Ocean Climate Trends Around Hawai`i Roger Lukas ± January 20, 2011 © 2011 ± HOT co-PIs and other collaborators

Sea level Temperature Salinity Circulation Mixing Biogeochemistry (CO 2, pH, O 2, nutrients, primary production, …) What is changing in the ocean? Density; stratification complex coupling

Main Focus and Summary Oceanic component of hydrological cycle is crucial in determining warming and other impacts Hydrological cycle acceleration -> much of subtropical gyre is receiving less rainfall; more rainfall midlatitudes Salinity matters! Upper ocean increase around Hawaii -> reduced stratification and enhanced mixing Enhanced CO 2 global warming can paradoxically lead to regional ocean cooling trends Impacts of salinity changes on biology?

A simple summary (IPCC Chapter 5) HI SL rise

Global mean sea level is rising,

… but not uniformly (as Mark showed) – How much is due to ocean warming? Surface height trend dominated by ocean density changes, and those are dominated by dynamics. More recent period, surface buoyancy flux trends may be emerging. (50-year GECCO reanalysis by Kohl and Stammer, 2008) Total SSH trend Steric height trend – +8 mm/yr-9 – +9 mm/yr

Salinity important for determining SL changes due to ocean density changes (Kohl and Stammer, 2008) Ocean dynamics are important – subtropical gyre intensification, thermocline/halocline displacements thermosteric height trend halosteric height trend – +13 mm/yr-8 – +8 mm/yr

Steric ALOHA 0/1000 dbar cm/decade (all casts) cm/decade (annual averages) 80/1000 dbar 1.6 cm/decade (with and without averaging) mixed layer salinity increase overcompensated warming 1.6 mm/yr 1.6 cm/decade Comparable to Hawaii tide gauges

A simple summary (IPCC Chapter 5) HI SL rise SL rise due to density change warmingaccelerated H 2 O cycle salinity increase – SL↓ Freshening SL↑

Surface Salinity subtropical gyres salty, getting saltier Durack and Wijffels (2010) S mean Freshwater flux (m 3 /yr) (Josey et al., 1998) 50-year salinity trend (psu)

Thermohaline ALOHA Warming over much of upper ocean (x m!) Peak warming m (S max ), not at surface Cooling below 700 m Salinity increasing in upper 200 m Freshening in the thermocline θ(z)S(z) 0.4 °C/decade 0.16/decade

Station ALOHA Lukas and Santiago-Mandujano (2008) surface salinity 1 psu

Salinity Trends on Isopycnals Freshening σ θ ( m) Max /decade large! Analysis of 21 annual means Analysis of 217 cruise means layer Trend with decadal cycle Trend w/o decadal cycle S max 0.041/ decade 0.039/ decade 0.011/ decade main thermocline / decade / decade / decade The freshening signal is very robust on isopycnal surfaces Small signal, but correlated with O 2 and nutrients

Subduction of ML salinity anomalies + anomalous subduction Analysis of Argo float data Sasaki et al. (2010, GRL); see also Ren and Riser (2010, (Deep-Sea Research II – Suginohara memorial issue) Evolution of a fresh/cold anomaly

Salinity Trends on Neutral Surfaces Durack and Wijffels (2010) zonally averaged salinity trends (Δpsu/ ) 70 S70 N

Salinity trends on Isopycnals water on heavier isopycnals comes from farther away (Δpsu/50 yrs) 24 σ θ 25 σ θ Durack and Wijffels (2010) σ θ 27.5 σ θ

Large vertical excursions of isopycnals due to internal waves and eddies – Poor signal/noise Depth of isopycnals (color scale on right) over ALOHA Depth   Density 25 m Z s.d. S mean

Isopycnal Depth Trends shoaling deepening Weakening stratification 12 m/decade deepening consistent with gyre intensification increasing stratification

Density and Stratification m density ↑ density ↓ σ θ (z) 0.1 kg m -3 /decade

Density and Stratification m density ↑ density ↓ Stratification ↓ in upper ocean σ θ (z)N(z)

Density and Stratification m density ↑ density ↓ Stratification ↓ in upper ocean m ↑ σ θ (z)N(z) dark blue – annual means light blue – cruise means Less stable more stable 0 m 200 m

Mixed Layer Deepening ~4 m/dec Warming ~ 0.05 °C/dec Salinity ↑ ~0.15/dec Density ↑ ~0.1 kg m -3 /dec Freeland et al. (1997) found 6 m/decade Station Papa Carton et al. (2008) found deepening in NPAC over 45 years

A Simple Summary (IPCC Chapter 5) HI acidification salinity increase warmingaccelerated H 2 O cycle SL rise SL rise due to density change

Updated and adapted from Dore et al. (2009, Proc Natl Acad Sci USA 106:12235 ) pH of surface ocean ALOHA pH trend vs depth Annual, interannual, decadal and longer term changes in surface forcing, mixing, and advection Local and remote physics are crucial, not just pCO 2, temperature and biology pH DIC This point was made in the paper Maximum not in surface layer

Dissolved Oxygen and Nutrients O 2 9 μmol/kg/decade Phosphate 0.06 μmol/kg/decade Remineralization of organic material along streamlines?

Summary warming upper ocean increasing upper ocean salinity (with decreasing[?] subducted signal below) decreasing density (ex ML)  increasing dynamic height dynamics vs thermohaline forcing? MLD increasing isopycnal depths and stratification trends show complex vertical structure Acknowledgements Funding for HOT has been provided by the National Science Foundation and the State of Hawaii. We thank the many HOT staff and ships’ crews for their dedication. Sea level rise, ocean warming and biogeochemistry are complicated by ocean dynamics, and by thermodynamic influences of salinity

Climate Change and Hawai`i Long recognized as important problem for Hawai`i Not widely recognized as an important problem for Hawai`i until recently

Moberly and Mackenzie (1985) Focused on sea level rise and warming Sound advice offered to the State on appropriate responses UH can play an important role in the research required to adapt and mitigate “Regretfully, the infrastructure for research activity at University of Hawaii has not kept pace with these or other lines of active research in the past decade. The University’s indirect costs, such as secretaries salaries and equipment repair, are the State’s responsibilities, and if met properly the University will be able to contribute beyond