Mixed Layer Depth in the Arctic Seas in observations and models The seasonal cycle of Mixed Layer Depth in the Arctic Seas in observations and models Michael Steele Mark Wensnahan Jinlun Zhang Polar Science Center Applied Physics Laboratory University of Washington & NCAR, NOC, NPS, AWI…

Phytoplankton Growth = Primary Production depends on… Michael Steele Polar Science Center / APL University of Washington Phytoplankton Growth = Primary Production depends on… light lead deep MLD: light deficit? shallow MLD: nutrient deficit? nutrients Skyllingstad & Denbo, 1998

Def’n: Mixed Layer Depth Michael Steele Polar Science Center / APL University of Washington Def’n: Mixed Layer Depth MLD  Max “gradient method” …less common: requires smoothing more complicated etc  = 0.05 0.125  z 0.2 0.3  z Depth (m)  0-10m 2) 0-10m +  “increment method” …more common: easier! Sigma-theta (kg/m3)

Def’n: Mixed Layer Depth Michael Steele Polar Science Center / APL University of Washington Def’n: Mixed Layer Depth MLD  Max “gradient method” …less common: requires smoothing more complicated etc  = 0.05 0.125  z 0.2 0.3  z Depth (m)  0-10m 2) 0-10m +  “increment method” …more common: easier! Sigma-theta (kg/m3)

Part 1: World Ocean Database’05 (Observed Profiles)

Profile data: obs. by month and 3-year bins 1980s & 90s CTD Bottles Profiling floats Buoys 1960s J F M A M J J A S O N D Month 50 250 450 650 850 1050 1250 1450 1650 # obs

Profile data: obs. by month and 3-year bins 1980s & 90s late spring CTD Bottles Profiling floats Buoys 1960s summer J F M A M J J A S O N D Month 50 250 450 650 850 1050 1250 1450 1650 # obs

# profiles per 100 km EASE grid cell 185,020 81,288 # profiles Good profiles Rejected profiles 1 5 15 50 200 1000

Reasons for rejection # rejected 1 5 No data above 10 m 15 50 200 1000 No data above 10 m <4 pts in profile # rejected unstable profile No data below 10 m

Monthly MLD (m) >1000 m ~10 m March June September November 1000 400 150 75 25 10 >1000 m few data in Arctic Ocean September November ~10 m

Regional Means Russia Russia Alaska

Central Arctic Ocean cute lil’ sine wave ~25% MLD (m) St. Dev / MLD 120 80 40 % total 1 4 7 10 13 16% 2000 1990 1980 1970 1960 1950 MLD (m) cute lil’ sine wave 0.6 0.4 0.2 ~25% St. Dev / MLD J F M A M J J A S O N D J F M A M J J A S O N D Month Month

Greenland-Iceland-Norwegian Seas 120 80 40 % total 1 4 7 10 13 16% 2000 1990 1980 1970 1960 1950 MLD (m) 0.6 0.4 0.2 ~50% ! St. Dev / MLD J F M A M J J A S O N D J F M A M J J A S O N D Month Month

Part 2: PIOMAS Model & …some CCSM

PIOMAS: daily vs. monthly MLD 2000-2009 output 5 -5 -10 -15 -20 MLD  MLD using T,Smonth minus MLD using T,Sday MLD (m) -0.1 -0.2 -0.3 -0.4 -0.5 MLD MLDmon , MLDday Monthly mean output: underestimates MLD by < ~15 m or < ~50% J F M A M J J A S O N D Month

models have a larger seasonal cycle amplitude relative to WOD. WOD vs. PIOMAS vs. CCSM Models: monthly means over > 50 years 20 40 60 80 Region 10: central Arctic Ocean WOD MLD (m) CCSM In most regions, models have a larger seasonal cycle amplitude relative to WOD. PIOMAS J F M A M J J A S O N D Month

models have a larger seasonal cycle amplitude relative to WOD. WOD vs. PIOMAS vs. CCSM Models: monthly means over > 50 years 20 40 60 80 Region 10: central Arctic Ocean ? WOD MLD (m) CCSM In most regions, models have a larger seasonal cycle amplitude relative to WOD. Salt rejection in leads? refs: Smith, Nguyen, Jin PIOMAS J F M A M J J A S O N D Month

WOD vs. PIOMAS vs. CCSM PIOMAS CCSM WOD In Nordic Seas, (?!?) monthly means 100 200 300 400 Region 7: GIN Sea PIOMAS MLD (m) CCSM WOD In Nordic Seas, (?!?) J F M A M J J A S O N D Month

PIOMAS vs. WOD vs. CCSM PIOMAS ~zE CCSM WOD In Nordic Seas, (?!?) monthly means 100 200 300 400 Region 7: GIN Sea PIOMAS ~zE MLD (m) CCSM WOD In Nordic Seas, (?!?) 300 m! … but it’s all below the euphotic depth zE J F M A M J J A S O N D Month

Multi-year PIOMAS monthly means MLD (m) Region 1: Beaufort Sea 10 20 30 40 50 60 MLD (m) Region 1: Beaufort Sea 1947 1957 1967 1977 1987 1997 2007

Summary To do list MLD stdev / MLD = 25-50% Monthly model output underestimates MLD <~50% Not shown: MLD  stratification below the MLD To do list Optimal increment? More models Interannual variability

ESS + Laptev Seas

Beaufort Sea

PIOMAS (monthly model) vs. WOD (obs.) 80 40 60 Region 10: central Arctic Ocean MLD (m) J F M A M J J A S O N D Month

PIOMAS (monthly model) vs. WOD (obs.) Region 7: GIN Sea

Data Sets WOD’05 Original profile data: require T & S (early 1900s-present)  poor winter, central Arctic Ocean coverage PIOMAS Jinlun Zhang’s ice-ocean model (daily 2000-2009) (monthly 1948-2009) excellent coverage everywhere! CCSM & other models… (monthly…)