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

Breakup of land-fast sea ice in Lutzow-Holm Bay, East Antarctica, and its teleconnection to tropical Pacific sea surface temperatures

A large land-fast sea ice breakup occurred in 2016 in Lutzow-Holm Bay, East Antarctica. The breakup caused calving from the Shirase Glacier Tongue. Although similar breakups and calving have been observed in the past, the timing and magnitudes are not well-constrained. The ice's breakup latitude during 1997-2016 was analyzed to investigate the variables controlling breakup and examine correlation with local calving for a longer period. The breakup latitude in April had a persistently high correlation with sea surface temperature (SST) in the tropical Pacific, which exceeds correlations with local atmospheric variables. The years of five out of six observed calving events from the mid-20th century can correspond to those of warm SST episodes and calving-front retreat in the 1980s to warmer SST shift. Our proposed teleconnection between tropical SST and Antarctic sea ice could lead to better predictions of breakup and might impact the glacier flux for a wider region. Plain Language Summary Land-fast sea ice forms along the Antarctic coast, and it occasionally breaks up significantly. The breakup event influences the flow of glaciers, which is otherwise held back by the fast ice. The breakup of land-fast sea ice and the discharge of glaciers have significant multidecadal variability as well as interannual variability. This study explores what controls the breakup phenomena of land-fast sea ice in Antarctica and finds the linkage with tropical sea surface temperatures. We find the environmental factors which are relevant to the ice breakup, and those variables are originally driven by the teleconnection from the tropical Pacific. We believe that our study makes a significant contribution in climate science by offering a causal mechanism that explains the previously observed multidecadal variability in ice extent in this region. Our model can explain five out of the last six calving events in a major glacier connected to this bay, offering hope for future predictions of ice behavior. This will also merit the logistics to Antarctic research stations

The Southern Ocean sea surface temperature response to ozone depletion: a multi-model comparison

The effect of the Antarctic ozone hole extends downward from the stratosphere, with clear signatures in surface weather patterns including a positive trend in the southern annular mode (SAM). Several recent studies have used coupled climate models to investigate the impact of these changes on Southern Ocean sea surface temperature (SST), notably motivated by the observed cooling from the late 1970s. Here we examine the robustness of these model results through comparison of both previously published and new simulations. We focus on the calculation of climate response functions (CRFs), transient responses to an instantaneous step change in ozone concentrations. The CRF for most models consists of a rapid cooling of SST followed by a slower warming trend. However, intermodel comparison reveals large uncertainties, such that even the sign of the impact of ozone depletion on historical SST, when reconstructed from the CRF, remains unconstrained. Comparison of these CRFs with SST responses to a hypothetical step change in the SAM, inferred through lagged linear regression, shows broadly similar results. Causes of uncertainty are explored by examining relationships between model climatologies and their CRFs. The intermodel spread in CRFs can be reproduced by varying a single subgrid-scale mixing parameter within a single model. Antarctic sea ice CRFs are also calculated: these do not generally exhibit the two-time-scale behavior of SST, suggesting a complex relationship between the two. Finally, by constraining model climatology–response relationships with observational values, we conclude that ozone depletion is unlikely to have been the primary driver of the observed SST cooling trend

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

Fast and slow responses of Southern Ocean sea surface temperature to SAM in coupled climate models

We investigate how sea surface temperatures (SSTs) around Antarctica respond to the Southern An- nular Mode (SAM) on multiple timescales. To that end we examine the relationship between SAM and SST within unperturbed preindustrial control simulations of coupled general circulation models (GCMs) included in the Climate Modeling Intercomparison Project phase 5 (CMIP5). We develop a technique to extract the re- sponse of the Southern Ocean SST (55◦S−70◦S) to a hypothetical step increase in the SAM index. We demonstrate that in many GCMs, the expected SST step re- sponse function is nonmonotonic in time. Following a shift to a positive SAM anomaly, an initial cooling regime can transition into surface warming around Antarctica. However, there are large differences across the CMIP5 ensemble. In some models the step response function never changes sign and cooling persists, while in other GCMs the SST anomaly crosses over from negative to positive values only three years after a step increase in the SAM. This intermodel diversity can be related to differences in the models’ climatological thermal ocean stratification in the region of seasonal sea ice around Antarctica. Exploiting this relationship, we use obser- vational data for the time-mean meridional and vertical temperature gradients to constrain the real Southern Ocean response to SAM on fast and slow timescales

Challenges and opportunities for improved understanding of regional climate dynamics

Dynamical processes in the atmosphere and ocean are central to determining the large-scale drivers of regional climate change, yet their predictive understanding is poor. Here, we identify three frontline challenges in climate dynamics where significant progress can be made to inform adaptation: response of storms, blocks and jet streams to external forcing; basin-to-basin and tropical–extratropical teleconnections; and the development of non-linear predictive theory. We highlight opportunities and techniques for making immediate progress in these areas, which critically involve the development of high-resolution coupled model simulations, partial coupling or pacemaker experiments, as well as the development and use of dynamical metrics and exploitation of hierarchies of models