Climate Modelling Group
School of Earth and Ocean Sciences

NSERC Operating Grant

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Principal Investigator:
Dr. Andrew J. Weaver
School of Earth and Ocean Sciences
University of Victoria
PO Box 1700
Victoria, British Columbia
CANADA V8W 2Y2

tel: (250) 472-4001
fax: (250) 472-4004
e-mail: weaver@ocean.seos.uvic.ca
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Preamble

I currently receive strategic (targeted) research grant support in two main areas: under the Climate System History and Dynamics Project I have salary support for two graduate students and one postdoctoral research associate to investigate the climate of the Last Glacial Maximum (LGM) and its transition to the Holocene; under my NSERC Strategic grant (together with my industrial partner funding) I have salary support for two additional postdocs to examine the role of the Arctic in climate change and climate variability, as well as two graduate students to undertake research in the area of global ocean modelling. In this proposal I am applying for support for my research efforts in the area of climate modelling and analysis. My overall objective is to acquire full support for my two technicians (E. Wiebe and M. Eby) who are continuing employees and whose efforts are essential for the maintenance and operation of my large computer network and climate modelling activities. I am also applying for salary support for one graduate student.

My research is ultimately phenomenologically, rather than methodologically, driven and this nature is reflected in the following discussion. As evident in my NSERC PDF 100, I have conducted research in several fields. My nature is not to focus on one specific problem in a narrowly focused area for many years but rather to work on many diverse problems at the same time. I find that this philosophy both heightens my keen interest in science and allows me to work with highest efficiency and productivity. Rather than go into excessive detail for any one particular project, I provide a more brief discussion of my methodology and anticipated goals over a wider range of problems, all of which I hope to complete over the four year term of my operating grant.

Over the last few years I have invested a large amount of effort to develop an atmospheric model suitable for coupling to ocean models, for the purpose of undertaking the long-timescale integrations required to investigate climate variability on the decadal-millennial timescale. To this end we have coupled an ocean general circulation model (OGCM) to a newly-developed energy-moisture balance atmosphere model (EMBM), into which a dynamic sea ice model has been incorporated (referred to as the UVic coupled model hereafter). We have also recently incorporated a parameterization which allows for wind stress feedbacks. The virtue of the atmospheric component of the coupled model is that we do not need to employ explicit flux adjustments to keep the simulation of the present climate stable. Thus, it is ideally suited for both climate and paleoclimate modelling. The developmental phase of this research has now been completed and I believe I am now poised to make fundamental advances to our understanding of climate variability on the decadal-millennial timescale. In addition, I believe it is essential to retain the personnel (M. Eby and E. Wiebe), responsible for the maintenance, upkeep and improvement of our coupled model code, on a continuous basis. As this code is essential to meet the objectives of a number of my research projects listed below, it is not possible nor appropriate for grad students or postdocs, who come and go, to be responsible for this task, nor is it an effective use of my time.

I will begin by discussing some new directions for my research into the role of the Arctic in climate change and climate variability. I will follow this with a brief description of my recent paleoclimatic modelling efforts whose continued success requires ongoing technical support. Finally I will propose three new areas of research to be supported by NSERC Operating grant funding.

1) The Role of the Arctic in Climate Change and Climate Variability


  • • Is Arctic freshwater export an important driver of North Atlantic interdecadal climate variability?
  • •Is the recent observation of warm, subsurface North Atlantic water intrusion into the Arctic consistent with the response expected from enhanced anthropogenic greenhouse gas forcing?
  • Approximately 10% of the world’s river runoff, accounting for ~3300 km3 yr-1 (with large seasonal and interannual variability – Cattle, 1985), enters the Arctic Ocean, which occupies only ~5% of the total ocean surface area and ~1.5% of its volume. Bering Strait inflow represents the second largest freshwater source for the Arctic Ocean (~1670 km3 yr-1), with precipitation minus evaporation (~900 km3 yr-1) and the import of freshwater in the Norwegian coastal current (~330 km3 yr-1) accounting for the remainder of the freshwater sources. The sources of freshwater for the Arctic are balanced primarily by the export of sea ice in the East Greenland Current, which accounts for a freshwater loss to the Arctic and a gain by the Greenland Sea of ~2800 km3 yr-1 (Aagaard & Carmack 1989). The exchange of water through the Canadian Archipelago and Fram Strait results in a loss of approximately 900 km3 yr-1 and 820 km3 yr-1 of freshwater, respectively. At low temperatures the density of sea water is largely controlled by salinity. As such, variations in the freshwater exchange (via both ocean and sea ice transports) between the Arctic and Atlantic Oceans are likely to affect the formation of deep and intermediate water masses there. Indeed, modelling results from Mauritzen & Häkkinen (1997) show that the thermohaline circulation increased by 10-20% in response to a decrease in sea ice export of 800 km3. The relative strength of the freshwater sources to the Nordic and Labrador Seas from the Arctic will also likely influence the preferred location and relative strengths of deep water formation. From a relatively short timeseries (1979-1985), Steele et al (1996) show that simulated interannual variability in the outflow through the Canadian Archipelago is anticorrelated with the outflow through Fram Strait, with the Fram Strait anomalies leading the Canadian Archipelago anomalies by one year. This may explain why deep water formation in the Nordic and Labrador Seas have been observed to be out of phase in the past few decades.

    The large changes that occur in Arctic/North Atlantic freshwater exchange are epitomized by the Great Salinity Anomaly (GSA) of the late 1960s. This event freshened the upper 500 m of the northern North Atlantic with a freshwater excess of approximately 2000 km3 (or 0.032 Sv over a two year period). Dickson et al (1988) trace this fresh anomaly as it was advected around the subpolar gyre for over 14 years. It originated north of Iceland in the late 1960s, moving southwestward into the Labrador Sea (1971-1973) and then proceeding across the north Atlantic, returning to the Greenland Sea in 1981-1982.

    Several studies have examined the cause of the GSA and have generally determined that it was a result of Arctic/North Atlantic interactions. Both modelling (Häkkinen 1993) and observational (Walsh & Chapman 1990; Wohlleben & Weaver 1995) studies concluded that strong northerly winds caused an increased sea ice export into the Greenland Sea. The large freshwater flux anomaly that was associated with this transport was likely enhanced by the relatively large advection of thick ice from north of Greenland. Additionally, as simulated by Häkkinen (1993), increased oceanic transport of freshwater from the Arctic occurred. This was caused by fresh anomalies within the Siberian Sea that were advected across the Arctic, entering the Greenland Sea approximately 4 years later. During the GSA, the anomalous sea ice and oceanic freshwater transports were coincident, resulting in a significant and persistent freshening of the north Atlantic. This appears to have resulted in a reduction of deep water formation with winter convection in the Labrador sea limited to the upper 200 m (compared to 1000-1500 m for 1971-1973) (Lazier 1980).

    North Atlantic SST records for the past century reveal slowly varying basin-scale changes including cold anomalies prior to 1920, warming from 1930—1940, and cooling again in the 1960s. Kushnir (1994) described the SST pattern associated with these long-term changes as uni-polar with a strong maximum around Iceland and in the Labrador Sea and a weaker maximum in a band near 35°N across the central Atlantic. The atmospheric pattern associated with the cooling in the 1960s has a negative pressure anomaly to the east of positive SST anomalies (also see Deser & Blackmon 1993 who suggest the pattern resembles the North Atlantic Oscillation [NAO]). Because the SLP anomalies appear downstream of the SST anomalies, these authors suggest that the atmosphere is responding to the ocean on these timescales.

    Evidence for changes in the subpolar North Atlantic Ocean over similar timescales, compiled by Dickson et al (1996), indicates that synchronous with the cooling in the late 1960s, convective activity reached a maximum in the Greenland Sea and a minimum in the Labrador Sea. These convective extremes occurred at the approximate time of the GSA. Since the early 1970s, the Greenland Sea has become progressively more saline and warmer through horizontal exchange with the deep waters of the Arctic Ocean. At the same time, the Labrador Sea has become colder and fresher as a result of local deep convection. Reverdin et al (1997) explored patterns associated with salinity anomalies and found a single pattern explains 70% of the variance of lagged salinity anomalies. The pattern represents a signal originating in the Labrador Sea that propagates from the west to the northeast in the subpolar gyre. The strong correlation between salinity and sea ice in the Labrador Sea lead Reverdin et al (1997) to link the salinity anomalies to the export of Arctic freshwater.

    Delworth et al (1993) described the first coupled ocean-atmosphere GCM study of long-term thermohaline variability. They associate the variability primarily with oceanic processes. Later Delworth et al (1997) found salinity anomalies in the surface layer of the Arctic Ocean precede anomalies of the thermohaline intensity by 10—15 years. In agreement with the proposed climate cycle of Wohlleben & Weaver (1995), these Arctic freshwater anomalies are connected to the North Atlantic through SLP anomalies in the Greenland Sea resembling the pattern that Walsh & Chapman (1990) report preceded the GSA. Weaver & Valcke (1998) gave further evidence that the GFDL model thermohaline variability is a mode of the fully coupled atmosphere-ocean-ice system.

    Based on these studies my conjecture is that Arctic sea ice export (and its relationship to the NAO) plays an integral role in both the observed decadal variability and that found in complicated atmosphere-ocean GCMs (e.g., Delworth et al 1993). The impact of ice export on climate variability will be addressed by applying an anomalous wind stress forcing to the UVic coupled model. The first 20 EOFs from NCEP reanalysis SLP data have been used to generate a synthetic anomalous wind stress field which has been applied over the North Atlantic ocean. Initial results are extremely encouraging as the model reveals decadal variability around the North Atlantic which is intimately linked to the export of sea ice from the Arctic. Nevertheless, we have much analysis to undertake to isolate the dominant mechanism and timescale for the variability and to unequivocally prove that sea ice dynamics are crucial to the oscillation. These sensitivity analyses will involve 1) applying the anomalous wind forcing only over ice; 2) shutting off the oceanic ice advection; 3) adding the anomalous wind forcing effects to the model calculation of latent and sensible heat fluxes; 4) changing the number of categories in the thermodynamic component of the sea ice model; 5) removing the oceanic effects of brine rejection and sea ice melting; 6) using climatological (from the spin up of the coupled model) freshwater fluxes or heat fluxes to determine whether heat or freshwater flux changes amplify or diminish the variability. In addition, it will be important for us to determine, through a sensitivity analyses to internal sea ice parameters (e.g., number of categories, shear strength, vertical temperature resolution, ice-ocean coupling parameter, albedo), whether or not the UVic coupled model allows self-sustained oscillations within a particular parameter range.

    Recent observations (e.g. Carmack et al 1995; McLaughlin et al 1996) indicate that the Atlantic layer within the Arctic Ocean has undergone large changes since 1990. These include a shift in the frontal structure which separates different Atlantic layer water masses (from the Lomonosov to the Mendeleyev ridge), and a significant warming of the Atlantic layer. By 1994, this warming extended across the Nansen, Amundsen and Makarov Basins. Swift et al (1998) show that these changes are likely caused by an increase in the temperature of the Atlantic waters which enter the Arctic Basin through Fram Strait. The anomalous warmth of these waters appears to be correlated with the NAO which corresponds to relatively warm air temperatures in the Greenland sea region and thus a reduction in oceanic heat loss. The temperature signal of these waters is transported into the Arctic Ocean by topographically-steered boundary currents. It then enters the interior ocean through intrusive layers which extend laterally into the ocean basins (Carmack et al 1998). An open question remains as to where the Arctic waters displaced through the intrusion of the Atlantic layer went, although enhanced transport through the Canadian Archipelago is plausible. This would be consistent with recent observations of anomalous cold and fresh waters in the Labrador Sea since the late 1980s (Dickson et al 1996).

    In some recent experiments using the UVic coupled model (Wiebe & Weaver 1998) to examine the transient ocean response of the climate system to increasing anthropogenic CO2, we found that subsurface warm waters intruded into the Arctic. Once in the Arctic, these waters were slowly advected and diffused throughout the basin, filling the middle layers with an anomalously warm water mass of Atlantic origin (see a movie of the pehnomenon). While this warming of the mid-depth Arctic Ocean is qualitatively similar to the aforementioned recent observed patterns of subsurface warming, there are several discrepancies, including too slow a spreading timescale and a maximum warming which is slightly too deep. The lack of a proper representation of the Arctic Ocean in the Wiebe & Weaver (1998) version of the couple model severely limited their ability to quantitatively capture the dynamics of the region. Nevertheless this intriguing finding warrants further exploration and I propose to do this with the more recent version of the UVic coupled model which now uses a rotated coordinate grid (allowing for better resolution of the Arctic), and includes sea ice dynamics.

    2) Northern hemisphere glaciation and the transition to the Holocene

  • • Was the closure of the Isthmus of Panama (IP) responsible for the onset of northern hemisphere glaciation?
  • • Can the transition between the last glacial maximum and the present be reproduced in a coupled atmosphere-ocean-sea ice-continental ice sheet - carbon cycle model?
  • Approximately 3 Ma ago the Late Pliocene event occurred, marked by northern hemisphere ice sheet formation and the onset of glacial/interglacial cycles (see Kennett, 1982 for a review). Luyendyk et al. (1972) and Berggren & Hollister (1974) hypothesized that oceanic circulation changes coincident with the IP final elevation were the probable cause. Berggren (1982) suggested that upon IP closure, increased amounts of warm subtropical waters moved northward in the Atlantic. He hypothesized that the stronger and warmer Gulf Stream after closure could have lead to intensified evaporation at midlatitudes and hence precipitation over eastern Canada, Greenland and Western Europe, providing the moisture required for glaciation. Ruddiman et al. (1980) suggested that an intensified, warmer Gulf Stream could have contributed to conditions favorable to ice sheet growth by causing a vigorous meridional atmospheric circulation associated with a strong temperature gradient at the eastern coast of North America. Murdock et al (1997) recently examined the effects of IP closure using the UVic coupled model and did indeed find that deep water formation in the North Atlantic initiated upon IP closure, leading to a warmer North Atlantic. Associated with the warmer North Atlantic was increased area-averaged evaporation by approximately 1.0 cm/yr from 23°N-49°N and precipitation by 0.4 cm/yr from 49°N-88°N. Thus their results were consistent with the Berggren hypothesis but due to their lack of a continental ice sheet model, were unable to quantitatively verify it.

    To address this question we are in the process of coupling a 3-D, thermomechanical ice sheet model which employs continuum mixture theory to incorporate ice streams (Marshall & Clarke 1997a,b), into the UVic coupled model. The goal of this project is to determine whether the increased moisture availability, upon IP closure, is capable, in concert with orbital forcing, of causing northern hemispheric glaciation to begin.

    A second area of paleoclimatic modelling involves the inclusion (by P. Poussart) of ocean carbon cycle model into the UVic coupled model (which has the land ice sheet model incorporated) to investigate the transition of the climate system from the LGM to the Holocene and to examine its stability as a function of mean climatic state. We will begin by only including carbon chemistry perhaps moving towards an inclusion of variations of the biological pump (in a manner similar to Sarmiento et al 1998).

    The capturing of millennial timescale variability and its packaging into Bond Cycles in cold climates, its association with Heinrich events, and its dependence on the mean climatic state remains one of the greatest challenges for paleoclimate modelers. These two projects must be viewed as extremely ambitious and high risk. If successful, the knowledge gained from these two projects would be extremely fundamental and of utmost importance.

    3) The Role of Boundary Layer Mixing on the Large Scale Ocean Circulation

  • • Are realistic ocean models sensitive to the horizontal distribution of sub grid scale vertical mixing?
  • • Do simple scaling theories exist to explain the dependence of the large-scale ocean response to enhanced lateral boundary layer mixing?
  • While the use of models to simulate past climatic events is an important avenue of investigation if one is to have confidence in their application to future climate change, it is also fundamental to understand the processes involved in the real ocean and how they are parameterized in these models. To this end, I would like to investigate how oceanic mixing in boundary layers, versus the interior of the ocean, affects the large-scale thermohaline circulation. More specifically, previous attempts at modelling the ocean’s large-scale thermohaline circulation have ignored the observation that diapycnal mixing in the ocean is enhanced near lateral boundaries (Ledwell & Hickey 1995; Ledwell & Bratkovitch (1995), topographic seamounts (Lueck & Mudge, 1997) and the bottom (Toole et al 1994). Indeed direct and indirect measurements of the vertical eddy diffusivity (Kv) in the thermocline of the interior of the ocean suggest a magnitude of about 10—5 m2s—1 (e.g Ledwell et al 1993), both much smaller than used in OGCMs and that required to accomplish meridional buoyancy transports equivalent to O(1PW) (De Szoeke 1995).

    In order to investigate the dependence of the large scale ocean circulation on Kv in the boundaries I propose to set up both idealised (single basin) and more realistic (global domain) ocean basin models and to undertake a systematic sensitivity analysis to the spatial dependence of Kv. Unlike Marotzke (1997), I would choose a regularly spaced model to avoid potential numerical diffusion which would affect the results. In addition, it is important to undertake this analysis using a rotated (isopycnal/diapycnal) diffusion tensor to avoid potential spurious diapycnal diffusivities in regions of steeply sloping isopycnals (e.g. western boundary). To interpret the numerical results I hope to develop a simple set of scaling relationship to explain the large-scale ocean response.

    4) Decadal-interdecadal variability in the Pacific Ocean

  • Can a simple delayed oscillator model be developed to capture the observed decadal-interdecadal variability of ENSO?
  • Interannual variability in the Pacific Ocean is well known to be dominated by El Niño/ Southern Oscillation (ENSO) variability and its associated teleconnection through the atmosphere to the North Pacific (Bjerknes 1969; Weare et al 1976; Horel & Wallace 1981; Wallace & Gutzler 1981; Deser & Blackmon 1995; Zhang et al 1997). Recently, however, it has become apparent that the North Pacific possesses its own rich modes of decadal-interdecadal variability (see reviews of Trenberth 1990; Nakamura et al 1997). A number of theories exist to explain the observed decadal variability in the North Pacific. Because ENSO is a nonlinear coupled tropical atmosphere-ocean phenomenon, it is possible that decadal modulation of ENSO and its subsequent teleconnection to the North Pacific could explain the observed low frequency variability there (Trenberth & Hurrel 1994). It may not be necessary to invoke a local nonlinearity of ENSO in the tropics. As pointed out by Gu & Philander (1997), a delayed negative feedback can be achieved through extratropical oceanic subduction of thermal anomalies (generated through the atmospheric teleconnection response to tropical SST anomalies) which slowly propagate along isopycnals towards the equator where they reverse the sign of equatorial SSTs. Finally, Latif & Barnett (1994, 1996) have suggested a mode of decadal-interdecadal North Pacific variability solely involving midlatitude coupled atmosphere-ocean interactions and the strength of the subpolar gyre and its associated northward heat transport. As pointed out by Nakamura et al (1997), subtropical gyre SST variability on the decadal-interdecadal timescale is not solely explained through the tropical source, and some combination of the mechanisms may exist in reality.

    It is the second of these hypotheses that I propose to address through the development of a simple delayed oscillator model to understand mechanisms for tropical/subtropical interactions and interdecadal variability. In the Battisti & Hirst (1989) model (which explained the essence of the ENSO mechanism of Cane & Zebiak, 1985) they exploited a relationship between the anomalous eastern equatorial pycnocline depth (h = hreflected Kelvin + hlocal), zonal wind stress anomalies and surface temperature anomalies. I propose to extend this model by including a term hmeridional which would account for a meridional delay (through pycnocline subduction) between extratropical SST anomalies and the eastern equatorial pycnocline depth. In order to estimate the timescale for the meridional delay and the magnitude of the pycnocline anomaly displacement for a particular extratropical anomaly, an ocean-only GCM has recently been integrated with a 2°C SST anomaly imposed in the extratropics (see a movie of the phenomenon). I will also need to develop a statistical relationship, based on historical observations, between the magnitude of a tropical and related extratropical SST anomalies.

    5) Trends in precipitation and extreme precipitation events over Canada

  • • Is there statistically significant evidence for a trend towards increased precipitation and frequency of extreme precipitation events over Canada over the last 100 years?
  • • To what extent are these statistics related to variability of the El Niño/Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) natural climatic modes?
  • In recent years anecdotal evidence (e.g., Saguenay flood of 1996; record west coast snowfall of 1996; Winnipeg floods of 1997; eastern ice storm of 1998) has been presented by scientists, the public and the media to suggest that the climate appears to be getting more extreme in response to anthropogenic global warming. While the jury is still out on this issue, it is apparent that the extreme economic costs of such events warrants a detailed understanding as to their trend. In addition, there is growing evidence to suggest that global warming may have significant ramifications for El Niño (which has great influence on the seasonal Canadian climate). Since the energy for El Niño ultimately comes from the warm water in the equatorial Pacific, it has been suggested that increased warming could lead to more extreme and more frequent El Niños (e.g., Trenberth, 1995).While the jury is also out on this issue, more anecdotal evidence exists: The two most extreme El Niño events this century have happened in the last two decades, with the 1997 event surpassing the 1982—83 event in strength. In addition, from 1991—1995 we had the "El Niño which didn’t go away", with only a small cooling (La Niña) in 1996. In a recent review of available literature, IPCC (1996) reported that:

    "Overall, there is no evidence that extreme weather events, or climate variability, has increased, in a global sense, through the 20th century, although data and analyses are poor and not comprehensive . On regional scales there is clear evidence of changes in some extremes and climate variability indicators. Some of these changes have been toward greater variability; some have been toward lower variability." p. 173

    While the data is inconclusive at this stage, this same report (IPCC, 1996 – p336) notes that many numerical models find increases in extreme warm events (decrease in extreme winter cool events) and increases in extreme precipitation events and severe drought periods under anthropogenic warming.

    In an attempt to further our understanding of the Canadian climatological record I will examine Canadian station data to determine whether or not there appear to be any trends in extreme events during the last century. Specifically, we will look for any regional and national trends in extreme precipitation events, droughts (consecutive days without precipitation) and extreme temperature events (both warm and cold). This work will be collaborative with Drs. Hogg, Zhang and Mekis at the CCRM and Zwiers at the CCCma Divisions of the AES Climate Research Branch. Corrected precipitation data from 69 stations across Canada will initially be analysed. It is anticipated that as more stations become available we will use them in our extreme value analysis

    Karl et al (1995) recently undertook a comprehensive analysis of trends in extreme weather events in the 48 conterminous states. They suggested, through the construction of a Climate Extreme Index (defined by an aggregate set of conventional climate extreme indicators) that the climate of the United States has become more extreme in recent decades. Their analysis however used rather crude definitions of an extreme event (e.g., for precipitation more than 2 inches of precipitation in 24 hours). Rather than use the methodology of Karl et al. (1995) for determining extreme events and whether or not there are any changes in their frequency, we will approach the problem using more sophisticated statistical techniques. Specifically, we intend to use the extreme value analysis discussed in Zwiers & Kharin (1997) as the underlying analytical tool. In addition, we will examine whether or not there is any relationship between seasonal precipitation timeseries and extreme precipitation events and the ENSO and NAO indices. If such a correlation exists then this will be useful in attempting to determine the statistical predictability of periods likely to be subject to fewer or more extreme precipitation events. This follows since significant advances are forthcoming with regards to the seasonal prediction of both ENSO and more recently the NAO.

     

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