Summary of the Observed Changes in the Freshwater Cycle of the
Hudson Bay System (HBS) comprised of Hudson, James and Ungava
Bays, Foxe Basin and Hudson Strait
A contribution to the NSF Freshwater Initiative Changes and Attribution Working Group
compiled by Fiamma Straneo (Woods Hole Oceanographic. Inst., fstraneo@whoi.edu).
This is a companion document to an annotated bibliography. It attempts to summarize and
synthesize some of the recent findings on changes in the freshwater cycle of the HBS.
Comments and suggestions are welcome.
Background
The Hudson Bay System (HBS) is a large (106 km2) but shallow (mean depth is 160m)
marginal sea composed of Hudson Bay (including James Bay), Foxe Basin, Ungava Bay and
Hudson Strait. The latter connects the first two basins to the third, and ultimately the entire
system to the Labrador Sea. The estimated mean exchange between HBS and the Labrador Sea is
approximately 106 m3/s (1 Sv). A second, smaller strait, Fury and Hecla Strait, connects Foxe
Basin to the Arctic Ocean with an estimated mean exchange of order 0.04 Sv (Sadler 1982) .
A large amount of freshwater (on the order of 103 km3/yr or 30 mSv) collected from an
extensive drainage basin is carried by rivers into HBS. An additional, but approximately equal,
amount of freshwater is seasonally involved in the sea-ice cycle. Indeed the HBS is ice-covered
for much of year, with September and October being the only too months where the entire system
is ice free. Thickness of sea-ice is largest in Foxe Basin (approximately 2m) and smallest in the
southern part of Hudson Bay (James Bay, approximately 1m) with considerable ice-ridging
(Prinsenberg 1988).
River runoff and freshwater from sea-ice formation/melting make the HBS a large-scale
estuarine system with strong Arctic characteristics, notwithstanding its lower latitude. It is
characterized by unusually high vertical stratification due to the large vertical stability provided
by the freshwater and most of the seasonal heating/cooling cycle is used up in the freezing and
melting of sea-ice (Saucier et al. 2004). Based on river runoff alone, it is the third largest
contributor to freshwater flow into the North Atlantic (after Davis and Fram Straits, Mertz et al.
1993). Water from this region flows directly into the Labrador Sea, and freshwater variability has
the potential to impact dense water formation in the Labrador Sea, as well as the highly
productive downstream shelf region.
Observed Hydrologic Changes in the Hudson Bay System
Very few hydrographic surveys of the region have been conducted, both due to its
remoteness and due to its extensive seasonal ice-cover. As a result, there is little data to document
any long-term change in properties, or in the freshwater content of the region. A number of
studies, however, indicate that the general conditions and hydrologic cycle of the entire region is
changing. Most notably, Dery et al. (2005) have shown that the total river runoff into the HBS is
decreasing. Their analysis is based on data from 1964 to 2000, with a net difference (from 1964
to 2000) of about 100 km3 (or 3 mSv). Similar results for a shorter record are discussed by
Prinsenberg (1988). This decrease is attributed to a reduction in the amount of precipitation
falling over the northern Canada (Dery and Wood, 2005). It is interesting to note that a
diminished river runoff is opposite to the accelerated hydrologic cycle that is observed in many
high latitude regions, such as the increase of river discharge into the Arctic Ocean from the
Eurasian continent discussed in Peterson et al. (2002).
Further evidence of a changing hydrologic cycle comes from observations of a decrease
in the sea-ice cover: from 1982 to 1998 discussed in Laine (2004) and 1979 to 1999 discussed in
Parkinson and Cavalieri (2002, as cited in Stewart and Lockhart 2005). This is mostly inferred
from observations of a reduction in the extent of ice cover, later ice formation and earlier ice-melt
from satellite microwave passive data. Measurements of ice-thickness are too scarce to provide
information on any thickness change (Parkinson and Cavalieri, 2002, as cited in Stewart and
Lockhart 2005). Finally, while analysis presented in a recent study suggests that precipitation
over the region may have increased by 3 cm/yr from the 1960s to the 1990s (from the ERA40 and
NCEP Reanalyses products, Myers et al. 2005), this amounts to an increase of 1 mSv, and hence
is smaller of the river runoff change.
Besides these long-term trends, interannual and decadal variability in the HBS regions is
found to be significantly correlated with several large scales mode of variability such as the
Arctic Oscillation (AO), the similar North Atlantic Oscillation (NAO) and the Southern
Oscillation (SO). A positive phase of the AO/NAO is associated with lower air temperatures, a
decrease in precipitation (over the drainage basin), evaporation and river discharge and an
increase in sea-ice cover (Wang et al., 1994; and Dery and Wood, 2004). Physically, these
authors suggest that this is a result of the anomalous northwesterly flow associated with positive
AO/NAO events that carried dry, cold air from the Arctic region over the HBS. Negative events
of the SO are also tied to a negative air temperature anomaly in the HBS (especially during the
summer and fall) giving rise to an increase in the duration of sea-ice cover (Wang et al. 1994).
When a negative SO coincides with a positive NAO/AO these patterns are further enhanced as
reported by Mysak et al. (1995) for 1972/1973, 1982/1983 and 1991/1992.
Model Response to Global Warming Scenario
A number of models have shown that warming due to an increase in greenhouse gases
will lead to a substantial decrease in the sea-ice cover over HBS (Gough and Wolfe, 2001;
Saucier and Donne, 1998; Boer et al. 2000).
Conclusion
This summary indicates that the HBS is undergoing a substantial change in its hydrologic
cycle as shown by trends in sea-ice and river runoff. If sustained, the observed changes will likely
lead to a decrease in the freshwater export from HBS, and to a decrease in the albedo of the
region (as already observed by Laine 2004). Because of its relatively large surface area,
compared to its volume, and to the relatively small exchange with the Arctic Ocean and the
Labrador Sea, any change in this system is likely to be quickly amplified by internal feedbacks
and processes. Freshwater changes are likely to cause dramatic changes in the vertical structure
of the waters in the basin and, consequently, in their nutrient cycles and ecosystem, since these
are currently strongly controlled by the annual freshwater cycle. Furthermore, these changes are
likely to be quickly communicated to the Labrador Sea and have a potential to impact North
Atlantic Deep Water Formation.