North Atlantic Ocean-atmosphere interaction on intraseasonal time scales

Summer 2004.Also issued as author's thesis (M.S.) -- Colorado State University, 2004.NSF grant ATM-320959, NSF grant ATM-0132190, and National Science Foundation on cover.Includes bibliographical references.A substantial fraction of midlatitude sea surface temperature (SST) variability on time scales ranging from months to years can be interpreted as the passive thermodynamic response of the ocean mixed layer to stochastic atmospheric forcing. Subsequently, the dominant structures of monthly and seasonal mean Northern Hemisphere SST variability owe their existence to variations in the extratropical atmosphere. To what extent midlatitude SST variability, in tum, gives rise to anomalies in the dominant structures of extratropical atmospheric variability remains unclear. Presumably, if the extratropical atmosphere exhibits a deep and statistically significant response to midlatitude SST anomalies, the dynamics of the response should occur on time scales shorter than the monthly and seasonal mean data used in most observational analyses of midlatitude atmosphere-ocean interaction. The motivation of the thesis is to investigate the interaction between North Atlantic SST variability and the extratropical atmospheric circulation on intraseasonal time scales. First, the climatology of the North Atlantic SST field and the overlying atmospheric circulation is described. The largest variance in intraseasonal and seasonal mean SST anomalies is located within a zone of enhanced SST gradients in the Gulf Stream extension. The region of maximum SST variance also underlies a region of marked wintertime cyclogenesis over the western edge of the North Atlantic storm track. Patterns of North Atlantic weekly SST variability are further investigated using Empirical Orthogonal Function (EOF) analysis. EOFs of both weekly summertime and wintertime SST anomalies reflect a mix of two patterns, variability in the Gulf Stream extension region and a meridionally banded structure of SST anomalies commonly referred to as the tripole. These patterns are most clearly evident in EOFs based on intraseasonal wintertime SST anomalies. Wintertime atmosphere-ocean interaction on intraseasonal time scales is then examined using lagged correlation/regression analysis. The results show that the tripole and variability in the Gulf Stream extension region emerge not only as the leading EOFs of intraseasonal wintertime SST variability but also in association with the leading pattern of Northern Hemisphere atmospheric variability, referred to as the Northern Annular Mode (NAM). Consistent with previous results, the strongest correlations between midlatitude SSTs and the NAM occur when variations in the NAM lead the tripole by ~2 weeks. However, the present results also show a coherent and statistically significant pattern of SST anomalies over the Gulf Stream extension region that precedes changes in the NAM by ~2 weeks

On the Observed Relationships between Variability in Gulf Stream Sea Surface Temperatures and the Atmospheric Circulation over the North Atlantic

The advent of increasingly high-resolution satellite observations and numerical models has led to a series of advances in understanding the role of midlatitude sea surface temperature (SST) in climate variability, especially near western boundary currents (WBC). Observational analyses suggest that ocean dynamics play a central role in driving interannual SST variability over the Kuroshio–Oyashio and Gulf Stream extensions. Numerical experiments suggest that variations in the SST field within these WBC regions may have a much more pronounced influence on the atmospheric circulation than previously thought. In this study, the authors examine the observational support for (or against) a robust atmospheric response to midlatitude SST variability in the Gulf Stream extension. To do so, they apply lead–lag analysis based on daily mean data to assess the evidence for two-way coupling between SST anomalies and the atmospheric circulation on transient time scales, building off of previous studies that have utilized weekly data. A novel decomposition approach is employed to demonstrate that atmospheric circulation anomalies over the Gulf Stream extension can be separated into two distinct patterns of midlatitude atmosphere–ocean interaction: 1) a pattern that peaks 2–3 weeks before the largest SST anomalies in the Gulf Stream extension, which can be viewed as the “atmospheric forcing,” and 2) a pattern that peaks several weeks after the largest SST anomalies, which the authors argue can be viewed as the “atmospheric response.” The latter pattern is linearly independent of the former and is interpreted as the potential response of the atmospheric circulation to SST variability in the Gulf Stream extension

Tropical forcing of increased Southern Ocean climate variability revealed by a 140-year subantarctic temperate reconstruction

Occupying 14% of the world’s surface, the Southern Ocean plays a fundamental role in global climate, ocean circulation, carbon cycling and Antarctic ice-sheet stability. Unfortunately, high interannual variability and a dearth of instrumental observations before the 1950s limits our understanding of how marine-atmosphere-ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing. Here we integrate climate-sensitive tree growth with ocean and atmospheric observations on southwest Pacific subantarctic islands that lie at the boundary of polar and subtropical climates (52–54˚S). Our annually-resolved temperature reconstruction captures regional change since the 1870s and demonstrates a significant increase in variability from the mid-twentieth century, a phenomenon predating the observational record. Climate reanalysis and modelling shows a parallel change in tropical Pacific sea surface temperatures that generate an atmospheric Rossby wave train which propagates across a large part of the Southern Hemisphere during the austral spring and summer

Observational evidence of reemergence in the extratropical Southern Hemisphere

Observations of subsurface temperatures are used to examine the winter-to-winter “reemergence” of sea surface temperature (SST) anomalies in the extratropical South Pacific Ocean. Reemergence is the mechanism through which SST anomalies formed in the late winter are sequestered beneath the relatively shallow summer mixed layer and then reentrained into the deepening mixed layer during the following autumn/winter. Although several studies have extensively examined reemergence in the Northern Hemisphere (NH), this is the first study to use observations of subsurface temperatures to document reemergence in the extratropical Southern Hemisphere (SH). The SH subsurface data reveal a pronounced reemergence signal in the western extratropical South Pacific. In this region, surface thermal anomalies formed during late SH winter are observed to persist below the summertime mixed layer and reemerge at the surface during the following early winter months. As such, SST anomalies formed during late winter are strongly correlated with SST anomalies during the following early winter but are not significantly correlated with SST anomalies during the intervening summer months. The results based on subsurface data are robust to small changes in the period of analysis and are qualitatively similar to existing evidence of reemergence in the NH. Analyses of independent SST data reveal that reemergence is widespread in the western extratropical South Pacific basin but is less discernible in SST anomalies over the eastern part of the basin

Observations of large-scale ocean-atmosphere interaction in the Southern Hemisphere

The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer

North Atlantic atmosphere-ocean interaction on intraseasonal time scales

The authors examine wintertime atmosphere–ocean interaction on weekly time scales over the North Atlantic sector. Consistent with previous results, it is found that the strongest interactions between the ocean and atmosphere occur when the atmosphere leads. However, the authors also find a spatially coherent and statistically significant pattern of sea surface temperature anomalies over the Gulf Stream extension region that precedes changes in the leading mode of Northern Hemisphere atmospheric variablilty by ∼2 weeks

Southern hemisphere ocean-atmosphere interaction

The motivation of this observational study is to provide a comprehensive analysis of ocean-atmosphere interaction in the Southern Ocean, with an emphasis on the relationships between the Southern Annular Mode (SAM) and observed variations in Southern Ocean sea-surface temperatures (SSTs). Lagged and contemporaneous relationships between SSTs and the SAM are examined on both monthly and weekly time scales. A complete diagnosis of the physical mechanisms that underlie the observed relationships is also provided. The implications of the results for large scale Southern Hemisphere SST variability and for observed trends in the Southern Ocean are discussed.Pages: 1253-125

Seasonal relationships between large-scale climate variability and Antarctic sea ice concentration

The observed relationships between anomalous Antarctic sea ice concentration (SIC) and the leading patterns of Southern Hemisphere (SH) large-scale climate variability are examined as a function of season over 1980–2008. Particular emphasis is placed on 1) the interactions between SIC, the southern annular mode (SAM), and El Niño–Southern Oscillation (ENSO); and 2) the contribution of these two leading modes to the 29-yr trends in sea ice. Regression, composite, and principal component analyses highlight a seasonality in SH sea ice–atmosphere interactions, whereby Antarctic sea ice variability exhibits the strongest linkages to the SAM and ENSO during the austral cold season months. As noted in previous work, a dipole in SIC anomalies emerges in relation to the SAM, characterized by centers of action located near the Bellingshausen/Weddell and Amundsen/eastern Ross Seas. The structure and magnitude of this SIC dipole is found to vary considerably as a function of season, consistent with the seasonality of the overlying atmospheric circulation anomalies. Relative to the SAM, the pattern of sea ice anomalies linked to ENSO exhibits a similar seasonality but tends to be weaker in amplitude and more diffuse in structure. The relationships between ENSO and sea ice also exhibit a substantial nonlinear component, highlighting the need to consider both season and phase of the ENSO cycle when diagnosing ENSO–SIC linkages. Trends in SIC over 1980–2008 are not significantly related to trends in either the SAM or ENSO during any season, including austral summer when the trend in the SAM is most pronounced