On the nature of winter cooling and the recent temperature shift on the northern Gulf of Alaska shelf Thomas Weingartner1, Markus Janout1, Seth Danielson1 and Tom Royer2 1Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska 99775(Contactweingart@ims.uaf.edu)1 2A Beach in Hawaii 1) Introduction The 40-year temperature (T) and salinity (S) time series from the coastal hydrographic station GAK 1 on the northern Gulf of Alaska (GOA) shelf, NCEP meteorological data, and moored T/S records from GAK 1 are used to describe the anomalous cooling that began in the winter of 2006-07 and continued through winter 2008-09. The recent cooling halted a ~1ºC/30 yr increase in deep (>150 m) temperatures on the shelf and recent spring temperature anomalies were <1ºC over much of the water column. Key variables that regulate the late winter temperature distribution are winter air-sea heat fluxes AND runoff, which sets the vertical stratification (Janout et al., accepted). Hence fluxes of salt or freshwater have a profound influence on mixing and the temperature distribution on this shelf. The winter of 2008-09 appears unusual in that the shelf developed anomalously strong stratification below 150 m due to high salinity, which prevented the deeper waters from cooling substantially. The source of the high salinity was enhanced upwelling (western Gulf) and reduced downwelling (eastern Gulf), which forced saline slope water onto the shelf. Winter Atmospheric Conditions for Cold and Warm Years Figure 7: Average sea level pressure distribution during anomalously warm (left) and cold (right) winter months in the northern Gulf of Alaska. . 46082 * Figure 1: Northern coastal GOA including dominant currents, Seward Line with GAK1 in red. Above normal air temperatures occur with strong Aleutian Lows (centered over the Aleutians) that transport warm and moist oceanic air from southern latitudes to the northern GOA Below normal air temperatures occur with a less pronounced low pressure system at the head of the Gulf, that causes advection of cold and dry continental air over the northern GOA Figure 4: a) 0-100 m and b) 100-250 m T anomalies compared to c) 20-100 m salinity difference computed from Feb-May averaged CTD profiles and compared to Nov-Mar anomalies in d) Air temp, e) Freshwater discharge, f) zonal, g) meridional winds, h) air-sea heat flux, and i) # of months with anomalous air-temps between Nov-Mar. (Data for 2009 incomplete.) Adapted from Janout et al., (accepted). GAK1 May Temperature and Salinity Profiles Multiple regression results indicate that 81% of the deep temperature variance is accounted for by: air-sea heat fluxes and winter discharge!! while other variables are less important Figure 2: Historic average May GAK1 temperature (left) and salinity (right) profiles (red) with the profiles collected in MAY 2007 (solid), May 2008 (dashed), and May 2009 (dash-dotted). Gray-shading encompasses +1 standard deviation Figure 8: Map showing location of met buoys used to compute cumulative winter heat losses over the Gulf of Alaska (left). Winter Air-sea heat fluxes are greatest in Shelikof Strait and Prince William Sound and smaller over the basin, SE Alaska and the outer shelf. May 2007: - 1-20C colder than average & weak stratification throughout the water column saltier than normal above 150 m and fresher than normal below 150 m May 2008: 1.50C colder than average above 100 m - normal salinities, but weaklystratified in upper 60 m May 2009: -1.50C colder than average above 100 m above normal salinities throughout the water column, weaker 0-100 m stratification, but strong stratified below150 m. Summary: 1. The winters of 2006-07, 2007-08, and 2008-09 were unusually cold and comparable to the early 1970s; 2. Deep (>100 m) cold temperature anomalies depend upon strong air-sea heat fluxes that cool the ocean and low winter runoff that permits deep mixing; 3. However, deep cooling in 2009 was inhibited by deep stratification established bu the intrusion of saline slope onto the shelf due to weak downwelling winds in the eastern Gulf and upwelling winds in western Gulf. 4. Surface temperature anomalies are not a good index of deep temperature anomalies on this shelf. 5. Cold winters (high heat loss and low runoff) are characterized by a weak low in the northern Gulf. Warm winters (low heat loss and high runoff) are characterized by a strong low centered over the Aleutians. 6. There are large spatial gradients in wintere air-sea heat loss. Heat fluxes in Shelikof Strait are 1.5-2.5 times greater than elsewhere over the shelf and basin!! Figure 5: Contours of temperature (left) and salinity (right) as a function of time and depth for selected years based on hourly T and S data from the GAK 1 mooring. The winter of 2000-01 is a “normal” year , while other years are colder than normal. Salt stratification affects the depth to which temperature anomalies penetrate. Unusually salty water below 150 m led to deep stratification and precluded deep cooling in 2009. The 2009 data are still being analyzed, but the high salinities appear due to weak winter downwelling and frequent upwelling over the northern shelf (Fig. 6), which enabled salty slope water to migrate inshore at the bottom. Monthly Temperature Anomalies at GAK1 The last 3 winters were: the coldest since the early 1970s & consist of the longest continuous “cold” period since then. Western Gulf Eastern Gulf 2009 References: Royer, T. C. and C. E. Grosch, 2006, Ocean warming and freshening in the northern Gulf of Alaska, Geophysical Research Letters, 33 (16). Janout, M. J., T. Weingartner, T. C. Royer, and S. Danielson: On the nature of winter cooling and the recent temperature shift on the northern Gulf of Alaska shelf. Journal of Geophysical Research (accepted). This work was supported under EVOSTC Project 070340 and NPRB projects 520, 603 & 708.  Figure 3: Detrended 0-100 m (top) and 100-250 m (bottom) GAK1 monthly temperature anomalies computed from ~1970 to May 2009. The dotted lines are +/- 2s. Updated from Royer and Grosch, (2006). Figure 6: Along-shore wind anomalies in the western (left) and eastern (right) Gulf of Alaska, 1948-2009.