The Setting - Climatology of the Hawaiian Archipelago. Link to Video of Maui Waves

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1 The Setting - Climatology of the Hawaiian Archipelago Link to Video of Maui Waves

2 What caused this week s weather?

3 What caused this weekend s weather?

4 Today s Objective: Provide overview and description of the properties of the atmosphere and the ocean around Hawai i: marine climate water properties

5 The Setting - Climatology of the Hawaiian archipelago

6 What drives surface ocean currents? What drives the wind patterns?

7 Uneven Heating Earth s surface is heated unevenly by energy from the Sun The Sun Tilt Angle: 23.4 degrees

8 Winds Redistribute Uneven Heat Tropic of Cancer 23.5 deg N Cold Air Cool Air Heating of the Earth s surface leads to seawater evaporation Evaporation Equator Tropic of Capricorn 23.5 deg. S Warm Air Cool Air Evaporation leads to lower sea-level atmospheric pressure Cold Air

9 Winds Redistribute Uneven Heat Cool Dry Air Sinks Tropic of Cancer 23.5 deg N Equator Tropic of Capricorn 23.5 deg. S Precipitation Low Sea-Level Atmospheric Pressure Precipitation Cool Dry Air Sinks

10 Surface Winds 90 N 60 N 30 N High Pressure Equator 30 S Low Pressure Low Pressure High Pressure 60 S 90 S Remember: The Earth rotates to the East

11 Wind Cells High Low High Remember: At the Earth s surface, winds flow from areas of high to low sea-level atmospheric pressure

12 Atmospheric Pressure - Terminology Subpolar Low Mid-lat Cell (Ferrell) Subtropical High Tropical Cell (Hadley) Equatorial Trough Tropical Cell (Hadley) Subtropical High Mid-lat Cell (Ferrell) Subpolar Low

13 Winds - Terminology Westerlies Northeasterly Trades Equatorial Trough Southeasterly Trades Westerlies Remember: Winds named by the direction they blow from

14 Wind Maps Winter: Summer: Scatterometer Climatology of Ocean Winds (SCOW) based on 8 years (Sept 1999 Aug 2007) of QuikSCAT scatterometer data.

15 Precipitation Cells High Low High Remember: At the Earth s surface, winds flow from areas of high to low sea-level atmospheric pressure

16 Precipitation Maps 30 N Equator 30 S 30 N Equator 30 S Tropical Precipitation Band (Equator); Low Rainfall (15-45 degrees)

Regional Salinity Patterns Annual average surface salinity. Period: 1895-1993.

17 Regional Salinity Patterns Annual average surface salinity. Period: Source: World Ocean Atlas, Ocean Climate Laboratory, NOAA. Units: ppt (parts-per-thousand).

Regional SST Patterns Summer Winter Mean surface water

coldest period) and August to October ("summer", the

18 Regional SST Patterns Summer Winter Mean surface water temperatures for February to April ("winter", the coldest period) and August to October ("summer", the warmest period). Period: 1987 to Units: degrees Celsius. Source: AVHRR weekly Multichannel Sea-Surface Temperature (MCSST) Project, Jet Propulsion Laboratory, NASA.

19 Regional SST Patterns SSTs have a strong north to south gradient, and a small annual cycle, being lowest around March 15, and highest around September 15. The variations of temperature parallel the island chain, (i.e. surface waters are warmer to the west at a given latitude).

Regional Atmospheric Patterns Northeasterly trade winds (from the east) are the dominant atmospheric pattern over Hawaiian archipelago; westerlies (from the west) occur farther to the north.

20 Regional Atmospheric Patterns Northeasterly trade winds (from the east) are the dominant atmospheric pattern over Hawaiian archipelago; westerlies (from the west) occur farther to the north. This circulation follows the North Pacific anticyclone (subtropical high). This High Pressure Cell shifts northward in summer, when trade winds intensify and reach on average 35 deg. N, and southward in winter, when westerlies extend as far south as 28 deg. N

21 Regional Pressure Patterns Summer Winter Mean sea-level pressure in the Central Pacific in Summer (June to August) and Winter (December to February). Period: Source: Comprehensive Ocean-Atmosphere Data Set, Environmental Research Laboratory, NOAA. Units: mbar.

22 Regional Wind Patterns Summer Winter Mean surface winds in the Central Pacific for Summer (June to August) and Winter (December to February). Period: Source: Comprehensive Ocean-Atmosphere Data Set, Environmental Research Laboratory, NOAA. Units: m/s (10 m/s = 22 mph).

23 Large-Scale Wind Patterns Annual average trade wind speed is 6 m/s (13.5 mph) at 20 N. Evaporation exceeds precipitation between 15 N and 36 N; further south and north, precipitation exceeds evaporation. There is a net cooling of the ocean by the atmosphere over the entire region.

24 Large-scale Surface Currents

Regional Currents Patterns Mean depth of the 10 C isotherm. Arrows indicate the direction of the flow. Period: 1895-1993. Source: World Ocean Atlas, Ocean Climate Laboratory, NOAA. Units: meters.

25 Regional Currents Patterns Mean depth of the 10 C isotherm. Arrows indicate the direction of the flow. Period: Source: World Ocean Atlas, Ocean Climate Laboratory, NOAA. Units: meters. Mean currents form a large basin-scale clockwise circulation, called gyre, centered at about 28 N. At the latitude of Hawai'i, the circulation is roughly from east to west and intensifies southward.

26 Local Current Patterns Mean surface flow, based on 40,000 ship drift observations, 85,000 drifting buoy data points, and 8,000 current measurements. Units: cm/s (25 cm/s = 0.5 knot). North Equatorial Current (NEC) reaches a mean westward speed of 17 cm/s (0.35 knot) at 13 N. Between N, currents strongly influenced by islands. A clockwise circulation at 19 N, merges with southern branch of NEC.

Waves and Storms The significant wave height

27 Waves and Storms The significant wave height (SWH or H s ) is the mean wave height (trough to crest) of the highest third of the waves (H 1/3 ) Winter Summer Mean significant wave height (H1/3). Period: 1992 to Source: TOPEX Altimeter mission, NASA and CNES. Units: meters.

28 Waves and Storms Strong seasonal cycle in wave height Large monthly variability (storm events)

29 Waves and Storms Strong seasonal cycle in wave direction NOTE: wave direction quantified by angle heading

30 Waves and Storms

31 Waves and Storms

Regional Swell Regimes Four dominant regimes responsible for swell in Hawai i: north Pacific swell trade wind swell south swell Kona storms Regions influenced by the four

32 Regional Swell Regimes Four dominant regimes responsible for swell in Hawai i: north Pacific swell trade wind swell south swell Kona storms Regions influenced by the four dominant swell regimes outlined by Moberly & Chamberlain (1964). Wave rose depicting annual swell heights and directions added to original graphic (Vitousek & Fletcher 2008).

33 Regimes North Pacific Swell Hawai i receives large ocean swell from extra-tropical storms that track predominantly eastward from the northwest Pacific. The north Pacific storminess reaches a peak in the boreal winter, as the Aleutian low intensifies and the north Pacific high moves southward. The strong winds associated with these storms produce large swell events, which can travel for thousands of miles until reaching the shores of Hawai i. In summer, the north Pacific high moves northward and storms become infrequent (Flament et al. 1996).

34 Regimes North Pacific Swell The mean winter wave heights in the north Pacific are 3 m while the mean summer wave heights are 2 m. Seasonal cycle high in winter and low in summer, with mean daily wave height of 4 m. However the system involves eastward storm wave heights of 5-10 m. Satellite (JASON-1) derived average wave heights [m] in the summer and winter.

swells to Aleutians and Pacific Northwest.

35 Regimes North Pacific Swell Many north Pacific storm swells do not reach Hawai i. Storms that originate in high latitudes and those that track northeast send swells to Aleutians and Pacific Northwest. Swells from lower latitude storms, and those that track to southeast, reach Hawai i with the largest wave heights.

Regimes Trade Wind Swell Occur ~ 75% of the year, with an average speed of 15.7 +/- 5 (SD) mph and direction 73 +/- 23 (SD).

36 Regimes Trade Wind Swell Occur ~ 75% of the year, with an average speed of /- 5 (SD) mph and direction 73 +/- 23 (SD). In winter, the north Pacific high weakens and moves south, decreasing trade wind persistence. Although the number of windy days in summer increases, the mean speed in summer and winter months is relatively similar. Number of days per season when trade winds occur with a certain speed ( NOAA Buoy 51001) Notice the persistence of typical trade winds (~ 16 mph) during summer.

Regimes Southern Swell Southern swell arriving in Hawai i is generated farther away than north Pacific swell. Generated by storms south of the equator: Australia, New Zealand and Southern Ocean.

37 Regimes Southern Swell Southern swell arriving in Hawai i is generated farther away than north Pacific swell. Generated by storms south of the equator: Australia, New Zealand and Southern Ocean. South swells occur in summer months (Southern winter) and reach Hawai i with significant wave heights of m. The largest South Swell waves on record (June 1955) had faces over 6 m (20 ft) breaking in shallow water. This first large Antarctic storm of the 2001 season produced significant wave heights of ft (10-15 m)

38 Regimes Southern Swell Peak storm surf reached Hawai i on 11 July 2001 (after 6 days) Data from NOAA/NCEP.

39 Regimes Kona Storms Giambelluca & Schroeder (1998) describe Kona storms as: Low-pressure areas (cyclones) of subtropical origin that develop NW of Hawai i in winter and move slowly eastward. They are accompanied by southerly winds (from whose direction the storm derives its name) and by rain. Kona storms generate wave heights of 3-4 m.

Regimes Kona Storms While minor Kona storms occur practically every year in Hawai i, major Kona storms with strong winds and large wave heights occur every 5-10

40 Regimes Kona Storms While minor Kona storms occur practically every year in Hawai i, major Kona storms with strong winds and large wave heights occur every 5-10 years, during La Niña years and the negative phase of the PDO cycle. El Niño years (warm phase) and the positive (warm) PDO phases seem to suppress Kona storms.

41 El Niño Southern Oscillation Walker Cell Low Pressure High Pressure Southern Oscillation: SL Pressure Tahiti - SL Pressure Darwin

42 Regional ENSO Effects El Niño Pressure La Niña

43 Local ENSO Effects Weaker trades Warm surface water flows east Thermocline depth deepens Stronger trades Warm surface water flows west Thermocline depth shoals

44 Your Tasks After This Lecture - Understand and use the terminology employed in this lecture - Describe the oceanographic / climatological setting of Hawai i

45 References Flament, P., Keenan, S., Lumpkin, R., Sawyer, M., Stroup, E.D., The Ocean Atlas of Hawai i. Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai i ( Giambelluca, T.W., Schroeder, T.A., Climate. In: Juvik, S.P., and Juvik, J.O. (eds.), Atlas of Hawai i. Honolulu: University of Hawai i Press, pp Moberly, R., Chamberlain, T Hawaiian Beach Systems. Hawai i Institute of Geophysics Report 64-2, University of Hawai i.

Lecture 13 El Niño/La Niña Ocean-Atmosphere Interaction. Idealized 3-Cell Model of Wind Patterns on a Rotating Earth. Previous Lecture!

Lecture 13 El Niño/La Niña Ocean-Atmosphere Interaction Previous Lecture! Global Winds General Circulation of winds at the surface and aloft Polar Jet Stream Subtropical Jet Stream Monsoons 1 2 Radiation