El Nino-Southern Oscillation Sea Surface Temperature Research Articles

ENSO impacts on salinity in Tampa Bay, Florida

Estuarine salinity distributions reflect a dynamic balance between the processes that control estuarine circulation. At seasonal and longer time scales, freshwater inputs into estuaries represent the primary control on salinity distribution and estuarine circulation. El Nino-Southern Oscillation (ENSO) conditions influence seasonal rainfall and stream discharge patterns in the Tampa Bay, Florida region. The resulting variability in freshwater input to Tampa Bay influences its seasonal salinity distribution. During El Nino events, ENSO sea surface temperature anomalies (SSTAs) are significantly and inversely correlated with salinity in the bay during winter and spring. These patterns reflect the elevated rainfall over the drainage basin and the resulting elevated stream discharge and runoff, which depress salinity levels. Spatially, the correlations are strongest at the head of the bay, especially in bay sections with long residence times. During La Nina conditions, significant inverse correlations between ENSO SSTAs and salinity occur during spring. Dry conditions and depressed stream discharge characterize La Nina winters and springs, and the higher salinity levels during La Nina springs reflect the lower freshwater input levels.

The BMRC Coupled General Circulation Model ENSO Forecast System

An El Nino-Southern Oscillation (ENSO) prediction system with a coupled general circulation model and an ocean data assimilation scheme has been developed at the Australian Bureau of Meteorology Research Centre (BMRC). The coupled model consists of an R21L9 version of the BMRC climate model and a global version of the Geophysical Fluid Dynamics Laboratory modular ocean general circulation model with resolution focused in the tropical region and 25 vertical levels. A univariate statistical interpolation method, with 10-day data ingestion windows, is used to assimilate ocean temperature data and initialize the coupled model. The coupling procedure does not use any flux corrections. Hindcasts have been carried out for the period 1981-95 for each season (60 in all), for up to a lead time of 12 months. This paper will describe these initial experiments and show that the skill of sea surface temperature (SST) hindcasts in the tropical Pacific is comparable to other published coupled models. The skill of the model is strongest in the central Pacific. SST skill tends to be lower during the earlier 1990s than during 1980s in the eastern Pacific but not in the central Pacific. Since the ENSO SST anomaly in the central Pacific is the most important forcing of regional and global climate anomalies, the high SST prediction skill and its insensitivity over the hindcast period in this region in this model give grounds for optimism in the use of coupled general circulation models.

Seasonality and atmospheric dynamics of the teleconnection between African rainfall and tropical sea‐surface temperature: Atlantic vs. ENSO

AbstractA 47‐year record (1951–1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea‐surface temperatures (SST) in key tropical areas, as depicted by the NIÑO3, South Atlantic and North Atlantic indices. El Niño–Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south–north dipole (positive/negative correlation) which shifts northward following the Inter‐tropical Convergence Zone (ITCZ) translation between the northern low‐sun and high‐sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space–time impacts of the three SST indices. Decadal‐scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIÑO3 index since 1970–1975 in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period.The circulation anomalies associated with the teleconnections were assessed using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. A study of the dataset accuracy in depicting long‐term climatic variations revealed that a major shift, mainly artificial, is found in 1967–1968 in the time‐series of most of the variables. The rest of the work thus concentrated on the 1968–1997 period. A number of changes in east–west circulation patterns have been found to be associated to ENSO variations. Over West Africa, El Niño events tend to result in enhanced northeasterlies/reduced monsoon flow, coupled to weakened upper easterlies, and hence dry conditions over West Africa close to the surface position of the ITCZ, in July–September, as well as January–March. Over the southwestern Indian Ocean, the positive equatorial temperature/geopotential height anomalies, which at 200 hPa accompany El Niño events, are conducive to an eastward shift of the mid‐latitude upper troughs, thus being detrimental to summer rainfall over South Africa. Abnormally wet ‘short rains’ in East Africa can be accounted for by an ENSO‐forced weakening of the equatorial Walker‐type (east–west) cell which is found over the Indian Ocean during that season. By contrast, the impact of South Atlantic warmings is mostly shown in low‐level dynamics, as exemplified by the weakened trades and monsoon flow which directly result in a southward shift of the ITCZ. The combination of ENSO and Atlantic SST anomalies are found to give rise to complex wind flow changes in the near‐equatorial Atlantic. In addition to large‐scale SST‐forced atmospheric dynamics, a few regional atmospheric signals are found to explain residual parts of rainfall variance. For instance, a strengthening of the African Easterly Jet, or northerly wind anomalies across the Sahara, are shown to be related to drought conditions in the Sahel (July–September) and the Gulf of Guinea area (January–March), once the remote effect of SST anomalies is removed. Copyright © 2001 Royal Meteorological Society

El Niño-Southern Oscillation and Rainfall Variability in Central and Southern Sudan

In this study, the effects of the El Niño-Southern Oscillation (ENSO) on the rainfall variability in the central (Savannah) and southern (Equatorial) regions of Sudan are examined. The annual rainfall data from 12 rainfall stations for 49 years are used in this examination. The results of the study show that the areal annual regionally averaged rainfall values in the two regions have decreased markedly since the early 1960s, with co-existence between the driest years and the warm ENSO events. The correlation between the annual regional rainfall values and the ENSO events is found to be relatively higher for the Savannah region than for the Equatorial region. Two regional ENSO-rainfall prediction models are developed, one for each region. These models use the ENSO sea surface temperature. The results of the models test show that both models can significantly improve the predictability of the annual rainfall values, which is essential for the planning and the management of water resources in Sudan

Fluctuations of the relationship between ENSO and northeast Australian rainfall

It is well known that during an El Nino-Southern Oscillation (ENSO) warm event, drought occurs in regions of northeastern (NE) Australia, leading to anomalously low annual rainfall. The present study explores fluctuations of this ENSO-rainfall relationship. It is found that the relationship tends to weaken when the linearly detrended global mean temperature is rising or particularly high, as in the period of 1931–45 period and since the late 1970s. Prior to a weakening, a correlation pattern of increased rainfall during El Nino events is seen first in northwestern Australia, then in eastern and southeastern Australia, and eventually in NE Australia. The 1931–45 period was particularly intriguing, when in terms of rainfall variability over NE Australia, the interannual ENSO-rainfall relationship went through a process of weakening, reversal, and rapid recovery. Features associated with the reversal are therefore examined and these features are: (1) the global background anomaly pattern (upon which internnal ENSO events operate) is ENSO-like; (2) ENSO sea surface temperature (SST) anomalies in tropical Pacific are weaker compared with those averaged over all ENSO events, whereas SST anomalies in the mid- to-high latitude Pacific (which have opposing polarity to those in tropical Pacific) are larger; (3) there is strong coherence between ENSO and variability in northern mid- to high-latitudes; and (4) the relationship that an El Nino event contributes to a warming anomaly of global mean SST weakens. Possible interrelationship among these features are discussed.

Observations of Warm Water Volume Changes in the Equatorial Pacific and Their Relationship to El Niño and La Niña

This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Niño–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Niño SST anomalies are larger than the corresponding cold La Niña anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences in the relative importance of physical processes controlling SST during El Niño and La Niña events.

Shortwave feedbacks and El Niño‐Southern Oscillation: Forced ocean and coupled ocean‐atmosphere experiments

Changes in tropical sea surface temperature (SST) can produce changes in cloudiness that modify incoming solar shortwave (SW) radiation, which in turn affects SST. The effects of this negative feedback on Pacific interannual variability are examined in forced ocean model and hybrid coupled ocean‐atmosphere model simulations. Two empirical schemes are used to model the large‐scale, low‐frequency response of surface SW to SST anomalies. The first scheme attempts to account for the nonlocal nature of the atmospheric response to SST based on patterns of covariability analyzed through singular value decomposition. In the observations the primary coupled mode of variability is composed of a SW anomaly in the central Pacific that covaries with anomalous SST in the eastern Pacific. This is applied in the model as a nonlocal SW feedback. The second scheme examines the effects of a purely local feedback with a spatially varying coefficient of magnitude chosen similar to the first scheme. In almost all cases the second scheme behaved similarly to the first, presumably because the correlation scale of SST is large enough for El Niño‐Southern Oscillation (ENSO) dynamics that there is little sensitivity to the local approximation in the SW feedback. In simulations forced by time series of observed wind stress the SW feedback induced very minor SST damping. Results from a simplified heat budget analysis showed that while the SW feedback increased the local heat flux damping on SST, it also induced a mean shallowing of the mixed layer. The resulting changes in both the local mean vertical temperature gradient and the zonal velocity response to the wind stress acted to oppose the local heat flux damping effects. When observed SW anomalies were applied to forced simulations, the simulated SST anomalies were modified as expected, and agreement with observed SST improved. In coupled simulations the SW feedbacks had greater impact than in the case of specified stress. The main effects were to decrease the magnitude of the warm and cold SST anomalies in the central Pacific, while leaving the pattern and evolution of ENSO anomalies essentially unchanged elsewhere. The SW feedbacks thus produce a modest improvement of the model ENSO SST pattern compared with observations, although they tended to shorten the period of the model ENSO cycle. Overall the results suggest that large‐scale SW feedbacks are of quantitative importance to simulating some aspects of the ENSO cycle but are not critical to the overall occurrence of the phenomenon.