Atlantic Sea Surface Temperature Variability Research Articles

Why Is North Tropical Atlantic SST Variability Stronger in Boreal Spring?

It is suggested that the seasonal dependence of interannual variability displayed by observed sea surface temperature (SST) over the north tropical Atlantic primarily reflects the seasonality of the remote forcing associated with the North Atlantic and Southern Oscillations and is controlled by the mean damping time scale of SST anomaly. A stochastic model including a seasonally dependent forcing is used to test this hypothesis against observations.

Holocene climate variability as derived from alkenone sea surface temperature and coupled ocean-atmosphere model experiments

Holocene climate modes are identified by the statistical analysis of reconstructed sea surface temperatures (SSTs) from the tropical and North Atlantic regions. The leading mode of Holocene SST variability in the tropical region indicates a rapid warming from the early to mid Holocene followed by a relatively weak warming during the late Holocene. The dominant mode of the North Atlantic region SST captures the transition from relatively warm (cold) conditions in the eastern North Atlantic and the western Mediterranean Sea (the northern Red Sea) to relatively cold (warm) conditions in these regions from the early to late Holocene. This pattern of Holocene SST variability resembles the signature of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). The second mode of both tropical and North Atlantic regions captures a warming towards the mid Holocene and a subsequent cooling. The dominant modes of Holocene SST variability emphasize enhanced variability around 2300 and 1000 years. The leading mode of the coupled tropical-North Atlantic Holocene SST variability shows that an increase of tropical SST is accompanied by a decrease of SST in the eastern North Atlantic. An analogy with the instrumental period as well as the analysis of a long-term integration of a coupled ocean-atmosphere general circulation model suggest that the AO/NAO is one dominant mode of climate variability at millennial time scales.

Remote Impact on Tropical Atlantic Climate Variability: Statistical Assessment and Dynamic Assessment*

Abstract The remote impact of tropical Pacific and North Atlantic climate forcing on the tropical Atlantic sea surface temperature variability is assessed using both a traditional statistical correlation method and a model-aided dynamic method. Consistently, both assessment methods suggest that the remote impact contributes to nearly half of the variance of the tropical Atlantic sea surface temperature variability at interannual and decadal time scales. In the meantime, the other half of the sea surface temperature variability is generated predominantly in the tropical Atlantic climate system, with local ocean–atmosphere coupling playing a critical role. Furthermore, the leading sea surface temperature variability modes seem also to originate predominantly internally in the tropical Atlantic climate system. The main effect of the remote impact is therefore an enhancement of the variance of these variability modes. This model study also shows some differences between the statistical and dynamic assessment ...

Summer U.K. Temperature and Its Links to Preceding Eurasian Snow Cover, North Atlantic SSTs, and the NAO

Motivated by an attempt to predict summer (June‐August) U.K. temperatures, the time-lagged correlations between summer U.K. and European temperatures and prior snow cover, North Atlantic sea surface temperatures (SSTs), and the North Atlantic Oscillation (NAO) are examined. The analysis centers on the 30-yr period 1972‐ 2001 corresponding to the interval of reliable satellite-derived land snow cover data. A significant association is found between late winter Eurasian snow cover and upcoming summer temperatures over the British Isles and adjacent areas, this link being strongest with January‐March snow cover. Significant links are also observed between summer temperatures and the preceding late winter NAO index and with a leading principal component of North Atlantic SST variability. The physical mechanisms underlying these time-lagged correlations are investigated by studying the associated variability in large-scale atmospheric circulation over the Euro‐Atlantic sector. Seasonal expansion in the Azores high pressure system may play an important role in the time-lagged relationships. The potential seasonal predictability of summer U.K. temperatures during the period 1972‐2001 is assessed by cross-validated hindcasts and usable predictive skill is found. However, the presence and cause of temporal instability in the time-lagged relationships over longer periods of time requires further investigation.

Multiple landslide clusters record Quaternary climate changes in the northwestern Argentine Andes

The chronology of multiple landslide deposits and related lake sediments in the semi-arid eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary, i.e. between 40 and 25 and after 5 14C kyr BP. These clusters may correspond to the Minchin (maximum at around 28–27 14C kyr BP) and Titicaca wet periods (after 3.9 14C kyr BP). The more humid conditions apparently caused enhanced landsliding in this environment. In contrast, no landslide-related damming and associated lake sediments occurred during the Coipasa (11.5–10 14C yr BP) and Tauca wet periods (14.5–11 14C yr BP). The two clusters at 40–25 and after 5 14C kyr BP may correspond to periods where the El Niño–Southern Oscillation (ENSO) and Tropical Atlantic Sea Surface Temperature Variability (TAV) were active. This, however, was not the case during the Coipasa and Tauca wet periods. Lake-balance modelling of a landslide-dammed lake suggests a 10–15% increase in precipitation and a 3–4°C decrease in temperature at ∼30 14C kyr BP as compared to the present. In addition, time-series analysis reveals a strong ENSO and TAV during that time. The landslide clusters in northwestern Argentina are therefore best explained by periods of more humid and more variable climates.

Independent component analysis of multivariate time series: Application to the tropical SST variability

With the aim of identifying the physical causes of variability of a given dynamical system, the geophysical community has made an extensive use of classical component extraction techniques such as principal component analysis (PCA) or rotational techniques (RT). We introduce a recently developed algorithm based on information theory: independent component analysis (ICA). This new technique presents two major advantages over classical methods. First, it aims at extracting statistically independent components where classical techniques search for decorrelated components (i.e., a weaker constraint). Second, the linear hypothesis for the mixture of components is not required. In this paper, after having briefly summarized the essentials of classical techniques, we present the new method in the context of geophysical time series analysis. We then illustrate the ICA algorithm by applying it to the study of the variability of the tropical sea surface temperature (SST), with a particular emphasis on the analysis of the links between El Niño Southern Oscillation (ENSO) and Atlantic SST variability. The new algorithm appears to be particularly efficient in describing the complexity of the phenomena and their various sources of variability in space and time.

Decadal variability of Russian winter snow accumulation and its associations with Atlantic sea surface temperature anomalies

Russian winter snow depth data over a 48-year period (1936–1983) are analysed to reveal variation characteristics and associations to Atlantic sea surface temperature (SST) anomalies using methods of rotated principal component analysis (RPCA), singular spectrum analysis (SSA), and singular value decomposition (SVD) analysis. The study demonstrates that four time scales (4 years interannual, 11.8 years quasidecadal, 20 years bi-decadal and trend) characterize Russian winter snow depth variations. The decadal and longer time scale variations are found to be significantly associated with Atlantic SST anomalies. The trend, which occurred over much of the study region, is associated with SST trends over the northern north and tropical south Atlantic. Bi-decadal snow depth variation over central Siberia is associated with western tropical north Atlantic SSTs. A quasi-decadal variation over western European Russia is connected to a major Atlantic SST variation pattern of opposite signs over alternative latitudinal belts. This study suggests that the connections between the Atlantic Ocean and regional climate may be better reflected at decadal time scales than interannual and seasonal ones, as the dominant variability over the ocean is at slow modes. Copyright © 2000 Royal Meteorological Society

Tropical‐extratropical connection in the Atlantic atmosphere‐ocean variability

A question how sea surface temperature (SST) fluctuations in the tropical Atlantic affect the North Atlantic atmosphere‐ocean system is addressed using an atmospheric general circulation model coupled to a 50‐m‐deep slab ocean (MGCM), and a linear baroclinic model. A comparison of a 60‐yr integration, in which the tropical SSTs are prescribed to observations (TOGA‐ML), with the control MGCM run shows that the tropical SST variations selectively enhance variance of the North Atlantic Oscillation (NAO) by about 80%. This is partly due to extratropical height anomalies associated with two dominant modes, monopole and dipole, of tropical Atlantic SST variability because they both have a projection on to the NAO in the TOGA‐ML run. The simulated midlatitude height anomalies accompany characteristic SST anomaly patterns beneath, and they are consistent with observations. Analyses of steady linear responses to the heating in the tropical Atlantic and transient eddy feedback in midlatitude reveal that the substantial part of height anomalies is generated by the latter rather than the former. The anomalous eddy forcing is, however, organized not only by the heating in the tropical Atlantic but also by the tropical Pacific SST variations through the teleconnection over North America. It is suggested that the tropical Atlantic SST variability does affect the North Atlantic atmosphere and ocean, but the dynamical connection is tenuous in comparison with the Pacific.

Variability of the Tropical Ocean Surface Temperatures at Decadal–Multidecadal Timescales. Part I: The Atlantic Ocean

Abstract Gridded time series from the Global Ocean Surface Temperature Atlas were analyzed with a variety of techniques to identify spatial structures and oscillation periods of the tropical Atlantic sea surface temperature (SST) variations at decadal timescales, and to develop physical interpretations of statistical patterns of decadal SST variations. Each time series was 110 yr (1882–1991) long. The tropical Atlantic SST variations were compared with decadal variations in a 74-yr-long (1912–85) north Nordeste Brazil rainfall time series and a 106-yr-long (1886–1991) tropical Atlantic cyclone activity index time series. The tropical Atlantic SST variations were also compared with decadal variations in the extratropical Atlantic SST. Multiyear to multidecadal variations in the cross-equatorial dipole pattern identified as a dominant empirical pattern of the tropical Atlantic SST variations in earlier and present studies are shown to be variations in the approximately north–south gradient of SST anomalies....

Prediction of tropical Atlantic sea surface temperature

The variability of sea surface temperature (SST) in the tropical Atlantic Ocean has a significant influence on the rainfall over Northeast Brazil and the Sahel. The predictability of the SST fluctuations in this region has, therefore, been a central concern for Atlantic climate studies. Here we show that the north tropical Atlantic SST variability has significant predictability using statistical models at short lead times (≤ 1 year), and using a coupled ocean‐atmosphere model for longer‐range forecasts (lead times up to about 3 years). While the remote influence of the IndoPacific, primarily due to El Niño, has been found to enhance the predictability of SST in the north Atlantic at short lead times, local ocean‐atmosphere interactions dominate the predictable dynamics farther south, and at longer time scales.

The northwest european shelf temperature and salinity variability

From long temperature and salinity time series of the Northwest European shelf and 39 years of numerical model transport calculations, it is possible to derive the long-term hydrographic variability of the North Sea system and its possible causes. This is shown by means of trend analyses, cross-correlations, and power spectra. Initially there is a weak linear long-term positive trend in the SST series, which is in agreement with the global warming trend. In all series investigated, several periodicities are found on corresponding band widths, and we argue that this is due to long-term oscillations in the North Atlantic circulation system and interaction with the atmosphere. Cross-correlations between Sea Surface Temperature (SST)/ Sea Surface Salinity (SSS) and some transports are significant, with time lags of about 5 to 6 years. These time lags are unexplained; we assume that differences in the subpolar and subtropical gyre circulation might be responsible. A two-year time lag between the SSS in the Rockall Channel and east of Scotland points to lateral exchanges by shear dispersion, not to advective transports. It seems that the North Sea SST longterm fluctuations are coupled to the Atlantic subpolar gyre SST variability. In contrast, the Bay of Biscay behaves very similarly to the Atlantic subtropical gyre system. The ratio between the inflow of saline Atlantic water into the North Sea and continental run-off seems to have been rather constant in the last 100 years.

A decadal climate variation in the tropical Atlantic Ocean from thermodynamic air-sea interactions

Rainfall variability in northeast South America1 and the Sahel region of Africa2–4 is profoundly influenced by the sea surface temperature (SST) of the tropical Atlantic Ocean. Of particular importance are relative changes in SST between the hemispheres on decadal timescales, a phenomenon often called the Atlantic SST dipole1,5. Here we propose that the decadal variation in the tropical SST dipole may be attributed to an unstable thermodynamic ocean–atmosphere interaction between wind-induced heat fluxes and SST. Using coupled ocean–atmosphere models, we show that the coupled dipole mode has a typical oscillation period of about a decade. The notion that the Atlantic dipole-like SST variability may be related to an oscillatory coupled mode might assist attempts to predict decadal climate variability in the tropical Atlantic region.

Tropical Atlantic sea surface temperature variability and its relation to El Niño‐Southern Oscillation

Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43‐year (1950–1992) record of SST [Smith et al, 1996] and other data derived from the Comprehensive Ocean‐Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre [1992]. Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is correlated with Pacific El Niño‐Southern Oscillation (ENSO) variability in several regions. The major region affected is the North Atlantic area of NE trades west of 40°W along 10°N–20°N and extending into the Caribbean. There, about 50–80% of the anomalous SST variability is associated with the Pacific ENSO, with Atlantic warmings occurring 4–5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO‐related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in question, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North Atlantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the development of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15°–30°S.

Decadal Variability of the Tropical Atlantic Ocean Surface Temperature in Shipboard Measurements and in a Global Ocean-Atmosphere Model

Sea surface temperature (SST) variability was investigated in a 200-yr integration of a global model of the coupled oceanic and atmospheric general circulations developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The second 100 yr of SST in the coupled model's tropical Atlantic region were analyzed with a variety of techniques. Analyses of SST time series, averaged over approximately the same subregions as the Global Ocean Surface Temperature Atlas (GOSTA) time series, showed that the GFDL SST anomalies also undergo pronounced quasi-oscillatory decadal and multidecadal variability but at somewhat shorter timescales than the GOSTA SST anomalies. Further analyses of the horizontal structures of the decadal timescale variability in the GFDL coupled model showed the existence of two types of variability in general agreement with results of the GOSTA SST time series analyses. One type, characterized by timescales between 8 and 11 yr, has high spatial coherence within each hemisphere but not between the two hemispheres of the tropical Atlantic. A second type, characterized by timescales between 12 and 20 yr, has high spatial coherence between the two hemispheres. The second type of variability is considerably weaker than the first. As in the GOSTA time series, the multidecadal variability in the GFDL SST time series has approximately opposite phases between the tropical North and South Atlantic Oceans. Empirical orthogonal function analyses of the tropical Atlantic SST anomalies revealed a north-south bipolar pattern as the dominant pattern of decadal variability. It is suggested that the bipolar pattern can be interpreted as decadal variability of the interhemispheric gradient of SST anomalies. The decadal and multidecadal timescale variability of the tropical Atlantic SST, both in the actual and in the GFDL model, stands out significantly above the background 'red noise' and is coherent within each of the time series, suggesting that specific sets of processes may be responsible for the choice of the decadal and multidecadal timescales. Finally, it must be emphasized that the GFDL coupled ocean-atmosphere model generates the decadal and multidecadal timescale variability without any externally applied force, solar or lunar, at those timescales.

Influences of High‐ and Low‐Latitude Processes on African Terrestrial Climate: Pleistocene Eolian Records from Equatorial Atlantic Ocean Drilling Program Site 663

High‐ and low‐latitude forcing of terrestrial African paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from Ocean Drilling Program site 663 in the eastern equatorial Atlantic. Terrigenous (eolian dust) and phytolith (savannah grass cuticle) accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high‐latitude forcing. The abundance of freshwater diatoms (Melosira) transported from dry African lake beds varies coherently at 23–19 kyr orbital periodicities and at a phasing which implicates low‐latitude precessional monsoon forcing. Modeling studies demonstrate that African climate is sensitive to both high‐ and low‐latitude boundary conditions. African monsoon intensity is modulated by direct insolation variations due to orbital precession, whereas remote high‐latitude forcing can be related to cool North Atlantic sea surface temperatures (SSTs) which promote African aridity and enhance dust‐transporting wind speeds. The site 663 terrigenous and phytolith records covary with North Atlantic SST variability at 41 °N (site 607). We suggest that Pleistocene African climate has responded to both high‐latitude North Atlantic SST variability as well as low‐latitude precessional monsoon forcing; the high‐latitude influence dominates the sedimentary record. Prior to circa 2.4 Ma, terrigenous variations occurred primarily at precessional periodicities (23–19 kyr), indicating that African climate was largely controlled by low‐latitude insolation variations prior to the onset of high‐amplitude glacial‐interglacial climate change.

Interannual variability in the tropical Atlantic

The anomalous meteorological and oceanic conditions in the tropical Atlantic basin during 1983–84 can be seen as an extreme example of a common historical pattern. Here we present time series of rainfall in subsaharan West Africa (SWA) and north-east Brazil (NEB), which show that departures in 1983–84 were severe, but not unprecedented. The associated sea surface temperature (SST) anomaly patterns included one that had previously been shown to accompany earlier years of extreme rainfall in SWA and NEB, and others that contained key elements of that pattern. The pattern concerned was recently shown to be a major mode of tropical Atlantic SST variation.