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.     

Remote sensing of the Indo-Pacific region: ocean colour,sea level,winds and sea surface temperatures

The 1997–1998 ENSO (El Niño-Southern Oscillation) was not only the largest event of the century but also the most comprehensively observed. Satellite data were employed for ocean colour, sea level, winds, sea surface temperature (SST), and outgoing longwave radiation (OLR) were used to describe the response of the surface marine ecosystem associated with the ENSO event. Some of the large-scale anomalies in ocean colour include elevated biological activity to the north of the Equator in the Pacific coincident with lower sea levels associated with the classic ENSO-horseshoe pattern ecosystem response to the anomalous upwelling in the eastern Indian Ocean caused by the 1997–1998 dipole event, and the dramatic eastward propagating feature in the Equatorial Pacific in response to the La Niña dynamics. Ocean general circulation model (OGCM) experiments show that capturing the high-frequency wind changes is crucial for simulating the La Niña and the coupled biological–physical model (OBGCM) runs clearly show that higher frequency winds are also important for capturing the mean upwelling and nutrient supply into the euphotic zone. Thus, the QuickSCAT winds are expected to play a major role in ecosystem modelling in the future. This study shows the utility of satellite data for understanding not only ocean circulation but also the coupled ecosystem variability. Morcover, it is also shown that spatio-temporal resolution of the satellite winds will directly affect the accuracy of oceanic and ecosystem simulations.     

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.     

Characteristics of atmospheric divergence and convergence in the Indian Ocean inferred from scatterometer winds

Large-scale surface atmospheric convergence and divergence patterns in the Indian Ocean are mapped using high-spatial resolution, merged scatterometer wind vectors during 1991-2000. The convergence zone evolves to north of 15°S as a result of convection promoted by warm (> 28 °C) equatorial sea surface temperature (SST), and it exhibits strong intensity during boreal summer and winter. A divergence zone evolves to the south of 15°S as a result of subdued convection caused by colder SST (< 24 °C) that reduces outgoing long-wave radiation; it exhibits enhanced intensity in the eastern Indian Ocean during boreal winter. The interannual variability shows that the divergence in the eastern Indian Ocean lags its western counterpart by 5-7 months. The convergence in the eastern Indian Ocean is stronger than its western counterpart during boreal summer. Relationship between Southern Oscillation Index and spatially averaged convergence time series indicate that the latter weakened during strong El Niño years 1994 and 1997. Spatially averaged divergence time series show a near-contemporaneous relationship with all-India rainfall, with a temporal lag of 1∼2 months.     

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.     

Impact on the chlorophyll concentration in the Bay of Bengal and Arabian Sea during Indian Ocean dipole mode

The chlorophyll (Chl) concentration in the seawater of the Bay of Bengal (BoB) and the Arabian Sea (AS) changes most during the Indian Ocean dipole (IOD) mode event. The empirical orthogonal function (EOF) of several oceanographic and atmospheric data was studied, using 9 years (1998–2006) of reanalysed satellite data. The variation of Chl during the period of September to November (SON) over the 9 years has been studied in this article. It has been found that significant enhancement of Chl in the BoB takes place during IOD years owing to the surface wind action, that is, wind-stress curl (WC), which favours the upwelling process, whereas the AS shows a decline in concentration of Chl owing to a reduced open ocean upwelling process.     

Global land vegetation and marine fishery responses to atmospheric and oceanic decadal variability

The decadal variability of sea surface temperature (SST) and sea level pressure (SLP) anomalies, as well as the response of global land vegetation and marine fisheries, are investigated for three periods: 1982–1988, 1989–1998, and 1999–2008, separated by the 1988–89 and 1998–99 regime shifts. The goal is to develop a global-scale ecosystem concept to support an improved understanding of the corresponding changes in atmospheric, oceanic, and biological responses. The analysis is based on global SST, SLP, precipitable water content (PWC), land vegetation condition index (VCI), and the United Nations Food and Agriculture Organization’s (FAO) fish capture data. The results show that SST and SLP displayed significant decadal variability. The decadal variability of sea surface temperature anomalies (SSTA) associated with sea level pressure anomalies (SLPA) has an influence on the land vegetation moisture condition (VCI). Positive SSTA tends to be associated with negative SLPA, and vice versa, in the corresponding ocean areas and most land areas. Consequently, clearly opposing distributions of SSTA and SLPA are observed in the periods 1982–1988 and 1999–2008. With positive SSTA and negative SLPA, VCI tends to increase in value representing more favourable vegetation conditions. Negative SSTA and positive SLPA is generally unfavourable for global vegetation development. The decadal variability of SSTA is closely related to the number of fish species (NFS) doing better or worse based on normalized fish landing data. However, the fishery responses show different yet consistent trends in the three ocean basins. When SSTA is negative, it appears more beneficial for the number of fish species with improved landings in the Atlantic Ocean. However, positive SSTA leads to more fish species with improved landings in the Indian and Pacific Oceans.     

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.     

Comparative study of the 1982–1983 and 1997–1998 El Niño events over different types of vegetation in South America

This work presents results which help to understand the behaviour of the Normalized Difference Vegetation Index (NDVI) anomalies over the South America continent during the two strongest El Niño events of the last century (1982–1983 and 1997–1998). The data used are parts of a long-term series (July 1981 to December 1999) of calibrated NDVI data derived from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) datasets. Special emphasis has been given to the analysis of the response of the major Brazilian vegetation types. This paper introduces an approach that enhances NDVI anomalies relative to the long-term climatology of the region. We find a negative NDVI anomaly for most of the region during the 1982–1983 event, whereas for the 1997–1998 event positive NDVI anomalies were observed over most regions. Only the ‘Nordeste’ region showed a similar vegetation response for both events. We identify three possible factors that may play a role in the different NDVI responses to the two El Niño events. Firstly, poor intercalibration of sensors may account for some, but not all of the differences. Secondly the response of the vegetation may depend upon the climate conditions prior to the El Niño events. Thirdly, the difference in the onset date and the duration of the mature phase of the two El Niño events, associated with very different Atlantic surface temperatures are shown to have dynamical consequences which may impact upon the vegetation.     

Observing the coupling effect between warm pool and “rain pool” in the Pacific Ocean

Traditionally, the tropical zone is known as the “heat reservoir” of the ocean and the “firebox” of the atmosphere. The western equatorial Pacific has been identified as both the warmest portion of the heat reservoir, named “warm pool” (WP), and the hottest portion of the firebox where a huge amount of precipitation-induced latent-heat release is accumulated. The latter mirrors a fact that the western tropical Pacific is also the wettest area on the globe, termed “rain pool” (RP), where the maximum annual precipitation is observed. The accumulation of continuous satellite data has reached a point that decade-long simultaneous observations of many important geophysical parameters have become available in recent years. One such example is the availability of a concurrent dataset of sea surface temperature, oceanic precipitation, and sea surface wind field for 1993-2002 derived from NOAA/AVHRR (Advanced Very High Resolution Radiometer), TOPEX/TMR (TOPEX Microwave Radiometer), and ERS-1,2/QuikSCAT, respectively. In the present study, this dataset is used to demonstrate and investigate the coupling and covarying effects of the Pacific WP and RP, leading to a number of interesting findings on their structural similarity, locational shift, phase lag, and evolutional coherency in association with the development of and the vacillation between El Niño and La Niña events.     

Phytoplankton biomass distribution and identification of productive habitats within the Galapagos Marine Reserve by MODIS, a surface acquisition system, and in-situ measurements

The Galapagos Marine Reserve (GMR) is one of the most diverse ecosystems in the world. Phytoplankton are the base of the ecosystem food chain for many higher trophic organisms, so identifying phytoplankton biomass distribution is the first step in understanding the dynamic environment for effective management of the GMR. Moderate Resolution Imaging Spectroradiometer (MODIS) and hyperspectral surface acquisition system derived chlorophyll, in-situ chlorophyll fluorescence, nitrate, salinity, and temperature were collected from March 2005 to the onset of a mild El Niño in November 2006. Islands in the eastern GMR, such as San Cristobal and Espanola, are the first to experience impacts of El Niño and southern migration of the Equatorial Front. Productive habitats were defined as surface waters with salinities > 34, temperatures < 24 °C, and chlorophyll a > 0.4 mg m^− 3. Six temporally variable productive habitats identified were: west of Isabela Island, southwest of Floreana Island, south of Santa Cruz, between Santiago and Santa Cruz Islands, and on the eastern side near San Cristobal Island. Model results coupled with surface acquisition system derived chlorophyll indicated productive habitats may also occur for short periods and at a distance from islands such as when the Equatorial Undercurrent (EUC) and South Equatorial Current (SEC) collide over the seamounts north of Isabela Island. All productive habitats were related to topographic upwelling from the EUC into surface waters.     

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.     

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.     

Monsoon and eddy forcing of chlorophyll-a variation in the northeast South China Sea

The physical and biological environments of the northeast South China Sea (SCS) were investigated using 11 year satellite and reanalysis data, including ocean chlorophyll-a (Chl-a) concentrations, sea surface wind (SSW) values, sea surface temperatures (SSTs), sea surface height anomalies (SSHAs), etc. The findings reveal that ocean Chl-a concentrations west of the Luzon Strait have the most significant annual cycles in the SCS. The dominant forcing mechanisms are monsoon winds and mesoscale eddies studied by multiple regression analysis. In the offshore regions, strong winds directly caused the enhancements by local vertical mixing and entrainment. As in the near-shore regions, the alongshore winds indirectly caused the enhancements by inducing coastal upwelling. Although SST is highly correlated with Chl-a concentration, SST cooling is difficult to observe in the bloom region. It is considered a consequence of monsoon and eddy forcings. Other mechanisms, such as local Ekman pumping by the wind stress curl and the geostrophic potential vorticity, have little effect on Chl-a seasonal variations.     

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.     

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.     

Observed chlorophyll-a bloom in the southern Bay of Bengal during winter 2006–2007

The analysis of 6-year chlorophyll-a data provided by the Moderate Resolution Imaging Spectral (MODIS) radiometer revealed anomalous chlorophyll-a bloom in the southern Bay of Bengal during the winter 2006–2007. The plausible causative mechanisms for such a large chlorophyll-a during the winter 2006–2007 are analysed through surface wind field, surface net heat flux and sea surface height anomaly (SSHA) data. The chlorophyll-a bloom developed in November 2006 near the northern tip of Sumatra. It was intensified and propagated slowly westward during December 2006 to January 2007 and then weakened by February 2007. The combined effect of shallowing of thermocline and weakening of barrier layer due to anomalous westward propagating upwelling Rossby waves associated with the Indian Ocean Dipole (IOD) event and relatively strong wind field causing entrainment of subsurface nutrient-rich water to euphotic zone leads to the bloom in the southern Bay of Bengal.     

Seasonal and temporal study of the northwest African upwelling system

The upwelling index (UI) obtained from sea surface temperature (SST) images for the period 1987–2006 and remote sensing wind stress were used to analyse the features of the coastal upwelling region off northwest Africa. The seasonal distribution shows a persistent upwelling throughout the year from 20° N to 33° N, seasonal behaviour from 12° N to south of 20° N, and an almost total lack of upwelling throughout the year from 5° N to 12° N. The major centres of active upwelling are located around Cape Ghir, Cape Juby and Cape Blanc. The UI shows an intensification of the upwelling system off northwest Africa during the 20-year period while the alongshore wind stress remains almost stable. During this period, upwelled waters off Cape Blanc have increased their offshore spreading.     

Study on the characteristics of the Indonesian seas using satellite remote-sensing data for 1998–2007

This study analyses the characteristics of the Indonesian seas using satellite remote-sensing data for the 10-year period from 1998 to 2007. Statistical properties and monthly average data or climatological data of sea surface temperature (SST), wind speed (WS), wind direction (WD), chlorophyll (CH) and sea surface height anomaly (SSHA) for the Indonesian seas are investigated. The results indicate negative dependence between SST and WS. The correlation between SST and WS has a 1-month phase difference. CH and SSHA are considered local data and reveal no apparent characteristics for the Indonesian seas as a whole. Seasonal changes of the indices, coupled with a north–south change, are observed. This analysis confirms several characteristics of the Indonesian seas.     

Space-time variations of the TOPEX/POSEIDON-derived heat storage anomaly over the Kuroshio upstream regions

I processed 9.6 years of TOPEX/POSEIDON (T/P) altimeter data and computed the time series of heat storage (HS) anomaly over the Kuroshio upstream regions. Coherence between the heat storage anomaly and the wind stress curl is significant (>0.8) at the annual component over the studied area. Using wavelet analysis, the semiannual component has significantly westward propagating signals at 19 and 27°N. The annual component is significantly strong at 11 and 27°N while westward-propagating signals can be found at the North Equatorial Current regions. The peaks of the annual components of heat storage anomalies occur in summer and the peaks of the annual components of wind stress curl exit in winter. However, generally, the wind stress curl lows are 1 month ahead of the peaks of heat storage anomalies at 1-year component.