El Nino-Southern Oscillation Sea Surface Temperature Anomalies Research Articles

Impact of ENSO longitudinal position on teleconnections to the NAO

While significant improvements have been made in understanding how the El Nino–Southern Oscillation (ENSO) impacts both North American and Asian climate, its relationship with the North Atlantic Oscillation (NAO) remains less clear. Observations indicate that ENSO exhibits a highly complex relationship with the NAO-associated atmospheric circulation. One critical contribution to this ambiguous ENSO/NAO relationship originates from ENSO’s diversity in its spatial structure. In general, both eastern (EP) and central Pacific (CP) El Nino events tend to be accompanied by a negative NAO-like atmospheric response. However, for two different types of La Nina the NAO response is almost opposite. Thus, the NAO responses for the CP ENSO are mostly linear, while nonlinear NAO responses dominate for the EP ENSO. These contrasting extra-tropical atmospheric responses are mainly attributed to nonlinear air-sea interactions in the tropical eastern Pacific. The local atmospheric response to the CP ENSO sea surface temperature (SST) anomalies is highly linear since the air-sea action center is located within the Pacific warm pool, characterized by relatively high climatological SSTs. In contrast, the EP ENSO SST anomalies are located in an area of relatively low climatological SSTs in the eastern equatorial Pacific. Here only sufficiently high positive SST anomalies during EP El Nino events are able to overcome the SST threshold for deep convection, while hardly any anomalous convection is associated with EP La Nina SSTs that are below this threshold. This ENSO/NAO relationship has important implications for NAO seasonal prediction and places a higher requirement on models in reproducing the full diversity of ENSO.

Interdecadal Change in the Relationship Between the North Pacific Oscillation and the Pacific Meridional Mode and Its Impact on ENSO

Two leading but independent modes of Northern Pacific atmospheric circulation: the North Pacific Oscillation (NPO) and the Pacific Meridional Mode (PMM), are known external triggers of the El Nino-Southern Oscillation (ENSO) by the sequential migration of sea surface temperature (SST) anomalies into the tropics possibly by means of wind-evaporation-SST (WES) feedbacks. Because of the similar roles of NPO and PMM, most previous studies have explored them with no separation. Here, we investigate their independent and combined effects in triggering ENSO, and find that when the NPO and PMM occur simultaneously during spring, ENSO or ENSO-like SST anomalies are generated during the following winter; whereas when either the NPO or PMM occur alone, ENSO events rarely occur. Furthermore, the relationship between NPO and PMM shows noticeable interdecadal variability, which is related to decadal changes in the mean upper-level jet stream over the North Pacific. Changes in the upper-level jet stream modify the location of the center of the Aleutian Low, which plays a role in bridging the NPO and PMM processes, especially when it migrates to the southwest. The period when NPO and PMM are well correlated coincides somewhat with the active ENSO period, and vice versa, indicating that a more efficient trigger due to combined NPO-PMM processes results in a higher variation of ENSO. Finally, analysis of the coupled model control simulations strongly supports our observational analysis results.

Estimating ENSO predictability based on multi-model hindcasts

Based on hindcasts of seasonal forecast systems participating in the North American Multi-Model Ensemble, the seasonal dependence of predictability of the El Nino–Southern Oscillation (ENSO) was estimated. The results were consistent with earlier analyses in that the predictability of ENSO was highest in winter and lowest in spring and summer. Further, predictability as measured by the relative amplitude of predictable and unpredictable components was dominated by the ensemble mean instead of the spread (or dispersion) among ensemble members. This result was consistent with previous analysis that most of ENSO predictability resides in the shift of the probability density function (PDF) of ENSO sea surface temperature (SST) anomalies (i.e., changes in the first moment of the PDF that is associated with the ensemble mean of ENSO SST anomalies) rather than due to changes in the spread of the PDF. The analysis establishes our current best estimate of ENSO predictability that can serve as a benchmark for quantifying further improvements resulting from advances in observing, assimilation, and seasonal prediction systems.

Zonal phase propagation of ENSO sea surface temperature anomalies: Revisited

AbstractPrevious studies have identified a reversal in the phase propagation characteristics (from westward to eastward propagation) of the El Niño–Southern Oscillation (ENSO) during the 1976 climate shift. These studies relied on the assumption of a stationary seasonal cycle in Sea Surface Temperature (SST), ignoring the possible effect of ENSO and decadal variability on the seasonal cycle strength. Here we demonstrate using complex empirical orthogonal functions combined with Radon transform methods that mutidecadal amplitude modulations of the seasonal cycle, the semiannual cycle, and the El Niño Modoki mode played an important role in determining the phase propagation characteristics of equatorial SST anomalies in past decades. We find very little evidence for changes in the propagation characteristics of the Cold Tongue ENSO mode.

Modulation of convectively coupled Kelvin wave activity in the tropical Pacific by ENSO

The influence of El Nino-Southern Oscillation (ENSO) on the convectively coupled Kelvin waves over the tropical Pacific is investigated by comparing the Kelvin wave activity in the eastern Pacific (EP) El Nino, central Pacific (CP) El Nino, and La Nina years, respectively, to 30-yr (1982–2011) mean statistics. The convectively coupled Kelvin waves in this study are represented by the two leading modes of empirical orthogonal function (EOF) of 2–25-day band-pass filtered daily outgoing longwave radiation (OLR), with the estimated zonal wavenumber of 3 or 4, period of 8 days, and eastward propagating speed of 17 m s−1. The most significant impact of ENSO on the Kelvin wave activity is the intensification of the Kelvin waves during the EP El Ninos. The impact of La Nina on the reduction of the Kelvin wave intensity is relatively weaker, reflecting the nonlinearity of tropical deep convection and the associated Kelvin waves in response to ENSO sea surface temperature (SST) anomalies. The impact of the CP El Nino on the Kelvin waves is less significant due to relatively weaker SST anomalies and smaller spatial coverage. ENSO may also alter the frequency, wavelength, and phase speed of the Kelvin waves. This study demonstrates that low-frequency ENSO SST anomalies modulate high-frequency tropical disturbances, an example of weather-climate linkage.

Changing dependence of Zimbabwean rainfall variability on ENSO and the Indian Ocean dipole/zonal mode

A novel approach of using variability as a major descriptive parameter for observational time series is adopted to investigate how southeastern African (Zimbabwe) summer rainfall may have been forced by the El Nino southern oscillation (ENSO) and/or the Indian Ocean dipole/zonal mode (IODZM), concurrently with the changing sea surface temperature (SST) background of the Indo-Pacific region. The period is from 1901 to 2007. Wavelet power spectrum and linear statistical analysis, including simple and partial correlation techniques, are utilized to achieve this end. The results reveal epochal changes in teleconnections and other statistical properties linking southeastern African rainfall variability to the Indo-Pacific SSTs. These epochs conspicuously exhibit the period around 1945/1946, 1960/1961, 1972/1973, and 1997/1998 as demarcating distinct systematic climate turning points during the century. The role of the 1976/1977 climate regime shift seems not to be that apparent but instead, 1960/1961 and 1997/1998 appear to be the major turning points, both in the teleconnections and Indo-Pacific SST temporal characteristics. From the early 1960s to 1997, as much as 28% of the rainfall variability was linearly related to the IODZM while the ENSO hardly explained a quarter of this value. During this period, droughts are especially strongly connected to the positive IODZM events but insignificantly linked to El Nino, thus contradicting the conventional knowledge attributing most of regional rainfall suppression to the warm ENSO phase. However, the post 1997 epoch saw the reversal of the two climate modes’ influences. ENSO influence evidently became activated, attaining the previously assumed dominant role which had proved elusive during the earlier epochs. But this increased ENSO control is attributed to only less than 12% of the rainfall variance while IODZM seem to be largely decoupled from the local rainfall influencing processes by hardly explaining a drastically reduced 1% of the rainfall variance. This transformation occurred despite of neither ENSO nor IODZM SST anomalies showing any statistically significant changes. As far as could be established, this epochal behavior may not be forced by the frequency and magnitude of ENSO and IODZM events but most probably by the slow processes inherent in the SST background, especially of the tropical Indian Ocean. Thus, the apparent simultaneous decoupling of the IODZM and coupling of the ENSO to the regional rainfall controlling mechanisms seems to be the result of the unprecedented warming of the intervening tropical western Indian Ocean SSTs in the last decade. Consequently, the knowledge of the states of epochal Indian Ocean SST background variability should be an important regional scientific issue, since these epochal variations may dictate the nature (and successful prediction) of interannual as well as decadal climate fluctuations over southeastern Africa.

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.