1996-1997 and 2001-2002) uncorrelated with ENSO are also evident.     

Abnormal features of the Pacific wind system during the 1997-1998 El Niño

The 1997-1998 El Niño, the strongest in recorded history, manifested itself with a number of unusual features associated with the Pacific wind system. These features include: (1) an annual cycle of an east-west migration of a weakened wind speed zone between 2° N-9° N; (2) an asymmetric see-saw process of trade wind variations between the two hemispheres in terms of relative intensity and central position; and (3) an 18-month cycle of meridional oscillations of the Pacific doldrums and trade wind belts. In addition, the commonly-used argument of trade wind relaxation in association with El Niños appears to be partly introduced, at least for the present case, by the 'tilt effect' of the Pacific zonal winds. These novel findings, revealed by the newly available multi-year TOPEX altimeter data, may help to improve existing theories on El Niño formation, and may also contribute to its future prediction.     

Application of altimeter observation to El Niño prediction

A new version of the Lamont forecast model is used to assess the impact of TOPEX/POSEIDON altimeter data on predicting short-term climate change, with emphasis on the 1997/98 El Niño and subsequent La Niña. As compared to forecasts initialized with only wind data, the model's predictive accuracy was improved when the altimeter sea level data are used for model initialization. This is due to the effectiveness of sea level data in correcting the model ocean state. For this particular application, the effect of altimeter sea level observations is comparable to that of tide gauge measurements.     

Comparisons of chlorophyll variability between the four major global eastern boundary currents

The first two years of SeaWiFS (Sea viewing Wide Field of view Sensor) data (1997–1999) are used to document the variability of large-scale surface chlorophyll patterns within the coastal region along the full latitudinal extent of each of the four major global eastern boundary currents; the California, Humboldt, Benguela and Canary Currents. Seasonal chlorophyll patterns are compared to coincident seasonal cycles of Ekman transport calculated from satellite scatterometer data. In all four regions, maximum chlorophyll concentrations are generally temporally and latitudinally coincident with the seasonal maximum in upwelling (offshore Ekman transport) over most of their latitudinal range, but exceptions are documented. Interannual differences are evident in each region, most notably in the two Pacific regions where the 1997–1998 chlorophyll seasonality was affected by El Niño conditions. Significant differences between previously published chlorophyll seasonality deduced from the relatively sparse coverage of the Coastal Zone Color Scanner (CZCS) and the more complete coverage of SeaWiFS in both Southern Hemisphere regions are evident.     

Altimeter observations of the MJO/ENSO connection through Kelvin waves

Altimeter‐derived Pacific Ocean sea surface height anomaly (SSHA) data are used to confirm the relationship between the Madden Julian Oscillation (MJO) and El Niño Southern Oscillation (ENSO), through Kelvin waves, previously identified in sea surface temperature (SST) data. The altimeter data, filtered for Kelvin waves, have been cross‐correlated with MJO indices, as an alternative way of exploring and quantifying the possible connection between the MJO and ENSO. The results of the cross‐correlations of the SSHA filtered data and the MJO indices generally show higher values in the periods preceding the three ENSO warm events and lower values during, and for a short time after, these events. The significance of this correlation pattern appears to link the MJO to the ENSO warm events, through Kelvin waves. Whether this can be used for predictive purposes needs to be confirmed by further studies.     

Galapagos indicator of El Niño using monthly SST from NASA Giovanni system

One geographical point could be sufficient to track El Niño Southern Oscillation (ENSO). This paper describes an approach to such tracking based on environmental modeling of sea surface temperature (SST). The key model of the approach is the most anomalous indicator of SST as a point near Isabella Island, Galapagos. This point can be computed using monthly SST data from NASA Giovanni system. Comparison with most popular indices demonstrates that such Galapagos indicator can clear indicate both El Niño and La Niña events.