JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C05009, doi:10.1029/2009JC005334, 2010 Click Here for Full Article Atmospheric forcing of sea ice in Hudson Bay during the fall period, 1980–2005 K. P. Hochheim1 and D. G. Barber1 Received 18 February 2009; revised 2 November 2009; accepted 18 November 2009; published 11 May 2010. [1] The principal objective of this study is to describe the autumn sea ice regime of Hudson Bay in the context of atmospheric forcing from 1980 to 2005. Both gridded Canadian Ice Service (CIS) data and Passive Microwave (PMW) data are used to examine the freezeup period for weeks of year (WOY) 43–52. Sea ice concentration (SIC) anomalies reveal statistically significant trends, ranging from −23.3% to −26.9% per decade, during WOY 43–48 using the CIS data and trends ranging from −12.7% to −16.8% per decade during WOY 45–50 using the PMW data. Surface air temperature (SAT) anomalies are highly correlated with SIC anomalies (r2 = 0.52–0.72) and with sea ice extents (r2 = 0.53–0.72). CIS data show that mean sea ice extents based on SICs ≥80% (consolidated ice) have decreased by 1.05 × 105 to 1.17 × 105 km2 for every 1°C increase in temperature in late November; PMW data show similar results. Regression analysis between SAT and standardized climate indices over the 1951–2005 period show that the East Pacific/North Pacific index is highly predictive of interannual SATs followed by the North Atlantic Oscillation and Arctic Oscillation indices. The data show that the Hudson Bay area has recently undergone a climate regime shift, in the mid 1990s, which has resulted in a significant reduction in sea ice during the freezeup period and that these changes appear to be related to atmospheric indices. Citation: Hochheim, K. P., and D. G. Barber (2010), Atmospheric forcing of sea ice in Hudson Bay during the fall period, 1980–2005, J. Geophys. Res., 115, C05009, doi:10.1029/2009JC005334. 1. Introduction the Sea of Okhotsk had large negative trends. In 1993–2007 SIC trends were consistently negative throughout the Arctic [2] Over the past several decades Arctic sea ice has and subarctic seas. Summer trends during the first half of the undergone significant changes in ice extent and concentra- satellite record showed negative trends in the eastern Siberian tion. In this paper we define sea ice extent (SIE) as the geo- Sea and positive trends in the Barents, Kara, and eastern graphic distribution of sea ice (presence/absence) within the Beaufort seas, in contrast to the second half of the satellite study region and sea ice concentration (SIC) as the percentage record, which was dominated by negative trends throughout concentration of sea ice within a particular subset of the study the Arctic. area. From 1953 to 2006 the total SIE at the end of the [4] In Hudson Bay (HB) a number of studies have exam- summer melt season in September declined at a rate of ined trends in SIE. Parkinson et al. [1999] showed that during −7.8% per decade [Stroeve et al., 2007]. The trends in SIC 1979–1996, only very slight negative trends were detectable vary depending on the time period examined and the geo- within HB (including Foxe Basin): annual trends were graphic location. Passive microwave (PMW) data show that − 3 3 2 – 1.4 × 10 ± 1.4 × 10 km /yr; autumn trends were larger, trends in SIC during the 1979 1996 period were relatively − 3 3 2 − − at 2.9 × 10 ± 3.6 × 10 km /yr; and none of the seasonal small throughout the Arctic, 2.2 and 3.0% per decade, in trends were statistically significant. Gough et al. [2004] contrast to the 1997–2007 period, which showed that declines found no significant trends in freezeup dates for the fall in SIC accelerated to −10.1 and −10.7% per decade [Comiso period in southwestern HB (1971–2003) using Canadian Ice et al., 2008]. Service (CIS) data (Environment Canada, CIS daily analysis [3] Deser and Teng [2008] showed that during the early ice charts; available at http://ice‐glaces.ec.gc.ca). part of the PMW period (1979–1993), ice trends in the ice [5] Gagnon and Gough [2005], on the contrary, found marginal zones within the polar seas varied geographically. statistically significant trends in freezeup dates using point During the winter the Labrador and Bering seas had large observations. Of the 25 points used throughout HB during positive trends in SIC; the Greenland and Barents seas and the freezeup period, only 6 points, located in the northern reaches of HB, showed statistically significant freezeup date 1 trends (based on an SIC ≥50%); results indicated that Centre for Earth Observation Science, University of Manitoba, Winnipeg, – – Manitoba, Canada. freezeup was occurring 0.32 0.55 day/yr earlier (1971 2003). Kinnard et al. [2006] showed no significant trends in Copyright 2010 by the American Geophysical Union. SICs based on CIS data from 1980 to 2004. The most recent 0148‐0227/10/2009JC005334 C05009 1of20 C05009 HOCHHEIM AND BARBER: ATMOSPHERIC FORCING OF SEA ICE IN HB C05009 work by Parkinson and Cavalieri [2008] showed statistically ability is the main factor controlling temperature variation in significant annual trends for SIE in HB (including Foxe the winter season over eastern Canada, with positive NAO Basin), with decreases of −4.5 × 103 ± 0.9 × 103 km2/yr indices coinciding with early formation of sea ice in HB. In (or −5.3% ± 1.1% per decade); fall trends were −8.5 × 103 ± addition to the NAO, Kinnard et al. [2006] showed that the 1.9 × 103 km2/yr (or −12.93% ± 2.9% per decade). ice regime in HB was significantly correlated with the East [6] Gagnon and Gough [2006] used ice thickness data Pacific/North Pacific oscillation (EP/NP) index during the from the CIS to examine trends in thickness. The data used spring (r = 0.63) and summer (r = 0.57), both being signifi- in their study were collected from the early 1960s to the cant at the p < 0.05 level. A positive phase of the EP/NP index early 1990s (the data collection program was terminated in corresponds to a high pressure located over Alaska/western ∼1990). Temperature trends were predominantly negative Canada and a low pressure over the central North Pacific and and ice thickness trends were predominantly positive in HB eastern North America. This configuration acts to draw cool during the fall and winter periods. Arctic air south to eastern North America including the HB [7] The variations in SIC and SIE throughout the Arctic and region. sub‐Arctic have been variously attributed to some combina- [9] In summary, previous work has shown that the dis- tion of anthropogenic forcing due to greenhouse gases and tribution of sea ice anomalies throughout the Arctic and low‐frequency oscillations in atmospheric circulation and subarctic seas have not been uniform over the PMW satellite associated positive feedback mechanisms [Johannessen et record (1978 to now). This observation is significant for the al., 2004; Holland et al., 2006]. Interannual variations in HB region and eastern Canada in general. Whereas much of SIC anomalies in the Arctic from 1960 to the mid 1990s are the Arctic was warming, the HB region was actually cooling partly explained by variations in the Arctic Oscillation (AO) (1979–1993), hence the positive sea ice anomalies early in and North Atlantic Oscillation (NAO) [Venegas and Mysak, the PMW record [Deser and Teng, 2008], the lack of signifi- 2000; Deser, 2000; Polyakov and Johnson, 2000; Comiso cant statistical trends in SIE from 1979 to 1996 [Parkinson et et al., 2008; Deser and Teng 2008; Overland et al., 2008] al., 1999], and the increasing sea ice thickness from 1960 to the and their effects on ice circulation (ice export) [Rigor et al., early 1990s [Gagnon and Gough,2006].Morerecentdata 2002], air temperature [Polyakov et al., 2003], and oceanic have shown that warming has occurred in HB since 1999–2003 heat transport [J. Zhang et al., 2004]. In addition to the [Gagnon and Gough, 2005; Ford et al., 2009; Laidler et al., gradual warming of the Arctic over the last 50 years, 2009] and that statistically significant negative SIC trends Lindsay and Zhang [2005] have also suggested that the are now evident in the Foxe Basin and HB [Parkinson and temporary phase change associated with the Pacific Decadal Cavalieri,2008]. Oscillation (PDO) together with the AO in 1988 may have [10] This paper seeks to build on previous work as it relates contributed significantly to the flushing of older ice out to the HB region by examining both SIE and SIC and then of the Arctic. More recently, warming in the high Arctic examining the possible atmospheric forcing mechanisms has accelerated, independent of any indices, even beyond linked to these sea ice metrics. In this paper we (1) provide worst‐case scenarios using greenhouse gas forcing, sug- detailed gridded representations of SAT trends of the land gesting that factors such as the sea ice‐albedo feedback surrounding HB to provide a context for the observed mechanism are contributing significantly to recent decreases changes in SIC and SIE; (2) show the weekly evolution of in SIE [Lindsay and Zhang, 2005; Holland et al., 2006]. sea ice cover during the fall period from 1980 to 2005, [8] The HB region differs from the Arctic Ocean and provide gridded maps of SIC trends over 1980–2005, and adjacent seas in that it is essentially a closed system and, provide SIC difference maps comparing the “cool period” therefore, isolated from the effects of open‐ocean circulation (1980–1995) to the “warm period” (1996–2005); (3) quantify [Wang et al., 1994] (e.g., warm‐water intrusions and sea ice the relationship between SAT anomalies and SIC anomalies export) and more reflective of atmospheric forcing, specif- and SIE; and (4) examine the relationships between SAT ically changes in air temperature and winds.