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Michael S. Pritchard
,
Andrew B. G. Bush
, and
Shawn J. Marshall

Abstract

To inform the ongoing development of earth system models that aim to incorporate interactive ice, the potential impact of interannual variability associated with synoptic variability and El Niño–Southern Oscillation (ENSO) at the Last Glacial Maximum (LGM) on the evolution of a large continental ice sheet is explored through a series of targeted numerical modeling experiments. Global and North American signatures of ENSO at the LGM are described based on a multidecadal paleoclimate simulation using an atmosphere–ocean general circulation model (AOGCM). Experiments in which a thermomechanical North American ice sheet model (ISM) was forced with persistent LGM ENSO composite anomaly maps derived from the AOGCM showed only modest ice sheet thickness sensitivity to ENSO teleconnections. In contrast, very high model sensitivity was found when North American climate variations were incorporated directly in the ISM as a looping interannual time series. Under this configuration, localized transient cold anomalies in the atmospheric record instigated substantial new ice formation through a dynamically mediated feedback at the ice sheet margin, altering the equilibrium geometry and resulting in a bulk 10% growth of the Laurentide ice sheet volume over 5 kyr.

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Gareth J. Marshall
,
Rebecca M. Vignols
, and
W. G. Rees

Abstract

The authors provide a detailed climatology and evaluation of recent climate change in the Kola Peninsula, Arctic Russia, a region influenced by both the North Atlantic and Arctic Oceans. The analysis is based on 50 years of monthly surface air temperature (SAT), precipitation (PPN), and sea level pressure (SLP) data from 10 meteorological stations for 1966–2015. Regional mean annual SAT is ~0°C: the moderating effect of the ocean is such that coastal (inland) stations have a positive (negative) value. Examined mean annual PPN totals rise from ~430 mm in the northeast of the region to ~600 mm in the west. Annual SAT in the Kola Peninsula has increased by 2.3° ± 1.0°C over the past 50 years. Seasonally, statistically significant warming has taken place in spring and fall, although the largest trend has occurred in winter. Although there has been no significant change in annual PPN, spring has become significantly wetter and fall drier. The former is associated with the only significant seasonal SLP trend (decrease). A positive winter North Atlantic Oscillation (NAO) index is generally associated with a warmer and wetter Kola Peninsula whereas a positive Siberian high (SH) index has the opposite impact. The relationship between both the NAO and SH and the SAT is broadly coherent across the region whereas their relationship with PPN varies markedly, although none of the relationships is temporally invariant. Reduced sea ice in the Barents and White Seas and associated circulation changes are likely to be the principal drivers behind the observed changes.

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Tsing-Chang Chen
,
Hal G. Marshall
, and
J. Shukla

Abstract

Wavenumber-frequency spectral analysis of a 90-day winter (15 January - 15 April) wind field simulated by a climate experiment of the GLAS (Goddard laboratory for Atmospheric Sciences) atmospheric circulation model is made using the space-time Fourier analysis which is modified with Tukey's numerical spectral analysis. Computations are also made to examine the nonlinear interactions of model wave disturbances in the wavenumber-frequency domain linear interactions. Results are compared with observation, especially Kao and Lee's (1977) study.

It is found that equatorial easterlies do not show up in this climate experiment at 200 mb. The zonal kinetic energy and momentum transport spectral of the model are generally in good agreement with observation. However, some distinct features of the model's spectra are revealed. The wavenumber spectra of kinetic energy show that the eastward moving waves of low wavenumbers have larger meridional motion compared with observation. Furthermore, the eastward moving waves show a band of large of spectral value in the medium-frequency regime. The frequency spectra in the high-frequency regime decrease faster than observation as frequency increases. The scheme proposed by Kao and Lee for the contribution to kinetic energy spectra by nonlinear interactions in middle latitudes is not applicable over the whole model globe because of the disappearance of equational easterlies. The contribution to momentum flux spectra by nonlinear interactions in Northern Hemisphere middle latitudes is similar to that of kinetic energy spectra. The primary nonlinear interactions of kinetic energy and momentum flux are contributed by those between mean zonal flow and long and medium waves with low and medium frequencies. The stationary waves do not play a significant role in the nonlinear interactions as found in observation.

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John C. Marshall
,
Richard G. Williams
, and
A. J. George Nurser

Abstract

The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate S ann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, S ann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of S ann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping.

The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate S ann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.

Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.

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P. O. Canziani
,
A. O'Neill
,
R. Schofield
,
M. Raphael
,
G. J. Marshall
, and
G. Redaelli
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Catherine H. Gregory
,
Neil J. Holbrook
,
Andrew G. Marshall
, and
Claire M. Spillman

Abstract

Marine heatwaves (MHWs) can severely impact marine biodiversity, fisheries, and aquaculture. Consequently, there is an increasing desire to understand the drivers of these events to inform their predictability so that proactive decisions may be made to reduce potential impacts. In the Tasman Sea (TS), several relatively intense and broad-scale MHWs have caused significant damage to marine fisheries and aquaculture industries. To assess the potential predictability of these events, we first determined the main driver of each MHW event in the TS from 1993 to 2021. We found that those MHWs driven by ocean advection—approximately 45% of all events—are generally longer in duration and less intense and affected a smaller area compared with the remaining 55%, which are driven by air–sea heat fluxes, are shorter in duration, and are more surface intense. As ocean advection–driven events in the TS have been closely studied and reported previously, we focus here on atmospherically driven MHWs. The predictability of these events is assessed by identifying the patterns of atmospheric pressure, winds, and air–sea heat fluxes in the Southern Hemisphere that coincide with MHWs in the Tasman Sea. We found that atmospherically driven MHWs in this region are more likely to occur during the positive phase of the asymmetric Southern Annular Mode (A-SAM)—which presents as an atmospheric zonal wave-3 pattern and is more likely to occur during La Niña years. These A-SAM events are linked to low wind speeds and increased downward solar radiation in the TS, which lead to increased surface ocean temperatures through the reduction of mixing.

Significance Statement

The purpose of this study is to understand factors of the atmosphere that contribute to an accumulation of heat in the upper ocean in the Tasman Sea to better inform predictability. Higher incidences of ocean extreme thermal events (known as marine heatwaves) in this region are becoming increasingly more common and threatening the important marine industries that support the people of both Australia and New Zealand. We need to know the sources of this extra heat to understand whether such events can be predicted. Previous studies have found the East Australian Current to be responsible for around half of these events, and our results show a connection between a known atmospheric pattern and the other half. As we continue to improve our ability to anticipate this pattern, this suggests that we may also be able to predict these extreme heating events.

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M. J. Pook
,
J. S. Risbey
,
P. C. McIntosh
,
C. C. Ummenhofer
,
A. G. Marshall
, and
G. A. Meyers

Abstract

The seasonal cycle of blocking in the Australian region is shown to be associated with major seasonal temperature changes over continental Antarctica (approximately 15°–35°C) and Australia (about 8°–17°C) and with minor changes over the surrounding oceans (below 5°C). These changes are superimposed on a favorable background state for blocking in the region resulting from a conjunction of physical influences. These include the geographical configuration and topography of the Australian and Antarctic continents and the positive west to east gradient of sea surface temperature in the Indo-Australian sector of the Southern Ocean. Blocking is represented by a blocking index (BI) developed by the Australian Bureau of Meteorology. The BI has a marked seasonal cycle that reflects seasonal changes in the strength of the westerly winds in the midtroposphere at selected latitudes. Significant correlations between the BI at Australian longitudes and rainfall have been demonstrated in southern and central Australia for the austral autumn, winter, and spring. Patchy positive correlations are evident in the south during summer but significant negative correlations are apparent in the central tropical north. By decomposing the rainfall into its contributions from identifiable synoptic types during the April–October growing season, it is shown that the high correlation between blocking and rainfall in southern Australia is explained by the component of rainfall associated with cutoff lows. These systems form the cyclonic components of blocking dipoles. In contrast, there is no significant correlation between the BI and rainfall from Southern Ocean fronts.

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M. N. Raphael
,
G. J. Marshall
,
J. Turner
,
R. L. Fogt
,
D. Schneider
,
D. A. Dixon
,
J. S. Hosking
,
J. M. Jones
, and
W. R. Hobbs

Abstract

The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.

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L. Illari
,
J. Marshall
,
P. Bannon
,
J. Botella
,
R. Clark
,
T. Haine
,
A. Kumar
,
S. Lee
,
K. J. Mackin
,
G. A. McKinley
,
M. Morgan
,
R. Najjar
,
T. Sikora
, and
A. Tandon

A collaboration between faculty and students at six universities in a project called Weather in a Tank is described, in which ways of teaching atmosphere, ocean, and climate dynamics are explored that bring students into contact with real fluids and fundamental ideas. Exploiting the use of classic rotating laboratory experiments, real-time meteorological data and associated theory, teaching tools, curricular, and evaluation materials have been developed that focus on fundamental aspects of atmospheric and oceanographic dynamics for use in undergraduate teaching. The intent of the project is to help students learn how to move between phenomena in the real world, theory, and models.

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Elaine M. Prins
,
Christopher S. Velden
,
Jeffrey D. Hawkins
,
F. Joseph Turk
,
Jaime M. Daniels
,
Gerald J. Dittberner
,
Kenneth Holmlund
,
Robbie E. Hood
,
Arlene G. Laing
,
Shaima L. Nasiri
,
Jeffery J. Puschell
,
J. Marshall Shepherd
, and
John V. Zapotocny
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